A tribological and biomimetic study of PI–CNT composites for cartilage replacement

This article presents an investigation into the possible matching of mechanical properties of a polyimide (PI)–carbon nanotube (CNT) composite system to natural cartilage tissue. Currently used ultrahigh molecular weight polyethylene (UHMWPE) used in total joint replacements presents certain drawbacks due to a mismatch in mechanical and tribological properties with those of a natural bone joint. Natural cartilage tissue is a composite material itself, being composed of collagen fibers, hydrophilic proteoglycan molecules, cells and other constituents. The current investigation attempts to mimic the mechanical and tribological properties of natural cartilage tissue by varying the CNT concentration in a PI matrix. Nanoindentation and pin-on-flat tribological tests were conducted for this purpose. It was found that the coefficient of friction (COF) reached a minimum at a concentration of 0.5% CNT (by volume) when articulated against Ti6Al4V alloy. When articulated against Ti6Al4V alloy in the presence of a lubricant, the minimum COF was obtained at a concentration of 0.2% CNT. The maximum penetration depth under nanoindentation varied with CNT concentration and indicated that the mechanical properties could be tailored to match that of cartilage tissue. A closer investigation into this behavior was carried out using scanning electron, transmission electron, and atomic force microscopy. It was noticed that there is good bonding between the CNTs and polyimide matrix. There was a ductile to brittle transition as the concentration of CNT was increased. Competing interactions between nanotube–matrix and nanotube–nanotube are possible reasons for the deformation and friction behavior identified.