4D Additive Manufacturing Method of Self-Growing Artificial Root System Based on Negative Poisson's Ratio Materials

Abstract

The present invention provides a self-growing artificial root 4D additive manufacturing method based on auxetic materials, which relates to the field of metamaterial biological structure construction and manufacturing technology. The artificial root system is constructed by repeating unit cell structures, and the method includes the following steps: (1) parametrically designing the core unit cell structure to construct a three-dimensional auxetic core structure; (2) parametrically designing the epidermal unit cell structure to construct a three-dimensional auxetic tubular structure; (3) combining the core structure and the tubular structure into an overall structure, and 3D printing the overall structure to form a single root; (4) by adjusting the design parameters, printing multiple root systems and assembling them into a complex three-dimensional root system to simulate the growth of single roots and complex root systems. The present invention can manufacture structures with good and comprehensive real root system performance, can fully simulate the mechanical, water absorption and water retention, and friction characteristics of the root system, and has broad application prospects in the fields of plant root system and ecological restoration research.

Description

Technical Field

[0001] The present invention relates to the technical field of the construction and manufacturing of metamaterial biological structures, and particularly to a 4D additive manufacturing method for self-growing artificial roots based on negative Poisson's ratio materials. Background Art

[0002] In the implementation of the technology of using forest tree roots to reinforce soil on a large number of exposed slopes generated by engineering construction, the root morphology directly affects the effect of soil reinforcement and slope protection. Due to the complexity of vegetation roots and their deep burial underground, it brings great difficulties to the acquisition of roots and the study of root morphology. Obtaining a root model through 3D printing is a solution. However, the commonly used methods for establishing root models at home and abroad still remain in the aspects of static graphics drawing and model making, and it is difficult to reflect the dynamic growth of plants and comprehensively simulate the mechanical properties, water absorption and water retention, and surface friction properties of roots. Summary of the Invention

[0003] In order to solve the technical problems existing in the prior art, an embodiment of the present invention provides a 4D additive manufacturing method for self-growing artificial roots based on negative Poisson's ratio materials. The technical solution is as follows:

[0004] A 4D additive manufacturing method for self-growing artificial roots based on negative Poisson's ratio materials, wherein the artificial roots are constructed by repeating unit cell structures, and the method includes the following steps:

[0005] (1) Parametrically design the unit cell structure of the core, and thus construct a three-dimensional negative Poisson's ratio core structure;

[0006] (2) Parametrically design the unit cell structure of the epidermis, and thus construct a three-dimensional negative Poisson's ratio tubular structure;

[0007] (3) Combine the core structure and the tubular structure into an overall structure, and 3D print the overall structure to form a single root;

[0008] (4) By adjusting the design parameters, print multiple roots and assemble them into a complex three-dimensional root system to simulate the growth of single roots and complex root systems;

[0009] Among them, a three-dimensional negative Poisson's ratio material is used. The three-dimensional negative Poisson's ratio material is constructed by repeating unit cell structures, wherein a negative Poisson's ratio square column serves as the root core, and a negative Poisson's ratio cylinder serves as the root epidermis. Under the action of an external force, it can be compressed into a solid with 20% - 40% of the original volume, and in the case of changes in the water environment and thermal environment, it can self-grow into long roots to achieve the 4D effect.

[0010] Optionally, the method further includes: testing the tensile expansion, compression, elastic modulus, tensile strength, water absorption and water retention capacity, and surface friction coefficient of a single root, wherein the manufactured root system has physical and mechanical properties similar to those of real roots, including tensile properties, bending properties, water absorption and water retention properties, and root surface friction characteristics.

[0011] Optionally, the 3D printing material used is thermoplastic polyurethane (TPU).

[0012] Optionally, the core unit cell is a symmetric hexagon, composed of two hexagons, an inner one and an outer one. The upper and lower sides of the outer hexagon are horizontal line segments, and the left and right sides are the hypotenuses of isosceles triangles. The inner hexagon has the same shape as the outer hexagon but smaller dimensions, and a uniform annular border area is formed between the two. Two short cuboid arms extend from both sides in the middle of the unit cell for connection between unit cells, for simulating the root system core;

[0013] And / or, the epidermis is a tubular porous structure, composed of multiple continuously arranged unit cells. The shape of the epidermis unit cell is the same as that of the core unit cell. The unit cells are connected to each other through adjacent sides, so that adjacent unit cells share edges, forming a closely arranged tubular whole, for simulating the root system epidermis;

[0014] And / or, the root system core and the tubular epidermis form a single whole root system through shared contact and are used for 3D printing and forming.

