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

Abstract

The invention discloses a method for designing a bionic femoral stent based on fusion of a lattice 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 CAD model and an Iso Truss unit homogeneous porous structure CAD model; creating a Ramp fusion function, and fusing the homogeneous porous structure of the Diamond unit and the homogeneous porous structure of the Iso Truss unit to generate a preliminary ITD fusion CAD model; then fusing the preliminary ITD fusion CAD model and a newly established cylinder III by using a Boolean intersection instruction to form a final ITD fusion CAD model, and finally dividing a finite element simulation grid model and outputting an inp-format simulation model file; importing the simulation model file in the inp format into HyperMesh software for repairing, and dividing grids to export the model in the inp file format. The bionic bone scaffold designed by the invention has higher bearing capacity and stability, and the formed mechanical property gradient adapts to a complex mechanical environment. Meanwhile, cell adhesion, proliferation and differentiation can be promoted, nutrient substances can be transported, metabolic waste can be discharged, and the cell survival rate can be increased.

Description

Technical Field

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

[0002] The actual structure of the human femur can be seen as consisting of an outer circular ring of cortical bone and an inner layer of cancellous bone in the radial direction. The cortical bone has a lower porosity and mainly bears the load, while the cancellous bone (bone marrow) has a porosity of 50% to 90%. Its higher porosity is mainly for transporting nutrients and providing growth space and landing points for bone cell proliferation and differentiation. That is, the femur itself has the hierarchical structural characteristics of cortical bone and cancellous bone.

[0003] In bone tissue engineering, triply periodic minimal surfaces (TPMS) and lattice structures are two common structural types used in the design of biomimetic bone scaffolds. For example, the Diamond (D) unit in the TPMS structure has structural characteristics such as zero average curvature, periodic repetition in three dimensions in space, high connectivity, and precise controllability. This makes the surface of the D unit very smooth, without sharp edges or sudden curvature changes. Smooth surfaces are conducive to cell adhesion and growth, allowing cells to be more evenly distributed and spread on such surfaces, improving cell-scaffold interaction. When lattice structures such as Iso Truss (IT) units are used to design biomimetic bone scaffolds, the structure itself is characterized by spatial symmetry and high porosity. The resulting porous scaffold also has excellent mechanical and biological properties, such as high load-bearing capacity, stress dispersion, and the ability to promote cell growth and material transport.

[0004] However, existing research on the design of bionic bone scaffolds only focuses on using one unit within the same type to correspond to one unit cell to design a homogeneous porous structure or perform gradient design in a certain single direction. This design method is firstly inconsistent with the actual structure of the femur and fails to fully reflect the hierarchical structural characteristics of natural bone. Secondly, there is no research on combining different types of structures (such as using TPMS units and lattice structure units at the same time) to perform cross-category heterogeneous units and adopt a radial smooth fusion method to design a porous structure as a bionic bone scaffold. Summary of the Invention

[0005] In view of the above shortcomings of the prior art, the present invention provides a method for designing a bionic femoral stent based on the fusion of lattice structure and TPMS structure.

[0006] To achieve the above object, the technical solution adopted by the present invention is:

[0007] A method for designing a bionic femoral stent based on the fusion of lattice structure and TPMS structure, comprising the following steps:

[0008] (1) Use Ntopology software and change the Midsurface Offset parameter to create a CAD model of a homogeneous porous structure of a cylinder Diamond unit with a diameter of R1 and a height of H1 with a target porosity;

[0009] (2) Use Ntopology software and change the wall thickness parameter to create a CAD model of a homogeneous porous structure of a cylinder Iso Truss unit with a diameter of R1 and a height of H1 with a target porosity;

[0010] (3) Use Ntopology software to create a cylinder CAD model II with a diameter of R2 and a height of H2. Then, call the Ramp command, substitute the cylinder CAD model II, and set the parameters In Min as a, In Max as b, Out Min as 0, Out Max as 1, and Continuity as Geometric (C0) to obtain a Ramp fusion function;

[0011] (4) Call the Mix command, and substitute the CAD model of the cylinder Diamond unit homogeneous porous structure, the CAD model of the cylinder Iso Truss unit homogeneous porous structure, and the Ramp fusion function into the Mix instruction respectively to generate a preliminary ITD fusion CAD model in the form of a cylinder with a radius and height of R1×H1;

