Theoretical and experimental evaluation of the radial force of self-expanding braided bioabsorbable stents

This study was carried out to evaluate if the analytical model developed by Jedwab and Clerc for calculating the mechanical properties of metallic braided stents is also valid for bioabsorbable braided stents. An analytical model could be used to shorten the development cycle of stents by reducing the amount of in vitro testing. Jedwab and Clerc derived formulae for longitudinal stiffness and radial pressure stiffness. The longitudinal stiffness was defined by measuring the stent elongation under load. The radial pressure stiffness was defined from the slope of the load-displacement curve measured with the testing method described by Agrawal and Clark where a collar is placed around the stent to compress it. The radial pressure stiffness was measured with and without lubrication to evaluate the effects of friction between the stent and collar and in the stent structure itself. Two bioabsorbable braided stents and one metallic braided stent were used in the measurements. The metal stent test results were consistent with what was reported by Jedwab and Clerc. However, the analytical model was not applicable to bioabsorbable stents. This was mainly due to the larger fibre diameter of the bioabsorbable stents, which prevents the fibres from freely collapsing when the stent diameter decreases. The analytical model is based on an assumption that the fibres behave independently. However, the testing method described by Agrawal and Clark provided a useful tool to compare the radial force of self-expanding stents.