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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: Characterisation and evaluation of hydroxyapatite- and Bioglass®-reinforced polyethylene composites for medical implants

Author: HUANG, Jie

Year: 1997

Supervisor(s): Bill Bonfield

The microstructure of hydroxyapatite (HA)- and Bioglass®-particles reinforced polyethylene (HDPE) composites (HA/HDPE and Bioglass®/HDPE) was characterised and the mechanical properties, bioactivity and in vitro biocompatibility were evaluated. The median particle sizes of the HA were 4 and 6 m, while those of the Bioglass® were 17, 46 and 56 m, respectively, with volume percentages (Vp) ranging from 0 to 45%. The dispersion and distribution of particles during various stages of processing were assessed with microscopy and image analysis. The ultimate tensile strength of Bioglass®/HDPE composite decreased with filler content, in contrast to a slight increase for HA/HDPE composite. However, the Young's modulus of Bioglass®/HDPE composites increased with filler content and decreasing particles size, as for HA/HDPE.
A significant decrease in the Young's modulus and ultimate tensile strength of Bioglass®/HDPE composite after 6 months in simulated body fluid (SBF) at 37°C was observed, accompanied by an increase in the strain to fracture. This behaviour is different from that of HA/HDPE composite, for which immersion in SBF at 37°C and accelerated ageing at 70°C did not significantly alter the Young's modulus, tensile strength and strain to fracture, demonstrating the advantage of using the insoluble HA particles.

The surface structure of the composites after immersion in SBF was investigated by infrared spectroscopy, X-ray diffraction and scanning electron microscopy. Formation of an apatite layer was observed on the surface of Bioglass®/HDPE composites for the three different particle sizes tested within 7 days of immersion when Vp > 30%, indicating in vitro bioactivity. The apatite formation rate slowed down with decreasing Bioglass® content, but the ability to induce apatite formation persisted. The in vitro biocompatibility of the composites was assessed with a primary human osteoblast-like (HOB) cell culture model. Neither the non-surface reactive HA/HDPE composite, nor the surface reactive Bioglass®/HDPE composite, released any toxic leachables detrimental to cell viability. A 'stimulating' effect on the HOB cell activity was observed with the extract from Bioglass®/HDPE composite.