High phosphate content significantly increases apatite formation of fluoride-containing bioactive glasses

Bioactive glass-containing toothpastes for treating dentine hypersensitivity work by precipitating hydroxycarbonate apatite (HCA) onto the tooth surface, but concerns exist over the long-term durability of HCA in the mouth. Fluoride-containing bioactive glasses form fluorapatite (FAp) in physiological solutions, which is more chemically stable against acid attack. The influence of phosphate content on apatite formation was investigated by producing a low-phosphate (about 1 mol% P2O5) and a high-phosphate (about 6 mol%) series of melt-derived bioactive glasses in the system SiO2P2O5CaONa2O; increasing amounts of CaF2 were added by keeping the ratio of all other components constant. pH change, ion release and apatite formation during immersion in Tris buffer at 37 °C over up to 7 days were investigated. Crystal phases formed in Tris buffer were characterized using infrared spectroscopy, X-ray diffraction and solid-state nuclear magnetic resonance (NMR) spectroscopy. An increase in phosphate or fluoride content allowed for apatite formation at lower pH; fluoride enhanced apatite formation due to lower solubility of FAp compared to hydroxyapatite or HCA. High phosphate content glasses formed apatite significantly faster (within 6 h) than low phosphate content glasses (within 3 days). In addition, an increase in phosphate content favoured apatite formation rather than fluorite (CaF2). 19F magic angle spinning NMR showed the apatite formed by fluoride-containing glasses to be FAp, which makes these glasses of particular interest for dental applications. This study shows that by varying the phosphate content, the reactivity and apatite formation of bioactive glasses can be controlled successfully.