Mechanical and tribological characterisations of PEG-based hydrogel coatings on XLPE surfaces

Hydrophilic hydrogel coatings can impart enhanced tribological and antifouling properties to biomedical device surfaces. The influence of crosslinking on the elastic moduli of poly (ethylene glycol) (PEG) hydrogels is well established, however, the effect of crosslinking on the ability of the hydrogels to form coatings on crosslinked polyethylene (XLPE) substrates is not fully understood, nor are the mechanics and tribological performance of the resultant hydrogel coated substrates. PEG hydrogels of four different crosslinking levels (5, 7.5, 10 and 12.5% crosslinker concentrations) were deposited onto XLPE substrates. Crosslinked matched hydrogel plugs were also manufactured for mechanical analysis. The wear performance and friction evolution of coated pins were assessed against sterilised cobalt chromium discs at a contact pressure of 0.08 MPa under an elliptical orbital motion. The indentation results showed an increase in the elastic modulus with increasing crosslinker concentration, which was augmented further by gamma sterilisation treatment. Hydrogel coated pins exhibited reduced friction levels compared to uncoated pins, and confocal imaging in conjuction with the roughness monitoring indicated that the coatings protected the asperities from being removed. The friction values increased as the tests progressed, in line with the coverage of the hydrogel coating decreasing and forming a hybrid XLPE/gel vs CoCr contact. The 10% cross-linker hydrogel coating produced the lowest friction and wear of all the coatings tested.