UV irradiation and aging effects on nanoscale mechanical properties of ultra high molecular weight polyethylene for biomedical implants H. Fouad Mohamed 1 , A.-H. I. Mourad* 2 and D. C. Barton 3 As a result of its relatively high strength, high chemical resistance, low creep and low wear rate, ultra high molecular weight polyethylene (UHMWPE) has been widely used as the ‘soft’ articulating surface for total hip and knee arthroplasty. However, for long term artificial joint replacements, accelerated wear as a result of aging of UHMWPE is one of the most important problems that can lead to joint failure. Therefore, the present work is focused on investigating the effects of thermal and serum aging and UV irradiation dose on the nanomechanical properties (elastic modulus, hardness and visco-elasticity) of UHMWPE (type GUR410) specimens under different deformation rates. The continuous stiffness measurement (CSM) nanoindentation technique is used in the present work to measure the nanomechanical properties. The results show a considerable increase in the nanomechanical properties with increasing deformation rate. It is also demonstrated that the nanomechanical properties of the thermally and serum aged UHMWPE specimens decrease compared to the virgin specimens, while their visco-elastic behaviour increases. For the UV irradiated specimens, the nanohardness and nano-elastic modulus show an increase with irradiation dose especially for small penetration depths. Moreover, a considerable decrease in visco-elastic behaviour was observed for the UV irradiated specimens as a result of the crosslinking effect of the radiation. Keywords: Nanomechanical properties, UV irradiation, Accelerated aging, UHMWPE Nomenclature A c , A c max projected contact area A c max projected contact area at maximum pene- tration depth c 1 …c 8 constants CSM continuous stiffness measurement E, E i , E s elastic modulus, elastic modulus of inden- ter and specimen E r reduced elastic modulus h c , h m actual contact depth, maximum penetra- tion depth H hardness m, n fitting parameters P, P max load and peak load S h max contact stiffness at maximum penetration depth UHMWPE ultra high molecular weigh polyethylene UV ultraviolet b indenter geometry constant51?034 e indenter geometry constant50?75 n i , n s Poisson’s ratio of indenter and specimen Introduction UHMWPE has been used extensively as a bearing surface in knee, ankle, shoulder and hip prostheses and as a prosthetic cruciate ligament. 1 The most frequently implanted type of prosthesis used in total hip arthroplasty is a system in which a metal femoral head rubs against an UHMWPE cup. The choice of UHMWPE depends on the desirable mechanical and physical properties of this material: relatively high strength, low creep, low friction coefficient, low wear rate and good biocompatibility. In spite of these good properties, the biotribological condi- tions at the articular surface can lead to degradation of the UHMWPE especially for long term implants. 2–4 The mechanical properties deteriorate as a result of this material degradation which may lead to enhanced wear and ultimately implant loosening or failure. It is known that polymeric material degradation can be attributed to changes in its chemical structure which can have a strong influence on physical and mechanical properties. 5,6 Moreover, the chemical structure of 1 Department of Applied Medical Science, Community College, King Soud University, Riyadh, Saudi Arabia 2 Current address: Mechanical Engineering Department, College of Engineering, United Arab Emirates University, P.O. Box 17555, Al-Ain, UAE; on leave from: Mechanical Design Department, College of Enginnering – Mataria, Helwan University, P.O. Box 11718 Cairo, Egypt 3 School of Mechanical Engineering, University of Leeds, Leeds LS2 9JT, UK *Corresponding author, email ahmourad@uaeu.ac.ae 346 ß 2008 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 21 February 2008; accepted 2 May 2008 DOI 10.1179/174328908X314370 Plastics, Rubber and Composites 2008 VOL 37 NO 8