Thesis (M.S.)--University of Wisconsin--Madison, 1984. / Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 112-114).

No description available.

No description available.

Thesis (Ph. D.)--University of Hong Kong, 2008. / Also available in print.

Thesis (Ph. D.)--University of Hong Kong, 2007. / Also available in print.

Thesis (B.S.)--University of Missouri, School of Mines and Metallurgy, 1904. / G.W. Harris determined to be George William Harris and H.A. Grine determined to be Harry Adam Grine from "Forty-First Annual Catalogue. School of Mines and Metallurgy, University of Missouri." The entire thesis text is included in file. Typescript. Title from title screen of thesis/dissertation PDF file (viewed Feb. 3, 2009).

Thesis (Ph. D.)--University of Hong Kong, 1990. / Includes bibliographical references (p. 200-214).

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.

published_or_final_version / abstract / Dentistry / Doctoral / Doctor of Philosophy

Various aspects of the precipitation behaviour of calcium phosphate in aqueous media have been investigated using seeded growth in conjuction with constant-volume and constant-composition techniques under different physical and chemical conditions. In each case, precipitation was allowed to proceed for up to seven days. The solid precipitates thus obtained were characterised by means of scanning electron microscopy, powder X-ray diffractometry and wavelength dispersive spectroscopy. During these precipitation experiments, the formation of the thermodynamically most stable and most supersaturated phase was invariably preceded by the appearance of less supersaturated precursor phase(s). These precursors subsequently underwent step-wise phase transformation into more stable phases. The preferred precursor and the rates of precipitation and phase transformation were dependent on the physical conditions and the chemical composition of the calcifying medium. Under physiological conditions, precipitation experiments were also carried out with the addition of certain non-collagenous bone-specific bio-chemicals. Phosphoserine dramatically accelerated the precipitation of a large quantity of small plate-like crystals, while osteonectin and phosphatidylserine induced the formation of quasi-cubic crystals at a slow rate. Bone protein extract displayed the strongest inhibitory effect on calcification. Bovine serum albumin showed signs of being irreversibly adsorbed to the crystal surface, thereupon inducing a high degree of calcium deficiency in the precipitate stoichiometry. Using a number of phosphorylated amino acids of different molecular masses, it was found that the processes of precipitation and phase transformation were facilitated by organic molecules whose phosphoryl functional groups were sterically accessible and highly electronegative. However, the acceleration brought about by the presence of a phosphorylated amino acid was maximised at an optimum concentration. The existence of such an optimum was very likely to be consequent of the competition for free calcium ions by the ongoing complexation and precipitation reactions. A model has also been developed to describe and predict the precipitation behaviour of calcium phosphate. The model is based on the Avrami-Johnson-Mehl expression for threedimensional nucleation and growth processes. Appropriate modifications to the original equation have been made, in order to adapt to this multi-ionic aqueous system. The resulting model has been found to describe the actual precipitation process accurately. It has also been applied to systems in which organic additives were present, and has again furnished predictions closely resembling the behaviour as observed experimentally.