Showing papers by "University of Twente published in 1993"

Effect of comonomer hydrophilicity and ionization on the lower critical solution temperature of N-isopropylacrylamide copolymers

Abstract: DSC was performed on aqueous solutions of poly(N-isopropylecrylamide-co-butyl methacrylate-co-X), with X being hydrophilic, hydrophobic, cationic, or anionic comonomers, to elucidate the mechanism of temperature-induced phase separation and the effect of comonomer content, hydrophilicity, and charge on the lower critical solution temperature (LCST). The endothermic heat of phase separation, which is related to the breaking of hydrogen bonds between water molecules surrounding hydrophobic moieties on the polymer, was a linear, decreasing function of the LCST. This suggests that the hydrophobic interactions between polymer side groups, which are the major driving force for phase separation, ere enhanced at elevated temperatures due to a decrease in the structuring of water around hydrophobic side groups

The efficiency of instructional conditions: An approach to combine mental-effort and performance measures

Abstract: This article reports on a calculational approach for combining measures of mental workload and task performance that allows one to obtain information on the relative efficiency of instructional conditions. The method is based on the standardization of raw scores for mental effort and task performance to z scores, which are displayed in a cross of axes. Relative condition efficiency is calculated as the perpendicular distance to the line that is assumed to represent an efficiency of zero. We conclude that the method for calculating and representing relative condition efficiency discussed here can be a valuable addition to research on the training and performance of complex cognitive tasks.

Abstracting Object Interactions Using Composition Filters

Abstract: It is generally claimed that object-based models are very suitable for building distributed system architectures since object interactions follow the client-server model. To cope with the complexity of today's distributed systems, however, we think that high-level linguistic mechanisms are needed to effectively structure, abstract and reuse object interactions. For example, the conventional object-oriented model does not provide high-level language mechanisms to model layered system architectures. Moreover, we consider the message passing model of the conventional object-oriented model as being too low-level because it can only specify object interactions that involve two partner objects at a time and its semantics cannot be extended easily. This paper introduces Abstract Communication Types (ACTs), which are objects that abstract interactions among objects. ACTs make it easier to model layered communication architectures, to enforce the invariant behavior among objects, to reduce the complexity of programs by hiding the interaction details in separate modules and to improve reusability through the application of object-oriented principles to ACT classes. We illustrate the concept of ACTs using the composition filters model.

Performance of a highly sensitive optical waveguide Mach-Zehnder interferometer immunosensor

Abstract: We describe a highly sensitive sensor which uses the evanescent field of a reusable planar optical waveguide as the sensing element. The waveguide used is optimized to obtain a steep dependence of the propagation velocity on the refractive-index profile near the surface. The adsorption of a layer of proteins thus results in a phase change, which is measured interferometrically using a Mach-Zehnder interferometer set-up. The stability of the interferometer is such that phase changes = (1 × 10-2)2pi per hour can be measured. Immunoreactions have been monitored down to concentrations of 5 × 10-11 M of a 40 kDa protein.

Condensed Monte Carlo simulations for the description of light transport

Abstract: A novel method, condensed Monte Carlo simulation, is presented that applies the results of a single Monte Carlo simulation for a given albedo μs/(μa + μs) to obtaining results for other albedos; μs and μa are the scattering and absorption coefficients, respectively. The method requires only the storage of the number of interactions of each photon with the medium. The reflectance and transmittance of turbid slabs can thus be found from a limited number of condensed Monte Carlo simulations. We can use an inversion procedure to obtain the absorption and scattering coefficients from the total reflectance and total transmittance of slabs. Remitted photon densities from a semi-infinite medium as a function of the distance between the light source and the detector for all albedos can be found even from the results of a single condensed Monte Carlo simulation. The application of similarity rules may reduce further the number of Monte Carlo simulations that are needed to describe the influence of the distribution of scattering angles on the results.

Optical properties of human dermis in vitro and in vivo

Abstract: Condensed Monte Carlo simulation results have been used for calculating absorption and reduced scattering coefficients from the literature data on the measured total transmittance and total reflectance of samples of the human skin in vitro. The results of several measuring methods have been compared. We have also estimated the range for absorption coefficients and reduced scattering coefficients at 660 and 940 nm from measured intensities at the skin surface as a function of the distance from the location where the light enters the skin by using condensed Monte Carlo simulations for a homogeneous semi-infinite medium. The in vivo values for the absorption coefficients and the reduced scattering coefficients appear to be much smaller than the values from the in vitro measurements, that have been assumed until now. The discrepancies have been discussed in detail. Our in vivo results are in agreement with other in vivo measurements that are available in the literature.

