Kinetics and mechanisms of converting bioactive borate glasses to hydroxyapatite in aqueous phosphate solution

Borate bioactive glasses are receiving increasing attention as scaffold materials for bone repair and regeneration. In this study, the kinetics and mechanisms of converting three groups of sodium–calcium–borate glasses with varying CaO:B₂O₃ ratio to hydroxyapatite (HA) in 0.25 M K₂HPO₄ solution were investigated at 10–70 °C. Glass disks with the composition 2Na₂O·(2 − x)CaO·(6 + x)B₂O₃ (x = 0, 0.5, and 1.0) were immersed for up to 8 days in the potassium phosphate solution. The conversion kinetics to HA were monitored by measuring the weight loss of the glass, while X-ray diffraction, scanning electron microscopy, and Fourier transform infrared spectroscopy were used to study structural and compositional changes. All three groups of glasses formed HA on their surfaces, showing that the glasses were bioactive. At 10–37 °C, the conversion kinetics was well fitted by the contracting sphere model. Also, the contracting sphere model has a good fit for the early stage of conversion at 70 °C, whereas a three-dimensional (3D) diffusion model provided a good fit to the data of the later stage. The results of this study provide kinetic and structural data for the design of borate bioactive glasses for potential applications in bone tissue engineering.