Showing papers by "University of Twente published in 2001"

Noncovalent synthesis using hydrogen bonding

Abstract: Hydrogen bonds are like human beings in the sense that they exhibit typical grouplike behavior. As an individual they are feeble, easy to break, and sometimes hard to detect. However, when acting together they become much stronger and lean on each other. This phenomenon, which in scientific terms is called cooperativity, is based on the fact that 1+1 is more than 2. By using this principle, chemists have developed a wide variety of chemically stable structures that are based on the reversible formation of multiple hydrogen bonds. More than 20 years of fundamental studies on these phenomena have gradually developed into a new discipline within the field of organic synthesis, and is nowadays called noncovalent synthesis. This review describes noncovalent synthesis based on the reversible formation of multiple hydrogen bonds. Starting with a thorough description of what the hydrogen bond really is, it guides the reader through a variety of bimolecular and higher order assemblies and exemplifies the general principles that determine their stability. Special focus is given to reversible capsules based on hydrogen-bonding interactions that exhibit interesting encapsulation phenomena. Furthermore, the role of hydrogen-bond formation in self-replicating processes is actively discussed, and finally the review briefly summarizes the development of novel materials (nanotubes, liquid crystals, polymers, etc.) and principles (dynamic libraries) that recently have emanated from this intriguing field of research.

Coupling of two superconductors through a ferromagnet: evidence for a pi junction.

Abstract: We report measurements of the temperature dependence of the critical current, Ic, in Josephson junctions consisting of conventional superconducting banks of Nb and a weakly ferromagnetic interlayer of a CuxNi1-x alloy, with x around 0.5. With decreasing temperature Ic generally increases, but for specific thicknesses of the ferromagnetic interlayer, a maximum is found followed by a strong decrease down to zero, after which Ic rises again. Such a sharp cusp can be explained only by assuming that the junction changes from a 0-phase state at high temperatures to a I� phase state at low temperatures.

Putting energy back in control

Abstract: Energy is one of the fundamental concepts in science and engineering practice, where it is common to view dynamical systems as energy-transformation devices. This perspective is particularly useful in studying complex nonlinear systems by decomposing them into simpler subsystems that, upon interconnection, add up their energies to determine the full system's behavior. The action of a controller may also be understood in energy terms as another dynamical system. The control problem can then be recast as finding a dynamical system and an interconnection pattern such that the overall energy function takes the desired form. This energy-shaping approach is the essence of passivity-based control (PBC), a controller design technique that is very well known in mechanical systems. Our objectives in the article are threefold. First, to call attention to the fact that PBC does not rely on some particular structural properties of mechanical systems, but hinges on the more fundamental (and universal) property of energy balancing. Second, to identify the physical obstacles that hamper the use of standard PBC in applications other than mechanical systems. In particular, we show that standard PBC is stymied by the presence of unbounded energy dissipation, hence it is applicable only to systems that are stabilizable with passive controllers. Third, to revisit a PBC theory that has been developed to overcome the dissipation obstacle as well as to make the incorporation of process prior knowledge more systematic. These two important features allow us to design energy-based controllers for a wide range of physical systems.

Formation of a Cobalt Magnetic Dot Array via Block Copolymer Lithography

Abstract: Single-domain cobalt dot arrayswith high magnetic particle density, patterned over large areas (e.g., 10 cm diameter wafers) are fabricated by self-assembled block copolymer lithography, using a polystyrene-poly(ferrocenyldimethylsilane) copolymer as a template. By varying the copolymer type and etching conditions the magnetic properties can be tuned. The Figure shows a typical array of Co dots with tungsten caps obtained via this procedure.

Situating Technoscience: An Inquiry into Spatialities

Abstract: This paper explores the spatial characteristics of science and technology. Originally seen as universal, and therefore outside space and place, studies in science, technology, and society (STS) located it first in specific locations -- laboratories -- and then in narrow networks linking laboratories. This double location implied that science is caught up in and enacts two topological forms -- region and network -- since objects in networks hold their shape by freezing relations rather than fixing Euclidean coordinates. More recent STS work suggests that science and technology also exist in and help to enact additional spatial forms. Thus some technoscience objects are fluid, holding their form by shifting their relations. And yet others achieve constancy by enacting simultaneous absence and presence, a topological possibility which we call here fire . The paper concludes by arguing that the 'global' includes and is enacted in all four of these topological systems.

