CN115099077B

CN115099077B - Optimized design method of blood vessel self-folding bracket structure and blood vessel self-folding bracket structure

Info

Publication number: CN115099077B
Application number: CN202210440770.2A
Authority: CN
Inventors: 张俐; 李嘉诚; 张国涛; 田傲翔; 陈鑫硕; 陈彦儒
Original assignee: Huazhong University of Science and Technology
Current assignee: Huazhong University of Science and Technology
Priority date: 2022-04-25
Filing date: 2022-04-25
Publication date: 2024-06-25
Anticipated expiration: 2042-04-25
Also published as: CN115099077A

Abstract

The invention provides an optimized design method of a blood vessel self-folding bracket structure and a blood vessel bracket structure. According to the method, a shape memory polymer is used as a stent material, a moment loading mode is used for calculating the minimum moment of the vascular stent required to be folded on the basis of the Prony series of the material, and the forward-reverse design of the optimized stent structure is completed by combining a reverse engineering technology, so that the vascular self-folding stent structure is obtained. The structure has a porous structure, compared with an unoptimized structure, the bearing capacity is higher than 1.08N, the structure has equivalent blood flow pressure bearing capacity, and the flow velocity of blood flow in the porous structure is lower; under comparable load conditions, the maximum stress increases by 4.9% under tensile load. The stress in the flexibility test is increased by about 3MPa, the stress on the vessel wall is lower and is 0.3755MPa, the damage to the vessel caused by expansion and stability of the vessel stent can be effectively reduced, and further the occurrence of restenosis of the vessel can be effectively resisted.

Description

Optimized design method of blood vessel self-folding bracket structure and blood vessel self-folding bracket structure

Technical Field

The invention relates to the technical field of medical equipment, in particular to an optimized design method of a blood vessel self-folding bracket structure and the blood vessel self-folding bracket structure.

Background

The application of the vascular stent often needs to carry out intensive research on material science, and the design of the vascular stent needs to fully consider the comprehensive unification of materials and structures. Wherein China patent 2015106248077 discloses a novel vascular stent, which consists of a cutting stent and a woven stent. The vascular stent comprises a supporting gold rib and a connecting rib, wherein the supporting rib parts are connected by the connecting rib. The supporting rib is a medical stainless steel cutting tower, the structure is a wave-shaped bracket ring, the interface of the supporting rib adopts a variable cross-section structure, and the cross-section area of the supporting rib corner is the largest; the connecting rib is a self-expansion type braided bracket braided by metal wires formed by shape memory alloy. The cross-sectional area of the supporting gold is larger than that of the connecting rib metal wires, the supporting gold is a main supporting part of the bracket, the connecting rib formed by the self-expansion type braided bracket is braided by thin metal wires, the flexibility is larger than that of the cutting bracket, the flexibility of the whole bracket is improved, and certain supporting performance is provided. The design process of the vascular stent adopts a genetic algorithm, and the four-unit cell, six-unit cell and eight-unit cell modes are respectively applied to assembly in the assembly process of the structure, and finally the polylactic acid resin modified shape memory polymer is used for completing the manufacturing of the stent structure, and the high reliability of the eight-unit cell stent structure is verified through three-point bending and compression tests, so that the self-recovery property of the material is verified under the loading condition of a high temperature environment.

In the structural design and manufacturing process of the first type of vascular stent, the traditional shape memory alloy is selected as a material, the manufacturing is realized by applying a braiding mode, the structural deformability after braiding is generally poor, the balloon structure is required to be used for re-expanding, and the implantation and the extraction of the balloon are both required to be performed by a secondary operation, so that the pain of patients is improved; the second type of stent uses shape memory polymer and verifies the self-recovery characteristic of the shape memory polymer, but the structural design is too simple, and genetic algorithm is applied, and single-layer four-unit, six-unit and eight-unit assembly modes are adopted, obviously, the tube diameter gradually increases along with the increase of the number of units, if the tube diameter is kept to float up and down in a small interval, the size of the single unit is also changed, the design of the whole structure is meaningless, and the occurrence of restenosis of the blood vessel is not effectively solved.

Disclosure of Invention

The invention aims to provide an optimized design method of a blood vessel self-folding bracket structure and the blood vessel self-folding bracket structure aiming at the defects in the prior art.

