Soong-Hyuck Suh

The biological effects of topical alginate treatment in an animal model of skin wound healing.

Wound healing is a dynamic and complex process of tissue repair that involves a number of cellular and molecular events. It proceeds from inflammatory response to reepithelialization and finally to formation of a permanent scar. Alginate is a polymer of guluronic and mannuronic acid that is used as a scaffolding material in biomedical applications. For the purpose of studying wound healing, full‐thickness skin defects were produced on the dorsal area in rats. We measured the relative sizes of the wounds on days 3, 5, 7, 14, and 28. The wound sizes were decreased in the alginate‐treated group compared with the control group and the vaseline‐treated group. The expressions of transforming growth factor‐β1, fibronectin, and vascular endothelial growth factor were significantly decreased in the alginate‐treated group compared with the control group, while the expression of collagen‐I was increased in the alginate‐treated group, as indicated by Western blotting and immunohistochemical staining. These data suggest that alginate has significant wound healing promoting activity. The results from the present study indicate that the effect of alginate on wound healing may involve biological mechanisms associated with the expression of transforming growth factor‐β1, fibronectin, vascular endothelial growth factor, and collagen‐I.

WEIGHTED-DENSITY APPROXIMATION AND ITS APPLICATION TO CLASSICAL FLUIDS

A simple weighted‐density approximation based on both local average and bulk densities is proposed. The weighting function is constructed to agree with that of the hybrid weighted‐density approximation (HWDA) proposed by Leidl and Wagner [J. Chem. Phys. 98, 4142 (1993)] for the homogeneous fluid; it has the advantage of being simpler to apply. The new approximation is applied to predict the homogeneous and inhomogeneous properties of classical fluids. For the homogeneous classical fluids, the new approximation generates the same accurate higher‐order direct correlation functions as those of the HWDA. For the properties of inhomogeneous classical fluids such as the density profiles of hard‐sphere and Lennard‐Jones fluids restricted in spherical cages, the results are in good agreement with the computer simulations, and comparable with those of the HWDA. Through these calculations, the density‐functional perturbation theory based on the second‐order perturbation theory of the uniform liquid has also been ex...

Inhomogeneous structure of penetrable spheres with bounded interactions

The density functional theory (DFT) based on the bridge density functional and the fundamental-measure theory (FMT) have been used to investigate the structural properties of one- and two-component penetrable spheres in a spherical pore. The Monte Carlo simulations have been carried out to compare with the theoretical results. The result shows that at low temperature the FMT functional is better than the DFT based on the bridge density functional and compares well with the computer simulations. At high temperature the DFT based on the bridge density functional is better than the FMT functional. These results suggest the reliable accuracy of the modified Verlet closure for the penetrable spheres at high temperature. However, the accuracy of both the FMT functional and the DFT based on the bridge density functional deteriorates if the packing fraction is increased.

Molecular Simulations of Phase Transitions in Pores

Abstract Methods for simulating phase transitions in narrow pores are reviewed, and the advantages and disadvantages of different techniques are discussed. Examples are given of applications to vapor-liquid, liquid-liquid, melting and freezing, solid-solid and layering transitions. While there has been a considerable body of simulation work on vapor-liquid, wetting and layering transitions for simple fluids and pore geometries, much remains to be done on more complex geometries and network effects, on heterogeneous surfaces, and on liquid-liquid, melting and solid-solid transitions in pores.

Self-diffusion in single-file pores of finite length

Molecular dynamics simulation results are reported for single-file self-diffusion in pores of infinite and finite length. The model system involves moderately dense, nonovertaking, hard-sphere fluids in cylindrical pores, and the particle mobility is investigated as a function of time and pore length. It is shown that, while stationary Fickian diffusion in infinitely long pores with diffusely reflecting pore walls is nonexistent, a diffusivity may be defined for single-file pores of finite length. For pores of moderate length, it is also demonstrated that the self-diffusion coefficient scales as 1/√L where L is the pore length.

Molecular dynamics simulation study of self-diffusion for penetrable-sphere model fluids.

