Mechanism of apatite formation on a sodium silicate glass in a simulated body fluid

²⁹Si and ³¹P magic-angle-spinning nuclear magnetic resonance (MAS-NMR) spectra were used to analyze 20Na₂O·80SiO₂ glass particles before and after soaking in a simulated body fluid (SBF; Kokubo solution, pH 7.4) for various periods (up to 14 d). The structure of the bulk glass and the glass surface, as well as the chemical states of the calcium phosphates adsorbed on the glass surface, were examined. The bulk glass before soaking in the SBF showed two ²⁹Si MAS-NMR resonances, at -100 and -110 ppm, which were attributable to Si(OSi)₃ONa and Si(OSi)₄ units, respectively. A new -105 ppm peak, which was due to Si(OSi)₃OH units, appeared after soaking in the SBF and grew as the soaking period increased. Thus, in the SBF, the glass network was degraded and hydrolyzed; that is, dissolution of the Na⁺ ions from the glass network and the formation of Si-O^- groups occurred. After soaking for 1 d, a ³¹P MAS-NMR resonance peak of orthophosphate (PO₄^3-) ions that were deposited on the glass surface appeared at approx. 3.0 ppm and grew as the soaking period increased. The chemical shift shows that the local structural environment of phosphorus atoms and ions was very similar to that of hydroxyapatite or rabbit cortical bone. Inductively coupled plasma atomic emission spectroscopic analysis confirmed the adsorption of calcium phosphates on the glasses and the close relationship between the dissolution of the Na^PLU ions and the precipitation of the calcium phosphates. It was concluded that the layer of hydrated silica gel, which was composed of Si(OSi)₃O^- units, provided the negatively charged sites that induced or promoted the precipitation of calcium phosphates, which led to apatite nucleation and crystallization.