Effects of Si speciation on the in vitro bioactivity of glasses.

The surface reactivity of glasses belonging to the (mol%) 31SiO2-11P2O5-(58-x)CaO-xMgO series, with x ranging from 0 to 32, was studied in Kokubo's simulated body fluid (SBF). Scanning electron microscopy, inductively coupled plasma spectroscopy and Fourier transform infrared spectroscopy were used to characterise the glass surface and the SBF compositional changes. All glasses develop surface layers rich in silica and calcium phosphate. An increasing surface activity with increasing MgO/CaO ratio was observed. In a previous investigation using magic-angle spinning nuclear magnetic resonance it was found that there is an increasing abundance of Q0 species in the glass structure with increasing MgO content. The present work shows that, when immersed in SBF, Q0-rich glasses are easily leached to form a silica gel layer. It is concluded that MgO in the glass indirectly improves the early stages of mineralisation by favouring Q0 speciation. This mechanism plays an important role in glass bioactivity.