Influence of the non-bridging oxygen groups on the bioactivity of silicate glasses

Abstract

The effect of the composition and bonding configuration of the bioactive silica-based glasses on the initial stage in vitro bioactivity is presented. Information of the IR active Si–O groups of glass in the system SiO₂–P₂O₅–CaO–Na₂O–K₂O–MgO–B₂O₃ was obtained by fourier transform Infrared (FTIR) spectroscopy. Two different bands associated to non-bridging oxygen stretching vibrations (Si–O–1NBO and Si–O–2NBO) and a gradual shifting of the bridging oxygen stretching vibration (Si–O) have been observed and evaluated. Both effects are attributed to a decrease of the local symmetry originating from the incorporation of alkali ions into the vitreous silica network. The Si–O–NBO(s)/Si–O(s) absorbance intensity ratio increases with a gradual incorporation of the alkali ions (diminution of SiO₂ content) following a linear dependence up to values close to 50 wt % of SiO₂. In vitro test analysis by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDXA) showed a correlation between the amount and type of the non-bridging oxygen functional groups and the growth of the silica-rich and CaP layers. It was found that a minimum concentration of Si–O–NBO bonds in the glass network is required in order to have an efficient ion exchange and dissolution of the silica network. Finally, the bioactivity of the glass is favored by the presence of the Si–O–2NBO groups in the glassy network. The role of these functional groups in the dissolution of the silica network through the formation of silanol groups and the adsorption of water is discussed.