In order to systematically evaluate the safety and reliability of self expanding nitinol stent, this study refers to the dynamic safety factor recommended by the International Organization for Standardization to quantitatively reflect the safety performance of the stents. Using the superelastic memory alloy material in the nonlinear finite element software Abaqus as the constitutive model, combined with the experimental data of uniaxial stretching of nitinol alloy pipes, the finite element simulation of the insertion process of Φ8x30mm, Φ10x30mm, and Φ12x30mm of three L-shaped brackets is carried out after first pressing and then self-expanding, and the shape change process under the action of high and low blood pressure pulsation after insertion. By analyzing the stress changes of the vascular stent,the maximum stress strain, stress concentration position, fatigue strength size and possible failure forms during the use of the vascular stent are studied, and the accelerated fatigue life test of the vascular stent in vitro is carried out. Finite element analysis and experimental results show that with the increase of the pressure grip, the maximum stress and maximum plastic strain of the bracket show an increasing trend, but there is no significant change in the maximum stress and strain distribution area of the bracket, which are concentrated on the inside of the arc of the connecting area of the support body and the connecting body. The dynamic safety coefficients of the three brackets are 1.31,1.23 and 1.14 ,respectively, the values are greater than 1, indicating that the safety performance of the three brackets is good, can meet the fatigue life requirements of more than 10 years, and the safety of the bracket has a tendency to decrease with the increase of the original diameter of the bracket.

The release mechanism of the self-expanding metal nitinol stent is to deliver the stent pressed into the sheath tube to the lesion site and release it. After the nitinol stent reaches the human body temperature, it automatically expands back to a predetermined shape, and produces enough support force to dilate the stenosis site of blood vessels. Nitinol alloy material has excellent shape memory and superelasticity, fatigue resistance, NMR compatibility, biocompatibility, adherent and corrosion resistance, etc., excellent material properties and process characteristics make it widely used in the treatment of intracranial stenosis blood vessels. American Society for Testing and Materials, ASTM F2477-07 standard clearly requires that nitinol stents must be able to withstand not less than 380 million pulsation and relaxation of blood vessels (10 years of fatigue life), so how to optimize the structure of nitinol stents to improve the safety performance of stents has gradually become a hot topic in scientific research.

Grujicic of the United States and others combined the finite element calculation method of fluid-solid coupling with advanced fatigue durability analysis technology to improve the fatigue life of self-expanding nitinol alloy brackets. Nes et al. introduced a numerical fatigue life evaluation method for self-expanding nitinol alloy scaffolds due to blood corrosion. Tokutake et al. studied the effect of multiple vascular stenosis of the femoral artery on the fatigue life of the self-expanding stent through finite element analysis. Marrey and others used the accelerated fatigue testing machine developed by Bose Company in the United States to detect the fatigue failure form of nitinol alloy stent in blood vessels. Zhang Huijuan of China and others studied the influence of stent production materials and stent geometry parameters on the successful service of self-expanding vascular stents and so on.

In the above study, finite element simulation and experimental verification of the fatigue life of the titanium alloy bracket were carried out, but the overall safety of the bracket was not discussed in detail. This study refers to the dynamic safety coefficient of the stent prescribed by the International Organization for Standardization to quantitatively evaluate the overall safety performance of the stent. For the material characteristics of the superelastic pseudo-deformation of the nitinol alloy, the superelastic memory alloy material in the nonlinear finite element software Abaqus is used as a constitutive model to simulate the insertion process of the nitinol stent after first pressing and then self-expansion, as well as the shape change process under the action of high and low blood pressure pulsation after placement. Combined with the Goodman chart method recommended by the United States Food and Drug Administration, the hazardous stress concentration area, fatigue strength and dynamic safety coefficient of three different sizes of stents were evaluated, and the accelerated fatigue test of vascular stents in vitro was carried out. The purpose of this study is to reveal the law of the impact of the diameter of the nitinol stent on its safety performance, in order to provide a theoretical basis for improving the safety and reliability of the vascular stent.