Trends in miniaturized total analysis systems for point-of-care testing in clinical chemistry

Abstract: A currently emerging approach enables more widespread monitoring of health parameters in disease prevention and biomarker monitoring. Miniaturisation provides the means for the production of small, fast and easy-to-operate devices for reduced-cost healthcare testing at the point-of-care (POC) or even for household use. A critical overview is given on the present state and requirements of POC testing, on µTAS elements suited for implementation in future µTAS devices for POC testing and µTAS systems for the determination of clinical parameters.

Thermal convection for large Prandtl numbers

Abstract: The Rayleigh-Benard theory by Grossmann and Lohse [J. Fluid Mech. 407, 27 (2000)] is extended towards very large Prandtl numbers Pr. The Nusselt number Nu is found here to be independent of Pr. However, for fixed Rayleigh numbers Ra a maximum in the Nu(Pr) dependence is predicted. We moreover offer the full functional dependences of Nu(Ra,Pr) and Re(Ra,Pr) within this extended theory, rather than only give the limiting power laws as done in J. Fluid. Mech. 407, 27 (2000). This enables us to more realistically describe the transitions between the various scaling regimes.

Bayesian estimation of a multilevel IRT model using Gibbs sampling

Abstract: In this article, a two-level regression model is imposed on the ability parameters in an item response theory (IRT) model. The advantage of using latent rather than observed scores as dependent variables of a multilevel model is that it offers the possibility of separating the influence of item difficulty and ability level and modeling response variation and measurement error. Another advantage is that, contrary to observed scores, latent scores are test-independent, which offers the possibility of using results from different tests in one analysis where the parameters of the IRT model and the multilevel model can be concurrently estimated. The two-parameter normal ogive model is used for the IRT measurement model. It will be shown that the parameters of the two-parameter normal ogive model and the multilevel model can be estimated in a Bayesian framework using Gibbs sampling. Examples using simulated and real data are given.

Membrane separation processes: Current relevance and future opportunities

Abstract: During the last 35 years membranes have evolved from a laboratory tool to industrial products with significant technical and commercial impact. Today, membranes are used for desalination of sea and brackish water and for treating industrial effluents. They are efficient tools for the concentration and purification of food and pharmaceutical products and the production of base chemicals. Furthermore, membranes are key components in artificial organs, drug delivery devices, and energy conversion systems. In combination with conventional techniques membranes often provide cleaner and more energy-efficient production routes for high-quality products. Fundamental aspects of membranes and membrane processes are discussed, as well as technically and economically effective applications. The present and future membrane market is assessed and new developments and future research needs are discussed.

Composing crosscutting concerns using composition filters

Abstract: It has been demonstrated that certain design concerns, such as access control, synchronization, and object interactions cannot be expressed in current OO languages as a separate software module [4, 7]. These so-called crosscutting concerns generally result in implementations scattered over multiple operations. If a crosscutting concern cannot be treated as a single module, its adaptability and reusability are likely to be reduced. A number of programming techniques have been proposed to express crosscutting concerns, for example, adaptive programming [9], AspectJ [8], Hyperspaces [10], and Composition Filters [1]. Here, we present the Composition Filters (CF) model and illustrate how it addresses evolving crosscutting concerns.

MCMC estimation and some model-fit analysis of multidimensional IRT models

Abstract: A Bayesian procedure to estimate the three-parameter normal ogive model and a generalization of the procedure to a model with multidimensional ability parameters are presented. The procedure is a generalization of a procedure by Albert (1992) for estimating the two-parameter normal ogive model. The procedure supports analyzing data from multiple populations and incomplete designs. It is shown that restrictions can be imposed on the factor matrix for testing specific hypotheses about the ability structure. The technique is illustrated using simulated and real data.

Unfolding individual nucleosomes by stretching single chromatin fibers with optical tweezers.

Abstract: Single chromatin fibers were assembled directly in the flow cell of an optical tweezers setup. A single λ phage DNA molecule, suspended between two polystyrene beads, was exposed to a Xenopus laevis egg extract, leading to chromatin assembly with concomitant apparent shortening of the DNA molecule. Assembly was force-dependent and could not take place at forces exceeding 10 pN. The assembled single chromatin fiber was subjected to stretching by controlled movement of one of the beads with the force generated in the molecule continuously monitored with the second bead trapped in the optical trap. The force displayed discrete, sudden drops upon fiber stretching, reflecting discrete opening events in fiber structure. These opening events were quantized at increments in fiber length of ∼65 nm and are attributed to unwrapping of the DNA from around individual histone octamers. Repeated stretching and relaxing of the fiber in the absence of egg extract showed that the loss of histone octamers was irreversible. The forces measured for individual nucleosome disruptions are in the range of 20–40 pN, comparable to forces reported for RNA- and DNA-polymerases.

