Porous bioactive glasses are attractive for use as bone scaffolds. There is increasing interest in strontium containing bone grafts, since strontium ions are known to up-regulate osteoblasts and down regulate osteoclasts. This paper investigates the influence of partial to full substitution of strontium for calcium on the dissolution and phase formation of a multicomponent high phosphate content bioactive glass. The glasses were synthesised by a high temperature melt quench route and ground to a powder of <38 microns. The dissolution of this powder and its ability to form apatite like phases after immersion in Tris buffer (pH 7.4) and simulated body fluid (SBF) was followed by inductively coupled plasma optical emission spectroscopy (ICP), Fourier transform infra red spectroscopy (FTIR), X-ray powder diffraction (XRD) and 31P solid state nuclear magnetic resonance spectroscopy up to 42 days of immersion. ICP indicated that all three glasses dissolved at approximately the same rate. The all calcium (SP-0Sr-35Ca) glass showed evidence of apatite like phase formation in both Tris buffer and SBF, as demonstrated after 3 days by FTIR and XRD analysis of the precipitate that formed during the acellular dissolution bioactivity studies. The strontium substituted SP-17Sr-17Ca glass showed no clear evidence of apatite like phase formation in Tris, but evidence of an apatite like phase was observed after 7 days incubation in SBF. The SP-35Sr-0Ca glass formed a new crystalline phase termed “X Phase” in Tris buffer which FTIR indicated was a form of crystalline orthophosphate. The SP-35Sr-0Ca glass appeared to support apatite like phase formation in SBF by 28 days incubation. The results indicate that strontium substitution for calcium in high phosphate content bioactive glasses can retard apatite like phase formation. It is proposed that apatite formation with high phosphate bioactive glasses occurs via an octacalcium phosphate (OCP) precursor phase that subsequently transforms to apatite. The equivalent octa-strontium phosphate does not exist and consequently in the absence of calcium, apatite formation does not occur. The amount of strontium that can be substituted for calcium in OCP probably determines the amount of strontium in the final apatite phase and the speed with which it forms.

Bragg coherent diffraction imaging to visualize the effects of lysine and magnesium on the internal structures of calcite crystals.

The presence of fluoride in bioactive glass-containing toothpastes can potentially lead to the formation of a fluoridated apatite, which may result in improved clinical effectiveness and durability. However, this should be further tested intra-orally.

Recently, the porous bioactive glasses have attracted a lot of attention for use as scaffolds for tissue engineering bone, such glasses include the high phosphate, strontium containing glass (Stronbone P TM). However, the previous studies suggest that strontium can have a detrimental effect on the ability of apatite-like phase formation of the glass. The previously studied high phosphate all Sr glass showed an unidentified phase rather than an apatite-like phase upon immersion. Octa-calcium phosphate (OCP) is believed to be a precursor phase to apatite, however octastrontium phosphate does not exist. Fluoride is known to knock out the OCP formation and promotes fluorapatite formation. This work presents the incorporation of a small amount of fluoride into calcium/strontium bioactive glasses. Differential 2 scanning calorimetry was used to estimate the glass thermal properties. All of the studied glass compositions were subjected to bioactivity studies in Tris buffer (pH=7.4) for up to 21 days. The initial glasses and the resultant precipitates were analysed using fourier transform infra red spectroscopy, X-ray diffraction and magic angle spinning-nuclear magnetic resonance. The findings showed that all the fluoride containing glasses were amorphous and there was a marked increase in the rate of apatite formation in vitro compared to the equivalent fluoride free glasses, particularly for the all strontium containing glass. This indicates that the presence of fluoride affects the pathway of apatite formation, forming fluorapatite directly instead of via the transformation from OCP to hydroxyapatite. Therefore, fluoride may have potential future clinical applications as an additive to increase apatite formation.