A design method for a vascular stent structure with bistable performance

Abstract

The present invention provides a method for designing a vascular stent structure with bistability, comprising: topologically optimizing a unit cell using a bidirectional evolutionary structural optimization method; calculating sensitivity as the sum of the total structural strain energy U, which includes elastic strain energy Ue and plastic strain energy Up, i.e., U=Ue+Up; and obtaining a unit cell structure with bistability through multiple iterative evolutions; further determining the relationship between the bistability of the unit cell and its structural dimensions based on the optimized bistable unit cell structure; selecting a set of dimensional parameters as verification and using finite element simulation analysis to obtain energy-displacement and force-displacement curves of the unit cell structure to determine whether they conform to the bistability mechanical performance curve; and arranging the unit cell structure and curling it into a tubular vascular stent. The vascular stent structure designed by the present invention has bistability, i.e., the vascular stent has two stable states, both of which enable the vascular stent to maintain structural stability.

Description

Technical Field

[0001] The invention provides a method for designing a vascular stent structure with bistable performance, belonging to the technical field of medical devices. Background Art

[0002] A vascular stent is a precision medical device in the form of a mesh tube that is used to prop open blocked diseased blood vessels to restore normal blood flow. Vascular stents are an indispensable treatment for cardiovascular disease. After being inserted into the diseased vessel, the stent expands radially through balloon inflation or self-expansion, propping open the blocked vessel to restore blood flow and treat atherosclerosis. Stent structural design is a highly complex engineering task that must take into account multiple performance requirements. Design considerations include, but are not limited to, tissue interaction, hemodynamic performance, stent mechanical behavior, and stress caused by drug delivery (in the case of drug-eluting stents).

[0003] One of the main challenges in designing stents is that the stent cannot fully expand in the radial direction, which is a common phenomenon. The inability to fully expand may be due to insufficient mechanical strength of the vascular stent structure, which causes radial collapse under the pulsating pressure exerted by the blood vessel wall, and fails to achieve the therapeutic purpose of opening the blocked blood vessel. Or it may be uneven expansion caused by improper design of the stent structure layout, that is, different positions along the axis of the stent do not expand uniformly, but rather non-uniform expansion of varying degrees, which may cause damage to the blood vessel, thereby triggering an inflammatory response and leading to restenosis of the blood vessel. Therefore, the clinical performance of vascular stents is highly dependent on the mechanical properties of the stent. The design of vascular stents should avoid the radial rebound phenomenon caused by insufficient radial mechanical properties, so that the stent can maintain a radially expanded state after expansion.

[0004] Existing vascular stents primarily expand and maintain their expanded configuration through plastic deformation. This approach is limited by the stent material and is primarily applicable to metals, which are prone to plastic deformation. Highly elastic biopolymers, however, are difficult to deform plastically, or even nonexistent. Furthermore, during balloon inflation, because significant force is required to expand the stent and cause plastic deformation, overinflation can easily lead to balloon rupture or localized plastic deformation, resulting in stent damage, seriously endangering patient safety.

[0005] The design structure of a vascular stent using shape memory alloys primarily exploits the alloy's memory properties. The stent's expanded state is used as its initial state. After being compressed and delivered to the affected blood vessel, the stent is released, at which point it automatically expands to its designed initial state, thus opening the vessel. This method is limited to vascular stent materials, which utilize shape memory alloys and inherent material properties. Furthermore, due to the alloy's excessive mechanical strength, the alloy can straighten the vessel when implanted, causing damage to the vessel wall. Summary of the Invention

[0006] Due to the insufficient radial mechanical properties of existing vascular stents, they are unable to maintain their expanded shape. Even if plastic deformation is achieved, there are problems of radial rebound and collapse under the pulsating pressure of the blood vessels. The imprecision of relying on plastic expansion may cause the vascular stent structure to be unable to fully expand, and may also cause unevenness in the stent expansion structure. The present invention proposes a vascular stent with a bistable structure. By designing a new bistable structure, the stent has two stable states, namely a compression stable state and an expansion stable state. The stent can maintain a stable state only by relying on the structure, which can effectively solve the problem of insufficient radial mechanical properties of the above-mentioned stent. Since the vascular stent is formed by the arrangement and combination of unit cells of the same structure, the design of the present invention starts from the unit cell, and finally the unit cells are arranged and curled into a tubular vascular stent.

