Method for designing bionic femoral stent based on fusion of VT structure and TPMS structure

Abstract

The invention discloses a method for designing a bionic femoral stent based on fusion of a VT structure and a TPMS structure, and belongs to the technical field of bionic bone stents. The method comprises the following steps: respectively creating a Diamond unit homogeneous porous structure and a VT porous structure; and creating a Ramp fusion function, substituting the homogeneous porous structure of the Diamond unit and the Ramp fusion function into a Mix instruction, and then sequentially substituting the VT porous structures into the Mix instruction to generate a fusion model. The homogeneous porous structure of the Diamond unit and the VT porous structure are subjected to radial smooth fusion, smooth connection is realized at the fusion position, the service life of the porous structure serving as the bionic bone scaffold is shortened, the smooth surface of the D unit and the periodic structure uniformly conduct stress, the rounded Thiessen polygonal structure further disperses stress, and the stress dispersion and synergistic effects are achieved. The bionic femoral stent constructed by the invention can simulate natural bones, promote cell adhesion, proliferation and differentiation, and optimize transportation of nutrient substances and metabolic wastes. And the smooth fusion interface reduces cell damage and reduces immunological rejection.

Description

Technical Field

[0001] The invention belongs to the technical field of bionic bone scaffolds, and in particular relates to a method for designing a bionic femoral scaffold based on the fusion of a VT structure and a TPMS structure. Background Art

[0002] In the field of bone tissue engineering, triply periodic minimal surface (TPMS) is a commonly used structural type when designing bionic bone scaffolds. For example, the Diamond (D) unit in the TPMS structure has structural characteristics including minimal surface characteristics, periodicity, high connectivity and precise controllability, which makes the surface of the D unit very smooth, without sharp edges or sudden curvatures. When designed into a bionic bone scaffold, it can have excellent mechanical properties, excellent biocompatibility and efficient material transfer. However, the current design of bionic femoral scaffolds based on the TPMS structure is mostly focused on using one unit to correspond to one unit cell for homogeneous porous structure design and gradient design in a certain single axial direction or radial direction. In addition, the porous structure generated based on the traditional Voronoi-Tessellation (VT) principle has unique structural characteristics such as spatial division, nearest neighbor, isometry, high connectivity and controllability. In addition, after rounding, the VT porous structure is very similar to the human cancellous bone obtained by medical CT scan. At present, when using the rounded VT porous structure as a bionic bone scaffold, the focus is also on controlling the overall homogeneous porosity or performing gradient design in the axial direction. For example, the number of points generated below is dense and the number of points generated above is relatively small. The VT porous structure generated by the interconnection between the points can be seen as a certain density gradient change in the axial direction. However, the actual structure of the human femur can be seen as consisting of an outer ring part of cortical bone and an inner part of cancellous bone in the radial direction. The cortical bone has a low porosity and mainly bears the load, while the cancellous bone (bone marrow) part mainly transports nutrients and provides growth space and landing points for the proliferation and differentiation of bone cells. That is, the femur itself has the hierarchical structural characteristics of cortical bone and cancellous bone. Therefore, the design method of the above-mentioned VT porous structure is inconsistent with the actual structure of the femur and fails to fully reflect the hierarchical structural characteristics of natural bones. Summary of the invention

[0003] In view of the above-mentioned shortcomings of the prior art, the present invention provides a method for designing a bionic femoral stent based on the fusion of a VT structure and a TPMS structure.

[0004] To achieve the above purpose, the technical solution adopted by the present invention is:

[0005] A method for designing a bionic femoral stent based on the fusion of a VT structure and a TPMS structure comprises the following steps:

[0006] (1) Use Ntopology software and change the Midsurface Offset parameter to create a cylindrical Diamond unit homogeneous porous structure CAD model with a target porosity and a diameter of R1 and a height of H1;

[0007] (2) Using Ntopology software, a rounded irregular VT porous structure CAD model and a rounded regular VT porous structure CAD model with a target porosity of diameter R1 and height H1 were established respectively;

[0008] (3) Use Ntopology software to create a cylindrical CAD model II with a diameter of R2 and a height of H2. Then call the Ramp command, substitute the cylindrical CAD model II into it and set the parameters In Min to a, In Max to b, Out Min to 0, OutMax to 1 and Continuity to Geometric (C0) to obtain the Ramp fusion function;

