Structural Design And Mechanical Property Test Of Biodegradable Polymeric Stent

How to treat cardiovascular diseases is a focus of medical research all over the world.Percutaneous coronary intervention with stents has become the main method to treat this kind of disease for its advantages of less trauma,high efficiency and surgical success rate,etc.Biodegradable vascular stents can overcome the core complications induced by the permanent retention of traditional metallic stents including vasomotor dysfunction,late in-stent restenosis and revascularization of advanced target lesions,which are hailed as the fourth revolution in coronary intervention and are the research frontiers in the field of stents.However,due to the relatively weak mechanical properties of polymers comparing with metals,the resulted polymeric stents usually have worse mechanical performance,which makes them difficult to meet the clinical requirements.Therefore,how to improve the mechanical properties of biodegradable polymeric stents has become the main challenge.Moreover,the differences in mechanical properties and processability between polymers and metals make polymeric stents different from metal stents in design,manufacture and in vitro mechanical property test.As a result,in order to improve the mechanical properties of biodegradable polymeric stents,a systematic study and exploration of structural design,manufacture and in vitro mechanical property test of polymeric stent were carried out,the main contents are as follows:(1)In view of the poor mechanical properties of biodegradable polymeric stents,a structural design method of biodegradable polymeric stent with high radial supporting capability was proposed,and three biodegradable polymeric stents were designed accordingly.They are J-shaped stent(JS),open C-shaped stent(OCS)and closed C-shaped stent(CCS),respectively.Using finite element method,key mechanical properties of the proposed stents are investigated and compared with those of the BVS.Based on the FEA results,it is found that the radial force of the JS,OCS and CCS are 14%、30%and 42%higher than those of the BVS,respectively.The radial recoils of the designed three stents are about 21%lower than those of the BVS.All designed stents were not shortened during expansion.The bending stiffness of the JS and OCS are comparable to that of BVS and are about 73%lower than that of the CCS.(2)Based on the analysis of the mechanical properties of designed biodegradable polymeric stents with different outer diameters and structures,the effects of the stent structures on the radial force,radial recoil,axial foreshortening and bending stiffness were studied.The results show that radial force and radial recoil are mainly dependent on the supporting ring structure,the utilization of designed unequal-height supporting ring can effectively improve these two properties.Axial foreshortening is mainly determined by the bridge geometry as well as the connecting position of the bridge with the adjacent supporting rings.It is feasible to improve the axial foreshortening by using the bridges with a curved structure and locating the connecting position in the middle of the straight section of the supporting elements.Bending stiffness is mainly affected by the connection form between the bridges and the supporting rings.The bending stiffness is better if the stent has open cells.(3)In view of the difficulty in controlling dimension and shape accuracy of polylactic acid micro tubes used for biodegradable polymeric stents,extrusion process of multi-specification polylactic acid micro tubes by a single extrusion die was proposed and the effects of gas flow rate,screw speed and pulling speed on outer diameter,wall thickness and ovality of the micro tubes were studied.The results show that the effects of gas flow rate,screw speed and pulling speed on micro tubes outer diameter and wall thickness are significant and nonlinear.Screw speed and pulling speed have greater effects on the micro tube outer diameter than gas flow rate.The effects of screw speed,pulling speed and gas flow rate on the thickness of the micro tube weaken in order.Gas flow rate directly affects the ovality of the micro tubes,the ovality decreases obviously with the increase of gas flow rate.When the gas flow rate is at a lower level,the ovality can be reduced by 68%as the gas injection volume is doubled.Combination of process parameters for extruding the three specification micro tubes were obtained and the extruded micro tubes meet design requirements.The micro tubes were then cut through femtosecond laser technology.There was no obvious heat affected zone at the cutting edges,and the error of strut width is less than±5 μm.(4)In view of the inconsistency of the in vitro mechanical test of polymeric stents for radial supporting property and flexibility,a comparative study of planar,V-groove and radial compression methods for the radial support property test were performed,and the effects of compression rate and circumferential positions on the test results were conducted.Based on the three-point bending method,the influences of compression rate and circumferential positions on flexibility were also explored.According to the above results,a better testing proposal for the radial supporting properties and flexibilities of biodegradable polymeric stents was determined to evaluate the properties of the three self-designed stents and the comparative Abbott BVS with different outer diameters.The results show that the developing trends of the compression load with the compression displacement measured by the three radial supporting property test methods are the same,but radial force values are quite different.The planar compression method is more suitable for comparing the radial supporting properties of stents with different diameters and structures.Compression rate has no obvious effect on the testing results of both the radial supporting property and flexibility.Compression circumferential positions have a great impact on testing radial support property with the planar or V-groove compression methods and testing flexibility with three-point bending method.Different circumferential position should be compressed for testing these two properties,compression circumferential position should be determined according to the structural characteristic and the periodicity of the supporting element.Test results should contain confidence interval of mechanical property parameters with a certain confidence level,the maximum fluctuation ranges of mechanical properties are reflected by sample range.The mechanical properties of the stents with different outer diameters and structures obtained from experiments are in good agreement with the results of finite element analysis.