[0007] A method for designing a vascular stent structure with bistable performance comprises the following steps:

[0008] S1. Use a bidirectional evolutionary structural optimization method to perform topology optimization on an 8 mm × 6 mm unit cell. The unit cell is divided into a design domain and a non-design domain. At the intersection of the design and non-design domains, the unit cell is fixed along the X-direction. An external force of 100 N is applied to the unit cell at its lower center. The Young's modulus of the unit cell material is 2.0E+11 Pa, the Poisson's ratio is 0.3, and the density is 7800 kg / m3. The initial number of iterations is set to 50.

[0009] S2. Based on the bidirectional evolutionary structural optimization algorithm, the sensitivity is calculated as the sum of the total structural strain energy U, which includes the elastic strain energy Ue and the plastic strain energy Up, i.e., U = Ue + Up. The elastic strain energy Ue of the bistable structure has a local minimum. After multiple iterative evolutions, a unit cell structure with bistable properties is obtained.

[0010] S3. Based on the optimized bistable unit cell structure, further determine the relationship between the bistable performance of the unit cell and its structural size.

[0011] S4. Based on the design schemes from the first three steps, select a set of dimensional parameters for verification. Use finite element simulation to obtain the energy-displacement and force-displacement curves of the unit cell structure and determine whether they conform to the bistable mechanical performance curves. If so, proceed to the next step.

[0012] S5. Arrange the unit cell structure and curl it into a tubular vascular stent.

[0013] The present invention proposes a vascular stent with a bistable structure, which belongs to the field of medical device technology. The designed vascular stent structure has bistable performance, that is, the vascular stent has two stable states, both of which can enable the vascular stent to maintain structural stability. The vascular stent with bistable performance has a default contraction stable state. Once radial force is applied to the stent through a balloon, the stent will quickly jump to another completely stable expansion structural state. This gets rid of the dependence on the plastic deformation of the vascular stent material, but depends on the bistable structural performance. Through this design method, the vascular stent will be better controlled during the expansion process, and can maintain the expanded state of the stent without radial rebound and collapse under the pulsating pressure of the blood vessels, thereby achieving the purpose of effectively treating blocked blood vessels and restoring normal blood circulation in the human body.

[0014] The beneficial effects brought about by the technical solution of the present invention are:

[0015] 1. The bi-stable vascular stent structure designed in this invention can maintain its expanded state solely through structural stability, improving the mechanical strength of the stent, breaking free from the constraints of stent materials, and is compatible with existing stent materials, possessing broad application value.

[0016] 2. The bistable vascular stent of the present invention can be used to load or coat drugs, and has the dual functions of restoring blood circulation in blocked blood vessels and delivering drugs for treatment.

[0017] 3. The bistable stent of the present invention has a simple structure and can be processed in a flexible manner according to different materials. For example, metal materials can be cut by laser, and biopolymer materials can be manufactured by extrusion 3D printing. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 Bistable Vascular Stent Design Flowchart

[0019] Figure 2 Optimizing unit cell structure based on bidirectional evolutionary structure algorithm

[0020] Figure 3 Topology optimization results in a bistable unit cell structure

[0021] Figure 4 Theoretical model of bistable unit cell structure parameters

[0022] Figure 5 Verification of unit cell bistability performance

[0023] Figure 6 Bistable vascular stent DETAILED DESCRIPTION

[0024] The present invention proposes a method for designing a vascular stent with a bistable structure as follows: Figure 1 , is the design flow chart of bistable vascular stent:

[0025] A method for designing a vascular stent structure with bistable performance comprises the following steps:

[0026] S1. Using the bidirectional evolutionary structural optimization method, the topology optimization of the unit cell with a size of 8mm×6mm is performed, such as Figure 2 As shown, the unit cell is divided into the design domain ( Figure 2 Medium-light gray) and non-design domain ( Figure 2 Medium-dark gray). At the intersection of the design domain and the non-design domain, the unit cell is fixed along the X-direction degree of freedom. An external force of 100 N is applied to the unit cell at the bottom center. The Young's modulus of the unit cell material is 2.0E+11 Pa, the Poisson's ratio is 0.3, and the density is 7800 kg / m3. The initial number of iterations is set to 50.

[0027] S2. According to the bidirectional evolutionary structural optimization algorithm, the sensitivity can be calculated as the sum of the total structural strain energy U, which includes the elastic strain energy Ue and the plastic strain energy Up, that is, U = Ue + Up. The elastic strain energy Ue of the bistable structure has a local minimum. After multiple iterative evolutions, a unit cell structure with bistable performance is obtained, such as Figure 3 As shown in the figure, its energy has a local minimum. By differentiating the energy-displacement, we can obtain the force-displacement curve, in which the force has a positive local maximum and a negative local minimum. Therefore, according to the energy-displacement curve and the force-displacement curve, the optimized structure has bistability.