In order to achieve the above purpose, the present invention adopts the following technical scheme:

the invention provides an optimized design method of a blood vessel self-folding bracket structure, which adopts a shape memory polymer as a bracket material, calculates the minimum moment of the blood vessel bracket required to be folded by applying a moment loading mode on the basis of the Prony series of the material, and completes the forward-reverse design of the optimized bracket structure by combining a reverse engineering technology to obtain the blood vessel self-folding bracket structure, and the method comprises the following specific steps:

Step S1, establishing a shape memory polymer viscoelastic material mathematical model to obtain Prony series;

Step S2, establishing an initial configuration of the vascular stent, wherein the radius of the vascular stent structure of the initial configuration is calculated as follows:

c _min＝πd_min.apprxeq. 10.242mm (2)

L=2×0.7mm×7=9.8 mm (formula 4)

SPATIAL REST = 10.242mm-9.8 mm=0.442 mm (formula 5)

Wherein R _min is the inner diameter of the stent when the vessel radius is the minimum value, C _min is the inner circumference of the vessel stent when the vessel radius is the minimum value, l is the length of one side of the flat plate structure, N _dual _units is the number of dual units, and R _total is the outer diameter of the reconstruction structure;

Step S3, an optimization model is established, self-folding load and boundary conditions are defined, based on a SIMP method frame, structural division is carried out by taking minimum flexibility as a structural optimization target, and forward modeling software is adopted for optimization, so that the minimum moment required to turn over the vascular stent is obtained;

s4, reversely modeling the optimized result of the forward modeling software by adopting reverse modeling software, and evaluating the reversely modeled model to obtain a vascular stent unit structure;

and S5, taking the vascular stent unit structure obtained in the step S4 as an initial unit structure, and obtaining an optimized vascular stent structure through model measurement, and reassembling and folding by Solidworks.

Further, the shape memory polymer is one of polylactic resin, polycaprolactone and polyurethane.

Further, in the step S2, the structural unit of the initial configuration of the vascular stent is a diamond-shaped reticular structure, the designed initial configuration of the vascular stent has an inner diameter of 3 mm-4 mm, an outer diameter of 3 mm-4 mm and a wall thickness of 0.1 mm-0.2 mm; preferably 3.26mm in inner diameter, 3.39mm in outer diameter and 0.13mm in wall thickness.

Further, the material Prony series viscoelastic expression is:

Where G (t) = 0.5574Mpa when t= infinity.

Further, in step S3, the forward modeling software includes Hyperworks software.

Further, the self-folding load is defined as blood flow normal pressure, and the self-folding boundary condition is defined as moment.

Further, the structure dividing process is as follows: and (3) adopting HYPERMESH regular tetrahedron units to carry out structural division on the geometric model obtained in the step (S2), wherein 70% of the volume is reserved and 30% of the volume is used for optimal design.

Further, the reverse modeling software comprises geomic software.

Further, in step S4, the vascular stent unit has a porous structure.

The invention also provides a blood vessel self-folding bracket structure which is obtained by optimizing the blood vessel self-folding bracket structure optimizing design method.

The technical scheme provided by the invention has the beneficial effects that:

(1) The invention provides an optimized design method of a blood vessel self-folding stent structure, which adopts a shape memory polymer as a stent material to establish an initial configuration of the blood vessel stent, and based on the Prony series of the material, the numerical calculation of mechanical loading boundary conditions is completed by applying torsional moment, elastic modulus with temperature change effect and section moment, and model establishment and reverse modeling of the structure after topological optimization are respectively completed by means of HYPERMESH and Geomagic software. The optimized vascular stent structure has a porous structure, has equivalent blood flow pressure bearing capacity compared with an unoptimized structure, and has lower flow velocity of blood flow in the porous structure; under comparable load conditions, the maximum stress increases by 4.9% under tensile load. The stress in the compliance aspect test was increased by about 3MPa.

(2) The self-folding simulation analysis of the optimized vascular stent shows that the porous structure can improve stress distribution in the turnover and compression processes and reduce the risk of neck fracture; the expansion force simulation shows that under the same pressure load of the blood vessel, the stress of the optimized structure on the wall of the blood vessel is lower and is 0.3755MPa, so that the damage to the blood vessel caused by expansion and stability of the blood vessel stent can be effectively reduced.

(3) The optimized structure has better restorability, and the structure is closer to the initial structure after being restored; in the radial test, the load-displacement curves of the original model and the optimized model are obtained by applying a flat compression method, and the test further shows that the porous structure has better mechanical bearing performance, so that the porous structure has higher supporting capability when being subjected to the extrusion action from the atherosclerosis tumor after being actually implanted; finally, through a flexibility test analysis, the structural mechanical response capability of the optimized stent unit structure under the action of three-point load is superior to that of an unoptimized structure, the bearing capacity is higher than that of the unoptimized structure by 1.8N, and the occurrence of restenosis of blood vessels can be effectively resisted.

