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IEEE TRANSACTIONS ON ROBOTICS AND AUTOMATION, VOL. 20, NO. 2, APRIL 2004243Coverage Control for Mobile Sensing NetworksJorge Cortés, Member, IEEE, Sonia Martínez, Member, IEEE, Timur Karatas, and ¸ Francesco Bullo, Senior Member, IEEEAbstract—This paper presents control and coordination algorithms for groups of vehicles. The focus is on autonomous vehicle networks performing distributed sensing tasks, where each vehicle plays the role of a mobile tunable sensor. The paper proposes gradient descent algorithms for a class of utility functions which encode optimal coverage and sensing policies. The resulting closed-loop behavior is adaptive, distributed, asynchronous, and verifiably correct. Index Terms—Centroidal Voronoi partitions, coverage control, distributed and asynchronous algorithms, sensor networks.rents, and other distributed oceanographic signals. The vehicles communicate via an acoustic local area network and coordinate their motion in response to local sensing information and to evolving global data. This mobile sensing network is meant to provide the ability to sample the environment adaptively in space and time. By identifying evolving temperature and current gradients with higher accuracy and resolution than current static sensors, this technology could lead to the development and validation of improved oceanographic models. B. Optimal Sensor Allocation and Coverage Problems A fundamental prototype problem in this paper is that of characterizing and optimizing notions of quality-of-service (QoS) provided by an adaptive sensor network in a dynamic environment. To this goal, we introduce a notion of sensor coverage that formalizes an optimal sensor placement problem. This spatial resource-allocation problem is the subject of a discipline called locational optimization [5]–[9]. Locational optimization problems pervade a broad spectrum of scientific disciplines. Biologists rely on locational optimization tools to study how animals share territory and to characterize the behavior of animal groups obeying the following interaction rule: each animal establishes a region of dominance and moves toward its center. Locational optimization problems are spatial resource-allocation problems (e.g., where to place mailboxes in a city or cache servers on the internet) and play a central role in quantization and information theory (e.g., how to design a minimum-distortion fixed-rate vector quantizer). Other technologies affected by locational optimization include mesh and grid optimization methods, clustering analysis, data compression, and statistical pattern recognition. Because locational optimization problems are so widely studied, it is not surprising that methods are indeed available to tackle coverage problems; see [5], and [8]–[10]. However, most currently available algorithms are not applicable to mobile sensing networks because they inherently assume a centralized computation for a limited-size problem in a known static environment. This is not the case in multivehicle networks which, instead, rely on a distributed communication and computation architecture. Although an ad-hoc wireless network provides the ability to share some information, no global omniscient leader might be present to coordinate the group. The inherent spatially distributed nature and limited communication capabilities of a mobile network invalidate classic approaches to algorithm design. C. Distributed Asynchronous Algorithms for Coverage Control In this paper, we design coordination algorithms implementable by a multivehicle network with limited sensing andI. INTRODUCTION A. Mobile Sensing NetworksTHE deployment of large groups of autonomous vehicles is rapidly becoming possible because of technological advances in networking and in miniaturization of electromechanical systems. In the near future, large numbers of robots will coordinate their actions through ad-hoc communication networks, and will perform challenging tasks, including search and recovery operations, manipulation in hazardous environments, exploration, surveillance, and environmental monitoring for pollution detection and estimation. The potential advantages of employing teams of agents are numerous. For instance, certain tasks are difficult, if not impossible, when performed by a single vehicle agent. Further, a group of vehicles inherently provides robustness to failures of single agents or communication links. Working prototypes of active sensing networks have already been developed; see [1]–[4]. In [3], launchable miniature mobile robots communicate through a wireless network. The vehicles are equipped with sensors for vibrations, acoustic, magnetic, and infrared (IR) signals as well as an active video module (i.e., the camera or micro-radar is controlled via a pan-tilt unit). A second system is suggested in [4] under the name of Autonomous Oceanographic Sampling Network. In this case, underwater vehicles are envisioned measuring temperature, cur-Manuscript received November 4, 2002; revised June 26, 2003. This paper was recommended for publication by Associate Editor L. Parker and Editor A. De Luca upon evaluation of the reviewers’ comments. This work was supported in part by the Army Research Office (ARO) under Grant DAAD 190110716, and in part by the Defense Advanced Research Projects Agency/Air Force Office of Scientific Research (DARPA/AFOSR) under MURI Award F49620-02-1-0325. This paper was presented in part at the IEEE Conference on Robotics and Automation, Arlington, VA, May 2002, and in part at the Mediterranean Conference on Control and Automation, Lisbon, Portugal, July 2002. J. Cortés, T. Karatas, and F. Bullo are with the Coordinated Science Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL 61801 USA (e-mail: jcortes@; tkaratas@; bullo@). S. Martínez is with the Escola Universitària Politècnica de Vilanova i la Geltrú, Universidad Politécnica de Cataluña, Vilanova i la Geltrú 08800, Spain (e-mail: soniam@mat.upc.es). Digital Object Identifier 10.1109/TRA.2004.8246981042-296X/04$20.00 © 2004 IEEE244IEEE TRANSACTIONS ON ROBOTICS AND AUTOMATION, VOL. 20, NO. 2, APRIL 2004communication capabilities. Our approach is related to the classic Lloyd algorithm from quantization theory; see [11] for a reprint of the original report and [12] for a historical overview. We present Lloyd descent algorithms that take into careful consideration all constraints on the mobile sensing network. In particular, we design coverage algorithms that are adaptive, distributed, asynchronous, and verifiably asymptotically correct. Adaptive: Our coverage algorithms provide the network with the ability to address changing environments, sensing task, and network topology (due to agents’ departures, arrivals, or failures). Distributed: Our coverage algorithms are distributed in the sense that the behavior of each vehicle depends only on the location of its neighbors. Also, our algorithms do not require a fixed-topology communication graph, i.e., the neighborhood relationships do change as the network evolves. The advantages of distributed algorithms are scalability and robustness. Asynchronous: Our coverage algorithms are amenable to asynchronous implementation. This means that the algorithms can be implemented in a network composed of agents evolving at different speeds, with different computation and communication capabilities. Furthermore, our algorithms do not require a global synchronization, and convergence properties are preserved, even if information about neighboring vehicles propagates with some delay. An advantage of asynchronism is a minimized communication overhead. Verifiable Asymptotically Correct: Our algorithms guarantee monotonic descent of the cost function encoding the sensing task. Asymptotically, the evolution of the mobile sensing network is guaranteed to converge to so-called centroidal Voronoi configurations (i.e., configurations where the location of each generator coincides with the centroid of the corresponding Voronoi cell) that are critical points of the optimal sensor-coverage problem. Let us describe the contributions of this paper in some detail. Section II reviews certain locational optimization problems and their solutions as centroidal Voronoi partitions. Section III provides a continuous-time version of the classic Lloyd algorithm from vector quantization and applies it to the setting of multivehicle networks. In discrete time, we propose a family of Lloyd algorithms. We carefully characterize convergence properties for both continuous and discrete-time versions (Appendix I collects some relevant facts on descent flows). We discuss a worst-case optimization problem, we investigate a simple uniform planar setting, and we present simulation results. Section IV presents two asynchronous distributed implementations of Lloyd algorithm for ad-hoc networks with communication and sensing capabilities. Our treatment carefully accounts for the constraints imposed by the distributed nature of the vehicle network. We present two asynchronous implementations, one based on classic results on distributed gradient flows, the other based on the structure of the coverage problem. (Appendix II briefly reviews some known results on asynchronous gradient algorithms.)Section V-A considers vehicle models with more realistic dynamics. We present two formal results on passive vehicle dynamics and on vehicles equipped with individual local controllers. We present numerical simulations of passive vehicle models and of unicycle mobile vehicles. Next, Section V-B describes density functions that lead the multivehicle network to predetermined geometric patterns. We present our conclusions and directions for future research in Section VI. D. Review of Distributed Algorithms for Cooperative Control Recent years have witnessed a large research effort focused on motion planning and coordination problems for multivehicle systems. Issues include geometric patterns [13]–[16], formation control [17], [18], gradient climbing [19], and conflict avoidance [20]. It is only recently, however, that truly distributed coordination laws for dynamic networks are being proposed; e.g., see [21]–[23]. Heuristic approaches to the design of interaction rules and emerging behaviors have been throughly investigated within the literature on behavior-based robotics; see [17], and [24]–[28]. An example of coverage control is discussed in [29]. Along this line of research, algorithms have been designed for sophisticated cooperative tasks. However, no formal results are currently available on how to design reactive control laws, ensure their correctness, and guarantee their optimality with respect to an aggregate objective. The study of distributed algorithms is concerned with providing mathematical models, devising precise specifications for their behavior, and formally proving their correctness and complexity. Via an automata-theoretic approach, the references [30] and [31] treat distributed consensus, resource allocation, communication, and data consistency problems. From a numerical optimization viewpoint, the works in [32] and [33] discuss distributed asynchronous algorithms as networking algorithms, rate and flow control, and gradient descent flows. Typically, both these sets of references consider networks with fixed topology, and do not address algorithms over ad-hoc dynamically changing networks. Another common assumption is that any time an agent communicates its location, it broadcasts it to every other agent in the network. In our setting, this would require a nondistributed communication setup. Finally, we note that the terminology “coverage” is also used in [34] and [35] and references therein to refer to a different problem called the coverage path-planning problem, where a single robot equipped with a limited footprint sensor needs to visit all points in its environment. II. FROM LOCATION OPTIMIZATION TO CENTROIDAL VORONOI PARTITIONS A. Locational Optimization In this section, we describe a collection of known facts about a meaningful optimization problem. References include the theory and applications of centroidal Voronoi partitions, see [10], and the discipline of facility location, see [6]. In the paper, we interchangeably refer to the elements of the network be the set as sensors, agents, vehicles, or robots. We letCORTÉS et al.: COVERAGE CONTROL FOR MOBILE SENSING NETWORKS245the position of the sensors and the partition of the space. This problem is referred to as a facility location problem and, in particular, as a continuous -median problem in [6]. Remark 2.2: Note that if we interchange the positions of any two agents, along with their associated regions of dominance, is not afthe value of the locational optimization function denotes the discrete group of perfected. Equivalently, if mutations of elements, then for all . To eliminate this discrete redundancy, one could take natural action on , and consider as the configuration space of for the position of the vehicles. B. Voronoi PartitionsFig. 1. Contour plot on a polygonal environment of the Gaussian density y ). function  = exp( x0 0of nonnegative real numbers, be the set of positive natural numbers, and . be a convex polytope in , including its interior, Let denote the Euclidean distance function. We call and let a distribution density function if it reprea map sents a measure of information or probability that some event takes place over . In equivalent words, we can consider to be the bounded support of the function . Let be the location of sensors, each moving in the space . Because of noise and loss of resolution, the sensing performance at point taken from th sensor at the position degrades with the between and ; we describe this degradadistance . tion with a nondecreasing differentiable function Accordingly, provides a quantitative assessment of how poor the sensing performance is (see Fig. 1). mobile robots Remark 2.1: As an example, consider equipped with microphones attempting to detect, identify, and localize a sound source. How should we plan the robots’ motion in order to maximize the detection probability? Assuming the source emits a known signal, the optimal detection algorithm is a matched filter (i.e., convolve the known waveform with the received signal and threshold). The source is detected depending on the signal-to-noise-ratio (SNR), which is inversely proportional to the distance between the microphone and the source. Various electromagnetic and sound sensors have SNRs inversely proportional to distance. Within the context of this paper, a partition of is a collecwith disjoint interiors, tion of polytopes and are whose union is . We say that two partitions equal if and only differ by a set of -measure zero, for . all We consider the task of minimizing the locational optimization function (1)One can easily see that, at a fixed-sensors location, the optimal partition of is the Voronoi partition generated by the pointsWe refer to [9] for a comprehensive treatment on Voronoi diagrams, and briefly present some relevant concepts. The set is called the Voronoi diagram for the of regions . When the two Voronoi regions generators and are adjacent (i.e., they share an edge), is called a (and vice versa). The set of indexes (Voronoi) neighbor of of the Voronoi neighbors of is denoted by . Clearly, if and only if . We also define the face as . Voronoi diagrams can be defined with , and respect to various distance functions, e.g., the 1-, 2-, -norm over , see [36]. Some useful facts about the Euclidean setting are the following: if is a convex polytope in an -dimensional Euclidean space, the boundary of each is the union of -dimensional convex polytopes. In what follows, we shall writeNote that using the definition of the Voronoi partition, we have for all . Therefore(2) that is, the locational optimization function can be interpreted as an expected value composed with a “min” operation. This is the usual way in which the problem is presented in the facility location and operations research literature [6]. Remarkably, one can show [10] that(3) where we assume that the th sensor is responsible for measure. Note that the function ments over its “dominance region” is to be minimized with respect to both 1) the sensors location , and 2) the assignment of the dominance regions . The optimization is, therefore, to be performed with respect to with respect to the th sensor i.e., the partial derivative of only depends on its own position and the position of its Voronoi neighbors. Therefore, the computation of the derivative of with respect to the sensors’ location is decentralized in the sense246IEEE TRANSACTIONS ON ROBOTICS AND AUTOMATION, VOL. 20, NO. 2, APRIL 2004of Voronoi. Moreover, one can deduce some smoothness prop: since the Voronoi partition depends at least conerties of , for all , the function tinuously on is at least continuously differentiable on for some . C. Centroidal Voronoi Partitions Let us recall some basic quantities associated with a region and a mass density function . The (generalized) mass, centroid (or center of mass), and polar moment of inertia are defined asIII. CONTINUOUS AND DISCRETE-TIME LLOYD DESCENT FOR COVERAGE CONTROL In this section, we describe algorithms to compute the location of sensors that minimize the cost , both in continuous and in discrete time. In Section III-A, we propose a continuous-time version of the classic Lloyd algorithm. Here, both the positions and partitions evolve in continuous time, whereas the Lloyd algorithm for vector quantization is designed in discrete time. In Section III-B, we develop a family of variations of Lloyd algorithm in discrete time. In both settings, we prove that the proposed algorithms are gradient descent flows. A. A Continuous-Time Lloyd Algorithm Assume the sensors location obeys a first-order dynamical behavior described byAdditionally, by the parallel axis theorem, one can write (4) is defined as the polar moment of inertia of where . the region about its centroid Let us consider again the locational optimization problem (1), and suppose now we are strictly interested in the setting (5) . The parallel axis that is, we assume and theorem leads to simplifications for both the function its partial derivative Consider a cost function to be minimized and impose that follows a gradient descent. In equivalent conthe location a Lyapunov function, and trol theoretical terms, consider stabilize the multivehicle system to one of its local minima via dissipative control. Formally, we set (6) where is a positive gain, and where we assume that the is continuously updated. partition Proposition 3.1 (Continuous-Time Lloyd Descent): For the closed-loop system induced by (6), the sensors location con, i.e., the verges asymptotically to the set of critical points of set of centroidal Voronoi configurations on . Assuming this set is finite, the sensors location converges to a centroidal Voronoi configuration. Proof: Under the control law (6), we haveHere, the mass density function is define. It is convenient toTherefore, the (not necessarily unique) local minimum points are centroids of their for the location optimization function Voronoi cells, i.e., each location satisfies two properties simultaneously: it is the generator for the Voronoi cell , and it is its centroidAccordingly, the critical partitions and points for are called centroidal Voronoi partitions. We will refer to a sensors’ configuration as a centroidal Voronoi configuration if it gives rise to a centroidal Voronoi partition. Of course, centroidal Voronoi configurations depend on the specific distribution density function , and an arbitrary pair admits, in general, multiple centroidal Voronoi configurations. This discussion provides a proof alternative to the one given in [10] for the necessity of centroidal Voronoi partitions as solutions to the continuous -median location problem.By LaSalle’s principle, the sensors location converges to the , which is precisely the largest invariant set contained in set of centroidal Voronoi configurations. Since this set is clearly consists of invariant for (6), we get the stated result. If converges to one of them a finite collection of points, then (see Corollary 1.2). Remark 3.2: If is finite, and , then a sufficient condition that guarantees exponential convergence is be positive definite at . Establishing that the Hessian of this property is a known open problem, see [10]. Note that this gradient descent is not guaranteed to find the global minimum. For example, in the vector quantization and signal processing literature [12], it is known that for bimodal distribution density functions, the solution to the gradient flow reaches local minima where the number of generators allocated to the two region of maxima are not optimally partitioned.CORTÉS et al.: COVERAGE CONTROL FOR MOBILE SENSING NETWORKS247B. A Family of Discrete-Time Lloyd Algorithms Let us consider the following variations of Lloyd algorithm. verifying the Let be a continuous mapping following two properties: , 1) for all , where denotes the th component of ; is not centroidal, then there exists a such that 2) if . Property 1) guarantees that, if moved according to , the agents of the network do not increase their distance to its corresponding centroid. Property 2) ensures that at least one robot moves at each iteration and strictly approaches the centroid of its Voronoi region. Because of this property, the fixed points of are the set of centroidal Voronoi configurations. Proposition 3.3 (Discrete-Time Lloyd Descent): Let be a continuous mapping satisfying properties denote the initial sensors’ location. 1) and 2). Let converges to the set Then, the sequence of centroidal Voronoi configurations. If this set is finite, then converges to a centroidal the sequence Voronoi configuration. as an objective funcProof: Consider tion for the algorithm . Using the parallel axis theorem, , and therefore (7) for all , as long as with strict inequality if for any , . , with strict inequality if In particular, , where denotes the set of centroids of the partition . Moreover, since the Voronoi partition is the optimal one for fixed , we also have (8) with strict inequality if . Now, because of property 1) of , inequality (7) yieldsalgorithm can also be seen as a fixed-point iteration. Consider for the mappingsLet be defined by . is continuous (indeed, ), and corresponds to Clearly, , Lloyd algorithm. Now, for all . Moreover, if is not centroidal, then . Therefore, verifies the inequality is strict for all properties 1) and 2). C. Remarks 1) Note that different sensor performance functions in (1) correspond to different optimization problems. Provided one [cf. (1)], uses the Euclidean distance in the definition of the standard Voronoi partition computed with respect to the Euclidean metric remains the optimal partition. For arbitrary , it is not possible anymore to decompose into the sum and . Nevertheless, it is still of terms similar to possible to implement the gradient flow via the expression for the partial derivative (3). Proposition 3.5: Assume the sensors location obeys a first. Then, for the closed-loop order dynamical behavior, system induced by the gradient law (3), , the converges asymptotically to sensors location . Assuming this set is finite, the the set of critical points of sensors location converges to a critical point. 2) More generally, various distance notions can be used to define locational optimization functions. Different performance function gives rise to corresponding notions of “center of a region” (any notion of geometric center, mean, or average is an interesting candidate). These can then be adopted in designing coverage algorithms. We refer to [36] for a discussion on Voronoi partitions based on non-Euclidean distance functions, and to [5] and [8] for a discussion on the corresponding locational optimization problems. 3) Next, let us discuss an interesting variation of the original problem. In [6], minimizing the expected minimum distance in (2) is referred to as the continuous -median function problem. It is instructive to consider the worst-case minimum distance function, corresponding to the scenario where no information is available on the distribution density function. In other words, the network seeks to minimize the largest possible distance from any point in to any of the sensor locations, i.e., to minimize the functionand the inequality is strict if of . In additionis not centroidal by property 2)because of (8). Hence, , and the inequality is a is strict if is not centroidal. We then conclude that descent function for the algorithm . The result now follows from the global convergence Theorem 1.3 and Proposition 1.4 in Appendix I. Remark 3.4: Lloyd algorithm in quantization theory [11], [12] is usually presented as follows. Given the location of agents, : 1) construct the Voronoi partition corresponding to ; 2) compute the mass centroids of the Voronoi regions found in step 1). Set the new location of the agents to these centroids, and return to step 1). LloydThis optimization is referred to as the -center problem in [6] and [7]. One can design a strategy for the -center problem analog to the Lloyd algorithm for the -median problem. Each vehicle moves, in continuous or discrete time, toward the center of the minimum-radius sphere enclosing the polytope. We refer to [37] for a convergence analysis of the continuous-time algorithms. In what follows, we shall restrict our attention to the -median problem and to centroidal Voronoi partitions.248IEEE TRANSACTIONS ON ROBOTICS AND AUTOMATION, VOL. 20, NO. 2, APRIL 2004E. Numerical Simulations To illustrate the performance of the continuous-time Lloyd algorithm, we include some simulation results. The as a single cenalgorithm is implemented in setting, the code computes the tralized program. For the package bounded Voronoi diagram using the , and computes mass, centroid, and polar moment of inertia of polygons via the numerical . Careful attention was paid to integration routine numerical accuracy issues in the computation of the Voronoi diagram and in the integration. We illustrate the performance of the closed-loop system in Fig. 3. IV. ASYNCHRONOUS DISTRIBUTED IMPLEMENTATIONS In this section, we show how the Lloyd gradient algorithm can be implemented in an asynchronous distributed fashion. In Section IV-A, we describe our model for a network of robotic agents, and we introduce a precise notion of distributed evolution. Next, we provide two distributed algorithms for the local computation and maintenance of the Voronoi cells. Finally, in Section IV-C, we propose two distributed asynchronous implementations of Lloyd algorithm. The first one is based on the gradient optimization algorithms, as described in [32], and the second one relies on the special structure of the coverage problem. A. Modeling an Asynchronous Distributed Network of Mobile Robotic Agents We start by modeling a robotic agent that performs sensing, communication, computation, and control actions. We are interested in the behavior of the asynchronous network resulting from the interaction of finitely many robotic agents. A framework to formalize the following concepts is the theory of distributed algorithms; see [30]. Let us here introduce the notion of robotic agent with computation, communication, and control capabilities as the th element of a network. The th agent has a processor with the ability of allocating continuous and discrete states and performing operations on them. Each vehicle has access to its unique identifier . The th agent occupies a location and it is cafor any period of pable of moving in space, at any time time , according to a first-order dynamics of the form (11) The processor has access to the agent’s location and deter. The processor of the th agent mines the control pair has access to a local clock , and a scheduling sequence, such i.e., an increasing sequence of times and . The that processor of the th agent is capable of transmitting information to any other agent within a closed disk of radius . We to be a quantity controlassume the communication radius lable by the th processor and the corresponding communication bandwidth to be limited. We represent the information flow between the agents by means of “send” (within specified radius ) and “receive” commands with a finite number of arguments.Fig. 2. Notation conventions for a convex polygon.D. Computations Over Polygons With Uniform Density In this section, we investigate closed-form expression for the control laws introduced above. Assume the Voronoi region is a convex polygon (i.e., a polytope in ) with vertexes such as in Fig. 2. It is labeled convenient to define . Furthermore, we . By evaluating assume that the density function is the corresponding integrals, one can obtain the following closed-form expressions:(9) To present a simple formula for the polar moment of inertia, let and , for . Then, the polar moment of inertia of a polygon about its centroid becomesThe proof of these formulas is based on decomposing the polygon into the union of disjoint triangles. We refer to [38] for . analog expressions over Note also that the Voronoi polygon’s vertexes can be expressed as a function of the neighboring vehicles. The vertexes of the th Voronoi polygon that lie in the interior of are the circumcenters of the triangles formed by and any two neighbors adjacent to . The circumcenter of the triangle determined by , , and is(10) is the area of the triangle, and . where Equation (9) for a polygon’s centroid and (10) for the Voronoi cell’s vertexes lead to a closed-form algebraic expression for the control law in (6) as a function of the neighboring vehicles’ location.。

