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.

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

Bioactive glasses convert to a biomimetic apatite when in contact with physiological solutions; however, the number and type of phases precipitating depends on glass composition and reactivity. This process is typically followed by X-ray diffraction and infrared spectroscopy. Here, we visualise surface mineralisation in a series of sodium-free bioactive glasses, using transmission electron microscopy (TEM) with energy-dispersive X-ray spectroscopy (EDXS) and X-ray nano-computed tomography (nano-CT). In the glasses, the phosphate content was increased while adding stoichiometric amounts of calcium to maintain phosphate in an orthophosphate environment in the glass. Calcium fluoride was added to keep the melting temperature low. TEM brought to light the presence of phosphate clustering and nearly crystalline calcium fluoride environments in the glasses. A combination of analytical methods, including solid-state NMR, shows how with increasing phosphate content in the glass, precipitation of calcium fluoride during immersion is superseded by fluorapatite precipitation. Nano-CT gives insight into bioactive glass particle morphology after immersion, while TEM illustrates how compositional changes in the glass affect microstructure at a sub-micron to nanometre-level.