[0015] Optionally, the root length of the root system is at least the length of one unit cell, and the diameter of a single root is 2 - 200 mm, capable of simulating the root systems of arbor plants such as pine, cypress, and locust tree.

[0016] Optionally, only one core unit cell and one epidermis unit cell need to be designed, and then periodically repeated arrangement in space is used to generate a single root structure, enabling it to have three-dimensional tensile expansion and compression characteristics, which can cause an expansion effect under changes in water environment or thermal environment, thereby simulating the growth effect of the root system.

[0017] Optionally, a negative Poisson's ratio square column is used to simulate the root system core, for simulating the tensile and bending mechanical properties of the root system;

[0018] And / or, a negative Poisson's ratio cylinder is used to simulate the root system epidermis, for simulating the surface friction characteristics of the root system.

[0019] Optionally, the mechanical properties of the artificial root system are as follows: density 1.1•10 3 -1.2•10 3 kg / m 3 , tensile elastic modulus is 10 - 25 MPa, tensile strength is 30 - 60 MPa, and bending elastic modulus is 10 - 25 MPa;

[0020] And / or, the artificial root system has water absorption and water retention properties. The water absorption property is generated by the siphon effect of the porous structure, with the lower limit of the water absorption height being 5 cm and the upper limit being 30 cm. The water retention property is controlled by the water tension film of the porous structure, and the water content is 5-25%;

[0021] And / or, the surface of the artificial root system has friction characteristics similar to those of real roots, with an equivalent friction coefficient of 0.2-0.5; the root-soil pull-out curve shows a non-linear form. The initial stage is the elastic deformation stage. As the pulling force increases, it gradually enters the plastic deformation stage, and finally the roots are detached from the soil when the maximum pull-out force is reached; it can simulate the pull-out failure mode, and when the frictional resistance at the root-soil interface is large, it can also simulate the root pulling fracture mode.

[0022] Optionally, the roots with complex shapes are composed of the single roots. This root system is composed of multiple single roots with different angles, diameters, and levels, specifically including:

[0023] (a) Root angle:

[0024] - Vertical roots, referring to the main roots growing vertically downward, with the angle between the root growth direction and the horizontal plane being 60-90 degrees;

[0025] - Horizontal roots, referring to the horizontal or nearly horizontal roots growing from the main roots or other lateral roots, with the angle between the root growth direction and the horizontal plane being 0-30 degrees;

[0026] - Oblique roots, referring to the obliquely growing roots between vertical roots and horizontal roots, with the angle between the root growth direction and the horizontal plane being 30-60 degrees;

[0027] (b) Root diameter:

[0028] - The diameter of the first-level main root, referring to the thickest root measured at the root neck;

[0029] - The diameter of the second-level lateral root, referring to the diameter of the lateral root directly growing from the main root;

[0030] - The diameter of the third-level lateral root, referring to the diameter of the root growing from the second-level lateral root;

[0031] (c) Root level:

[0032] - First-level roots, namely the main roots;

[0033] - Second-level roots, the lateral roots branching out from the first-level roots;

[0034] - Third-level roots, the lateral roots branching out from the second-level roots.

[0035] Optionally, the root systems with complex morphology are composed of the single roots. Both the single root system and the complex root system can form aggregates at room temperature through compression. The aggregates can grow into the original morphology when placed in water at 50°C to 80°C, thereby simulating the root growth process.

[0036] A self-growing artificial root system based on auxetic materials manufactured by the method described above.

[0037] The beneficial effects brought by the technical solution provided in the embodiments of the present invention at least include:

[0038] In view of the complexity of vegetation root systems, the common methods for establishing root system models at home and abroad still remain in static graphics drawing and model making. It is difficult to reflect the dynamic growth of plants and comprehensively simulate the mechanics, water absorption and water holding, and the friction performance of the root surface of the root system. The present invention proposes a 4D additive manufacturing method for a self-growing artificial root system based on auxetic materials, which can not only reflect the complex growth conditions of plant root systems, but also simulate the mechanical properties, water absorption and water holding performance, and the root-soil interface friction of the root system under the influence of various environmental factors, making it similar to real root systems.

[0039] Among them, the porous 4D printing artificial root system manufacturing technology based on the principle of auxetic materials can not only reflect the complex growth conditions of plant root systems, but also simulate the mechanical properties, water absorption and water holding performance, and the root-soil interface friction of the root system under the influence of various environmental factors, effectively detecting the application effects of different plant root systems in slope protection, and having broad application prospects.

