Self-growing artificial root system 4D additive manufacturing method based on negative Poisson's ratio material

Abstract

The invention provides a self-growing artificial root system 4D additive manufacturing method based on a negative Poisson's ratio material, and relates to the technical field of metamaterial biological structure construction and manufacturing. The artificial root system is constructed by a repeated unit cell structure, and the method comprises the following steps: (1) parametrically designing a core unit cell structure so as to construct a three-dimensional negative Poisson's ratio core structure; (2) parametrically designing an epidermal unit cell structure, thereby constructing a three-dimensional negative Poisson's ratio tubular structure; (3) the core body structure and the tubular structure are combined into an integral structure, and the integral structure is subjected to 3D printing to form single roots; and (4) printing a plurality of root systems and assembling the root systems into a complex three-dimensional root system by adjusting design parameters, and simulating the growth of a single root and the complex root system. A structure with good and comprehensive real root system performance can be manufactured, mechanical, water absorption, water retention and friction characteristics of the root system can be fully simulated, and the device has a wide application prospect in the field of plant root system and ecological restoration research.

Description

Technical Field

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

[0002] In the implementation of forest root soil consolidation technology to deal with the large number of exposed slopes caused by engineering construction, the root morphology directly affects the soil consolidation and slope protection effect. Since the root system of vegetation is complex and deeply buried underground, it is difficult to obtain the root system and study the root morphology. Obtaining a root model through 3D printing is a solution. However, the common methods for establishing root models at home and abroad are still in the aspects of static graphics drawing and model making, which is difficult to reflect the dynamic growth of plants and comprehensively simulate the mechanics, water absorption and water retention of the root system, and the friction performance of the root surface. Summary of the invention

[0003] In order to solve the technical problems existing in the prior art, the 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 a self-growing artificial root system based on a negative Poisson's ratio material, wherein the artificial root system is constructed by a repeated unit cell structure, and the method comprises the following steps:

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

[0006] (2) Parametrically design the epidermal unit cell structure to construct a three-dimensional negative Poisson's ratio tubular structure;

[0007] (3) combining the core structure and the tubular structure into an integral structure, and 3D printing the integral structure to form a single root;

[0008] (4) By adjusting the design parameters, multiple root systems are printed and assembled 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 a repeated unit cell structure, in which 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 external force, it can be compressed into a solid body of 20% to 40% of its original volume, and under the changes in water and thermal environments, it can self-grow into a long root system to achieve a 4D effect.

[0010] Optionally, the method further comprises: 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.

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

[0012] Optionally, 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, the left and right sides are the hypotenuses of an isosceles triangle, 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, and two short rectangular arms extend from both sides of the middle of the unit cell for connecting the unit cells to simulate the root core;

[0013] And / or, the epidermis is a tubular porous structure, composed of a plurality of continuously arranged unit cells, the shape of the epidermis unit cells is the same as the shape of the core unit cells, and the unit cells are connected to each other through adjacent edges, so that adjacent unit cells share edges to form a tightly arranged tubular whole, which is used to simulate the root epidermis;

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

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

[0016] Optionally, only one core unit cell and one epidermis unit cell need to be designed, and then they are arranged periodically and repeatedly in space to generate a single root structure, so that it has three-dimensional tensile and compressive properties, which can enable it to have an expansion effect under changes in water or thermal environment, thereby simulating the growth effect of the root system.

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

[0018] And / or, negative Poisson's ratio cylinders are used to simulate the root epidermis, which is used to simulate the surface friction characteristics of the root system.

[0019] Optionally, the mechanical properties of the artificial root system are: 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;

[0020] 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%;

[0021] 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; the root-soil pull-out curve presents a nonlinear form, with the initial stage being the elastic deformation stage, and gradually entering the plastic deformation stage as the pulling force increases, and finally the root system is separated from the soil when the maximum pull-out force is reached; the pull-out failure mode can be simulated, and when the friction resistance at the root-soil interface is large, the root pull-out fracture mode can also be simulated.

