Nano-imaging confirms improved apatite precipitation for high phosphate/silicate ratio bioactive glasses

Bioactive glass-ceramics containing fluorapatite, xonotlite, cuspidine and wollastonite form apatite faster than their corresponding glasses

Crystallisation of bioactive glasses has been claimed to negatively affect the ion release from bioactive glasses. Here, we compare ion release and mineralisation in Tris-HCl buffer solution for a series of glass-ceramics and their parent glasses in the system SiO2-CaO-P2O5-CaF2. Time-resolved X-ray diffraction analysis of glass-ceramic degradation, including quantification of crystal fractions by full pattern refinement, show that the glass-ceramics precipitated apatite faster than the corresponding glasses, in agreement with faster ion release from the glass-ceramics. Imaging by transmission electron microscopy and X-ray nano-computed tomography suggest that this accelerated degradation may be caused by the presence of nano-sized channels along the internal crystal/glassy matrix interfaces. In addition, the presence of crystalline fluorapatite in the glass-ceramics facilitated apatite nucleation and crystallisation during immersion. These results suggest that the popular view of bioactive glass crystallisation being a disadvantage for degradation, apatite formation and, subsequently, bioactivity may depend on the actual system study and, thus, has to be reconsidered.

Clustering of fluoride and phosphate ions in bioactive glass from computer simulation

In order to understand the nature of ionic clustering in bioactive glass compositions, computer simulation was used to model four different compositions of bioactive glass with various amounts of flouride and phosphate. Fluoride ions were chemically bonded only to sodium and calcium, creating regions rich in fluoride and modifiers, and fluoride clustering was seen to be present in all compositions. The majority of phosphate groups are present as orthophosphate and phosphate clustering is also seen, and shown to be stronger in compositions with a lower phosphate content. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 1)'.