Drawings

FIG. 1 is a schematic flow chart of a method for optimizing the design of a blood vessel self-folding stent structure;

FIG. 2 is a graph showing a 45℃fit of stress relaxation data for the shape memory polymer of example 1;

FIG. 3 is a line graph of Prony terms versus residual;

FIG. 4 is a model of a stent unit and a dimensional map, where a is an open stent model map, b is a map of a single stent structure, and c and d are single stent structure parameter maps;

FIG. 5 is a diagram of the original structure of a reconstructed vascular stent;

FIG. 6 is a schematic diagram of design domain and non-design domain partitioning and load application;

FIG. 7 is an iterative view of the structural optimization of a vascular stent unit;

FIG. 8 is a reverse diagram of an optimized structure and a processing model thereof, wherein a is a structure optimized by Hypeworks Optistruct modules, b is a model diagram of reverse modeling by Geomagic software, c shows that the matching degree of a reverse modeling model and an STL file is higher, the corresponding reverse modeling precision is higher, and d is a partial model structure diagram after turnover;

FIG. 9 is a plan view of an optimized stent and a tubular structural view thereof;

FIG. 10 is a comparative view of a static simulation of a tubular structure;

FIG. 11 is a finite element model of meshing a stent-vessel coupled structure in combination with Fluent meshing;

FIG. 12 is a comparative graph of fluid-solid coupling simulation of a tubular structure;

FIG. 13 is a graph of the maximum stress trend during self-folding of a tubular structure;

FIG. 14 is a schematic illustration of a simulation of the implantation of a stent into a vessel;

FIG. 15 is a comparison of vascular stent structure, wherein a is an optimized vascular stent structure object diagram and b is an un-optimized vascular stent structure object diagram;

FIG. 16 is a diagram showing the initial structural shape of the stent obtained in example 1;

FIG. 17 is a graph of spontaneous bending deformation of a stent of non-optimized configuration;

FIG. 18 is a spontaneous bending deformation diagram of an optimized structure;

FIG. 19 is a compressive load-displacement curve of a vascular stent structure;

FIG. 20 is a graph comparing three-point bending load versus displacement curves of a vascular stent structure.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the following detailed description of the specific embodiments of the present invention will be given with reference to the accompanying drawings.

The main components of the shape memory polymer used in the application are prepared by polylactic acid resin (PLA), polycaprolactone (PCL), polyurethane (TPU) and other high molecular organic matters through complex chemical reactions such as polycondensation reaction, addition reaction and the like, the shape memory polymer material selected in the specific embodiment is purchased from Shenzhen Guanghua Wenyujin company, the deformation temperature interval given by the shape memory polymer material is 45-90 ℃, and the inventor carries out the analysis and test of stress relaxation, high-temperature stretching, dynamic thermal mechanical detection, thermal pyrolysis and differential thermal scanning on the shape memory polymer material to obtain the performance characteristics: the initial decomposition temperature was starting from 165.411 ℃ and continuing to 291.057 ℃ and rapidly decomposing after 291.057 ℃; the crystallization temperature starts at 78 ℃ and ends at 90 ℃, and the accurate temperature value is 85.86 ℃; the elastic modulus at 25 ℃, 35 ℃,45 ℃, 55 ℃, 65 ℃ is 695.48MPa, 529.35MPa, 535.88MPa, 438.65MPa and 472.72MPa respectively.

Example 1

Referring to fig. 1, a flow chart of the optimized design method of the self-folding stent structure of the present invention includes the following steps:

the inventor describes the shape memory characteristic of the shape memory polymer by adopting a multi-element bonding pot model, and couples a WLF equation to complete the physical property parameter input under the material thermodynamics, and finally establishes a shear modulus-time stress relaxation curve conforming to ANSYS application by means of a Prony series fitting algorithm, thereby providing a set of material library with high feasibility and large data volume for subsequent topological optimization and finite element simulation.

The inventor can obtain according to the deduction of the constitutive theory of the shape memory polymer: the fitting of the stress relaxation curve and the establishment of the Prony series require a larger amount of separation, and the shear modulus G (t) value is calculated using the stress relaxation data in the main curve and equation (8).

TABLE 1 calculation of time and shear modulus experimental data

Inputting the values of Table 1 into the SHEAR DATA-Viscoelastic list of ANSYSDatabase, performing the Curve Fit operation, a Curve Fit map as shown in FIG. 2 can be obtained. And then, completing the establishment of the shape memory polymer viscoelasticity mathematical model by means of the rapid inverse function of the Prony series. The initial value setting of Prony series can improve the fitting precision and can be rapidly calculated according to the equation of the fitting curve.

To ensure the fitting accuracy and the calculation complexity, the number of terms of the Prony series is determined in advance. The least squares code is written using Python and the modulus at the Prony scale and the modulus actually measured for stress relaxation are shown by E _prony、E_meas, respectively. The effect of the Prony term number on the fitting error is now calculated by calculating the difference between each fitting point and the fitted curve. As can be seen from fig. 3, when the number of Prony series terms reaches 4, the error R ² fluctuates in a range between 2.5-2.8 (and R ² _opt≈0.75R₀ ²). It should be noted that as the number of Prony series items increases, the fitting accuracy of the material naturally increases, but a large number of items approaching zero appear, which reduces the calculation efficiency.

