Effects of ions in aqueous media on hydroxyapatite induction by silica gel and its relevance to bioactivity of bioactive glasses and glass-ceramics.

Hydroxyapatite induction by a synthesized pure silica hydrogel was examined in various simulated body fluids (SBFs) having different magnesium, calcium, and phosphate ion concentrations as well as pH values. The silica hydrogel generated biologically active apatite on its surface by taking up calcium and phosphorous ionic groups from a surrounding SBF that was prepared to emulate the human plasma in inorganic composition. The induction period for apatite nucleation on the surface of the silica was largely decreased with the addition of a small amount of the calcium or phosphate ions to the SBF and with an increase in pH, but increased with the addition of magnesium ion. Bioactivity of bioactive materials like Bioglass and glass-ceramic A-W was well interpreted in terms of the rate of apatite formation reflected in these results. Moreover, the results provide the basic knowledge for designing new bioactive materials.