Showing papers by "University of Twente published in 2004"
TL;DR: In this article, the authors provide a theoretical basis for understanding the current phase relation (CPhiR) for the stationary Josephson effect in various types of superconducting junctions.
Abstract: This review provides a theoretical basis for understanding the current-phase relation (CPhiR) for the stationary (dc) Josephson effect in various types of superconducting junctions The authors summarize recent theoretical developments with an emphasis on the fundamental physical mechanisms of the deviations of the CPhiR from the standard sinusoidal form A new experimental tool for measuring the CPhiR is described and its practical applications are discussed The method allows one to measure the electrical currents in Josephson junctions with a small coupling energy as compared to the thermal energy A number of examples illustrate the importance of the CPhiR measurements for both fundamental physics and applications
1,084 citations
TL;DR: In this paper, the spectral distribution and time-dependent decay of light emitted from excitons confined in the quantum dots are controlled by the host photonic crystal, and both inhibited and enhanced decay rates are observed depending on the optical emission frequency.
Abstract: Control of spontaneously emitted light lies at the heart of quantum optics. It is essential for diverse applications ranging from miniature lasers and light-emitting diodes, to single-photon sources for quantum information, and to solar energy harvesting. To explore such new quantum optics applications, a suitably tailored dielectric environment is required in which the vacuum fluctuations that control spontaneous emission can be manipulated. Photonic crystals provide such an environment: they strongly modify the vacuum fluctuations, causing the decay of emitted light to be accelerated or slowed down, to reveal unusual statistics, or to be completely inhibited in the ideal case of a photonic bandgap. Here we study spontaneous emission from semiconductor quantum dots embedded in inverse opal photonic crystals. We show that the spectral distribution and time-dependent decay of light emitted from excitons confined in the quantum dots are controlled by the host photonic crystal. Modified emission is observed over large frequency bandwidths of 10%, orders of magnitude larger than reported for resonant optical microcavities. Both inhibited and enhanced decay rates are observed depending on the optical emission frequency, and they are controlled by the crystals’ lattice parameter. Our experimental results provide a basis for all-solid-state dynamic control
of optical quantum systems.
1,046 citations
TL;DR: In this article, a feed-forward noise-canceling technique is proposed to cancel the noise and distortion contributions of the matching device, which allows for designing wide-band impedance-matching amplifiers with noise figure (NF) well below 3 dB.
Abstract: Known elementary wide-band amplifiers suffer from a fundamental tradeoff between noise figure (NF) and source impedance matching, which limits the NF to values typically above 3 dB. Global negative feedback can be used to break this tradeoff, however, at the price of potential instability. In contrast, this paper presents a feedforward noise-canceling technique, which allows for simultaneous noise and impedance matching, while canceling the noise and distortion contributions of the matching device. This allows for designing wide-band impedance-matching amplifiers with NF well below 3 dB, without suffering from instability issues. An amplifier realized in 0.25-/spl mu/m standard CMOS shows NF values below 2.4 dB over more than one decade of bandwidth (i.e., 150-2000 MHz) and below 2 dB over more than two octaves (i.e., 250-1100 MHz). Furthermore, the total voltage gain is 13.7 dB, the -3-dB bandwidth is from 2 MHz to 1.6 GHz, the IIP2 is +12 dBm, and the IIP3 is 0 dBm. The LNA drains 14 mA from a 2.5-V supply and the die area is 0.3/spl times/0.25 mm/sup 2/.
749 citations
TL;DR: A review of the various types of catalysts that have been used in several research works to reduce the tars in the producer gas derived from the biomass gasification process is presented in this paper.
Abstract: A project is carried out to develop a process for tar elimination downstream of a gasifier making use of cheap and active materials as catalysts. In the first stage of the project, screening of catalysts was carried out in a fixed-bed tubular reactor. The results of the fixed-bed experiments will be used in the design of the process. This paper presents a review of the various types of catalysts that have been used in several research works to reduce the tars in the producer gas derived from the biomass gasification process. The catalysts are divided into two classes according to their production method: minerals and synthetic catalysts. A summary of the review and recommendations for good catalyst candidates and future work are also provided.
