Characterisation of a Novel Bioactive Composite: MAS-NMR and SEM Study

Objectives: To study the dissolution rate and the potential of fluorapatite (FAP) formation of a bioactive glass (BAG) composite in an acidic solution in reference to a neutral solutions, using the magic angle spinning-nuclear magnetic resonance (MAS-NMR) technique and the scanning electron microscope (SEM).
Methods: Composite disks (90) were prepared from a novel melt derived fluoride- containing BAG and a light cure resin. Three sample groups (n=30) of disks were immersed in Tris buffer pH=7.3 (TB), artificial saliva pH=7 (AS7) and artificial saliva pH=4 (AS4) for ten time points (6hours-6months) with three replicas. The immersed disks of each time point were crushed into a powder and investigated by the solid state MAS-NMR. SEM studies were undertaken after embedding half of the immersed disk in a self-cure acrylic where the fracture surface is facing the top and then polished, carbon coated and imaged.
Results: MAS-NMR results show that the BAG composite dissolves significantly faster in AS4 compared to TB and AS7. At the end of the immersion period (six months) all glass particles in AS4 has reacted, evidenced by the sharp peak at 2.82ppm compared to a broader peak in TB and AS7. It also shows evidence of FAP formation indicated by peaks at -103ppm, while peaks around -108ppm indicate the formation of calcium fluoride from the excess Ca⁺² and F^- especially on longer immersion. SEM images confirm higher dissolution rate of the BAG composite in AS4 and reveal the impact of time on the dissolution of more glass particles. The images also indicate apatite formation around the glass particles in TB and AS4, while it forms predominantly over the disk surface in AS7.
Conclusions: BAG composite has a smart reactivity in response to pH, suggesting a potential clinical benefit against enamel demineralisation. The composite is forming FAP under acidic condition, which is essential for remineralisation.