This research establishes the fundamental link between microstructure and cyclic damage mechanisms in advanced polymers used in structural and medical applications. An illustration of this problem with worldwide significance is found in total joint replacements where ultra-high molecular weight polyethylene (PE) is used as the bearing surface and serves to replace diseased or damaged cartilage. In this application, PE sustains large cyclic contact stresses as the joint is articulated and the polymer is set in motion against a polished metal or ceramic component. In these prostheses, the polyethylene component usually articulates against a metallic or ceramic component, which leads to the generation of wear debris. Submicrometer size particulate debris is known to cause bone resorption and implant loosening, necessitating early revision surgery to replace the implant. In recent years, radiation crosslinking has emerged as a processing technique that can dramatically decrease the generation of particulate wear of polyethylene. This has led to its implementation in the processing of acetabular cups for total hip replacement prostheses. However, there are concerns about its use in knees and shoulders where high contact stresses are expected. This concern is valid as radiation crosslinking decreases certain mechanical properties, such as ultimate tensile properties, fracture toughness and resistance to fatigue crack propagation. Fatigue strength is the most important property from a clinical standpoint since these load-bearing components are subjected to cyclic loads. In this study, the investigator and her collaborator are using novel processing conditions that combine radiation crosslinking with high pressure, gas assisted processing techniques to improve the performance of PE as a bearing material. The processing of polyethylene using a pressurized soluble gas leads to improvement in fusion of polyethylene resin particles and improves tensile, fracture and fatigue properties. This is coupled with crosslinking that provides wear resistance. In this work, the researchers (I) fabricate bulk components of polyethylene using (a) radiation crosslinking, (b) gas-assisted processing utilizing carbon dioxide and an inert diluent and (c) high pressure processing in the presence of an inert gas, and combinations thereof; (II) characterize the morphology at the nano- and micro- scales and (III) evaluate concomitant mechanical properties of all types of polyethylene by conducting short term tensile and fracture toughness tests, long-term fatigue tests, and measurement of the tribological properties at nanoscale and microscale using nanoindentation and wear tests. The novelty of this interdisciplinary work stems from multiscale experimental studies that yield insight into the fundamental mechanisms of fatigue, fracture and wear in advanced polymers such as PE. The intellectual merit of this work is the systematic evaluation of morphology and micromechanisms of fatigue, fracture, and wear at the nanoscale and microscale levels. The broad impact of this work extends to clinical orthopedics, polymer materials science, mechanical engineering and bioengineering. This study involves the synergy of bioengineering, mechanical engineering, materials science and medicine, and provides educational opportunities at all levels including undergraduates, graduates, postdocs, and faculty.
