Kinetics of apatite deposition of silica gel dependent on the inorganic ion composition of simulated body fluids

Nucleation and crystal growth mechanism of apatite formation on porous silica gel derived by a sol-gel procedure was investigated by the use of several simulated body fluids (SBF's) that had different concentrations of Ca(II), P(V), and OH^- but had the same degree of supersaturation for hydroxyapatite. Induction time of apatite crystallization in SBF's was evaluated by thin film X-ray diffractometry. The effect of each ion on the induction time increased in the order: Ca(II)-rich SBF < P(V)-rich SBF ≪ OH^--rich SBF, while that for the rate of initial crystal growth of apatite increased in the order: P(V)-rich SBF < Ca(II)-rich SBF < OH^--rich SBF. It suggested that amorphous calcium phosphate (ACP) deposited as the precursor of apatite and ACP transformed into apatite nuclei by the incorporation of OH^-. Moreover, the transformation was accelerated by large amount of ACP and was assisted by the rearrangement of silicate units in the gel surface. Ca(II) was richer in ACP deposited in Ca(II)-rich SBF and OH^--rich SBF than that deposited in P(V)-rich SBF. Ca(II)-rich ACP was favorable for crystal growth.