[0022] Optionally, a root system of complex morphology is composed of the single roots, and the root system is composed of multiple single roots at different angles, diameters and levels, specifically including:

[0023] (a) Root system perspective:

[0024] - 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;

[0025] - 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;

[0026] - 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;

[0027] (b) Root diameter:

[0028] - primary taproot diameter, the thickest root measured at the root collar;

[0029] - Secondary lateral root diameter, which refers to the diameter of the lateral roots growing directly from the taproot;

[0030] - Tertiary lateral root diameter, which refers to the diameter of the root growing from the secondary lateral root;

[0031] (c) Root level:

[0032] - Primary root, also known as taproot;

[0033] - Secondary roots, lateral roots branching from the primary roots;

[0034] - Tertiary roots, lateral roots branching off from secondary roots.

[0035] Optionally, the complex root system is composed of the single roots, and both the single root system and the complex root system can form aggregates by compression at room temperature. The aggregates are placed in 50℃~80℃ water and can grow into their original form, thereby simulating the root growth process.

[0036] A self-growing artificial root system based on a negative Poisson's ratio material manufactured by the method.

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

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

[0039] Among them, the porous 4D printing artificial root manufacturing technology based on the principle of negative Poisson's ratio materials can not only reflect the complex growth conditions of plant roots, but also simulate the mechanical properties, water absorption and water retention capabilities, and root-soil interface friction of the roots under the influence of various environmental factors. It can effectively detect the application effects of different plant roots in slope protection and has broad application prospects.

[0040] The present invention can manufacture structures with good and comprehensive real root performance, which not only breaks through the limitation of traditional root model reconstruction methods that are only static models, but also can fully simulate the mechanics, water absorption and water retention, and friction characteristics of the root system, and has broad application prospects in the field of plant root system and ecological restoration research. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0042] Figure 1 is a geometric parameter definition diagram of a single cell structure in a single artificial root structure of the present invention; wherein, Figure 1 (a) is a schematic diagram of the unit cell length parameter. Figure 1 (b) is a schematic diagram of the unit cell area parameter;

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

[0044] Figure 3 A flow chart for constructing the epidermis of a single artificial root structure of the present invention;

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

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

[0047] Figure 6 It is a tensile stress-strain curve diagram of a single artificial root system of the present invention;

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

[0049] Figure 8 A stress-strain curve diagram of a single artificial root system according to the present invention;

[0050] Fig. 9 This is a physical picture of a single artificial root core structure of the present invention;

[0051] Fig.10 This is a physical picture of the epidermal structure of a single artificial root system of the present invention;

[0052] Fig.11 This is a physical picture of the overall structure of a single artificial root system of the present invention. DETAILED DESCRIPTION

[0053] The technical solution of the present invention is described below in conjunction with the accompanying drawings.

[0054] The following describes the embodiments of the present invention through specific examples, and those skilled in the art can easily understand other advantages and benefits of the present invention from the contents disclosed in this specification. The present invention can be applied or implemented through other different embodiments, and the details in the specification can be modified or adjusted according to different application perspectives and requirements without deviating from the core spirit of the present invention. It should be noted that the illustrations provided in the following embodiments only illustrate the basic concept of the present invention in a schematic manner, and the following embodiments and the features in the embodiments can be combined with each other without conflict.

[0055] Among them, the drawings are only used for illustrative explanations, and they only represent schematic diagrams rather than actual product drawings, and should not be understood as limitations on the present invention. In order to better illustrate the embodiments of the present invention, some parts of the drawings may be omitted, enlarged or reduced, and do 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 drawings may be omitted.

[0056] The same or similar numbers in the drawings of the embodiments of the present invention correspond to the same or similar parts; in the description of the present invention, it should be understood that if the terms "upper", "lower", "left", "right", "front", "back" and the like indicate directions or positional relationships, they are based on the directions or positional relationships shown in the drawings, which 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 direction, be constructed and operated in a specific direction. Therefore, the terms describing the positional relationship in the drawings are only used for illustrative purposes and should not be regarded as limitations on the present invention. Technicians can understand these terms based on actual conditions.

[0057] Negative Poisson's Ratio Materials (NPR) are materials that exhibit deformation characteristics opposite to those of conventional materials in the lateral direction when subjected to external forces. Such materials expand in the lateral direction when stretched in one direction and contract in the lateral direction when compressed in one direction, so they are also called "Lattice Materials". Poisson's ratio describes the ratio of the lateral 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 in the lateral direction when stretched and contracts in the lateral direction when compressed. This characteristic gives negative Poisson's ratio materials significant advantages in mechanical properties, such as improved shear resistance and energy absorption capacity. The preparation methods of negative Poisson's ratio materials mainly include porous structures and fiber-filled composite materials. Porous materials such as two-dimensional honeycomb solid materials with concave pore structures can achieve negative Poisson's ratio effects by designing macroscopic structures. Fiber-filled composite materials achieve negative Poisson's ratio effects by controlling the order of stacking layers of different scales.