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一般说，量价关系，如同水与船的关系、水涨船高的关系。

Roles of Conformational and Configurational Defects on the Physical Aging of Amorphous Poly(lactic acid)Kaoru Aou†and Shaw Ling Hsu*Department of Polymer Science and Engineering and Materials Research Science and Engineering Center,Uni V ersity of Massachusetts,Amherst,Massachusetts01003Lothar W.Kleiner and Fuh-Wei TangAbbott Vascular,Cardiac Therapies,3200Lakeside Dri V e,Santa Clara,California95054Recei V ed:June11,2007;In Final Form:August8,2007The roles of poly(lactic acid)chain conformation and configuration on the enthalpy relaxation kinetics ofamorphous poly(lactic acid)were examined.Enthalpic relaxation data,which were scaled to the samesupercooling from the initial fictive temperature,were taken for three types of the polymer containing variousD-lactyl monomers(5.7%,13.0%,50%)to assess the effects of configurational defects.The kinetics datawere very similar from sample to sample.The effects of configurational defects were assessed using thegeneralized Kohlrausch-Williams-Watts(KWW)equation solved by the Tool-Narayanaswamy-Moynihan(TNM)equation.The major effect of increasing the D-lactyl contents was to lower the PLA glass transitiontemperature,thereby accelerating the kinetics of enthalpic relaxation.Configurational defects showed nosignificant effect on the other KWW/TNM fit parameters(x,∆h,ln A).A slightly larger KWW stretchedexponential parameter is observed for greater(50%D)than for lower(5.7%D)amount of D-lactyl monomer,although these differences are just within the experimental error.Raman spectroscopy showed that conformationdoes not change appreciably during physical aging.IntroductionThe changes in mechanical and transport properties,which accompany physical aging,1,2are critically relevant to both industrial and medical applications of poly(lactic acid),or PLA. This polymer,derived from renewable resources,3,4was recently introduced in commercial applications as a viable commodity-scale thermoplastic.5-7PLA has also been studied as a drug-carrier matrix in drug delivery systems as well as the bulk component of other medical devices as it has been established to be biocompatible and bioresorbable.8-11It is important to understand the kinetics of PLA physical aging and its impact on the performance of PLA-based drug delivery systems,as it impacts device shelf life,which needs to be evaluated for any pharmaceutical device.Physical aging is the densification,or isotropic volumetric shrinkage,of amorphous materials below their glass transition temperature.This is in contrast to anisotropic shrinkage,in particular fiber shrinkage,which we previously studied using a spectroscopic method and in relation to molecular deformation in PLA fibers.12The same method,originally developed in another study,7is used here to evaluate any changes in the rotational isomeric populations of PLA during physical aging, in order to determine any changes in PLA conformation during physical aging.Conformational changes have been reported to cease below the glass transition for polystyrene and poly(methyl methacrylate),13,14as reported from wide-angle X-ray scattering, whereas others have used a model that assumes conformational changes during physical aging.15,16There are also studies based on thermal density fluctuations,which show that physical aging is accompanied by an increase in local order.17,18Therefore,it was important to study whether or not conformational changes are observable during the physical aging of PLA.We are also interested in the effect of configurational defects, or polymer tacticity,on the kinetics of physical aging.To our knowledge,there are only a few studies,namely,on poly(methyl methacrylate),that report an effect of polymer tacticity on the activation enthalpy.19,20The effect of stereochemical defects on physical aging is a relatively unexplored and important aspect that we pursue through this study.From our past investigations, we know that D-lactyl defects perturb the distribution of rotational isomer populations at D-L junctions,thereby render-ing the overall chain more flexible.21We know that a greater fraction of configurational defects leads to lower glass transition temperatures(T g),which speeds up the kinetics of physical aging.We wish to see any effects of these defects above and beyond this change in the T g.PLA amorphous dynamics have previously been studied using dielectric spectroscopy and calorimetric methods.22-27However, no well-known physical aging models were used to analyze those aging kinetics data.We use the generalized Kohlrausch-Williams-Watts(KWW)equation,solved by the Tool-Narayanaswamy-Moynihan(TNM)description of relaxation time,to characterize the physical aging kinetics of PLA,as it is an accepted model for analysis of physical aging.28-30In this study,calorimetric methods are employed to monitor enthalpic relaxation of PLA,which is a measure of physical aging. Materials and Experimental TechniquesMaterials.Poly(DL-lactic acid),or PLA50,was obtained from Abbott Vascular,Inc.Poly(L-lactic acid)with 5.7%D,or*To whom correspondence should be addressed.Phone:413-577-1125.E-mail:slhsu@.†Current address:PU Global Product R&D,The Dow ChemicalCompany,Freeport,TX77541.12322J.Phys.Chem.B2007,111,12322-1232710.1021/jp074509t CCC:$37.00©2007American Chemical SocietyPublished on Web10/04/2007PLA5.7,and 13.0%D -lactyl content,or PLA13,were re-ceived from NatureWorks and were the same samples used in previous studies.21The D -lactyl contents were chosen such that quiescent crystallization was slow and crystallization could be readily suppressed by a rapid thermal quench,at the same time covering a wide range of tacticity.Residual lactides were found for the as-received PLA50sample,as indicated in its Raman spectra by the extra peak at 656cm -1,as shown in Figure 1.DL -Lactide was used as received from Aldrich.The PLA50sample was therefore purified by first preparing a solution in dichloromethane at a concentration of 10%by weight and then precipitating in methanol.The sample was dried in ambient atmosphere and then further dried in vacuum at 60°C for 2h.Raman spectra of the resulting samples were taken to ensure that no solvent peaks are present.The same treatment was performed for the PLA13and PLA5.7samples.Molecular Weight Characterization.An in-house-as-sembled size exclusion chromatograph was used to measure the molecular weight parameters of various PLA samples.A Knauer HPLC pump K-501and a Polymer Laboratories PL DataStream detector were used on a Polymer Laboratories 5µm mixed-D column,with chloroform at 30°C as the eluting medium.Narrow molecular weight polystyrene standards ranging from 2.7to 390kg mol -1were used for calibration.Differential Scanning Calorimetry.A TA Instruments model Q100differential scanning calorimeter equipped with an RCS cooling system and nitrogen gas purge set at 50mL/min flow rate was used for all calorimetric measurements.Aluminum pans with PLA samples were hermetically sealed before measurement.Polymer sample masses varied from 6to 13mg.The top part of the pan was pressed down to improve thermal contact with the sample,taking care that the flatness of the bottom face contacting the temperature sensor in the differential scanning calorimetry (DSC)chamber was maintained.Indium and water were used as a standard to calibrate the temperature at their onset melting points of 156.6and 0.0°C,respectively.The indium heat of fusion (28.6J g -1)was used to calibrate heat flow.31Heat flow and not absolute heat capacity was the quantity measured,since for the purposes of enthalpy relaxation kinetics,it was only necessary to measure the total heat release.19The difference between heat flows of aged samples and quenched glass was integrated over a temperature interval of 10-100°C to compute the excess enthalpy that characterizes the extent of physical aging,in accordance with the method used by Petrie.32The “glass transition temperature”in heating scans was taken as the halfway point of the tangent line containing the inflection point and hereafter referred to as T g,h .The fictive temperature option available on the Universal Analysis software from TA Instruments was not used,in largepart because in our measurements,the value thus measured did not differ by more than 0.05°C compared with T g,h .The onset of glass transition upon cooling,or T g,onset ,has been taken to be the temperature of the intersection point between the liquid baseline and the glass transition inflection point in constant rate cooling scans.Dispersive Raman Microspectroscopy.A Jobin-Yvon Horiba LabRam HR800dispersive Raman spectrometer was used to obtain Raman spectra of PLA50.The 632.8nm line of the helium -neon gas laser was used for excitation.A spectral resolution of 4cm -1was maintained near the laser line.Circularly polarized incident light was obtained with a quarter-wave retardation plate in order to perform a signal aver-aging over all possible orientations.For the purposes of acquiring temperature-dependent Raman spectra,an in-house-built aluminum heating block was used.A copper -constantan thermocouple (diameter ∼0.08mm),calibrated with boiling water (100.0°C)and ice water (0.0°C),was placed close to the sample for temperature measurement.The PLA50sample was further wrapped with aluminum foil for good thermal conduction,with a small hole at the top (∼2mm)to allow for Raman spectral acquisition.The sample was conditioned at 55°C (well above both its glass transition and fictive temperatures)for 1.5h to initialize thermal history to the equilibrium liquid state.Computer Program for Curve Fitting to KWW/TNM Equations.A computer program code written in Visual Basics for Applications (VBA)was implemented in a spreadsheet written in Microsoft Excel 2000.This VBA program was used to fit the data to the nonlinear,generalized KWW/TNM equations.A standard Levenberg -Marquardt algorithm was used to fit the experimental aging data to those equations.33,34ModelingIn order to evaluate the effect of configurational defects,the theories on physical aging kinetics are reviewed.The aging behavior of small molecular glass and atactic polymers has been studied extensively in the past.Many phenomenological models of sub-glass-transition amorphous dynamics exist.28-30,35,36One that is commonly encountered in literature is the Cowie -Ferguson equation,37or the linear KWW equation,which requires a simple linear three-parameter fit and has been used most often with small molecular and inorganic glasses.38,39Its theoretical explanation has been presented with minimal as-sumptions,40which is the major reason for its adoption in the interpretation of physical aging in many systems.However,the linear KWW equation has problems in theory and in application in the context of enthalpic relaxation.41In fact,the asymmetry-of-approach and the memory effect experiments by Kovacs showed that experimental phenomena could not be explained by a constant relaxation time,36,42which means that a variable relaxation time has to be incorporated in any aging kinetics model.A recent study also indicates that the fit value of ∆H endo approaching time infinity from a linear KWW analysis of short-time data cannot predict the equilibrium value of ∆H endo that was obtained experimentally.43In order to reconcile nonlinear effects,the generalized KWW equation and TNM formalism for relaxation time are used to analyze the physical aging most commonly of polymeric glasses.28-30,36The TNM model modifies the Arrhenius equation by incorporating the fictive temperature as a measure of the instantaneous structure,in addition to the annealing temperature,of the material during aging.The generalized KWW equation is expressed as follows:Figure 1.Raman spectra of DL -lactide,PLA50pellet as-received,and PLA50after purification.Physical Aging of Amorphous Poly(lactic acid)J.Phys.Chem.B,Vol.111,No.42,200712323whereδ/δ0represents the deviation from thermodynamic equilibrium,t is the aging time,τ0is the characteristic relaxation time(variable over the course of enthalpic relaxation),and is a stretched exponential parameter.The calorimetricδ/δ0is calculated from experimental quantities as follows:where∆H∞is the endotherm at long time(equilibrium)and ∆H t is the endotherm at time t.As eq1is not solvable analytically for known expressions of the relaxation time,it is numerically solved.The solution uses the well-known TNM equation,28,29,36which has temperature-(T a)and structure-dependent(T f)terms:where ln A is a pre-exponential factor,R is the universal gas constant,T a is the annealing temperature,∆h is the activation energy for enthalpy relaxation,and x partitions the temperature and structure terms.T f is the instantaneous fictive temperature of the material that represents the instantaneous structure of the glass during aging,as shown:where T a is the aging temperature and∆C p is the change in heat capacity as the system passes through the glass transition temperature.Although there are criticisms concerning the shortcoming of the model,37,44,45this is the preferred model usedin studies of polymer physical aging.46,47In the actual application of the TNM formalism to the analysis of physical aging,finding a stable solution can be problematic if a four-parameter fit( ,x,∆h,ln A)is adopted.This owes to the fact that the solution can be overly sensitive to experimental error.This aspect is remedied by reducing the data analysis to a three-parameter fit.This is done by fixing one of the parameters out of the four-parameter fit.It has been noted previously that,due to the fact that T a/T f is close to unity on the absolute temperature scale,then eq3simplifies to where T a∼T f∼T so that two of the fitting parameters(ln Aand∆h)would be correlated linearly.19Owing to this correlation, the curve fit becomes extremely sensitive to small changes in δ(of the order of magnitude of experimental uncertainty).To address this issue,the following approximation was used:48where q is the cooling rate and T g,onset is the onset of glass transition.The parameter∆h was thus determined from a set of cooling rate dependence experiments,the result of which is summarized in Figure2.The activation energy for enthalpy relaxation obtained through eq6for amorphous PLA of varying tacticity was in the range of750-900kJ/mol for the three PLA samples,which was on the same order of magnitude as,or higher than,for other polymers.47,49-51With the value of∆h determined,the analysis of enthalpic relaxation now involves a stable three-parameter fit.Results and DiscussionThe PLA samples were measured using size exclusion chromatography in order to assess their molecular weight and molecular weight distribution.Mark Houwink parameters of (K/mL g-1,a))(4.9×10-3,0.794)were used for polysty-rene,52whereas(5.45×10-2,0.73)and(2.27×10-2,0.77) were used for all-L and DL-PLA,respectively.53The result is shown in Table1.It is noted that the molecular weights are all above the entanglement molecular weight of8700and10500 g mol-1of a PLA melt(reported at180and200°C,respec-tively).54,55DSC was used to obtain the T g,h of all the samples so that subsequent physical aging studies are done for each samples at nearly the same supercooling from the respective T g,h values.In addition,with different heating rate,the heat capacity change at the glass transition(∆C p)did not vary more than2%,2,19and the aging endotherm(∆H endo)at a given time and temperature also did not vary significantly(see Table2). The average of the∆C p value for the different heating rates was used in the analysis of enthalpy relaxation kinetics.We then further studied the role of conformational and configura-tional defects of these samples associated with physical aging. From past studies we know that the conformer distribution is perturbed by D-lactyl configurational defects.21Here,in the context of physical aging,we were more concerned with the change in the conformer distribution during physical aging rather than the difference in the initial values of the tg’t conformer population of the PLA samples of varying tacticity.We used aδ(Ta ,t)/δ(Ta,0))exp[-(∫0t d t/τ0(T,t)) ](1)δ(Ta ,t)/δ(Ta,0))(∆H∞-∆Ht)/∆H∞(2)lnτ0)ln A+x∆hRTa+(1-x)∆hRTf(δ)(3)T f (δ))Tf(Ta,t))Ta+δ(Ta,t)∆Cp(4)lnτ≈ln A+∆hRT(5)d ln|q|d(1/Tg,onset)≈-∆hR(6)Figure2.Determination of the TNM∆h parameter by the coolingrate dependence of the onset of the glass transition for(a)PLA5.7%D,(b)13.0%D,and(c)50%D.The uncertainty of∆h is stated as onestandard deviation.12324J.Phys.Chem.B,Vol.111,No.42,2007Aou etal.method from our previous studies,which relied on the fact that ratio of the Raman spectral intensities at the 1128and 1044cm -1can be related linearly with the tg ’t conformer popula-tion.7,56The conformer population of the PLA50was observed as a function of time at 37°C during the physical aging process.In the annealing time range and beyond where the DSC results from Figure 3show that physical aging is underway,Raman spectra from Figure 4shows no discernible difference during physical aging at 37°C.This is especially true in the present case where there are no detectable differences in the tg ’t population during physical aging.This is consistent with what has been observed for other amorphous polymers,where neither interchain spacing nor conformation change below their glass transition temperatures.13,14However,we should recognize the possibility that the changes in conformer population are small and therefore experimentally undetectable.This may result from the fact that the tg ’t conformer (∼80%)is dominant in PLA.7,56Therefore,conformational defects have little or no detectable role in physical aging of PLA.On the other hand,configurational defects are known to lower the T g of PLA by as much as 10°C and therefore are expected to have significant impact on theaging kinetics.In addition,any impact of configurational defects beyond their effect on the T g was studied.To analyze these effects of configurational defects,DSC was used,as relatively small samples needed to be tested.46When referring specifically to physical aging measured through DSC,the term enthalpic relaxation would be used.The experimental details and analysis of excess enthalpy using calorimetry is not as trivial as in the case of crystallinity analysis,and the issues are discussed here.Enthalpic relaxation kinetics is very sensitive to its thermal history,and therefore certain precautions needed to be taken.After every heating scan,the sample was quenched to 0°C at the maximum cooling rate accessible by the equipment (∼40°C/min near T g ),before reheating to the annealing temperature for physical aging.This was done to approach the annealing temperature from the low-temperature side,which would minimize the extent of aging prior to reaching isothermal conditions,46as well as to suppress crystallization for the PLA5.7sample,which can crystallize when cooled down slowly from the melt.Also,in order to initialize the thermal history of the sample at the melt state,heating scans were performed in the temperature range of 0to 120,160,and 160°C for PLA50,PLA13,and PLA5.7,respectively.The difference between annealing temperature and the glass transition is a very important parameter to consider,since enthalpic relaxation kinetics accelerates as the temperature approaches the glass transition.The purpose of measuring T g,h was to determine the set of supercooling temperatures to use for the annealing temperatures with respect to the approximate T g,h of the ter measurement of T g,h upon cooling at 5°C min -1yielded values that were separated by about the same amount from sample to sample.In order to evaluate the enthalpy relaxation kinetics,the initial fictive temperature is necessary,and this is determined by the equilibrium excess enthalpy for samples annealed at 5°C lower than the T g,h .46,47It should be noted that true glass transition temperature in practice is determined during cooling scans;57however,as our purpose does not lie in the determination of the true glass transition temperature,this was not done.As annealing required isothermal conditions,stability of temperature as well as reproducibility of data were examined.Sub-T g annealing was performed in the DSC cell for the same sample at temperatures 14,12,10,7,and 5°C lower than the T g,h ,and aging experiments conducted under identical conditions (annealing time and temperature)were repeated three times.Data were reproducible as shown in Figure 5.The cell temperature was stable to (0.02°C for isothermal tests programmed at 30.0,40.0,and 50.0°C for 10min,and the temperature range covers the set of annealing temperatures used in the present study.Heating scans of melt-quenched states were compared before and after the set of aging experiments were carried out toTABLE 1:Summary of Molecular Weights and Glass Transition Temperatures of PLA SamplesPLLA standardPDLLA standardM n /kg mol -1M w /kg mol -1PDI M n /kg mol -1M w /kg mol -1PDI T g,h /°C at 10°C min -1PLA 5.7%D 3569 1.964687 1.9059.3PLA 13.0%D 3365 1.954382 1.9058.4PLA 50%Dn/an/an/a21401.9253.0TABLE 2:Summary of ∆C p and ∆H endo of the Three PLA Samples at 5,10,and 20°C Min -1Heating RatesPLA 5.7%D (T a )46.0°C)PLA 13.0%D (T a )45.0°C)PLA 50%D (T a )40.0°C)heating rate/°C min -1510205102051020∆H endo after 30min aging at T a 1.1 1.2 1.2 1.1 1.2 1.2 1.0 1.2 1.2∆C p at T g,h /J g -1°C -10.540.540.540.530.520.520.530.540.53T g,h /°C58.359.361.357.258.460.751.853.054.9Figure 3.DSC traces of PLA 50%D annealed at 37.0°C.Conditioning took place after initializing thermal history at 120°C for 1min.Heating rate is 20°C min -1.Figure 4.Raman spectra of PLA50during physical aging at 37.0°C.The sample was preconditioned at 55°C for 1.5h to remove thermal history.Physical Aging of Amorphous Poly(lactic acid)J.Phys.Chem.B,Vol.111,No.42,200712325confirm the absence of sample degradation due to the DSC heating scans.In the context of physical aging/enthalpic relaxation,the most significant effect of configurational defects on the PLA back-bone may be their effect on the T g,58which in turn affects the enthalpic relaxation paring the samples of PLA5.7 and PLA50,we find that the addition of more D-lactyl units resulted in the lowering of T g,h by about6°C,as seen from Table2.Consequently,the initial fictive temperature(T f,o)is also lowered by about6°C,as shown in Table3.Since physical aging occurs only below T g,where glassy behavior is observed, then for a given annealing temperature,enthalpic relaxation of PLA50should proceed faster than that of PLA5.7,all other things being equal.So a difference in T g in fact should have a very large impact on aging kinetics.In order to investigate effects of configurational defects above and beyond their impact on T g,the experiments were set up such that a series of approximately constant supercoolings relative to T g,h,which is related to the T f,o,were applied to the three types of PLA samples to facilitate comparison of enthalpic relaxation kinetics.The KWW/TNM equations require T f,o to be included as a parameter so that using these equations allow us to separate the two effects of configurational defects on the enthalpic relaxation kinetics:one on the T g,and thus T f,o,and the other on the KWW/TNM fit parameters.The data and curve of best fit to the KWW/TNM equations are presented in Figure5.We note that it is difficult to discern an obvious difference in enthalpy relaxation kinetics between the three different samples.The KWW/TNM fitting parameters are shown in Figure6.The error bars of∆h in that figure correspond to the propagation of the curve-fitting error combined with the experimental error,as shown in Figure2.The error bars for the other three parameters in Figure6represent the lower and higher values obtained when the∆h parameter is used in the KWW/TNM fit is fixed to its lower or higher value.For clarity,the actual values are tabulated in Table3. We note that smaller error bars may be achieved if both the curve-fitting and experimental errors were reduced,which can be accomplished if a greater range of cooling rates were covered in obtaining the value of∆h.In future attempts,one must still keep in mind that too slow a cooling rate would be affected by noise of the calorimeter,while too fast a cooling rate could suffer from thermal lags that would render the measured values of T g,onset meaningless.That is,there is a limited range of cooling rates under which meaningful values of∆h can be determined. Figure6shows that aside from ,none of the fitting param-eters are different beyond the uncertainty limits.Since the values of∆h and ln A are correlated to each other,then their un-certainties would also be correlated.Unless the∆h were deter-mined to higher accuracies,the sample-to-sample differences for these two parameters would not be distinguishable.The x parameter,which partitions the relaxation time into temperature-and structure-dependent terms,also does not vary beyond significance across the different PLA samples.Since the physical meaning of x is not well understood,further analysis is difficult at this point.From Figure6,we find that the parameter is all within experimental error for the three PLA samples.The relative magnitude for PLA13compared with the other two samples is not obvious owing to its large uncertainty.It should be kept in mind that a value of about0.5has been found in polystyrene where KWW/TNM analysis was used,with the∆h determined separately as in the present study.43However,as the value of is just within the error bar for PLA5.7and PLA50,it merits some discussion,since it is conceivable that difference may be significant if the uncertainty of∆h can be reduced in futureFigure5.Extent of enthalpy relaxation as a function of time for(a) PLA5.7%D,(b)PLA13.0%D,and(c)PLA50%D.The black dotted lines represent the curve fit to the generalized KWW/TNM equations. The gray dotted lines correspond to the upper and lower uncertainty limits of the∆h of each sample.TABLE3:TNM Fitting Parameters for PLA5.7%D,13.0%D,and50%DPLA5.7%D PLA13.0%D PLA50%D T f,o/°C56.955.950.4∆h/104J mol-176(1591(1687(15x0.63-0.150.47-0.110.48-0.11 0.41(0.050.49(0.070.51(0.08 ln(A/s)-272-55-328-59-319-56Figure6.KWW/TNM fitting parameters of PLA5.7%D,13.0%D, and50%D.The error bars correspond to the upper and lower uncertainty limits of the∆h of each sample.12326J.Phys.Chem.B,Vol.111,No.42,2007Aou etal.experiments.There is the possibility that the difference is simply due to the fact that PLA5.7can crystallize,whereas PLA50 cannot.59However,as care was taken to suppress crystallization by quenching rapidly from the melt,and subsequent DSC scans showed no detectable crystallinity,there is the possibility that difference in crystallinity does not exist and cannot explain the difference in .In order to explain the difference in the parameter,it has been suggested that in glassy polymers there is a crossover size limit(“Gaussian blobs”)between Rouse dynamics( )0.5)and R-relaxation dynamics( <0.5).60-63 In those studies,it is proposed that Rouse dynamics is dominant above,and R-relaxation dynamics below,the crossover limit.64,65 If the precision obtained for the values of are improved through better estimation of∆h and there are significant differences between the values,then this crossover size limit concept should certainly be revisited in greater depth. ConclusionsConformational analysis using Raman spectroscopy reveals little or no change in helical tg’t conformer population during the physical aging process,which is consistent with previous observations of other polymers.We find that the use of cooling rate dependence of T g,onset to obtain the activation energy for enthalpy relaxation(∆h)is required in order to have a stable and reliable curve fit.The values of∆h were therefore calculated for each PLA sample and had similar magnitude as other polymeric glasses.The effects of polymer tacticity on the KWW/ TNM fit parameters were analyzed.The major effect of configurational defects was to lower the T g,h of PLA,thereby accelerating the enthalpic relaxation kinetics at a given annealing temperature.All KWW/TNM fit parameters were found not to change significantly from sample to sample.Further attempts to improve accuracy of the∆h and thus of the curve-fitting parameters should be done in the future. 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以兴趣为起点，以方法为推手——浅谈初中语文阅读教学发布时间：2021-09-07T01:03:46.749Z 来源：《教学与研究》2021年第7月19期作者：张用兰[导读] 书籍是造就灵魂的工具，阅读则是帮助我们了解作者灵魂的过程，读一本好书，就如同交一个益友。

