• finite element method
• polymeric coating
• Stent

The aim of this work was to study stresses and strains generated within polymeric coatings of endovascular devices (stents) during the stent deployment and the squeezing of the atherosclerotic plaque. Three different stent geometries were considered (Palmaz-Schatz, Cypher, and Multi link mini vision) and three different poly(methylmethacrylate-co-butylmethacrylate) copolymers with a different macromolecular composition were analyzed as stent coatings. Mechanical properties of coatings were modeled using experimental parameters obtained by loading the polymeric samples with three different amounts of an active principle (1%, 5% and 10% in weight of drug with respect to the polymer).
The preliminary analysis focused on the free expansion of the bare metal stent, imposing the nodes of the internal surface of the created models a fixed radial displacement. The obtained results highlight higher von Mises stress values in those areas designed for the radial expansion.
A simplified approach, which would help to reduce the computational costs and the simulation time, was of the utmost importance. Therefore, we planned a comparison between the whole stent structure, its half and a single mesh of the device. From the analysis it could be seen that von Mises stress values showed small differences between the three considered models. Therefore, this analysis allowed to select a small, but significant portion (the stent mesh), that showed an identical mechanical response with respect to the whole structure.
A further analysis focused on the stent expansion when the device comes into contact with the atherosclerotic plaque. This study was carried out in order to verify the degree of the stresses on the polymeric coating, caused by the contact with the atherosclerotic plaque. The von Mises stress values showed that the contact with the plaque affected the polymeric coating, but maximum reached values are lower than the ultimate stress for each tested material and no ruptures occurred during the deployment and the plaque squeezing.
Furthermore, from the obtained von Mises elastic strain values, it resulted that the atherosclerotic plaque was squeezed by the stent deployment and so, the correct vessel diameter is restored.
In conclusion, obtained results showed that stresses and strains within the coating were smaller than the first yield stress and therefore the coating integrity was ensured.