[0012] (5) Create a cylinder III with a radius of R3 and a height of H3 in the Create module, and fuse the cylinder III and the preliminary ITD fusion CAD model using the Boolean intersection instruction to form a final ITD fusion CAD model of a cylinder with a radius of R3 and a height of H3, where 0 < R2 < R3 < R1, 0 < H2 ≤ H3 < H1, R3 / H3 = 1 - 2, 0 ≤ a < b < 2(R3 - R2), R1 / H1 = 1 - 2, R2 / H2 = 1 - 2;

[0013] (6) Use the Mesh From Implicit Body command in Ntopology software to preliminarily divide the final ITD fusion CAD model into a finite element simulation mesh model, and finally use the output command to output a simulation model file in inp format;

[0014] (7) Import the simulation model file in inp format into HyperMesh software for repair and re-meshing. Use the tetrahedral mesh division form, with the mesh type being C3D4 and the mesh element size being 0.2 mm, and then export and save the inp format file model again.

[0015] The present invention imports the inp file format model obtained by step (7) directly into the abaqus dynamics simulation module to carry out the simulation of quasi-static compression simulation, obtains the result data after 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, the model established is the target bionic bone scaffold. The target bionic bone scaffold model of design can be carried out bionic bone support construction by means of routine in this area, such as: the inp file format model obtained by step (7) is exported to the model file of STL form by Export command, then the computer is connected to a 3D printer, the model file of STL form is imported into the 3D printer and can be printed to obtain the target bionic bone scaffold.

[0016] As a preferred embodiment of the present invention, in step (1), the relationship between the Midsurface Offset parameter M and the porosity P1 is P1 = (0.5-1.1741923M) × 100%; through the relationship between the Midsurface Offset parameter M and the porosity P1, the Midsurface Offset parameter is set to adjust the porosity of the Diamond unit homogeneous porous structure CAD model.

[0017] As a preferred embodiment of the present invention, in step (2), the relationship between the wall thickness parameter T and the porosity P2 is P2 = (1.199177-1.2805811T) × 100%; through the relationship between the wall thickness parameter T and the porosity P2, the wall thickness parameter T is set to adjust the porosity of the Iso Truss unit homogeneous porous structure CAD model.

[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 and Y planes is (ba) / 2≥half of the maximum unit cell side length in the Diamond unit and the Iso Truss 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 and the Iso Truss unit homogeneous porous structure are both 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 Diamond unit homogeneous porous structure and the Iso Truss unit homogeneous porous structure in the three-dimensional annular area in the Z-axis direction to form a mechanical property gradient and adapt to complex mechanical environments. Moreover, the two unit structures work together to disperse stress and improve bearing capacity and stability. In addition, radial smooth fusion improves the stability of the overall structure and avoids weak links. In terms of biological performance, the multi-level pore structure after fusion described in the present invention simulates natural bone and promotes cell adhesion, proliferation and differentiation. Moreover, the formed pore network efficiently transports nutrients, discharges metabolic waste, and improves cell survival rate. At the same time, the smooth fusion interface promotes cell migration, reduces foreign body reactions, and enhances integration. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Schematic diagrams of the cylindrical Diamond unit homogeneous porous structure CAD model and the cylindrical Iso Truss unit homogeneous porous structure CAD model. (a) is the cylindrical Diamond unit homogeneous porous structure CAD model, and (b) is the Iso Truss unit homogeneous porous structure CAD model.

[0023] Figure 2 These are the linear fitting relationship curves of the porosity of the cylindrical Diamond unit homogeneous porous structure and the cylindrical Iso Truss unit homogeneous porous structure and the numerical value M or wall thickness T. (a) is the linear fitting relationship curve of the cylindrical Diamond unit homogeneous porous structure and the numerical value M, and (b) is the linear fitting relationship curve of the porosity of the cylindrical Iso Truss unit homogeneous porous structure and the wall thickness T.

[0024] Figure 3 Schematic diagram of the change from the preliminary ITD fusion CAD model to the final ITD fusion CAD model.

