The effect of strontium substitution on apatite-wollastonite glass-ceramics

The aim of this work was to synthesise novel strontium containing glass-ceramics based on the apatite-wollastonite system. Parent glasses were formed via the melt quench route with strontium being substituted for calcium according to: 35.4SiO2•7.1P2O5•0.4CaF2•7.1MgO• (49.9-X)CaO•XSrO, where X = 6.2, 12.5, 18.7 or 24.9. Glass frits were ground and sieved and size distribution shown to be constant. Successful substitution of strontium into the parent glass was confirmed with energy dispersive x-ray (EDX) analysis, x-ray diffraction (XRD) and density measurements Glass rods were successfully cast for all the compositions tested. The x-ray attenuation of the material increased with increasing strontium substitution in line with theoretical values.
Differential scanning calorimetry (DSC) revealed two major exotherms, TP1 and TP2. TP1 moved to lower temperatures with increasing strontium substitution while TP2 moved to higher temperatures. XRD and Rietveld refinement identified TP1 as corresponding to fluorapatite and TP2 as to wollastonite, both phases took in strontium following substitution. Pseudowollastonite formed at higher strontium contents. The weight percentage of wollastonite decreased with increasing strontium substitution. Scanning electron microscopy (SEM) and EDX revealed the bulk morphology and composition of the material’s phases.
Porous scaffolds were formed by sintering the parent glasses in carbon moulds. Increasing strontium substitution shifted sintering from a solid state mechanism towards liquid phase resulting in a loss of porosity. In all but the highest substitution, the flexural strength decreased with increasing strontium.
Simulated body fluid (SBF) testing indicated an increase in solution pH with increasing strontium substitution and a shift in the ability of the material to form a surface apatite layer. Human bone mesenchymal stem cells (hMSCs) were able to attach, expand and differentiate on the materials. Strontium doping was shown to significantly affect the osteogenic capacity of apatite-wollastonite glass-ceramic.