[0040] The present invention can manufacture a structure with good and comprehensive real root system performance, which not only breaks through the limitation of the traditional root system model reconstruction method being only a static model, but also can fully simulate the mechanics, water absorption and water holding, and friction characteristics of the root system, and has broad application prospects in the fields of plant root systems and ecological restoration research. Description of the Drawings

[0041] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0042] Figure 1 It is a definition diagram of the geometric parameters of the single-cell structure in the single artificial root system structure of the present invention; among them, Figure 1 Figure (a) is a schematic diagram of the single-cell length parameter, Figure 1 Figure (b) is a schematic diagram of the single-cell area parameter;

[0043] Figure 2Flow chart for constructing a single artificial root system structure core of the present invention;

[0044] Figure 3 Flow chart for constructing the epidermis of a single artificial root system of the present invention;

[0045] Figure 4 Schematic diagram of the overall structure of a single artificial root system of the present invention;

[0046] Figure 5 Schematic diagram of the overall structure of a complex artificial root system of the present invention;

[0047] Figure 6 Tensile stress-strain curve of a single artificial root system of the present invention;

[0048] Figure 7 Flexural stress-strain curve of a single artificial root system of the present invention; wherein, Figure 7 In (a) is the three-point bending stress-strain curve, Figure 7 In (b) is the schematic diagram of the three-point bending simulation model;

[0049] Figure 8 Pull-out stress-strain curve of a single artificial root system of the present invention;

[0050] Figure 9 Physical diagram of the core structure of a single artificial root system of the present invention;

[0051] Figure 10 Physical diagram of the epidermis structure of a single artificial root system of the present invention;

[0052] Figure 11 Physical diagram of the overall structure of a single artificial root system of the present invention. Detailed implementation manners

[0053] The following describes the technical solutions in the present invention with reference to the accompanying drawings.

[0054] The following illustrates the implementation manners of the present invention through specific examples. Those skilled in the art can easily understand other advantages and benefits of the present invention from the content disclosed in this specification. The present invention can be applied or implemented through other different implementation manners, and the details in the specification can be modified or adjusted according to different application perspectives and requirements without departing from the core spirit of the present invention. It should be noted that the diagrams provided in the following examples only illustrate the basic concept of the present invention schematically. Without conflict, the following examples and the features in the examples can be combined with each other.

[0055] Among them, the attached drawings are only for illustrative purposes, showing only schematic diagrams rather than actual product drawings, and should not be construed as a limitation on the present invention; in order to better illustrate the embodiments of the present invention, some components in the attached drawings will be omitted, enlarged or reduced, which does not represent the size of the actual product; for those skilled in the art, it is understandable that some well-known structures and their descriptions in the attached drawings may be omitted.

[0056] In the attached drawings of the embodiments of the present invention, the same or similar reference numerals correspond to the same or similar components; in the description of the present invention, it should be understood that if there are terms such as "upper", "lower", "left", "right", "front", "rear", etc. indicating the orientation or positional relationship, they are based on the orientation or positional relationship shown in the attached drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. Therefore, the terms describing the positional relationship in the attached drawings are only for illustrative purposes and should not be construed as a limitation on the present invention. Those skilled in the art can understand these terms according to the actual situation.

[0057] Negative Poisson's Ratio Materials (abbreviated as NPR) are materials that exhibit deformation characteristics opposite to those of conventional materials in the transverse direction when subjected to external forces. When this material is uniaxially stretched, it expands laterally, and when uniaxially compressed, it contracts laterally. Therefore, it is also called "auxetic materials" (LatticeMaterials). The Poisson's ratio describes the ratio of the transverse deformation to the longitudinal deformation of a material when subjected to external forces. The Poisson's ratio of conventional materials is positive, usually between 0 and 0.5. The Poisson's ratio of negative Poisson's ratio materials is negative, which means that the material expands laterally when stretched and contracts laterally when compressed. This property gives negative Poisson's ratio materials significant advantages in mechanical properties, such as improving shear resistance and energy absorption capacity. The preparation methods of negative Poisson's ratio materials mainly include porous structures and fiber-filled composites. Porous materials such as two-dimensional honeycomb solid materials with concave bubble cell structures can achieve the negative Poisson's ratio effect by designing the macroscopic structure. Fiber-filled composites obtain the negative Poisson's ratio effect by controlling the order of different-scale laminates.

