Ga-Modified (Si-Ca-P) Sol-Gel Glasses: Possible Relationships between Surface Chemical Properties and Bioactivity

In vitro bioactivity features of a Ga-modified sol gel Si-Ca-P glass (SGGa) were investigated, in comparison with a plain ternary Si-Ca-P system (SG). Reaction/dissolution of the glass at increasing soaking times in simulated body fluids (SBF) and the consequent growth of an apatite-like layer, monitoring bioactivity, were studied by employing a variety of chemical and physical techniques. The growth of a crystalline apatitic layer at the Ga-modified-glass/SBF interface is severely delayed with respect to the Ga-free glass, and the reasons for it have been looked for in the dramatic changes induced, at the glass/SBF interface, by the presence of the Ga2O3 component. In situ Fourier transform infrared spectroscopy allowed to describe the nature/structure of surface terminations for the two glasses and to reveal/quantify the acidic strength of different Ga species exposed at the SGGa glass surface. 2,6-Dimethylpyridine and carbon monoxide were employed as molecular probes to reveal Bronsted and Lewis acidity. At the surface of the Ga-modified glass, both Bronsted and strong Lewis acidic sites are present. The enhanced surface acitiy of SGGa glass, with respect to the plain glass SG, has been proposed to be responsible for the slower glass dissolution in SBF and for the delayed deposition/crystallization of an apatite-like layer at the glass/SBF interface.