Production and characterisation of hydroxyapatite/multi-walled carbon nanotube composites

Hydroxyapatite (HA) is a biologically active ceramic that is used in surgery to replace bone. While HA promotes bone growth along its surface, its mechanical properties are not sufficient for major load-bearing medical devices. Carbon nanotubes (CNTs), as one of the strongest and stiffest materials available, have the potential to strengthen and toughen HA, thus expanding the range of clinical uses for the material. Furthermore, studies have suggested that the nanotubes themselves possess some bioactive properties. This work sought to develop and characterise HA-CNT composites in four main areas: 1) production and characterisation of green materials, 2) investigation of appropriate sintering atmospheres, 3) evaluation of mechanical properties, and 4) assessment of biological response to in vitro cell culture. HA was synthesised by a precipitation reaction between Ca(OH)2 and H3PO4, and multi-walled CNTs were produced by chemical vapour deposition. Composites were produced by adding the CNTs to the Ca(OH)2 solution as the HA was precipitating. Both as-made (nfCNTs) and acid-treated CNTs (fCNTs) were used to make composites with loadings of 0.5 – 5 wt.% CNTs. The resulting slurry was shear mixed and then processed to make a powder. The powder was then uniaxially pressed into tablets of ~45% theoretical density. Characterisation of the green material with XRD and FTIR found that the primary phase was HA which was well hydroxylated. The powder particles were found to have a bimodal size distribution, and all materials had similar surface areas, as determined by BET. Composites made with fCNTs were found to have a better dispersion of CNTs in the HA matrix and better interaction between the HA and CNTs compared with nfCNT composites. CNTs oxidise at the high temperatures needed to sinter HA, yet water is necessary to prevent dehydroxylation and decomposition of the HA. Using 5 wt.% fCNT composite, fourteen sintering atmospheres were investigated to determine their effect on phase purity, hydroxylation, sintered density, and remaining CNT content after sintering. An atmosphere of CO + H2 bubbled through ice water resulted in optimal properties. Additionally, it was found that increasing the gas flow rate and the number of samples sintered in one batch increased CNT retention. However, this came at the expense of the density of the sintered samples, as composites with a higher CNT content were more porous. To optimise the composite microstructure for mechanical studies, six different sintering time / temperature profiles were examined to determine their effect on density (balanced with CNT retention) and grain size. HA and both nfCNT and fCNT composites with CNT loadings of 0.5, 1, 2 and 5 wt.% were produced using the optimised atmosphere and profile, and then tested to determine tensile strength (using diametral compression) and hardness, and to look for evidence of toughening. It was found that CNTs had little reinforcing effect; instead, mechanical behaviour results were mainly attributed to differences in porosity, due at least in part to the CNTs’ presence. The in vitro cellular response to the materials was examined by culturing human osteoblast-like cells on HA and nfCNT (0.88 wt.%) and fCNT (3.3 wt.%) composites for 12 days. Cells were found to attach and grow well on HA and the nfCNT composite, with slightly enhanced response on the composite. The fCNT composite, on the other hand, showed a decrease in cell viability between days 1 and 12. These results were mainly attributed to the effects of a lower local pH due to remnant acid on the fCNTs and differences in material characteristics, such as CNT loading and surface roughness. This systematic study of the production and properties of HA-CNT composites has resulted in improved understanding of the production and processing of these materials and the effects of a wide range of sintering atmospheres on their characteristics. Additionally, it has yielded interesting preliminary results of their mechanical reinforcement potential and biological behaviour