[0058] A 4D additive manufacturing method for self-growing artificial roots based on negative Poisson's ratio materials uses three-dimensional negative Poisson's ratio materials, which are constructed by repeating unit cell structures. Among them, the negative Poisson's ratio square column serves as the root core, and the negative Poisson's ratio cylinder serves as the root epidermis. Under the action of external forces, it can be compressed into an aggregate with a volume of 20% - 40%. When the water environment and thermal environment change, it can self-grow into roots, thus achieving the 4D effect.

[0059] As an artificial root system 4D additive manufacturing method, the manufactured root system has mechanical properties similar to those of real root systems, including tensile properties, bending properties, water absorption and water retention properties, and the friction characteristics of the root surface.

[0060] As a 4D additive manufacturing method, the 3D printing material used is thermoplastic polyurethane (TPU).

[0061] As an artificial root system 4D additive manufacturing method, the core unit cell is a symmetric hexagon, composed of two hexagons, an inner one and an outer one. The upper and lower sides of the outer hexagon are horizontal line segments, and the left and right sides are the hypotenuses of isosceles triangles. The inner hexagon has the same shape as the outer hexagon but smaller dimensions, and a uniform annular border area is formed between them. The aspect ratio of the core unit cell structure is defined as l / w, and the range of l / w is 0.1 - 0.5. The thickness-to-width ratio of the core unit cell structure is defined as t / w, and the range of t / w is 0.1 - 0.5. The core unit cell has the property of negative Poisson's ratio during uniaxial tension and compression in both the longitudinal and transverse directions, and is used to simulate the root system core.

[0062] As an artificial root system 4D additive manufacturing method, the root epidermis is a tubular porous structure, composed of multiple continuously arranged unit cells. The shape of the epidermal unit cell is the same as that of the core unit cell, so that adjacent unit cells share edges, forming a closely arranged whole. The aspect ratio of the epidermal unit cell structure is defined as T / L, and the range of T / L is 0.1 - 0.5. As T / L increases, the yield stress of the root system decreases under the same strain. The hollow area of the epidermal unit cell structure is defined as A t =A(total area) - A s (solid area), and the hollow area ratio of the epidermal unit cell structure is defined as A t / A, A t / A ranges from 0.1 to 0.5. As the value of A t / A increases, the friction coefficient of the root epidermis decreases, and it is used to simulate the root epidermis.

[0063] As an artificial root system 4D additive manufacturing method, the root length of the root system is at least one unit cell length, and the single root diameter is 2 - 200 mm, which can simulate the root systems of arbor plants such as pine, cypress, and locust tree.

[0064] As an artificial root system 4D additive manufacturing method, the negative Poisson's ratio square column is used to simulate the root system core, mainly for simulating the tensile and bending mechanical properties of the root system.

[0065] As an artificial root system 4D additive manufacturing method, the mechanical properties of the artificial root system are as follows: the tensile strength is 30 - 60 MPa, the tensile elastic modulus is defined as E = σ / ε, and the range is 10 - 25 MPa. The bending elastic modulus is defined as E f =σf / ε f (where σ f is the bending stress and ε f is the bending strain), ranging from 10 to 25 MPa. The density is defined as ρ = M / V0 (where M is the root mass and V0 is the apparent volume of the root system), ranging from 1.1•10 3 -1.2•10 3 kg / m 3 ;

[0066] As an artificial root system 4D additive manufacturing method, the manufactured root system has water absorption and water retention properties. The water absorption property is generated by the siphon effect of the porous structure, and its water absorption height can be greater than 10 cm. The water retention property is controlled by the water tension film of the porous structure to retain water. The porosity per unit length of the single cell is defined as (V is the absolute dense volume), ranging from 5 to 25%, and the water content change range is from 5 to 25%. Take a group of artificial root systems, measure the weights of 49.7 g, 51.1 g, and 51.3 g. Then completely immerse the root systems in the aqueous solution. After 2 minutes, take out the root systems and weigh them to get the weights of 58.9 g, 60.2 g, and 60.9 g, and calculate the equal water contents of 16%, 15%, and 16%.

[0067] As an artificial root system 4D additive manufacturing method, negative Poisson's ratio cylinders are used to simulate the root epidermis, mainly for simulating the surface friction characteristics of the root system.

