Characterizing the expansive deformation of a bioresorbable polymer fiber stent

Polymeric vascular stents must employ other strategies than malleable deformation, as generally practiced with metal stents, to expand and withstand compressive stresses in situ. The stent expansion strategy must further consider induced flow perturbations and wall stresses that may injure the vessel wall and promote thrombogenesis. Analyzing the stresses furled stents undergo during balloon-assisted expansion is an important first step in achieving a better understanding of stent-wall mechanical interactions, thereby to improve stent function. To this end, we performed finite element (FE) analysis of the balloon-induced unfurling of an internally coiled, bioresorbable polymeric stent employing a 3D FE solid model of a 120 degrees symmetric stent segment and a large deformation finite strain formulation. Uni-axial tensile testing of stent fiber elastic to plastic yielding provided the mechanical property information, and the von Mises criterion was employed to establish the elastic-plastic transition in the FE model. The model was validated with pressure and deformation measurements obtained during stent expansion tests. The internal coils of this inner coil-outer coil design twisted as the stent expanded, leading to plastic yielding at the point of tangency of the inner and outer coils. The remaining stent fiber portions underwent elastic bending. Cross-sections revealed only the outside surface layer of the coiled fiber underwent plastic yielding. The interior elastic fiber was supported by this plastic shell. The analysis suggests that during balloon-induced expansion, local plastic yielding in torsion sets the stent fibers, imparting high radial collapse resistance. The results further suggest that the stent exerts non-uniform mechanical forces on the vessel wall during expansion.