A method for designing a biomimetic femoral support based on lattice structure and TPMS structure fusion

Abstract

The application discloses a method for designing a bionic femur support based on lattice structure and TPMS structure fusion, and belongs to the technical field of bionic bone support. The application respectively creates a CAD model of a Diamond unit homogeneous porous structure and a CAD model of an Iso Truss unit homogeneous porous structure; a Ramp fusion function is created to fuse the Diamond unit homogeneous porous structure and the Iso Truss unit homogeneous porous structure to generate a preliminary ITD fusion CAD model; then the preliminary ITD fusion CAD model and a newly established cylinder III are fused by using a Boolean intersection instruction to form a final ITD fusion CAD model, finally, the final ITD fusion CAD model is divided into a finite element simulation grid model and an inp format simulation model file is output; the inp format simulation model file is imported into HyperMesh software to repair and divide a grid and an inp file format model is exported. The bionic bone support designed by the application has higher bearing capacity and stability, forms a mechanical property gradient, and is suitable for complex mechanical environments. Meanwhile, the bionic bone support can promote cell adhesion, proliferation and differentiation, transport nutrients, discharge metabolic waste, and improve cell survival rate.

Description

TECHNICAL FIELD

[0001] The application 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 lattice structures and TPMS structures. BACKGROUND

[0002] The actual structure of the human femur can be seen as being composed of an outer circular ring part of cortical bone and an inner part of cancellous bone in the radial direction, wherein the cortical bone has a low porosity and mainly bears the load, and the cancellous bone (bone marrow) has a porosity of 50% to 90%, and mainly transports nutrients and provides growth space and landing points for bone cell proliferation and differentiation, that is, the femur itself has the hierarchical structure characteristics of cortical bone and cancellous bone.

[0003] In the field of bone tissue engineering, triply periodic minimal surface (TPMS) and lattice structure are two commonly used structure types when designing bionic bone scaffolds. For example, the Diamond (D) unit in the TPMS structure has the characteristics of zero average curvature, periodicity in three spatial dimensions, high connectivity, and precise controllability, which makes the surface of the D unit very smooth without sharp corners or abrupt curvature. The smooth surface is conducive to cell adhesion and growth, and cells can be more uniformly distributed and spread on such a surface, improving the interaction between cells and the scaffold. When designing a bionic bone scaffold using a lattice structure such as the Iso Truss (IT) unit, the structure itself has spatial symmetry and high porosity, and the generated porous scaffold also has good 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 designing bionic bone scaffolds only focuses on using a single unit corresponding to a single cell to design homogeneous porous structures or performing gradient design in a single direction within the same type. Such design methods first do not match the actual structure of the femur and do not fully reflect the hierarchical structure characteristics of natural bone, and secondly, there is no research on designing porous structures as bionic bone scaffolds by combining different types of structures (such as using TPMS units and lattice structure units) and adopting a radial smooth fusion method. SUMMARY

[0005] To overcome the shortcomings of the prior art, the application provides a method for designing a bionic femoral scaffold based on the fusion of lattice structures and TPMS structures.

[0006] To achieve the above object, the technical scheme adopted by the application is as follows:

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

[0008] (1) using Ntopology software and changing Midsurface Offset parameter to create a cylindrical Diamond element homogeneous porous structure CAD model with a target porosity, a diameter of R1 and a height of H1;

[0009] (2) using Ntopology software and changing wall thickness parameter to create a cylindrical Iso Truss element homogeneous porous structure CAD model with a target porosity, a diameter of R1 and a height of H1;

[0010] (3) using Ntopology software to create a cylindrical CAD model II with a diameter of R2 and a height of H2, then calling Ramp command, substituting cylindrical CAD model II and setting 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) calling Mix command, substituting cylindrical Diamond element homogeneous porous structure CAD model, cylindrical Iso Truss element homogeneous porous structure CAD model and Ramp fusion function into Mix instruction to generate a preliminary ITD fusion CAD model in the form of a cylinder with a radius and a height of R1xH1;

[0012] (5) creating a cylinder III with a radius of R3 and a height of H3 in Create module, fusing cylinder III with preliminary ITD fusion CAD model by Boolean intersection instruction to form a final ITD fusion CAD model in the form of a cylinder with a radius of R3 and a height of H3, wherein 0

[0013] (6) preliminarily dividing finite element simulation mesh model of final ITD fusion CAD model by Mesh From Implicit Body command in Ntopology software, and finally outputting simulation model file in inp format by output command;

[0014] (7) importing simulation model file in inp format into HyperMesh software to repair and re-divide mesh, adopting tetrahedral mesh division form, and mesh type is C3D4 and mesh element size is 0.2mm, then exporting and saving inp format file model again.

