An experimental and computational investigation of the mechanics of degradation in polymer bioresorbable scaffolds
Fiuza, Constantino
Fiuza, Constantino
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Publication Date
2023-10-03
Type
Thesis
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Abstract
Drug-eluting stents (DES) have established themselves as the gold-standard in treating arterial blockages, with over 1.3 million stent procedures carried out across Europe in coronary vessels alone. However, these permanent metallic devices remain in the body well beyond the timeframe that their structural role has been completed, which can cause long-term complications, such as strut failure, latestage thrombosis and restenosis. Polymer bioresorbable scaffolds (BRS) showed great potential as the next generation of coronary stents, whereby they would support the vessel during the healing period and subsequently being resorbed into the surrounding tissue once their functional role was complete. However, the development of polymer BRS have encountered a number of setbacks in recent years that has led to the withdrawal of several commercial devices from the market due to inferior long-term performance compared to DES. While the underlying reasons for this poor performance are complicated, some of the primary contributing factors is thought to be the mechanical changes that take place during the degradation process, which remain poorly understood strut thickness and vessel sizing. The objective of this thesis is to investigate the physical and mechanical degradation behaviour of polymer BRS through a combined experimental and computational approach. Detailed experimental bench-top studies using an accelerated degradation protocol were carried out to evaluate the long-term physical and mechanical performance of two polymer BRS. In parallel, a phenomenological degradation framework was developed and implemented within the Abaqus finite element code and the model parameters were calibrated to fully predict the radial response of one of these devices over the course of degradation. Experiments were carried out to validate the calibrated model by implanting the polymer BRS within a silicone vessel and subjecting it the accelerated degradation protocol and comparing the long-term degradation performance to the computational prediction. The developed computational framework was used to conduct a detailed investigation of the individual roles of scaffolds geometry/design and material properties on both the short-and long-term performance of polymer BRS. Finally, the computational degradation framework was integrated into an in silico clinical trial platform for the development of coronary stents and used to predict the long-term performance of several BRS in a range of clinical scenarios. It was found that both polymer BRS were highly effective in maintaining their radial stiffness and strength during short-and medium-term degradation, but underwent a ductile to brittle transition in later stages of degradation. This brittle behaviour coincided with distinct increases in relative crystallinity of the polymer and highlighted a possible reason for polymer BRS poor long-term performance in clinical settings. The computational degradation framework was able to successfully capture the short-term deployment behaviour and the long-term degradation response under radial loading, including all aspects of elastic, yield and post-yield behaviour throughout all time-points. However, in an effort to validate the degradation model, the polymer BRS showed distinct creep behaviour in the early stages of the degradation response, where the diameter of the BRS was greatly reduced when implanted in a silicone vessel and under constant load from a parallel plate test. This was not captured by the computational model and suggests that a further understanding of the creep performance of these devices is required, both experimentally and computationally to enable future BRS development. It was found that optimising the geometry of the polymer BRS generally improved only the short-term deployment performance, with design changes only providing modest benefits to long-term degradation behaviour. This indicates that material development is the primary route that can be targeted to enhance the degradation performance of BRS. The work in this thesis enhances the current understanding of the mechanics of degradation in polymer BRS and provides a benchmark for the future development in this area.
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NUI Galway