[0009] (4) Call the Mix command, substitute the cylindrical Diamond unit homogeneous porous structure CAD model and the Ramp fusion function into the two parameters of Scalar Field B and Factor in the Mix command respectively; then substitute the rounded irregular VT porous structure CAD model or the rounded regular VT porous structure CAD model into Scalar Field A in the Mix command to generate a fusion model in the form of a cylinder with a radius and height of R1×H1; where 0<R2<R1, 0<H2≤H1, 0≤a<b<2(R1-R2), R1 / H1=1~2, R2 / H2=1~2;

[0010] (5) The fused model is then preliminarily divided into a finite element simulation mesh model using the Mesh From Implicit Body command in the Ntopology software, and finally the simulation model file in inp format is output using the output command;

[0011] (6) Import the simulation model file in inp format into the HyperMesh software to repair and re-divide the mesh, using tetrahedral meshing, with the mesh type of C3D4 and the mesh unit size of 0.2 mm, and export and save the inp format file model again.

[0012] The present invention directly imports the inp file format model obtained by step (6) into the abaqus dynamics simulation module to simulate the quasi-static compression simulation, obtains the result data after the quasi-static compression finite element simulation, evaluates whether the yield strength and elastic modulus of the model are respectively within the yield strength and elastic modulus range of cancellous bone and cortical bone, if within the range, then the model established is the target bionic bone scaffold. The target bionic bone scaffold model designed can be constructed by conventional means in the art, such as: the inp file format model obtained by step (6) is output as a model file in the form of STL using the Export command, then the computer is connected to a 3D printer, and the model file in the form of STL is imported into the 3D printer to print, and the target bionic bone scaffold is obtained.

[0013] As a preferred embodiment of the present invention, the establishment of the rounded irregular VT porous structure CAD model specifically includes: using the Mesh From Implicit Body command in the Ntopology software to divide the surface mesh of the cylindrical CAD model, then calling the Random points in Body command to generate disordered random seed points, and then calling the Voronoi Volume Lattice command to set the wall thickness, and substituting the established Randompoints in Body command and Mesh From ImplicitBody command into the Seedpoints and Boundary in the Voronoi Volume Lattice command respectively to connect the disordered random seed points to preliminarily form an irregular VT porous structure, and then using the Smooth command to smooth the irregular VT porous structure to form a rounded irregular VT porous structure CAD model.

[0014] As a preferred embodiment of the present invention, the establishment of the rounded regular VT porous structure CAD model specifically includes: using the Mesh From Implicit Body command in the Ntopology software to divide the surface mesh of the cylindrical CAD model, calling the Volume Mesh command to further divide the internal body mesh of the Mesh From Implicit Body on the basis of the divided surface mesh, and then calling the Random points in Volume Mesh command to generate relatively ordered random points based on the cylindrical body mesh, and then calling the Voronoi Volume Lattice command to set the wall thickness, and the established Random points in Volume Mesh command and Volume Mesh command are respectively substituted into the Seedpoints and Boundary in the Voronoi Volume Lattice command to connect the relatively ordered random points generated based on the cylindrical body mesh to preliminarily form a regular VT porous structure, and then using the Smooth command to smooth the regular VT porous structure to form a rounded regular VT porous structure CAD model.

[0015] As a preferred embodiment of the present invention, in the step (1), the Midsurface Offset parameter is set to adjust the porosity of the Diamond unit homogeneous porous structure CAD model through the relationship between the Midsurface Offset parameter M and the porosity P1; the relationship between the Midsurface Offset parameter M and the porosity P1 is P1 = (0.5-1.1741923M) × 100%.

[0016] As a preferred embodiment of the present invention, the porosity of the rounded irregular VT porous structure CAD model is adjusted by setting the wall thickness parameter T1 through the relationship between the wall thickness parameter T1 and the porosity P2 of the rounded irregular VT porous structure CAD model. The relationship between the wall thickness parameter T1 and the porosity P2 is P2 = (1.49462-3.2448T2) × 100%.

