Optimisation of hydroxyapatite (HAp) for orthopaedic application via the chemical precipitation technique
Kehoe, Sharon (2008) Optimisation of hydroxyapatite (HAp) for orthopaedic application via the chemical precipitation technique. PhD thesis, Dublin City University.
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Hydroxyapatite, (HAp), Ca₁₀(PO₄)₆(OH)₂, is a naturally occurring mineral found in the inorganic component of human bone and enamel. The constituent elements of HAp are primarily calcium and phosphorous, with a stoichiometric calcium to phosphorous ratio of 1.667 capable of promoting intimate bone growth onto femoral implants. HAp is rendered a bioactive material since it has the ability to promote such growth with rapid fixation in what is also considered an osteoconductive process. The performance, lifespan and quality of the resultant biological coating in vivo is largely dependent on the coating morphology, phase composition, particle size and crystallinity of the powders pre-coating application. The present study aims to synthesise phase pure HAp powders via the wet chemical precipitation technique, in order to evaluate the critical process parameters and their effect (main and interaction) on controlling the final HAp powder characteristics, such as, phase composition, purity, crystallinity, crystallite size, lattice parameters, particle size and particle size distribution. These powders were synthesised with a view to satisfying regulatory requirements. These effects were quantified using Design of Experiments (Design Expert) to develop mathematical models, in terms of the chemical precipitation process parameters. HAp possessing optimum powder characteristics for orthopaedic application via the thermal plasma spray technique can therefore be prepared using the following chemical precipitation process parameters: reaction temperature 60ºC, ripening time 48h and stirring speed 1500rpm using high reagent concentrations. In the case of HAp synthesis at a lower reaction temperature, synthesis under a controlled environment (inert atmosphere) is desirable in order to achieve a high level of crystallinity: with an increase from 86.32% synthesised at 20ºC to 95.19% synthesised at 60ºC. The presence of an inert atmosphere appears less critical at synthesis under increased levels of reaction temperature. Ripening time and stirring speed significantly affect the final phase purity. An increase in both the ripening time (0.5 - 48h) and stirring speed (600 - 1500rpm) can result in an increase of purity by 77.74% (21.15 – 98.89%). Crystallite size, lattice parameters and mean particle size were also optimised within the research to find desired settings to achieve results suitable for FDA regulations.
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