EPR and kinetic studies of overall free radical decay in Gamma-irradiated UHMWPE used in joint arthroplasty

Medical-grade UHMWPE is widely used in an artificial joint and is commonly treated with γ-irradiation for sterilization and crosslinking. A vacuum annealing treatment is essential in eliminating the radicals generated by γ-irradiation and in maintaining the long-term stability of the material. It is important to better understand the kinetics of radical decay in an annealing process. In this study, the kinetics of overall radical decay in a γ-irradiated UHMWPE with doses of 25, 50, and 100 kGy at various temperatures was investigated. The overall free radical concentration was monitored by EPR technique and found to exhibit a fast-to-slow transition over time, which has been successfully simulated by a parallel 2nd-order reaction model proposed herein. The model includes two simultaneous but isolated reactions to describe the fast radical decay in amorphous and the slow radical decay in crystalline regions. Results showed that irradiation dose has limited effect on reaction rate constant and their activation energies in amorphous and crystalline regions are approximately 3-4 and 12-14 kcal/mol, respectively. The current parallel 2nd-order reaction model can be served as a useful tool in predicting the residue radical level of a vacuum annealing process.