Computational modelling of the degradation of poly-L-lactide for a bioresorbable polymeric stent
Shine, Rosa Connor
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The interest in biodegradable polymers for the design of bioresorbable stents, temporary vascular scaffolds designed to restore patency to obstructed vessels, has witnessed a dramatic growth over the last ten to fifteen years. While bioresorbable polymeric stents (BPS) offer possibilities to help address the long-term complications (e.g. in-stent restenosis, stent thrombosis) associated with permanent devices, in-vivo degradation behaviours are not yet fully understood. The application of computational modelling, for example finite element analysis (FEA), to predict and analyse BPS degradation behaviour provides a means to investigate in-vivo performance and further enhance BPS design. Current computational modelling techniques for the degradation of BPS predominately focus on the phenomenological aspects of degradation, with little emphasis given to the inherent microstructure changes (bulk degradation, crystallisation) which occur in the degrading polymer. This research aims to advance the computational modelling techniques for examination of BPS degradation behaviours, though development of co-simulation techniques which are applied to evaluate the physio-chemical degradation of BPS and assess the impacts of material, design and degradation product boundary condition on the physio-chemical degradation, and on the subsequent mechanical performance and scaffolding ability of the device. Physio-chemical degradation of BPS geometries and materials is simulated through adaptation of the heat equation in FE. Model predictions reveal a significant dependency of degradation on the imposed degradation product boundary conditions. Predictions indicate that BPS design does not have a significant impact on molecular weight reduction rates; however, material crystallinity and heterogeneities in crystallinity emerge as key contributors to device performance. Consideration of the device mechanical boundary conditions shows a reliance of the scaffolding ability of degrading BPS on the imposed initial loading. This research proposes considerations for the modelling requirements surrounding BPS regulatory approval. In conclusion, the work performed in this thesis has led to an enhanced understanding of the in-vivo degradation behaviours and mechanical performance of BPS. This work has generated new insight into the expected clinical performance of BPS and presents a solid framework for the development of further design and analysis techniques for BPS.