731 citations
TL;DR: This book analyses the factors affecting the online consumer's behavior and examines how e-marketers can influence the outcome of the virtual interaction and buying process by focusing their marketing efforts on elements shaping the customer's virtual experience, the Web experience.
Abstract: Addresses one of the fundamental issues of e-marketing: how to attract and win over the consumer in the highly competitive Internet marketplace Analyses the factors affecting the online consumer's behavior and examines how e-marketers can influence the outcome of the virtual interaction and buying process by focusing their marketing efforts on elements shaping the customer's virtual experience, the Web experience Identifying the Web experience components and understanding their role as inputs in the online customer's decision-making process are the first step in developing and delivering an attractive online presence likely to have the maximum impact on Internet users Click-and-mortar firms delivering superior Web experience influence their physical clients’ perceptions and attitudes, driving additional traffic to traditional sales outlets Provides a contribution to the theoretical debate around the factors influencing the online consumer's behavior and outlines some noticeable similarities and differences between the traditional and virtual consumers
600 citations
TL;DR: 3D-deposited scaffolds seeded with bovine articular chondrocytes supported a homogeneous cell distribution and subsequent cartilage-like tissue formation following in vitro culture as well as subcutaneous implantation in nude mice.
582 citations
TL;DR: The findings suggest that developing PMSs should to a large extent be understood as a co-ordination effort rather than a design effort and the lessons learned cannot have universal validity, but may be helpful in similar kinds of initiatives.
495 citations
01 Jan 2004
TL;DR: An energy-efficient medium access protocol designed for wireless sensor networks that reduces the number of transceiver state switches and hence the energy wasted in preamble transmissions and is compared to SMAC and EMACs by simulation.
Abstract: In this paper, we present an energy-efficient medium access protocol designed for wireless sensor networks. Although the protocol uses TDMA to give nodes in the WSN the opportunity to communicate collision-free, the network is self-organizing in terms of time slot assignment and synchronization. The main goal of the medium access protocol is to minimize overhead of the physical layer. The protocol reduces the number of transceiver state switches and hence the energy wasted in preamble transmissions. The protocol is compared to SMAC and EMACs by simulation. The LMAC protocol is able to extend the network lifetime by a factor 2.4 and 3.8, compared to EMACs and SMAC respectively.
473 citations
01 Jan 2004
TL;DR: The primary aim of this introductory chapter is to outline some basic notions on curriculum and curriculum development as discussed by the authors, and it is not intended to provide a comprehensive overview and analysis of the scholarly field of curriculum theory.
Abstract: The primary aim of this introductory chapter is to outline some basic notions on curriculum and curriculum development. It is certainly not my aspiration to offer a comprehensive overview and analysis of the scholarly field of curriculum theory. For that purpose a number of thorough, extensive books are available, for example The Handbook of Research on Curriculum (Jackson, 1992a) or, more recently, Curriculum Books, The First Hundred Years (Schubert, Lopez Schubert, Thomas & Carroll, 2002). Nor is it my intention to provide a synoptic guide for the study of curriculum. For that purpose many valuable books have been published, for example Fundamentals of Curriculum (Walker, 1990, 2003) and Curriculum: Alternative Approaches and Ongoing Concerns (Marsh & Willis, 2003), to name two of my favorites.
450 citations
TL;DR: This first view of any multi-component membrane shows the relative positions and associations of the photosynthetic complexes and reveals crucial new features of the organization of the network: it was found that the membrane is divided into specialized domains each with a different network organization and in which one type of complex predominates.
Abstract: In photosynthesis, the harvesting of solar energy and its subsequent conversion into a stable charge separation are dependent upon an interconnected macromolecular network of membraneassociated chlorophyll–protein complexes. Although the detailed structure of each complex has been determined1–4, the size and organization of this network are unknown. Here we show the use of atomic force microscopy to directly reveal a native bacterial photosynthetic membrane. This first view of any multi-component membrane shows the relative positions and associations of the photosynthetic complexes and reveals crucial new features of the organization of the network: we found that the membrane is divided into specialized domains each with a different network organization and in which one type of complex predominates. Two types of organization were found
for the peripheral light-harvesting LH2 complex. In the first, groups of 10–20 molecules of LH2 form light-capture domains that interconnect linear arrays of dimers of core reaction centre (RC)–light-harvesting 1 (RC–LH1–PufX) complexes; in the second they were found outside these arrays in larger clusters. The LH1 complex is ideally positioned to function as an energy collection hub, temporarily storing it before transfer to the RC where photochemistry occurs: the elegant economy of the photosynthetic membrane is demonstrated by the close packing of these linear arrays, which are often only separated by narrow ‘energy conduits’ of LH2 just two or three complexes wide.