[0025] Figure 4 Schematic diagram of the final ITD fusion CAD model formed by radially smooth fusion of Diamond unit homogeneous porous structure and Iso Truss unit homogeneous porous structure.

[0026] Figure 5 Schematic diagram of the specific location distribution of the Diamond unit homogeneous porous structure and the Iso Truss unit homogeneous porous structure in the final ITD fusion CAD model.

[0027] Figure 6 Schematic diagram of the deformation behavior and stress concentration distribution of the axial center section during the quasi-static compression simulation of the finite element model of the final ITD fused CAD model. DETAILED DESCRIPTION

[0028] 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.

[0029] Example 1

[0030] A method for designing a bionic femoral stent based on the fusion of a lattice structure and a TPMS structure comprises the following steps:

[0031] (1) First, a cylindrical CAD model (cylinder I) with a radius (R1) and height (H1) of 5 mm × 14 mm was created 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, the D unit cell was selected using the TPMS Unite cell with Offset command, and the Midsurface Offset (M) parameter was set to -0.0433 using the RectangularVolumeLattice command to generate a D homogeneous unit porous structure CAD model with a radius and height of 5 mm × 14 mm and a porosity of 55% (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). Similarly, the GraphUnite Cell command is used to select the IT unit cell. Then, the Rectangular Volume Lattice command is used to set the wall thickness parameter (Thickness, T) to 0.5097mm, resulting in a cylindrical IT homogeneous unit porous structure CAD model with a radius and height of 5mm×14mm and a porosity of 55%. The other parameters in the Rectangular Volume Lattice command used in both cases are: the unit cell size is set to 2mm×2mm×2mm, and the Frame and Trim commands are set to the default settings. The schematic diagram of the generated porous structure model is shown below. Figure 1 shown.

[0032] In this Example 1, the overall porosity is 55% to design the D and IT homogeneous unit porous structure models. The porosity range can be selected within the range of 50% to 90% based on the cancellous bone porosity. However, since the D unit is the outer layer simulating cortical bone during the later fusion, the porosity of the D unit should not exceed the porosity of the IT unit model so that the fusion structure better conforms to the hierarchical structure characteristics of natural bone. The unit cell size is 2mm×2mm×2mm because the pore size is between 100μm and 1200μm at this time, which can meet the pore size requirements for bone cell proliferation and differentiation. The overall size is a cylindrical CAD model with a diameter and height of 10mm×14mm. It is based on the ISO 13314:2011 (E) standard: when the plate structure size is compressed, the height to diameter ratio should be between 1 and 2. The porosity-related parameter settings (-0.0433 and 0.5097 mm) were obtained through quantitative calculations. The specific derivation process is as follows: when the unit cell size is 2 mm × 2 mm × 2 mm, the M parameter range is between -0.5 and 0.5, and the T parameter range is between 0 and 1.05. When both are taken to the minimum value, the corresponding theoretical model porosity is 100%, but no solid model can be generated. When both are taken to the maximum value, the corresponding D and IT homogeneous unit models are solid cylindrical models with zero porosity. Therefore, five M values ​​and five T values ​​are selected within the range. The specific correspondence between the porosity (P) of the generated D and IT homogeneous unit models is shown in Table 1 below:

[0033] Table 1

[0034] M (mm) -0.3 -0.2 0 0.2 0.3 <![CDATA[P1(%)]]> 85.32 73.34 50 26.67 14.67 T(mm) 0.3 0.5 0.6 0.7 0.8 <![CDATA[P2(%)]]> 81.49 56.34 42.74 29.66 17.99

[0035] By performing linear fitting on the data in Table 1, the fitting results are as follows Figure 2 As shown in (a) and (b), the linear relationships between the porosity (P1) and M values ​​for the D homogeneous unit porous structure model and the porosity (P2) and T values ​​for the IT homogeneous unit porous structure model are P1 = (0.5-1.1741923M) × 100% and P2 = (1.199177-1.2805811T) × 100%, respectively, with reliability levels of 99.998% and 99.947%, respectively. Both reliability levels meet the requirement for a reliability (R) of at least 98% for the fitting results. Substituting a porosity of 55% into P1 = (0.5-1.1741923M) × 100% and P2 = (1.199177-1.2805811T) × 100%, respectively, yields an M value of -0.0433 and a T value of 0.5097.