Mechanism and kinetics of the selective hydrogenation of ethyne and ethene

Abstract: A review of the kinetics and mechanism for the selective hydrogenation of ethyne and ethene on palladium catalysts is presented. The progress made in the last fifteen years is mainly discussed. It has become clear that the classical view, where the selectivity of the reaction was believed to be due to the thermodynamic factor is an over-simplification. Currently, it is generally assumed that at least two different sites are active during the selective hydrogenation, one of these might possibly involve the support. Ethene hydrogenation also occurs in the presence of high ethyne concentrations, which cannot be explained by the classical theory. Besides the two main hydrogenation reactions and the oligomerisation, there exists a direct route from ethyne to ethane, which, however, is only of minor importance. Possibly due to the rather complex nature of the system, there have been relatively few kinetic studies presenting practical rate expressions.

Micro-liquid flow sensor

Abstract: A simple to realise micro-liquid flow sensor with high sensitivity is presented. The sensor is based on well known thermal anemometer principles. An analytical model for the sensor behaviour applicable for gas/liquid fluids is presented. The realisation process of the sensor is described. Model and experimental results agree well. The sensor is simple to integrate with other micro-liquid handling components such as pumps, mixers, etc.

The Regional Extraction Method: EC Input–Output Comparisons

Abstract: This paper presents a method for quantifying inter-regional linkages on the basis of a many-region input—output table. The extraction method proposed can be seen as an extension of Strassert's hypothetical extraction method. Instead of extracting one sector from a sector-based model, the effects of hypothetically extracting a region from a many-region model are considered. It is shown that the method calculates so-called backward linkages. Forward linkages are obtained analogously from the matrix of allocation coefficients. The method is applied to intercountry linkages for the EC in the years 1970 and 1980.

Calibration and evaluation of scanning-force-microscopy probes

Abstract: It is demonstrated that a stepped (305) surface of a SrTiO3 crystal can be used routinely to evaluate the probing profile of scanning-force-microscopy probes. This provides a means to select optimal surface probes, and to evaluate possible image distortions within the range of the atomic and nanometer scale. The scope and limitations of the resolution of structural defects are discussed as a criterion for a true atomic resolution.

Sintering kinetics and microstructure development of nanoscale Y-TZP ceramics

Abstract: Zirconia samples doped with 3 mol% yttria were prepared by gel precipitation from a metal chloride solution and their sintering behaviour compared with that of a commercial powder. Dense (relative density 97%) nanoscale ceramics with a mean grain size of 60 nm are obtained after sintering at 1050°C for 7h. Important densification mechanisms in the initial sintering stage are grain boundary sliding and grain boundary diffusion. Grain growth in the final sintering stage seems to be impurity drag controlled. Extremely low activation energies are obtained for both densification and grain growth in the initial sintering stages. Special attention has been paid to the effect of aggregate size of the precursor powder on the final grain size.

Determination of hyperpolarizability tensor components by depolarized hyper Rayleigh scattering

Abstract: Depolarized hyper Rayleigh scattering of para-nitroaniline (C2v symmetry) and nitrocalix[4]arene (C4v symmetry) in solution has been measured. Using linearly and circularly polarized fundamental radiation information about the ratios between the several hyperpolarizability tensor components, including their sign, was obtained. Results are consistent with the theory developed for both symmetry groups. Comparison between experimental depolarization ratios and ratios obtained from ab initio calculated hyperpolarizability tensor components shows good agreement.

Paresthesia thresholds in spinal cord stimulation: a comparison of theoretical results with clinical data

Abstract: The potential distributions produced in the spinal cord and surrounding tissues by dorsal epidural stimulation at the midcervical, midthoracic, and low thoracic levels were calculated with the use of a volume conductor model. Stimulus thresholds of myelinated dorsal column fibers and dorsal root fibers were calculated at each level in models in which the thickness of the dorsal cerebrospinal fluid (CSF) layer was varied. Calculated stimulus thresholds were compared with paresthesia thresholds obtained from measurements at the corresponding spinal levels in patients. The influences of the CSF layer thickness, the contact separation in bipolar stimulation and the laterality of the electrodes on the calculated thresholds were in general agreement with the clinical data. >

Nonlinear State Space H ∞ Control Theory

Abstract: Although the H ∞ control problem was originally formulated [55] as a linear design problem in the frequency domain (in fact, H ∞ stands for the Hardy space of complex functions bounded and analytic in the open righthalf complex plane), it can be naturally translated to the time-domain and extended to nonlinear state-space systems. Indeed, the standard H ∞ control problem can be equivalently formulated as the optimal attenuation of the L 2-induced norm from exogenous inputs (inputs with unknown power spectrum) to the to-be-controlled outputs, under the constraint of internal stability. Also, although early research in H ∞ control was conducted solely using frequency domain methods, a satisfactory state space solution to the linear H ∞ (sub-)optimal control problem was reached by the end of the eighties (see especially [12], [31], [17], [46], [16], [30], [48], [49], [47]). Moreover, this state space solution relies on tools familiar from LQ and LQG theory, in particular Riccati equations and Hamiltonian matrices. In the classical paper by Willems on LQ control [52] the relations of these tools with the underlying notion of dissipativity were being stressed; while in [53] dissipativity was defined for general nonlinear systems, encompassing notions of passivity of physical systems and input-output stability of nonlinear (feedback) systems. The resulting dissipation inequalities were fruitfully explored in e.g. [35], [20], [21], also linking them to the Hamilton-Jacobi equation from classical nonlinear optimal control (see also [36]).

