Design And Structural Optimization Of Braided Degrading Magnesium Alloy Airway Stent

Magnesium alloy is a green and degradable material.Its degradation products can be absorbed by the body and have no obvious side effects on the human body.Due to their good biocompatibility and biosafety,and magnesium alloy has potential for development of degradable airway stents.Compared with traditional non-degradable airway stents,magnesium alloy airway stents can gradually degrade as the narrow airways progress,eliminating the need for secondary surgery.Considered that,the elasticity and plasticity of magnesium alloys are inferior to those of conventional materials,the structures designed for traditional airway stent are not suitable for degradable magnesium alloys.Therefore,structural optimization is necessary to obtain better service performance.To date,there is no public report on the use of magnesium alloy materials for airway stents.In this paper,three-dimensional models of braided airway stents with different structures were established in order to study their influence on elasto-plasticity,deficient radial support,radial recovery and axial recovery,and obvious dog bone effect in degradable magnesium alloy airway stents.Finite element simulation software ABAQUS was used to simulate the mechanical properties of braided magnesium alloy airway stents,including compression process simulation,expansion process simulation,and compliance simulation.First,three-dimensional models of single-wire stents and double-wire stents with different wire diameters,different wire number and different pitches were established.Then ABAQUS analysis software was used to investigate the effect of wire diameter,number of wires and pitch on the radial stiffness,dog bone rate,radial recovery,axial recovery,axial shortening and bending stiffness of the stents.According to the law of influence,the best level of the three factors of wire diameter,wire number and pitch was chosen.Secondly,through the orthogonal experiments,the comprehensive effects of multiple factors were analyzed,and the optimal combination of stent parameter was obtained.By using orthogonal experiment method,the stent structure was further optimized.Further,the mold was designed according to the optimized stent,and an optimized braided stent was prepared.Finally,the optimized braided airway stent was characterized to verify the correctness of the simulation design.The results show that the wire diameter,the pitch and the number of wires have an important influence on mechanical properties of the single-wire and double-wire stents.With an increase of the diameter of the wire,the radial stiffness both of the single-wire and double-wire stents increases.The longitudinal recoil and longitudinal shortening and flexibility decrease with an increase in wire diameter.There is no obvious regularity between the radial recoil and the wire diameter of the single-wire stents.The radial recoil of double-wire stents first showed a decreasing trend with the increase of wire diameter and then showed an increasing trend.With increasing of the number of wires,the radial stiffness of the single-wire and double-wire stents increase,and the longitudinal recoil and longitudinal shortening are reduced.Dog bone effect of single-wire stent is more and more obvious with the increase of the number of wires.There is no obvious regularity between the dog bone rate and the number of the wire.Their flexibility is getting worse as the number of wire increases.With an increase in the pitch,the radial stiffness of the single-wire and double-wire stents becomes smaller,and their radial recoil is becoming more and more serious.The dog bone rate of the single-wire and double-wire stents generally increases with the increase of pitch.The flexibility of both single-wire and double-wire stent is getting worse with the increase of the pitch.Both the single-wire and double-wire stents have the dog bone effect during expansion.In the expansion process longitudinal recoil and longitudinal shortening of single-wire and double-wire stent are negative,indicating that length of the stent is extended.The orthogonal experimental results of the single-wire stent show that when the stent parameters of wire diameter,number of the wires,and pitch is 0.26 mm,26 and 30 mm respectively,the comprehensive performance of stent is the best.For the double-wire stent,the comprehensive performance of the double wire stent is the best when the diameter of the wire,the number of the wires,and pitch is 26 mm,28,and 20 mm,respectively.The mechanical properties of the double-wire stent after the orthogonal optimization are similar to those of single-wire stent except for the radial stiffness.The radial stiffness of the double-wire stent is very large,which could cause airway tissue necrosis.In addition,the actual weaving and other issues also should be considered.In view of these problems above,the single-wire stent optimized after orthogonal optimization is still the object of further optimization design.After further optimization of the single-wire stent,the dog bone effect did not appear by the variable pitch treatment.When the length of the variable pitch segment was 4 mm and the pitch was 24 mm,the performance of the stent is better.The actual experimental test results show that the performance of the optimized magnesium alloy stent has a significant improvement compared with the one before optimization.Even,the maximal radial support and flexibility of magnesium alloy stents are better than that of medical nickel-titanium alloys.In this paper,we optimized the overall structure of magnesium alloy braided airway stents,improved its mechanical properties,and reduced the dog bone effect.In addition,a basic parameter system for braided stents was established which provides a design reference for the clinical application of magnesium braided airway stents.