[0017] As a preferred embodiment of the present invention, the porosity of the rounded irregular VT porous structure CAD model is adjusted by setting the wall thickness parameter T2 through the relationship between the wall thickness parameter T2 and the porosity P3 of the rounded regular VT porous structure CAD model; the relationship between the wall thickness parameter T2 and the porosity P3 is P3 = (1.7426-3.1428T3) × 100%.

[0018] As a preferred embodiment of the present invention, the fusion area in the Ramp fusion function is a ring, the length of the ring on the Z axis is H2, the width of the ring on the X, Y plane is (ba) / 2≥half of the maximum unit cell side length in the Diamond unit and the rounded irregular VT porous structure unit or the rounded regular VT porous structure unit, and the axial direction of the ring is the Z axis direction.

[0019] As a preferred embodiment of the present invention, the unit cell size of the Diamond unit homogeneous porous structure, the rounded irregular VT porous structure unit or the rounded regular VT porous structure unit is 2 mm×2 mm×2 mm.

[0020] As a preferred embodiment of the present invention, the target porosity is 50% to 90%.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows: the present invention radially and smoothly fuses the three-dimensional annular region of the Diamond unit homogeneous porous structure and the VT porous structure in the Z-axis direction, and realizes a smooth connection at the fusion interface, thereby avoiding the large stress concentration that causes the fusion interface to break first, thereby reducing the service life of the porous structure as a bionic bone scaffold. The smooth surface of the D unit and the periodic structure evenly conduct stress, and the rounded Thiessen polygonal structure further disperses stress, improves the overall bearing capacity, and plays a stress dispersion and synergistic role. The bionic femoral scaffold constructed by the present invention can simulate natural bone, promote cell adhesion, proliferation and differentiation, and optimize the transport of nutrients and metabolic waste. In addition, the smooth fusion interface reduces cell damage, reduces immune rejection, and promotes the integration of the scaffold into the physiological environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 The linear fitting relationship curves of porosity and numerical value M or wall thickness T, (a) is the linear fitting relationship curve of porosity of D homogeneous unit porous structure and numerical value M, (b) is a schematic diagram of the linear fitting relationship curve of porosity of rounded VT homogeneous unit porous structure and wall thickness T.

[0023] Figure 2 Schematic diagram of the D homogeneous unit porous structure model.

[0024] Figure 3 Schematic diagram of random points of irregular and regular VT homogeneous porous structures. (a) is a schematic diagram of random points of irregular VT homogeneous porous structure, and (b) is a schematic diagram of random points of regular VT homogeneous porous structure.

[0025] Figure 4 Schematic diagram of the formation of rounded regular and irregular VT homogeneous porous structures. (a) is a schematic diagram of random points of a rounded irregular VT homogeneous porous structure, and (b) is a schematic diagram of the formation of a rounded regular VT homogeneous porous structure.

[0026] Figure 5 Schematic diagram of the VTD model with radially smooth fusion of rounded regular and irregular VT homogeneous porous structures and D homogeneous units.

[0027] Figure 6 Schematic diagram of the specific location distribution of D homogeneous unit and VT homogeneous porous structure in the VTD fusion model.

[0028] Figure 7 Schematic diagram of the deformation behavior and stress concentration distribution of the axial center section during quasi-static compression simulation of the VTD1 and VTD2 finite element models, (a) for VTD1 and (b) for VTD2. DETAILED DESCRIPTION

[0029] In order to better illustrate the purpose, technical solutions and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments.

[0030] Example 1

[0031] A method for designing a bionic femoral stent based on the fusion of a VT structure and a TPMS structure comprises the following steps:

[0032] (1) First, create a cylindrical CAD model (cylinder I) with a radius (R1) and height (H1) of 4.5mm×12mm in the Create module of the Ntopology software (all model structures in the present invention are CAD models that are symmetrical in the vertical and horizontal directions with the origin (0, 0, 0) as the center point in the three-dimensional Cartesian coordinate system). Then use the TPMS Unite cell with Offset command to select the D unit cell, and use the RectangularVolume Lattice command to set the Midsurface Offset (M) parameter to 0 to generate a cylindrical D homogeneous unit porous structure CAD model with a radius and height of 4.5mm×12mm and a porosity of 50% (the M parameter indicates that the wall thickness can be increased or decreased and the corresponding range will also change when the unit cell size changes). The unit cell size (Unit CellSize) is designed to be 2mm×2mm×2mm, and the default settings of the Frame and Trim commands can be Optional and Trim respectively. The schematic diagram of the generated D homogeneous unit porous structure model is as shown in the figure. Figure 2 shown.