441 citations
TL;DR: Results suggest that certain antibiotics are able to bind/chelate with calcium, resulting in a better incorporation into the CHA coating and a slower release, which might be used to prevent post-surgical infections and to promote bone-bonding of orthopedic devices.
TL;DR: In this article, the authors make an attempt to contribute from an acertain perspective on governance studies to the ongoing debate onthe challenges ''internationalisation'' or ''globalisation'' bringup for higher education policy analyses and especially for comparative research in that area.
Abstract: The new phenomenon of European integration has again challenged ourconceptual and empirical tools for higher education studies tointegrate the international dimension into frameworks that tendto concentrate on the single nation state and domestic policieseven where international comparisons are made. It drives as wellthe awareness of certain blind spots: namely (1) the concentrationon policy effects, neglecting the input side of policy formation,and (2) the concern with macro level policy-making and meso levelorganisational adaptation, neglecting to some extend the microdynamics and effects in the actual practices and performances ofacademic work. This paper makes an attempt to contribute from acertain perspective on governance studies to the ongoing debate onthe challenges ``internationalisation'' or ``globalisation'' bringup for higher education policy analyses and especially for comparativeresearch in that area. The development of governance theory towardsa multi-level and multi-actor approach is discussed and its strengthsand weaknesses for higher education studies in an internationalisingenvironment are addressed.
TL;DR: An overview of the state of the art of laser beam machining in general with special emphasis on applications of short and ultrashort lasers is given in this article, where the driving force is higher accuracy at reasonable cost, which is realised by compact systems delivering short laser pulses of high beam quality.
TL;DR: In this article, the authors investigated the generality vs. domain-specificity of maturing metacognitive skills, and found that metAC skills appeared to develop and contribute to learning performance, partly independent of intelligence.
TL;DR: This tutorial review deals with the most representative contributions in the field of supramolecular chirality, and centers on several examples that represent the control over chorality for self-assembled systems in solution.
Abstract: Self-assembly plays an important role in the formation of many (chiral) biological structures, such as DNA, α-helices or β-sheets of proteins. This process, which is the main tool of Supramolecular Chemistry (i.e. the chemistry of the molecular assemblies and of the intermolecular bonds), starts to play a significant role in nanotechnology for the construction of functional synthetic structures of nanometer size. The control of chirality in synthetic self-assembled systems is very important for applications of these systems e.g. in molecular recognition or mimicking of the catalytic activity of enzymes. This tutorial review deals with the most representative contributions in the field of supramolecular chirality. Specifically, the discussion centers on several examples that represent the control over chirality for self-assembled systems in solution.
TL;DR: The tool described here is the first part of a tool set called GROOVE (GRaph-based Object-Oriented VErification) for software model checking of object-oriented systems using graphs to represent state snapshots; transitions arise from the application of graph production rules.
Abstract: The tool described here is the first part of a tool set called GROOVE (GRaph-based Object-Oriented VErification) for software model checking of object-oriented systems. The special feature of GROOVE, which sets it apart from other model checking approaches, is that it is based on graph transformations. It uses graphs to represent state snapshots; transitions arise from the application of graph production rules. This yields so-called Graph Transition Systems (GTSrsquos) as computational models.
TL;DR: In this paper, the authors present a maintenance framework that shows management cycles of maintenance activities during the product life cycle, identifying technical issues of maintenance and discuss the advances of technologies supporting the change in the role of maintenance.
01 Jan 2004
TL;DR: Internal model control is also a linear multivariable control technique that makes use of a process model in order to compute the input moves that have to be made.
Abstract: Internal model control is also a linear multivariable control technique that makes use of a process model in order to compute the input moves that have to be made. Although the technique is not as popular as model predictive control, such as DMC, it is nevertheless useful for a short explanation. Also some difference with model predictive control will be pointed out.