[0058] A 4D additive manufacturing method for self-growing artificial roots based on negative Poisson's ratio materials uses a three-dimensional negative Poisson's ratio material, which is constructed of a repeated unit cell structure, in which negative Poisson's ratio square columns serve as the root core and negative Poisson's ratio cylinders serve as the root epidermis. Under the action of external forces, they can be compressed into aggregates with a volume of 20% to 40%. When the water environment and thermal environment change, the roots can self-grow, thereby achieving a 4D effect.

[0059] As a 4D additive manufacturing method for artificial roots, the manufactured roots have mechanical properties similar to those of real roots, including tensile properties, bending resistance, water absorption and water retention properties, as well as root surface friction characteristics.

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

[0061] As a 4D additive manufacturing method for artificial roots, 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 an isosceles triangle. The inner hexagon has the same shape as the outer hexagon but is smaller in size, and a uniform annular border area is formed between the two. The aspect ratio of the core unit cell structure is defined as l / w, and the range of l / w is 0.1 to 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 to 0.5. The core unit cell has a negative Poisson's ratio property under longitudinal and transverse uniaxial tension and compression, and is used to simulate the root core.

[0062] As a 4D additive manufacturing method for artificial roots, the root 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, so that adjacent unit cells share edges to form a tightly arranged whole. The aspect ratio of the epidermis unit cell structure is defined as T / L, and the range of T / L is 0.1 to 0.5. As T / L increases, the yield stress of the root system decreases under the same strain. The hollow area of ​​the epidermis unit cell structure is defined as A t =A(total area)-A s (solid area), the hollow area ratio of the epidermal unit cell structure is defined as A t / A,A t The range of / A is 0.1~0.5, and it varies with A t As the / A value increases, the friction coefficient of the root epidermis decreases, which is used to simulate the root epidermis.

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

[0064] As a 4D additive manufacturing method for artificial roots, negative Poisson's ratio square columns are used to simulate the root core, mainly to simulate the tensile and bending mechanical properties of the roots.

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

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

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

[0068] As a 4D additive manufacturing method for artificial roots, the surface of the artificial root system has similar friction characteristics to real roots, with an equivalent friction coefficient of 0.2-0.5; the root-soil pull-out curve presents a nonlinear form, with the initial stage being the elastic deformation stage, and gradually entering the plastic deformation stage as the pulling force increases, and finally the root system is separated from the soil when the maximum pull-out force is reached; the pull-out failure mode can be simulated, and when the friction resistance at the root-soil interface is large, the root pull-out fracture mode can also be simulated. A single root is placed horizontally on a surface with a density of 1.5-1.7 g•cm -3 The soil was buried at a depth of 5 cm. Uniform vertical loads of 0 kPa, 10 kPa, 20 kPa, 50 kPa, and 100 kPa were applied to the soil surface, and horizontal pull-out tests were carried out. The equivalent friction coefficient was 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 tensile force, π is the circumference, d is the root diameter, L R is the root length);

[0069] As a 4D additive manufacturing method for artificial root systems, the root system consists of multiple single roots at different angles, diameters and levels, including:

[0070] (a) Root system perspective:

[0071] - 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;

[0072] - 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;

[0073] - 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;

[0074] (b) Root diameter:

[0075] - primary taproot diameter, the thickest root measured at the root collar;

[0076] - Secondary lateral root diameter, which refers to the diameter of the lateral roots growing directly from the taproot;

[0077] - Tertiary lateral root diameter, which refers to the diameter of the root growing from the secondary lateral root;

[0078] (c) Root level:

[0079] - Primary root, also known as taproot;

[0080] - Secondary roots, lateral roots branching from the primary roots;

[0081] - Tertiary roots, lateral roots branching off from secondary roots;

[0082] As a 4D additive manufacturing method for artificial roots, it only requires designing a core unit cell and an epidermal unit cell, and then periodically arranging them in space to generate a single root structure, which has tensile and compressive properties. Both single roots and complex roots can form aggregates at room temperature through compression, and can be restored to their original form in 50-80℃ water, thereby simulating the root growth process.

