X-ray diffraction and solid state NMR studies of the growth of hydroxyapatite on bioactive calcia:silica sol-gel glasses

An advanced materials characterisation methodology has been used to examine systematically a range of sol-gel glass materials of contemporary biomedical interest. High energy x-ray total diffraction, x-ray fluorescence and Raman spectroscopy, and isotope-enriched ¹⁷O and ³¹P MAS NMR have been used to study the local calcium environment in four sol-gel derived bioactive calcium silicate glasses of the general formula (CaO)_x(SiO₂)_1−x, and to gain an atomic scale insight into the nature of the processes initiated when this bioactive glass is immersed in vitro in simulated body fluid (SBF). The results reveal a rapid dissolution of the labile surface calcium ions from the glass within 30 min of immersion in SBF and the later formation of a hydroxyapatite like layer, which MAS NMR suggests is partially ordered locally, but which x-ray diffraction confirms as amorphous overall. Annealing the samples at 650°C allows this layer to crystallise, revealing the anticipated hydroxyapatite layer more clearly. The results presented here are consistent with, but greatly extend our previous XRD, ⁴³Ca and ²⁹Si MAS-NMR, XANES and EXAFS studies of these materials.