The purpose of this study involves reducing the friction coefficient of cross-linked polyethylene (CLPE) by optimizing the grafting thickness of zwitterionic poly(2-(methacryloyloxy) ethyl dimethyl-(3-sulfopro pyl) ammonium hydroxide) (PMEDSAH) via a photo-induced free radical polymerization process. Cross-sectional TEM images and XPS spectra indicated that the PMEDSAH layers were successfully grafted on the CLPE. The grafting thickness increased when UV-irradiation time, UV-irradiation energy, and MEDSAH monomer concentration increased. The contact angle data indicated that the PMEDSAH layers successfully imparted hydrophilicity (below 20 degrees) on the CLPE surface. The friction coefficient test and the resulting 3-D confocal images confirmed a significant decrease in the friction coefficient with increases in the grafting thickness. A minimum grafting thickness of 200 nm PMEDSAH was required to minimize the friction coefficient in an aqueous medium. However, a PMEDSAH-grafting thickness of approximately 40 nm was sufficient to reveal the minimized friction coefficient in simulated body fluids (ionic buffer solution) due to the salt-induced hydration effect. The results defined the minimum required thickness of PMEDSAH grafting to minimize the friction coefficient on CLPE surface that can potentially be applied for artificial hip joints. (C) 2018 Elsevier B.V. All rights reserved.