Investigation into the use of carbon nanofilaments in bone repair applications.

The potential of carbon nanofilaments for use in surface modification of implants and as fillers in biocompatible polymer composites was investigated with particular respect to nanofilament size and structure. Carbon nanofilaments were synthesised using chemical vapour deposition or obtained commercially, which provided a range of carbon nanofilament average diameters (13 nm, 134 nm, 142 nm, 155 nm) and structures (platelet, platelet and herringbone, multi-walled nanotubes and vapour-grown nanofibres). The topography and texture of pressed nanofilament substrates was dependent on the nature of the nanofilaments, producing lower micron-scale roughness (Ra) values in the GNF samples (0.5-2.0 µm) compared to the MWNT9 and PR19PS substrates (3-4 µm), but no significant differences in nanoscale roughness (Ra~150 nm). Human osteoblast response to these substrates was measured. Cells attached and spread to substrates with average nanofilament diameters of 134-155 nm (GNF1, GNF3 and PR19PS) rather than 13 nm (MWNT9) after 90 minutes, but proliferated and differentiated greater on the rougher nanotube samples over 14 days (MWNT9 and PR19PS). Investigation of polymer/carbon nanofilament composites revealed the following. Low concentrations of nanofilament addition into poly(ethyl methacrylate)/tetra furfuryl methacrylate reduced the surface roughness of the polymer (Ra: 1.7 µm) by up to 88 % (5 wt% GNF composite), and reduced the storage modulus by 26-68 % of the unfilled polymer (1591 MPa at 37 °C). The electrical resistivity of the composites was significantly reduced due to addition of nanofilaments; all samples reaching percolation just above 10 wt% but with different resistivities (~30 Ω.m at 15 wt% PR19PS, ~10 Ω.m at 15 wt% GNF and ~0.15 Ω.m at 15 wt% MWNT9). Human osteoblast attachment on the PEMA/THFMA composites followed trends in roughness, attaching in higher quantities but with less spreading to rougher surfaces (i.e. higher nanofilament concentrations) on all samples except on the 5 wt% MWNT9 composite, which showed high spreading and attachment. This sample also showed the greatest degree of proliferation and differentiation over 14 days of culture. Faradic stimulation of human osteoblasts was investigated by pulsing 10 µA of electrical current through 5 wt% MWNT9 composite samples for 6 hours daily over 14 days. There was a slight increase in osteoblast proliferation when stimulating the 5 wt% MWNT9 composite sample with pulsed current compared to unstimulated 5 wt% MWNT9 composite controls. The investigation indicated that the size and nature of carbon nanofilaments affected the surface and bulk properties of pressed nanofilament substrates and nanofilament -PEMA/THFMA composites. Human osteoblasts responded to the size of nanofilaments, especially their diameter, but also with respect to their effect on surface roughness. This was thought to be related to their dimensional similarity to extracellular matrix components in bone tissue. Carbon nanofilaments could therefore potentially be used to texture surfaces and improve bulk properties in biomaterials, particularly in total joint components, bone cements, or tissue engineered scaffolds that could also be electrically stimulated to promote osseointegration. This work also instigates further investigation into the toxicity and reinforcing capabilities of carbon nanofilaments.