NUI Galway – UL Alliance First Annual ENGINEERING AND - ARAN ...

Modelling Corrosion in Bioabsorbable Metallic Stents
J.A. Grogan, B.J. O’ Brien, S.B. Leen, P.E. McHugh
Mechanical and Biomedical Engineering and National Centre for Biomedical Engineering
Science, National University of Ireland, Galway, Ireland.
j.grogan1@nuigalway.ie
Abstract
The design of bioabsorbable stents, tiny absorbable
scaffolds that are used in the treatment of heart disease,
is highly challenging due to the complex mechanical
and chemical interaction between them and their
surroundings in the body. The aim of this work is to
facilitate the development of these devices through the
development and experimental calibration of a finite
element based stent assessment and design tool.
1. Introduction
A new generation of stents that are gradually
absorbed in the body are showing great promise in
terms of reducing long-term health risks associated with
permanent metallic implants [1]. However, the
corrosion of these devices in the body is still not well
understood, making their design difficult.
Computational modelling is extensively used in the
design of conventional, permanent stents [2]. The ability
to include the effects of device corrosion in a finite
element (FE) based stent design framework will greatly
facilitate the development of absorbable metallic stents
(AMS). This ability is developed in this work through
the creation on a novel FE corrosion model and its
calibration based on the results of experimental
corrosion studies on a bioabsorbable alloy.
2. Methods
The corrosion behavior of thin biodegradable alloy
(AZ31) foils is determined in simulated physiological
solution, including a determination of alloy corrosion
rate and the effects of mechanical loading on alloy
corrosion behavior.
A FE based corrosion model is developed and
calibrated based on the results of the experimental
corrosion tests. The model captures the effects of
corrosion through the simulation of a stochastic pit
growth process and the use of a corrosion damage
parameter.
The model is applied in simulating the corrosion of a
3-D AMS geometry in a three-layer artery and is used in
predicting the reduction in stent mechanical integrity
over time due to corrosion.
3. Results and Discussion
A localized, pitting corrosion attack is observed
experimentally in the alloy, resulting in a significant
reduction in specimen mechanical integrity with
relatively little mass loss. The effects of loading and
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corrosion on foil integrity are captured very well by the
FE model, as shown in Fig. 1.
Fig. 1 – The experimental and simulated results of a
constant load immersion test on AZ31 alloy.
The FE AMS model, shown in Fig. 2, predicts a
localized pitting corrosion in the stent, resulting in a
greater reduction in scaffolding support than that
predicted in the ideal (homogeneous) corrosion case.
This predictive capability is significant in both the
selection of effective candidate alloys for AMS
application and in the optimization of AMS geometries
for improved long-term scaffolding support.
Fig. 2 – A FE stent, delivery system and artery model
with simulated corrosion pits shown in a single
hinge. An SEM of a corrosion pit is also shown.
4. Acknowledgements
Research funded by an ICRSET scholarship under the
EMBARK initiative.
5. References
[1] Erbel R, et al., Lancet, 2007, 369: 1869–75.
[2] Mortier P, et al., Ann Biomed Eng, 2009, 38: 88-99.