The Mechanical Performance of Permanent and Bioabsorbable Metal Stents
Grogan, James A.
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Developments in metallic coronary stents are toward permanent devices with thinner struts and bioabsorbable devices. To facilitate the development of these promising technologies an improved understanding of thin-strut and bioabsorbable stent mechanical performance is important. The work presented in this thesis uses computational mechanics to investigate the mechanical performance of corroding bioabsorbable metal stents and computational micromechanics to investigate the performance of thin metallic struts undergoing large plastic deformation. The short-term mechanical performance of stents consisting of a range of bioabsorbable and permanent metals is first investigated using typical stent modelling approaches. It is found that devices consisting of bioabsorbable metals can match the short-term scaffolding performance of permanent stents, however the former has a greater risk of fracture. The mechanical performance of permanent and bioabsorbable stent struts undergoing large deformations is then investigated in more detail, using micromechanical analyses based on crystal plasticity theory. Lower limits on suitable strut thicknesses for bioabsorbable and permanent stents are identified, based on predictions of microstructure-dependent ductility size effects. A corrosion model is developed for bioabsorbable metal stents and is calibrated and validated based on immersion experiments on thin bioabsorbable alloy foils. The model is used to predict the scaffolding performance of stents undergoing corrosion in the body. It is found that the form of corrosion undergone by the device in the body has a strong influence on scaffolding performance. In particular, it is predicted that experimentally observed localized corrosion leads to a significant decrease in scaffolding duration relative to an ideal uniform corrosion behaviour, pointing to recommendations for future alloy development for such applications.
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