Developing a thermal grafting process for zwitterionic polymers on cross-linked polyethylene with geometry-independent grafting thickness

Abstract

To overcome the drawbacks of the UV grafting method, an alternative, thermal grafting process is suggested. The uniform and geometry-independent grafting of zwitterionic polymers on curved cross-linked polyethylene (CLPE), which is used in artificial hip joints, surface was successfully achieved. Poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) and poly(2-(methacryloyloxy)ethyl)dimethyl(3-sulfopropyl)ammonium hydroxide) (PMEDSAH) were grafted on the CLPE by two methods: a UV-based process and a thermal process. The thermal method yielded zwitterionic surfaces with similar hydrophilicities and graft layer thicknesses to those prepared via the UV grafting method. The X-ray photoelectron spectra and surface zeta potential results showed that the PMPC and PMEDSAH layers were successfully grafted onto the CLPE surface. In addition, 3-D confocal microscopy, as well as friction and wear volume tests, confirmed that there was a significant decrease in the friction coefficient and wear, which indicates that the thermal grafting method can successfully substitute the UV grafting method. The thermally grafted polymer showed uniform graft layer thickness on the curved CLPE surface, whereas the UV-grafted polymer showed a geometry-dependent heterogeneous graft layer thickness. Thus, we confirmed that the thermal grafting method is advantageous for the preparation of uniform grafting layers on artificial joint surfaces with complicated shapes. STATEMENT OF SIGNIFICANCE: Formation of uniform grafting thickness of the zwitterionic polymers on the implant materials is a very important issue in the field of biomaterials. In this study, a thermal grafting process was developed for the formation of the uniform grafting thickness of the zwitterionic polymers on the curved cross-linked polyethylene (CLPE) surface used in artificial hip-joint. This method yielded zwitterionized CLPE surfaces with similar hydrophilicities and friction coefficient to those prepared via the UV grafting method which has been widely used process to modify the implant surfaces. Furthermore, the thermally grafted CLPE surface showed geometry-independent uniform grafting thickness on the curved CLPE surface while UV-grafted one showed uneven grafting thickness. This grafting method could help the development of complex, personalized, and biocompatible artificial liner surfaces.

Keywords: Artificial joint; Cross-linked PE; Friction coefficient; Surface-initiated polymerization; Thermal grafting; UV grafting; Zwitterion.