An adaptive model of sensory integration in a dynamic environment applied to human stance control.

Abstract: An adaptive estimator model of human spatial orientation is presented. The adaptive model dynamically weights sensory error signals. More specific, the model weights the difference between expected and actual sensory signals as a function of environmental conditions. The model does not require any changes in model parameters. Differences with existing models of spatial orientation are that: (1) environmental conditions are not specified but estimated, (2) the sensor noise characteristics are the only parameters supplied by the model designer, (3) history-dependent effects and mental resources can be modelled, and (4) vestibular thresholds are not included in the model; instead vestibular-related threshold effects are predicted by the model. The model was applied to human stance control and evaluated with results of a visually induced sway experiment. From these experiments it is known that the amplitude of visually induced sway reaches a saturation level as the stimulus level increases. This saturation level is higher when the support base is sway referenced. For subjects experiencing vestibular loss, these saturation effects do not occur. Unknown sensory noise characteristics were found by matching model predictions with these experimental results. Using only five model parameters, far more than five data points were successfully predicted. Model predictions showed that both the saturation levels are vestibular related since removal of the vestibular organs in the model removed the saturation effects, as was also shown in the experiments. It seems that the nature of these vestibular-related threshold effects is not physical, since in the model no threshold is included. The model results suggest that vestibular-related thresholds are the result of the processing of noisy sensory and motor output signals. Model analysis suggests that, especially for slow and small movements, the environment postural orientation can not be estimated optimally, which causes sensory illusions. The model also confirms the experimental finding that postural orientation is history dependent and can be shaped by instruction or mental knowledge. In addition the model predicts that: (1) vestibular-loss patients cannot handle sensory conflicting situations and will fall down, (2) during sinusoidal support-base translations vestibular function is needed to prevent falling, (3) loss of somatosensory information from the feet results in larger postural sway for sinusoidal support-base translations, and (4) loss of vestibular function results in falling for large support-base rotations with the eyes closed. These predictions are in agreement with experimental results.

Positional Preference: Prevalence in Infants and Follow-Up After Two Years

Abstract: Objectives 1) To determine the prevalence of positional preference in the general population of infants up to the age of 6 months; 2) to gather information on possible risk factors; 3) to determine the percentage of children with positional preference undergoing diagnostic evaluation and/or treatment; and 4) to assess the overall outcome of positional preference in infants and toddlers, with currently used diagnostic and treatment practices Setting Infant health care centers in The Netherlands Methods Seven thousand six hundred nine infants below the age of 6 months were screened for positional preference (cases:n = 623) Anamnestic data and physical signs of asymmetry of the range of motion and the shape of the head were recorded These data were also registered of an immediate next child visiting the infant health care center with the same sex and about the same age but without positional preference (controls:n = 554) In a first follow-up study, 6 to 8 months after the original study, 468 of the 623 children with positional preference were reexamined for asymmetry of the range of motion and the shape of the head In a second follow-up study, 24 to 32 months after the original study, 129 of 259 children who still had shown signs of asymmetry in the first follow-up study were again reexamined Results The prevalence of positional preference was 82% and was highest in children below 16 weeks of age The boy:girl ratio was 3:2 Firstborns, premature children, and children with breech position at the time of delivery proved to have a higher risk for positional preference The supine sleeping position of the child and a strong preference in offering the feeding always from the right or the left side were positively correlated with positional preference In the first follow-up study, 12% still showed restricted active range of motion, 8% restricted passive range of motion, 47% asymmetric flattening of the occiput, and 23% of the forehead Thirty-two percent of the children with positional preference had been referred for diagnostical/therapeutical intervention In the second follow-up study, active range of motion was restricted in 6%, passive rotation in 2%, 45% had an asymmetric flattening of the occiput, and 21% of the forehead Conclusion Positional preference is frequently observed (82%) in The Netherlands It leads to referral, additional diagnostics and, if necessary, treatment of almost 1 of every 3 affected children Extrapolated to the original population in 1995, 24% of all children would still have a restricted range of motion and/or flattening of the skull at the age of 2 to 3 years The high prevalence of positional preference in infancy, the persistency of accompanying signs, the large number of children referred for further diagnostic and/or treatment, and the resulting high medical expenses strongly call for a primary preventive approach positional preference, deformational plagiocephaly, asymmetry, infants, population-based study

Dynamics of coarsening foams: accelerated and self-limiting drainage.