Grujicic of the United States and others combined the finite element calculation method of fluid-solid coupling with advanced fatigue durability analysis technology to improve the fatigue life of self-expanding nitinol alloy brackets. Nes et al. introduced a numerical fatigue life evaluation method for self-expanding nitinol alloy scaffolds due to blood corrosion. Tokutake et al. studied the effect of multiple vascular stenosis of the femoral artery on the fatigue life of the self-expanding stent through finite element analysis. Marrey and others used the accelerated fatigue testing machine developed by Bose Company in the United States to detect the fatigue failure form of nitinol alloy stent in blood vessels. Zhang Huijuan of China and others studied the influence of stent production materials and stent geometry parameters on the successful service of self-expanding vascular stents and so on.

In the above study, finite element simulation and experimental verification of the fatigue life of the titanium alloy bracket were carried out, but the overall safety of the bracket was not discussed in detail. This study refers to the dynamic safety coefficient of the stent prescribed by the International Organization for Standardization to quantitatively evaluate the overall safety performance of the stent. For the material characteristics of the superelastic pseudo-deformation of the nitinol alloy, the superelastic memory alloy material in the nonlinear finite element software Abaqus is used as a constitutive model to simulate the insertion process of the nitinol stent after first pressing and then self-expansion, as well as the shape change process under the action of high and low blood pressure pulsation after placement. Combined with the Goodman chart method recommended by the United States Food and Drug Administration, the hazardous stress concentration area, fatigue strength and dynamic safety coefficient of three different sizes of stents were evaluated, and the accelerated fatigue test of vascular stents in vitro was carried out. The purpose of this study is to reveal the law of the impact of the diameter of the nitinol stent on its safety performance, in order to provide a theoretical basis for improving the safety and reliability of the vascular stent.

The sample material of the nitinol stent is a medical laser-engraved nitinol tube with an alloy composition of Ni (55.92%) – Ti (44.06%). The elastic-plastic deformation behavior of the stent is described by the vonMises yield criterion and the isotropic strengthening criterion. Combined with the uniaxial tensile experimental data of the laboratory Y8000 series universal material testing machine,the stress-strain curve of the material is drawn as shown in Figure 1, where B: the starting stress of the transition from martensite to austenite; C: the final stress of the transition from martensite to Austenite; D : the starting stress of the transition from Austenite to Martensite; E:the final stress of the transition from austenite to martensite.

First, the expansion plan of the vascular stent is established in Autocad2014,and then the expansion map is inserted into the three-dimensional software Solidwork2014 sketch interface,and the model of the three vascular stent systems Φ8x30mm, Φ10x30mm, and Φ12x30mm is established by covering, stitching, mirroring, deleting faces and other features. The shape of the bracket is based on the research group’s patent(Stent. I. P. Patent 3146103), the outer diameter is 8,10,12 mm, the length is 30mm, wherein the connecting body is type i,and the bracket support body and the connecting body are symmetrically distributed in the circumferential direction,the width is 0.12 mm,and the wall thickness is 0.2 mm. The three-dimensional model of the stent, blood vessel, and pressure-grip shell was guided to HyperMeshlO in IGS format. O two-dimensional and three-dimensional meshing, using 8-node hexahedral units(C3D8 )for grid dispersion ,to ensure that the grid quality Jacobi is 0.7,greater than the tolerance value, and finally imported nonlinear Abaqus6.14 for finite element analysis. Figure 2 is a grid model of the stent connector site and vascular stent system.

