An experimental and computational investigation of the mechanics of degradation in polymer bioresorbable scaffolds
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2023-10-03Author
Fiuza, Constantino
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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.