Research On Flexibility Of Degradable Polymer Stents

Stent implantation is an effective clinical treatment for coronary disease.At present,most of the clinical applications are permanent stents.And degradable polylactic acid(PLA)stents have a wide application prospect in clinic because of its many advantages.However,the PLA material has low strength,so the stent structure is designed to be thick enough to provide sufficient support performance which results in poor flexibility.Therefore,in the case of a certain thickness of the stent,the flexibility of PLA stent is studied in this paper from two aspect: the material and the structure.And it can provided a theoretical basis for improving the flexibility of PLA stents.The main contents of this paper are the following three aspects:(1)The flexibility of the PLA stent is improved in terms of materials.The uniaxial tensile test were performed on composites of PLA and seven concentrations of hydroxyapatite,and its mechanical properties were measured.And the results are compared with the mechanical properties of ordinary PLA materials.In this way,it can improve the flexibility of stents in terms of materials.(2)The influence of flexibility on structure parameters of stent is studied.Referring to the BVS stents of Abbott and ensuring that other design parameters remain unchanged,stent models with different length of connecting bar and different width of connecting bar were built.Then,analyzed the flexibility of the stent in natural state and crimping state.In the same time,with the material parameters obtained by experiment,the influence of the structure parameters of crimping stents on its flexibility were studied using finite element simulation.(3)The flexibility of stents with different length of connecting bar during implantation is studied.The three-dimensional model of the patient's real coronary artery was constructed to simulate the process of implantation of the stents with different length of connecting bar.And analyzed the force required of different stents to achieve deformation during implantation and evaluate its flexibility in this process.The results show that the mechanical properties are the best when PLA is combined with 0.2% hydroxyapatite,the elongation of the material at break reaches 142.8% and the rupture stress reaches 41.82 MPa.Compared with ordinary PLA,the mechanical property of modified PLA material have been improved.With the modified PLA material,the finite element simulation is used to compare the bending stiffness of the same stent under the natural state and the crimping state.The difference of the model is more than 10%,and the difference between two states is significant.The influence of stent structure parameters on its flexibility under the crimping state was studied based on the parameter of bending stiffness.When the parameter of the connecting bar is changed within a certain range,the longer the connecting bar is,the greater the bending stiffness of the stents is;the wider the connection bar is,the greater the bending stiffness of the stents is.In the process of implanting into the real coronary model with different lengths of connecting bar stents,the stent will move into the coronary artery along the rigid catheter in the direction of the centerline of the vessel.In order to simplify the operation,the normal contact force of rigid catheter is equivalent to the force required for stents to reach the bend to characterize the flexibility of stents.The maximum normal contact force of the rigid catheter are 0.38 N?0.41N?0.44 N and 0.46 N when the length of stent connecting bar is 0.9mm,1.0mm,1.1mm and 1.2mm.And the maximum normal contact force increase with the increase of the length of connecting bar.It can be seen from the above results that the mechanical properties of the modified PLA material have been improved,and using this material to the finite element simulation can improve the flexibility of the stents in the material.In terms of the structural parameters of the stent,the length and width of the connecting bar have a certain effect on the flexibility of the stent,and the length of the connecting bar is more significant to the flexibility of the stent.The longer the connecting bar is,the poorer the flexibility is,the wider the connecting bar is,the poorer the flexibility is.The virtual implant process of stent is used to analyze the flexibility,and the results are consistent with the results of analysis of stent flexibility based on bending stiffness.This study provides a theoretical basis for the design and optimization of stents,and provides a new idea for studying the flexibility of stents.