Structure of fluoride-containing bioactive glasses

Fluoride prevents dental cavities, stimulates bone mineralisation and decreases the melting temperature of glasses and is therefore an interesting component of bioactive glasses for use as dental or orthopaedic biomaterials. However, when designing new glass compositions, the structural role of fluoride in the glass needs to be better understood. We have characterised a glass series in the system SiO2-P2O5-CaO-Na2O with increasing concentrations of CaF2. Network connectivity was fixed at 2.13 by adding CaF2 while the ratio of all other components was kept constant. F-19 and Si-29 MAS NMR spectra showed that addition of CaF2 does not cause disruption of the glass network by formation of Si-F bonds but forms mixed calcium sodium fluoride species. P-31 MAS NMR showed phosphate being present as orthophosphate. Hence it does not form part of the actual glass network backbone and no Si-O-P bonds are present. Na-23 MAS NMR showed the presence of multiple sodium sites with an increase in the mean coordination number of sodium with increasing CaF2 content. The glass transition temperature decreased with increasing amounts of CaF2. As no Si-F bonds were formed, this can be explained by formation of hypothetical CaF+ species. The results can be used for designing new fluoride-containing bioactive glass compositions for specific applications.