And setting a fourth-order Prony series for solving. The time τ is selected to be equal to 1, 100, 1000, and 1E5 as initial values, and the Seed Value (guess Value) and Calculated Value (actual Value) are obtained through ANSYS solving and calculation, as shown in table 2.

TABLE 2 Seed Value and Calculated Value Value

And performing fitting calculation on the Seed Value and Calculated Value in the table again to obtain the series fitting data of the table.

TABLE 3 Prony series fitting Table

Note that: SQRT represents the square root rounded off by comparison of Table and equation 7 to infinitely small amounts, the Prony series viscoelastic expression for the shape memory polymers of the present application can be obtained:

where G (t= infinity) = 0.5574MPa, which is 1.34% different from 0.55MPa obtained in the experimental data, is within the numerical allowable range.

Step S2, establishing an initial configuration of the vascular stent:

Applicant uses the stent cell structure as shown in fig. 4a as an initial cell structure, and uses SolidWorks to regenerate an initial geometric model through model measurements. An open stent model is shown in fig. 4a, the structure itself has highly symmetrical features, a graphical representation of a single stent structure is shown in fig. 4b, and its structural parameters are shown in fig. 4c and fig. 4 d. According to the industry standard of implantable medical devices of the people's republic of China, the diameter range of human blood vessels is 3.26 mm-4.16 mm, the wall thickness of blood vessels is 0.2 mm-0.5 mm, and the thickness of the allowed implanted stent is 0.025 mm-0.177 mm. Here, in order to ensure the effectiveness of stent design and triggering of the self-folding properties of the shape memory polymer, the present application reconstructs the structure to effectively avoid incomplete numbers of stent units in the tubular structure.

Meanwhile, the whole vascular stent structure is not closed by the application of the integral vascular stents, and the self-folding characteristic triggering of the vascular stents can be further ensured by the increase of the gaps (SPATIAL REST). The overall inner circumference of the stent was approximately 10.242mm (pi. Times.1.63.times.2 mm) as calculated by reconstruction. Meanwhile, the thickness of the bracket structure is 0.13mm, the inner diameter is 3.26mm (the total radius R _total of the whole structure is about 1.76 mm), and the specific calculation process is shown in the formulas 1 to 6.

C _min＝πd_min.apprxeq. 10.242mm (2)

L=2×0.7mm×7=9.8 mm (formula 4)

SPATIAL REST = 10.242mm-9.8 mm=0.442 mm (formula 5)

Where r _min and C _min represent stent radius and circumference. l is the length of one side of the flat structure (as shown in fig. 4 a), N _dual _units is the number of dual units, and R _total is calculated to avoid errors in design. Numerical calculation shows that the reconstruction meets the medical standard, and the reconstructed model is shown in figure 5.

Step S3, an optimization model is established, self-folding load and boundary conditions are defined, based on a SIMP method frame, structural division is carried out by taking the minimum flexibility as a structural optimization target, forward modeling software is adopted for optimization, and the minimum moment required to turn over the vascular stent is obtained:

The Prony series parameters established in step S1 are input Hyperworks into the software and the module Optistruct is started, and the load application schematic diagram is shown in FIG. 6. To avoid optimization failure, red areas are set as optimization areas, gray areas are non-design areas, and normal tetrahedral units of HYPERMESH are applied for structural division, and the unit tables are shown in table 4.

TABLE 4 mesh node partition number

According to the bending characteristics of the shape memory polymer, the deformation recovery moment M of the shape memory polymer can be represented by (formula 8) when the influence of the self weight of the shape memory polymer is ignored.

M=m ₁+M₂ (8)

Wherein M ₁ is a restoring moment generated by the shape memory polymer material itself; m ₂ is the restoring moment generated by the shear stress of the structure, which is compounded by a plurality of layers of shape memory polymers of different materials. In this context, M ₂＝0,M₁ can be represented by the meaning that the vascular stents are made of the same material.

To properly apply the load moment, the model is cut along line AD in fig. 6c, and the moment M required to be applied to the rectangular surface and the section moment I _S are calculated, and the calculation processes are shown in (formula 10) to (formula 12).

