Characterisation of the inhomogeneity of sol–gel-derived SiO₂–CaO bioactive glass and a strategy for its improvement

Sol–gel derived bioactive glasses are one of the most promising materials for bone regeneration. The nanostructure of the glasses is critical regarding their interaction with the physiological environment. Composition is one of the most important factors affecting the nanostructure. In this study, inhomogeneity (a translucent component surrounded by an opaque component) was found within sol–gel derived bioactive glass monoliths (70 mol% SiO₂ and 30 mol% CaO) produced by a standard procedure. The nanostructure and composition of the two components were characterised by electron microscopy, nitrogen sorption, secondary ion mass spectroscopy, X-ray fluorescence and solid state nuclear magnetic resonance. Results showed that calcium concentration and nanoscale porosity were much higher in the opaque region than in the translucent component. This is believed to be caused by calcium nitrate accumulation on the outer surface of the monoliths during the drying stage of the sol–gel process. The homogeneity of monoliths was successfully improved by using Teflon moulds. These findings are important for the large scale commercial production of homogeneous sol–gel glasses.