Characterization and tribology of PEG-like coatings on UHMWPE for total hip replacements

A crosslinked hydrogel coating similar to poly(ethylene glycol) (PEG) was covalently bonded to the surface of ultrahigh molecular weight polyethylene (UHMWPE) to improve the lubricity and wear resistance of the UHWMPE for use in total joint replacements. The chemistry, hydrophilicity, and protein adsorption resistance of the coatings were determined, and the wear behavior of the PEG-like coating was examined by two methods: pin-on-disk tribometry to evaluate macroscale behavior, and atomic force microscopy (AFM) to simulate asperity wear. As expected, the coating was found to be highly PEG-like, with approximately 83% ether content by x-ray photoelectron spectroscopy and more hydrophilic and resistant to protein adsorption than uncoated UHMWPE. Pin-on-disk testing showed that the PEG-like coating could survive 3 MPa of contact pressure, comparable to that experienced by total hip replacements. AFM nanoscratching experiments uncovered three damage mechanisms for the coatings: adhesion/microfracture, pure adhesion, and delamination. The latter two mechanisms appear to correlate well with wear patterns induced by pin-on-disk testing and evaluated by attenuated total reflection Fourier transform infrared spectroscopy mapping. Understanding the mechanisms by which the PEG-like coatings wear is critical for improving the behavior of subsequent generations of wear-resistant hydrogel coatings.