TL;DR: It is concluded that until today, microfluidic devices have not been used extensively in tissue engineering, and major contributions are expected in two areas: growth of complex tissue, and the development of in vitro physiological systems for studying fundamental biological phenomena.
Abstract: An introductory overview of the use of microfluidic devices for tissue engineering is presented. After a brief description of the background of tissue engineering, different application areas of microfluidic devices are examined. Among these are methods for patterning cells, topographical control over cells and tissues, and bioreactors. Examples where microfluidic devices have been employed are presented such as basal lamina, vascular tissue, liver, bone, cartilage and neurons. It is concluded that until today, microfluidic devices have not been used extensively in tissue engineering. Major contributions are expected in two areas. The first is growth of complex tissue, where microfluidic structures ensure a steady blood supply, thereby circumventing the well-known problem of providing larger tissue structures with a continuous flow of oxygen and nutrition, and withdrawal of waste products. The second, and probably more important function of microfluidics, combined with micro/nanotechnology, lies in the development of in vitro physiological systems for studying fundamental biological phenomena.
TL;DR: The ability to measure and predict local oxygen tensions offers new opportunities to obtain more insight in the relation between oxygen tension and chondrogenesis.
Abstract: The supply of oxygen within three-dimensional tissue-engineered (TE) cartilage polymer constructs is mainly by diffusion. Oxygen consumption by cells results in gradients in the oxygen concentration. The aims of this study were, firstly, to identify the gradients within TE cartilage polymer constructs and, secondly, to predict the profiles during in vitro culture. A glass microelectrode system was adapted and used to penetrate cartilage and TE cartilaginous constructs, yielding reproducible measurements with high spatial resolution. Cartilage polymer constructs were cultured for up to 41 days in vitro. Oxygen concentrations, as low as 2-5%, were measured within the center of these constructs. At the beginning of in vitro culture, the oxygen gradients were steeper in TE constructs in comparison to native tissue. Nevertheless, during the course of culture, oxygen concentrations approached the values measured in native tissue. A mathematical model was developed which yields oxygen profiles within cartilage explants and TE constructs. Model input parameters were assessed, including the diffusion coefficient of cartilage (2.2 × 10-9) + (0.4 × 10-9 m2 s-1), 70% of the diffusion coefficient of water and the diffusion coefficient of constructs (3.8 × 10-10 m2 s-1). The model confirmed that chondrocytes in polymer constructs cultured for 27 days have low oxygen requirements (0.8 × 10-19 mol m-3 s-1), even lower than chondrocytes in native cartilage. The ability to measure and predict local oxygen tensions offers new opportunities to obtain more insight in the relation between oxygen tension and chondrogenesis.
TL;DR: In this article, the extent of mixing and segregation induced by a single bubble injected in a monodisperse and bidisperse fluidised bed at incipient fluidisation conditions and in freely bubbling fluidised beds was studied both with well-defined experiments and with a 3D Euler?Lagrangian model.
15 Mar 2004
TL;DR: It is shown that for measuring trunk and pelvis inclination during the functional three-dimensional activity of stacking crates, the inclination error that is made is nearly twice as accurate as compared to current methods based on low-pass filtering of accelerometer signals.
Abstract: In the medical field, accelerometers are often used for measuring inclination of body segments and activity of daily living (ADL) because they are small and require little power. A drawback of using accelerometers is the poor quality of inclination estimate for movements with large accelerations. This paper describes the design and performance of a Kalman filter to estimate inclination from the signals of a triaxial accelerometer. This design is based on assumptions concerning the frequency content of the acceleration of the movement that is measured, the knowledge that the magnitude of the gravity is 1 g and taking into account a fluctuating sensor offset. It is shown that for measuring trunk and pelvis inclination during the functional three-dimensional activity of stacking crates, the inclination error that is made is approximately 2/spl deg/ root-mean square. This is nearly twice as accurate as compared to current methods based on low-pass filtering of accelerometer signals.
TL;DR: These longitudinal recordings of network firing have brought to light a reproducible pattern of complex changes in spontaneous firing dynamics of bursts during the development of isolated cortical neurons into synaptically interconnected networks.
