Conversion kinetics of silicate, borosilicate, and borate bioactive glasses to hydroxyapatite

Bioactive 45S5 glass has been studied extensively both in vivo and in vitro, and it is relatively well known that when placed in a phosphate containing solution the glass will react to form the bone-like material hydroxyapatite (HA). In the present work, a kinetic analysis of previously measured weight loss data was done to determine reaction rate constants for four bioactive glasses; one silicate glass, two borosilicate glasses, and one borate glass, via the contracting volume model. The reaction rate increased with increasing B₂O₃ content, with the borate glass reacting nearly five times faster than the silicate 45S5 glass. The three silica containing glasses all deviated from the contracting volume model after approximately 50–70% of the total weight loss; however, when compared to the 3D diffusion model, the normalised data were in good agreement to 100% of the total weight loss. The deviation from the contracting volume model to the slower 3D diffusion model indicates a change in conversion model for the silica containing glasses and can likely be attributed to the formation of a silica rich layer of a certain thickness that began controlling the release of ions from the unreacted glass by diffusion.