Surface Reactions of Mesoporous Bioactive Glasses Monitored by Solid-State NMR: Concentration Effects in Simulated Body Fluid

A bone-mineral-mimicking layer of hydroxycarbonate apatite (HCA) forms at the surface of a bioactive glass on its contact with body fluids. We report a solid-state Si-29 nuclear magnetic resonance (NMR) spectroscopy study of the surface reactions preceding the HCA formation at three CaO-SiO2-(P2O5) mesoporous bioactive glasses (MBGs) with distinct compositions, surface areas, and mesoporous arrangements, during their immersion in simulated body fluid (SBF) out to 30 days. The evolution of the various populations of coexisting silicate species associated with the bulk-arid surface portions of the pore-walls were monitored. The MBGs revealed drastically different surface alterations between the scenarios of low (0.6 g/L) and high (20 g/L) MBG concentrations in the SBF: for the low MBG dose, which is expected to be more relevant for in vivo conditions, all MBGs follow a "universal" dissolution mechanism beyond approximate to 24 h of SBF soaking, regardless of their precise compositions and textural properties. The only essential difference among the specimens occurs during the first hour of soaking when their variable Ca2+ reservoirs are depleted. In contrast, for high MBG concentrations, the surface reactions and their associated silicate network degradation retard for Ca-poor MBGs, whereas the reactions are completely quenched for Ca-rich compositions. These findings rationalize previously reported discrepancies in the correlation between the HCA formation and the MBG composition for distinct concentrations during SBF testing, and simplify future MBG design by identifying which compositional and textural factors are relevant for a rapid and substantial HCA formation in vitro.