Abstract: The evolution of a foam is determined by drainage flow of the continuous (liquid) phase and coarsening (aging) of the dispersed phase (gas bubbles). Free-drainage experiments with slow- and fast-coarsening gases show markedly different dynamics and elucidate the importance of the coupling of the two effects. Strong coarsening leads to drainage times that are shorter (accelerated drainage) and independent of the initial liquid content (self-limiting drainage). A model incorporating the physics of both drainage and diffusive coarsening shows quantitative agreement with experiment.

Discussing aspects of AOP

Abstract: Aspect-oriented programming is a new evolution in the line of technology for separation of concerns technology that allows design and code to be structured to reflect the way developers want to think about the system. AOP builds on existing technologies and provides additional mechanisms that make it possible to affect the implementation of systems in a crosscutting way.

Segmentation of Fingerprint Images

Abstract: An important step in an automatic fingerprint recognition system is the segmentation of fingerprint images. The task of a fingerprint segmentation algorithm is to decide which part of the image belongs to the foreground, originating from the contact of a fingertip with the sensor, and which part to the background, which is the noisy area at the borders of the image. In this paper, an algorithm for the segmentation of fingerprints is presented. The method uses three pixel features, being the coherence, the mean and the variance. An optimal linear classifier is trained for the classification per pixel, while morphology is applied as postprocessing to obtain compact clusters and to reduce the number of classification errors. Manual inspection shows that the proposed method provides accurate high-resolution segmentation results. Only 6.8% of the pixels is misclassified while the postprocessing further reduces this ratio. Experiments show that the proposed segmentation method and manual segmentation perform equally well in rejecting false fingerprint features from the noisy background. Keywords— Image processing, fingerprint image segmentation, pixel features, coherence, linear classification, neural network.

Integrated Optical sensors for the chemical domain

Abstract: During the last decade there has been a rapidly growing interest in integrated optical (IO) sensors, expecially because many of them principally allow for sensitive, real time, label-free-on-site measurements of the concentration of (bio-)chemical species. This review aims at giving an overview of the most relevant developments in this area. After a general introduction into the field of IO sensors for the chemical domain, relevant aspects of integrated optics and chemical sensing are presented in short. A large variety of IO sensing platforms are introduced and discussed: interferometers, resonators, coupling-based devices such as grating couplers and surface plasmon resonance based sensors and finally a new class of sensors based on chemically induced field profile changes. Strong and weak points of principle and of configuration based on these principles are indicated and the main performance data of the IO sensing platforms, especially the obtained resolution, are indicated. Best resolutions of the chemically induced refractive indices on the order of magnitude $10^{-6}-10^{-8}RIU$ can be obtained, corresponding to a resolution of $10^{-3}-10^{-5}$nm in the chemically induced growth of layer thickness of chemo-optical transducer materials. Depending on the anlalyte and the type of transduction layer chemical concentrations down to some ppb or some pg $ml^{-1}$can be determined. Several IO sensing systems are commercially available. Extension of individual sensors to sensor arrays is treated and finally an outlook for the future is given.

Cell biology beyond the diffraction limit: near-field scanning optical microscopy.

Abstract: Throughout the years, fluorescence microscopy has proven to be an extremely versatile tool for cell biologists to study live cells. Its high sensitivity and non-invasiveness, together with the ever-growing spectrum of sophisticated fluorescent indicators, ensure that it will continue to have a prominent role in the future. A drawback of light microscopy is the fundamental limit of the attainable spatial resolution - similar to 250 urn - dictated by the laws of diffraction. The challenge to break this diffraction limit has led to the development of several novel imaging techniques. o­ne of them, near-field scanning optical microscopy (NSOM), allows fluorescence imaging at a resolution of o­nly a few tens of nanometers and, because of the extremely small near-field excitation volume, reduces background fluorescence from the cytoplasm to the extent that single-molecule detection sensitivity becomes within reach. NSOM allows detection of individual fluorescent proteins as part of multimolecular complexes o­n the surface of fixed cells, and similar results should be achievable under physiological conditions in the near future