[0083] like Figure 5 A method for constructing an artificial root structure with a negative Poisson's ratio is shown, wherein the negative Poisson's ratio structure is constructed as a whole by repeating basic cell structures.

[0084] Example 1

[0085] The method for constructing a negative Poisson's ratio artificial root structure specifically comprises the following steps:

[0086] A. Construct a core unit cell structure model (modeling is performed using Fusion software). 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 an isosceles triangle. The inner hexagon is the same shape as the outer hexagon but 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 to connect the unit cells. The length of the basic unit cell structure is 1620 (the following lengths are all proportional lengths, without units, and can be scaled proportionally), the width is 1180, and the aspect ratio is 1.373:1; the width of the basic unit cell structure is 1180, the thickness is 200, and the aspect ratio is 5.9:1; the basic unit cell structure has a negative Poisson's ratio property under longitudinal and transverse uniaxial compression, such as Figure 1 As shown; a unit cell structure is provided, which includes a hollow part and a rectangular parallelepiped structure on the upper and lower sides; the unit cell structure constructed by standard software is periodically arranged horizontally and vertically in the space along the hollow surface direction to form an approximately cubic structure with a common vertex and thickness, with an overall length of 82 mm, an overall width of 20 mm, and an overall height of 20 mm, as shown Figure 2 As shown; these cubic structures are arranged periodically to form an artificial root core; wherein the periodic horizontal and vertical arrangement is as follows: the rectangular parts on the upper and lower sides are removed from the unit cell structure, and the remaining structure in the middle is retained, the two remaining structures are rotated 90° respectively, and placed on the upper side of the unit cell in a mirrored manner, the mirrored part is rotated 180° and moved, and placed on one end of the upper side of the unit cell, and then the rotated and mirrored structure is arrayed, and multiple copies are made 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 the structure can present a periodic arrangement, such as Figure 2 As shown; the actual core structure diagram can be found in Fig. 9 ;

[0087] B. Construct the epidermal cell structure model (modeling is performed using Fusion software). The epidermis is a tubular porous structure composed of multiple continuously arranged unit cells. Each unit cell is connected to each other through adjacent edges, so that adjacent unit cells share edges to form a tightly arranged whole. 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. As T / L increases, the yield stress of the root system decreases under the same strain. The hollow area ratio of the epidermal unit cell structure is defined as A t / A,A t The range of / A is 0.1~0.5, and it varies with A t As the value of / A increases, the friction coefficient of the root surface decreases. Figure 3 As shown; for the actual picture of the epidermal structure, see Fig.10 ;

[0088] C. Assemble and construct a three-dimensional negative Poisson's ratio tubular structure. The core structure and the tubular structure are integrated into an overall structure. The actual picture of the overall structure is as follows Fig.11 As shown, the 3D printed overall structure forms a single root, such as Figure 4 As shown;

[0089] D. Modify the parameters of individual root systems, and bond individual root systems with different parameters into an overall structure, and 3D print the overall structure to form a complex root, such as Figure 5 As shown;

[0090] D. Thermoplastic polyurethane (TPU) filaments are selected as the raw materials for 3D printing; the 3D printer uses a fused deposition modeling 3D printer, inputs a suitable file format into the printing software, sets the printing process according to the viscosity and flow characteristics of different printing materials, and does not need to add any support materials during the printing process; slices the model to form a printing path, builds the model, and uploads the model to the 3D printer; due to the characteristics of TPU materials, in actual printing, most of the support type is used, but due to the small surface gaps of the artificial root model, it is easy for the support material to remain in the pores, affecting the shape and mechanical properties of the model. Through support-free printing, the model accuracy can be effectively improved, and the molding of the three-dimensional negative Poisson's ratio artificial root structure can be better completed.

[0091]

[0092] Example 2

[0093] The tensile expansion and compression test method of the artificial root structure with negative Poisson's ratio specifically comprises the following steps;

[0094] A universal testing machine (WANCE ETM-104B, China) was used with a tensile speed of 5 mm / min. A single core specimen of TPU material with a length of 82 mm, a width of 20 mm, and a height of 20 mm was selected for tensile testing. The stress-strain curve is shown in Figure 6 As shown, the experimental results are in good agreement with the finite element results.

[0095] ABAQUS is used to simulate the root tensile test of the root system. The displacement load is set at the reference point of the upper pressure head, and the stress-strain numerical curve is generated. The results are shown in Figure 6 As shown, the experimental results are in good agreement with the finite element results.