The constraints of the stent, the pressure grip shell, the balloon and the blood vessel are all constrained by the axial degree of freedom at one end, and the circumferential degree of freedom at the other end is completely constrained. Boundary constraints are applied to the entire system in a column coordinate environment,where the Z direction represents the axial direction of the model system, r represents the circumferential direction, and r is radial. The friction-free contact type is defined between the grip shell and the frame balloon and the bracket. The specific settings are as follows：

① Constrain the circumferential displacement of one end of the entire model: Ut=0 ;

② Constrain the axial displacement at one end of each entity in the entire model: Uz=0;

③ The radial displacement R of the entire model is defined according to the external loading situation and the contact of each surface.

The self-expanding nitinol alloy stent is compressed and transported to the blood vessels,and the nitinol stent reaches the phase transition temperature(36 degrees Celsius), which will induce the stent shape memory effect to automatically expand and deform to the release size. After the stent is placed, it is also affected by the pulsating effect of the vascular wall. In order to ensure the authenticity of the simulation, the load addition is divided into two processes: static pressure grip expansion and dynamic alternating load action, and the static expansion process of the stent includes two stages of pressure grip contraction and self-expansion expansion. In order to better analyze the maximum equivalent force and the maximum plastic strain trend,radial displacement constraints were applied to the three stent grip shells,so that the stent was gradually pressed from the initial outer diameter to 4mm, and then released to the normal size;the dynamic loading process simulates the process of changes in human vascular blood pressure,adding physiological pulsation loads of high pressure 0.0213 MPa, low pressure 0.0107 MPa stress values (equivalent to pulse pressure 160 mm Hg and 80 mm Hg), the loading frequency is 60 Hz, as shown in Figure 3.

The self-expansion process of the pressure grip of the vascular stent involves nonlinear problems such as material elastic plasticity and large geometric deformation.The Newton-Raphson algorithm of the Abaqus6. 14 standard solver is used to numerically solve the problem. In order to study the dangerous areas of stress concentration and possible fracture sites of vascular stents during the pressure grip process, the three vascular stents Φ8x30mm, Φ10x30mm, and Φ12x30mm were gradually pressed to 4mm through the pressure grip shell, and the maximum equivalent force and plastic strain changes of the stent were analyzed. The results showed that the maximum equivalent force and maximum plastic strain of the three brackets showed an increasing trend with the increase of the amount of pressure during the clamping process, but there was no significant change in the maximum stress and strain distribution area of the bracket, all concentrated on the inside of the arc of the connecting area of the support body and the connecting body. When the bracket is pressed to 4mm, its equivalent force and plastic strain value are the largest. During the pressing process, the maximum equivalent force and plastic strain of the bracket are shown in Table 2, and the corresponding equivalent force and plastic strain are shown in Figure 2.

Under the action of long-term relaxation pulsation of the vascular wall, the nitinol stent is prone to deformation, collapse and even fracture and other fatigue failure, thereby reducing the expected service life of the stent. Using the Goodman chart method recommended by the vascular stent industry standards formulated by the United States Food and Drug Administration and the European health department, the stent meets the fatigue life of 10 years(380 million recuperation pulsations)to assess whether it meets the fatigue life of 10 years (380 million recuperation pulsations). The bracket stress cloud diagram of 80 and 160 mm Hg obtained by finite element analysis shows that the stress at the inner node of the transition arc between the support body and the connecting body is higher, which is consistent with the concentrated position of the bracket to withstand the stress during the clamping process, indicating that this area is a dangerous part in the use of the bracket. The equivalent alternating stress and equivalent average stress of the three nickel-titanium brackets are obtained from the formula (1) and (2), and the calculation results are shown in Table 3.

σmean =( σ80 mm Hg + σ160 mm Hg ) / 2 ( 1 )

σalt= (σ80 mm Hg – σ160 mm Hg ) / 2 ( 2 )

Where: σalt is the alternating stress of the pulsating load,and σmean is the average stress of the pulsating load.

