In vitro bioactivity and degradation behavior of silica xerogels intended as controlled release materials

Room temperature-processed silica-based sol-gel, termed silica xerogel, is a novel type of controlled release material. As shown previously, these materials are porous, degradable and can release biologically functional molecules in a controlled manner. It was also demonstrated that these materials are biocompatible in vivo. Herein we report on the ability of silica-based xerogels to form a bioactive, apatite-like (AP) surface and the effect of AP surface on the xerogel stability in vitro. Formation of a crystalline, carbonated AP (c-AP) was found on all silica xerogels studied, with or without Ca- and P-oxides. Calcium and phosphate (Ca-P) free xerogels showed long times to Ca-P precipitation and to formation of a detectable AP-layer (up to 2 weeks). In contrast, the times to precipitation were reduced by 2-3 orders of magnitude, and the c-AP layer was formed within 24 h on all Ca-P containing xerogels. Mechanisms of the c-AP formation on these xerogels were similar to those typical for Ca-P based ceramics: dissolution of calcium and phosphate ions, solution oversaturation with respect to AP and subsequent precipitation of bone-like minerals. The presence of the c-AP surface film produced a remarkable surface stabilizing effect: the rates and the amounts of Si release were significantly reduced in comparison to those for xerogels without the film. This evidence of in vitro bioactivity and controlled degradation, combined with previous in vitro and in vivo reports, suggests that silica xerogel is a promising controlled release material.