[0068] As an artificial root system 4D additive manufacturing method, the surface of the artificial root system has friction characteristics similar to those of real root systems, and the equivalent friction coefficient is 0.2 - 0.5; the root-soil pull-out curve shows a non-linear shape. The initial stage is the elastic deformation stage. As the pulling force increases, it gradually enters the plastic deformation stage, and finally the root system detaches from the soil when the maximum pull-out force is reached; the pull-out failure mode can be simulated. When the frictional resistance at the root-soil interface is large, the root system pull-out fracture mode can also be simulated. Place a single root horizontally in the soil with a density of 1.5 - 1.7 g•cm -3 , with a burial depth of 5 cm. Apply uniform vertical loads of 0 kPa, 10 kPa, 20 kPa, 50 kPa, and 100 kPa on the soil surface respectively, and conduct horizontal pull-out experiments. The equivalent friction coefficient is defined as μ = σ v / τ (where σ v is the vertical load, τ = F / πdL R is the equivalent shear stress, F is the maximum pulling force, π is the pi, d is the root diameter, and L R is the root length);

[0069] As an artificial root system 4D additive manufacturing method, the root system is composed of multiple single roots with different angles, diameters, and levels, specifically including:

[0070] (a) Root angle:

[0071] - Vertical roots refer to the main roots that grow vertically downward, and the growth direction of the root system forms an angle of 60 - 90 degrees with the horizontal plane;

[0072] - Horizontal roots refer to the horizontal or nearly horizontal roots that grow from the main root or other lateral roots, and the growth direction of the root system forms an angle of 0 - 30 degrees with the horizontal plane;

[0073] - Oblique roots refer to the obliquely growing roots between vertical roots and horizontal roots, and the growth direction of the root system forms an angle of 30 - 60 degrees with the horizontal plane;

[0074] (b) Root diameter:

[0075] - The diameter of the primary main root refers to the thickest root measured at the root neck;

[0076] - The diameter of the secondary lateral root refers to the diameter of the lateral root that grows directly from the main root;

[0077] - The diameter of the tertiary lateral root refers to the diameter of the root that grows from the secondary lateral root;

[0078] (c) Root level:

[0079] - The primary root, i.e., the main root;

[0080] - The secondary root, the lateral root branched from the primary root;

[0081] - The tertiary root, the lateral root branched from the secondary root;

[0082] As an artificial root system 4D additive manufacturing method, only one core unit cell and one skin unit cell need to be designed, and then the single - root structure is generated by periodic repetition arrangement in space, so that it has auxetic and compressive properties. Both single roots and complex root systems can form aggregates at room temperature through compression and can recover to the original shape in water at 50 - 80 °C, thus simulating the root growth process;

[0083] Such as Figure 5 shown in a construction method of an artificial root system structure with negative Poisson's ratio, the overall negative Poisson's ratio structure is constructed by repeating the basic unit cell structure.

[0084] Example 1

[0085] The construction method of the artificial root system structure with negative Poisson's ratio specifically includes the following steps:

[0086] A. Construct a core unit cell structure model (modeling using Fusion software). The core unit cell is a symmetric hexagon composed of two hexagons, an inner one and an outer one. The upper and lower sides of the outer hexagon are horizontal line segments, and the left and right sides are the hypotenuses of isosceles triangles. The inner hexagon has the same shape as the outer hexagon but smaller dimensions, and a uniform annular border area is formed between them. Two short cuboid arms extend from both sides in the middle of the unit cell for connecting between unit cells. The length of the basic cell structure is 1620 (the following lengths are all proportional lengths without units and can be scaled proportionally), the width is 1180, and the length-width ratio is 1.373:1; the width of the basic cell structure is 1180, and the thickness is 200, and the width-thickness ratio is 5.9:1; the basic cell structure has the property of negative Poisson's ratio during uniaxial compression in both the longitudinal and transverse directions, as Figure 1 shown; Provide a unit cell structure, which includes a hollow part and cuboid structures on both the upper and lower sides; The unit cell structures constructed by standard software are periodically arranged horizontally and vertically in space along the direction of the hollow surface to form an approximately cubic structure with a shared vertex and thickness. The overall length is 82 mm, the overall width is 20 mm, and the overall height is 20 mm, as Figure 2 shown; By periodically arranging these cubic structures, an artificial root core is formed; Among them, the method of the periodic horizontal and vertical arrangement is as follows: Remove the cuboid parts on both the upper and lower sides from the unit cell structure, retain the remaining structure in the middle, rotate the two remaining structures by 90° respectively, and place them on the upper side of the unit cell in a mirror image manner. Rotate and move the mirrored part by 180° and place it at one end of the upper side of the unit cell. Subsequently, perform an array operation on the above-mentioned structures that have been rotated and mirrored, and replicate them multiple times at different angles to form a structure with multiple identical unit cells. Through the above array process, a three-dimensional structure with a complex geometric shape is finally obtained, and this structure can present a periodic arrangement pattern, as Figure 2 shown; See the physical diagram of the core structure in Figure 9 ;

