Showing papers by "University of Twente published in 1987"
TL;DR: LOTOS is a specification language that has been specifically developed for the formal description of the OSI (Open Systems Interconnection) architecture, although it is applicable to distributed, concurrent systems in general.
Abstract: LOTOS is a specification language that has been specifically developed for the formal description of the OSI (Open Systems Interconnection) architecture, although it is applicable to distributed, concurrent systems in general. In LOTOS a system is seen as a set of processes which interact and exchange data with each other and with their environment. LOTOS is expected to become an ISO international standard by 1988.
1,446 citations
TL;DR: The development and application of computer-simulation methods to predict stress-related adaptive bone remodeling, in accordance with 'Wolff's Law' are developed and applied to investigate the relation between 'stress shielding' and bone resorption in the femoral cortex around intramedullary prostheses.
1,085 citations
TL;DR: In this article, the ternary diffusion process that occurs in a cellulose acetate (CA) -acetone casting solution immersed into a water bath has been investigated and the necessary concentration dependent thermodynamic and hydrodynamic parameters have been derived from experimental data on the three limiting binary mixtures.
377 citations
TL;DR: In this article, the authors derived boundary conditions for the isothermal diffusion processes in the coagulation bath and in the polymer solution after immersion of a cast (ternary) polymer solution into a (binary) coagulated bath and expressed the mass transfer in terms of thermodynamic driving forces and frictional coefficients.
374 citations
TL;DR: In this article, the maximum allowable concentration for a (micro)porous membrane under process conditions can be determined, by means of theoretical considerations, for a homogeneous smooth material (gq < 90/deg) can be calculated.
374 citations
TL;DR: In this article, the effects of rubber concentration and particle size on the tensile modulus, torsion modulus and notched impact strength of the EPDM-rubber blends were studied.
373 citations
TL;DR: HEC adhesion in CMS onto (co)polymers of hydroxyethyl methacrylate (HEMA) and methyl methacylate (MMA) was found to be optimal on the moderately wettable copolymer (mol ratio 25 HEMA/75 MMA) and in CM, complete cell spreading in CM was only observed on the positively-charged copolymers.
366 citations
TL;DR: A simple CMOS circuit technique for realizing both linear transconductance and a precision square-law function is described, which is versatile in application and diverse applications are demonstrated in the fields of linear amplifiers, continuous-time filters, and nonlinear function implementation.
Abstract: A simple CMOS circuit technique for realizing both linear transconductance and a precision square-law function is described. The circuit provides two separate outputs in the linear as well as square-law modes. The linear outputs both have a range of 100% or more of the total quiescent current value. The theory of operation is presented and effects of transistor nonidealities on the performance are investigated. Design optimization techniques are developed. Experimental results measured on nonoptimized prototypes are: distortion of 0.2% for input signals up to 2.4 V/SUB p-p/ in the case of linear transfer function and 1.3% in the case of the square-law transfer function, with a DC to -3-dB bandwidth of up to 20 MHz. Improved performance is expected when the optimization techniques developed are applied. The circuit is versatile in application: diverse applications are demonstrated in the fields of linear amplifiers, continuous-time filters, and nonlinear function implementation.
327 citations
TL;DR: In this article, a novel type of hydrophobic zeolite has been used for the purpose of adding a sorptive filler with a high selectivity towards alcohol to improve both selectivity and flux.
301 citations
TL;DR: Desorption of these proteins and exchange for, e.g., cellular Fn may result in cell spreading and proliferation of HEC upon TCPS, which may lead to the development of small-diameter vascular grafts.
Abstract: A systematic study of the effects of polymer surface properties on the interaction with human endothelial cells (HEC) may lead to the development of small-diameter vascular grafts. HEC, suspended in culture medium containing 20% serum adhered and spread onto moderately wettable polymers such as TCPS (tissue culture polystyrene). Reduced or no adhesion of HEC was observed upon the hydrophobic polymers PETP (polyethyleneterephthalate, Dacron) and FEP (fluoroethylenepropylene copolymer, Teflon). Polymers precoated with the proteins albumin (Alb), high density lipoprotein (HDL), and immunoglobulin G (IgG) inhibited the adhesion of HEC, whereas fibronectin (Fn) coatings promoted cell adhesion. Endothelialization of PETP and FEP only occurred after precoating of these materials with Fn. The adsorption of Fn, Alb, HDL, and IgG from solutions of different serum concentrations onto TCPS, PETP, and FEP was related to the adhesion of HEC. Serum Fn only adsorbed onto TCPS, with the maximum at 0.1% serum concentration. Maximal cell adhesion onto TCPS was also observed after pretreatment with a solution containing 0.1% serum. The cell adhesion inhibiting proteins Alb and HDL preferentially adsorbed at higher serum concentrations. Desorption of these proteins and exchange for, e. g., cellular Fn may result in cell spreading and proliferation of HEC upon TCPS.