[0033] (2) Create a rounded irregular VT unit homogeneous porous structure: First, use the Mesh From Implicit Body command in the Ntopology software to divide the surface mesh of the cylindrical CAD model with a radius and height of 4.5 mm × 12 mm, where Tolerance is set to 1 (the size of a single triangle mesh is 1 mm, and the larger the number, the larger the size of a single mesh in the divided surface mesh). The remaining parameters can be set by default, that is, Min.feature size: Optional, select Sharpen, and do not select Simplify. Then call the Randompoints in Body command and substitute a cylinder with a radius and height of 4.5mm×12mm into the Body parameters, set point to 0.85mm (the average distance between points is 0.85mm, and the parameter value can be selected according to the cancellous bone pore diameter of 100μm to 1200μm), and set Random Seed to the default setting, which is 2 (representing the random seed parameter and must be a positive integer greater than 0. Each time an integer is determined, the position of the generated point will be changed accordingly, that is, the layout of the points will be changed). After setting the three parameters of Randompoints in Body, the overall representation is that disordered random seed points with an average distance of 0.85mm will be generated within the volume frame of a cylinder with a radius and height of 4.5mm×12mm. The schematic diagram is as follows Figure 3 (a) is shown. Then call the Voronoi Volume Lattice command, and then substitute the Random points in Body command and Mesh From Implicit Body command into the Seed points and Boundary in the Voronoi Volume Lattice command respectively. The wall thickness (Thickness) parameter in the Voronoi Volume Lattice command is set to 0.2999. After setting the parameters, it means that random points with an average distance of 0.85mm will be generated in the 4.5mm×12mm cylindrical volume frame. Connecting them to each other can generate a preliminary irregular VT porous structure. At the same time, use the Smooth Body command as shown in Figure 4As shown in (a), the preliminary irregular VT porous structure is smoothed to remove sharp corners and edges to form a rounded irregular VT porous structure. In the smooth command, the Grid size and Smoothen iterations parameters are set to 0.05 and 4 respectively (the more times the smooth command is used, the smoother it is, but the thickness will also be thinner. Therefore, when using this command, at least the points can be connected to each other, and the overall porosity of the rounded irregular VT porous structure formed is between 50% and 90%. The parameter value set this time is equivalent to grinding 4 times with 0.05mm thick sandpaper. If the Grid size parameter value is larger, the number of grinding times should be reduced accordingly). The remaining parameters in the smooth command can be set by default, that is, Interpolation type is linear, Domain: Optional, Extrapolation: Outside, and the overall average porosity P2 of the rounded irregular VT porous structure is 50%.

