Calcium phosphate compounds have been widely utilised in the interdisciplinary field of biomaterials research and are used clinically in a number of medical devices. In the present work, calcium phosphate compositions were prepared by two precipitation methods: a classic aqueous precipitation method and a novel biomimetic precipitation method. The aqueous precipitation method was used to produce a range of hydroxyapatite and carbonate-substituted hydroxyapatite compositions that would act as reference materials, but more importantly to allow a method of determining the Ca/P molar ratio of these compositions using X-ray diffraction data. The importance of this was that a simple technique would be required for studying the calcium phosphates produced later by biomimetic methods, as the very low yields prevents traditional methods such as XRF. It was found that Rietveld refinement of XRD data from precipitated apatites that had been heated at between 800 and 1200°C in air provided quantitative phase compositions that could then be used to calculate Ca/P molar ratios. In order to use synthesis conditions that were more similar to in vivo mineralisation, calcium phosphate compositions were directly prepared by using alkali-induced 'biomimetic' precipitation from Simulated Body Fluid (SBF) under controlled conditions. The key parameters that were studied were the effect of precipitation pH, the SBF concentration, the reaction time, the soaking time, and the magnesium and the carbonate ion concentration of the SBF solution. The key finding was that while the precipitated product obtained from 'conventional' SBF was an amorphous calcium phosphate with a Ca/P molar ratio of 1.5, irrespective of the pH, SBF concentration, reaction time or carbonate ion concentration, the gradual removal of magnesium ions from the SBF solution led to the precipitation of firstly a Ca-deficient apatite and eventually (for low Mg concentrations and a Mg-free SBF) a stoichiometric hydroxyapatite with a Ca/P molar ratio of 1.67. This method resulted in very low yields, typically (50 mg from 100 ml of SBF) but control of parameters could yield different phase compositions and could also affect the crystal size and the crystallinity of the product. These observations highlighting the important role of magnesium ions in the SBF on the composition of the calcium phosphate phase that would precipitate raised the question of what effect does SBF magnesium ion content have on the surface apatite precipitation on various biomaterials when incubated in SBF, which is the classic in vitro bioactivity test. Studies that incubated bioactive glass discs and powder (45S5 composition) and dense hydroxyapatite ceramic discs in conventional SBF and an Mg-free SBF showed that the rate of surface apatite precipitation was significantly affected by the Mg content in the SBF. For the case of bioactive glass discs, a surface apatite layer formed in 2 hours when incubated in an Mg-free SBF compared to 24 hours in conventional SBF.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:694692 |
| Date | January 2016 |
| Creators | Thammakan, Nirawan |
| Publisher | University of Aberdeen |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://digitool.abdn.ac.uk:80/webclient/DeliveryManager?pid=230656 |
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