[0036] (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 2.5 mm × 14 mm in the Create module of the Ntopology software. 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.

[0037] The In Min and In Max parameters constitute the fusion region. In the present invention, the fusion region forms a circular fusion region within the height range of Z = -H2 / 2mm to Z = -H2 / 2mmmm. In the present invention, the In Min and In Max parameters are constants a and b, respectively, and the ranges of a and b are 0≤a<b<2(R1-R2)(mm). In order to achieve a smooth connection and a better connection effect, the fusion region length (ba) / 2(mm) should be ≥ half of the maximum unit cell side length of the two fusion units. In the present invention, the D and IT homogeneous unit models are fused and the unit cell side lengths are both 2mm×2mm×2mm, so ba=2 / 2=1=In Max-In Min=1-0.

[0038] In this embodiment, In Min is 0, indicating that fusion starts from R2=2.5mm around the edge of cylinder II, and In Max is 1, indicating that the fusion will grow symmetrically by 0.5mm along the radius of the small cylinder and end at R=3mm. That is, the same straight line will be extended symmetrically and equally in opposite directions, and the sum of the extensions is 1, eventually forming a circular fusion area within the height range of Z=-7mm to Z=7mm.

[0039] In this embodiment, Out Min is 0 and Out Max is 1 to ensure that the overall fusion model is consistent with the initial modeling framework (cylinder I) at a ratio of 1:1, that is, the overall model will not be widened or heightened.

[0040] (3) The third step: First, call the Mix command and substitute the D homogeneous unit and IT homogeneous unit models created in step (1) and the Ramp fusion function created in step (2) into the Mix command to generate a preliminary ITD fusion CAD model in the form of a cylinder with an overall morphology radius and height of 5 mm × 14 mm.

[0041] Here is the preliminary ITD fusion CAD model as follows Figure 3 As shown in the figure, it is found that the fusion effect of the fusion model with only one surface layer is not good, so a smaller cylinder (cylinder III) is established to remove the surface layer. Cylinder III can be smaller than the cylinder I model but larger than the volume frame of cylinder II, and the ratio of the overall height to diameter of cylinder III should be between 1 and 2.

[0042] (4) Therefore, in the Create module, a cylinder (Cylinder III) with a radius (R3) of 4.5 mm and a height (H3) of 12 mm is created. The Cylinder III and the preliminary ITD fusion model are fused using the Boolean Intersect command to form the final ITD fusion CAD model of a cylinder with a radius and height of 4.5 mm × 12 mm. At this time, the porosity of the final fusion model ITD is 55.67%. The schematic diagram of the model is as shown in Figure 4 and 5 shown.

[0043] The final ITD fusion CAD model of a cylinder with a radius and height of 4.5 mm × 12 mm will also limit Cylinder II in step (2). That is, finally, 0 mm < R2 < R3 = 4.5 mm and 0 mm < H2 ≤ H3 = 12 mm. And when the height does not match H3, the IT unit will be completely wrapped by the D unit. It will also limit the In Min and In Max parameters in the second step to constants a and b, where a and b are 0 ≤ a < b < 2(R3 - R2) (mm).

[0044] (5) The final ITD fusion CAD model is initially divided into a finite element simulation mesh model using the Mesh From Implicit Body command in Ntopology software, and finally, the simulation model file in inp format is output using the Export FE Mesh command.

[0045] (6) The simulation model file in inp format is imported into HyperMesh software for repair and re-meshing. The tetrahedral mesh division form is adopted, the mesh type is C3D4, and the mesh element size is 0.2 mm. Then, it is exported and saved again as an inp format file model.

[0046] (7) The generated inp format file model is imported again into the explicit dynamics module in Abaqus software to perform quasi-static compression finite element simulations respectively. An elastic modulus of 110 GPa, a Poisson's ratio of 0.3, and 4.51 g / cm 3The 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 reaches 1.5. At the same time, the boundary conditions for the compression of the porous structure are defined. 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 and bottom surfaces 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.

