The Structure and Properties of Soluble Phosphate Based Glasses

The recent discipline of tissue engineering has developed innovative degradable and non-degradable (dependant upon application) materials in combination with low toxicity and good biocompatibility. The major focus of these new technologies is to guide the regenerative process. The aim of this project was to take the regenerative tissue healing process a step further by developing a material which not only guides the tissue regeneration process, but also enhances it and has a degradation profile that is tailored to the tissue and on degradation, leaves no toxic or irritating debris behind to cause any tissue reaction. The chosen material was a soluble phosphate based glass, modified with CaO, Na2O, MgO, K2O and/or CaF2. Materials have been developed in order to work as closely as possible to the natural phase of bone, within the limitations imposed by the glass forming process. 5 different glass systems have been synthesised via conventional glass making procedures and the solubility process has been investigated via weight loss experiments and ion and pH measurements. All materials were soluble with different degradation processes, depending on the composition of the glass. Glasses with low CaO content showed a linear relationship between weight loss per unit area and time. Glasses with higher CaO content show an increasing non-linearity in their weight loss behaviour. The pH showed a significant increase in the first stages of degradation, which was explained by cation-exchange processes taking place from the material to the solution and vice versa. The ion concentration in solution was found to increase with time as expected and it mirrored the weight loss curves. Preliminary cell culture tests (MTT tests) using the MG63 human osteoblast cell line were established to test the biocompatibility of the soluble extracts from the different glasses. The tests revealed that glasses with low CaO content, i.e. high solubility, showed reduced proliferation below the control line of tissue culture plastic. Proliferation was however similar to the control line or above for glasses with CaO contents higher than 30 mol%, i.e. low solubility. The best test results, with enhanced proliferation was seen for glass with the highest CaO content. The MTT test results look very promising for materials with a high calcium oxide content, indicating biocompatibility with enhanced cell proliferation. There was also evidence that small amounts of K2O and MgO affected cell proliferation. Structural analysis was carried out using DTA and MAS-NMR spectroscopy. The results from the glasses were found to be in line with X-ray analysis of similar glass-ceramics, which had been analysed in earlier studies. Thermal analysis revealed multi-crystallisation events as evidenced by the presence of one or two crystallisation peaks with more than two or three corresponding melting points. The use of MAS-NMR spectroscopy showed that two species, Q1 and Q2 formed the basis of the glass structure and it was possible to identify the dominant Q2 species as Na4Ca(PO3)6. The Q1 species is represented by (Ca3(PO4)2 or Na2P2O7). However, it was up to the present time not possible to identify the Q1 species unambiguously. The work has shown that it is possible to synthesise biodegradable glasses for hard tissue surgery, whose composition is close to the inorganic phase of bone. Biocompatibility studies have helped to define optimal compositions and it is therefore hoped that the results from this study will contribute towards future implant research utilising soluble glasses