Effect of nitrogen and fluorine on mechanical properties and bioactivity in two series of bioactive glasses

Abstract

Bioactive glasses are able to bond to bone through formation of carbonated hydroxyapatite in body fluids, and fluoride-releasing bioactive glasses are of interest for both orthopaedic and, in particular, dental applications for caries inhibition. However, because of their poor strength their use is restricted to non-load-bearing applications. In order to increase their mechanical properties, doping with nitrogen has been performed on two series of bioactive glasses: series (I) was a “bioglass” composition (without P2O5) within the quaternary system SiO2Na2OCaOSi3N4 and series (II) was a simple substitution of CaF2 for CaO in series (I) glasses keeping the Na:Ca ratio constant. The objective of this work was to evaluate the effect of the variation in nitrogen and fluorine content on the properties of these glasses. The density, glass transition temperature, hardness and elastic modulus all increased linearly with nitrogen content which indicates that the incorporation of nitrogen stiffens the glass network because N is mainly in 3-fold coordination with Si atoms. Fluorine addition significantly decreases the thermal property values but the mechanical properties of these glasses remain unchanged with fluorine. The combination of both nitrogen and fluorine in oxyfluoronitride glasses gives better mechanical properties at much lower melting temperatures since fluorine reduces the melting point, allows higher solubility of nitrogen and does not affect the higher mechanical properties arising from incorporation of nitrogen. The characterization of these N and F substituted bioactive glasses using 29Si MAS NMR has shown that the increase in rigidity of the glass network can be explained by the formation of SiO3N, SiO2N2 tetrahedra and Q4 units with extra bridging anions at the expense of Q3 units. Bioactivity of the glasses was investigated in vitro by examining apatite formation on the surface of glasses treated in acellular simulated body fluid (SBF) with ion concentrations similar to those in human blood plasma. Formation of a bioactive apatite layer on the samples treated in SBF was confirmed by grazing incidence X-ray diffraction and scanning electron microscopy (SEM) combined with energy dispersive X-ray spectroscopy (EDS). The crystallinity of this layer decreases with increasing N content suggesting that N may decrease bioactivity slightly.