[0096] Example 3

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

[0098] A universal testing machine (WANCE ETM-104B, China) was used to select a single core specimen of TPU material with a length of 82 mm, a width of 20 mm, and a height of 20 mm for a three-point bending test. The stress-strain curve is shown in Figure 7 Middle (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 specimen to simulate the pressure head and two supports in the test. The results are shown in Figure 2. Figure 7 As shown in (b), the displacement load is set at the reference point in the figure, the density is 1.1g / cm, and the stress-strain curve is as follows Figure 7 As shown in (a).

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

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

[0102] Example 4

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

[0104] A. Carry out a water absorption test on the root system, insert a dry single root vertically into the water, and obtain the siphon height under different size design parameters, with a range of 5cm~30cm;

[0105] B. Take a group of artificial roots and measure their weights to 49.7g, 51.1g, and 51.3g. Then immerse the roots completely in the aqueous solution. After 2 minutes, take out the roots and weigh them to obtain weights of 58.9g, 60.2g, and 60.9g. Calculate the water contents to be 16%, 15%, and 16%;

[0106] Example 5

[0107] The friction performance test method of the artificial root structure with negative Poisson's ratio 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 test box in layers and compact it to make the soil density between 1.5 and 1.8 g•cm -3 . Bury a single root horizontally at a depth of 5 cm;

[0109] Uniform vertical loads of 0 kPa, 10 kPa, 20 kPa, 50 kPa, and 100 kPa were applied to the soil surface;

[0110] The root length outside the specimen box was left at 20 mm for the clamping part of the chuck, and the horizontal pull-out test was carried out, and the tensile speed was set to 100 mm / min;

[0111] The friction force displacement curve can be obtained by performing a pull-out friction test on the root system. 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 circumference, d is the root diameter, L R is the root length, and the equivalent friction coefficient is 0.2~0.5); the root-soil pull-out curve presents a nonlinear shape. The initial stage is the elastic deformation stage. As the pulling force increases, it gradually enters the plastic deformation stage. Finally, the root is separated from the soil when the maximum pull-out force is reached. The pull-out failure mode can be simulated. When the friction 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 artificial root structure with negative Poisson's ratio specifically comprises the following steps:

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

[0115] B. Immerse the aggregates in 50-80℃ water, and the root shape will slowly and automatically recover, thus simulating the growth of a single root system;

[0116] C. Compress the printed complex artificial root system into an aggregate, shrinking it to a nearly spherical structure with a diameter of 20 to 60 mm;

[0117] D. Immerse the aggregates in 50-80℃ water, and the root shape will slowly and automatically recover, thereby simulating the growth of complex root systems.

[0118] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art can easily think of changes or substitutions within the technical scope disclosed by the present invention, which should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

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 to form a single root; (4) By adjusting the design parameters, multiple root systems 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 serves as the root core and a negative Poisson's ratio cylinder serves 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 in water and thermal environments, it can self-grow into a long root system to achieve a 4D effect.

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 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, the left and right sides are the hypotenuses of isosceles triangles, the inner hexagon is the same shape as the outer hexagon but 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 connection between the unit cells, and the core unit cell is used to simulate the root core; And / or, the epidermis is a tubular porous structure, composed of a plurality of continuously arranged unit cells, the shape of the epidermis unit cells is the same as the shape of the core unit cells, and the unit cells are connected to each other through adjacent edges, so that adjacent unit cells share edges to form a tightly arranged tubular whole, which is used to simulate the root epidermis; And / or, the root core and the tubular epidermis are formed into a single integral root system through shared contact and used for 3D printing.

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 length is at least one unit cell length, and the diameter of a single root is 2-200mm, which can simulate the root system of arbor plants such as 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; the root-soil pull-out curve presents a nonlinear form, with the initial stage being the elastic deformation stage, and gradually entering the plastic deformation stage as the pulling force increases, and finally the root system is separated from the soil when the maximum pull-out force is reached; the pull-out failure mode can be simulated, and when the friction resistance at the root-soil interface is large, the root pull-out fracture mode can also be 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 described in claim 1, and the root system 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 described in claim 1 or claim 2. Both the single root system and the complex root system can form aggregates by compression at room temperature. The aggregates are placed in 50°C ~ 80°C water and can grow into their 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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