Nichtkovalente Synthese mit Wasserstoffbrücken

Abstract: Wasserstoffbrucken ahneln Menschen insofern, als dass sie zur Gruppenbildung neigen. Einzeln sind sie schwach, leicht zu brechen und manchmal schwer zu finden. Handeln sie jedoch gemeinsam, so werden sie viel starker und unterstutzen einander. Bei diesem, unter dem Begriff Kooperativitat bekannten Phanomen ist 1+1 mehr als 2. Unter Nutzung dieses Prinzips haben Chemiker eine grose Vielzahl chemisch stabiler Strukturen entwickelt, deren Bildung auf der reversiblen Knupfung mehrerer Wasserstoffbrucken beruht. Mehr als 20 Jahre des Studiums dieser Phanomene haben zur Entwicklung einer neuen Disziplin innerhalb der organischen Synthese gefuhrt, die man heute als nichtkovalente Synthese bezeichnet. Dieser Aufsatz beschreibt die nichtkovalente Synthese auf der Basis der reversiblen Bildung mehrerer Wasserstoffbrucken. Er beginnt mit einer eingehenden Beschreibung des Wesens der Wasserstoffbrucke, fuhrt dann durch eine Vielzahl von bimolekularen Aggregaten und Assoziaten hoherer Ordnung und erlautert die allgemeinen Prinzipien, die deren Stabilitat zugrunde liegen. Besondere Aufmerksamkeit wird reversibel uber Wasserstoffbrucken gebildeten Kapseln gewidmet, die interessante Einschlussphanomene aufweisen. Weiterhin wird die Rolle von Wasserstoffbrucken in Selbstreplikationsprozessen diskutiert, und abschliesend werden neue Materialien (Nanorohren, Flussigkristalle, Polymere usw.) und Prinzipien (dynamische Bibliotheken) vorgestellt, die in jungster Zeit aus diesem faszinierenden Forschungsgebiet hervorgegangen sind.

Bicontinuous Nanoporous Polymers by Carbon Dioxide Foaming

Abstract: We investigate the physical foaming process of glassy poly(ether imide) and poly(ether sulfone) using carbon dioxide and report temperature-concentration diagrams ("foam diagrams") marking out the foaming envelope in which dense CO2-saturated films expand and microvoids are introduced. Two types of porosities are observed. Closed microcellular structures occur at carbon dioxide saturation levels below 50 cm3 (STP)/cm3 (polymer); nanoporous bicontinuous (open) structures with pore sizes as small as 40 nm occur above this CO2 concentration threshold, which is identical for both polymers. The cellular-to-bicontinuous transition is characterized in detail on the basis of gas permeation measurements and is represented as a separate window inside the foaming diagram. In this paper, the transition to bicontinuous structures is reported for the first time, and its generic physical basis is critically reviewed.

Three-dimensional finite element modeling of skeletal muscle using a two-domain approach: Linked fiber-matrix mesh model

Abstract: In previous applications of the finite element method in modeling mechanical behavior of skeletal muscle, the passive and active properties of muscle tissue were lumped in one finite element. Although this approach yields increased understanding of effects of force transmission, it does not support an assessment of the interaction between the intracellular structures and extracellular matrix. In the present study, skeletal muscle is considered in two domains: (1) the intracellular domain and (2) extracellular matrix domain. The two domains are represented by two separate meshes that are linked elastically to account for the trans-sarcolemmal attachments of the muscle fibers’ cytoskeleton and extracellular matrix. With this approach a finite element skeletal muscle model is developed, which allows force transmission between these domains with the possibility of investigating their interaction as well as the role of the trans-sarcolemmal systems. The model is applied to show the significance of myofascial force transmission by investigating possible mechanical consequences due to any missing link within the trans-sarcolemmal connections such as found in muscular dystrophies. This is realized by making the links between the two meshes highly compliant at selected intramuscular locations. The results indicate the role of extracellular matrix for a muscle in sustaining its physiological condition. It is shown that if there is an inadequate linking to the extracellular matrix, the myofibers become deformed beyond physiological limits due to the lacking of mechanical support and impairment of a pathway of force transmission by the extracellular matrix. This leads to calculation of a drop of muscle force and if the impairment is located more towards the center of the muscle model, its effects are more pronounced. These results indicate the significance of non-myotendinous force transmission pathways.