张用兰海南省东方市感城中学 572633摘要：书籍是造就灵魂的工具，阅读则是帮助我们了解作者灵魂的过程，读一本好书，就如同交一个益友。

从小学到中学，课标都强调了阅读的重要性，并对学生阅读提出了明确要求。

在“大语文”时代背景下，阅读更不容忽视。

下文从初中语文阅读教学切入，简要分析初中学生阅读现状，重点探讨提高初中语文阅读教学效率的有效策略。

关键词：基础教育；阅读教学；兴趣；方法一、初中学生阅读现状（一）阅读兴趣低下从我班上学生阅读表现来看，大部分学生阅读兴趣低下，对文学书籍,尤其是文学名著缺乏兴趣。

课程标准要求要求学生每学期阅读名著不少于2部，但实际上很少有学生完成了这一阅读任务，大部分学生最喜欢读的是《骆驼祥子》,但能够读完整本书的学生却只占班上所有学生四分之一，一般学生表示对《朝花夕拾》、《钢铁是怎样炼成的》表示有阅读兴趣，但真正读完整本书的学生大概占六分之一，更多学生喜欢读言情小说、玄幻小说、漫画连载小说，唯独对文学类书籍兴趣淡薄。

（二）缺乏正确阅读方法《语文课程标准》明确要求初中学生要掌握一定的阅读方法，能够用普通话正确、流利、有感情地朗读；能够养成自主默读习惯，并且有一定速度地阅读；在通读文章的基础上能够理清文章行文脉络，提炼出文章的核心思想；对阅读过的文本能够提出个人看法和疑问，能够运用自主阅读、合作阅读、探究性阅读等多种方式解决阅读过程中的疑难问题。

事实上，大部分学生无论是课内阅读还是课外阅读，都只是循规蹈矩地读读书罢了，课堂上老师说怎么读就怎么读，课后自己想怎么读就怎么读，没有吸收知识的阅读无疑是效率低下的。

