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Network Connectivity and Bio-Mineralization of 0.45SiO2–(0.45-x)MgO–xK2O–0.1(3CaO:P2O5) Glasses

This work investigates the structure of phosphosilicate glasses from the SiO2MgO K2O3CaO.P2O5 system, as determined by 29 Si and 31 P MAS-NMR. According to the 29 Si NMR, the actual network connectivity of the glasses generally exhibits a tendency for decreasing with increasing the alkali additive content. 31 P NMR shows that the glass structure includes orthophosphate groups (PO4 3) closer to Ca 2+ ions than to Mg 2+ ions, which are probably deeply inserted within the silicate network. Structural features were related with acellular bioactivity performance, in terms of in vitro capability of glass surface for inducing apatite mineralization in simulated body fluid. Needle-like apatite precipitates at the surface of the investigated bulk glass samples. A relatively flat and poorly crystallised hydroxyapatite (HAp) layer forms on the Kenriched glasses. For glasses without or with smaller K content, nearly amorphous octacalciumphosphate (OCP) seems to predominate over HAp. In this case apatite balls, built up from thin needles, are spread over the glass surface. Both the crystallinity degree and the thickness of the apatite precipitates, as observed by SEM, seem to evolve with increasing K content, consistently with the decrease in the average network connectivity detected by NMR. Introduction Silicate glass structure generally comprises tetrahedral Si-centred units of [SiO4 4]. Depending on chemical composition, in terms of elements and their proportion in the glass, tetrahedra can appear isolated or covalently connected with one, two, three or four atoms of oxygen in their neighbouring tetrahedra. Network connectivity can thus generally range from completely free tetrahedra to fourfold connected units. The network connectivity was shown to be related with the surface reactivity of bioglasses in in vivo situations [1]. It is now generally accepted that the same kind of apatite layer that acts as a bonding interface between a bioactive glass and the live bone [2], can be obtained on the surface of that glass after immersion in an acellular simulated body fluid (SBF) [3]. The possibility of studying the surface behaviour of glasses in synthetic inorganic media and relate it with the basic silicate glass structure leads to the challenge of modifying the bioactive properties of glasses even before the in vivo essays. The glasses investigated here had nominal molar compositions of 0.45SiO2(0.45-x)MgO xK2O0.1(3CaO.P2O5) (x=0, 0.045 and 0.09). For this work a short glass series was built from K2O for MgO substitutions corresponding to the ratios K2O/(K2O+MgO)=0.0, 0.1, 0.2. The main purpose was to investigate the influence of K2O additions on the structure and in vitro mineralization of those glasses.