Abstract: Extracellular action potentials were recorded from developing dissociated rat neocortical networks continuously for up to 49 days in vitro using planar multielectrode arrays. Spontaneous neuronal activity emerged toward the end of the first week in vitro and from then on exhibited periods of elevated firing rates, lasting for a few days up to weeks, which were largely uncorrelated among different recording sites. On a time scale of seconds to minutes, network activity typically displayed an ongoing repetition of distinctive firing patterns, including short episodes of synchronous firing at many sites ( network bursts). Network bursts were highly variable in their individual spatio-temporal firing patterns but showed a remarkably stable underlying probabilistic structure (obtained by summing consecutive bursts) on a time scale of hours. On still longer time scales, network bursts evolved gradually, with a significant broadening (to about 2 s) in the third week in vitro, followed by a drastic shortening after about one month in vitro. Bursts at this age were characterized by highly synchronized onsets reaching peak firing levels within less than ca. 60 ms. This pattern persisted for the rest of the culture period. Throughout the recording period, active sites showed highly persistent temporal relationships within network bursts. These longitudinal recordings of network firing have, thus, brought to light a reproducible pattern of complex changes in spontaneous firing dynamics of bursts during the development of isolated cortical neurons into synaptically interconnected networks.
TL;DR: The results of this study suggest that bone tissue engineering could benefit from the activation of critical levels of Wnt signaling at defined stages of differentiation and suggest that hMSCs provide a valid in vitro model to study the role of WNT signaling in mesenchymal biology.
Abstract: Mesenchymal stem cells are pluripotent cells from bone marrow, which can be differentiated into the osteogenic, chondrogenic, and adipogenic lineages in vitro and are a source of cells in bone and cartilage tissue engineering. An improvement in current tissue-engineering protocols requires more detailed insight into the molecular cues that regulate the distinct steps of osteochondral differentiation. Because Wnt signaling has been widely implicated in mesenchymal differentiation, we analyzed the role of Wnt signaling in human mesenchymal stem cell (hMSC) biology by stimulation of the pathway with lithium chloride and Wnt3A-conditioned medium. We demonstrate a role for low levels of Wnt signaling in proliferation of uncommitted hMSCs and confirm that Wnt signaling controls osteoprogenitor proliferation. On the other hand, at high Wnt levels we observed a block in adipogenic differentiation and an increase in the expression of alkaline phosphatase, suggesting a role in the initiation of osteogenesis. The results of this study suggest that bone tissue engineering could benefit from the activation of critical levels of Wnt signaling at defined stages of differentiation. Moreover, our data suggest that hMSCs provide a valid in vitro model to study the role of Wnt signaling in mesenchymal biology.
TL;DR: An array of very uniform cylindrical nanopores with a pore diameter as small as 25 nm has been fabricated in an ultrathin micromachined silicon nitride membrane using focused ion beam (FIB) etching as mentioned in this paper.
Abstract: An array of very uniform cylindrical nanopores with a pore diameter as small as 25 nm has been fabricated in an ultrathin micromachined silicon nitride membrane using focused ion beam (FIB) etching The pore size of this nanosieve membrane was further reduced to below 10 nm by coating it with another silicon nitride layer This nanosieve membrane possesses adequate mechanical strength up to several bars of transmembrane pressure, and it can withstand high temperatures up to 900 C In addition, it is inert to many aggressive chemicals such as hot concentrated potassium hydroxide (KOH), piranha (H2SO4 + H2O2), and nitric acid (HNO3)
Journal Article•
TL;DR: In this article, the authors have identified the candidates of block ciphers suitable for WSNs based on existing literature and devised a systematic framework that not only considers the security properties but also the storage and energy-efficency of the candidates.
Abstract: Choosing the most storage- and energy-efficient block cipher specifically for wireless sensor networks (WSNs) is not as straightforward as it seems. To our knowledge so far, there is no systematic evaluation framework for the purpose. In this paper, we have identified the candidates of block ciphers suitable for WSNs based on existing literature. For evaluating and assessing these candidates, we have devised a systematic framework that not only considers the security properties but also the storage- and energy-efficency of the candidates. Finally, based on the evaluation results, we have selected the suitable ciphers for WSNs, namely Rijndael for high security and energy efficiency requirements; and MISTY1 for good storage and energy efficiency.