[0034] (3) Create a rounded regular VT unit homogeneous porous structure: First, use the MeshFrom Implicit Body command in the Ntopology software to divide the surface mesh of the cylindrical CAD model with a radius and height of 4.5mm×12mm, where Tolerance is set to 1 (indicates that the size of a single triangle mesh is 1mm, and the larger the number, the larger the size of the single mesh in the divided surface mesh). The remaining parameters can be set by default, that is, Min.feature size:Optional, select Sharpen, and do not select Simplify. Then call the Volume Mesh command to further divide the internal volume mesh based on the surface mesh divided by Mesh From Implicit Body. The other two parameters Edge Length and GrowthRate in the Volume Mesh command can be set to 0.5 and 2 respectively, indicating that the size of the divided tetrahedral body mesh is between 0.5mm and 1mm. Then call the Random points in Volume Mesh command, substitute the Volume Mesh command with the set parameters into the Random points in Volume Mesh command, set the Point Count parameter to 1200 (indicates that the number of random points generated is 1200 based on the volume mesh framework divided by a cylinder with a radius and height of 4.5mm×12mm. At this time, the frame area remains unchanged. The more points there are, the smaller the distance between the points. The number of generated points can be controlled so that the distance between the points is between 100μm and 1200μm), set the Relaxation Lterations parameter to 300 (the larger the Relaxation Lterations parameter, the more regular the arrangement between the points, and the overall arrangement is in multiple circular rings), Random Seed is 1, and SpatialWeighting is Optional. The schematic diagram is as follows Figure 3 (b) As shown. Then call the Voronoi Volume Lattice command, and then substitute the Random points in Volume Mesh command and Volume Mesh command into the Seed points and Boundary in the Voronoi Volume Lattice command respectively. The wall thickness parameter in the Voronoi Volume Lattice command is set to 0.39215. After setting the parameters, the 1200 generated random points will be connected to each other to generate a preliminary regular VT porous structure. At the same time, the Smooth Body command is used as shown in Figure 4As shown in (b), the preliminary irregular VT porous structure is smoothed to remove sharp corners and edges to form a rounded irregular VT porous structure (the more times it is polished, the smoother it is, but the thickness will also become thinner. Therefore, when using this command, it is at least necessary to ensure that the points can be connected to each other, and the overall porosity of the formed rounded regular VT porous structure should be between 50% and 90% of the porosity of the cancellous bone in the real femur). This time, the Grid size and Smoothen iterations parameters in the smooth command are set to 0.05 and 8 respectively (equivalent to polishing 8 times with 0.05mm thick sandpaper), and the remaining parameters in the smooth command can be set by default, namely: Interpolation type is linear, Domain: Optional, Extrapolation: Outside, and the overall average porosity P3 of the rounded regular VT porous structure is 50%. It is worth noting that in steps (2) and (3), after the respective smoothing treatment conditions, the pores formed are all 50% porosity rounded (irregular and regular) VT porous structures, and their porosities are obtained by quantitative calculation. The wall thickness is selected and the homogeneous rounded VT porous structures are generated as shown in Table 2:

[0035] Table 2

[0036] <![CDATA[T2(mm)]]> 0.2 0.25 0.3 0.35 0.4 <![CDATA[P2(%)]]> 86.79 67.14 49.97 34.90 21.79 <![CDATA[T3(mm)]]> 0.25 0.3 0.35 0.4 0.45 <![CDATA[P3(%)]]> 96.00 80.53 63.65 47.65 33.87

[0037] The linear fitting is performed on the above data, and the fitting results are as follows Figure 1 As shown in (b), the linear relationship between the porosity (P2) and the wall thickness T2 of the rounded irregular homogeneous porous structure model is P2 = (1.49462-3.2448T2) × 100%, and its reliability is 99.378%. The linear relationship between the porosity (P3) and the wall thickness T3 of the rounded regular homogeneous porous structure model is P3 = (1.7426-3.1428T3) × 100%, and its reliability is 99.867%. Both reliability levels meet the requirement that the reliability (R) of the fitting result should be above 98%. The design scheme for this time is a porosity of 50%. Substituting P2 = (1.49462-3.2448T2) × 100% and P3 = (1.7426-3.1428T3) × 100%, we can get the corresponding T2 and T3 values ​​​​of 0.2999 and 0.39215 respectively.

[0038] (2) The second step is to create a fusion gradient command: First, create a cylindrical CAD model (cylinder II) with a radius (R2) and height (H2) of 2.5mm×12mm in the Create module of the Ntopology software. The unit structure in the cylinder II area is to simulate the cancellous bone area, so it cannot exceed the initial modeling diameter and height of the cylindrical model framework (cylinder I). In summary, according to this design plan, 0mm<R2<R1=4.5mm and 0mm<H2≤H1=12mm should be maintained. When the height is not consistent with the initial modeling height (H1), the rounded porous structure unit in this design case will be completely wrapped by the D unit.

[0039] Then call the Ramp command, substitute Cylinder II into it and set the parameters In Min to 0, In Max to 1, Out Min to 0, Out Max to 1 and Continuity to Geometric (C0) to obtain the Ramp fusion function.

[0040] The In Min and In Max parameters form the fusion area. In Min is 0, which means that the fusion starts from R2=2.5mm around the edge of the cylinder II. In Max is 1, which means that it will grow symmetrically along the radius of the small cylinder by 0.5mm to r=3mm, that is, the same straight line will extend in the opposite direction, symmetrically, and equally, and the sum of the extensions is 1, and finally a circular fusion area is formed in the height range of Z=-6mm to Z=6mm. The In Min and In Max parameters are constants a and b, respectively. The range of a and b can be summarized as 0≤a<b<2(R1-R2)(mm). In order to reflect the smooth connection and better connection effect, the length of the fusion area (ba) / 2(mm) should be ≥ half of the maximum unit cell side length in the D unit. The unit cell side length of the D homogeneous unit model is 2mm×2mm×2mm, so ba=2 / 2=1.