[0028] S3. Based on the optimized bistable unit cell structure, further determine the relationship between the bistable performance of the unit cell and its structural size. Figure 4 As shown in (a), assume that the unit cell structure dimensions are length L mm, width W mm, and brace length c mm. The length of rod 1-2 is a mm, the length of rod 2-3 is b mm, and the angle between rods 1-2 and 1-3 is θ. The unit cell structure is initially in a stable configuration, where the strain energy is zero, as shown in Figure 4 As shown in (a), since the deformation of the four inclined rods in the unit cell structure is the same, the inclined rod 13 is selected for investigation.

[0029]

[0030]

[0031] By determining the angle θ and strain ε c The relationship between the two is used to determine the bistability of the structure. When the displacement boundary condition is applied to the unit cell structure, the unit cell structure begins to move and generates strain energy. Assuming that the deformation of the diagonal rod is a linear elastic material, its strain ε c It can be expressed as,

[0032]

[0033] Further derive formula (3) to determine the angle θ and strain ε c Assume that the angle θ0 in equation (4) is the initial angle, and θ is the variable in the process of unit cell structure movement and deformation.

[0034]

[0035] After calculation, when designing the unit cell structure, its angle θ should be within the following range to ensure that the structure has bistability,

[0036]

[0037] Therefore, according to the angle θ, the relationship between other parameters of the unit cell structure can be deduced as follows:

[0038]

[0039] Therefore, to design a unit cell structure with bistable performance, it is only necessary to determine the angle θ and the size b, thereby determining other parameters.

[0040] S4. Based on the design scheme of the first three steps, the present invention only selects a set of size parameters for verification, such as Figure 5 As shown in the figure, finite element simulation analysis is used to fix the degrees of freedom on both sides of the unit cell, and displacements of +44.16mm and -44.16mm are applied to the upper and lower triangular vertices respectively. The energy-displacement and force-displacement curves of the unit cell structure are obtained, which are consistent with the bistable mechanical performance curve.

[0041] S5. Arrange the unit cell structure and roll it into a tubular vascular stent, such as Figure 6 As shown, the design of a vascular stent structure with bistable performance is completed.

[0042] The present invention proposes a novel vascular stent design method with bistable performance, which avoids the need for the stent to achieve plastic deformation in order to maintain an expanded stable configuration. It can effectively solve the problem of insufficient radial mechanical properties of the stent and radial rebound after expansion, which makes it impossible to expand the blood vessel. The stent designed by the present invention is a bistable structure with rapid penetration behavior. The structure has a default contraction state, that is, a stable state in which the vascular stent is not expanded. Once an external force is applied to the structure, it will quickly jump to another completely stable state, that is, the expanded stable state of the vascular stent, without radial rebound. The stent does not need to achieve plastic deformation to maintain an expanded stable shape.

[0043] The bistable stent of the present invention can be used in application fields such as studying hemodynamic performance, stent mechanical behavior and drug delivery (in the case of drug eluting stents).

[0044] The bistable stent of the present invention has a simple structure and is not restricted by materials. The processing method can be selected according to different materials. For example, metal materials can be cut by laser, and biopolymer materials can be manufactured by extrusion 3D printing.

Claims

1. A method for designing a vascular stent structure with bistable performance, characterized in that: The following steps are involved: S1. Use a bidirectional evolutionary structural optimization method to perform topology optimization on the unit cell. Topology optimization was performed on an 8 mm × 6 mm unit cell. The unit cell was divided into a design domain and a non-design domain. At the intersection of the design and non-design domains, the unit cell was fixed along the X-direction. An external force of 100 N was applied to the unit cell at its lower center. The Young's modulus of the unit cell material was 2.0E+11 Pa, the Poisson's ratio was 0.3, and the density was 7800 kg / m³. The initial number of iterations was set to 50. S2. Based on the bidirectional evolutionary structural optimization algorithm, sensitivity is calculated as the sum of the total structural strain energy U, which includes elastic strain energy Ue and plastic strain energy Up, i.e., U = Ue + Up. The elastic strain energy Ue of the bistable structure has a local minimum. After multiple iterative evolutions, a unit cell structure with bistable properties is obtained. S3. Based on the optimized bistable unit cell structure, further determine the relationship between the bistability of the unit cell and its structural dimensions; S4. Based on the design schemes from the first three steps, select a set of dimensional parameters for verification. Use finite element simulation to obtain the energy-displacement and force-displacement curves of the unit cell structure to determine whether they conform to the bistable mechanical performance curves. If so, proceed to the next step. S5. Arrange the unit cell structure and curl it into a tubular vascular stent.

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