Wound is a growing healthcare challenge affecting several million worldwide. Lifestyle disorders such as diabetes increases the risk of wound complications. Effective management of wound is often difficult due to the complexity in the healing process. Addition to the conventional wound care practices, the bioactive polymers are gaining increased importance in wound care. Biopolymers are naturally occurring biomolecules synthesized by microbes, plants and animals with highest degree of biocompatibility. The bioactive properties such as antimicrobial, immune-modulatory, cell proliferative and angiogenic of the polymers create a microenvironment favorable for the healing process. The versatile properties of the biopolymers such as cellulose, alginate, hyaluronic acid, collagen, chitosan etc have been exploited in the current wound care market. With the technological advances in material science, regenerative medicine, nanotechnology, and bioengineering; the functional and structural characteristics of biopolymers can be improved to suit the current wound care demands such as tissue repair, restoration of lost tissue integrity and scarless healing. In this review we highlight on the sources, mechanism of action and bioengineering approaches adapted for commercial exploitation.

Biopolymers: Applications in wound healing and skin tissue engineering

Methods are reviewed for improving the mechanical properties of oxide glasses. These are divided into surface and bulk techniques and include thermal and chemical strengthening, controlled crystallization, and particle, fibre and whisker reinforcement. The merits and limitations of individual techniques are compared and discussed. In conclusion, a number of applications for these materials are briefly outlined.

Methods for improving the mechanical properties of oxide glasses

Nucleation and crystallization behaviour of glasses in SiO2-La2O3, SiO2-La2O3-Al2O3 and SiO2-La2O3-ZrO2 systems were investigated using glasses prepared by the fusion of “gels” and the mixtures of oxides in solar/image furnace. Two methods for the preparation of multicomponent homogeneous non-crystalline products in the form of gels were developed. The phase separation, devitrification and micro-hardness of the above glasses were investigated in relation to the starting materials and the composition. The results show that the glasses made from gels are more homogeneous than those made from oxide mixtures. The phase separation characteristics of glasses made from gels are markedly different from those of glasses made from a mixture of oxides. The addition of Al2O3 to the binary SiO2-La2O3 glasses improves the homogeneity but reduces the micro-hardness and the devitrification tendency, whereas the addition of ZrO2 causes a considerable increase in micro-hardness and enhances the devitrification. The rates of nucleation and crystallization of glasses of different compositions made from gels are much higher than those made from the mixture of oxides. The formation of the high temperature crystal form, (Β-La2Si2O7 is more evident with the crystallization of gel-glasses. When the rate of nucleation is low, (in the case of glasses from the mixture of oxides), the curve representing the relation between the micro-hardness and the time of heat-treatment shows a distinct minimum, whereas this minimum is not obtained with the gel-glasses. With most of the gel-glasses, the micro-hardness rises very sharply with the length of heat-treatment. The curve showing the relation between the micro-hardness and the volume fraction of the dispersed crystalline phase also gives a distinct minimum which can be explained on the basis of the fracture mechanism consisting of the processes of crack nucleation and of crack propagation around the dispersed crystalline particles.

A comparative study of “gels” and oxide mixtures as starting materials for the nucleation and crystallization of silicate glasses

The mechanical properties of silica alcogels and aerogels were measured by the three-point bending technique. Elastic moduli and the fracture strength were investigated as a function of concentration of silicon compound, catalysis conditions, and aging time. The evolution of modulus and strength was followed during gel to glass transformation. Toughness and fracture energy results of aerogels are presented.

Evolution of mechanical properties during the alcogel-aerogel-glass process

The compatibility of fibroin with chitosan was confirmed by means of the DSC method, measuring thermal characteristics of blends of these components in the form of films obtained from solutions in a common solvent such as HFIP. Polymers have the general temperatures of glass transition and the beginnings of degradation of crystal areas. The TG analysis of fibroin/cellulose blends in the form of films obtained from NMMO solutions has shown that the thermal stability of such blends is higher than that of cellulose owing to the strong interaction between the components, which is also shown by the decreased mass loss rate of the blend during thermal treatment, as compared to that of individual components.