而导致学生出现这些阅读现象的根本原因在于学生缺乏正确的阅读方法。

Japan.J.Math.8,147–183(2013)DOI:10.1007/s11537-013-1280-5About the Connes embedding conjectureAlgebraic approachesNarutaka Ozawa?Received:28December2012/Accepted:15January2013Published online:20March2013©The Mathematical Society of Japan and Springer Japan2013Communicated by:Yasuyuki KawahigashiAbstract.In his celebrated paper in1976,A.Connes casually remarked that anyfinite von Neu-mann algebra ought to be embedded into an ultraproduct of matrix algebras,which is now known as the Connes embedding conjecture or problem.This conjecture became one of the central open problems in thefield of operator algebras since E.Kirchberg’s seminal work in1993that proves it is equivalent to a variety of other seemingly totally unrelated but important conjectures in the field.Since then,many more equivalents of the conjecture have been found,also in some other branches of mathematics such as noncommutative real algebraic geometry and quantum infor-mation theory.In this note,we present a survey of this conjecture with a focus on the algebraic aspects of it.Keywords and phrases:Connes embedding conjecture,Kirchberg’s conjecture,Tsirelson’s prob-lem,semi-pre-C -algebras,noncommutative real algebraic geometryMathematics Subject Classification(2010):16W80,46L89,81P151.IntroductionThe Connes embedding conjecture([Co])is considered as one of the most im-portant open problems in thefield of operator algebras.It asserts that anyfi-nite von Neumann algebra is approximable by matrix algebras in a suitable sense.It turns out,most notably by Kirchberg’s seminal work([Ki1]),that the N.O ZAWAResearch Institute for Mathematical Sciences,Kyoto University,Kyoto606-8502,Japan(e-mail:)?Partially supported by JSPS(23540233)and by the Danish National Research Foundation (DNRF)through the Centre for Symmetry and Deformation.148N.Ozawa Connes embedding conjecture is equivalent to a variety of other important con-jectures,which touches most of the subfields of operator algebras,and also someother branches of mathematics such as noncommutative real algebraic geome-try([Sm])and quantum information theory.In this note,we look at the alge-braic aspects of this conjecture.(See[BO,Ki1,Oz1]for the analytic aspects.)This leads to a study of the C -algebraic aspect of noncommutative real alge-braic geometry in terms of semi-pre-C -algebras.Specifically,we treat someeasy parts of Positivstellensätze of Putinar([Pu]),Helton–McCullough([HM]),and Schmüdgen–Bakonyi–Timotin([BT]).We then treat their tracial analogueby Klep–Schweighofer([KS]),which is equivalent to the Connes embeddingconjecture.We give new proofs of Kirchberg’s theorems on the tensor productC F d˝B.`2/and on the equivalence between the Connes embedding conjec-ture and Kirchberg’s conjecture.We also look at Tsirelson’s problem in quantuminformation theory([Fr,J+,Ts]),and prove it is again equivalent to the Connesembedding conjecture.This paper is an expanded lecture note for the author’slecture for“Masterclass on sofic groups and applications to operator algebras”(University of Copenhagen,5–9November2012).The author gratefully ac-knowledges the kind hospitality provided by University of Copenhagen duringhis stay in Fall2012.He also would like to thank Professor Andreas Thom forvaluable comments on this note.2.Ground assumptionWe deal with unital -algebras over k2f C;R g,and every algebra is assumedto be unital,unless it is clearly not so.The unit of an algebra is simply denotedby1and all homomorphisms and representations between algebras are assumedto preserve the units.We denote by i the imaginary unit,and by the complexconjugate of 2C.In case k D R,one has D for all 2k.3.Semi-pre-C -algebrasWe will give the definition and examples of semi-pre-C -algebras.Recall thata unital algebra A is called a -algebra if it is equipped with a map x!xsatisfying the following properties:(i)1 D1and.x / D x for every x2A;(ii).xy/ D y x for every x;y2A;(iii). x C y/ D x C y for every x;y2A and 2k.The sets of hermitian elements and unitary(orthogonal)elements are writtenrespectively asA h WD f a2A W a D a g and A u WD f u2A W u u D1D uu g:About the Connes embedding conjecture149 Every element x2A decomposes uniquely as a sum x D a C b of an hermitian element a and a skew-hermitian element b.The set of hermitian elements is an R-vector space.We say a linear map'between -spaces is self-adjoint if ' D',where' is defined by' .x/D'.x / .We call a subset A C A h a -positive cone(commonly known as a quadratic module)if it satisfies the following:(i)R 01 A C and a C b2A C for every a;b2A C and 2R 0; (ii)x ax2A C for every a2A C and x2A.For a;b2A h,we write aÄb if b a2A C.We say a linear map'be-tween spaces with positivity is positive if it sends positive elements to positive elements(and often it is also required self-adjoint),and a positive linear map' is faithful if a 0and'.a/D0implies a D0.Given a -positive cone A C, we define the -subalgebra of bounded elements byA bdd D f x2A W9R>0such that x xÄR1g:This is indeed a -subalgebra of A.For example,if x is bounded and x xÄR1,then x is also bounded and xx ÄR1,because0ÄR 1.R1 xx /2D R1 2xx C R 1x.x x/x ÄR1 xx : Thus,if A is generated(as a -algebra)by S,then S A bdd implies A D A bdd.Definition.A unital -algebra A is called a semi-pre-C -algebra if it comes together with a -positive cone A C satisfying the Combes axiom(also called the archimedean property)that A D A bdd.Since hÄ.1C h2/=2for h2A h,one has A h D A C A C for a semi-pre-C -algebra.We define the ideal of infinitesimal elements byI.A/D f x2A W x xÄ"1for all">0gand the archimedean closure of the -positive cone A C(or any other cone)by arch.A C/D f a2A h W a C"12A C for all">0g:The cone A C is said to be archimedean closed if A C D arch.A C/.A C -alge-bra A is of course a semi-pre-C -algebra,with a zero infinitesimal ideal and an archimedean closed -positive coneA C D f x x W x2A g:If A B.H/(here B.H/denotes the C -algebra of the bounded linear opera-tors on a Hilbert space H over k),then one also hasA C D f a2A h W h a ; i 0for all 2H g:150N.Ozawa Note that the condition a being hermitian cannot be dropped when k D R.Itwill be shown(Theorem1)that if A is a semi-pre-C -algebra,then A=I.A/is a pre-C -algebra with a -positive cone arch.A C/.Definition.We define the universal C -algebra of a semi-pre-C -algebra Aas the C -algebra C u.A/together with a positive -homomorphismÃW A!C u.A/which satisfies the following properties:Ã.A/is dense in C u.A/andevery positive -representation of A on a Hilbert space H extends to a -representation N W C u.A/!B.H/,i.e., D N ıÃ.In other words,C u.A/isthe separation and completion of A under the C -semi-normsup fk .a/k B.H/W a positive -representation on a Hilbert space H g: (We may restrict the dimension of H by the cardinality of A.)We emphasize that only positive -representations are considered.Everypositive -homomorphism between semi-pre-C -algebras extends to a posi-tive -homomorphism between their universal C -algebras.It may happen that A C D A h and C u.A/D f0g,which is still considered as a unital(?)C -algebra.Every -homomorphism between C -algebras is automatically pos-itive,has a norm-closed range,and maps the positive cone onto the positivecone of the range.However,this is not at all the case for semi-pre-C -algebras,as we will exhibit a prototypical example in Example1.On the other hand,we note that if A is a norm-dense -subalgebra of a C -algebra A such thatarch.A C/D A\A C,then every positive -representation of A extends to a-representation of A,i.e.,A D Cu .A/.(Indeed,if x2A has k x k A<1,then1 x x2A C and hence k .x/k<1for any positive -representation ofA.)It should be easy to see that the following examples satisfy the axiom of semi-pre-C -algebras.Example1.Let be a discrete group and kŒ be its group algebra over k:.f g/.s/DXt2f.st 1/g.t/and f .s/D f.s 1/ for f;g2kŒ :The canonical -positive cone of kŒ is defined as the sums of hermitian squares,kŒ C Dn n Xi D1 i i W n2N; i2kŒo:Then,kŒ is a semi-pre-C -algebra such that C u.kŒ /D C ,the full group C -algebra of ,which is the universal C -algebra generated by the unitary representations of .There is another group C -algebra.Recall that the left regular representation of on`2 is defined by .s/ıt Dıst for s;t2 , or equivalently by .f/ D f for f2kŒ and 2`2 .The reducedAbout the Connes embedding conjecture151 group C -algebra C r of is the C -algebra obtained as the norm-closure of .kŒ /in B.`2 /.The group algebra kŒ is equipped with the corresponding -positive conek rŒ C D f f2kŒ W9f n2kŒ C such that f n!f pointwise gD f f2kŒ W f is of positive type g;and the resultant semi-pre-C -algebra k rŒ satisfies C u.k rŒ /D C r .Indeed, if f2kŒ \ 1.C r C/,then for D .f/1=2ı12`2 ,one has f D and f is the pointwise limit of n n2kŒ C,where n2kŒ are such that k n k2!0.On the other hand,if f is of positive type(i.e.,the kernel .x;y/!f.x 1y/is positive semi-definite),then f D f and h .f/Á;Ái 0 for everyÁ2`2 ,which implies .f/2C r C.It follows that k rŒ C D arch.kŒ C/if and only if is amenable(see Theorem1).Example2.The -algebra kŒx1;:::;x d of polynomials in d commuting hermitian variables x1;:::;x d is a semi-pre-C -algebra,equipped with the -positive conekŒx1;:::;x d C D -positive cone generated by f1 x2i W i D1;:::;d g:One has C u.kŒx1;:::;x d /D C.Œ 1;1 d/,the algebra of the continuous func-tions onŒ 1;1 d,and x i is identified with the i-th coordinate projection.Example3.The -algebra k h x1;:::;x d i of polynomials in d non-commuting hermitian variables x1;:::;x d is a semi-pre-C -algebra,equipped with the -positive conek h x1;:::;x d i C D -positive cone generated by f1 x2i W i D1;:::;d g: One has C u.k h x1;:::;x d i/D C.Œ 1;1 / C.Œ 1;1 /,the unital full free product of d-copies of C.Œ 1;1 /.Example4.Let A and B be semi-pre-C -algebras.We denote by A˝B the algebraic tensor product over k.There are two standard ways to make A˝B into a semi-pre-C -algebra.Thefirst one,called the maximal tensor product and denoted by A˝max B,is A˝B equipped with.A˝max B/C D -positive cone generated by f a˝b W a2A C;b2B C g: The second one,called the minimal tensor product and denoted by A˝min B, is A˝B equipped with.A˝min B/C D.A˝B/h\.ÃA˝ÃB/ 1..C u.A/˝min C u.B//C/: (See Theorem14for a“better”description.)One has C u.A˝˛B/D C u.A/˝˛C.B/for˛2f max;min g.The right hand side is the C -algebra maximal u152N.Ozawa (resp.minimal)tensor product(see[BO,Pi1]).For A1 A2and B1 B2, one has.A1˝min B1/C D.A1˝min B1/\.A2˝min B2/C;but the similar identity need not hold for the maximal tensor product. Example5.The unital algebraic free product A B of semi-pre-C -algebras A and B,equipped with.A B/C D -positive cone generated by.A C[B C/;is a semi-pre-C -algebra,and C u.A B/D C u.A/ C u.B/,the unital full free product of the C -algebras C u.A/and C u.B/.The following is very basic(cf.[Ci]and Proposition15in[Sm]).Theorem1.Let A be a semi-pre-C -algebra andÃW A!C u.A/be the uni-versal C -algebra of A.Then,one has the following.kerÃD I.A/,the ideal of the infinitesimal elements.A h\à 1.C.A/C/D arch.A C/,the archimedean closure of A C.uAlthough it follows from the above theorem,we give here a direct proof of the fact that arch.A C/\. arch.A C// I.A/.Indeed,if h2Ä1and "1<h<"1for"2.0;1/,then one has0Ä.1C h/." h/.1C h/D".1C h/2 h h.2C h/hÄ.4"C"/1 .2 "/h2; which implies h2<5"1.We postpone the proof and give corollaries to this theorem.4.PositivstellensätzeWe give a few results which say if an element a is positive in a certain class of representations,then it is positive for an obvious reason.Such results are re-ferred to as“Positivstellensätze.”Recall that a C -algebra A is said to be resid-uallyfinite dimensional(RFD)iffinite-dimensional -representations separate the elements of A,i.e., .a/ 0for allfinite-dimensional -representations implies a 0in A.All abelian C -algebras and full group C -algebras of residuallyfinite amenable groups are RFD.Moreover,it is a well-known result of Choi that the full group C -algebra C F d of the free group F d of rank d is RFD(see Theorem26).In fact,finite representations(i.e.,the unitary repre-sentations such that .F d/isfinite)separate the elements of C F d([LS]). However,we note that the full group C -algebra of a residuallyfinite group need not be RFD([Be1]).We also note that the unital full free products of RFDAbout the Connes embedding conjecture153 C -algebras is again RFD([EL]).In particular,C u.k h x1;:::;x d i/is RFD.The results mentioned here have been proven for complex C -algebras,but they are equally valid for real cases.See Sect.7.Theorem1,when combined with resid-ualfinite dimensionality,immediately implies the following Positivstellensätze (cf.[Pu,HM]).Corollary2.The following are true.Let f2kŒ h.Then, .f/ 0for every unitary representation if and only if f2arch.kŒ C/.The full group C -algebra C of a group is RFD if and only if the fol-lowing statement holds.If f2kŒ h is such that .f/ 0for every finite-dimensional unitary representation ,then f2arch.kŒ C/.Let f2kŒx1;:::;x d h.Then,f.t1;:::;t d/ 0for all.t1;:::;t d/2Œ0;1 d if and only if f2arch.kŒx1;:::;x d C/.(See Example2.)Let f2k h x1;:::;x d i h.Then,f.X1;:::;X d/ 0for all contractive her-mitian matrices X1;:::;X d if and only if f2arch.k h x1;:::;x d i C/.(See Example3.)In some cases,the -positive cones are already archimedean closed.We will see later(Theorem26)this phenomenon for the free group algebras kŒF d . 5.Eidelheit–Kakutani separation theoremThe most basic tool in functional analysis is the Hahn–Banach theorem.In this note,we will need an algebraic form of it,the Eidelheit–Kakutani separation theorem.We recall the algebraic topology on an R-vector space V.Let C V be a convex subset.An element c2C is called an algebraic interior point of C if for every v2V there is">0such that c C v2C for all j j<". The convex cone C is said to be algebraically solid if the set Cıof algebraic interior points of C is non-empty.Notice that for every c2Cıand x2C, one has c C.1 /x2Cıfor every 2.0;1 .In particular,CııD Cıfor every convex subset C.We can equip V with a locally convex topology,called the algebraic topology,by declaring that any convex set that coincides with its algebraic interior is open.Then,every linear functional on V is continuous with respect to the algebraic topology.Now Hahn–Banach separation theorem reads as follows.Theorem3(Eidelheit–Kakutani([Ba])).Let V be an R-vector space,C an algebraically solid cone,and v2V n C.Then,there is a non-zero linear func-tional'W V!R such that'.c/:'.v/Äinfc2CIn particular,'.v/<'.c/for any algebraic interior point c2C.154N.Ozawa Notice that the Combes axiom A D A bdd is equivalent to that the unit1is an algebraic interior point of A C A h and arch.A C/is the algebraic closure of A C in A h.(This is where the Combes axiom is needed and it can be dispensed when the cone A C is algebraically closed.See Sect.3.4in[Sm].)Let A be a semi-pre-C -algebra.A unital -subspace S A is called a semi-operator system.Here,a -subspace is a subspace which is closed under the -operation. Existence of1in S ensures that S C D S\A C has enough elements to span S h.A linear functional'W S!k is called a state if'is self-adjoint, positive,and'.1/D1.Note that if k D C,then S is spanned by S C and every positive linear functional is automatically self-adjoint.However,this is not the case when k D R.In any case,every R-linear functional'W S h!R extends uniquely to a self-adjoint linear functional'W S!k.We write S.S/ for the set of states on S.Corollary4.Let A be a semi-pre-C -algebra.Let W A be a -subspace and v2A h n.A C C W h/.Then,there is a state'on A such that'.W/D f0g and'.v/Ä0.(Krein’s extension theorem)Let S A be a semi-operator system.Then every state on S extends to a state on A.Proof.Since A C C W h is an algebraically solid cone in A h,one mayfind a non-zero linear functional'on A h such that'.v/Äinf f'.c/W c2A C C W h g:Since'is non-zero,'.1/>0and one may assume that'.1/D1.Thus the self-adjoint extension of'on A,still denoted by',is a state such that'.v/Ä0 and'.W h/D f0g.Let x2W.Then,for every 2k,one has'.x/C. '.x// D'.. x/C. x/ /D0:This implies'.W/D f0g in either case k2f C;R g.For the second assertion,let'2S.S/be given and consider the coneC D f x2S h W'.x/ 0g C A C:It is not too hard to see that C is an algebraically solid cone in A h and v…C for any v2S h such that'.v/<0.Hence,one mayfind a state N'on A such that N'.C/ R 0.In the same way as above,one has that N'is zero on ker', which means N'j S D'.About the Connes embedding conjecture155 6.GNS constructionWe recall the celebrated GNS construction(Gelfand–Naimark–Segal construc-tion),which provides -representations out of states.Let a semi-pre-C -algebra A and a state'2S.A/be given.Then,A is equipped with a semi-inner product h y;x i D'.x y/,and it gives rise to a Hilbert space,which will bedenoted by L2.A;'/.We denote by O x the vector in L2.A;'/that correspondsto x2A.Thus,h O y;O x i D'.x y/and k O x k D'.x x/1=2.The left multiplica-tion x!ax by an element a2A extends to a bounded linear operator '.a/on L2.A;'/such that '.a/O x D ca x for a;x2A.(Observe that a aÄR1implies k '.a/k2ÄR.)It follows that 'W A!B.L2.A;'//is a positive -representation of A such that h '.a/O1;O1i D'.a/.If W A!B.H/is a positive -representation having a unit cyclic vector,then'.a/D h .a/ ; i is a state on A and .x/ !O x extends to a uni-tary isomorphism between H and L2.A;'/which intertwines and '.Sinceevery positive -representation decomposes into a direct sum of cyclic repre-sentations,one may obtain the universal C -algebra C u.A/of A as the closureof the image under the positive -representationM '2S.A/ 'W A !BM'2S.A/L2.A;'/Á:We also make an observation that.A˝min B/C in Example4coincides withc2.A˝B/h W .'˝ /.z cz/ 0for all'2S.A/;2S.B/;z2A˝B:7.Real versus complexWe describe here the relation between real and complex semi-pre-C -algebras. Because the majority of the researches on C -algebras are carried out for com-plex C -algebras,we look for a method of reducing real problems to complex problems.Suppose A R is a real semi-pre-C -algebra.Then,the complexifica-tion of A R is the complex semi-pre-C -algebra A C D A R C i A R.The -algebra structure(over C)of A C is defined in an obvious way,and.A C/C is defined to be the -positive cone generated by.A R/C:.A C/C Dn n Xi D1z i a i z i W n2N;a i2.A R/C;z i2A Co:(This is a temporary definition,and the official one will be given later.See Lemma11.)Note that A R\.A C/C D.A R/C.The complexification A C has an involutive and conjugate-linear -automorphism defined by x C i y!156N.Ozawa x i y ,x;y 2A R .Every complex semi-pre-C -algebra with an involutive and conjugate-linear -automorphism arises in this way.Lemma 5.Let R W A R !B R be a -homomorphism between real semi-pre-C -algebras (resp.'R W A R !R be a self-adjoint linear functional).Then,the complexification C W A C !B C (resp.'C W A C !C )is positive if and only if R (resp.'R )is so.Proof.Weonly prove that 'C is positive if 'R is so.The rest is trivial.Let b D P i z i a i z i 2.A C /C be arbitrary,where a i 2.A R /C and z i D x i C i y i .Then,b D P i .x i a i x i C y i a i y i /C i P i .x i a i y i y i a i x i /.Since x i a i y i y i a i x i is skew-hermitian,one has 'R .x i a i y i y i a i x i /D 0,and 'C .b/D'R .P i x i a i x i C y i a i y i / 0.This shows 'C is positive.We note that if H C denotes the complexification of a real Hilbert space H R ,then B .H R /C D B .H C /.Thus every positive -representation of a real semi-pre-C -algebra A R on H R extends to a positive -representation of its complexification A C on H C .Conversely,if is a positive -representation of A C on a complex Hilbert space H C ,then its restriction to A R is a positive -representation on the realification of H C .The realification of a complex Hilbert space H C is the real Hilbert space H C equipped with the real inner product h Á; i R D <h Á; i .Therefore,we arrive at the conclusion that C u .A R /C D C u .A C /.We also see that .R Œ /C D C Œ ,.A R ˝B R /C D A C ˝B C ,.A R B R /C D A C B C ,etc.8.Proof of Theorem 1We only prove the first assertion of Theorem 1.The proof of the second is very similar.We will prove a stronger assertion thatk Ã.x/k C u.A /D inf f R >0W R 21 x x 2A C g :The inequality Ätrivially follows from the C -identity.For the converse,as-sume that the right hand side is non-zero,and choose >0such that 21 x x …A C .By Corollary 4,there is '2S.A /such that '. 21 x x/Ä0.Thus for the GNS representation ',one hask '.x/k k '.x/O 1k D '.x x/1=2 :It follows that k Ã.x/k .About the Connes embedding conjecture157 9.Trace positive elementsLet A be a semi-pre-C -algebra.A state on A is called a tracial state if .xy/D .yx/for all x;y2A,or equivalently if is zero on the -subspace K D span f xy yx W x;y2A g spanned by commutators in A.We denote by T.A/the set of tracial states on A(which may be empty).Associated with 2T.A/is afinite von Neumann algebra. .A/00; /,which is the von Neumann algebra generated by .A/ B.L2.A; //with the faithful normal tracial state .a/D h a O1;O1i that extends the original .Recall that afinite von Neumann algebra is a pair.M; /of a von Neumann algebra and a faithful normal tracial state on M.The following theorem is proved in[KS]for the algebra in Example3and in[JP]for the free group algebras,but the proof equally works in the general setting.We note that for some groups ,notably for D SL3.Z/([Be2]),it is possible to describe all the tracial states on kŒ . Theorem6([KS]).Let A be a semi-pre-C -algebra,and a2A h.Then,the following are equivalent..1/ .a/ 0for all 2T.A/..2/ . .a// 0for everyfinite von Neumann algebra.M; /and every positive -homomorphism W A!M..3/a2arch.A C C K h/,where K h D K\A h D span f x x xx W x2A g. Proof.The equivalence.1/,.2/follows from the GNS construction.We only prove.1/).3/,as the converse is trivial.Suppose a C"1…A C C K h for some">0.Then,by Corollary4,there is 2S.A/such that .K/D f0g (i.e., 2T.A/)and .a/Ä "<0.10.Connes embedding conjectureThe Connes embedding conjecture(CEC)asserts that anyfinite von Neumann algebra.M; /with separable predual is embeddable into the ultrapower R! of the hyperfinite II1-factor R(over k2f C;R g).Here an embedding means an injective -homomorphism which preserves the tracial state.We note that if is a tracial state on a semi-pre-C -algebra A andÂW A!N is a -preserving -homomorphism into afinite von Neumann algebra.N; /,thenÂextends to a -preserving -isomorphism from .A/00onto the von Neumann subalgebra generated byÂ.A/in N(which coincides with the ultraweak clo-sure ofÂ.A/).Hence,.M; /satisfies CEC if there is an ultraweakly dense -subalgebra A M which has a -preserving embedding into R!.In particular, CEC is equivalent to that for every countably generated semi-pre-C -algebra A and 2T.A/,there is a -preserving -homomorphism from A into R!.We will see that this is equivalent to the tracial analogue of Positivstellensätze in158N.Ozawa Corollary 2.We first state a few equivalent forms of CEC.We denote by tr the tracial state 1N Tr on M N .k /.Theorem 7.For a finite von Neumann algebra .M; /with separable predual,the following are equivalent..1/.M; /satisfies CEC,i.e.,M ,!R !..2/Let d 2N and x 1;:::;x d 2M be hermitian contractions.Then,forevery m 2N and ">0,there are N 2N and hermitian contractions X 1;:::;X d 2M N .k /such thatj .x i 1 x i k / tr .X i 1 X i k /j <"for all k Äm and i j 2f 1;:::;d g ..3/Assume k D C (or replace M with its complexification in case k D R ).Letd 2N and u 1;:::;u d 2M be unitary elements.Then,for every ">0,there are N 2N and unitary matrices U 1;:::;U d 2M N .C /such thatj .u i u j / tr .U i U j /j <"for all i;j 2f 1;:::;d g .In particular,CEC holds true if and only if every .M; /satisfies condition .2/and/or .3/.The equivalence .1/,.2/is a rather routine consequence of the ultra-product construction.For the equivalence to (3),see Theorem 27.Note that the assumption k D C in condition (3)is essential because the real analogue of it is actually true ([DJ]).Since any finite von Neumann algebra M with sep-arable predual is embeddable into a II 1-factor which is generated by two her-mitian elements (namely .M R/N ˝R ),to prove CEC,it is enough to verify the conjecture (2)for every .M; /and d D 2.We observe that a real finite von Neumann algebra .M R ; R /is embeddable into R !R (i.e.,it satisfies CEC)if and only if its complexification .M C ; C /is embeddable into R !C .The “only if”di-rection is trivial and the “if”direction follows from the real -homomorphism M N .C /,!M 2N .R /,a C i b ! a b b a .A complex finite von Neumann al-gebra .M; /need not be a complexification of a real von Neumann algebra,but M ˚M op is (isomorphic to the complexification of the realification of M ).Therefore,M satisfies CEC if and only if its realification satisfies it.For a finite von Neumann algebra .M; /and d 2N ,we denote by H d .M /the set of those f 2k h x 1;:::;x d i h such that .f.X 1;:::;X d // 0for all hermitian contractions X 1;:::;X d 2M .Further,let H d D\M H d .M /and H fin d D \NH d .M N .k //D H d .R/:Notice that H d D arch .k h x 1;:::;x d i C C K h /(see Example 3and Theorem 6).About the Connes embedding conjecture 159Corollary 8([KS]).Let k 2f C ;R g .Then one has the following.Let .M; /be a finite von Neumann algebra with separable predual.Then,M satisfies CEC if and only if H fin d H d .M /for all d .CEC holds true if and only if H fin d D arch .k h x 1;:::;x d i C C K h /for all/some d 2.Proof.It is easy to see that condition (2)in Theorem 7implies H fin d H d .M /.Conversely,suppose condition (2)does not hold for some d 2N ,x 1;:::;x d 2M ,m 2N ,and ">0.We introduce the multi-index notation.For i D .i 1;:::;i k /,i j 2f 1;:::;d g and k Äm ,we denote x i D x i 1 x i k .It may happen that i is the null string ;and x ;D 1.Then,C D closure f .tr .X i //i W N 2N ;X 1;:::;X d 2M N .k /h ;k X i k Ä1gis a convex set (consider a direct sum of matrices).Hence by Theorem 3,there are 2R and ˛i 2k such that <X i ˛i .x i /< Äinf 2C <X i˛i i :Replacing ˛i with .˛i C ˛ i/=2(here i is the reverse of i ),we may omit <from the above inequality.Further,arranging ˛;,we may assume D 0.Thus f D P i ˛i x i belongs to H fin d ,but not to H d .M /.This completes the proof of the first half.The second half follows from this and Theorem 6.An analogue to the above also holds for C ŒF d .Corollary 9([JP]).Let k D C .The following holds.Let .M; /be a finite von Neumann algebra with separable predual.Then,M satisfies CEC if and only if the following holds true:If d 2N and ˛2M d .C /h satisfies that tr .P ˛i;j U i U j / 0for every N 2N and U 1;:::;U d 2M N .C /u ,then it satisfies .P ˛i;j u i u j / 0for every uni-tary elements u 1;:::;u d 2M .CEC holds true if and only if for every d 2N and ˛2M d .C /h the follow-ing holds true:If tr .P ˛i;j U i U j / 0for every N 2N and U 1;:::;U d 2M N .C /u ,then P ˛i;j s i s j 2arch .C ŒF d C C K h /,where s 1;:::;s d are the free generators of F d .11.Matrix algebras over semi-pre-C -algebrasWe describe here how to make the n n matrix algebra M n .A /over a semi-pre-C -algebra A into a semi-pre-C -algebra.We note that x D Œx j;i i;j for x D Œx i;j i;j 2M n .A /.We often identify M n .A /with M n .k /˝A .There。

