Kinetics and mechanisms of the conversion of silicate (45S5), borate, and borosilicate glasses to hydroxyapatite in dilute phosphate solutions

Bioactive glasses with controllable conversion rates to hydroxyapatite (HA) may provide a novel class of scaffold materials for bone tissue engineering. The objective of the present work was to comprehensively characterize the conversion of a silicate bioactive glass (45S5), a borate glass, and two intermediate borosilicate glass compositions to HA in a dilute phosphate solution at 37 degrees C. The borate glass and the borosilicate glasses were derived from the 45S5 glass by fully or partially replacing the SiO2 with B2O3. Higher B2O3 content produced a more rapid conversion of the glass to HA and a lower pH value of the phosphate solution. Whereas the borate glass was fully converted to HA in less than 4 days, the silicate (45S5) and borosilicate compositions were only partially converted even after 70 days, and contained residual SiO2 in a Na-depleted core. The concentration of Na+ in the phosphate solution increased with reaction time whereas the PO43- concentration decreased, both reaching final limiting values at a rate that increased with the B2O3 content of the glass. However, the Ca2+ concentration in the solution remained low, below the detection limit of atomic absorption, throughout the reaction. Immersion of the glasses in a mixed solution of K2HPO4 and K2CO3 produced a carbonate-substituted HA but the presence of the K2CO3 had little effect on the kinetics of conversion to HA. The kinetics and mechanisms of the conversion process of the four glasses to HA are compared and used to develop a model for the process.