166 citations
TL;DR: This paper reviews the results that have been reported on ISFET based enzyme sensors and introduces a coulometric system that compensates for the analyte buffer capacity.
TL;DR: Measurement of depolarized orthogonal light scattering is a useful new parameter for flow-cytometric cell differentiation and shows that for small values of the detection aperture, the measured depolarization is caused by anisotropic cell structures and multiple scattering processes inside the cell.
Abstract: Polarization measurement of orthogonal light scattering is introduced as a new optical parameter in flow cytometry. In the experimental setup, the electrical field of the incident laser beam is polarized in the direction of the sample flow. The intensity of the orthogonal light scattering polarized along the direction of the incoming laser beam is called depolarized orthogonal light scattering. Theoretical analysis shows that for small values of the detection aperture, the measured depolarization is caused by anisotropic cell structures and multiple scattering processes inside the cell. Measurements of the orthogonal depolarized light scattering in combination with the normal orthogonal light scattering of human leucocytes revealed two populations of granulocytes. By means of cell sorting it was shown that the granulocytes with a relatively high depolarization are eosinophilic granulocytes. Similar experiments with human lymphocytes revealed a minor subpopulation of yet-unidentified lymphocytes with a relative large orthogonal light-scattering depolarization. The results were obtained with an argon ion laser tuned at different wavelengths as well as with a 630-nm helium neon laser. These results show that measurement of depolarized orthogonal light scattering is a useful new parameter for flow-cytometric cell differentiation.
TL;DR: It can be concluded that the best choice is to measure the normal component of the magnetic field at points which are situated in the spherical surface, which is the best possible fit for the head.
TL;DR: In this article, spray-dried fractions of α-lactose monohydrate and dicalcium phosphate dihydrate, respectively, suspended in solutions of lactose, were spray dried in order to obtain products with various amorphous lactose contents.
TL;DR: In the evaluation of groups of instructional strategies, the Reading approach has been found to be superior to the Expert and Spiral approaches, and six tactics which can be used both to design courses and to evaluate strategies are presented.
Abstract: This article offers an examination of instructional strategies and tactics for the design of introductory computer programming courses in high school. We distinguish the Expert, Spiral and Reading approach as groups of instructional strategies that mainly differ in their general design plan to control students' processing load. In order, they emphasize topdown program design, incremental learning, and program modification and amplification. In contrast, tactics are specific design plans that prescribe methods to reach desired learning outcomes under given circumstances. Based on ACT* (Anderson, 1983) and relevant research, we distinguish between declarative and procedural instruction and present six tactics which can be used both to design courses and to evaluate strategies. Three tactics for declarative instruction involve concrete computer models, programming plans and design diagrams; three tactics for procedural instruction involve worked-out examples, practice of basic cognitive skills and task variation. In our evaluation of groups of instructional strategies, the Reading approach has been found to be superior to the Expert and Spiral approaches.
TL;DR: The microstructure of the layers in the hippocampal CA1 area suggests that differences may exist between the electrical conductivities of these layers, and a one-dimensional CSD analysis of field potentials evoked by Schaffer collateral stimulation found that these differences did not lead to differences in the spatial distribution of sources and sinks.
Abstract: The microstructure of the layers in the hippocampal CA1 area suggests that differences may exist between the electrical conductivities of these layers. In order to quantify these differences a sinusoidal current was applied to hippocampal slices in a bathing medium and potential differences were measured between pairs of neighbouring electrodes from an array. The maximum relative conductivity (100%) was found in the middle part of str. radiatum, with a gradual decrease towards the fissure (84%). There was also a gradual decrease towards the alveus (70%), but in str. pyramidale the relative conductivity was only 42%. No differences were observed between the laminar conductivities of normal hippocampal slices and slices generating spontaneous interictal bursts. These results were used to carry out a one-dimensional CSD analysis of field potentials evoked by Schaffer collateral stimulation. Despite the differences in conductivity, the homogeneous and the inhomogeneous CSD approximations did not lead to differences in the spatial distribution of sources and sinks and only gave some differences in the current density, especially at the pyramidal layer and its close environment.
TL;DR: In this article, preliminary results for the oxidative coupling of methane to give ethane/ethylene mixtures over a series of different catalyst formulations were reported; the temperature range studied is 650-850°C.
TL;DR: In this paper, the effect of several spinning parameters (i.e., bore medium flow rate, spinning dope extrusion rate, fibre take-up rate, and spinning height) on the hollow fibre dimensions is reported.
TL;DR: In this article, a sequence of orthogonal polynomials defined by a three-term recurrence formula is considered and bounds for the endpoints of the smallest interval containing the (real and distinct) zeros of Pn in terms of the parameters in the recurrence relation are derived.
