Plasticity as a Lifesaver in the Design of Cardiovascular Stents

A common treatment to restore normal blood flow in an obstructed artery is the deployment of a stent (i.e. small tube-like structure). The vast majority of stents are crimped on a folded balloon and laser cut from 316L stainless steel tubes. Although, several numerical studies (exploiting the Finite Element Method) are dedicated to the mechanical behaviour of balloon expandable stents, there seems to be no consensus regarding the mechanical properties to describe the inelastic material behaviour of SS316L. Moreover, as the typical dimensions of stent struts (e.g. 100 μm for coronary stents) are of a similar order of magnitude as the average grain size in stainless steel (i.e. 25 μm), continuum approaches relying on macroscopic material properties may be questionable. In addition, an experimental study on stainless steel stent strut specimens showed a size-dependency of the failure strain. In this study the impact of the magnitude of the yield stress on the stent expansion behavior is examined. An increase in the yield stress (from 205 N/mm² to 375 N/mm²) results in an increase of the pressure (from about 0.3 N/mm² to approximately 0.4 N/mm²) which the clinician needs to exert for the balloon to unfold and to reach its cylindrical expanded shape. Furthermore, the effect of the size dependency behavior of the material is studied by monitoring the nominal strain during stent expansion. The maximum value of the nominal strain in the expanded stent (e.g. εn = 23 %) does not exceed the critical value of the failure strain, (i.e. εn = 33 %), moreover the critical values are nowhere exceeded in the whole stent during the expansion. Our numerical results - accounting for the presence of the balloon in its actual folded shape - correspond very well with pressure/diameter data supplied by the manufacturer. Consequently, this study shows that the free expansion of new generation balloon-expandable stents can be studied accurately with computational analysis based on the Finite Element Method (FEM) and relying on macroscopic material properties. In this context, there is no need to implement a size-based constitutive material model, but before accepting the results of the study, one should check in any case the maximum strain against the limit as shown above.