According to the calculated equivalent alternating stress and equivalent average stress values, the fatigue strength Goodman diagram of the three brackets is drawn, as shown in Figure 5. The closer the stress distribution point in Goodman’s diagram is to the fatigue strength limit, the lower the fatigue strength at that location, the easier it is to break, and the safer it is on the contrary.Similarly, if the stress point distribution area is close to the fatigue limit, it means that the bracket as a whole is prone to collapse and large deformation failure. The stress distribution point area diagrams in the three bracket Goodman diagrams are located below the fatigue limit straight line, that is, they are located in the safe area,indicating that they all meet the fatigue life requirements of more than 10 years,and can achieve a predetermined use effect.

Based on ASTM F2477-07 standard, the use of Bose SGT9210 stent fatigue tester in the United States, under experimental conditions with pulse pressure fluctuation range of 80 ~ 160 mm Hg, pulse frequency of 60 Hz, and temperature(37 ± 2)℃, three types of scaffolds were implanted into artificial blood vessels for fatigue testing. After 400 million pulse cycles equivalent to 10 years, the fracture position of the surface of the nitinol stent was observed. The test results show that the location of the fracture of the three types of brackets is in the arc part of the connection between the support body and the connecting body, which matches the position of the maximum stress point obtained by the previous finite element analysis, and then verifies the correctness of the finite element analysis results.

According to the evaluation guidelines of the International Organization for Standardization on the fatigue strength of vascular stents, fatigue strength tests can be divided into two categories：

One is the Goodman criterion,and the second is the dynamic safety factor.

Vascular stents are prone to extreme cases of failure or even fracture under the action of pulsating cyclic loads. The Goodman curve of the fatigue life evaluation method can show the fatigue strength of the stent, and the dynamic safety factor of the stent can quantitatively reflect the safety and reliability of the stent. The calculation formula is as follows:

l/SFDynamic =σmean/σuts+σalt/σe (3)

In the formula: SfDynamic is the dynamic safety factor;σe is the fatigue limit;σuts is the tensile strength limit.

The calculation results show that the three brackets are safe,and the safety factor is greater than 1. Among them, the safety performance of the bracket Φ8 is the best,and the dynamic safety factor is the most,which is 1.31;while the safety performance of Φ12 is the worst,and the dynamic safety factor is 1.14. The analysis and comparison of the safety coefficients of the three brackets show that with the increase of the original diameter of the nitinol bracket, the safety coefficient of the bracket has a tendency to decrease, that is, with the increase of the original diameter of the bracket, the safety reliability decreases.

This study uses the superelastic memory alloy material in the nonlinear finite element software Abaqus as a constitutive model, combined with the uniaxial tensile test data of nitinol alloy pipes, analyzes the complex phase transition mechanism and nonlinear superelastic mechanical properties of nitinol alloy, and realistically simulates the process of self-expanding nitinol alloy vascular stent pressure grip self-expansion, 80 mm Hg/160 mm Hg blood pressure pulsation of the vascular wall, and analyzes the gradual pressure grip of Φ8x30mm, Φ10x30mm, Φ12x30mm three L-shaped vascular stent to 4mm The results show that the maximum stress and maximum plastic strain of the three brackets show an increasing trend with the increase of the amount of pressure during the clamping process, but there is no significant change in the maximum stress and strain distribution area of the bracket, all concentrated on the inside of the arc of the connecting area between the support body and the connecting body, which provides theoretical guidance for optimizing the mechanical properties of the bracket at body temperature and determining the best heat treatment setting process. By calculating the alternating stress and average stress values of the three stents, the Goodman diagram of the fatigue strength of the stent is drawn, and the fatigue life of the vascular stent after placement is analyzed. The results show that the stress distribution area maps of the three brackets are located below the straight line of the fatigue limit, which meets the fatigue life requirements of more than 10 years, and can achieve the predetermined use effect. The maximum danger point of the three kinds of brackets is far from the fatigue limit, which means that the nitinol bracket will not have fatigue fracture. The stress point distribution area is far from the fatigue limit, which means that the bracket as a whole is not prone to collapse and large deformation failure.

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