Objectives: Fluoride prevents dental cavities, and bioactive glasses form apatite in physiological solutions. This makes fluoride-containing bioactive glasses interesting for use in dental biomaterials or dentifrices. Aim of this research was to investigate their structure-property relationship.
Methods: Glasses in the system SiO2-P2O5-CaO-Na2O-CaF2 with a fixed network connectivity of 2.13 and increasing CaF2 content (0 to 33mol%) were prepared by a melt-quench route and characterised using differential scanning calorimetry, X-ray diffraction, solid-state nuclear magnetic resonance (NMR) spectroscopy and degradation studies in simulated body fluid (SBF) over up to two weeks followed by infrared spectrometry (FTIR). Formation of hydroxycarbonate apatite (HAp) in SBF was taken as indication for bioactivity.
Results: 29Si and 19F-MAS NMR showed no formation of Si-F bonds but presence of mixed calcium sodium fluoride species. Glass transition temperature decreased with increasing CaF2. 31P-NMR showed phosphate being present as orthophosphate, i.e. it is not part of the silica network and no Si-O-P bonds are present. Bioactive glasses are known to give a pH rise in aqueous solution. This pH increase was less pronounced for higher fluoride contents (pH7.8 for 33mol% CaF2) than for low fluoride contents (pH8.1 for 0mol% CaF2). pH in SBF stayed constant over two weeks. FTIR showed formation of apatite after three days for the fluoride-free glass and glasses with high CaF2 content. Glasses with lower CaF2 content showed apatite only after one week, as for low CaF2 contents the pH might be too low for deposition of HAp. For higher CaF2 concentrations fluoroapatite, which is more stable than HAp, is formed instead as shown by NMR.
Conclusion: Addition of CaF2 does not disrupt the glass network. Addition of fluoride decreases the pH rise in aqueous solutions and for glasses with high fluoride content fluoroapatite is formed in SBF.
Acknowledgments: DTI grant HO669