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Short CommunicationMyths and facts of cone beam computed tomography in orthodonticsAhmad Abdelkarim a ,*aCare Planning and Restorative Sciences,University of Mississippi,School of Dentistry,Jackson,Mississippia r t i c l e i n f oArticle history:Received 9December 2011Received in revised form 16March 2012Accepted 9April 2012Available online 11May 2012Keywords:Advanced imagingCone beam CT (CBCT)in orthodontics Myths and facts of CBCT in orthodontics Orthodontic cone beam CTa b s t r a c tCone beam computed tomography (CBCT)is a revolutionary imaging modality.It has changed numerous aspects of dentistry and has added great value to its diagnostic phase as well as that of orthodontics.Three-dimensional imaging CBCT has the potential to improve the diagnosis and treatment planning of cases.However,there has been some confusion about CBCT and its implementation in orthodontics.This could be due to overmarketing or limited understanding of the imaging technique itself.The purpose of this article is to present 10myths about CBCT in orthodontics and replace them with facts about this imaging technique.Published by Elsevier Inc.1.IntroductionCone beam computed tomography (CBCT)is a revolutionary imaging modality that has changed numerous aspects of dentistry and has added great value to its diagnostic phase as well as that of orthodontics.CBCT three-dimensional (3D)imaging has the potential to improve the diagnosis and treatment planning.With the desire to enhance treatment by incorporating the highest technological advancements,CBCT has attracted signi ficant attention.The potential applications in orthodontics have been recognized and appreciated.One of the most common selection criteria of CBCT in ortho-dontics is evaluation of impacted teeth [1e 4].It allows visualization in three dimensions,and the relation to adjacent teeth.For ex-ample,visualization of an external resorption of a maxillary lateral incisor due to impaction of a maxillary canine can be precisely evaluated [5].Additionally,CBCT can reveal the presence or absence of the canine,size of the follicle,inclination of the long axis of the tooth,relative buccal and palatal positions,amount of the bone covering the tooth,local anatomic considerations,and overall stage of dental development [6].Supernumerary teeth can be evaluated as well [7].Additionally,CBCT examination is recommended in patients with dentofacial deformities,including severe facial asymmetry or facial disharmony [8],cleft palate [9],patients withobstructive sleep apnea,and when a patient has an airway study [10e 15].The clear advantage of CBCT in these cases is evaluation of 3D structures through a 3D imaging modality.Additionally,the nasopharyngeal and oropharyngeal airway can be assessed in volume size and shape.Moreover,if temporary anchorage devices are planned,CBCT can assist in initial site assessment [16e 24],or temporary anchorage device site status evaluation [25e 27].Therefore,CBCT applications in orthodontics are plentiful.However,it is easy to be seduced by the sheer beauty of CBCT image reconstructions.Following the ethical principle of nonmale ficence,practitioners in health care are supposed to minimize harm to patients.A cornerstone of radiation protection is to keep radiation “As Low As Reasonably Achievable,”the ALARA principle.There has been some confusion about CBCT and its imple-mentation in orthodontics.This could be due to overmarketing or limited understanding of the imaging technique itself.The purpose of this article is to present 10myths about CBCT in orthodontics and replace them with facts about this imaging technique.The 10myths of CBCT in orthodontics are (1)CBCT exposes orthodontic patients to low radiation;(2)if daily background radiation is 8m Sv,any CBCT acquisition would be justi fied because it has the effect of only few days of background radiation;(3)we must have totally new 3D CBCT cephalometric analyses that replace two-dimensional (2D)standardized analyses;(4)CBCT can replace impressions for orthodontics and be as accurate;(5)volume rendering is suf ficient to display external root resorption;(6)CBCT changes the final outcome of orthodontic treatment;(7)Boards of Orthodontics will eventually adopt CBCT for all orthodontic records and superimpositions;(8)CBCT is suf ficient and the best imaging*Corresponding author:Care Planning and Restorative Sciences,University of Mississippi,2500N.State Street,Jackson,MS 39216.E-mail address:ahmadozbrain@.Contents lists available at SciVerse ScienceDirectJournal of the World Federation of Orthodontistsjournal h omepage:ww2212-4438/$e see front matter Published by Elsevier Inc.doi:10.1016/j.ejwf.2012.04.002Journal of the World Federation of Orthodontists 1(2012)e3e e8modality to examine the temporomandibular joints (TMJ);(9)if CBCT is ordered,the orthodontist is at liability risk for any pathology in the scan;and (10)CBCT is an orthodontic practice builder.2.Myth 1CBCT exposes orthodontic patients to low radiation.2.1.FactThe effective dose of an imaging modality is a commonly used term in radiation biology and presents a numeric value in micro-Sieverts (m Sv).It provides a mechanism for assessing the radiation risk from partial body irradiations in terms of data derived from whole body irradiations.It is calculated as the weighted average of the mean absorbed dose to the various body organs and tissues,where the weighting factor is the radiation risk for a given organ (from a whole-body irradiation)as a fraction of the total radiation detriment [28].The effective dose of CBCT is quite flexible and the range is much wider than other imaging techniques used in dentistry.Table 1describes that the effective dose of panoramic radiography is esti-mated to be 2.7to 23m Sv [29e 34].Cephalometric radiography effective dose is approximately 1.7e 3.4m Sv [35].On the other hand,effective dose range of CBCT is very large and can be anywhere from 20to 1000m Sv,depending on the machine,field of view,and selected technique factors [29,31,34,36e 40].Only one CBCT machine with large field of view exceeded 1000m Sv [36].Each CBCT device uses different settings,which results in a wide range of the effective dose.The latest findings report that this wide effective dose range of different CBCT devices is strongly related to field size [41].Increasing the field of view would increase the effective dose [31].Alternatively,reducing the volume size is perhaps the best way to reduce radiation exposure for the patient.For example,an impacted tooth does not require imaging of the whole head.A smaller volume of 40Â40mm,for example,may be appropriate.This has the potential bene fit of increased resolution of the images because most CBCT machines that provide different scan volume options usually use larger voxel size on large field of view scans and smaller voxels in smaller field of views.Fortunately,CBCT effective dose is comparatively smaller than traditional medical CT imaging technique [42e 48].In other words,CBCT effective dose is higher than conventional panoramic and cephalometric radiography,but less than conventional CT.No approach regarding CBCT radiation risk assessment in chil-dren in particular has been examined yet.However,a CT study found that the smaller mass of children caused the corresponding effective doses to be higher than those in adults undergoing similar CT examinations [49].Generally,thyroid shielding with a leaded thyroid shield or collar is strongly recommended for children and pregnant women d these patients are particularly sensitive to radiation [50].This reduces the risk because lead shielding reduces the effective dose [38].3.Myth 2If daily background radiation is 8m Sv,any CBCT acquisition would be justi fied because it has the effect of only few days of background radiation.3.1.FactThere are certainly other sources of radiation from natural sources and human activities besides diagnostic imaging.Natural background radiation,terrestrial radiation,and long flights at high altitudes constantly expose humans to radiation.The annual global per capita effective dose due to natural background radiation sources alone is estimated to be about 2400m Sv at sea level,and may be of wide range between 1000and 3000m Sv.Consequently,the natural background radiation is estimated to be about 8m Sv per day.It is correct that a certain CBCT effective dose of 50m Sv is equivalent in magnitude to 6or 7days of background radiation;however,the effect is entirely different because the acute nature of CBCT exposure of few a seconds is unlike the very low,continuous,and chronic exposure of background radiation.Radiation hormesis concept suggests that very low doses of continuous and chronic ionizing radiation that are equivalent to natural background levels are,in fact,bene ficial.This is because they stimulate the activation of repair mechanisms that protect against the disease process [51,52].Therefore,it is better to adhere to the radiation protection concept of ALARA [53],than comparing CBCT to background radi-ation,a source that cannot be controlled anyway.Ideally,CBCT should be acquired in orthodontics when a 3D evaluation is required.Otherwise,conventional 2D imaging might be suf ficient.For example,ordering a full head CBCT scan just to build cephalo-metric and panoramic radiographs is not justi fied,because these two radiographs could have been ordered without exposing the patient to the additional radiation.As previously mentioned,a full head CBCT effective dose,in some machines and settings,can be much higher the combined dosage of digital panoramic and ceph-alometric radiographs.Consequently,ordering the higher radiation CBCT just to build panoramic and cephalometric radiographs is not consistent with the ALARA principle,and could not be justi fied if compared with the myth of “few days of background radiation.”Additionally,it is judicious to take advantage of the 3D capability of CBCT after adopting this technology.According to Buttke and Prof fit [54],only 15%of orthodontic patients are adults.Therefore,because most orthodontic patients are children,it is important to emphasize that the ALARA principle applies even more critically in the majority of orthodontic patients who are more sensitive to radiation and who have long years to live whereby radiation risks may manifest in their lives [55,56].4.Myth 3We must have totally new 3D CBCT cephalometric analyses that replace 2D standardized analyses.4.1.FactPanoramic and cephalometric radiographs are standard ortho-dontic records.Introducing new CBCT cephalometric analyses require some new landmarks,planes,and angles [57].CBCT also requires different analyses that may be dif ficult to memorize and apply.Table 1Effective doses of basic radiographic imaging versus CBCT and medical head CT Imaging modalityEstimated range of effective dose (m Sv)Digital panoramic radiography 2.7e 23Digital cephalometric radiography 1.7e 3.4CBCT 20e 1025Head CT2000A.Abdelkarim /Journal of the World Federation of Orthodontists 1(2012)e3e e8e4Realistically,few orthodontists perform tracing and full analyses on all their patients.In fact,the availability of2D analyses of conventional lateral cephalometric radiograph may not make a significant difference to the treatment decisions[58].Tracing the sagittal and frontal cephalometrics and analyzing CBCT acquisitions are unquestionably recommended.However, performing CBCT cephalometric analyses in three dimensions (including new numeric and angle values that are not present in conventional analyses such as Downs,Steiner,Ricketts,McNamara, and Tweed)may not be necessary.The3D CBCT should benefit clinicians in many different ways, other than complicated3D cephalometric analyses.This modality offers three orthogonal images of oral and maxillofacial structures (axial,sagittal,and coronal views),and any otherflat and curved slices of variable thickness.Therefore,3D superimpositions,asses-sment of treatment outcome,and growth change evaluation in three dimensions can be performed[59e63].Surgical outcomes can be evaluated,and this can be of great value for the orthodontist and the patient[64,65].Lastly,soft tissue change can be visualized and evaluated in the short-and long-term in cases of orthognathic surgery[66,67].Therefore,sophisticated3D analyses may improve the diag-nostic records in some cases,but they are questionable to be valuable as a new standard.5.Myth4CBCT can replace impressions for orthodontics and be as accurate.5.1.FactEven though CBCT digital“models”are sometimes accurate for making linear measurements for overjet,overbite,and crowding measurements[68],they are unlikely to be accurate for clear tray and orthodontic wire fabrications as conventional impressions.It takes only one amalgam restoration to create“beam hard-ening”artifact[69],which creates distortion.Beam hardening occurs around dense objects such as metal brackets and bands[70]. Other CBCT image artifacts include cupping,dark bands,noise,and scatter.Although CBCT acquisition during orthodontic treatment is possible,the images would be distorted due to the beam hardening and scatter around orthodontic appliances.Another significant limitation for these tasks is possible patient movement,creating motion artifact during the relatively long scans,especially in young orthodontic patients.All these limitations of CBCT technique should be considered as they may reduce image quality.In reality,image quality itself is not similar among different CBCT machines.This was found in testing different CBCT machines in detection of simulated canine impaction-induced external root resorption in maxillary lateral incisors,a common application of CBCT in orthodontics[71].Those who are not familiar with CBCT images may not be able to differentiate between different machines,in regards to image quality.These artifacts are not noted in digital or conventional impres-sions.One may argue,however,that CBCT will continue to improve to where it will be as accurate and precise as impressions.But the numerous CBCT machines already installed will not be replaced with newer ones to replace these impressions.6.Myth5Volume rendering is sufficient to display external root resorption.6.1.FactLike any other volumetric imaging,CBCT interpretation requires the use of computer software to provide multiplanar reformatted images and supplementary3D visual representations such as volume rendering.The volume rendering,usually provided by the software automatically,is similar to architectural exemplary illustrations that provide the exterior layout but not the interior details.Three-dimensional volume rendering images can only be utilized as an adjunctive aid where it can help the orthodontist,as well as a great visual aid for the patient or parent to understand the treatment plan.However,these attractive illustrations are computer-generated and are created upon software algorithms that may not be reliable.Selecting the volume rendering may obscure normal anatomy or create artifacts that are not present.Unfortunately,numerous presentations of CBCT images include the volume rendering only.This rendering may produce false-negatives or false-positives and is not sufficient to identify presence or absence of mild external root resorption that may be present on a maxillary lateral incisor,for example,due to impaction of a maxillary canine.Evaluation of multiple slices of the scan is necessary.Perhaps some clinicians opt to present the volume rendering because evaluating the axial,coronal,and sagittal views of the scan is more “technically demanding.”Nevertheless,examination of the scan through these views is generally required because this has higher sensitivity and specificity.7.Myth6CBCT changes thefinal outcome of orthodontic treatment.7.1.FactCBCT increases accuracy of orthodontic diagnosis.Increasing diagnostic accuracy eliminates false-positive and false-negative results.Also,the treatment plan becomes more appropriate for specific cases.This may change thefinal outcome in some cases,but not always.Until now,there have not been randomized clinical trials that examine whether there is a favorable difference between orthodontic patients who were imaged by CBCT and those who were not.The effects of information derived from these images in altering diagnosis and treatment decisions have not been estab-lished in several types of cases[72].This certainly does not mean that there is no benefit of CBCT for specific cases such as impacted and supernumerary teeth,tempo-rary anchorage device site assessment,pharyngeal airway assess-ment,craniofacial deformities,cleft palate,identification of root resorption,and orthognathic surgery planning[72].Moreover,retrospective evaluation of existing CBCT data may,in many cases,provide additional understanding of numerous aspects of orthodontics.At this point,it is still arguable whether CBCT in orthodontics always provides more diagnostic information than panoramic and cephalometric radiographs,changing thefinal outcome,in order to justify its routine use in all orthodontic patients.In fact,additional information or lack thereof cannot be discovered unless comprehensive CBCT evaluation is performed.It also should be remembered that incorporating CBCT routinely in regular orthodontic practice increases the collective dose to orthodontic patients as a whole,thereby increasing the probability of deleterious effects of radiation in a group that is relatively sensitive to radiation.A.Abdelkarim/Journal of the World Federation of Orthodontists1(2012)e3e e8e58.Myth7Boards of Orthodontics will eventually adopt CBCT for all orthodontic records and superimpositions.8.1.FactAs of today,these boards have not recommended CBCT for all cases.It is,however,likely that3D imaging will be required when it provides useful information that meets the treatment needs.For example,in2000,a position paper by the American Academy of Oral and Maxillofacial Radiology(AAOMR)recom-mended that some form of cross-sectional imaging be used for most patients receiving implants[73].Today,CBCT is the preferred imaging modality for implantology.Fortunately,most elderly patients receiving implants are less sensitive to radiation than most orthodontic patients who are typically young.In another position paper by the AAOMR and the American Association of Endodon-tists,CBCT was recommended for selected,but not all,cases in endodontics[74].It was recommended that CBCT must not be used routinely for endodontic diagnosis or for screening purposes in the absence of clinical signs and symptoms[74].In orthodontics,the frequency of CBCT use is likely to be equal to endodontics,unlike implant imaging,where CBCT is used more frequently.Selection of CBCT in orthodontics is clearly case specific, and wise clinical judgment should be used.In other words,CBCT is justified in selected,but not all cases in orthodontics[75].9.Myth8CBCT is sufficient and is the best imaging modality to examine the temporomandibular joints(TMJ).9.1.FactCBCT is excellent for imaging of the bony component of the temporomandibular joints,especially if compared with panoramic radiography.Therefore,it is a valuable diagnostic tool for TMJ evaluation[76e78].However,TMJ complex is composed of bony and soft tissue structures.Unfortunately,CBCT does not map out the muscle structures,and the articular disk cannot be visualized[79].The inability to visualize the articular disk and internal derangements through CBCT imaging is a significant disadvantage for TMJ imaging.Although degenerative bone changes(which can be depicted by CBCT)may be correlated with disk displacement without reduction [80],there is actually a poor correlation between condylar changes observed on CBCT images and pain,and with other clinical signs and symptoms of TMJ of osteoarthritic origin[81].Magnetic resonance imaging(MRI)is the imaging technique of choice if an evaluation of the articular disk is required[82,83]. Although CBCT can provide valuable information about TMJ bony changes,it is not the best imaging modality for TMJ evaluation.At least,it is not sufficient to create a comprehensive radiographic evaluation of the TMJ.10.Myth9If CBCT is ordered,the orthodontist is at liability risk for any pathology in the scan.10.1.FactLately,there has been considerable concern among dental practitioners regarding the liability of reporting any pathology or incidentalfinding present in the CBCT scan.Dentists are not typi-cally trained on CBCT interpretation in dental schools,so a full evaluation of CBCT scans can be a difficult task.Even though the radiographic anatomy of CBCT is the basic structure of the skull, differentiation between a patient with a normal anatomy and an abnormality can be challenging.Until now,there have been mixed opinions on this issue.No definitive guidelines have been formed at this time.Turpin [84]and Jerrold[85]advise that orthodontists,if ordering CBCT imaging,are liable for the interpretation of the CBCT volume.But it should be remembered that potential risks for the orthodontist include unidentified pathology in traditional radiographs and possibly photographs.If examined by an oral and maxillofacial radiologist,liability risks can be avoided.Afterwards,other risks in orthodontics may be avoid as well,because CBCT contributes to accurate diagnosis and, therefore,improved treatment plan[86].Some argue that a legal document can eliminate the risk.The patient can sign an informed consent that no interpretation of the volume would be performed,and only the prescribed diagnostic task would be evaluated.There is less discussion regarding the moral consideration of fully evaluating the CBCT for the patient’s benefit.Another way to reduce the risk is to use a smallerfield of view. This actually has another benefit of reduced effective dose.11.Myth10CBCT is an orthodontic practice builder.11.1.FactThis is probably the most ironic and debatable myth.One may claim that CBCT provides superior images that facilitate treatment plan presentation.As previously said,some believe that CBCT has the potential of replacing conventional impressions and intraoral and extraoral photos,and subsequently,one CBCT scan can be sufficient for initial orthodontic records.Furthermore,some believe that patients are attracted to this technology.However,this expensive technology that involves ionizing radiation is unlikely to replace conventional impressions and be as accurate and sufficient to create comprehensive diagnoses and build wires and clear trays.Additionally,progress andfinal photos and radiographs cannot be compared with a single3D radiographic scan.For consistency,an initial CBCT scan would require an addi-tionalfinal scan for comparison.In this case,the radiation dose is doubled,assuming no acquisition retakes are performed.Many parents are aware of ionizing radiation risks and are unlikely to be interested in higher radiation for their children if given the choice of whether or not to use CBCT.Three-dimensional evaluations through CBCT should continue to evolve in orthodontics.Unfortunately,this technology is not ubiquitous yet.At this point,it is still significantly more expensive than other technologies in standard orthodontic practice.As a result,CBCT may not be an orthodontic practice builder for everybody.12.ConclusionsCBCT is a valuable imaging modality in orthodontics.Its appli-cations in thisfield have been widely recognized.It is time to reevaluate the validity of some erroneous ideas that are based on blind faith and overmarketing,instead of scientific evidence and common sense.A.Abdelkarim/Journal of the World Federation of Orthodontists1(2012)e3e e8 e6The risk e benefit ratio of CBCT is usually favorable.Due to numerous overlapping carcinogenic factors in human life,it is impossible to assess the long-term stochastic effects of radio-graphic examinations.For that reason,the concept of ALARA,sug-gesting that radiation should be kept“As Low As Reasonably Achievable,”should be adhered to,rather than justifying CBCT acquisitions by comparing their dose exposure to background radiation.Replacing impressions with CBCT,ordering it for every patient,and claiming that it would build the orthodontic practice are tactics that should be debunked.Once acquired,orthodontists are encouraged to evaluate the scan thoroughly,primarily for the patient’s benefit.Because acquisition of CBCT is case specific,each patient may benefit differently,but some patients may not benefit from the procedure.In addition,CBCT usage will ultimately result in more validated research being performed and will become an advantage of our profession with secondary tangential benefits to those undergoing orthodontic therapy.References[1]Botticelli S,Verna C,Cattaneo PM,Heidmann J,Melsen B.Two-versus three-dimensional imaging in subjects with unerupted maxillary canines.Eur J Orthod2011;33:344e9.[2]Haney E,Gansky SA,Lee JS,et parative analysis of traditional radio-graphs and cone-beam computed tomography volumetric images in the diagnosis and treatment planning of maxillary impacted canines.Am J Orthod Dentofacial Orthop2010;137:590e7.[3]Liu DG,Zhang WL,Zhang ZY,Wu YT,Ma XC.Localization of impactedmaxillary canines and observation of adjacent incisor resorption with cone-beam computed tomography.Oral Surg Oral Med Oral Pathol Oral Radiol Endod2008;105:91e8.[4]Maverna R,Gracco A.Different diagnostic tools for the localization of impactedmaxillary canines:clinical considerations.Prog Orthod2007;8:28e44.[5]Alqerban A,Jacobs R,Lambrechts P,Loozen G,Willems G.Root resorption ofthe maxillary lateral incisor caused by impacted canine:a literature review.Clin Oral Investig2009;13:247e55.[6]Walker L,Enciso R,Mah J.Three-dimensional localization 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tomography.Eur J Orthod2010;32:735e40. [18]Fayed MM,Pazera P,Katsaros C.Optimal sites for orthodontic mini-implantplacement assessed by cone beam computed tomography.Angle Orthod 2010;80:939e51.[19]Heymann GC,Cevidanes L,Cornelis M,De Clerck HJ,Tulloch JF.Three-dimensional analysis of maxillary protraction with intermaxillary elastics to miniplates.Am J Orthod Dentofacial Orthop2010;137:274e84.[20]Baumgaertel S.Quantitative investigation of palatal bone depth and corticalbone thickness for mini-implant placement in adults.Am J Orthod Dentofacial Orthop2009;136:104e8.[21]Baumgaertel S,Hans MG.Buccal cortical bone thickness for mini-implantplacement.Am J Orthod Dentofacial Orthop2009;136:230e5.[22]Gracco A,Lombardo L,Cozzani M,Siciliani G.Quantitative cone-beamcomputed tomography evaluation of palatal bone thickness for orthodontic miniscrew placement.Am J Orthod Dentofacial Orthop2008;134:361e9. [23]Kim SH,Kang JM,Choi B,Nelson G.Clinical application of a stereolithographicsurgical guide for simple positioning of orthodontic mini-implants.World J Orthod2008;9:371e82.[24]Kim SH,Choi YS,Hwang EH,Chung KR,Kook YA,Nelson G.Surgical posi-tioning of orthodontic mini-implants with guides fabricated on models replicated with cone-beam computed tomography.Am J Orthod Dentofacial Orthop2007;131(4Suppl):S82e9.[25]Alves Jr M,Baratieri C,Nojima LI.Assessment of mini-implant displacementusing cone beam computed tomography.Clin Oral Implants Res2011;22: 1151e6.[26]Hong C,Truong P,Song HN,Wu BM,Moon W.Mechanical stability assessmentof novel orthodontic mini-implant designs:Part2.Angle Orthod2011;81: 1001e9.[27]Kau CH,English JD,Muller-Delgardo MG,Hamid H,Ellis RK,Winklemann S.Retrospective cone-beam computed tomography evaluation of temporary anchorage devices.Am J Orthod Dentofacial Orthop2010;137:166.e1e5.discussion166e7.[28]McCollough CH,Schueler BA.Calculation of effective dose.Med Phys2000;27:828e37.[29]Okano T,Harata Y,Sugihara Y,et al.Absorbed and effective doses from conebeam volumetric imaging for implant planning.Dentomaxillofac Radiol2009;38:79e85.[30]Garcia Silva MA,Wolf U,Heinicke F,Gründler K,Visser H,Hirsch E.Effectivedosages for recording Veraviewepocs dental panoramic images:analogfilm, digital,and panoramic scout for CBCT.Oral Surg Oral Med Oral Pathol Oral Radiol Endod2008;106:571e7.[31]Ludlow JB,Davies-Ludlow LE,Brooks SL,Howerton WB.Dosimetry of3CBCTdevices for oral and maxillofacial radiology:CB Mercuray,NewTom3G and i-CAT.Dentomaxillofac Radiol2006;35:219e26.[32]Gijbels F,Jacobs R,Bogaerts R,Debaveye D,Verlinden S,Sanderink G.Dosimetry of digital panoramic imaging.Part I:patient exposure.Dento-maxillofac Radiol2005;34:145e9.[33]Lecomber AR,Yoneyama Y,Lovelock DJ,Hosoi T,Adams parison ofpatient dose from imaging protocols for dental implant planning using conventional radiography and computed tomography.Dentomaxillofac Radiol 2001;30:255e9.[34]White S,Pharoah M.Oral radiology:principles and interpretation.6th ed.StLouis:Mosby;2009.35.[35]Gijbels F,Sanderink G,Wyatt J,Van Dam J,Nowak B,Jacobs R.Radiation dosesof indirect and direct digital cephalometric radiography.Br Dent J2004;197: 149e52.[36]Ludlow JB,Ivanovic parative dosimetry of dental CBCT devices and64-slice CT for oral and maxillofacial radiology.Oral Surg Oral Med Oral Pathol Oral Radiol Endod2008;106:106e14.[37]Ludlow JB,Davies-Ludlow LE,Brooks SL.Dosimetry of two extraoral directdigital imaging devices:NewTom cone beam CT and Orthophos Plus DS panoramic unit.Dentomaxillofacial Radiol2003;32:229e34.[38]Roberts JA,Drage NA,Davies J,Thomas DW.Effective dose from cone beam CTexaminations in dentistry.Br J Radiol2009;82:35e40.[39]Tsiklakis K,Donta C,Gavala S,Karayianni K,Kamenopoulou V,Hourdakis CJ.Dose reduction in maxillofacial imaging using low dose Cone Beam CT.Eur J Radiol2005;56:413e7.[40]Danforth RA,Dus I,Mah J.3-D volume imaging for dentistry:a new dimen-sion.J Calif Dent Assoc2003;31:817e23.[41]Pauwels R,Beinsberger J,Collaert B,et al.SEDENTEXCT Project Consortium.Effective dose range for dental cone beam computed tomography scanners.Eur J Radiol2012;81:267e71.[42]Brooks S.CBCT dosimetry:orthodontic considerations.Semin Orthod2009;15:14e8.[43]Silva MA,Wolf U,Heinicke F,Bumann A,Visser H,Hirsch E.Cone-beamcomputed tomography for routine orthodontic treatment planning:a radiation dose evaluation.Am J Orthod Dentofacial Orthop2008;133:640.e1e5.[44]Bianchi S,Anglesio S,Castellano S,Rizzi L,Ragona R.Absorbed doses and riskin implant planning:comparison between spiral CT and cone-beam CT.Dentomaxillofac Radiol2003;30:S28.[45]Danforth RA,Peck J,Hall P.Cone beam volume tomography:an imagingoption for diagnosis of complex mandibular third molar anatomical rela-tionships.J Calif Dent Assoc2003;31:847e52.[46]Sukovic P.Cone beam computed tomography in craniofacial imaging.OrthodCraniofac Res2003;6(suppl1):31e6.179e82.[47]Mah JK,Danforth RA,Bumann A,Hatcher D.Radiation absorbed in maxillo-facial imaging with a new dental computed tomography device.Oral Surg Oral Med Oral Pathol Oral Radiol Endod2003;96:508e13.[48]Mozzo P,Procacci C,Tacconi A,Martini PT,Andreis IA.A new volumetric CTmachine for dental imaging based on the cone-beam technique:preliminary results.Eur Radiol1998;8:1558e64.A.Abdelkarim/Journal of the World Federation of Orthodontists1(2012)e3e e8e7。

用激情点燃每一个渴望紧张而富有激情的学习生活几乎让人忘记了匆匆的时光，不经意的回眸，远程学习就已经拉下了帷幕了，感慨之余不免增添了几许惆怅。

短短的几天学习中，我们互助，我们反思，我们紧张，我们收获，我们快乐，我们成长。

新课改的种子已经在我们心头悄悄的滋润，萌动，发芽，生长……通过学习，必将会触动一颗颗心灵，带动一所所学校，成长一批批名师，造福一个个学子。

回头看看我们“武汉东西湖语文班”所走过的足迹，虽然有的深，有的浅，有的重，有的轻，但是我们前进的步伐是整齐的，方向是一致的，心情是豪迈的，气氛是热烈的，效果是自豪的。