According to the SIMP classical topology optimization model, taking the minimum flexibility as a structural optimization target, the mathematical model of topology optimization is expressed as:

Subjec to:

:KU＝F

:k_e＝(x_e)^pk₀

:

x _min is more than or equal to 0 and less than or equal to 1 (14)

Wherein K represents a rigidity matrix, U represents a displacement matrix, and U ^T represents a transposed matrix of the displacement matrix; k _e,u_e represents a unit stiffness matrix transpose matrix, a unit stiffness matrix and a unit displacement matrix respectively; f represents a volume fraction; r represents the outer diameter of the vascular stent; x _min represents the minimum value of the design variable (non-zero constant) avoiding the matrix singularity of the calculation process.

Moment can act as a boundary condition for self-collapse, but vascular stents are also routinely subjected to fluid pressure from the blood stream. Moment and blood flow are loaded on the model body at normal pressure in Hyperworks Optistruct, and the structural materials are set to be reserved for 70 percent and the rest 30 percent for optimal design, wherein the optimization process and the volume change are shown in figure 7.

The Hyperworks software optimization result is usually in STL format, the Geomagic software is applied to carry out reverse modeling, meanwhile, the reverse modeling model is evaluated, and the Color Bar under the Volume Evaluation module in Geomagic can show the feasibility and the accuracy of the reverse modeling.

As shown in fig. 8, fig. 8a is a Hypeworks Optistruct module optimized structure; FIG. 8b shows a model of the reverse modeling of Geomagic software; in the graph c, green shows that the matching degree of the reverse modeling model and the STL file is higher, and the corresponding reverse modeling precision is higher; and the graph d shows the part of the model structure after being folded.

The final optimized stent structure (as shown in fig. 9) is obtained by reassembling and folding the soludworks with the structure of fig. 8c as the initial unit structure.

In order to verify the performance of the vascular self-folding stent structure obtained by adopting the vascular self-folding stent structure optimization design method, the inventor performs the following research:

(1) Vascular stent statics simulation analysis

Half of the optimized and non-optimized structures of example 1 were selected for performance, respectively. One end of the ANSYS is fixed, and tensile simulation is carried out on the stress of 0.05MPa in the X positive direction; one end is fixed, and the other end is applied with 0.01N to complete the flexibility simulation; the intravascular wall fluid load at 0.06MPa was simulated to further verify the stress response of the structure under static load.

The simulation data are drawn into a histogram (as shown in fig. 10), and it can be seen that under the stress stretching condition of the structure at 0.05MPa, the stress difference between the optimized structure and the original structure is 0.4481MPa, which indicates that the bearing capacity of the original structure and the optimized structure in terms of stress is equivalent; for the flexibility test, the maximum stress of the optimized structure is increased by 4.7404MPa, the displacement is increased by 0.33082mm equally, and the structural holes are increased, so that the unidirectional load resistance performance is reduced to a certain extent, but the maximum stress does not exceed the strength limit of 20MPa.

(2) Vascular stent fluid-solid coupling simulation analysis

The static simulation analysis of the vascular stent can prove that the vascular stent performance subjected to structural optimization is generally superior to that of the vascular stent with the original structural configuration under the same load condition. Since vascular stents are actually implanted into the human body for working, and blood flow often appears in a turbulent flow form, the flow performance is complex, and the simulation of the vascular stents needs to be completed by means of ANSYS Fluent. It is noted that in fluid theory, when the flow rate is low, the flow can be considered to be laminar (Laminar Flow); while as the flow rate increases, the flow needs to take into account turbulence effects (Turbulent Flow) with the attendant occurrence of eddies, interlaminar slip, fluid mixing. Because the blood velocity in the human body flows faster, the flow is set to simulate the turbulence in order to consider the influence of the stent on the blood flow.

The inventor establishes a finite element model for coupling a vascular structure and a vascular stent, wherein the length of a blood vessel is 25mm, the inner diameter and the outer diameter respectively take 3.6mm and 4.6mm, and the wall thickness is 0.5mm. Meshing the stent-vessel coupled structure in combination with Fluent meshing functions results in a structure as shown in fig. 11, wherein the black portions represent vessel mesh structures and the green portions represent embedded vessel stents.

The data of fluid-solid coupling are plotted into a bar graph (as shown in fig. 12), and from the simulation data in the blood of the blood vessel, the data of the optimized model are generally lower than the original model, and the pressure and equivalent stress of the blood flow on the blood vessel stent are far lower than the material strength limit of 20MPa. Meanwhile, in the optimized model, due to the existence of the hole structure, the speed and the turbulence kinetic energy in the blood vessel are 0.9188m/s and 0.4312m ²/s² respectively, the speed of blood flow passing through the small hole is slowed down, and the turbulence of the restenosis part of the lesion blood vessel is reduced. In the simulation data of wall pressure and stress borne by the bracket, the bearing pressure is equivalent, and the shearing stress is smaller in value, so that the structure is more stable in a flowing environment. Clearly, a structure with lower shear stress has more reliable stability and longer service time.

