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In this investigation, three routes, namely, uniaxial pressing, slip casting and H₂O₂ foaming, were used to fabricate porous hydroxyapatite (HA). Processing parameters in each route were studied, pore characteristics in sintered bodies assessed, and mechanical properties of porous HA evaluated. Scanning electron microscopy, gas pycnometry and mercury intrusion porosimetry were used to assess pore characteristics in terms of porosity, pore size and pore shape. Mechanical properties of porous HA were evaluated using a biaxial testing fixture. The 2³ factorial design method was used to determine the influence of pore characteristics on mechanical properties. It was shown that pore characteristics were dependent on the manufacturing route, processing parameters, porosifier and the amount of porosifier. In the uniaxial pressing and slip casting routes, porosity, pore size and pore shape could be controlled using different porosifiers. Porosifiers were able to pass their geometrical characteristics to the pores they formed. Although H₂O₂ foaming was the simplest route and large pores could be formed through this route, pore characteristics were not easily controllable. It was found that porosity, pore size and pore shape all had effects on mechanical properties of sintered products. The interaction of pore size and pore shape affected mechanical properties in that it caused mechanical properties to vary differently according to pore shape (or pore size) when pore size (or pore shape) was at different levels.

Hydroxyapatite (HA) reinforced high density polyethylene (HDPE) composites (HAPEX™) have been developed as a bone analogue for medical applications. Conventionally processed HAPEX^TM containing 40vol% of HA possesses a stiffness approaching the lower bound of human cortical bone and is now used in minor- or non- load bearing areas in patients. In order to improve the mechanical properties of HAPEX™ and hence extend its application into major load bearing areas, HAPEX™ with various amounts of HA was hydrostatically extruded at different extrusion ratios. The extruded rods were subsequently tested under tension or bending. Their structure as well as fracture surfaces were examined under an SEM. The molecular mass of the HDPE matrix was analyzed at each processing stage. DSC thermograms were also obtained to study the effects of hydrostatic extrusion. It was found that the uniform distribution of HA in the HDPE matrix achieved by compounding was not altered by hydrostatic extrusion, but the average molecular weight of HDPE was reduced. DSC results indicated polymer chain alignment along the extrusion direction. Mechanical properties of HDPE and HAPEX™ were substantially increased by hydrostatic extrusion. It was evident that the higher the extrusion ratio, the stronger and the stiffer the HAPEX™ rod. Hydrostatically extruded HAPEX™ containing 40vol% of HA possessed a Young’s modulus of 11.4GPa and fracture strength of 91.2MPa, which are within the bounds for mechanical properties of human cortical bone. This clearly shows the potential of HAPEX™ for major load bearing applications.

A variety of bioactive particle filled polymers have been developed for tissue replacement since the early 1980s. In this investigation, hydroxyapatite (HA) reinforced polysulfone (PSU) composites were produced and evaluated for potential medical applications. Composites containing up to 15vol% of HA were studied at the preliminary stage. The HA/PSU composites were made through a standardised procedure by incorporating a commercially available HA powder into a polysulfone resin of medium viscosity that is suitable for injection moulding. Compounded materials and injection moulded parts were assessed using a variety of techniques. It was found that HA particles were well dispersed in the PSU matrix after the compounding process. Defect-free composite samples could be obtained by injection moulding. Thermogravimetric analysis (TGA) verified volume percentages of HA in the composites. Differential scanning calorimetry (DSC) results indicated that the glass transition temperature (Tg) of the polymer matrix was not affected by the incorporation of HA. It was shown through dynamic mechanical analysis (DMA) that the storage modulus of the composite was increased with an increase in HA content below ~185°C which is Tg of the polymer, while tan δ was maintain at nearly the same level for all composites. It was established that water uptake reached an equilibrium after 7 days’ immersion in distilled water for all composites and unfilled PSU. It was found that after 7 days’ immersion in distilled water, the storage modulus of HA/PSU composite was decreased less than that of unfilled PSU.