[0041] Out Min is 0 and Out Max is 1. The parameters are set to 0 and 1 to ensure that the overall fused model is consistent with the initial modeling framework (cylinder I) at 1:1, that is, the overall model will not have changes such as widening and heightening. When the TPMS unit and the VT unit are fused using the method of the present invention, both parameters can be set to 0 and 1.

[0042] (3) Step 3: Call the Mix command, substitute the cylindrical Diamond unit homogeneous porous structure CAD model and the Ramp fusion function into the Scalar Field B and Factor parameters in the Mix command respectively; then substitute the rounded irregular VT porous structure CAD model or the rounded regular VT porous structure CAD model into Scalar Field A in the Mix command to generate the fusion models VTD1 and VTD2 in the form of a cylinder with a radius and height of R1×H1 (see the schematic diagram). Figure 5 shown).

[0043] (4) The VTD1 or VTD2 fused CAD model with a final cylindrical shape of 4.5 mm × 12 mm in radius and height is again divided into a finite element simulation mesh model using the Mesh From Implicit Body command in the Ntopology software, and finally the simulation model file in inp format is output using the output command (Export FE Mesh).

[0044] (5) Import the simulation model file in inp format into the HyperMesh software to repair and re-divide the mesh, using tetrahedral meshing, with the mesh type of C3D4 and the mesh unit size of 0.2 mm, and export and save the inp format file model again.

[0045] (6) The generated inp format file model was imported into the display dynamics module of Abaqus software for quasi-static compression finite element simulation. The elastic modulus of 110 GPa, Poisson's ratio of 0.3 and 4.51 g / cm 3 The density is set as the property of Ti-6Al-4V material, and the mesh unit is set to yield failure when the maximum deformation ratio is 1.5. At the same time, the boundary conditions of the porous structure compression are defined, and a 60% displacement load is applied to the top surface (the height of this model is 12mm, that is, the compression stops when the pressure is 7.2mm). The bottom surface constrains the movement and rotational freedom of the porous structure in the X, Y, and Z coordinate directions. The top surface constrains the movement in the X, Y directions and the rotational freedom in the X, Y, and Z directions, and only allows movement in the Z direction. The top surface and the bottom surface are rigid surfaces, and the friction coefficient between the rigid surface and the porous structure is 0.2. The total simulation time is set to 0.1 seconds and the analysis step is 5e-6 seconds.

[0046] like Figure 7The deformation behavior and stress concentration distribution of the axial center section of the two fusion finite element models VTD1 and VTD2 during quasi-static compression simulation are shown. The elastic stage strain (ε) of 10% and 15%, the yield stage (ε) of 25%, 35% and 50%, and the densification stage (ε) of 65%, 75% and 100% (corresponding to compression to 7.2mm) are selected as representatives to show the changes in the full-stage compression simulation process of the model. Figure 7 The stress-strain cloud diagram shows that:

[0047] 1. In the elastic stage: Due to its complex and regular geometric structure, stress concentration usually occurs at the edges, corners and nodes of the D unit. However, due to the uniformity of its overall structure, the stress concentration is relatively low and the distribution is relatively uniform. At this time, the deformation of the D unit is small and uniform, and the shape of the unit remains relatively stable, mainly manifested as a slight overall compression. For the rounded VT polygonal porous structure unit, due to the two structures of irregularity and relative regularity of pore shape and distribution, the stress concentration presents a more dispersed and complex characteristic. There will be more obvious stress concentration at the edges and intersections of the pores. The deformation of the rounded VT polygonal porous structure is relatively large and uneven, and the pores are slightly twisted and compressed locally, but the overall structure still maintains a certain degree of connectivity. There will be a certain degree of transition in stress at the fusion interface, and the stress value will gradually change from the D unit to the rounded Thiessen polygonal porous structure unit. There will be a certain local stress concentration at the connection of the interface, but because it is a smooth connection, the concentration is relatively small. The deformation also shows a gradual transition at the interface. The deformation of the D unit is smaller, while the deformation of the rounded Thiessen polygonal porous structure unit is larger. A certain coordinated deformation occurs at the interface to adapt to the different deformation characteristics of the two units.

