Study Of A Progressive Expanding Stent With Nitinol And Biodegradable Polymers

Heart attack is the No.1human killer. Stent implementation is themost popular treatment for this disease. However, the main complicationof this procedure is in-stent restenosis, where the stented arteries werere-blockaded. While the cause of this complication is multifactorial, e.g.the stent design, individual difference, diabetes, or smoking habits, theinitial trauma caused by balloon expansion is assured to be one. Therefore,the reduction of this trauma is beneficial for lowering the rate of in-stentrestenosis.In the design of such a progressive expanding stent, how to achievethe progressive criteria meanwhile not results in too much complication instent structure is crucial. This study proposes a new stent design based onNitinol shape memory alloy and biodegradable polymers. Due to itshyperelastic effect, the stent material can undertake large elasticdeformation after being crimped. The recover speed was modulated withthe introduction of restraining units made from biodegradable polymers.As their name indicated, the restraining units would delay the expansionspeed thus made the expansion process a progressive one.The study begins with the study of mechanical properties of the twomaterials. The mechanical deformation of Nitinol was obtained withuniaxial test and the property of polymer was obtained through degradationexperiment. It turns out the elastic modulus of the biodegradable materialdoes not change as its degradation, however, the breaking deformationgradually loss as the material become brittle and bristle with time.It further compares two different design proposals with two differentself-expanding stent: braided and laser cut stent. The final design chooses the later one for its apparent advantage in stent thickness. To verify thedesign concept, annealed Nitinol wires were fabricated to form the basicstrut of the design and degradation experiment was conducted to observethe expansion and degradation. To expedite the design, finite elementmethod was adopted to analysis the whole process of stent fabrication andimplementation.