The Manufacture And Mechanical Properties Analysis And Evaluation Of Endovascular Stents

Endocascular stents are usually made of biocompatible metals by special manufacture process and they are used to treat the stricture and occlusion of the vascular lumen. Currently, coronary stents and peripheral vascular stents are widely used in clinic. There are very strict requirements for not only the biocompatibility of the stent material, but also the manufacture of the stents. Considering the embarrassing complexion that the stents depend greatly on importation in China, this paper first aims at solving the major technical problems included in the manufacture and post disposing of the stents. And then finite element method (FEM) and computational fluid dynamics (CFD) are used to help identify some valuable mechanical properties of the stent and its implantation, which cannot be easily obtained by routine methods. In this paper, the major mechanical properties of the stents with different design characters are analyzed and compared, and the major factors that influence the mechanical properties of the stents are summarized. All these would be very important and helpful for the developing of native stents.Firstly, the development of the manufacture of the stent and the requirements for the laser-cutting equipment and 316L stainless steel tubes are introduced in this paper. The parameters for laser cutting of 316L stainless steel stent and Nitinol stent are concluded by using high power optical fiber laser. The influence of acid pickling, polishing and passivation on thesurface quality, corrosion resistance and blood compatibility of the stent material is summarized. The surface quality of the stent, which is manufactured under the guide of this work, is even better than that of the importation stents.Secondly, FEM is used in the analysis and evaluation of the major mechanical properties of coronary stent, and the validity of FEM is confirmed by experiments. Factors that affect the mechanical properties of the stents are summarized after analyzing and comparing the radial strength and flexibility of six stents with different design characters. In order to achieve a better understanding of the biomechanical characteristics of intracoronary stent implantation (ICSI), a viscoplastic material model for the plaque and a superelastic material model for the artery are developed. FEM is used to simulate the stent implantation under the balloon inflation and deflation. The simulated results are helpful for the better understanding of the high restenosis ratio at the end of stent and the instant lumen recoil after stent implantation. The instant recoil ratio of the balloon-stent-plaque-artery model (representing ICSI) is muchlarger than that of the balloon-stent model (12.3% to 3.1%). Due to the existence of the stent, the recoil ratio of the balloon-stent-plaque-artery model is much less than that of the balloon-plaque-artery model (representing PTCA) (12.3% to 22.9%). This is well consistent with the clinical experiments.Thirdly, the major properties of Nitinol stents are analyzed and compared by using FEM. Owing to the complexities in modeling the highly non-linear behavior of Nitinol alloy, the analysis and evaluation of the mechanical properties of Nitinol stents has been a key problem that the designers face. A method of developing the material model for Nitinol alloys in FEM is introduced in this paper. And the radial strength, flexibility and self-expanding process of Nitinol stents are simulated. The simulated results show that the mechanical properties of the stents are greatly affected by the strut's width and thickness. After stent implantation, the maximal internal stress in Nitinol stent is about 300MPa, which is much lower than that of the 316L stainless steel stent (600-700MPa). Then it could be concluded that the injury to the artery will be reduced due to the better conformity of Nitinol stent. And this will be helpful for the long-term success of Nitinol stent implantation.Finally, CFD is used to investigate the influence of lumen stenosis and stent implantation on blood flow. It should be noted that the stent implantation affects not only the stress in the artery, but also the blood flow patterns within the stent. And the relating changes in the blood flow would affect the repairing of the injured endothelium and then lead to restenosis after stent implantation. The simulated results show that blood flow accelerates in the stenosed lumen, which would lead to high wall shear stress, evolution of atherosclerotic plaque, and formation of thrombosis. Flow stagnation exists in the outlet of the stenosis, which would induce the congregation of platelet and lipid in the blood. And both of these would accelerate the fibrosis and calcification of atherosclerotic plaque. After stent implantation, low wall shear stress area forms, which is closely related with the strut thickness and the stent design. All these lead to the point that more attention should be paid to CFD in the analysis and evaluation of endovascular stent design.