TL;DR: It is concluded that HEC deposit fibronectin onto TCPS, irrespective of the presence of a preadsorbed layer of proteins which delay cell adhesion.
Abstract: Human endothelial cells (HEC) suspended in a culture medium containing 20% human serum (CMS) adhere and spread on(to) moderately wettable polymers, such as tissue culture polystyrene (TCPS). We have previously shown that serum derived-fibronectin, which is a cell adhesion promoting protein, has a high affinity for TCPS, but that the amount of fibronectin which adsorbed from CMS was relatively small. In this study we investigated whether fibronectin derived from HEC contributes to the adhesion and spreading of the cells on(to) TCPS. Therefore, HEC were seeded in the presence of fibronectin-depleted CMS. The amount of fibronectin detected on TCPS increased with both cell seeding density and incubation time. Although initial HEC adhesion is delayed on TCPS which had been precoated with albumin (Alb), high density lipoprotein (HDL) or immunoglobulin G(IgG), maximal numbers of adhering and spreading HEC were found on these surfaces 6 h after seeding of HEC. Fibronectin was detected on these surfaces, but an exchange of preadsorbed Alb, HDL, or IgG for fibronectin could not be demonstrated. We conclude that HEC deposit fibronectin onto TCPS, irrespective of the presence of a preadsorbed layer of proteins which delay cell adhesion.
TL;DR: In vitro studies of PHPG–naltrexone conjugate in disk form did not show constant release because of the hydrophilic nature of the polymer backbone and the changing local chemical environment upon hydrolysis of drug–polymer linkages.
Abstract: The narcotic antagonist naltrexone (I) was modified at the 3 and 14 hydroxyl positions and covalently coupled to a biodegradable poly(α-amino acid) backbone through a labile bond. Selective acetylation of I with acetic anhydride gave naltrexone-3-acetate (II), which was subsequently succinoylated to naltrexone-3-acetate-14-hemisuccinate (III) with succinic anhydride. The polymeric backbone chosen for initial coupling experiments was poly-N
5-(3-hydroxypropyl)-L-glutamine (PHPG). The side-chain hydroxyl functionality permitted covalent bonding of III through an ester linkage. Hydrolysis of covalently bound drug to give naltrexone or its derivatives (II and III) should be much slower than diffusion of drug through the polymer matrix. While hydrolysis of naltrexone from the polymer side chain is first order, release of drug from the matrix can be zero order due to the geometry of the device and the physical and chemical interactions between naltrexone and the polymer matrix. In vitro studies of PHPG–naltrexone conjugate in disk form did not show constant release because of the hydrophilic nature of the polymer backbone and the changing local chemical environment upon hydrolysis of drug–polymer linkages. The conjugated system was made more hydrophobic by coupling drug to copolymers of hydroxypropyl-L-glutamine (HPG) and L-leucine. Conjugates of III coupled with copoly(HPG-70/Leu-30) demonstrated a nearly constant, but slightly declining release rate of naltrexone and its derivatives for 28 days in vitro.
TL;DR: Catheters coated with a conjugate of heparin and albumin showed a four- to five-fold reduction in platelet adhesion as compared to uncoated catheters, which may indicate that within the range of shear rates studied, the adhesion of platelets onto the catheter surface is mainly determined by the rate of the reaction between the platelets and the material surface.
Abstract: The adhesion of human blood platelets onto vascular catheters was studied using a specially designed perfusion chamber. Polyurethane catheters were exposed to citrated human blood for different periods (up to 20 min) and at different wall shear rates (190, 260, 330 sec-1). The rate of platelet adhesion was determined using 111In-labeled platelets, while the morphology of adhering platelets was investigated using scanning electron microscopy. A linear increase in platelet adhesion was found within the first 10 min of perfusion, after which a plateau value was reached. The number of adhering platelets did not vary significantly with the shear rates applied, which may indicate that within the range of shear rates studied, the adhesion of platelets onto the catheter surface is mainly determined by the rate of the reaction between the platelets and the material surface. Catheters coated with a conjugate of heparin and albumin showed a four- to five-fold reduction in platelet adhesion as compared to uncoated catheters. This reduction in platelet adhesion was not only due to the presence of albumin moieties at the surface but also to the presence of heparin residues in the adsorbed albumin-heparin conjugate.
TL;DR: Monitoring of simulated sleep suggests that the difficulty in separating wakefulness from stage 1 is due to poor modelling, and the maximum likelihood sleep stage monitor, that uses the sleep-related observations, has been derived and implemented.