我们没有理由不为武汉东西湖区新一届的高一语文老师而感到骄傲。

一、我班概况我们武汉东西湖语文一班有学员20人，男10人，女10人，来自吴家山中学6人（含辅导老师冯涛）、吴家山四中10人、睿升学校2人、职校1人、教研室1人、中太学校1人，除了睿升学校李永阳老师家在襄樊，吴威老师在家坐月子学习，还有吴四中明雪老师因家在武昌南湖来往不便在家学习，王雪梅老师因在华师学习每天晚上在家学习、交作业外，全班学员每天都按要求到职校集中学习，按要求交作业，发评论，办简报。

到30日为止根据全国中小学继续教育网站统计，全班学员的学分都是100分，提交作业144篇，优秀作业38篇；发表文章176篇，精华文章44篇，教育部新课程项目组有七期语文课程简报和工作简报提到“武汉东西湖语文班”，文章和片断被引用16篇（含班主任3篇），占学员总数的65%，其中班主任被提到5次，引用文章4篇（次），班主任刘伯飞作为“研修之星”被语文课程简报表扬一次。

二、学习回顾回顾我班所走过的路程，我们有许多感慨，但是最重要的有以下两点值得总结：1、领导重视，组织到位在我7月16日得知要当新课改远程培训班主任前，东西湖区教育局已经按教育部和省市文件精神进行了系列部署，东西湖教师进修学校也根据上级精神制定了《高中新课程教师国家远程培训说明》《班主任信息表》《辅导教师信息表》和《教师总表》，对培训目的、学科及组织、活动要求、注册说明都做了详细的安排，同时也在开班前几天对十几个班主任进行了岗前培训和指导，在东西湖区教育局的协调之下，把全区参加高一新课改的所有老师划分为两个学习中心（即教校片和职校片），两校都提供了技术保障和后勤保障，学员都免费学习，并且东西湖教师进修学校的分管领导天天坐镇现场，发现问题，及时解决。

A Highly Flexible Testbed for Studies ofad-hoc Network BehaviourHartmut Ritter, Min Tian, Thiemo Voigt, Jochen Schiller Freie Universität Berlin, GermanyInstitut für Informatik{hritter, tian, voigt, schiller}@inf.fu-berlin.deAbstractStudies of mobile ad-hoc networks are often based on simulation and their underlying, necessarily simplified assumptions of physical reality. In order to analyse the practical problems we built our own hardware and software. The hardware consists of a core Motorola controller and different wired and wireless interfaces like a Bluetooth and a 433/868 MHz RF module. It allows therefore analysing different scenarios: First, the deployment of a pure ad-hoc network using Bluetooth or 433/868 MHz RF modules. Scenarios going beyond this cover the connection of Bluetooth piconets using the complementary RF technology. This overcomes the proximity requirements of Bluetooth scatternets. Finally, hybrid scenarios with some nodes connected to the Internet and providing Web access over a multihop ad-hoc network can be studied.In this paper we present the testbed and solutions realized up to now. These include home automation scenarios as well as support for mobile ad-hoc games.1.IntroductionThe behaviour of wireless ad-hoc networks is strongly determined by physical limitations of radio communication. Integrating the underlying physical laws into a simulator increases complexity, so that either simplifications are used or the scope of the simulator has to be reduced to a radio-layer simulator. While simulation studies are useful, they should be complemented with real world experiments in order to reveal phenomena the simulation cannot cover. For example, doing real world experiments with IEEE 802.11b based wireless ad hoc networks, Lundgren et al. discovered the existence of so-called communication gray zones that previous simulations were not able to detect [6]. Furthermore, while network simulations usually model queuing and propagation delays very accurately, less emphasis is put on processor cost. If methods exist, they are usually limited. For example, in NS-2 the standard way to model node delays is by using a timer and performing some actions on the expiration of the timer. In our testbed, the capabilities of the hardware are limited and therefore tasks such as packet processing requires much more time than in traditional routers. Real world experiments inherently include these increased processing costs.For these reasons, we decided to build a testbed using fully-understood hard- and software that allows to perform measurements and to evaluate scenarios covering all layers beginning at the radio-layer and embracing all layers up to HTTP/TCP/IP and the application space. The hardware is an embedded device we call Embedded Wireless Module (EWM) consisting of a core controller (Motorola 68HC912) and I/O peripherals as well as different wireless and wired network connectors. The device is, for example, equipped with Bluetooth and a 433 MHz RF module.In this paper, we demonstrate some experiments we have performed with this hardware. We present some general observations as well as some real world experiments covering different application domains such as ad-hoc gaming and home automation. While vertical handover times between Bluetooth and the 433 MHz RF modules are in the order of milliseconds, our measurements show that the handover time for a Bluetooth slave between two different piconets is too slow to enable highly interactive multi-user games in an ad-hoc gaming infrastructure we present in this paper. The presented home automation scenario evaluates the efficiency of service usage in ad-hoc networks. While demonstrating the feasibility of this approach further optimizations of some of the steps are necessary.The remainder of this paper is outlined as follows: In the next section, we describe the hard- and software of this testbed. In Section 3, we discuss general observations found in our experiments and in Section 4 we present selected experiments. Finally we compare with related work and discuss the overall conclusions.2.Description of the TestbedIn this part we give a description of the hardware used in the testbed. It was designed to be as flexible as possible. Thus, the testbed does not use any special hardware, all components are fully documented and even the hardware-related low-level program code is available.2.1.Hardware of the TestbedOne of the hardware modules that builds our testbed is shown in Figure 1. The embedded module basically consists of a core controller (Motorola 68HC912) and I/O peripherals as well as different wireless and wired network connectors.On the upper left hand side resides a 433 MHz RF module from Radiometrix [7] used for long-range interconnections between embedded wireless modules. On the opposite side of the display the class 2 Bluetooth module from CSR [1] can be seen. Hidden behind the display resides the core controller. It communicates over serial connections with the 433 MHz and the Bluetooth module. In addition there are analogue and digital I/O-port on the bottom, an Ethernet connector on the left side, CAN bus connectors on the right and a serial interface for debugging purposes.Figure 1. Embedded Wireless ModuleIn most scenarios the core Motorola controller will maintain the state of all these peripherals and coordinates the communication and probably routing over the different network interfaces. The fixed Ethernet jacket serves in some wired scenarios as Internet connection of a Web Server realised on the embedded module. 2.2.SoftwareFrom the software point of view full access to nearly all communication layers is possible.The 433 MHz RF module basically offers a bitstream interface and converts all bits sent to it over the serial interface into an AM-modulated signal at 433 MHz frequency. Similarly, data received over the air is passed to the serial interface as an unmodified bitstream. As the module can not send and receive simultaneously, it defaults to the receive mode and switches to the send mode only if data is passed to it over the serial interface. Functions to achieve long-term parity between zero and one-bits as well as all kinds of framing and error checks have to be implemented in software running on the Motorola controller.The situation differs a bit with the Bluetooth module. As it would be too much overhead to realize all layers of the Bluetooth protocol stack on the Motorola controller, we take advantage of the fact that the Bluetooth chip itself can take over the lower layers of protocol handling. Beyond that, the Bluetooth module even allows to download and deploy small programs that can access all provided Bluetooth functionality like inquiry, service discovery, and data access on the RFCOMM level and above. These programs run on the Bluetooth module, thus relieving the Motorola controller from a lot of time-critical protocol handling. The air interface of the Bluetooth module follows the Bluetooth standards, providing for example serial port emulation or Internet access using BNEP, whereas the interface to the Motorola controller is a serial interface. Depending on the scenario, communication between the Motorola core controller and the Bluetooth module takes place either on the level of a simple ASCII protocol or by exchanging Ethernet frames over the serial line between both.We do not use an operating system as this might consume too much resources but instead deploy a simple loop that handles all tasks and that can be interrupted by events like expired timers and incoming data packets. However, as all direct hardware access is encapsulated in C functions, application programming does not require deep knowledge of the hardware while on the other hand keeping the full flexibility.3.General ObservationsIn this section, we present some results that are not specific for a certain scenario but results that we consider general for the kind of technologies used.3.1.Media AccessThe 433 MHz RF module used for our EWM modules incorporates hardware providing carrier sense. To evaluate if it is necessary to implement a complete MAC layer with carrier sense and collision detection protocols we performed several experiments. In an example experiment, one EWM module sends a broadcast which is received by two other EWM modules. Upon the reception of this broadcast, these two EWM modules immediately perform a rebroadcast. Performing this experiment we noticed very few collisions. The reason for this is that even though the EWM modules receive the first broadcast almost contemporaneously, they do not rebroadcast it exactly at the same time. With the cycle time of the EWM being 200 ns, the EWM modules do not perform the rebroadcast at exactly the same time and the carrier sense of the radio module is fast enough to note that the “faster” EWM has already started retransmitting the broadcast. The transmission of the packet is delayed until the “faster” EWM has transmitted its packet. Therefore, very few collisions occur. Since we only have a limited number of EWMs this allowed us to abstain from implementing a complete MAC layer. Of course, the situation is different in dense networks.3.2.Bluetooth Connection TerminationWhen making some preliminary experiments with Bluetooth, it turned out that the Bluetooth chip on the EWM modules needs several seconds until it detects that a slave is out of range. We experienced the same behaviour for connections between different Bluetooth-equipped entities, namely both for the Bluetooth chip on other EWM modules as well as for Bluetooth-equipped IPAQ 3970 PDAs.Since this behaviour is undesirable in many scenarios, we implemented another approach in which the Bluetooth chip on the EWM modules continuously measures the link quality of the connections to the slaves. If the link quality falls beyond a certain threshold, the master breaks the connection to the slave and the core controller is informed. One additional advantage of this approach is that it allows to shape the range of the Bluetooth piconet and to adapt it to the physical topology.4.ScenariosThe following four scenarios show the clear advantage of real-world implementations compared to simulations because they exhibit special hardware characteristics. 4.1.Vertical Handover Bluetooth to 433 MHzIn this scenario, we demonstrate a vertical handover from Bluetooth to the 433 MHz technology. Bluetooth is a mature technology that provides several features such as device and service discovery, media access control and connection management. While these features facilitate ad hoc communication, the indoor transmission range of Bluetooth is only about 10 meters. On the other hand, the range of our 433 MHz modules is about 50 meters indoors, but 433 MHz does not provide any higher layers.Therefore, it makes sense to use Bluetooth as the primary communication technology between EWM modules while these are in transmission range but to change to 433 MHz when Bluetooth communication is not possible, for example, when the EWM modules move out of range. Therefore, our system must be able to perform a handover between Bluetooth and the 433 MHz technology. This scenario shows that such a vertical handover is feasible with our EWM modules.In this scenario, we use two EWM modules: one immobile and one mobile. In the beginning, both EWM modules are within Bluetooth transmission range and therefore they establish a Bluetooth connection with the immobile EWM being the master and the mobile EWM being the slave1. The immobile EWM continuously measures the signal strength. As the mobile EWM is moving away, the signal strength falls beyond a predefined threshold. The BT chip therefore breaks the connection to the mobile EWM and informs the core controller about the teardown of the connection. Knowing that the Bluetooth connection has been broken, the core controller issues a broadcast over the 433 MHz module searching for the mobile EWM. As an identifier the Bluetooth address is used since these are guaranteed to be unique and the EWM already knows the Bluetooth address of the mobile peer. After the reception of the search request, the mobile EWM replies with a unicast message and the two EWM modules are ready to communicate using their 433 MHz modules. The whole procedure from the teardown of the Bluetooth connection until the establishment of the “connection” using 433 MHz takes only a few milliseconds.While in this scenario we use 433 MHz to extend the communication range between EWM beyond the meagre transmission range offered by Bluetooth, 433 MHz can also be seen a second wireless technology providing 1We say that the EWM performs actions but note that here the BT chip performs these actions.fault-tolerance in case Bluetooth for some reason does not work.4.2.GamingIn this scenario, we describe our experiences with an infrastructure for ad-hoc multiplayer games that uses EWM modules as the main components. Our goal is to develop an architecture that supports ad-hoc multi-user games on mobile devices such as PDAs and mobile telephones. The architecture should require a small and cheap infrastructure only and it should be possible to set up the infrastructure very quickly and in an ad-hoc fashion. We envision for example a place like a university, a subway station and other places where people meet and possibly wait for transportation, the beginning of a lecture etc. In these scenarios, spontaneous group formation will take place if it is supported by some technical means. For example, the system could provide high-scores, a gamer’s archive allowing persistent nicknames, activity control or cheating prevention.Based on the capabilities of our hardware we developed a system for gaming support that will be described in this section.4.2.1. System Overview For the envisioned kind of ad-hoc games, Bluetooth is very attractive since building small ad-hoc Bluetooth networks is straightforward. Furthermore, most modern PDAs and mobile telephones provide Bluetooth support. However, the transmission range of Bluetooth is about 10 meters and thus quite short. As argued in section 2.2, Bluetooth scatternet support has some major drawbacks. Therefore, we choose a different approach and set up what can be roughly defined as a roaming infrastructure: In order to overcome the problems of short-range Bluetooth piconets, we distribute several EWM modules across the field.We use the 433 MHz RF technology on the EWM module (with transmission ranges of up to 300 meters) as a second wireless technology. Bluetooth is used to transport gaming data, whereas the 433 MHz technology is used to maintain a global view of the game, to track the players and to exchange gaming data between the piconets. This way, the full bandwidth that Bluetooth offers can be used to transport gaming data. Only data that has to be routed to another piconet passes the 433 MHz RF link. Another usage of the 433 MHz technology is to find out whether players have left the game.The Bluetooth chips on the Embedded Web Server modules act as the masters of the Bluetooth piconets around them. As described in Section 3.2, the BT chip continuously measures the link quality to the slave. When the link quality of a connection passes a threshold value, the Bluetooth chip breaks the connection to the salve and sends a message to the core controller. Note that though the EWM module could also inform its neighbours when the Bluetooth chip breaks the connection to a slave, it turned out to be simpler to inform neighbours when a new slave (player) is found only.When the connection has been broken the slave (player) has left the piconet. When the player is within coverage of the next EWM module, the Bluetooth device of the player is inquired by the Bluetooth chip of this EWM module. The latter informs the adjacent EWM modules about the newly found player using the 433 MHz technology. If an EWM module looses a player and neither finds her again nor does it receive a notification that an adjacent EWM module has found that player, it can assume that the player no longer participates in the game. Obviously she has either left the game or turned off her Bluetooth device.For communication between the 433 MHz RF modules we use a very simple ASCII-based protocol. The messages contain a header consisting of the sender and receiver ID, a message type, the packet length and the payload. The protocol implementation supports both reliable and unreliable messages. For our purposes we have defined several message types (e.g. FOUND when a new player is found). Thus, it is sufficient that the payload contains the Bluetooth address of the affected player.In our current implementation the EWM modules are numbered and have a priori knowledge about their neighbours which is used for direct neighbour-to-neighbour communication. We leave the definition of a true zero configuration protocol as future work. Note that this is not a real problem. When players place the EWM modules in the field, they just have to make sure that they place the modules according to their consecutive numbering (Figure 2 shows an example).4.2.2. Evaluation The infrastructure supporting ad-hoc multiplayer games described so far was fully implemented and evaluated in the context of our lab building. This allows to get practical experiences and to uncover difficulties. We see it as a platform for future research of different kinds of games.Our testbed depicted in Figure 2 consists of four EWM modules that are placed inside the offices of our lab members. During the experiments the affected four office doors are closed. The distances between adjacent offices are 12 m (EWM 1 in office 1 to EWM 2), 15.3 m (EWM 2 to EWM 3) and 16 meters (EWM 3 to EWM 4). Fortunately, this is more than the transmission range of Bluetooth but less than the double of the transmission range. As expected, 20 meters is in the transmission range of the 433 MHz. The baud rate of the 433 MHz transmission is set to 19200 in our experiments. As clientdevice we use an iPAQ 3970 PDA equipped withBluetooth, running Windows CE 3.0.Figure 2. TestbedIn order to collect distributed timing information, we use a fifth EWM module, called time-EWM, this time without a Bluetooth chip. It serves for fine-granular logging of the distributed system’s behaviour. The other EWM modules inform the time-EWM by sending messages over 433 MHz RF technology. The message type depicts the nature of the event. Time differences between events can be computed by calculating how often the timer interrupt between two successive events is triggered. This timer interrupt is triggered by the hardware every 65 ms. Note, that the time-EWM is not needed at all for the proper functioning of the distributed gaming infrastructure, but serves only as a monitoring station.In order to get data about the usability of the testbed for gaming, we measure the Bluetooth handover time, which we define for the purpose of this paper as the time difference between the teardown of the connection by the first EWM module and the moment the mobile client is found by the second EWM module. This handover time is of extreme importance to determine the types of games that can be played on our architecture. Before conducting this experiment, we validated that when a player moves from one EWM module to the next, it is in the transmission range of the second EWM module before the connection is disrupted by the first EWM module because the quality of the connection falls beyond the threshold as described in Section 3.2.In this experiment, an EWM module notifies the time-EWM when it disrupts the connection to a client. When an EWM module finds a client, it also sends a message to the time-EWM but this time another type of message. The time-EWM can now compute the handover time, i.e. the time difference between the arrivals of the two messages. Note that a Bluetooth slave can belong to one piconet only; thus, it cannot be found by another Bluetooth master before it has left its current piconet.Our test runs resulted in an average handover time of 1.43 seconds. The results were very similar for the three sections of our setup. We also redid the same experiments using a laptop instead of the iPAQ and achieved similar resultsDue to the large handover time, we do not assume that highly interactive multiplayer games are feasible using our architecture. Unless, of course, handovers take place during non time-critical moments that are part of many games, for example before the start of a racing game or when switching between game levels. Note that, since our EWM modules monitor the signal strength, we have the possibility of warning players when they move into a zone with weak connectivity and may thus trigger a handover. Further results can be found in [8].In summary, this scenario realized a fully mobile infrastructure allowing cross-piconet coordination and communication using two wireless technologies available on the EWM modules.4.3.Home AutomationThe home automation scenario focuses on the aspects of heterogeneous ad-hoc networks and demonstrates once more, that not only functionality of an approach matters, but also measurements that give an impression of the real world behaviour.The main functionality that should be provided in the home automation scenario is provisioning of an interface for a mobile PDA user that allows the user to control and interact with devices in a room, like a coffee machine or even a light bulb. The basic interaction principle is shown in Figure 3: At first, a new hardware device (HW Dev) registers itself with a service repository. This is done using the service location protocol SLP [11]. A mobile user with a PDA entering a room would then perform SLP service discovery to find out what services are provided within the room. After receiving a complete service description, probably filtered according to its wishes, the user connects from the PDA via Bluetooth directly with the device. This third step comprises the exchange of an XML file that describes the capabilities and controls of the actual hardware device and the command issuing by the PDA user.In order to realize this scenario within our testbed, we use the Bluetooth part of an embedded wireless module as prototype HW device that allows to switch on and off a LED directly connected with the Bluetooth device. The core Motorola controller of this module is not used, therefore the hardware could be stripped down in a real product to dimensions of 1.5 x 2 cm. A second embedded wireless device could be used for hosting the SLPdaemon. Nevertheless, a full SLP implementation would consume too much resources of both the Motorola controller and the Bluetooth module of the prototype HWFigure 3. Home Automation Scenario Therefore, we modified the basic principle shown in Figure 3 by using a downstripped protocol that is translated into the SLP protocol format by a proxy running on a separate PC. We are currently porting this proxy from the PC to the Motorola controller on the embedded wireless module, so that also in full ad-hoc mode service usage will be possible. The proxy is announced via Bluetooth SDP as an additional service, so that the PDA and all HW devices have a fixed entry point for registration and service discovery respectively.Table 1. Interaction timesAction TimeneededComments1 ServiceRegistration ofthe HW device 5 s(excl.inquiry)Needed onlyat first deviceusage2 SLP proxydiscovery byPDA6 s3 Download of allparameters of aspecific service toPDA 2-6 s Could becached on thePDA4 Service usage(download ofXML file,commandtransfer)2.3 sFor analysis of the behaviour besides the bare functionality it is crucial to perform measurements of the timing behaviour of this setting. Our results using a Compaq iPAQ PDA, a Linux PC running SLP proxy and daemon and an embedded wireless module as HW device are presented in Table 1.The results show the problems with long inquiry and service discovery times of Bluetooth. Nevertheless, actual service usage is in an acceptable timeframe of 2 to 3 seconds. This indicates that caching mechanisms and service discovery in the background based on user profiles are promising. The embedded wireless modules of the presented testbed allow real user experience that is needed for determination of acceptable interaction scenarios.4.4.Hybrid ScenariosThe embedded wireless modules also allow Internet access using TCP/IP. Therefore, we ported the implementation of Dannenberg et al. for the Texas Instruments MSP 430 (described in [3]) to the Motorola controller. Based on this implementation an embedded wireless module can get an IP address using DHCP and offer Internet access over the different wireless links. We both implemented the transport of IP packets over the 433 MHz radio link and Internet access over Bluetooth. Multihop routing across embedded wireless modules is currently being implemented.For Internet access over Bluetooth we implemented the PAN profile using BNEP on board of the Bluetooth chip. Thus, any Bluetooth device that also implements this profile can connect with the Internet over the Bluetooth chip and the Motorola controller. The Bluetooth module handles all Bluetooth-related stuff and just exchanges Ethernet packets with the Motorola controller. The latter handles the Bluetooth device as just another Ethernet device.The throughput in this scenario is bounded to the rate of the serial line between Bluetooth device and Motorola controller, currently set to 115.2 kbit/s. We get delay times of up to 600 ms between an external Bluetooth device and the Motorola controller. These times include protocol handling on board of the Bluetooth chip and the Motorola controller, and the typical delay of the Bluetooth link layer.5.Related WorkPrototype devices built in the smart-Its project [9] are based on two different microcontroller platforms, Atmel and PIC. The Atmel platform is extended with an Ericsson’s Bluetooth module, while the PIC-based platform is augmented with an 868MHz module for mobile communication. The goal of the Smart-Its projects is to attach small and unobtrusive computer modules withwireless transmission capability to real world objects. These modules communicate with each other in an ad hoc fashion. In contrast to our EWM the Smart-Its devices are obviously targeted at networks of small, nearly invisible nodes such as sensor networks.Hartwig et al. describe the possibilities for applying general-purpose, pluggable microservers running on WAP over the Bluetooth technology for remote control purposes [4]. Their goal is to connect electronic devicesto the Web with inexpensive standard technologies. This comes close to our home automation scenario but requiresa WAP server on each hardware device.While these efforts have, precisely as the hardware described in this paper, produced artefacts that enable real world experimentation, the APE testbed [5] is designed to perform large-scale, reproducible experiments with IEEE 802.11b based ad hoc networks. APE aims at assessing several different routing protocols in a real-world environment instead of by simulation. The motivation for building such a testbed is that the accuracy of simulation results depends very much on the modelling of the physical layer [10]. Furthermore, simulations of one particular protocol implementation often lack the interactions with other layers [2].6.ConclusionsIn this paper we presented a testbed that enables real world evaluation of ad-hoc networks. The various scenarios highlight the flexibility of the embedded wireless modules. Our results show that testbeds are needed to uncover and deal with real-world performance problems in ad-hoc networking. Our scenarios demonstrate several performance flaws with Bluetooth. For example we identify the problem that the detection ofa connection breakdown is strongly delayed. The testbed assisted us with finding and evaluating a proper solution, showing the clear advantage of real-world implementations compared to simulations.7.References[1] Cambridge Silicon Radio. [2] D. Cavin, Y. Sasson, and A. Schiper. On the accuracy of manet simulators. Workshop on Principles of Mobile Computing (POMC'02), October 2002[3] Design&Elektronik Extra: Embedded Internet. September 2001. Available at: http://www.design-elektronik.de/extraheft. [4] S. Hartwig, J- Strömann, and P. Resch. Wireless Microservers. IEEE Pervasive computing, April-June, 2002. [5] H. Lundgren, D. Lundberg, J. Nielsen, E. Nordström, C. Tschudin. A Large-scale Testbed for Reproducible Ad hoc Protocol Evaluations.IEEE Wireless Communications and Networking Conference , 2002.[6] H. Lundgren, E. Nordström and C. Tschudin. Coping with communication gray zones in IEEE 802.11b based ad hoc networks. 5th ACM international workshop on Wireless mobile multimedia, Atlanta, Georgia, USA, 2002.[7] Radiometrix. [8] H. Ritter, T. Voigt, M. Tian and J.Schiller. Experiences Using a Dual WirelessTechnology Infrastructure to Support Ad-hoc Multiplayer Games. NetGames 2003, Redwood City, California, USA, May 2003.[9] [10] M. Takai, J. Martin and R. Bagrodia. Effects of Wireless Physical Layer Modeling in Mobile Ad Hoc Networks. ACM International Symposium on Mobile Ad Hoc Networking & Computing (MobiHoc 2001), October 2001.[11] J. Veizades, E. Guttman, C. Perkins. Service Location Protocol. RFC 2165, June 1997.。

