Optimization based simulation of self-expanding Nitinol stent

Self-expanding Nitinol (nickel–titanium alloy) stents are tubular, often mesh like structure, which are expanded inside a diseased (stenosed) artery segment to restore blood flow and keep the vessel open following angioplasty. The super-elastic and shape memory properties of Nitinol reduce the risk of damage to the stent both during delivery into the body and due to accidents while in operation. However, as Nitinol stents are subjected to a long-term cyclic pulsating load due to the heart beating (typically 4 × 107 cycles/year) fatigue fracture may occur. One of the major design requirements in medical implants is the device lifetime or, in engineering terms, fatigue life. In order to improve the mechanical properties of Nitinol stents, at first, a reliable procedure of finite element analysis (FEA) is established to provide quantitative measures of the stent’s strain amplitude and mean strain which are generated by the cyclic pulsating load. This allows prediction of the device’s life and optimization of stent designs. Secondly, the objective is to optimize the stent design by reducing the strain amplitude and mean strain over the stent, which are generated by the cyclic pulsating load. An optimization based simulation methodology was developed in order to improve the fatigue endurance of the stent. The design optimization approach is based on the Response Surface Method (RSM), which is used in conjunction with Kriging interpolation and Sequential Quadratic Programming (SQP) algorithm.