[0047] like Figure 6 The deformation behavior and stress concentration distribution of the axial center section during the quasi-static compression simulation of the ITD fusion model finite element model are shown, and the elastic stage strain (ε) of 5% 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 at this time) are selected as representatives to show the changes in the model's full-stage compression simulation process. Figure 6 The stress-strain contours reveal the following: 1. Elastic Stage: The D-cell surface is slightly concave, the overall height decreases slightly, the pores are slightly compressed, and stress is concentrated in the center of the surface. The T-cell struts elastically bend, their overall height decreases slightly, the pore shape changes slightly, and stress is evenly distributed at the strut nodes. The fusion interface smoothly connects and transmits deformation, with minimal elastic deformation and low stress concentration. 2. Yield Stage: The D-cell surface undergoes plastic deformation, the pores squeeze and close, the volume decreases, and high stress is concentrated in the yield region. The IT-cell struts undergo plastic deformation or breakage, porosity decreases, the structure shrinks, and high stress is concentrated in the deformed struts. The fusion interface adapts to different deformation rates, undergoing tensile deformation, with stress concentrated in the connection and transition region. 3. Densification Stage: The D-cell undergoes severe densification, with pores disappearing, a blocky structure, and high stress distributed throughout the cell. The IT-cell undergoes severe densification, with pore compression, a compact structure, and high stress distributed throughout the cell. The fusion interface undergoes compression, bending, and torsional deformation, resulting in high stress concentration, ultimately leading to cracks and fracture.

[0048] In summary, the bionic porous scaffold exhibits different deformation behaviors and stress distributions in different mechanical stages. The stress at the fusion interface is low in the elastic stage. As the yield and densification stages advance, the stress concentration increases, eventually leading to the destruction of the fusion interface.

[0049] The yield strength simulation result of the ITD fusion model is 191.78 MPa, which is within the range of the yield strength of the cortical bone in the femur (33-193 MPa). The elastic modulus after simulation is 7.59 GPa, which is also within the range of the elastic modulus of the cortical bone (3-20 GPa). Therefore, the porous structure is suitable as a bionic femoral scaffold implanted in the body and can avoid stress shielding problems.

[0050] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for designing a bionic femoral stent based on the fusion of lattice 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 and changing the wall thickness parameters, a cylindrical Iso Truss unit homogeneous porous structure CAD model with a target porosity and a diameter of R1 and a height of H1 was created; (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) Calling the Mix command, substituting the cylindrical Diamond unit homogeneous porous structure CAD model, the cylindrical Iso Truss unit homogeneous porous structure CAD model, and the Ramp fusion function into the Mix command to generate a preliminary ITD fusion CAD model in the form of a cylinder with a radius and height of R1×H1; (5) In the Create module, a cylinder III with a radius of R3 and a height of H3 is created, and the cylinder III is fused with the preliminary ITD fusion CAD model using the Boolean intersection command to form a final ITD fusion CAD model of a cylinder with a radius of R3 and a height of H3, where 0<R2<R3<R1, 0<H2≤H3<H1, R3 / H3=1~2, R1 / H1=1~2, R2 / H2=1~2, 0≤a<b<2(R3-R2); (6) The final ITD fused CAD 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; (7) 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 lattice structure and TPMS structure according to claim 1, characterized in that: In the step (1), the relationship between the Midsurface Offset parameter M and the porosity P1 is P1=(0.5-1.1741923M)×100%; 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.

3. The method for designing a bionic femoral stent based on the fusion of lattice structure and TPMS structure as claimed in claim 1, characterized in that: In the step (2), the relationship between the wall thickness parameter T and the porosity P2 is P2=(1.199177-1.2805811T)×100%; the wall thickness parameter T is set to adjust the porosity of the Iso Truss unit homogeneous porous structure CAD model through the relationship between the wall thickness parameter T and the porosity P2.

4. The method for designing a bionic femoral stent based on the fusion of lattice 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 Iso Truss unit.

5. The method for designing a bionic femoral stent based on the fusion of lattice structure and TPMS structure as claimed in claim 4, characterized in that: The unit cell size of the Diamond unit homogeneous porous structure and the Iso Truss unit homogeneous porous structure are both 2 mm×2 mm×2 mm.

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

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