博客教研，有效教研的新途径作者：何晓新来源：《新课程学习·中》2013年第02期摘要：因互联网的普及，教研的形式也发生着变化。

利用博客开展教研活动，越来越受到广大教师的欢迎和喜爱。

“有效教研”是一种民主、开放、科学的教研机制，是一种合作、坦诚、包容的学习文化。

它改变了传统教研定时、定点、定人的方式，使教研活动更灵活、更自主也更高效。

关键词：有效教研；博客教研；交流平台教育引领时代的发展，而时代的发展也在影响着教育，随着科学技术的进步，计算机网络应经走进千家万户，走进各行个业。

因互联网的普及，教研的形式也发生着变化。

利用博客开展教研活动，越来越受到广大教师的欢迎和喜爱。

北安市教师进修学校课题组把“有效教研”，作为研目标，通过课题的研究，改变过去教研活动低档次徘徊、原地打转、重复浪费的状况，解决教研活动的内驱力问题，使教研活动具有最大的“有效化”。

在教研活动中，我们把教研和博客结合起来，使教研活动更加高效、自觉。

博客教研，就是一种新兴的、实现教研有效性的途径之一。

它改变了传统教研定时、定点、定人的方式，使教研活动更灵活、更自主，也更高效，实现了教研活动的日常性。

一、利用博客开展网络教研的主要优势1.方便快捷，学习内容丰富现在网络已经走进家庭，上网学习、工作、娱乐成为一些教师的日常必不课少的活动，只有有网络，就可随时随地学习。

博客教研内容丰富，教师和教育人的博客涉及的内容丰富，有教育叙事研究；有的利用博客开展行动研究，在不断反思中进步；也有老师运用博客开展研究性学习……教育博客的运用对促进教师的专业发展起到了很好地推动作用。

