Thermodynamics of non-bridging oxygen in silica bio-compatible glass-ceramics

Correlations between the structural properties of Na2O-CaO-SiO2 glasses characterized by the activity of oxygen ions and the bioactivity were examined by comparing the compositional dependence of the structural parameters calculated on the basis of a thermodynamic consideration with that of the bioactivity. A simple model of characterizing the glass structure by considering the bridging and non-bridging oxygen ions was employed as the first step for this purpose. Further detailed thermodynamic analysis on the anionic constitution in the glass was performed and the compositional dependences of the relative proportions of bridging, non-bridging and free oxygen ions were calculated. The bioactive region corresponded to the compositional region characterized by the higher relative proportion of non-bridging oxygen ions with co-existing an appreciable concentration of bridging oxygen ions, suggesting a possible important role of the non-bridging oxygen ions on the surface chemical process of bone-like apatite layer formation.