[0087] B. Construct an epidermal cell structure model (modeling using Fusion software). The epidermis is a tubular porous structure composed of multiple continuously arranged unit cells. Each unit cell is connected to adjacent ones through adjacent sides, so that adjacent unit cells share edges and form a closely arranged whole. The length-width ratio of the epidermal unit cell structure is defined as T / L, and the variation range of T / L is 0.1 - 0.5. As T / L increases, the yield stress of the root system under the same strain decreases accordingly. The hollow area ratio of the epidermal unit cell structure is defined as A t / A, and the variation range of A t / A is 0.1 - 0.5. As the value of A t / A increases, the friction coefficient of the root epidermis decreases accordingly, as Figure 3 shown; See the physical diagram of the epidermal structure in Figure 10 ;

[0088] C. Assemble and construct a three-dimensional negative Poisson's ratio tubular structure, where the core structure and the tubular structure are fused into an integral structure. The physical diagram of the integral structure is shown in Figure 11 as follows. 3D print the integral structure to form a single root, as shown in Figure 4 follows;

[0089] D. Modify the parameters of a single root system, and bond and combine single root systems with different parameters into an integral structure. 3D print the integral structure to form a complex root, as shown in Figure 5 follows;

[0090] D. Select thermoplastic polyurethane (TPU) wire as the 3D printing raw material; use a fused deposition modeling 3D printer. Import a suitable file format into the printing software, set the printing process according to the viscoelastic properties of different printing materials, and no support material is required during the printing process; slice the model to form a printing path, build the model, and upload the model to the 3D printer; due to the material properties of TPU, in actual printing, support is mostly used. However, due to the small surface voids of the artificial root system model, it is easy for the support material to remain in the pores, affecting the shape and mechanical properties of the model. Through supportless printing, the model accuracy can be effectively improved, and the formation of the three-dimensional negative Poisson's ratio artificial root system structure can be better completed.

[0091]

[0092] Example 2

[0093] The tensile and compression test method for the negative Poisson's ratio artificial root system structure specifically includes the following steps;

[0094] Use a universal testing machine (WANCE ETM-104B, China), and set the tensile speed to 5 mm / min. Select a single-core sample made of TPU with a length of 82 mm, a width of 20 mm, and a height of 20 mm for tensile testing. The stress-strain curve is shown in Figure 6 as follows, and the experimental results are in good agreement with the finite element results.

[0095] Use ABAQUS to simulate the tensile test of the root system. Set the displacement load at the reference point of the upper punch to generate a stress-strain numerical curve. The results are shown in Figure 6 as follows, and the experimental results are in good agreement with the finite element results.

[0096] Example 3

[0097] The three-point bending performance test method for the negative Poisson's ratio artificial root system structure specifically includes the following steps:

[0098] The universal testing machine (WANCE ETM-104B, China) was used. A single-core sample of TPU material with a length of 82 mm, a width of 20 mm, and a height of 20 mm was selected for the three-point bending test, and the stress-strain curve is as shown in Figure 7 Figure (a).

[0099] ABAQUS was used to simulate the three-point bending test of the root system. Three rigid cylinders were set on both sides of the sample to simulate the indenter and two supports in the test. The results are as shown in Figure 7 Figure (b). The displacement load was set at the reference point in the figure, the density was 1.1 g / cm, and the stress-strain curve is as shown in Figure 7 Figure (a).

[0100] Figure 7 Figure (a) shows the experimental and numerical stress-strain curves of the two root system models, and the experimental and finite element results are in good agreement.

[0101] The three-point bending test was carried out on the root system to obtain the deflection change range under different size design parameters.

[0102] Example 4

[0103] The test method for the water absorption and water retention performance of the negative Poisson's ratio artificial root system structure specifically includes the following steps:

[0104] A. Conduct the water absorption test on the root system. Insert the dry single root vertically into the water to obtain the siphon height under different size design parameters, and the variation range is 5 cm to 30 cm;

[0105] B. Take a group of artificial root systems, measure the weights of 49.7 g, 51.1 g, and 51.3 g. Then immerse the root systems completely in the aqueous solution. After 2 minutes, take out the root systems and weigh them to get the weights of 58.9 g, 60.2 g, and 60.9 g, and calculate the equal moisture contents of 16%, 15%, and 16%;

[0106] Example 5

[0107] The test method for the friction performance of the negative Poisson's ratio artificial root system structure specifically includes the following steps:

[0108] Weigh an appropriate amount of soil sample, prepare the soil sample according to the natural moisture content, pour the prepared soil sample into the specimen box layer by layer and compact it to make the soil density 1.5 - 1.8 g•cm -3 . Horizontally bury the single root, and the burial depth is 5 cm;

[0109] Apply uniform vertical loads of 0 kPa, 10 kPa, 20 kPa, 50 kPa, and 100 kPa on the soil surface respectively;

[0110] Leave 20 mm of the root length outside the test piece box for the clamping part of the chuck, and conduct a horizontal pull-out test with a pulling speed set at 100 mm / min;

[0111] Conduct a pull-out friction test on the roots to obtain a friction force-displacement curve, and pull out at the maximum pull-out force, as Figure 8 ; The equivalent friction coefficient is defined as μ = σ v / τ (where σ v is the vertical load, τ = F / πdL R is the equivalent shear stress, F is the maximum pull-out force, π is the pi, d is the root diameter, and L R is the root length, and the equivalent friction coefficient is 0.2 - 0.5); The root-soil pull-out curve shows a non-linear form. The initial stage is the elastic deformation stage. As the pulling force increases, it gradually enters the plastic deformation stage, and finally the roots break away from the soil when the maximum pull-out force is reached; The pull-out failure mode can be simulated, and when the frictional resistance at the root-soil interface is large, the root pull-out fracture mode can also be simulated.

[0112] Example 6

[0113] The growth test method of the negative Poisson's ratio artificial root system structure specifically includes the following steps:

[0114] A. Compress the printed single artificial root into an aggregate, and shrink it to an approximate spherical structure with a diameter of 10 - 30 mm;

[0115] B. Immerse the aggregate in water at 50 - 80 °C, and the root shape slowly and automatically recovers, thereby simulating the growth of a single root;

[0116] C. Compress the printed complex artificial roots into an aggregate, and shrink it to an approximate spherical structure with a diameter of 20 - 60 mm;

[0117] D. Immerse the aggregate in water at 50 - 80 °C, and the root shape slowly and automatically recovers, thereby simulating the growth of complex roots.

[0118] The above is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention can easily think of changes or substitutions, which should all be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claimed rights.

Claims

1. A 4D additive manufacturing method for self-growing artificial roots based on negative Poisson's ratio materials, characterized in that: The artificial root system is constructed by repeating unit cell structures, and the method comprises the following steps: (1) Parametrically design the core unit cell structure and construct a three-dimensional negative Poisson's ratio core structure; (2) Parametrically design the epidermal unit cell structure to construct a three-dimensional negative Poisson's ratio tubular structure; (3) combining the core structure and the tubular structure into an integral structure, and 3D printing the integral structure into a single piece; (4) By adjusting the design parameters, multiple single roots are printed and assembled into a complex three-dimensional root system to simulate the growth of single roots and complex root systems; Among them, a three-dimensional negative Poisson's ratio material is used. The three-dimensional negative Poisson's ratio material is constructed by a repeated unit cell structure, in which a negative Poisson's ratio square column is used as the root core and a negative Poisson's ratio cylinder is used as the root epidermis. Under the action of external force, it can be compressed into a solid body of 20% to 40% of its original volume, and under the changes of water environment and thermal environment, it can grow into a long root system by itself, achieving a 4D effect. The core unit cell is a symmetrical hexagon, consisting of two inner and outer hexagons. The upper and lower sides of the outer hexagon are horizontal line segments, and the left and right sides are the hypotenuses of isosceles triangles. The inner hexagon has the same shape as the outer hexagon but is smaller in size, and a uniform annular frame area is formed between the two. Two short rectangular arms extend from both sides of the middle of the unit cell for connecting the unit cells. The core unit cell is used to simulate the root core. The epidermis is composed of a plurality of continuously arranged epidermal cells, the epidermis is a tubular porous structure, the shape of the epidermal cells is the same as that of the core cells, the epidermal cells are connected to each other through adjacent edges, so that adjacent epidermal cells share edges to form a tightly arranged tubular whole, which is used to simulate the root epidermis; The root core and tubular epidermis form a single integral root system through shared contact and are used for 3D printing.

2. The 4D additive manufacturing method for self-growing artificial root system based on negative Poisson's ratio material according to claim 1, characterized in that: The method also includes: testing the expansion, compression, elastic modulus, tensile strength, water absorption and water holding capacity, and surface friction coefficient of a single root, wherein the manufactured root system has physical and mechanical properties similar to those of a real root system, including tensile properties, bending properties, water absorption and water holding properties, and root surface friction characteristics.