[0015] The application directly imports the inp file format model obtained in step (7) into an abaqus dynamics simulation module to perform quasi-static compression simulation, obtains result data after quasi-static compression finite element simulation, and evaluates whether the yield strength and the elastic modulus of the established model are within the yield strength and the elastic modulus of the cancellous bone and the cortical bone, respectively. If within the range, the established model is the target bionic bone support. The designed target bionic bone support model can be constructed into a bionic bone support through conventional means in the art, for example, the inp file format model obtained in step (7) is exported in STL form by using an Export command, and then the computer is connected with a 3D printer, the STL form model file is transmitted into the 3D printer, and the target bionic bone support is obtained.

[0016] As a preferred embodiment of the application, 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.

[0017] As a preferred embodiment of the application, 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.

[0018] As a preferred embodiment of the application, in the Ramp fusion function, the fusion region is a circular ring, the length of the circular ring on the Z axis is H2, the width of the circular ring on the X, Y plane is (b-a) / 2≥half of the maximum unit cell size of the Diamond unit and the Iso Truss unit, and the axial direction of the circular ring is the Z axis direction.

[0019] As a preferred embodiment of the application, the unit cell size of the Diamond unit homogeneous porous structure and the Iso Truss unit homogeneous porous structure is 2mm×2mm×2mm.

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

[0021] Compared with the prior art, the present application has the beneficial effects that: the present application radially and smoothly fuses the stereoscopic annular region of the Diamond unit homogeneous porous structure and the Iso Truss unit homogeneous porous structure in the Z-axis direction, forms a mechanical property gradient, and adapts to a complex mechanical environment. Moreover, the two unit structures work together to disperse stress, improve the bearing capacity and stability. In addition, the radially smooth fusion achieves to improve the stability of the overall structure and avoid weak links. In terms of biological performance, the multi-level porous structure after fusion simulates natural bone, promotes cell adhesion, proliferation and differentiation. Moreover, the formed pore network efficiently transports nutrients and discharges metabolic waste, improves cell survival rate. At the same time, the smooth fusion interface promotes cell migration, reduces foreign body reaction, and enhances integration. BRIEF DESCRIPTION OF DRAWINGS

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

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

[0024] Figure 3 Fig. 3 is a schematic diagram of the change from the preliminary ITD fusion CAD model to the final ITD fusion CAD model.

[0025] Figure 4 Fig. 4 is a schematic diagram of the radially and smoothly fused final ITD fusion CAD model formed by the Diamond unit homogeneous porous structure and the Iso Truss unit homogeneous porous structure.

[0026] Figure 5 Fig. 5 is a schematic diagram of the specific position 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 Fig. 6 is a schematic diagram of the deformation behavior and stress concentration distribution of the axial center section when the finite element model of the final ITD fusion CAD model is subjected to quasi-static compression simulation. DETAILED DESCRIPTION

[0028] For better illustrating the purposes, technical solutions and advantages of the present application, the present application will be further described in combination with specific examples.

[0029] Example 1

[0030] A method for designing a biomimetic femoral support based on the fusion of lattice structure and TPMS structure, comprising the following steps:

[0031] (1) First, a CAD model of a cylinder (cylinder I) with a radius (R1) and height (H1) of 5 mm x 14 mm is created in the Create module of Ntopology software. Then, D unit cells are selected by using the TPMS Unite cell with Offset command, and the Midsurface Offset (M) parameter is set to -0.0433 by using the Rectangular Volume Lattice command to generate a CAD model of D homogeneous unit porous structure in the form of a cylinder with a radius and height of 5 mm x 14 mm, with a porosity of 55%. Similarly, IT unit cells are selected by using the Graph Unite Cell command, and then the wall thickness parameter (Thickness, T) is set to 0.5097 mm by using the Rectangular Volume Lattice command to generate a CAD model of IT homogeneous unit porous structure in the form of a cylinder with a radius and height of 5 mm x 14 mm, with a porosity of 55%. In the Rectangular Volume Lattice command used in the two cases, the unit cell size is designed to be 2 mm x 2 mm x 2 mm, and the Frame and Trim commands are set by default. The schematic diagram of the generated porous structure model is shown in Figure 1 .