(3) Vascular stent self-folding memory process simulation analysis

If the support structure is required to be folded from the planar structure to the curved surface structure, a column body with the same inner diameter as the tubular support is required to be placed right above the planar structure, so that simulation stopping conditions of the structure are arranged in ANSYS. The upper auxiliary column body is set to be Fixed constraint (Fixed Support) under an ANSYS LS-DYNA environment, the lower plate structure loads moment M along the central axis direction of each unit structure, and the upper surface of the plate structure is set to be a Contact surface and the outer surface of the cylinder is set to be a Target surface because the grid division density of the plate-type vascular stent structure is large and is the object mainly considered in the simulation process. In addition, a Contact mode is set on the Contact surface of the plane structure and the cylinder structure to be of a bound type, an asymmetric solution (ASYMETRICAL BEHAVIOR) is selected as a solution mode, and the stress limit (STRESS LIMIT) is 400MPa, namely, the Contact surface and the Target surface are fixed as long as the Contact surface is in Contact, and the simulation precision is improved by taking the Contact surface and the Target surface as conditions for stopping simulation.

The simulated stress and displacement values are plotted as shown in fig. 13. The original structure can be read out that the original structure has smaller maximum stress by virtue of the non-porous configuration in terms of maximum stress and withstanding the same magnitude of folding stress; when the structure subjected to topological optimization bears the same load, the stress concentration degree of the structure is reduced compared with that of the original structure due to the appearance of the small hole structure, so that the rapid fracture of the neck part in the original structure is avoided, the maximum stress of the structure after self-folding and folding is 12.85MPa, and the maximum stress is less than the strength limit of 20MPa, and the structure can still be used.

(4) Vascular stent distraction force simulation analysis

And selecting the condition of the vertebral artery blood vessel with the blockage rate of 25%, establishing a finite element model of the blood vessel in a blockage state, and implanting a vascular stent. Wherein the inner diameter of the blood vessel is 3.6mm, and the inner diameter of the blood vessel after blockage is 2.7mm. In the actual setting simulation of ANSYS MECHANICAL, the contact between plaque and vessel wall was set to bound. In order to simulate the blood flow pressure caused by heart beating, the normal pressure is set to be 0.15MPa on the inner wall of a plaque in the first simulation process, the average blood flow pressure is restored to be 0.06MPa in the second simulation process, bound contact is inhibited in the first process, and the second process is revalidated.

The simulation cloud image of the coupling stress of the blood vessel and the bracket implanted in the inner wall shown in fig. 14 can be obtained through simulation. It can be seen from the cloud chart that when the optimized stent structure is implanted into a blood vessel, the surface stress of the blood vessel is reduced from 0.37592MPa to 0.3755MPa, and the stress level is lower under the same blood pressure condition, namely the vascular stent is less traumatic to the inner wall of the blood vessel. Meanwhile, the stress level of the two stent structures is less than 0.1MPa and is far lower than the stress yield limit of the vascular stent material used in the process, so that long-term service can be realized in a blood vessel.

(5) Vascular stent structure mechanical property test

The inventor carries out amplification manufacturing and testing on the structure, and verifies the effectiveness of the simulation through an actual test experiment to indirectly reflect the feasibility of the actual structure. In terms of structural test and mechanical property characterization, the mechanical property test method of the degradable polymer vascular stent mainly comprises the following steps: radial retraction, axial collapse, radial support, and compliance testing. Because the stent structure and the materials used in the application have shape memory effect, the radial retraction and axial shortening test are characterized by shape memory characteristics; for the test of radial support, a flat compression method is adopted; finally, in the test of the flexibility, a three-point bending test method is adopted to finish the test.

Additive manufacturing: firstly, the shape memory polymer wire is installed into a 3D printer through a wire feeding head, and the pretreatment preparation work before printing is finished by adjusting the temperature of a printing platform (210 ℃) and the temperature of a spray head (35-55 ℃) to 45 ℃. The vascular stent model designed herein is magnified 20 times by [ scaling ] command by means of Solidworks software, and the STL format is selected for 3D printed slice processing when stored. The print boundary dimensions after enlargement were 66.8231mm x 55.825mm x 120.3076mm, and the actual print volumes of the structures before and after optimization were 16745.45mm ³、12688.83mm³, respectively. And adopting Cura slicing software, setting the printing layer height to be 0.15mm, and setting the first sacrificial layer number and the top layer number to be 5. After dicing, the G-code adapted for FDM additive manufacturing is loaded into the Go Print 3D printer through a storage medium (Flash memory card). The 3D printed stent structure is shown in fig. 15a, and fig. 15b is an initial structure.

