Nanomechanical and tribological characterization of the MPC phospholipid polymer photografted onto rough polyethylene implants

Grafting biomimetic polymers onto biomaterials such as implants is one of the promising approaches to increase their tribological performance and biocompatibility and to reduce wear. In this paper, poly(2-methacryloyloxyethyl phosphorylcholine) (p(MPC)) brushes were obtained by photografting MPC from the rough surface of ultra high molecular weight polyethylene (UHMWPE) joint implants. Such substrates have a high roughness (Ra ∼ 650 nm) which often has the same order of magnitude as the brush thickness, so it is very difficult to estimate the vertical density profile of the grafted content. The quality of the p(MPC) grafting was evaluated through a wide range of characterization techniques to reveal the effectiveness of the grafting: atomic force microcopy (AFM) imaging and force spectroscopy, contact angle, SEM/EDX, and confocal microscopy. After testing the methods on smooth glass substrate as reference, AFM nano-indentation proves to be a reliable non destructive method to characterize the thickness and the mechanical properties of the p(MPC) layer in liquid physiological medium. Tribological measurements using a homemade biotribometer confirm that, despite heterogeneity thickness (h = 0.5–6 μm), the p(MPC) layer covers the roughness of the UHMWPE substrate and acts as an efficient lubricant with low friction coefficient and no wear for 9 h of friction.