[0032] The D and IT homogeneous unit porous structure model with overall porosity of 55% was designed in this embodiment 1. The porosity range of the model can be selected within 50% to 90% of cancellous bone porosity. However, the porosity of the D unit should not exceed that of the IT unit model so as to make the fused structure better conform to the natural bone hierarchical structure characteristics, because the D unit is the outer layer simulating cortical bone in the later fusion. The cell size is 2 mm x 2 mm x 2 mm, because the pore size is between 100 μm and 1200 μm at this time, which can meet the pore size requirement for bone cell proliferation and differentiation. The overall size is a cylindrical CAD model with a diameter and height of 10 mm x 14 mm. The parameter settings related to porosity (-0.0433 and 0.5097 mm) are obtained by quantitative calculation. The specific derivation process is as follows: when the cell size is 2 mm x 2 mm x 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 the minimum values are taken, the corresponding theoretical model porosity is 100%, but the solid model cannot be generated. When the maximum parameter values are taken, the D and IT homogeneous unit generated is a solid cylindrical model with a porosity of 0. Therefore, 5 M values and 5 T values are selected within the range. At this time, the specific corresponding relationship between the generated D and IT homogeneous unit model porosity (P) and the M and T values is shown in Table 1.

[0033] Table 1

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

[0035] By linear fitting the data in Table 1, the fitting results are shown in Figure 2 (a) and (b). The linear relationship between the porosity (P1) of the D homogeneous unit porous structure model and the M value and the linear relationship between the porosity (P2) of the IT homogeneous unit porous structure model and the T value are P1 = (0.5-1.1741923M) x 100% and P2 = (1.199177-1.2805811T) x 100%, respectively. The reliabilities are 99.998% and 99.947%, respectively. Both of them meet the requirement that the reliability (R) of the fitting result should be above 98%. By substituting the porosity of 55% into P1 = (0.5-1.1741923M) x 100% and P2 = (1.199177-1.2805811T) x 100%, the corresponding M value is -0.0433 and the T value is 0.5097.

[0036] (2) The second step is to create a fusion gradient command: first, create a CAD model of a cylinder with a radius (R2) and height (H2) of 2.5 mm x 14 mm (cylinder II) in the Create module of 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 a Ramp fusion function.

[0037] The In Min and In Max parameters constitute a fusion region, and in the present application, the fusion region is a circular ring fusion region formed within a height range of Z = -H2 / 2 mm to Z = -H2 / 2 mm. In the present application, the In Min and In Max parameters are constants a and b, respectively, and the range of a and b is 0≤a

[0038] In the present embodiment, In Min is 0, indicating that the fusion starts from the edge of cylinder II at R2 = 2.5 mm, and In Max is 1, indicating that the fusion symmetrically increases by 0.5 mm along the radial direction of the small cylinder to R = 3 mm, i.e., the same linear direction is symmetrically and equally extended in the opposite direction, and the total extension is 1. Finally, a circular ring fusion region is formed within a height range of Z = -7 mm to Z = 7 mm.

[0039] In the present embodiment, Out Min is 0 and Out Max is 1, which is to ensure that the overall model after fusion is consistent with the initial modeling framework (cylinder I) at 1:1, i.e., the overall model will not have changes in width and height.

[0040] (3) The third step is to first call the Mix command, and then substitute the D homogeneous unit model and the IT homogeneous unit model 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 a radius and height of 5 mm x 14 mm.

[0041] Here, the preliminary ITD fusion CAD model is as shown in the following figure: Figure 3 As shown in the figure, it is found that the fusion model only has poor fusion effect on the surface layer, so a smaller cylinder (cylinder III) is established to remove the surface layer. Cylinder III is smaller than cylinder I model but larger than the volume framework of cylinder II, and the ratio of the overall height of cylinder III to the diameter should also be between 1 and 2.

[0042] (4) Thus, the Create module creates a cylinder (cylinder III) with a radius (R3) and height (H3) of 4.5 mm x 12 mm, and the cylinder III is fused with the preliminary ITD fusion model using the Boolean Intersect instruction to form a final ITD fusion CAD model of a cylinder with a radius and height of 4.5 mm x 12 mm, at which time the final fusion model ITD porosity is 55.67%, and a diagram of the model is shown in Figure 4 and 5 .

[0043] The final ITD fusion CAD model of a cylinder with a radius and height of 4.5 mm x 12 mm also limits the cylinder II in the first step (2), that is, the final should have 0 mm < R2 < R3 = 4.5 mm and 0 mm < H2 < H3 = 12 mm, and when the height is not consistent with H3, the IT unit is completely wrapped by the D unit. It will also limit the In Min and In Max parameters in the second step to be constants a and b, respectively, and a and b are 0 < a < b < 2 (R3-R2) (mm).

[0044] (5) The final ITD fusion CAD model is again preliminarily 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 the HyperMesh software to repair and re-divide the mesh, in the form of tetrahedral mesh division, and the mesh type is C3D4, and the mesh element size is 0.2 mm. The inp format file model is again exported and saved.