Shape memory property test of vascular stents: the model used in this part of the experiment was cylindrical in structure, with a water bath temperature of 42.15℃being selected. The specific experimental test steps are as follows:

1. Injecting a proper amount of hot water and cold water into the plastic water tank, uniformly stirring, and ensuring the temperature to be 42.15 ℃;

2. placing the vascular stent structures with two structural designs into a water tank, standing for 3 minutes, and softening the structures;

3. taking out the softening structure, and fixing the structure into a planar plate-shaped structure by using a shape fixing clamp;

4. standing for 3 minutes, and after the surface temperature of the structure is reduced to be lower than the self-deformation temperature, soaking the structural part in the water tank again, and observing the deformation of the structure.

The used fixing clamp is of a hollow structure, the inner thickness is equal to the thickness of the bracket, the length is equal to the maximum length of the bracket, the blood vessel bracket is quickly placed in the fixing clamp after being softened, and the structure of the blood vessel bracket after being fixed is shown in fig. 16. The recovery process of the structure is shown in fig. 17 and 18.

The self-bending deformation diagrams of the two structures show that the structure can be folded within 16s quickly, the folding degree of the structure with T=16s shows that the structure after optimization has higher folding property and stronger structural compactness. In the self-folding simulation of the optimized structure, the structural stress is higher, so that the folding performance of the structure is stronger than that of an unoptimized and non-hole structure.

Radial support test of vascular stents: radial support test of vascular stents: and the radial support performance analysis of the vascular stent is completed by adopting a flat plate compression test mode. To ensure the experimental temperature at the same time, the temperature of the incubator was set to 45℃and the instrument model was E1000 series manufactured by Instron group Co. The self-made flat plate is made of glass fiber plate material (shown in black), and the loading speed is set to be 2mm/min. The final plot yields a load-displacement graph as shown in fig. 19.

Testing of vascular stents often requires three phases: ① A nonlinear stage in initial compression, mainly because the contact between the flat plate and the bracket is linear; ② An elastic deformation stage, wherein the upper pressing plate is completely contacted with the bracket; ③ In the plastic deformation stage, as the deformation amount is increased continuously, the support is subjected to plastic deformation gradually, and when the deformation of the support reaches a certain amount, the connecting ribs are contacted with each other to further resist the deformation between the supports, so that the load and displacement curve is increased in a nonlinear manner. In practical tests, the non-linear curve of ① th stage appears during 0-15 mm; when the displacement of the upper pressing plate is 15mm, inflection points appear on load curves of the two models, the slope is a constant value, the increase of the load shows linear change, and the stage is the ② th stage, namely elastic deformation; when the displacement amounts reach 25.72mm and 26.49mm respectively, the curves of the two models are reduced and the curves are plastic deformation is shown when the curves are increased again, the main reason is that the structure is broken for the first time, the existence of the connecting ribs can avoid the complete breakage of the structure, and in the optimized structure, the rib parts of the structure are not subjected to main pressure any more due to the existence of the hole structures, and are converted into the rest of the structure, so that the structure can bear larger load values before the breakage. After the optimization is obtained through labeling, the maximum bearable load of the model is close to 50N, and the resistance of the model is increased by 3.5N compared with that of an unoptimized model.

Compliance testing of vascular stents: the test was performed on a stent unit structure at a loading rate of 1mm/min and at a test temperature of 45 c using an E1000 series instrument available from instron corporation. As shown in fig. 20, the material itself has exhibited viscoelastic properties due to the incubator action, and both of the structure test curves exhibit nonlinear increases. Compared with the non-optimized structure, the optimized vascular stent unit structure has the maximum load which can be carried by 1.8N more, and the compressed load is the same as the compressed load of the vascular stent, because the optimized structural unit has more structural distribution of the supporting rib type, the load caused by the downward pressing of the test head can be effectively improved, and the longer service time of the structure is realized. It should also be noted that the structure suddenly fails beyond the maximum load, but the optimized structural unit may obviously fail under a larger displacement load, further explaining the distribution of the structure in the form of the support ribs, which may effectively increase the load carrying capacity.

From a material point of view, the invention creatively uses polymer materials with shape memory characteristics to design and manufacture a porous vascular stent structure. In the structural design process, the minimum moment of the vascular stent which needs to be folded is calculated by applying a moment loading mode, and the forward-reverse integrated design of the optimized stent structure is completed by combining a reverse engineering technology. And meanwhile, after the design is finished, the optimized and non-optimized structures are subjected to multi-performance comparison, the superiority of the optimized support structure under the conditions of spontaneous turnover, flow characteristics and static stress response is verified through a simulation mode, and the structure has more excellent mechanical resistance under the action of mechanical load by combining with actual additive manufacturing.

