| Percutaneous coronary intervention (PCI) with coronary stent is an extensively usedtreatment procedure for patients with coronary artery disease. A stent is a medical devicedesigned to serve as a temporary or permanent internal scaffold to maintain or increase thelumen of a body conduit. Metallic materials including316L Stainless Steel (316L SS),Platinum–Iridium (Pt–Ir) alloy, Tantalum (Ta), Nitinol (Ni–Ti), Cobalt–Chromium (Co–Cr)alloy, Titanium (Ti), due to their high strength, ductiability, and good corrosion resistance,are an important class of materials used as stent, and other load-bearing implants for therepair or replacement of diseased or damaged tissues. But, these metallic materials are notbiodegradable in human body. Due to the limitation of metal stent researchers and engineersdiverted to investigate new material such as biodegradable materials. Biomedical implantswith temporary function for the vascular intervention are extensively studied in recent years.This feature can avoid the risk of long-term complications, such as restenosis or mechanicalinstability of the device when the vessel grows in size in pediatric patients. In fact, thestructure conditions of both ends of stent system influence a stent’s direct expansionbehavior.The research outlined in this thesis based on the numerical simulation, optimizationand development of new magnesium stent design. Polymeric material, iron and magnesiumalloy are the candidates for the biodegradable material stent. Magnesium alloy is one of themost potential materials for the cardiovascular coronary artery stent. The parametricmodeling of various stents is done in the Pro/Engineering Wild fire4. On the basis of designvariables e.g. thickness of strut, width of stent struts, element height and spacing, elementpattern various stent design are studied. The objective and basics criterion for designselection is stress concentration. Finite element analysis results show that the locations ofthe stress concentration at ends and central of the structure unit for different stent size ofstrut. The residual stress decreases with decreasing of the stent size (thickness, width).Stress concentration at the small curvature strut is obvious and higher than that at otherlocations of the stent strut. This investigation also reveals that the number of crest perelement effects on the expansion distribution and provide more supporting properties. Threeoptimized designs are further studied for the crimping and expansion behavior to identifythe mechanical properties of the stent. For this purpose a new method is used to get thecumulative stress concentration on the stent design. Finite element analysis was carried outby a commercial code ANSYS. It is found from the finite element results that the Model-3A shows the promising properties like radial recoil <5%at198MPa residual stresses.Whereas other model does not shows the better simulated results. Model-3A has a uniquesmall ring facilitating stent expansion, which become a cause of low cumulative stressconcentration on ring curvature. The simulation was carried out to analyze the flexibleproperties of the stent. The results show that the Model-3A is more flexible than the otherstent because1mm displacement occurs at small force0.06N, whereas the other stentsrequired higher force0.12N to reach same displacement. Subsequently Model-3A isnumerically studied for the small blood vessel stent of diameter2.5mm,1.8mm and1.5mm.Further finite element investigation of In-stent restenosis was carried out on theoptimized stent Model-3A. This probe points to a critical stress intensity in arteries wall,above which an aggressive healing response leads to in-stent restenosis in stented vessels.By using preclinical tool (numerical simulation) stent design are evaluated to get the criticalstress level on the vessel wall. The stent inflation renders the injury on the vessel wall. Thisinjury is measured in terms of stresses induced on the vessel wall by the stent. In responsethe arteries developed restenosis to the injury. Thus the stress induced on the vessel wallreflects the restenosis rate. The finite element simulation results show that the stress on theplaque and vessel induced by the Model-3A design are0.026MPa,0.009MPa, respectively.The force exerted by the vessel wall and plaque on the stent before and after degradation iscalculated by the finite element simulation. The results show that the pressure exerted on thecontact surface before degradation is0.0213N/mm2and the contact force by vessel on thestent is0.072633N whereas after degradation the contact pressure is0.02211N/mm2andthe contact force is0.0754N. The supportive properties in form of radial forces are alsoanalyzed by the numerical method. The results shows that Model-3A before degradationsupport0.73N force whereas the after degradation it still can support0.55N force. |
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