[0046] (7) The generated inp format file model is again imported into the display dynamics module of the Abaqus software to perform quasi-static compression finite element simulation, and the elastic modulus of 110 GPa, the Poisson's ratio of 0.3, and the density of 4.51 g / cm 3The density of the D unit is set as the property of the Ti-6Al-4V material, and the grid 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 redefined, the upper top surface is subjected to a displacement load of 60% (the height of this model is 12 mm, i.e. the compression stops when 7.2 mm is pressed down), the lower bottom surface is constrained in the X, Y, Z three coordinate directions, the upper top surface is constrained in the X, Y directions and the X, Y, Z directions, and only the movement in the Z direction is allowed. The upper top surface and the lower bottom surface are rigid surfaces, the friction coefficient between the rigid surface and the hole structure is 0.2, the total simulation time is set to 0.1 seconds, and the analysis step is 5e-6 seconds.

[0047] As shown in Figure 6 The deformation behavior and stress concentration distribution of the axial center section of the ITD fusion model finite element model during the quasi-static compression simulation are shown, and the strain (ε) of 5% and 15% in the elastic stage, the yield stage (ε) of 25%, 35% and 50%, the densification stage (ε) of 65%, 75% and 100% (at this time, it has been compressed to 7.2 mm) are shown as representative to show the changes of the model during the whole stage compression simulation. Through Figure 6 The stress-strain cloud chart can be seen: 1. Elastic stage: D unit curved surface micro-concave, overall height slightly reduced, pore slightly compressed, stress concentrated in the surface center area. T unit strut elastic bending, overall height slightly reduced, pore shape slightly changed, stress uniformly distributed in the strut node. The fusion interface is smoothly connected and deformed, slightly elastically deformed, and low stress concentrated. 2. Yield stage: D unit local curved surface plastic deformation, pore extrusion closed, volume reduced, high stress concentrated in the yield area. IT unit strut plastic deformation or broken, porosity decreased, structure contracted, high stress concentrated in the deformed strut. The fusion interface adapts to different deformation rates, tensile deformation, and stress concentration in the connection part and the transition area. 3. Densification stage: D unit serious densification, pore disappearance, block structure, high stress distribution in the whole unit. IT unit serious densification, pore compression, structure compact, high stress distribution in the whole unit. The fusion interface is extruded, bent and twisted, high stress concentrated, and finally leads to cracks and fractures.

[0048] In summary, the bionic porous scaffold shows different deformation behavior and stress distribution in different mechanical stages, the stress of the fusion interface is low in the elastic stage, and the stress concentration increases with the advancement of the yield and densification stages, and finally leads to the damage of the fusion interface.

[0049] The yield strength of the ITD fusion model is simulated to be 191.78 MPa, which is within the range of the yield strength of the cortical bone of femur (33-193 MPa), and 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), so the porous structure is suitable for being implanted into the body as a biomimetic femur stent and can avoid the stress shielding problem.

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

Claims

1. A method for designing a biomimetic femoral scaffold based on the fusion of lattice structure and TPMS structure, characterized in that, Includes the following steps: (1) Use Ntopology software and change the Midsurface Offset parameter to create a CAD model of a cylindrical Diamond unit homogeneous porous structure with a diameter of R1 and a height of H1 and a target porosity. (2) Use Ntopology software and change the wall thickness parameters to create a CAD model of a cylindrical Iso Truss unit homogeneous porous structure with a diameter of R1 and a height of H1 and a target porosity. (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 and substitute 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 with a radius and height of R1×H1 in the form of a cylinder. (5) Create a cylinder Ⅲ with radius R3 and height H3 in the Create module. Merge cylinder Ⅲ with the preliminary ITD fused CAD model using the Boolean intersection command to form the final ITD fused CAD model with radius R3 and height 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 used to preliminarily divide the 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 output command. (7) Import the simulation model file in .inp format into HyperMesh software to repair and re-mesh it. Use tetrahedral meshing, and set the mesh type to C3D4 with a mesh element size of 0.2mm. Then export and save the .inp format file model again.

2. The method for designing a biomimetic femoral scaffold based on the fusion of lattice structure and TPMS structure as described in claim 1, characterized in that, In step (1), the relationship between the Midsurface Offset parameter M and the porosity P1 is P1 = (0.5 - 1.1741923M) × 100%; the porosity of the Diamond unit homogeneous porous structure CAD model is adjusted by setting the Midsurface Offset parameter M and the porosity P1.

3. The method for designing a biomimetic femoral scaffold based on the fusion of lattice structure and TPMS structure as described in claim 1, characterized in that, In 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 by using the relationship between the wall thickness parameter T and the porosity P2.

4. The method for designing a biomimetic femoral scaffold based on the fusion of lattice structure and TPMS structure as described in claim 1, characterized in that, The fusion region 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 cell edge length of the Diamond unit and the Iso Truss unit.

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

6. The method for designing a biomimetic femoral scaffold based on the fusion of lattice structure and TPMS structure as described in claim 1, characterized in that, The target porosity is 50%–90%.

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