Research On Elastoplastic Deformation And Dynamic Behaviors Of Bioabsorbable Coronary Stents

Bioabsorbable coronary stents(BRSs)can not only provide the necessary support for the coronary vessels in the short term,but also degrade gradually,thus avoiding the in-stent restenosis caused by long-term stent retention.Therefore,the mechanical properties of BRSs are being an important issue to investigate in biomedical engineering.The expansion and springback deformation of the stent during implantation,as well as the dynamic behaviors after stent implantation,are the key mechanical problems that need to be further studied.In this paper,experimental analysis,theoretical modeling and solution,and finite element numerical simulation were adopted to systematically study the large elastoplastic deformation of coronary stent during expansion and springback and the harmonious vibration modal response of implanted stent.The main contents and results are as follows:(1)The simulated experiment was carried out for the expansion deformation of the coronary stent driven by the internal pressure load of a balloon.An aluminum alloy stent with similar shape and enlarged size to the medical magnesium alloy stent was used as the specimen to simulate the expansion and deformation of the metal stent driven by an inflatable ballon,and the corresponding experimental device was designed by ourselves.Using this device,the simulated expansion deformation experiments were carried out for the thin-walled rubber tube simulating the ballon under the internal pressure load,and the elastoplastic stent under the interaction between the stent and the inflatable ballon.The relation curves between the internal pressure loads and the radial expansion displacements were obtained.In addition,the relationship between axial load and principal elongation of the thin-walled rubber tube ballon specimen were obtained by tensile test.These results provide the experimental verification basis for the theoretical analysis.(2)The expansion deformation behaviors of the stent and ballon were analyzed theoretically.Firstly,the superelastic constitutive relation was derived by using the strain energy function expressed by the invariant of the elongation tensor,and the uniform finite deformation solutions of the thin-walled rubber tube ballon under tensile and internal pressure loads were solved respectively.The relationship curves between the internal pressure load and the main elongation of the thin-walled rubber tube ballon obtained by theoretical analysis are in good agreement with the experimental data.Secondly,considering the periodicity and symmetry of the stent structure,the characteristic substructure of the S-shaped curved beam is extracted from the overall structure of the stent as the research object.Based on the elastic-plastic theory of curved beams,the corresponding simplified mathematical model was established,and the analytical solutions of the displacement of curved beam in elastic deformation and local plastic deformation stage are obtained respectively.Furthermore,an analytical expression of the relationship between the contact pressure load on the inner wall of the stent and its radial expansion displacement was given by considering the geometric deformation relationship between the curved beam substructure and the whole stent structure and the balance equation between the contact pressure load and the internal force.Finally,the synergistic deformation effect of the interaction between the stent and the ballon was considered based on the continuity condition of the contact pressure and normal displacement of the stent and the ballon at the contact interface,and the analytical solution of the radial expansion displacement of the stent varying with the internal pressure load of the ballon was obtained.The theoretical analysis results are in good agreement with the experimental results,which shows the applicability of the proposed theoretical model and analytical method.(3)Based on elastoplasticfinite element simulation method,the elastoplastic deformation of the composite stent during expansion was analyzed.The core of the composite stent is magnesium alloy material,and the outer layer is poly-1-lactic acid material(PLLA).The Mises stress and residual deformation of the composite stent during expansion,after springback and after complete degradation of the outer layer were calculated.The radial springback displacement and axial contraction displacement of the stent after unloading are also calculated.The numerical simulation results showed that due to the wrapping of PLLA,the Mises stress on the dangerous cross section of composite stent was concentrated in the inner part,while that on the outer surface of single magnesium alloy stent.This indicates that the double-layer composite structure can reduce the stress corrosion caused by stress concentration.(4)By means of finite element modal analysis and harmonious response calculation,the dynamic performances of Magnesium alloy,PLLA and combined with them stents were analysized,considering the effects of stent materials,expansion and springback deformation,residual stress,outer layer degradation and vascular constraint.The natural frequencies and corresponding vibration modes of stents were numerically calculated,and the response under harmonic excitation was also obtained.The results show that each natural frequency in the first five modes of the composite stent is much less than that of the magnesium alloy stent.The natural frequency of the stent after large deformation expansion and springback is significantly lower than that of the original stent.The residual stress has little effect on the natural frequency of the stent and does not change the vibration modes of the stent.However,the degradation of the outer layer and vascular elastic constraints have significant effects on the natural frequency of the stent.The results of modal analysis showed that the low order vibration modes were bending,torsion and breathing modes,and the vibration modes would change after stent implantation.In addition,simulation caculations were performed to compare the natural frequency and vibration modes between healthy,plaque contained and stent-implanted vessels.