2.不受时空限制，有利于教师主动学习只要有网络，有时间，教师就可以学习。

利用博客开展网络教研，方便组织，易于实施，表达更加自由，阅读更加随意。

能有力地促进教研活动的决策者、设计者、组织者、实施者与参与者的多元对话，可有效地提高教研活动的实效。

3.思考更加充分，有利于教师积累和总结经验在常规教研活动中，教师的评课、议课由于受时间限制，思考不够充分，对问题的认识理解往往较为肤浅。

Activatable aptamer probe for contrast-enhanced in vivo cancer imaging based on cell membrane protein-triggered conformation alterationHui Shi,Xiaoxiao He,Kemin Wang 1,Xu Wu,Xiaosheng Ye,Qiuping Guo,Weihong Tan,Zhihe Qing,Xiaohai Yang,and Bing ZhouState Key Laboratory of Chemo/Biosensing and Chemometrics,College of Chemistry and Chemical Engineering,Institute of Biology,Hunan University,Key Laboratory for Bio-Nanotechnology and Molecule Engineering of Hunan Province,Changsha,China 410082Edited by Larry Gold,SomaLogic,Inc.,Boulder,CO,and approved January 21,2011(received for review October 29,2010)Aptamers have emerged as promising molecular probes for in vivo cancer imaging,but the reported “always-on ”aptamer probes remain problematic because of high background and limited con-trast.To address this problem,we designed an activatable aptamer probe (AAP)targeting membrane proteins of living cancer cells and achieved contrast-enhanced cancer visualization inside mice.The AAP displayed a quenched fluorescence in its free state and underwent a conformational alteration upon binding to target cancer cells with an activated fluorescence.As proof of concept,in vitro analysis and in vivo imaging of CCRF-CEM cancer cells were performed by using the specific aptamer,sgc8,as a demonstration.It was confirmed that the AAP could be specifically activated by target cancer cells with a dramatic fluorescence enhancement and exhibit improved sensitivity for CCRF-CEM cell analysis with the cell number of 118detected in 200μl binding buffer.In vivo studies demonstrated that activated fluorescence signals were obviously achieved in the CCRF-CEM tumor sites in pared to always-on aptamer probes,the AAP could substantially minimize the background signal originating from nontarget tissues,thus resulting in significantly enhanced image contrast and shortened diagnosis time to 15min.Furthermore,because of the specific affinity of sgc8to target cancer cells,the AAP also showed desir-able specificity in differentiating CCRF-CEM tumors from Ramos tumors and nontumor areas.The design concept can be widely adapted to other cancer cell-specific aptamer probes for in vivo molecular imaging of cancer.switchable aptamer probe ∣in vivo imaging ∣activatable fluorescent molecular imaging ∣cancer detection ∣cell surface proteinDevelopment of sensitive and specific molecular probes is one of the central challenges in cancer imaging (1,2).Aptamers are single-stranded RNA or DNA oligonucleotides with unique intramolecular conformations that hold distinct binding proper-ties to various targets,including small molecules,proteins,and even entire organisms (3–5).As a small,polyanionic and nonim-munogenic type of probe,aptamers may exhibit faster tissue penetration and uptake,shorter residence in blood and nontarget organs,and higher ratio of target accumulation,thus affording great potential for in vivo cancer imaging (6,7).Design of aptamer probes for cancer imaging has primarily relied on a strategy using the so-called “always-on ”probes (8),in which the reporter-bearing aptamers are bound to target cancer cells and accumulation of the reporters around cells then results in an elevated signal with reference to the surrounding environment.V arious signal reporters have been adopted,includ-ing radioactive,magnetic,and fluorescent agents (9–13).Our group has demonstrated that near-infrared dye-labeled aptamers could effectively recognize cancer cells in vivo and achieve cancer imaging with high specificity (13).However,because always-on aptamer probes had constant signals,the image contrast was cri-tically limited by a high background.And the cancer site could be clearly observed only after the physiological clearing of unboundaptamers,thus leading to a long diagnosis time,which further compromised contrast by substantial consumption of bound aptamers.Therefore,ideal aptamer probes for in vivo cancer imaging should preferably display signal alteration architectures,in which the normally quenched signal is activated only after successfully targeting cancer sites.In the context of molecular imaging,a variety of such probes have been developed (14–21).T ypical mechanisms include enzy-matic cleavage of a quencher group from the fluorescence moiety using cancer-related enzymes (15–18),intracellular degradation of fluorescence-quenched protein against cancer cell receptors (19,20),and surrounding-sensitized fluorescence labels conju-gated to macromolecular ligands selectively endocytosed by can-cer cells (8).However,these mechanisms cannot be utilized to develop aptamer-based activatable probes.Because the sequences of aptamer probes can be custom-designed,aptamers have the particular advantage of target recognition-triggered conforma-tional alteration,leading,in turn,to signal alteration.This strategy has been well demonstrated with molecular beacons for nucleic acid monitoring and free protein detection (22,23).Assuming that this strategy would work on surface membrane proteins of living cancer cells,we hypothesized that conformational alteration triggered by the specific binding of aptamers with cell membrane proteins could detect cancer cells,thus affording a substantial basis for the development of a unique kind of activatable mole-cular probe for in vivo cancer imaging.Herein we report such an activatable aptamer probe (AAP)designed based on cell membrane protein-triggered conformation alteration,as illustrated in Fig.1.The AAP is a single-stranded oligonucleotide consisting of three fragments:a cancer-targeted aptamer sequence (A-strand),a poly-T linker (T-strand),and a short DNA sequence (C-strand)complementary to a part of the A-strand,with a fluorophore and a quencher covalently attached at either terminus.Because of hybridization of the C-strand with its complementary part of the A-strand,the AAP is hairpin struc-tured.This conformation keeps the fluorophore in close proxi-mity to the quencher,resulting in quenched fluorescence in the absence of a target.However,when the probe encounters the target cancer cell,it is capable of binding with protein receptors on the cell surface,causing a spontaneous conformational reorga-nization of the hairpin structure and,consequently,forcing the fluorophore to separate far from the quencher.Finally,a fluores-cence signal is activated in response to the successful binding ofAuthor contributions:H.S.,X.H.,and K.W.designed research;H.S.,X.W.,and X.Ye performed research;H.S.,X.H.,K.W.,W.T.,and X.Yang analyzed data;Q.G.,Z.Q.,and B.Z.contributed new reagents/analytic tools;and H.S.,X.H.,K.W.,and W.T.wrote the paper.The authors declare no conflict of interest.This article is a PNAS Direct Submission.1To whom correspondence should be addressed.E-mail:kmwang@.This article contains supporting information online at /lookup/suppl/doi:10.1073/pnas.1016197108/-/DCSupplemental .3900–3905∣PNAS ∣March 8,2011∣vol.108∣no.10/cgi/doi/10.1073/pnas.1016197108the AAP to the target cancer cell.As a “signal-on ”probe,the low fluorescence-quenched background may display dramatically enhanced image contrast,thus leading to a much shorter diagnosis time.Note that in our design,the AAP not only acts as a molecular recognition probe but also serves as a transducer in generating an activated fluorescence signal as a result of cell membrane protein binding events.Thus,by integrating target specificity with sensi-tive signal transduction,these aptamer-based probes are afforded a unique advantage.Results and DiscussionConstruction and Sequence Optimization of AAP.An aptamer sgc8was used as a model system to demonstrate the feasibility of the AAP-based strategy for in vivo cancer imaging.The sgc8was selected by cell-SELEX against human acute lymphoblastic leukemia CCRF-CEM cells (24)and identified to interact with the cell membrane protein tyrosine kinase-7(PTK7),a protein closely associated with a number of cancers (25).The AAP for CCRF-CEM cancer imaging was constructed as a single-stranded oligonucleotide consisting of the aptamer sgc8,the T-strand,and the C-strand,with a fluorophore FAM and a quencher BHQ1covalently attached at either terminus.T o ensure that the fluor-escence resonance energy transfer (FRET)between FAM and BHQ1would be substantially eliminated after AAP activated,a total number of the nucleotides contained in the T-strand andC-strand was fixed at 25.Consequently,the signal-to-background ratio of the AAP was strongly dependent upon the C-strand sequence.More specifically,the hybridization of the C-strand with its complementary part of the A-strand resulted in low back-ground and increased aptamer stabilization.For obtaining the AAP sequence with the highest signal-to-background ratio,four aptamer probes —probe a,b,c,and d —with different C-strand compositions were designed.As listed in T able 1,the number of nucleotides contained in C-strand was gradually increased from six in probe a to nine in probe d.Fig.2A gives the fluores-cence responses of these aptamer probes to CCRF-CEM cells with reference to the control Ramos cells.After incubation with Ramos cells,the probes displayed decreased background responses with longer C-strand complementary to the A-strand.On the other hand,with shortened C-strand,the affinity of the probes increased and the resulting signals of these probes acti-vated by target CCRF-CEM cells were enhanced significantly.The signal-to-background ratios of these probes are given in Fig.2B .The best signal-to-background ratio was achieved with probe c that had an eight-nucleotide sequence complementary to the A-strand.With the optimized AAP sequence,a near-infra-red AAP was then constructed using a near-infrared dye,Cy5,as the reporter and BHQ2as an efficient quencher.The utilization of the near-infrared reporter could further facilitate applications of the AAP for in vivo imaging of CCRF-CEM cancer (26).Activation of AAP by Target Cancer Cells.Flow cytometry assayswere performed to investigate the fluorescence activation of the AAP by target cancer cells.Control probe 1,a negative con-trol probe for the AAP ,was constructed with the A-strand sub-jected to an arbitrary alteration such that it showed little affinity to target cancer cells.Fig.3depicts the fluorescence signals of the AAP and control probe 1in response to different cell lines.It was observed that the AAP showed much higher labeling of CCRF-CEM cells than control probe 1.This revealed that the AAP was substantially activated after binding with membrane proteins of the target cancer cell with an elevated fluorescence.The activation efficiency for several nontarget cell lines,including two cancer cell lines,such as Ramos cells and U266cells,as well as one normal cell line,such as B95-8cells,was then inves-tigated.We observed that fluorescence responses of the AAP to these cells did not exhibit significant difference from those obtained with control probe 1,thus showing that the AAP was not activated by these nontarget cells.This was further confirmed by flow cytometry assays of cancer cells in mouse serum with the AAP (Fig.S1).Although the surrounding environment was changed from binding buffer to mouse serum,which was more complex and unstable for the AAP ,the fluorescence elevation was still detected after the AAP was activated by CCRF-CEM cells,while there was little signal enhancement observed for Ramos cells.These results implied that the AAP strategy held the desired ability to work on cell membrane proteins bothinFig.1.Schematic representation of the novel strategy for in vivo cancer imaging using activatable aptamer probe (AAP)based on cell membrane pro-tein-triggered conformation alteration.The AAP consists of three fragments:a cancer-targeted aptamer sequence (A-strand),a poly-T linker (T-strand),and a short DNA sequence (C-strand)complementary to a part of the A-strand,with a fluorophore and a quencher attached at either terminus.In the absence of a target,the AAP is hairpin structured,resulting in a quenched fluorescence.When the probe is bound to membrane receptors of the target cancer cell,its conformation is altered,thus resulting in an activated fluorescence signal.Table 1.All of the oligonucleotides used in this work*ProbeSequenceProbe a 5′-FAM-TCTAAC TTTTTTTTTTTTTTTTTTT ATCTAACTG CTGCGCCGCCGGGAAAATACTGTACGGT TAGA-BHQ1-3′Probe b 5′-FAM-TCTAACC TTTTTTTTTTTTTTTTTT ATCTAACTG CTGCGCCGCCGGGAAAATACTGTACGGTTAGA -BHQ1-3′Probe c 5′-FAM-CTAACCGT TTTTTTTTTTTTTTTTT ATCTAACTG CTGCGCCGCCGGGAAAATACTGTACGGTTAGA -BHQ1-3′Probe d5′-FAM-TCTAACCGT TTTTTTTTTTTTTTTT ATCTAACTG CTGCGCCGCCGGGAAAATACTGTACGGTTAGA -BHQ1-3′Activatable aptamer probe (AAP)5′-Cy5-CTAACCGT TTTTTTTTTTTTTTTTT ATCTAACTG CTGCGCCGCCGGGAAAATACTGTACGGTTAGA -BHQ2-3′Control probe 1†5′-Cy5-ACGGTTAG TTTTTTTTTTTTTTTTT ATACGGTGA CTGCGCCGCCGGGAAAATACTGTCTAACCGTA -BHQ2-3′“Always-on ”aptamer probe 5′-Cy5-ATCTAACTGCTGCGCCGCCGGGAAAATACTGT ACGGTTAGA -3′Control probe 2‡5′-Cy5-ATACGGTGACTGCGCCGCCGGGAAAATACTGT CTAACCGTA -3′*In all sequences,the A-strand is presented in italic,the T-strand is presented underlined,and the C-strand is presented in bold.†Control probe 1is a negative control probe for AAP ,which is constructed with the A-strand subjected to an arbitrary alteration such that it shows little affinity to target cancer cells.‡Control probe 2is a negative control probe for always-on aptamer probe,the sequence of which is identical with the altered aptamer sequence of control probe 1.Shi et al.PNAS ∣March 8,2011∣vol.108∣no.10∣3901C H E M I S T R YA P P L I E DB I O L O G IC A L S C I E N C E Sbuffer and serum,and that the activation of the AAP was criti-cally dependent on protein expression patterns of cells.Interest-ingly,it was found that the AAP labeled with Cy5fluorophore and BHQ2quencher displayed a substantially improved signal-to-background ratio (S ∕B ≈7.38)than the AAP labeled with FAM and BHQ1(S ∕B ≈1.53),which might be attributed to the alleviation of endogenetic fluorescence of cells in the near-infrared spectral region.Then,the binding assays of the AAP with CCRF-CEM cancer cells were further performed by using flow cytometry (Fig.S2).By subtracting the mean fluorescence intensity of nonspecific binding from control probe 1,the AAP was confirmed to have high affinity for CCRF-CEM cells with calculated equilibrium dissociation constants (K d )in the nanomolar-to-picomolar range (K d ¼0.26Æ0.02nM).Moreover,according to the results of the binding assays,the maximum number of the AAP bound to,on average,each CCRF-CEM cell was calculated to be about 2.57×105.Detection of Cancer cells with AAP.Before in vivo implementation,the AAP was tested for cancer cell detection and compared with an always-on aptamer probe that was constructed using aptamer sgc8with a Cy5label.As shown in Fig.4A ,after incubation with CCRF-CEM cells,the AAP achieved much higher relative fluorescence intensity,and its signal-to-background ratio was enhanced to approximately 2.5times of that obtained by the always-on probe.It was revealed that the detection sensitivity was substantially improved by the AAP-based strategy over thatof the always-on probe,resulting from the dramatically reduced background endowed by the unbound AAP in solution.This was also demonstrated through flow cytometry assays of the spiked mouse serum samples (Fig.S3).In order to further investigate sensitivity of the AAP for detec-tion of CCRF-CEM cells,samples with varying CCEF-CEM cell numbers ranging from 118to 392,000in 200μl binding buffer were obtained by serial dilution.T o quantify target cell number,statistical analyses were performed according to the AAP-labeled events appearing in the upright (UR)region.As cell number decreased,the number of events located in the UR region decreased accordingly (Fig.4B ).Counts less than background count plus three times standard deviation were considered to be negative.Background count was determined using the same procedure without the addition of CCRF-CEM cells.For each sample,the number of CCRF-CEM cells detected by the AAP ,Y ,was plotted versus that measured using hemocytometer,X ,as shown in Fig.4C .The regression equation was log Y ¼0.9757×log X −0.3321with the smallest cell number of 118detected in our real experiments.This low concentration was comparable to those obtained using the existing cancer detection methods that exploited aptamer-conjugated nanomaterials-based signal amplification technologies (27,28).In addition,the specificity of the AAP was also determined through detection of target cancer cells in mixed cell samples with different concentration ratios of CCRF-CEM to Ramos cells (Fig.S4).With the concentration ratio of CCRF-CEM to Ramos cells reduced from 9∶1to 1∶9,the percentage of positivesignalsFig.2.Sequence optimization results of the AAP using flow cytometry.(A )Flow cytometry assays of target CCRF-CEM cells or nontarget Ramos cells incubated with FAM-sgc8,probe a,probe b,probe c,and probe d,respectively.(B )The corresponding histogram of the fluorescence ratios of CCRF-CEM cells to Ramos cells for theprobes.Fig.3.Flow cytometry assays of CCRF-CEM cells,Ramos cells,U266cells,and B95-8cells after incubation with the AAP and control probe 1,respectively.The flow cytometry assays were performed by counting 10,000events,and the used concentration of the probes was 25nM.3902∣/cgi/doi/10.1073/pnas.1016197108Shi etal.obtained using the AAP decreased accordingly from 70.36%to 14.91%.Note that,though the always-on probe could achieve an equivalent signal-to-background ratio to the AAP through the traditional sample preparation method with time-consuming washing process (Fig.S5),our assay allows a simple implementa-tion through 15-min incubation of the AAP with the cancer cell samples before detection.This reveals that the AAP holds considerable potential as a simple,rapid,sensitive,and specific cancer cell detection strategy with no required washing step.In Vivo Contrast-Enhanced Cancer Imaging with AAP.We next imple-mented the developed AAP for in vivo imaging of CCRF-CEM tumors implanted in nude mice.Fig.5displays time-dependent in vivo fluorescence images of the CCRF-CEM tumor-bearingmice after intravenous injection of 0.35nmol labeled probes with 4.5nmol unlabeled random oligonucleotide.The latter was injected to slow down the degradation of aptamer probes in blood.As expected,the AAP rapidly circulated within 5min throughout the animal,and weak fluorescence signals could be seen in most parts of the body including the tumor site (Fig.5A ).At 15min,a fluorescence image was evident in the whole body,a phenomenon that might have originated from the distribution of the AAP in subcutaneous tissues and possible degradation of the AAP in blood.More importantly,prominent fluorescence signals appeared in the tumor site at this time.This demonstrated that the AAP could be activated by target cancer cells with a faster restoration of fluorescence achieved in the tumor site than other areas.Thereafter,fluorescence signals fadedcontinuouslyFig. 4.Detection of CCRF-CEM cancer cells with the AAP.(A )Flow cytometry assay of CCRF-CEM cells detected by the AAP ,in comparison with the results achieved by the “always-on ”aptamer probe.The detector voltage used for control probe 1and the AAP was 540.The detector voltage used for control probe 2and the always-on probe was 420.(B )Flow cytometry assays of CCRF-CEM cells with decreasing cell amounts in 200μl binding buffer using the AAP-based cancer cell detection strategy.(C )Calibra-tion curve illustrating the relationship between the amount of CCRF-CEM cells counted by hemocyt-ometer and the number of CCRF-CEM cells detected by flow cytometer with the AAP.Data represent mean Æstandard deviation of three cell samples pergroup.Fig.5.In vivo time-dependent fluorescence imaging of CCRF-CEM RF-CEM tumor-bearing nude mice were intravenously injected with (A )the AAP ,(B )control probe 1,and (C )the “always-on ”aptamer probe,respectively.Fluorescence images of the dorsal side of live mice were then taken at several specified postinjection time points.Time of exposure for every fluorescence image was 1,000ms in groups A and B ,100ms in group C .The pink circle in every image locates the tumor site.Shi et al.PNAS ∣March 8,2011∣vol.108∣no.10∣3903C H E M I S T R YA P P L I E DB I O L O G IC A L S C I E N C E Sover most regions of the body,but a high signal contrast remained in the tumor.At 60min,the AAP was almost cleared from blood and nontarget tissues,and the tumor was still the brightest structure visualized.Interestingly,we observed that the clearance rate of the AAP in the tumor site was much slower than that seen in other areas.This seemed to be attributed to the fact that the high affinity of the AAP to target cancer cells might protect the probes from degradation and metabolism.Even at 180min after injection,a visible fluorescence signal was still obtained in the tumor site,again demonstrating slow clearance of the AAP in target areas.T o verify that the aforementioned observations supported the feasibility of the AAP for in vivo cancer imaging,a control experiment was performed by injecting control probe 1,the fluor-escence-quenched near-infrared probe,into CCRF-CEM tumor-bearing mice.As shown in Fig.5B ,control probe 1was visualized to be distributed throughout the animal via circulation followed by a continuous clearance over most regions of the body.At 60min,control probe 1was almost cleared in most regions of the body,and fluorescence signals nearly vanished at 120min.During the whole imaging process,no prominent fluorescence signal was obtained in the tumor site,indicating that control probe 1was not accumulated or activated at the tumor site.This also strongly suggested that the fluorescence contrast achieved using the AAP did arise from specific residence or activation of the probe by the CCRF-CEM tumor.Further comparison of the AAP-based cancer imaging strategy was then performed with the always-on aptamer probe.Unlike the AAP ,we observed that these always-on probes suffered high fluorescence background in the whole body from the earliest time point,even though the exposure time used for the always-on probe was as short as one tenth that of the AAP (Fig.5C ).After 15min,fluorescence signals faded gradually over the whole body,including the tumor site.At 60min,the tumor site was roughly distinguishable in fluorescence signals from nontarget sites with a very limited signal-to-background ratio,revealing that the high affinity of aptamer probes to target cancer cells might actually slow down their clearance in the target region.Thereafter,the probes were cleared continuously until fluorescence signals entirely disap-peared in the whole body.While these findings demonstrated that the always-on aptamer probe was capable of targeting CCRF-CEM tumors in vivo,a desirable imaging contrast could not be achieved because of high fluorescence background.With distinctly different behavior,the AAP could substantially mini-mize the background signal originating from nontarget tissues,as often noted in this study,and it displays highly contrast-enhanced imaging from only 15min through more than 2h after injection.In Vivo Specific Cancer Imaging with AAP.T o further validate theimaging specificity,the ability of the AAP to discriminate be-tween the CCRF-CEM tumor-bearing mice and mice with no tumor or Ramos tumor implanted was tested in vivo.According to the previous observations,a postinjection time of 60min was selected as the optimal imaging time.As displayed in Fig.6,a clear imaging of the CCRF-CEM tumor with substantially enhanced fluorescence signals was observed in the tumor-implanted site,the right forelimb of the tested mouse (plot C ).In contrast,there was almost no fluorescence detected either in the right forelimb region of the mouse with no tumor implanted or in the Ramos tumor site (plots A and B ).Although the AAP could be activated and accumulated by the target tumor,its nonspecific activation and residence in the normal tissue and nontarget tumor were minimized.These results positively support AAP as a promising specific molecular probe for in vivo cancer imaging.ConclusionWe have developed a unique strategy of AAP for in vivo cancer imaging.The AAP underwent a conformational switch upon binding to proteins on target cancer cell surface with an activated fluorescence.Flow cytometry assays revealed that the AAP was specifically activated by target cancer cells and showed the improved sensitivity for detection of CCRF-CEM cells,both in buffer and serum.In vivo imaging applications demonstrated that,when compared to always-on aptamer probes,the AAP displayed substantially enhanced contrast,which will facil-itate a more sensitive detection of cancer at its early stage.Furthermore,the AAP was able to give clear fluorescence ima-ging specific to the CCRF-CEM tumor site within 15min after injection.This stands in contrast to always-on probes,which required a long postinjection time to obtain contrast from differ-entiated rates in clearing unbound and bound probes.Consider-ing the expansion of aptamer discovery for varying cancer targets (29),the developed AAP strategy might hold great potential as a versatile molecular probe for in vivo cancer imaging with high sensitivity and specificity.Materials and MethodsChemicals and Materials.All the DNA probes reported in this article were custom-designed and then synthesized by Takara Bio Inc.Sequences of the oligos are listed in Table 1.Dulbecco ’s phosphate buffered saline was purchased from Sigma.Mouse serum was obtained from WACAY .All other reagents were of the highest grade available.Deionized water was obtained through the Nanopure Infinity ™ultrapure water system (Barnstead/Thermo-lyne Corp.).Binding buffer was prepared by adding 1mg ∕mL BSA and 10%fetal bovine serum into the Dulbecco ’s PBS containing 4.5g ∕L glucose and 5mM MgCl 2.RF-CEM cells (T cell line,human acute lymphoblastic leukemia)were obtained from Cell Bank of the Committee on Type Culture Collection of the Chinese Academy of Sciences.Ramos cells (B cell line,human Burkitt ’s lymphoma)and U266cells (B lymphocyte,human myeloma,plasmacytoma)were purchased from the Cancer Institute &Hospital (Chinese Academy of Medical Sciences).B95-8cells (EBV-producing marmoset B-cell line)were pro-vided by the Cell Center of our lab.Cells were cultured in RPMI 1640medium supplemented with 15%fetal bovine serum (FBS)and 100IU ∕mL penicillin-streptomycin and incubated at 37°C in a humidified incubator containing 5%wt ∕vol CO 2.The cell density was determined using a hemocytometer,and this was performed prior to any experiments.Animals.Male athymic BALB/c (Balb/C-nu)mice were obtained from the Shanghai SLAC Laboratory Animal Co.,Ltd.(BALB/c).They were 4–6weeks old at the start of each experiment and weighed 20–25g.All animalopera-Fig.6.In vivo specific fluorescence imaging of the CCRF-CEM tumor with the AAP.The AAP was intravenously injected into (A )the mouse with no tumor implanted,(B )the Ramos tumor-bearing mouse,and (C )the CCRF-CEM tumor-bearing mouse,respectively.Fluorescence images of the dorsal side of live mice were taken at 60-min postinjection time.Time of exposure for every fluorescence image was 1,000ms,and the pink circles locate the tumor site.3904∣/cgi/doi/10.1073/pnas.1016197108Shi etal.tions were in accord with institutional animal use and care regulations,ac-cording to protocol No.SYXK (Xiang)2008-0001,approved by the Laboratory Animal Center of Hunan.Flow Cytometry Assays.Generally,probes were incubated with 2×105cells in 200μl binding buffer at normal temperature for 15min in the dark and then immediately determined with a FACScan cytometer (BD Biosciences)by counting 10,000events.Especially for the detection sensitivity assay,differ-ent amounts of CCRF-CEM cells were stained by 25nM activatable aptamer probe (AAP)in 200μl binding buffer at normal temperature in the dark.After incubation for 15min,the samples were immediately detected with flow cytometer at high rate by counting the AAP-labeled events appearing in the upright (UR)region for 2min.The number of samples used to derive statistical information for each cell concentration was 3.The binding affinity of the AAP was determined by incubating CCRF-CEM cells (approximately 1.55×105)on ice for 50min in the dark with varying concentrations of the AAP in a 250-μl volume of binding buffer containing 20%FBS and 0.1mg ∕ml yeast tRNA.Cells were then washed twice with 0.3ml of the binding buffer with 0.1%sodium azide,suspended in 0.2ml of binding buffer with 0.1%sodium azide,and subjected to flow-cytometric analysis.Control probe 1was used as a negative control to determine non-specific binding.All of the experiments for binding assay were repeated three times.The mean fluorescence intensity of CCRF-CEM cells labeled by the AAP was used to calculate for specific binding by subtracting the mean fluorescence intensity of nonspecific binding from control probe 1.The equi-librium dissociation constant (K d )of the AAP –cell interaction was obtained by fitting the dependence of fluorescence intensity of specific binding onthe concentration of the AAP to the equation Y ¼B max X ∕ðK d þX Þ,using SigmaPlot.In Vivo Fluorescence Imaging.Four-week-old male BALB/c nude mice received a subcutaneous injection of 5×106in vitro-propagated cancer cells into the backside.Tumors were then allowed to grow for 3–4weeks to 1–2cm in diameter.Before imaging,BALB/c nude mice,with or without tumors,were anesthetized with the combined use of tranquilizer and anesthetic.In detail,a 2mg ∕kg dose of chlorpromazine hydrochloride was first injected intramuscularly,and several minutes later,an intraceliac injection was performed with an 80mg ∕kg dose of pentobarbital sodium solution.Once the mice were anesthetized to be motionless,a 140μl volume of physiolo-gical saline containing 0.35nmol of labeled probes and 4.5nmol of unlabeled random oligonucleotide was injected intravenously via the tail vein.At specified times,fluorescence images of the dorsal side of live mice were taken by a Maestro ™in vivo fluorescence imaging system (Cambridge Research &Instrumentation,Inc.).A 640nm (Æ25nm)bandpass filter and a 680nm longpass filter were selected to be used as the excitation filter and the emission filter,respectively.All the fluorescence images were presented after processing by the Image J software (version 1.38x).ACKNOWLEDGMENTS.This work was supported by Program for Innovative Research Team of Hunan National Science Foundation (10JJ7002),International Science &Technology Cooperation Program of China (2010DFB30300),Program for Changjiang Scholar and Innovative Research Team in University Program for New Century Excellent Talents in University (NCET-06-0697,NCET-09-0338),and National Science Foundation of P .R.China (90606003,20775021).1.Becker A,et al.(2001)Receptor-targeted optical imaging of tumors with near-infrared fluorescent ligands.Nat Biotechnol 19:327–331.2.Thekkek N,Richards-Kortum R (2008)Optical imaging for cervical cancer detection:Solutions for a continuing global problem.Nat Rev Cancer 8:725–731.3.Ellington A-D,Szostak J-W (1990)In vitro selection of RNA molecules that bind specific ligands.Nature 346:818–822.4.Tuerk C,Gold L (1990)Systematic evolution of ligands by exponential enrichment:RNA ligands to bacteriophage T4DNA polymerase.Science 249:505–510.5.Daniels D-A,Chen H,Hicke B-J,Swiderek K-M,Gold L (2003)A tenascin-C aptamer identified by tumor cell SELEX:systematic evolution of ligands by exponential enrich-ment.Proc Natl Acad Sci USA 100:15416–15421.6.Tavitian B,et al.(1998)In vivo imaging of oligonucleotides with positron emission tomography.Nat Med 4:467–471.7.Schmidt K-S,et al.(2004)Application of locked nucleic acids to improve aptamer in vivo stability and targeting function.Nucleic Acids Res 32:5757–5765.8.Urano Y ,et al.(2009)Selective molecular imaging of viable cancer cells with pH-activatable fluorescence probes.Nat Med 15:104–109.9.Hicke B-J,et al.(2006)Tumor targeting by an aptamer.J Nucl Med 47:668–678.10.Wang A-Z,et al.(2008)Superparamagnetic iron oxide nanoparticle-aptamer biocon-jugates for combined prostate cancer imaging and therapy.ChemMedChem 3:1311–1315.11.Hwang D-W,et al.(2010)A nucleolin-targeted multimodal nanoparticle imagingprobe for tracking cancer cells using an aptamer.J Nucl Med 51:98–105.12.Wu Y ,Sefaha K,Liu H,Wang R,Tan W (2010)DNA aptamer-micelle as an efficientdetection/delivery vehicle toward cancer cells.Proc Natl Acad Sci USA 107:5–10.13.Shi H,et al.(2010)In vivo fluorescence imaging of tumors using molecular aptamersgenerated by cell-SELEX.Chem-Asian J 5:2209–2213.14.Hama Y ,Urano Y ,Koyama Y ,Choyke P-L,Kobayashi H (2007)Activatable fluorescentmolecular imaging of peritoneal metastases following pretargeting with a biotiny-lated monoclonal antibody.Cancer Res 67:3809–3817.15.Weissleder R,Tung C-H,Mahmood U,Bogdanov A,Jr (1999)In vivo imaging of tumorswith protease activated near-infrared fluorescent probes.Nat Biotechnol 17:375–378.16.Jiang T,et al.(2004)Tumor imaging by means of proteolytic activation ofcell-penetrating peptides.Proc Natl Acad Sci USA 101:17867–17872.17.Blum G,von Degenfeld G,Merchant M-J,Blau H-M,Bogyo M (2007)Noninvasiveoptical imaging of cysteine protease activity using fluorescently quenched activity-based probes.Nat Chem Biol 3:668–677.18.Olson E-S,et al.(2010)Activatable cell penetrating peptides linked to nanoparticlesas dual probes for in vivo fluorescence and MR imaging of proteases.Proc Natl Acad Sci USA 107:4311–4316.19.Hama Y ,et al.(2007)A target cell-specific activatable fluorescence probe for in vivomolecular imaging of cancer based on a self-quenched avidin-rhodamine conjugate.Cancer Res 67:2791–2799.20.Ogawa M,et al.(2009)Fluorophore-quencher based activatable targeted opticalprobes for detecting in vivo cancer metastases.Mol Pharm 6:386–395.21.Ogawa M,Kosaka N,Choyke P-L,Kobayashi H (2009)H-type dimer formation offluorophores:A mechanism for activatable,in vivo optical molecular imaging.ACS Chem Biol 4:535–546.22.Wang K,et al.(2009)Molecular engineering of DNA:Molecular beacons.Angew ChemInt Edit 48:856–870.23.Tang Z,et al.(2008)Aptamer switch probe based on intramolecular displacement.J Am Chem Soc 130:11268–11269.24.Shangguan D,et al.(2006)Aptamers evolved from live cells as effective molecularprobes for cancer study.Proc Natl Acad Sci USA 103:11838–11843.25.Shangguan D,et al.(2008)Cell-specific aptamer probes for membrane proteinelucidation in cancer cells.J Proteome Res 7:2133–2139.26.Frangioni J-V (2003)In vivo near-infrared fluorescence imaging.Curr Opin Chem Biol7:626–634.27.Smith J,et al.(2007)Aptamer-conjugated nanoparticles for the collection anddetection of multiple cancer cells.Anal Chem 79:3075–3082.28.Medley C-D,et 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菜鸟PK高手
王淼
【期刊名称】《黑龙江教育（中学教学案例与研究）》
【年(卷),期】2009(000)001
【摘要】5年前，我大学毕业后，被分配到家乡县城的一所中学任教。

初到学校，校长便任命我为初三（5）班的班主任。

原来，这是一个全校有名的问题班。

全班45名学生，个个毛病不少，迟到的、早退的、抽烟的、喝酒的，应有尽有。

上课
不注意听讲，发出怪声怪调；下课打架斗殴，班上就只有5名女生，还都谈起了“朋友”。

别班的学生见了他们，就跟见了黄鼠狼似的，唯恐一不小心惹上一身臊。

背地里其他班学生都叫这个班“垃圾”班，科任老师不愿上他们的课，初中二年班主任就换了9个。

【总页数】2页(P78-79)
【作者】王淼
【作者单位】(Missing)
【正文语种】中文
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因版权原因，仅展示原文概要，查看原文内容请购买。

2020年江西省宜春市店下中学高二语文期末试题含解析一、现代文阅读（35分，共3题）1. 阅读下面的作品，完成各题。

材料一人工智能“阿法狗”出自谷歌旗下深度思维（DeepMind）公司。

最早在2016年1月，《自然》杂志以封面位置报道该程序在和所有其他围棋程序的对抗中获得了99．8％的胜率，并且以5比0的成绩打败了欧洲围棋冠军。

随后同年3月，“阿法狗”挑战世界围棋冠军李世石，经五局鏖战，人类1：4不敌人工智能，引发巨大轰动。

但“阿法狗”仍在积攒实力。

2017年1月，一个神秘账号Master公布自己正是“阿法狗”新版。

该账号在知名围棋平台上先后挑战柯洁、朴廷桓和井山裕太等顶级高手，豪取胜利，随后战赢“棋圣”聂卫平，直至60盘时自曝身份，决胜收官。

英国《自然》杂志19日最新论文中，英国伦敦深度思维公司“阿法狗”项目的主要负责人报告了新版“阿法狗”软件——“阿法元”。

“阿法元”最引人注目之处在于其无师自通的本领，在每次自我对弈中进步。

通过3天的训练——包括近500万局自我对弈——阿法元以其100比0的不败战绩，狠狠打击了曾书写历史的前任“阿法狗”。

材料二AI和计算机之父阿兰?图灵来自英国，英国可谓是AI的发源地，多年来对AI的研发投入已经形成一套独有机制。

英国工程和物理科学委员会（EPSRC）已经对143个科学项目进行研发拨款，目的是研发出智能技术和系统，其跨学科研究体系有助于AI工具和技术满足伦理和应用需求。

估计，在2035年之前，AI将为英国经济带来8140亿美元的产值，其年产值在国民生产总值中的比例将由现在的2.5％增加到3.9％。

2015年，EPSRC联合剑桥、爱丁堡、牛津、华威与伦敦大学学院这5所英国名校，出资4200万美元成立了阿兰?图灵研究所，研究AI在国防安全、健康、计算技术、数据中心工程，以及金融和智能城市等领域的应用，并帮助培训新一代数据科学家。

此外，英国还建立了阿兰数据研究集团、开放数据研究所、国家数据创新中心和皇家统计协会数据科学分部等，在AI的一些关键研究领域取得突破性进展。

励志人生三部曲咬定教研不放松——记南昌市东湖区教研中
心主任汪智星
吴亮
【期刊名称】《成功密码：综合版》
【年(卷),期】2017(000)006
【摘要】汪智星,特级教师,江西省首批正高级教师,现任南昌市东湖区教研中心主任,全国小语会青年教师教学研宄中心委员.荣获全国百姓学习之星、教育部“十二五”规划课题研宄标兵、“全国小学作文教学名师”提名奖;江西省先进工作者、
江西省首届“中小学名师培养计划”培养对象、江西省“基础教育研宄智库”专家、江西省首批语文学科带头人、江西省师德先进个人、江西省“五一”劳动奖章、南昌市“五一”劳动奖章、南昌市人民政府特殊津贴获得者等称号,同时是南昌市“十佳”教师,“全省著名特级教师”重点培养对象.
【总页数】8页(P4-11)
【作者】吴亮
【作者单位】
【正文语种】中文
【中图分类】G40
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