Design Optimization And Finite Element Analysis Of Superelastic Nitinol Stents

Atherosclerosis is a common vascular disease that can cause complications such as coronary heart disease and peripheral vascular disease.The self-expanding stents can treat this disease by supporting blocked blood vessels.Superelastic nitinol stent is a typical self-expanding stent with automatic expansion,mild radial force and good flexibility.However,clinical results show that it has unstable efficacy,insufficient radial strength and relatively low vascular patency.It also lacks studies on self-expanding stents in medical research area.In order to solve these problems,this paper uses the finite element method to study the mechanical properties of commercial stents after crimping and expansion,and a new nitinol stent called Bowknot is designed which has excellent radial strength,flexibility,clinical efficacy and complete stress-induced martensitic transformation.In addition,the geometry of stent is optimized in this paper.The main work and results are as follows:1.The model of commercial stent called SMART CONTROL is constructed using software Siemens NX 11.0,and a stent-artery-plaque system is also established in the finite element analysis software ABAQUS EXPLICIT.The superelastic model and the Ogden model are used to describe the material properties of the stent and the artery-plaque system respectively.Furthermore,the accuracy of the material properties is verified by the stress-stretch curve.The mesh sensitivity of the finite element model is studied to balance the accuracy of the results and the required computational resources.The results show that the optimal mesh size of the stent and the artery-plaque are 4×4 and 2 layers respectively.2.The mechanical properties of commercial stent during in vitro crimping and expansion in body are analyzed.The results show that:(1)The rings of stent after crimping have similar stress-strain distributions with the maximum stress of 350.2 MPa mainly concentrated in Ubends.In addition,the strain peaks at 3.48% which is located in the inner corner of the Ubends.This indicates that the stent has an incomplete martensitic transformation.(2)After the release of stent,the diameter at both ends is larger than that in the middle,and the stress and strain of the middle rings are higher.The stress-strain level of the stent decreases with the reverse phase transformation,and it still locates at the U-bends indicating that this area is most prone to failure.The maximum stress and strain are 181.61 MPa and 3.10% respectively.(3)The diameter of both ends and the middle of the stent are 6.25 mm and 4.50mm respectively,so the diameter unevenness is 28%.(4)The maximum stress of the arteryplaque system is 0.3789 MPa located at both ends of the plaque,and the maximum strain is 47.4% which is concentrated on the inner wall of the plaque.This suggests that these areas are easily broken and cause restenosis.The patency rate of blood vessel is 78.3%,and it still can be improved by optimizing stent design.3.The new self-expanding stent Bowknot is designed and geometrically optimized.The optimal size is determined by considering the mechanical performance and size constraints.Finally,the radial strength and flexibility of complete stent are evaluated.It is found that:(1)The radial strength decreases with the rise of the length,while the width and thickness have opposite trend.In addition,the length and width have a greater impact on the results.(2)The circumferential compression and three-point bending are simulated for the commercial and designed stent.It is found that the Bowknot stent improves the radial strength by 26.3% without affecting the flexibility.(3)Bowknot stent is more suitable for implantation in longer and high-stenosis diseased blood vessels due to its characteristics.4.The mechanical performance of the new stent after crimping and expansion is studied by finite element method.The results show that:(1)The stress and strain of the stent after crimping are mainly concentrated on the U-bends and the intersection of the struts,and the peak values are 512.88.MPa and 7.30% respectively.(2)After the expansion of stent,the diameter of both ends is larger than that in the middle,and the stress owns reverse trend.The maximum stress and strain are 242.57 MPa and 4.75% respectively which locate at the intersection of the struts.(3)The inner corner of U-bends and the outer side of the struts have complete martensitic transformation,and the overall strain level is still within the safe range indicating that the stent fully exerts the superelasticity of nitinol.(4)The diameter of the end of the new stent is 6.94 mm,while the intermediate diameter is 5.10 mm.Therefore,the diameter unevenness is 26.51% that is lower than that of the commercial stent suggesting that the new stent has higher fatigue strength.(5)The ends of plaque still have the highest stress(0.4089 MPa)and strain(48.04%).Furthermore,the diameter is increased from 3 mm to 5.10 mm,so the patency rate is 85% that is close to the upper limit of clinical results of self-expanding stent(90%).This indicates the new stent has better clinical efficacy.