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Queen Mary University of LondonQueen Mary University of London
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School of Engineering and Materials Science
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PhD Thesis: Fabrication and characterisation of pourous hydroxyapatite

Author: SHAW, John

Year: 1996

Supervisor(s): Bill Bonfield

Hydroxyapatite is a calcium phosphate compound which closely resembles the mineral component in bone and is consequently biocompatible in vivo. When implanted, the presence of porosity permits bone ingrowth, anchoring the prosthesis in place.
Porous hydroxyapatite prepared by three ceramic processing routes was characterised in the present thesis. The first preparation method, termed under-sintering, involved cold pressing discs of hydroxyapatite powder and sintering the discs at temperatures (1100-1350°C) up to and above that required for 100% densification (1260°C). The second technique, graphite burn-out, involved casting an aqueous slip of graphite and hydroxyapatite powders into a porous mould. The dried green body was sintered in air at 1250°C, the graphite particles burning out to generate pores. In the third method, designated the hydrogen peroxide route, H2O2 was catalytically decomposed in an aqueous hydroxyapatite slip, while simultaneously drying the slip. Oxygen bubbles liberated from the H2O2 became entrapped in the green body forming pores.

The largest pore size in the under sintering, graphite and H2O2 routes was, respectively, 4-5 m, 100 m and 2000 m. X-ray diffraction and Fourier Transform infrared spectroscopy indicated the presence of pure, single phase hydroxyapatite. Biaxial failure stress, , and elastic modulus, E, varied with respect to porosity P as follows

= (80.6MPa).e-4.91P; R = 0.88

and

E = (89.5GPa).e-5.1P; R = 0.92.

Fracture toughness and Vicker's hardness were also investigated. It was concluded the percentage porosity was the strongest factor in influencing mechanical properties, while stress concentration due to pore shape appeared to be of secondary importance.