Diagnosing and solving over-determined constraint satisfaction problems

Abstract: Constraint relaxation is a frequently used technique for managing over-determined constraint satisfaction problems. A problem in constraint relaxation is the selection of the appropriate constraints. We show that methods developed in model-based diagnosis solve this problem. The resulting method, DOC, an abbreviation for Diagnosis of Over-determined Constraint Satisfaction Problems, identifies the set of least important constraints that should be relaxed to solve the remaining constraint satisfaction problem. If the solution is not acceptable for a user, DOC selects next-best sets of least-important constraints until an acceptable solution has been generated. The power of DOC is illustrated by a case study of scheduling the Dutch major league soccer competition. The current schedule is made using human insight and Operations Research methods. Using DOC, the 1992-1993 schedule has been improved by reducing the number and importance of the violated constraints by 56%. The case study revealed that efficiency improvement is a major issue in order to apply this method to large-scale over-determined scheduling and constraint satisfaction problems.

Hard cases: a procedural approach

Abstract: Much work on legal knowledge systems treats legal reasoning as arguments that lead from a description of the law and the facts of a case, to the legal conclusion for the case. The reasoning steps of the inference engine parallel the logical steps by means of which the legal conclusion is derived from the factual and legal premises. In short, the relation between the input and the output of a legal inference engine is a logical one. The truth of the conclusion only depends on the premises, and is independent of the argument that leads to the conclusion. This paper opposes the logical approach, and defends a procedural approach to legal reasoning. Legal conclusions are not true or false independent of the reasoning process that ended in these conclusions. In critical cases this reasoning process consists of an adversarial procedure in which several parties are involved. The course of the argument determines whether the conclusion is true or false. The phenomenon of hard cases is used to demonstrate this essential procedural nature of legal reasoning. Dialogical Reason Based Logic offers a framework that makes it possible to model legal dialogues. We use Dialogical Reason Based Logic to specify hard cases in dialogical terms. Moreover, we analyse an actual Dutch hard case in terms of Dialogical Reason Based Logic, to demonstrate both the possibilities and the shortcomings of this approach. It turns out that there is no one set of rational dialogue rules. There are many concurring sets of rules that govern particular types of dialogues. The rules for legal procedures are as much part of the law as the more substantial rules. As a consequence, it is not possible to offer an universal set of dialogue rules. Dialogical Reason Based Logic rather provides a framework which can be filled with dialogue rules that determine which dialogues are valid and which ones are invalid.

The effect of phagocytosis of poly(L-lactic acid) fragments on cellular morphology and viability

Abstract: The aim of this study was to investigate the effect of phagocytosed poly(L-lactic acid) particles on the morpholgy and viability of phagocytes, mainly macrophages. Therefore, predegraded poly(L-lactic acid) (P-PLLA) and nontreated PLLA (N-PLLA) particles, both having diameters not exceeding 38 µm, were injected intraperitoneally in mice. P-PLLA particles were obtained by 25 kGy γ-irradiation of N-PLLA particles. N-PLLA and P-PLLA particles were injected using an 0.3% ethanol/0.9% saline solution intraperitoneally to the mice. We also studied the release of the absorbed ethanol as a possible model for the release of low molecular weight, potentially toxic products. As control, nondegradable polytetrafluoroethylene (PTFE) particles and the carrier solution were used. After 1, 2, 3, 4, 5, and 7 days, the cells of the abdominal cavity were harvested to study the effect of phagocytosis of polymer particles on phagocytic cell morphology and viability. Studies with transmission electron microscopy indicated that, upon injection of particles in the peritoneal cavity, macrophages demonstrated signs of cell damage, cell death, and cell lysis due to phagocytosis of a large amount of P-PLLA particles. The morphology of the cells that had phagocytosed the N-PLLA and PTFE particles did not differ substantially from those of control animals in which only the solution was injected. Also, in the controls, hardly any cell death and no debris was observed. When the PLLA particles were injected as a suspension in a 0.3% ethanol/0.9% saline solution, no difference was observed between N-PLLA and P-PLLA. After phagocytosis, both cause cell damage, sometimes leading to cell death. The highest numbers of necrotic cells were observed on day 2. The effects could be caused by the (peak) release of degradation products from P-PLLA fragments or by the release of the absorbed ethanol when the 0.3 ethanol/0.9 saline solution was used to administer the particles. In conclusion, it can be stated that cell damage, sometimes leading to cell death, may be caused by phagocytosed poly(L-lactic acid) particles.