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  • Publication
    Investigations of Tribological Performance of Structured Surfaces on Bioimplants
    (University College Dublin. School of Mechanical and Materials Engineering, 2022)
    Bioimplants are man-made medical devices to replace the malfunctional natural joints. In the past six decades, total joint replacement is credited as one of the most successful surgical operations. Metal alloy matching with polymer is the most common material combination used in the bearing parts of artificial joints. However, most commercial products have a relatively short in-vivo lifespan due to the unsatisfactory tribological performance of bearing parts. This study aims at increasing the longevity of bioimplants by modifying the surface topographies of bearing parts, including surface roughness and surface texturing. This thesis starts with the motivation of carrying out this project, followed by a comprehensive literature review. In Chapter 3, the validity of molecular-mechanical frictional theory is tested for the bioimplant application using the pin-on-disk tribometer. By a long-term sliding test (10 km) and the dynamic analysis, the study firstly highlights the importance for the metallic bearing surface to keep its original surface finish after implantation other than only getting a superfinished surface before implantation. As a result, surface texturing approach is proposed to achieve this goal. In Chapter 4, the impact of four potential working mechanisms related to surface texturing in bioimplants are investigated. An important finding on the role of hydrodynamic pressure on the tribological performance of textured implants is presented: the extra lifting force provided by hydrodynamic pressure is negligible. This unique property distinguishes the bioimplants application from other conventional bearing systems. Further numerical simulation and experimental experiments attribute this novel finding to the working conditions of implanted joints: slow sliding speed and low viscosity lubricating solution. Alternatively, a new understanding, namely squeezing effect, is established to explain the increased thickness of lubricant film which helps to improve the tribological performance of textured bioimplants. Meanwhile, a novel failure mechanism, interlocking effect (stress concentration and two-body abrasive wear), is put forward to explain why some pattern designs, such as sharp-corner structure, are detrimental to the bioimplants. Afterwards, a technical solution, namely round-corner structure, is developed to resolve the interlocking effect. In Chapter 5, sliding orientation is found to play a minor role on the tribological performance of textured bioimplants and this phenomenon is explained by the proposed working mechanism of squeezing effect. Furthermore, the regularly arranged texturing patterns are proved to be more suitable for bioimplants than the irregularly arranged micro patterns. Finally, an orthogonal experiment is designed to reveal the influential level of five main pattern parameters: area density > size > shape > depth > distribution mode. The conclusion is that the optimal pattern design with specific parameters: triangle structure with 200 µm side length, 8-10 µm depth, 10% area density and square distribution mode, can provide the optimized tribological performance. In Chapter 6, a long-term wear experiment with 1 million cycles is carried out to compare the tribological performance between the only polished bioimplants and the ones with optimal structured bearing part. The study confirms that, by applying the optimal structured surface, the in-vivo longevity of polymer-based bioimplants can be effectively increased.
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