Numerical Researches And Design Optimization Of Mechanical Properties Of Coronary Artery Stent

Coronary artery heart disease (CHD) has become the most dangerous reason which threatens human health. It is reported by national authority that three million people die of CHD every year and the number reaches 45% of total death. Annually, it costs more than 130 billion RMB in this field. Coronary artery stent implantation is the latest effective therapy. As stent technology develops rapidly, stent is used in more heart surgery instead of CABG (coronary artery bypass graft surgery). The mechanical properties of stent are important. Cardiologists from hospitals, engineers who design the products and scientists of mechanics community share the same interest on this subject. This dissertation studies the mechanical properties of stent utilizing finite element method.At the beginning of this dissertation, a bare stent is studied. Firstly, attention is paid to how element type and mesh density influence the simulation results of deformation. Secondly, the expansion speed curve is derived from the expansion curve, and an instinct relationship between the expansion speed and the recommended expansion range is obtained. Thirdly, the deformation during crimp stage to assemble stent and balloon together is simulated and the results show that it does not affect expansion and compression. However, the expanding process will change the resistance to external pressure of stent greatly. At last, beam element is employed to construct the finite element model. In terms of computational accuracy and effectiveness, beam element model is compared with those of shell and solid element.Contact analysis involving stent and balloon structures is accomplished. The results are in accordance with the actual deformation. The contact pressure obtained from contact elements, which should have been average, varies greatly due to the coarse meshes. In order to validate the simulation, strain energies of all elements are collected from results when the non-linear iteration converges every time. By re-calculating the expanding load according to strain energy distribution, real contact pressure is obtained and the results match the expanding force used for bare stent very well.A cantilever model is proposed to mimic stent’s flexibility, which has some advantages over 3-point load method and the simple beam model. The IBS (Instant Bending Stiffness) of stent is studied and it is found that IBS maybe anisotropic when curvature grows large enough. In addition, a deflection and a rotation angle occur to the stent because of the anisotropic IBS.Design optimization is the most important content of this research. Optimization theory is applied to improve the mechanical properties of the stent. A program which is automatic, parametric, integrated and robust is developed using ANSYS parametric design language. According to the findings that obtained respectively from the simulations about expansion and flexibility of chapter 2 and 4, objective functions to improve the expansion properties and flexibility are proposed. And then, the proper design variables are chosen and the entire optimization models are built in mathematical formulation. Finally, the optimization tasks are performed and the optimal designs are obtained successfully. This research introduces the optimization theory to the design of coronary stent, which improves the design efficiency in this field.