Compatibility of fibroin/chitosan and fibroin/cellulose blends studied by thermal analysis

Flexural strength and fracture surface energy were determined for lead borate glasses whose compositions lie in the immiscible region of the PbO-B2O3 system. The microstructural characterization indicated that the glasses are typical particulate composites which consist of two immiscible phases. For the glasses whose microstructure consists of PbO-rich particles/B2O3-rich matrix (B2O3-rich side of the miscibility gap), the fracture surface energy was found to decrease with increasing second-phase particles. To explain this behaviour, a crack propagation model in a brittle composite containing “penetrable” particles was proposed. On the other hand, for the glasses whose microstructure consists of B2O3-rich particles/PbO-rich matrix (PbO-rich side of the miscibility gap), an increase in fracture surface energy with volume fraction of dispersed particles was observed. This phenomenon could be best explained by Lange-Evans theory of fracture in brittle composites containing “impenetrable” particles. It was concluded that, when the critical crack size in a non-dispersed host glass is much larger than the particle size, the crack size in particulate composites is invariant with microstructure and also that the variation of strength results entirely from the variation of fracture toughness.

Strength and fracture surface energy of phase-separated glasses

A theoretical analysis is made of the indentation hardness of glass matrix, particulate composites. It is hypothesized that glass is an elastic-plastic solid on a microscopic scale. Based upon the Marsh theory of indentation, expressions are formulated for indentation hardness of two-phase composites containing spherical particles. When hard particles are dispersed in a soft glass matrix, the overall hardness depends upon the matrix hardness, the volume-fraction of dispersed phase, the elastic properties of the two phases and also the matrix flow stress. On the other hand, when soft particles are dispersed in a hard glass matrix, the hardness and the elastic moduli vary in parallel with the volume-fraction of dispersed phase. Furthermore, the present analysis indicates that the hardness of a composite is independent of the particle size and interparticle spacing if the volume-fraction of the particles is kept constant. Experimental results of the Vickers hardness of phase-separated glasses as well as published hardness data for a glass-ceramic are used for the verification of the theory. The proposed theory explains well the hardness behaviour of such glass matrix composites in terms of the properties and amounts of the individual phases and the microstructural effects.

Micromechanics approach to the indentation hardness of glass matrix particulate composites

This paper proposes a fracture theory for two-phase glass-crystal composites. It is hypothesized that the fracture mechanisms of such solids consist of the processes of crack nucleation and of crack propagation round the dispersed particles. At lower volume fractions of dispersed phase, macroscopic fracture will occur as a result of the growth of the micro-cracks originating in the vicinity of the pre-existing structural imperfections through a heterogeneous nucleation process; in this case, strength decreases with the proportion of the dispersed phase. At higher volume fractions where further crack propagation is prohibited by the hard crystalline particles, the process of crack propagation round the dispersed particles may be responsible for the macroscopic fracture of the composite; in this case, strength is an increasing function of the volume fraction. Expressions are formulated for mechanical strength of the glass-crystal composites, based upon the nucleation theory and Griffith's criterion. The published data on the strength of glass-alumina composites are used for the verification of the theory. The proposed theory explains well the strength behaviour of glass-alumina composites, and in particular, the dependence of the strength reduction on particle size at lower volume fractions.

Theoretical approach to the fracture of two-phase glass-crystal composites

On the basis of a theory previously developed by the authors for the indentation hardness of glass matrix, particulate composites, an attempt was made to interpret published hardness data for a ZnO-Al2O3-SiO2 glass-ceramic in which gahnite (ZnAl2O4) crystal particles are dispersed in a glass matrix as a major crystalline phase. The elastic moduli for gahnite were estimated using both the bulk modulus-molar volume relationship and the density-Poisson's ratio relationship, established for oxide crystals. After determining the variation of the matrix Young's modulus with heat-treatment, the variation of the overall hardness with volume fraction of crystal phase as well as the crystal-size effect were discussed. The hardness behaviour of the present glass-ceramic could be interpreted well in terms of the properties and amounts of the constituent phases and the microstructural effects.

H. Jinno

Papers

Strength and fracture surface energy of phase-separated glasses

Micromechanics approach to the indentation hardness of glass matrix particulate composites

Theoretical approach to the fracture of two-phase glass-crystal composites

Discussion of the indentation hardness of a glass-ceramic with participate microstructure