[0048] 2. In the yield stage: the stress value in the stress concentration area of ​​the D unit increases further, and local yield signs appear at some weak nodes, edges, and places where the curvature of the surface changes greatly. More obvious deformation begins to appear, and the shape of the unit begins to change, but the overall structure can still maintain a certain bearing capacity. The stress concentration of the rounded VT polygonal porous structure unit is more significant, and the deformation of the pore structure is aggravated, resulting in more stress concentration in the supporting parts of the pores. Its deformation increases rapidly, some pores collapse and merge, and the bearing capacity of the structure gradually decreases. The stress transition at the interface becomes more obvious, and stress concentration causes tiny cracks and local damage at the interface. At this time, the deformation coordination at the interface becomes more difficult, and tiny dislocations or local plastic deformations will occur to balance the different yield behaviors of the two units.

[0049] 3. In the densification stage: the deformation of the D unit continues to increase, the stress concentration area gradually expands, the structure tends to be densified, and the stress distribution gradually tends to be uniform. Its pores are greatly compressed, the structure becomes more dense, and the bearing capacity gradually reaches the limit. Most of the pores of the rounded VT polygonal porous structure unit have collapsed, and the stress concentration is mainly concentrated on the remaining supporting structure. The stress value reaches a high level and its deformation is very significant. The structure is almost completely densified and most of the pore space is lost. The stress distribution at the fusion interface tends to be complex, and a large stress difference occurs due to the different densification degrees of the two units. At this time, severe plastic deformation and material flow occur at the fusion interface, and some fusion areas show signs of interface separation, but due to the early smooth connection design, the integrity of the connection can still be maintained to a certain extent.

[0050] In summary, during the compression process, the stress concentration and deformation behaviors of the D unit, the rounded Thiessen polygon porous structure unit and the fusion interface affect each other. Through reasonable design and smooth fusion, the mechanical properties and stability of the structure can be optimized to a certain extent.

[0051] The yield strength simulation results of the VTD1 and VTD2 fusion models are 217.86MPa and 205.21MPa respectively, which are slightly larger than the yield strength of the cortical bone in the femur (33-193MPa), and can meet the load strength requirements. The elastic modulus after simulation is 13.68GPa and 12.72GPa respectively, which is within the range of the cortical bone elastic modulus (3-20GPa) and can avoid stress shielding problems. Therefore, these two porous structures are suitable for implantation as bionic femoral scaffolds in the body.

[0052] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the essence and scope of the technical solution of the present invention.

Claims

1. A method for designing a bionic femoral stent based on the fusion of VT structure and TPMS structure, characterized in that: The steps include: (1) Use Ntopology software and change the Midsurface Offset parameter to create a cylindrical Diamond unit homogeneous porous structure CAD model with a target porosity and a diameter of R1 and a height of H1; (2) Using Ntopology software, a rounded irregular VT porous structure CAD model and a rounded regular VT porous structure CAD model with a target porosity of diameter R1 and height H1 were established respectively; (3) Use Ntopology software to create a cylindrical CAD model II with a diameter of R2 and a height of H2. Then call the Ramp command, substitute the cylindrical CAD model II into it and set the parameters In Min to a, In Max to b, Out Min to 0, Out Max to 1 and Continuity to Geometric (C0) to obtain the Ramp fusion function; (4) Call the Mix command, substitute the cylindrical Diamond unit homogeneous porous structure CAD model and the Ramp fusion function into the two parameters of Scalar Field B and Factor in the Mix command respectively; then substitute the rounded irregular VT porous structure CAD model or the rounded regular VT porous structure CAD model into Scalar Field A in the Mix command to generate a fusion model in the form of a cylinder with a radius and height of R1×H1; where 0<R2<R1, 0<H2≤H1, 0≤a<b<2(R1-R2), R1 / H1=1~2, R2 / H2=1~2; (5) The fused model is then preliminarily divided into a finite element simulation mesh model using the Mesh From Implicit Body command in the Ntopology software, and finally the simulation model file in inp format is output using the output command; (6) Import the simulation model file in inp format into the HyperMesh software to repair and re-divide the mesh, using tetrahedral meshing, with the mesh type of C3D4 and the mesh unit size of 0.2 mm, and export and save the inp format file model again.