Molecular dynamics simulations are carried out to investigate the diffusion behavior of penetrable-sphere model fluids characterized by a finite energy barrier ϵ. The self-diffusion coefficient is evaluated from the time-dependent velocity autocorrelation function and mean-square displacement. Detailed insights into the cluster formation for penetrable spheres are gained from the Enskog factor, the effective particle volume fraction, the mean free path, and the collision frequency for both the soft-type penetrable and the hard-type reflective collisions. The simulation data are compared to theoretical predictions from the Boltzmann kinetic equation and from a simple extension to finite ϵ of the Enskog prediction for impenetrable hard spheres (ϵ→∞). A reasonable agreement between theoretical and simulation results is found in the cases of ϵ∗ ≡ ϵ/k(B)T=0.2, 0.5, and 1.0. However, for dense systems (packing fraction ϕ>0.6) with a highly repulsive energy barrier (ϵ∗ = 3.0), a poorer agreement was observed due to metastable static effects of clustering formation and dynamic effects of correlated collision processes among these cluster-forming particles.

Equilibrium and nonequilibrium molecular dynamics studies of diffusion in model one-dimensional micropores

Abstract Molecular dynamics simulation results are reported for self-diffusion of a hard-sphere fluid in a one-dimensional interconnected spherical cavity-cylindrical pore model of zeolites. The mode of transport within the cylindrical sections of the structure is by single-file diffusion and a scaling relationship between the self-diffusivity and the length of the particle files proposed in recent work is verified using two different nonequilibrium simulation techniques. It is also shown that strong nonlocal spatial correlations can exist between the fluid particles in pore systems containing single-file channels and these results in conjunction with the local spatial dependence of the transport and equilibrium properties indicate that care needs to be exercised when interpreting experimental measurements of diffusivities within zeolitic structures.

Effect of polymer size and chain length on depletion interactions between two colloids.

A density functional theory based on the weighted density has been developed to investigate the depletion interactions between two colloids immersed in a bath of the binary polymer mixtures, where the colloids are modeled as hard spheres and the polymers as freely jointed tangent hard-sphere chain mixtures. The theoretical calculations for the depletion forces between two colloids induced by the polymer are in good agreement with the computer simulations. The effects of polymer packing fraction, degree of polymerization, polymer/polymer size ratio, colloid/polymer size ratio on the depletion interactions, and colloid-colloid second virial coefficient B2 due to polymer-mediated interactions have been studied. With increasing the polymer packing fraction, the depletion interaction becomes more long ranged and the attractive interaction near the colloid becomes deeper. The effect of degree polymerization shows that the long chain gives a more stable dispersion for colloids rather than the short chain. The strong effective colloid-colloid attraction appears for the large colloid/polymer and polymer/polymer size ratio. The location of maximum repulsion Rmax is found to appear Rmax approximately sigmac+Rg2 for the low polymer packing fraction and this is shifted to smaller separation Rmax approximately sigmac+sigmap2 with increasing the polymer packing fraction, where sigmap2 and Rg2 are the small-particle diameter and the radius of gyration of the polymer with the small-particle diameter, respectively.

Molecular Dynamics Studies of Diffusion in Model Cylindrical Pores at Very Low Densities

Abstract Molecular dynamics simulation has been used to study diffusion of methane at ambient temperature in cylindrical pores at very low densities. The cylinders were modelled as a continuum solid which interacts with the methane in the radial direction only. At the lowest densities, the VACF method does not yield reliable values of the self diffusion coefficient, Ds , but a suitable choice of time step and run length enables values of Ds to be found from MSD plots that are below the classical Knudsen diffusion coefficients. When density is increased, Ds passes through a maximum although the adsorption isotherm remains inside the Henry law region. Maxima are found for two cylinder radii and for two adsorbent field strengths. The existence of a maximum is attributed to transient intermolecular interactions. Analysis of a molecular trajectory demonstrates that long diffusion paths can be triggered by the rare event of an intermolecular encounter which forces a molecule into the repulsive part of the wall ...

Depletion interactions in two-dimensional colloid–polymer mixtures: molecular dynamics simulations

The depletion interactions acting between two hard colloids immersed in a bath of polymers, in which the interaction potentials include the soft repulsion/attraction, are extensively studied by using the molecular dynamics simulations. The collision frequencies and collision angle distributions for both incidental and reflection conditions are computed to study the dynamic properties of the colloidal mixtures. The depletion effect induced by the polymer-polymer and colloid-polymer interactions are investigated as well as the size ratio of the colloid and polymer. The simulated results show that the strong depletion interaction between two hard colloids appears for the highly asymmetric hard-disc mixtures. The attractive depletion force at contact becomes deeper and the repulsive barrier becomes wider as the asymmetry in size ratio increases. The strong polymer-polymer attraction leads to the purely attractive depletion interaction between two hard colloids, whereas the purely repulsive depletion interaction is induced by the strong colloid-polymer attraction.