Role of pH and temperature on silica network formation and calcium incorporation into sol–gel derived bioactive glasses

Abstract

Bioactive glasses and inorganic/organic hybrids have great potential as implant materials. Bioactive glasses can bond to bone through the formation of a bone-like hydroxycarbonate apatite (HCA) layer and stimulate bone growth via their dissolution products. The brittle nature of these glasses can be combined with the toughness of a biodegradable polymer by forming a hybrid through the sol–gel process. However, for polymer incorporation, lower temperatures and milder pH conditions are required rather than the current method which uses pH < 1 and stabilisation at 600 °C. The impact of varying pH at low temperature processing (40 °C) on the inorganic component is unknown and must be investigated as a fundamental examination for hybrid research. This work seeks to combine pH control with a calcium source to examine the effect on porosity and silica network connectivity and to determine if raising the pH of synthesis can form a more highly connected glass without the need for high temperatures. Glass monoliths (70 mol% SiO2 and 30 mol% CaO) were fabricated, using calcium hydroxide as the calcium precursor and at different pH values (pH 0.5–5.5). With a view to hybrid synthesis, gels were dried at 40 °C and were compared with glasses made from identical gels that had been stabilised at 600 °C. When dried at 40 °C, gels synthesised at pH < 2 were not mesoporous whereas those formed at pH > 2 were mesoporous. This indicates a difference in gel formation about the isoelectric point of silicic acid, which was confirmed by 29Si solid state NMR. When immersed in simulated body fluid (SBF), the glasses stabilised at 600 °C were more porous, yet had a slower ion release rate than the gels dried at 40 °C. All gels and glasses formed an HCA layer in SBF; however, calcium was only incorporated into the silica network after stabilisation at 600 °C and thus a new way of incorporating calcium at low temperatures must still be found. This work is an important foundation for hybrid synthesis as raising the pH of the sol–gel process from pH < 1 to pH 5.5 was found to have no adverse effects on silica network formation and thus polymer can be incorporated into the sol–gel process at milder pH conditions without the concern of acid catalysed polymer degradation by chain scission.