2. The method for designing a bionic femoral stent based on the fusion of VT structure and TPMS structure according to claim 1, characterized in that: The establishment of the rounded irregular VT porous structure CAD model specifically includes: using the Mesh From Implicit Body command in the Ntopology software to divide the surface mesh of the cylindrical CAD model, then calling the Randompoints in Body command to generate disordered random seed points, and then calling the Voronoi Volume Lattice command to set the wall thickness, and substituting the established Randompoints in Body command and Mesh From Implicit Body command into the Seedpoints and Boundary in the Voronoi Volume Lattice command respectively to connect the disordered random seed points to preliminarily form an irregular VT porous structure, and then using the Smooth command to smooth the irregular VT porous structure to form a rounded irregular VT porous structure CAD model.

3. The method for designing a bionic femoral stent based on the fusion of VT structure and TPMS structure as claimed in claim 1, characterized in that: The establishment of the rounded regular VT porous structure CAD model specifically includes: using the Mesh From Implicit Body command in the Ntopology software to divide the surface mesh of the cylindrical CAD model, calling the Volume Mesh command to further divide the internal body mesh of the Mesh From Implicit Body on the basis of the divided surface mesh, calling the Random points in Volume Mesh command to generate relatively ordered random points based on the cylindrical body mesh, and then calling the Voronoi Volume Lattice command to set the wall thickness, and respectively substituting the established Random points in Volume Mesh command and Volume Mesh command into the Seedpoints and Boundary in the Voronoi Volume Lattice command to connect the relatively ordered random points generated based on the cylindrical body mesh to preliminarily form a regular VT porous structure, and then using the Smooth command to smooth the regular VT porous structure to form a rounded regular VT porous structure CAD model.

4. The method for designing a bionic femoral stent based on the fusion of VT structure and TPMS structure as claimed in claim 1, characterized in that: In the step (1), the MidsurfaceOffset parameter is set to adjust the porosity of the Diamond unit homogeneous porous structure CAD model through the relationship between the Midsurface Offset parameter M and the porosity P1; the relationship between the Midsurface Offset parameter M and the porosity P1 is P1=(0.5-1.1741923M)×100%.

5. The method for designing a bionic femoral stent based on the fusion of VT structure and TPMS structure as claimed in claim 2, characterized in that: The porosity of the rounded irregular VT porous structure CAD model is adjusted by setting the wall thickness parameter T1 through the relationship between the wall thickness parameter T1 and the porosity P2 of the rounded irregular VT porous structure CAD model. The relationship between the wall thickness parameter T1 and the porosity P2 is P2=(1.49462-3.2448T2)×100%.

6. The method for designing a bionic femoral stent based on the fusion of VT structure and TPMS structure as claimed in claim 3, characterized in that: The wall thickness parameter T2 is set to adjust the porosity of the rounded irregular VT porous structure CAD model through the relationship between the wall thickness parameter T2 and the porosity P3 of the rounded regular VT porous structure CAD model; the relationship between the wall thickness parameter T2 and the porosity P3 is P3=(1.7426-3.1428T3)×100%.

7. The method for designing a bionic femoral stent based on the fusion of VT structure and TPMS structure as claimed in claim 1, characterized in that: The fusion area in the Ramp fusion function is a ring, the length of the ring on the Z axis is H2, and the width of the ring on the X and Y planes is (ba) / 2≥half of the maximum unit cell side length in the Diamond unit and the rounded irregular VT porous structure unit or the rounded regular VT porous structure unit.

8. The method for designing a bionic femoral stent based on the fusion of VT structure and TPMS structure as claimed in claim 1, characterized in that: The unit cell size of the Diamond unit homogeneous porous structure, the rounded irregular VT porous structure unit or the rounded regular VT porous structure unit is 2 mm×2 mm×2 mm.

9. The method for designing a bionic femoral stent based on the fusion of VT structure and TPMS structure as claimed in claim 1, characterized in that: The target porosity is 50% to 90%.

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