Sol-gel-derived biomaterials of silica and carrageenans.

A new precursor, tetrakis(2-hydroxyethyl) orthosilicate (THEOS), introduced by Hoffmann et al. (J. Phys. Chem. B 106 (2002) 1528-1533), was used to synthesize monolithic hybrid biomaterials on the basis of silica and three main types of carrageenans, kappa-, iota-, and lambda-carrageenans. The advantage of THEOS over the currently applied TEOS and TMOS is in its complete solubility in water. This negated the need to add organic solvents, thus excluding a denaturating effect on biopolymers. In their turn, carrageenans introduced into the precursor solution made use of common catalysts unneeded to trigger the sol-gel transition. It was found that they promoted the mineralization, acting as a template for the inorganic component. The kinetics of sol-gel processes, mechanical properties, phase behavior, and structure of novel hybrid biomaterials were studied by dynamic rheology, differential scanning calorimetry, and scanning electron microscopy. The material properties were regulated by both the precursor and carrageenan. The increase of silicate concentration led to a rise in the stiffness and brittleness of the material, whereas the polysaccharide addition made it softer and more elastic. It was shown that the formation and properties of mixed gels were determined by the nature of carrageenan. kappa-Carrageenans brought about shrinkage of hybrid materials that led to water separation, while iota- and lambda-carrageenans did not induce the syneresis. This is in line with the difference in polysaccharide properties when they are in aqueous solutions without silicate. Furthermore, kappa- and iota-carrageenans experienced a thermoreversible phase transition in the hybrid materials owing to the helix-coil transition. This resulted in a step like change in the mechanical properties of mixed systems in the corresponding temperature range. lambda-Carrageenan is a nongelling polysaccharide, which is why the rheological parameters of its hybrid gel were unchanged with the temperature. It was found that the polysaccharides modified the structure of silica-based materials. They transformed a three-dimensional network of connected silica particles into that consisting of crossed fibers.