Abstract: Stochastic models are proposed for sleep and for the sleep related electroencephalogram (EEG), electrooculogram (EOG), and electromyogram (EMG). The evolution of sleep through its various stages is described as a Markov chain. The EEG is modelled using Wiener processes. The EOG and EMG are modelled as combinations of Poisson point processes and Gaussian processes, respectively. The EEG models contain a feedback structure that is based on physiological data. The maximum likelihood sleep stage monitor, that uses the sleep-related observations, has been derived and implemented. The agreement between automatic and human stage classifications of six sleep recordings was 70.6%, which was 4.5% worse than the average agreement between six human classifiers. Monitoring of simulated sleep suggests that the difficulty in separating wakefulness from stage 1 is due to poor modelling. If one ignores this difference, which, from a diagnostic point of view is fairly unimportant, the above mentioned agreement reaches 81.8%, which is 0.5% better than the corresponding average human vs human agreement.
TL;DR: In this paper, magnetic susceptibility, X' and X'', and ac resistivity, ρ, of two different samples of superconducting YBa2Cu3O7-I´ have been measured.
TL;DR: In this article, a coprecipitation method was proposed for the synthesis of monophasic, contamination free, dense and machineable ceramics suitable for practical applications.
TL;DR: Anilides 4a-c,e-k, lla-d in which the amide function is benzylated, or silylated and having different electron-withdrawing groups (EWG) at the methyl methyl in the ortho position, cyclize under the influence of potassium tert-butoxide to the corresponding indole derivatives 5a,c, e-k and 9a-d, respectively as mentioned in this paper.
TL;DR: In this paper, the authors examined performance and engagement in computer literacy of boys and girls (N = 873) with CAST, a Dutch version of the Minnesota Computer Literacy Awareness Assessment.
Abstract: This research study examines performance and engagement in computer literacy of boys and girls (N = 873). Performance and engagement in computer literacy are established with CAST. Computer Alfabetisme Schalen Twente, a Dutch version of the Minnesota Computer Literacy Awareness Assessment. The results of the study show that girls perform lower and are less engaged in computer literacy than boys. Research on sex differences in mathematics and science education shows that three factors are important for the design of action programs for girls, viz. the expectation and behaviour of significant others, the perception of the usefulness of the subject for a future career and a positive attitude towards the subject. This study shows that these factors seem to be relevant for computer literacy too. It has been found that a positive attitude towards mathematics and physics is positively related to a positive attitude towards computer literacy. An examination of the relation between performance in computer literacy and attitude towards mathematics and physics shows no differences in performance between boys and girls with a negative attitude towards mathematics and physics. For boys and girls with a positive attitude towards mathematics and physics however a difference in performance in computer literacy has been found in favour of boys
14 Oct 1987
TL;DR: In this article, two planar waveguide sensors for the chemical domain are introduced, which consist of multilayered structures coated by a very thin organic overlayer that is able to absorb the species to be measured out of the environment.
Abstract: Two novel types of planar waveguide sensors for the chemical domain are introduced. Both are realized by thin film technologies. They consist of multilayered structures coated by a very thin organic overlayer that is able to absorb the species to be measured out of the environment. This absorbtion results in a change in its dielectric function. In the first sensor to be demonstrated this change is measured using a surface plasmon mode as a probe. In the second one the radiationless energy transfer from luminescent centres incorporated in one of the layers to the overlayer serves as a probe.
TL;DR: In this paper, the authors examined the possible problems arising from such a scaling-down of manifolds with smaller dimensions, and the major advantage of their design seems to be the diminution of the outer size together with the integration of the various component parts of a manifold in a permanent and rigid position.
Abstract: Introduction Flow injection analysis (FIA) is a well established technique nowadays. One of the interesting features is its low reagent consumption in conjunction with the relatively small sample volumes required. This aspect will become even more important when a further reduction of the dimensions of the systems can be achieved. Technically such a miniaturisation does not present great problems as far as transport conduits or reaction compartments are concerned. Photolithographic techniques developed in chip-technology have reached a remarkable degree of perfection and offer excellent possibilities in this respect. A well known example is the miniature gas chromatograph etched on a silicon wafer of match box size as described by Angel1 et al. ‘. Some years ago RiiiiEka and Hansen213 suggested a similar approach for flow injection manifolds. In their so-called ‘integrated micro conduits’ a large part of the manifold was manufactured by engraving grooves and other parts in a small single block of perspex, also of match box size. However, because of the mechanical engraving technique used, they did not actually reduce the size of the conduits and the major advantage of their design seems to be the diminution of the outer size together with the integration of the various component parts of a manifold in a permanent and rigid position. This contributes significantly to the enhanced repeatability that was experimentally observed. Since lithographic techniques offer the possibility of manufacturing manifolds with smaller dimensions, it is worthwhile to examine the possible problems arising from such a scaling-down. To facilitate further discussions some technological concepts will be introduced first.