3. The 4D additive manufacturing method for self-growing artificial root system based on negative Poisson's ratio material according to claim 1, characterized in that: The 3D printing material used is thermoplastic polyurethane (TPU).

4. The 4D additive manufacturing method for self-growing artificial root system based on negative Poisson's ratio material according to claim 1, characterized in that: The aspect ratio of the core unit cell structure is defined as l / w, and the range of l / w is 0.1-0.

5. The thickness-to-width ratio of the core unit cell structure is defined as t / w, and the range of t / w is 0.1-0.

5. The core unit cell has a negative Poisson's ratio property in both longitudinal and transverse uniaxial stretching and compression, which is used to simulate the root core; And / or, the aspect ratio of the epidermal unit cell structure is defined as T / L, the range of T / L is 0.1 to 0.5, and the hollow area of ​​the epidermal unit cell structure is defined as A t = Total area of ​​A – A s The ratio of the solid area to the hollow area of ​​the epidermal unit cell structure is defined as A t / A,A t / A varies in the range of 0.1 to 0.5 and is used to simulate the root epidermis.

5. The 4D additive manufacturing method for self-growing artificial root system based on negative Poisson's ratio material according to claim 1, characterized in that: The root system has a root length of at least one unit cell, and a single root diameter of 2-200 mm, and can simulate the root system of tree plants, wherein the tree plants are pine, cypress and locust.

6. The 4D additive manufacturing method for self-growing artificial root system based on negative Poisson's ratio material according to claim 1, characterized in that: It is only necessary to design a core unit cell structure and an epidermal unit cell structure, and then arrange them periodically and repeatedly in space to generate a single root structure, so that it has three-dimensional tensile and compressive properties. This property enables it to have an expansion effect under changes in water or thermal environment, thereby simulating the growth effect of the root system.

7. The 4D additive manufacturing method for self-growing artificial root system based on negative Poisson's ratio material according to claim 2, characterized in that: The mechanical properties of the artificial root system are as follows: density 1.1•10 3 -1.2•10 3 kg / m 3 , tensile elastic modulus is 10~25MPa, tensile strength is 30-60MPa, bending elastic modulus is 10~25MPa; And / or, the artificial root system has water absorption and water holding properties, the water absorption property is generated by the siphon effect of the porous structure, the lower limit of the water absorption height is 5 cm, and the upper limit is 30 cm, and the water holding property is controlled by the water tension membrane of the porous structure, and the water content is 5-25%; And / or, the surface of the artificial root system has friction characteristics similar to those of a real root system, with an equivalent friction coefficient of 0.2~0.5; simulating the pull-out failure mode, the root-soil pull-out curve presents a nonlinear form, the initial stage is the elastic deformation stage, and as the pulling force increases, it gradually enters the plastic deformation stage, and finally the root system is separated from the soil when the maximum pull-out force is reached; when the friction resistance at the root-soil interface is large, the root pull-out fracture mode is simulated.

8. The 4D additive manufacturing method for self-growing artificial root system based on negative Poisson's ratio material according to claim 1, characterized in that: The root system of complex shape is composed of the single roots, and the root system of complex shape is composed of multiple single roots at different angles, diameters and levels, specifically including: (a) Root system perspective: - Vertical roots refer to the main roots that grow vertically downwards, with the angle between the root growth direction and the horizontal plane being 60 to 90 degrees; - Horizontal roots refer to horizontal or nearly horizontal roots growing from the main root or other lateral roots, and the angle between the root growth direction and the horizontal plane is 0 to 30 degrees; - Oblique roots refer to oblique roots between vertical roots and horizontal roots, with the root growth direction and the angle between the horizontal plane and 30 to 60 degrees; (b) Root diameter: - primary taproot diameter, the thickest root measured at the root collar; - Secondary lateral root diameter, which refers to the diameter of the lateral roots growing directly from the taproot; - Tertiary lateral root diameter, which refers to the diameter of the root growing from the secondary lateral root; (c) Root level: - Primary root, also known as taproot; - Secondary roots, lateral roots branching from the primary roots; - Tertiary roots, lateral roots branching off from secondary roots.

9. The 4D additive manufacturing method for self-growing artificial root system based on negative Poisson's ratio material according to claim 1, characterized in that: The complex root system is composed of the single roots. Both the single roots and the complex root system are formed into aggregates by compression at room temperature. The aggregates are placed in 50°C ~ 80°C water to grow into the original form, thereby simulating the root growth process.

10. A self-growing artificial root system based on a negative Poisson's ratio material manufactured by the method according to any one of claims 1 to 9.

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