Finite Element Analysis Of Thermally Induced Shape Memory Polymer Stent

Shape memory polymer(SMP), as a new smart material, has a wide application prospect due to its unique shape memory effect. The self-expanding vascular stent is one of important applications of SMP in medical field; however, the conventional method is very difficult to carry out an optimized design of stents since their shapes and service conditions are very complex. The finite element method can be conveniently used to simulate the process of the stent access the vessel. The difference among different designed schemes was discussed by changing the geometry and material parameters to implement an optimized design of the stents.The main contents of this work are listed as follows:1) Based on the viscoelastic polymer theory and time-temperature equivalence principle, a new time-temperature equivalent equation was established to describe the shape memory behavior of glassy thermally induced SMP. The comparison between simulations and Tobushi’s experiments shows that the improved constitutive model simulates well in the loading, cooling and unloading processes. After heating, the model does not consider the unrecoverable strain.2) The evolution equation of crystallinity proposed by Barot is simplified and is used to simulate Kim’s experiemnts. The results show the simplified model can predict the thermo-mechanical process of SMP:the accuracy in predicting stress-strain curves when the maximum loading strain is small; the error in predicting stress value increases with the increased maximum loading strain; the accuracy in predicting the temporarily fixedstrain at low temperature. After heating, the model does not consider the unrecoverable strain.3) The finite element models of glassy thermo-induced SMP stent were established to simulate the thermo-mechanical process the stent access the vessel and discuss the influence of different thicknesses of stent on the support performance. The results show that the thickness of stent affects the support performance remarkably:If the thickness of stent is too thin, the buckling will occur, which weakens the support performance of stent; With the increased thickness of stent, the stent presents more uniform deformation, which implied that the buckling is difficult to occur, i.e., the better support performance can be obtained; The flexibility of stent gradually decreases with the increased thickness. Thus, the thickness of stent should be so moderate that the stent can have enough both support ability and good flexibility.4) The finite element models of crystalline thermo-induced SMP stent were established to simulate the process of the stent access the vessel and discuss the influence of different material parameters on the support performance of stent. The results show that the radial stretched displacement of vascular does not matter with the moduli of reversible phase and crystallinity, but increases with the increased modulus of stationary phase. The ability of temporary shape fix decreases with the increased modulus of stationary phase and increases with the increased moduli of reversible phase and crystallinity. It implied that a larger modulus of stationary phase, relative ratio of the moduli reversible phase and stationary phase and crystallinity make the treatment of stent access the vessel procedure better.