Nano and Microscale Mechanisms of Fatigue, Fracture and Wear in Polyethylene

聚乙烯疲劳、断裂和磨损的纳米和微尺度机制（ei: Lisa Pruitt，加州大学伯克利分校生物工程与机械工程系）合作者：Anuj Bellare，哈佛医学院骨科学系。摘要本研究建立了用于结构和医疗应用的先进聚合物的微观结构和循环损伤机制之间的基本联系。在全关节置换术中发现了具有世界意义的这个问题的一个例证，其中超高分子量聚乙烯（PE）用作承载表面，用于替换患病或受损的软骨。在这种应用中，PE承受较大的循环接触应力，因为接头是铰接的，并且聚合物与抛光的金属或陶瓷部件一起运动。在这些假体中，聚乙烯组件通常与金属或陶瓷组件接合，从而导致磨损碎片的产生。已知亚微米大小的颗粒碎片会导致骨吸收和种植体松动，因此需要早期翻修手术来更换种植体。近年来，辐射交联作为一种可以显著减少聚乙烯颗粒磨损产生的加工技术而出现。这导致其在全髋关节置换术中髋臼杯的加工中得以实施。然而，它在膝盖和肩膀上的使用也令人担忧，因为那里预计会有很高的接触应力。这种担忧是有效的，因为辐射交联会降低某些机械性能，如极限拉伸性能、断裂韧性和抗疲劳裂纹扩展能力。从临床角度来看，疲劳强度是最重要的特性，因为这些承重部件受到循环载荷。在这项研究中，研究者和她的合作者正在使用新的加工条件，将辐射交联与高压、气体辅助加工技术相结合，以提高PE作为轴承材料的性能。采用加压可溶气体对聚乙烯进行加工，可以改善聚乙烯树脂颗粒的融合，提高拉伸、断裂和疲劳性能。这与提供耐磨性的交联相结合。在这项工作中，研究人员(I)使用(a)辐射交联(b)利用二氧化碳和惰性稀释剂的气体辅助处理(c)在惰性气体存在下的高压处理及其组合来制造聚乙烯的散装组件；（II）表征纳米和微观尺度上的形态特征；（III）通过进行短期拉伸和断裂韧性测试、长期疲劳测试，以及使用纳米压痕和磨损测试测量纳米和微观尺度上的摩擦学性能，评估所有类型聚乙烯的伴随机械性能。这项跨学科工作的新颖性源于多尺度实验研究，这些研究对PE等先进聚合物的疲劳、断裂和磨损的基本机制产生了深入的了解。这项工作的智力价值在于在纳米尺度和微观尺度上对疲劳、断裂和磨损的形态和微观机制进行了系统的评估。这项工作的广泛影响延伸到临床骨科，高分子材料科学，机械工程和生物工程。本研究涉及生物工程、机械工程、材料科学和医学的协同作用，并提供包括本科生、研究生、博士后和教师在内的各级教育机会。

Nano and Microscale Mechanisms of Fatigue, Fracture and Wear in PolyethylenePI: Lisa Pruitt, Departments of Bioengineering and Mechanical Engineering, UC BerkeleyCollaborator: Anuj Bellare, Department of Orthopedic Surgery, Harvard Medical SchoolAbstractThis 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.