New Sol-Gel Derived Bioactive Glasses and Organic/Inorganic Hybrids for Bone Regeneration

Sol-gel derived bioactive glasses have been considered as one of the most promising materials for bone regeneration. However they are brittle, therefore composites are needed if bioactive materials are to share load with bone. One strategy for production of composites with tailored mechanical properties and congruent degradation rates is the development of inorganic/ organic hybrids. Hybrids are particular types of nanocomposite synthesised by introducing a polymer into the sol-gel process so that the silica and polymer chains interact at the nanoscale. Calcium must be incorporated into glasses and hybrids if they are to be bioactive (e.g. bone to bone). Calcium nitrate is conventionally used in the sol-gel process as the calcium source. However, there are many disadvantages of using it. Calcium nitrate causes inhomogeneity by forming calcium rich regions and it requires high temperature treatment (>400⁰C) to be incorporated into the glass network. Calcium nitrate cannot be used in the synthesis of hybrids where the highest temperature used in the process is approximately 60⁰C. Therefore, a different precursor is needed to improve homogeneity of glasses and for low temperature synthesis of hybrids. In this work, two alternatives were investigated and compared to the conventional approach of using calcium nitrate: calcium chloride, an alternative calcium salt, and calcium methoxyethoxide (CME), a calcium alkoxide. The structure of the gels and glasses were investigated over a range of final processing temperatures from 60⁰C to 800⁰C, corresponding to hybrid and glass process temperatures using advanced probe techniques such as solid state NMR. The temperature at which calcium was incorporated into the network was identified for 70S30C (70 mol% SiO2, 30 mol% CaO) and 58S (60 mol% SiO2, 36 mol% CaO, 4 mol% P2O5) compositions synthesised with the three different calcium precursors. Using calcium nitrate, calcium did not enter the silica network until temperatures greater than 380⁰C were reached. When calcium chloride was used, the calcium did not seem to enter the network at any of the temperatures. In contrast, calcium from CME entered the silica network at room temperature, indicating CME is an improved calcium source for low temperature synthesis. An aim of this work was to synthesise poly(γ-glutamic acid)/ silica hybrids, containing calcium, using calcium chloride and CME. Calcium incorporation was much improved when CME was used and mechanical properties were much improved compared a sol-gel glass or hybrids synthesised with other calcium sources. A hydroxycarbonated apatite (HCA) formed on the hybrids after immersion in simulated body fluid (SBF), indicating bioactivity. Polylactide was also trialled as the organic phase for hybrid synthesis, using polylactide-diol (PLAD). However, synthesis of PLAD/bioactive glass hybrid was not successful as it was difficult to incorporate the functionalised polymer into the sol. Calcium incorporation into the silica network using the sol-gel process is therefore possible but challenging