In the aspect of performance characterization of materials, a multi-element Maxwell viscoelastic constitutive model is deduced from a single Maxwell unit and a Kelvin unit, after the accuracy requirement of numerical calculation is fully considered, a mathematical characterization method of the viscoelastic constitutive model is given out in an integral form, a Python least square code is written in combination with a numerical fitting method, and an optimal value of Prony order is calculated, so that a four-order Prony order is established to meet the characterization accuracy of a material constitutive equation, and finally topology optimization of the viscoelastic material is realized by means of a Hyperworks Optistruct module. The vascular stent obtained by optimization of the invention can flexibly adapt to the change of soft tissue implantation environment, and can be spontaneously folded under the external excitation of temperature to form the vascular stent, thereby providing a favorable environment for cell proliferation.

The embodiments described above and features of the embodiments herein may be combined with each other without conflict.

The foregoing description of the preferred embodiments of the invention is not intended to limit the invention to the precise form disclosed, and any such modifications, equivalents, and alternatives falling within the spirit and scope of the invention are intended to be included within the scope of the invention.

Claims

1. The optimized design method of the vascular self-folding stent structure is characterized in that a shape memory polymer is adopted as a stent material, a moment loading mode is applied to calculate the minimum moment of the vascular stent required to be folded on the basis of the Prony series of the stent material, and the forward-reverse design of the optimized stent structure is completed by combining a reverse engineering technology, so that the vascular self-folding stent structure is obtained, and the method comprises the following specific steps:

s1, establishing a shape memory polymer viscoelastic material mathematical model to obtain Prony series;

s2, establishing an initial configuration of the vascular stent, wherein the radius of the vascular stent structure of the initial configuration is calculated as follows:

C_min＝πd_min≈10.242mm (2)

l＝2×0.7mm×7＝9.8mm (4)

Spatial Rest＝10.242mm-9.8mm＝0.442mm (5)

Wherein R _min is the inner diameter of the stent when the vessel radius is the minimum value, C _min is the inner circumference of the vessel stent when the vessel radius is the minimum value, l is the length of one side of the flat plate structure, N _dualunits is the number of dual units, and R _total is the outer diameter of the reconstruction structure;

S3, establishing an optimization model, defining self-folding load and boundary conditions, carrying out structural division by taking the minimum flexibility as a structural optimization target based on a SIMP method frame, and optimizing by adopting forward modeling software to obtain the minimum moment required to turn over the vascular stent;

S4, reverse modeling is carried out on the result optimized by the forward modeling software by adopting reverse modeling software, and meanwhile, a reverse modeling model is evaluated to obtain a vascular stent unit structure;

s5, taking the vascular stent unit structure obtained in the step S4 as an initial unit structure, and obtaining an optimized vascular stent structure through model measurement, and reassembling and folding by Solidworks.

2. The method for optimizing the design of a vascular self-folding stent structure according to claim 1, wherein the shape memory polymer is one of polylactic resin, polycaprolactone and polyurethane.

3. The optimized design method of a vascular self-folding stent structure according to claim 1, wherein in the step S2, the structural units of the initial configuration of the vascular stent are diamond-shaped net structures, the designed initial configuration of the vascular stent has an inner diameter of 3 mm-4 mm, an outer diameter of 3 mm-4 mm and a wall thickness of 0.1 mm-0.2 mm.

4. A method of optimizing the design of a vascular self-folding stent structure as in claim 3, wherein the initial configuration of the vascular stent has an inner diameter of 3.26mm, an outer diameter of 3.39mm and a wall thickness of 0.13mm.

5. The optimized design method of a vascular self-folding stent structure according to claim 2, wherein the material Prony series viscoelasticity expression is:

Where G (t) = 0.5574Mpa when t= infinity.

6. A method for optimizing the design of a vascular self-folding stent structure as defined in claim 3, wherein in step S3, the forward modeling software includes Hyperworks software.

7. A method of optimizing the design of a vascular self-folding stent structure as defined in claim 3, wherein the self-folding load is defined as blood flow normal pressure and the self-folding boundary condition is defined as torque.

8. The method for optimizing the design of the self-folding stent structure of claim 6, wherein the structural division process is as follows: and (3) adopting HYPERMESH regular tetrahedron units to carry out structural division on the geometric model obtained in the step (S2), wherein 70% of volume is reserved, and the volume which is less than or equal to 30% is used for optimizing the design.

9. The method of optimizing the design of a vascular self-folding stent structure as defined in claim 7, wherein in step S4, the reverse modeling software includes geomic software.

10. The method for optimizing the design of a self-folding stent structure according to claim 7, wherein in the step S4, the stent unit structure has a porous structure.

11. A vascular self-folding stent structure, characterized in that the vascular self-folding stent structure is optimized by adopting the vascular self-folding stent structure optimizing design method as defined in any one of claims 1 to 10.