TL;DR: It is shown that, assuming that agents act under uncertainty in some states of the system, the notion of allowable strategy should be defined with some caution, and a subtle difference between an agent knowing that he has a suitable strategy and knowing the strategy itself is demonstrated.
Abstract: We look at ways to enrich Alternating-time Temporal Logic (ATL) - a logic for specification and verification of multi-agent systems - with a notion of knowledge. Starting point of our study is a recent proposal for a system called Alternating-time Temporal Epistemic Logic (ATEL). We show that, assuming that agents act under uncertainty in some states of the system, the notion of allowable strategy should be defined with some caution. Moreover, we demonstrate a subtle difference between an agent knowing that he has a suitable strategy and knowing the strategy itself. We also point out that the agents should be assumed similar epistemic capabilities in the semantics of both strategic and epistemic operators. Trying to implement these ideas, we propose two different modifications of ATEL. The first one, dubbed Alternating-time Temporal Observational Logic (ATOL), is a logic for agents with bounded recall of the past. With the second, ATEL-R*, we present a framework to reason about both perfect and imperfect recall, in which we also incorporate operators for reasoning about the past. We identify some feasible subsystems of this expressive system.
TL;DR: An online (recursive) algorithm is proposed that estimates the parameters of the mixture and that simultaneously selects the number of components to search for the maximum a posteriori (MAP) solution and to discard the irrelevant components.
Abstract: There are two open problems when finite mixture densities are used to model multivariate data: the selection of the number of components and the initialization. In this paper, we propose an online (recursive) algorithm that estimates the parameters of the mixture and that simultaneously selects the number of components. The new algorithm starts with a large number of randomly initialized components. A prior is used as a bias for maximally structured models. A stochastic approximation recursive learning algorithm is proposed to search for the maximum a posteriori (MAP) solution and to discard the irrelevant components.
TL;DR: In this paper, three different broadband sensitization concepts for optically active erbium ions are reviewed: silicon nanocrystals, with absorption over the full visible spectrum, efficiently couple their excitonic energy to Er3+, silver-related defect states in sodalime silicate glass, and organic cage complexes coordinated with well-chosen chromophores serve as broadband sensitizers in the visible.
Abstract: Three different broadband sensitization concepts for optically active erbium ions are reviewed: 1) silicon nanocrystals, with absorption over the full visible spectrum, efficiently couple their excitonic energy to Er3+, 2) silver-related defect states in sodalime silicate glass have absorption in the blue and transfer energy to Er3+, and 3) organic cage complexes coordinated with well-chosen chromophores serve as broadband sensitizers in the visible. Energy transfer rates, efficiencies, and limiting factors are addressed for each of these sensitizers. Implications of the use of strong sensitizers for planar waveguide design are illustrated by using a model for the sensitizing effect of ytterbium.
TL;DR: In this paper, theories that were validated for relatively big, free (not encapsulated) gas bubbles are compared with high-speed optical observations of insonified contrast agent microbubbles and it is concluded that most of the results obtained are consistent with free gas bubble theory.
Abstract: When encapsulated microbubbles are subjected to high-amplitude ultrasound, the following phenomena have been reported: oscillation, translation, coalescence, fragmentation, sonic cracking and jetting. In this paper, we explain these phenomena, based on theories that were validated for relatively big, free (not encapsulated) gas bubbles. These theories are compared with high-speed optical observations of insonified contrast agent microbubbles. Furthermore, the potential clinical applications of the bubble-ultrasound interaction are explored. We conclude that most of the results obtained are consistent with free gas bubble theory. Similar to cavitation theory, the number of fragments after bubble fission is in agreement with the dominant spherical harmonic oscillation mode. Remarkable are our observations of jetting through contrast agent microbubbles. The pressure at the tip of a jet is high enough to penetrate any human cell. Hence, liquid jets may act as remote-controlled microsyringes, delivering a drug to a region-of-interest. Encapsulated microbubbles have (potential) clinical applications in both diagnostics and therapeutics.