Biomedical subjects

Thijs J H Vlugt

Publications and source records attributed to Thijs J H Vlugt.

7 recordsLinked to original sources

PubMed2006

Sheared force networks: anisotropies, yielding, and geometry.

A scenario for the yielding of granular matter is presented by considering the ensemble of force networks for a given contact network and applied shear stress tau. As tau is increased, the probability distribution of contact forces becomes highly anisotropic, the difference between average contact forces along minor and major axes grows, and the allowed networks span a shrinking subspace of all force networks. Eventually, contacts start to break, and at the maximal shear stress the packing becomes effectively isostatic. The size of the allowed subspace exhibits simple scaling properties, which lead to a prediction for the yield stress for packings of an arbitrary contact number.

Journal Article↗

PubMed2006

Modeling the release of proteins from degrading crosslinked dextran microspheres using kinetic Monte Carlo simulations.

To optimize and predict the release of proteins from biodegradable microspheres based on crosslinked dextran, a fundamental understanding of the mechanisms controlling their release is necessary. For that purpose, a mathematical model has been developed to describe the release of proteins from these hydrogel-based microspheres. A kinetic Monte Carlo scheme for the degradation of a small domain inside the microsphere was developed. The results from this were used in a second kinetic Monte Carlo scheme to model the diffusion and the subsequent release of proteins. The only processes included in this model are diffusion and degradation. The general effects of diffusion, crosslink density, protein loading, and clustering of proteins on the release were investigated. The model crosslink density (Xmodel) and the model diffusivity (Dmodel) were fitted to experimental release data of BSA monomer from hydroxyethyl methacrylated dextran (dex-HEMA) microspheres. By using the experimental release curves of liposomes and BSA monomer, it was found that (1) the model crosslink density (Xmodel) scales with the hydrodynamic diameter (dh) as dh(1.64) and (2) the diffusivity of the protein (Dmodel) scales approximately with 1/dh (Stokes-Einstein). Using these scaling relations, quantitative predictions of the release curves of BSA dimer, immunoglobulin G and human growth hormone were possible. In conclusion, this model may play an important role in the optimization, understanding and prediction of the release of various proteins from degradable hydrogels.

Algorithms↗

PubMed2005

Molecular simulations of the adsorption of cycloalkanes in MFI-type silica.

A new force field for the simulation of the adsorption of cycloalkanes in nanoporous silica affords a significant improvement over any previously employed force field. The simulated isotherms reproduce the most salient features in the experimental isotherms extremely well. The study of cyclo-pentane, -hexane, and -heptane adsorption in MFI-type silica indicates an inflection for cyclopentane but not for cyclohexane at intermediate pressure. If corroborated by experiments, such an inflection point would afford an excellent calibration point for further force field developments. At low pressures, mixture isotherms of cyclohexane and n-hexane show a temperature dependence on the selectivity in accordance with recent results by J. P. Fox and S. P. Bates, J. Phys. Chem., 2004, 108, 17136. This dependence is caused by a difference in temperature dependence of the Henry coefficient for both molecules. At high pressures entropy effects due to packing always favor the sorption of n-hexane. Furthermore, the influence of the flexibility of the zeolite framework on the adsorption of these rather bulky molecules is investigated. It is found that this influence of the flexibility on the adsorption of cyclohexane is as small as with n-alkanes.

Adsorption↗

PubMed2005

Stretching a heteropolymer.

We study the elastic properties of single heteropolymers. By means of exact enumeration of conformations, Monte Carlo (MC) simulation, and variational principles, we calculate equilibrium force-extension curves of heterocopolymers for specific arrangements of the monomer types along the sequence. At a given extension z, the time averaged measured force is the weighted sum of restoring forces for various configurations. Using variational principles, we calculate force-extension (f-z) curves of heteropolymers with fixed extensions z. These results are compared with f-z curves obtained from MC simulations and exact enumeration of all conformations. Typical random sequences manifest several piecewise unfoldings of blocks of various size, which are overlapping due to thermal fluctuations. The shape of the elastic response of a heteropolymer reflects the disorder in the primary block structure and the binding energies of these blocks.

Journal Article↗

PubMed2004

Ensemble theory for force networks in hyperstatic granular matter.

An ensemble approach for force networks in static granular packings is developed. The framework is based on the separation of packing and force scales, together with an a priori flat measure in the force phase space under the constraints that the contact forces are repulsive and balance on every particle. In this paper we will give a general formulation of this force network ensemble, and derive the general expression for the force distribution P(f). For small regular packings these probability densities are obtained in closed form, while for larger packings we present a systematic numerical analysis. Since technically the problem can be written as a noninvertible matrix problem (where the matrix is determined by the contact geometry), we study what happens if we perturb the packing matrix or replace it by a random matrix. The resulting P(f) 's differ significantly from those of normal packings, which touches upon the deep question of how network statistics is related to the underlying network structure. Overall, the ensemble formulation opens up a different perspective on force networks that is analytically accessible, and which may find applications beyond granular matter.

Journal Article↗

PubMed2004

Understanding the role of sodium during adsorption: a force field for alkanes in sodium-exchanged faujasites.

We have developed a united atom force field able to accurately describe the adsorption properties of linear alkanes in the sodium form of FAU-type zeolites. This force field successfully reproduces experimental adsorption properties of n-alkanes over a wide range of sodium cation densities, temperatures, and pressures. The force field reproduces the sodium positions in dehydrated FAU-type zeolites known from crystallography, and it predicts how the sodium cations redistribute when n-alkanes adsorb. The cations in the sodalite cages are significantly more sensitive to the n-alkane loading than those in the supercages. We provide a simple expression that adequately describes the n-alkane Henry coefficient and adsorption enthalpy as a function of sodium density and temperature at low coverage. This expression affords an adequate substitute for complex configurational-bias Monte Carlo simulations. The applicability of the force field is by no means limited to low pressure and pure adsorbates, for it also successfully reproduces the adsorption from binary mixtures at high pressure.

Journal Article↗

PubMed2004

Force network ensemble: a new approach to static granular matter.

An ensemble approach for force distributions in static granular packings is developed. This framework is based on the separation of packing and force scales, together with an a priori flat measure in the force phase space under the constraints that the contact forces are repulsive and balance on every particle. We show how the formalism yields realistic results, both for disordered and regular triangular "snooker ball" configurations, and obtain a shear-induced unjamming transition of the type proposed recently for athermal media.

Journal Article↗