Calcium phosphate formation and ion dissolution rates in silica gel-PDLLA composites

Sol-gel derived silicas are potential biomaterials both for tissue regeneration and drug delivery applications. In this study, both SiO(2) and calcium and phosphate-containing SiO(2) (CaPSiO(2)) are combined with poly-(DL-lactide) to form a composite. The main properties studied are the ion release rates of biologically important ions (soluble SiO(2) and Ca(2+)) and the formation of bone mineral-like calcium phosphate (CaP) on the composite surface. These properties are studied by varying the quality, content and granule size of silica gel in the composite, and porosity of the polymer. The results indicate that release rates of SiO(2) and Ca(2+) depend mostly on the formed CaP layer, but in some extent also on the granule size of silicas and polymer porosity. The formation of the bone mineral-like CaP is suggested to be induced by a thin SiO(-) layer on the composite surface. However, due to absence of active SiO(2) or CaPSiO(2) granules on the outermost surface, the suitable nanoscale dimensions do not contribute the nucleation and growth and an extra source for calcium is needed instead. The result show also that all composites with varying amount of CaPSiO(2) (10-60 wt%) formed bone mineral-like CaP on their surfaces, which provides possibilities to optimise the mechanical properties of composites.