Study On Structural Optimization Of Zinc Vascular Stent And Its Selective Laser Melting Processing Parameters

In recent years,the incidence of cardiovascular disease is rising year by year.The vascular stent is the most important medical device for cardiovascular disease treatment.The degradable vascular stent is getting more and more attention,especially the zinc degradable vascular stent.Zinc has good biocompatibility and a suitable degradation rate,and it has great application prospects for degradable vascular stent material.However,the traditional structures of non-degradable stents are not suitable for zinc vascular stent.it is essential to further study the stent structure for degradable zinc-based vascular stent,which can improve the performance of the degradable vascular stent and reduce the probability of restenosis in application.Additive manufacturing can fabricate parts with complex geometric structures.However,due to the low melting point of zinc,zinc has severe evaporation during the forming process.It is difficult to produce zinc vascular stent by additive manufacturing.And related researches for manufacturing zinc vascular stents are limited.It is significant to study the structural design of degradable vascular stent to promote its clinical application.In this thesis,the finite element analysis（FEM）software was used to simulate a series of performance of three degradable metal stent structures in iron,magnesium,and zinc alloy.The effects of material and structure on the performance of vascular stents were studied,respectively.The vascular stent structure with good comprehensive performance was selected and the connecting rib and supporting rib structure of the vascular stent was further optimized.The performance differences of stent before and after optimization were compared by FEM.And the rationality of the optimized vascular stents was also verified by FEM.At the same time,the manufacturing parameters of selective laser melting（SLM）for metallic zinc was investigated in this work,the vascular stents and the extension structure of stents were manufactured by SLM.The extension structure of stent was stretched by experiment tests and compared with the results of FEM.The experimental results provide guideline for the design and manufacturing of personalized degradable zinc vascular stent.The main research results are as follows:The performance of three different vascular stent structures with three different materials have great difference.The longitudinal compression performance of the V-type stent was poor.The flexibility performance of the S-type stent was poor,and the comprehensive mechanical properties of the U-type stent were great.For the same stent structure,the iron vascular stent has good expansion performance,the magnesium-based vascular stent has good flexibility performance and the magnesium-based vascular stent and zinc-based vascular stent has good longitudinal support performance.In order to optimize the structure of the vascular stent,the curvature in its the arc portion of the support rib were increased from 0.36 mm^-1to 0.82 mm^-1,and its connecting rib a curved structure was added.After optimization,the maximum stress of the vascular stent was 313.44 MPa after expansion,which was 10.72 MPa lower than unoptimized stent.It reduced the risk of vascular stent fracture during expansion and improved the safety of stent.When the bending offset was 1.5 mm setting in FEM,the required torque was 0.47 N·mm,which was 0.2 N·mm lower than unoptimized stent,and the flexibility performance of the vascular stent was improved.The optimal SLM manufacturing parameters were studied and the simulation results were qualitatively verified with the experimental results.When the laser power was 100W and the scanning speed was 300 mm/s,high-density of SLM-built samples with a density of 99.86%were obtained.The elastic modulus,percentage elongation after fracture and ultimate strength of the manufactured samples were 48.6±2.4 GPa,8.9±0.7%,95.5±3.3 MPa,67.1±0.4 MPa,respectively.The displacement of extension structure before vascular stent broken for optimized and unoptimized stent was 4.3 mm and 2.3 mm,respectively.The breaking points both appeared near the junction of the arc and the S-shaped structure.It illustrates that the optimized stent structure needs larger amount of deformation to reach tensile strength and failure and the optimized stent is safer,which is consistent with the trend of simulation results.The location of the fracture point was consistent with the stress concentration area after the expansion of the stent in the FEM results.