Preparation of hollow hydroxyapatite microspheres by the conversion of borate glass at near room temperature

Hollow hydroxyapatite microspheres, consisting of a hollow core and a porous shell, were prepared by converting Li2O–CaO–B2O3 glass microspheres in dilute phosphate solution at 37 °C. The results confirmed that Li2O–CaO–B2O3 glass was transformed to hydroxyapatite without changing the external shape and dimension of the original glass object. Scanning electron microscopy images showed the shell wall of the microsphere was built from hydroxyapatite particles, and these particles spontaneously align with one another to form a porous sphere with an interior cavity. Increase in phosphate concentration resulted in an increase in the reaction rate, which in turn had an effect on shell wall structure of the hollow hydroxyapatite microsphere. For the Li2O–CaO–B2O3 glass microspheres reacted in low-concentration K2HPO4 solution, lower reaction rate and a multilayered microstructure were observed. On the other hand, the glass microspheres reacted in higher phosphate solution converted more rapidly and produced a single hydroxyapatite layer. Furthermore, the mechanism of forming hydroxyapatite hollow microsphere was described.