Method for preparing a three-dimensional structured hydrogel and use thereof

Abstract

The application discloses a preparation method and application of a three-dimensional structure hydrogel. The application has the advantages of low manufacturing cost, wide applicable materials, customizable complex three-dimensional structure, high shape fidelity and excellent mechanical properties. Compared with the existing preparation methods of hydrogel soft structure and device, the preparation method comprises three steps of low-temperature printing, low-temperature crosslinking and solvent post-treatment. The obtained hydrogel can contain multiple material components with different crosslinking mechanisms, and does not need specific photopolymerization reaction groups and rheological property adjustors. The obtained hydrogel has a self-supporting three-dimensional structure including overhanging, thin-walled, hollow and high aspect ratio features. The prepared hydrogel structure can withstand repeated bending, stretching and extrusion without breaking, and can still recover to the initial shape after the external force is removed, facilitating the assembly integration and function realization of soft devices and soft robots.

Description

Technical Field

[0001] The present invention relates to the field of hydrogel preparation, in particular to a preparation method and application of a hydrogel with a three-dimensional structure, and especially to a preparation method of a hydrogel with multiple material components and a three-dimensional structure. Background Art

[0002] In recent years, hydrogels, due to their high water content and unique advantages such as softness, low cost, and biosafety, have shown great potential in fields such as biomedical engineering, wearable devices, and underwater exploration. Soft devices and soft robots based on hydrogels are currently a hot topic in academic research, promising to bring rich functionality and novel capabilities to scenarios such as biomimetic design, surgical intervention, and artificial organs.

[0003] Due to the low density of hydrogels' inherent polymer networks and weak interfacial bonding, traditional hydrogels are typically integrated into devices using flat or thin film patterns, making complex devices and functions infeasible. Developed 3D hydrogel fabrication technologies generally target a single material or a single material system with the same forming mechanism, requiring individual material processing properties to be tailored, resulting in a narrow range of applications and limited versatility.

[0004] Therefore, it is necessary to develop a universal preparation method for hydrogels with multiple material components and three-dimensional structures. Summary of the Invention

[0005] The present invention addresses the challenges of the prior art by providing a method for preparing and applying a three-dimensional hydrogel. This method provides a simple, feasible, and customizable method for preparing hydrogels with a three-dimensional structure and multiple material components, enabling the integrated manufacturing of hydrogel soft devices and soft robots.

[0006] This traditional 3D printing method based on solute doping (specific photo- or thermal-polymerization reaction groups or rheological property modifiers) requires customized adjustment of the ink one by one, which is time-consuming and labor-intensive, and will result in the cross-linking method being specific, making it difficult to integrate and simultaneously cross-link heterogeneous structures. At the same time, this solute doping will produce a component competition effect (i.e., the mixture mainly exhibits the properties of its high-content component), which will weaken the properties of the polymer network (stretchability, conductivity, etc.). The present invention does not require the introduction of specific photo- or thermal-polymerization reaction groups or doping rheological property modifiers.

[0007] The purpose of the present invention can be achieved by the following solutions:

[0008] The technical solutions adopted by the present invention to solve the above technical problems are as follows:

[0009] The present invention provides a method for preparing a three-dimensional hydrogel, comprising the following steps:

[0010] S1. Low temperature printing

[0011] The hydrogel precursor dispersion is deposited and solidified as needed in a freezing space to form a crystalline structure;

[0012] S2, low temperature cross-linking

[0013] The crystalline structure is immersed in a cross-linking bath and cross-linked at low temperature to obtain a polymer structure, that is, a three-dimensional hydrogel.

[0014] In the first step of the present invention, the solidification phase transition of the hydrogel precursor dispersion from liquid to solid in a low-temperature environment is used to freeze and fix the shape. Combined with 3D printing, the deposited pattern can be programmed to achieve the formation of the desired crystalline structure. In the second step, the antifreeze and other ingredients contained in the cross-linking bath diffuse into the crystalline structure immersed therein, causing the melting point of the crystalline structure to change. The slow melting of the ice crystals inside the crystalline structure in a low-temperature environment provides a liquid environment (solvent) for the cross-linking reaction. Low-temperature cross-linking can control the reaction kinetics and is applicable to hydrogel precursors of different compositions to achieve the transformation from crystalline structure to hydrogel structure.

[0015] As an embodiment of the present invention, in step S1, the hydrogel precursor dispersions of different compositions include dispersions of different hydrogel precursor materials and / or dispersions of different hydrogel precursor contents.

[0016] As an embodiment of the present invention, in step S1, the hydrogel precursor includes one or more of polyvinyl alcohol (PVA), sodium alginate (SA), poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonic acid) (PEDOT:PSS), chitosan, hyaluronic acid, and poly(N-isopropylacrylamide).

[0017] The present invention is mainly a cross-linking reaction, from the precursor (polymer chain) to the cross-linked network, and does not involve monomer polymerization. In the prior art, taking polyvinyl alcohol materials as an example, the precursor dispersion of this type of material has strong fluidity, does not have thixotropic properties, and cannot undergo sol-gel transition. Conventional molding methods rely on mold-assisted molding, and cross-linking is achieved by freeze-thaw method or by adding low-concentration cross-linking agents to the precursor dispersion in advance. It is impossible to directly 3D print the three-dimensional structure of polyvinyl alcohol hydrogel. Taking sodium alginate materials as an example, although this type of material has sol-gel transition characteristics, its cross-linking mechanism mostly depends on calcium ion complexation, which has the limitations of fast cross-linking reaction rate and difficult regulation. The sodium alginate printed structure is directly immersed in the calcium ion cross-linking bath. The external cross-linked area restricts the penetration of calcium ions into the interior of the structure, resulting in uneven structural cross-linking and poor mechanical properties. Taking the heterogeneous structure composed of polyvinyl alcohol and sodium alginate as an example, the cross-linking mechanisms of the two materials are chemical cross-linking and physical cross-linking, respectively. It is difficult for a single cross-linking bath component to achieve integrated synchronous cross-linking of the heterogeneous structure.

[0018] As an embodiment of the present invention, in step S1, the solid mass fraction of the hydrogel precursor dispersion is 0.5%-15%, wherein polyvinyl alcohol is preferably 5-15%, sodium alginate is preferably 0.5-12%, and chitosan is preferably 2-6%.

[0019] The method of the present invention can be used for hydrogel precursor dispersions with low mass fractions. When the viscosity is low, it cannot be used for conventional 3D printing. For example, 2% sodium alginate has a viscosity of only 1 Pa·s, which is lower than the viscosity required for conventional 3D printing and cannot be directly printed.

[0020] Hydrogel precursor dispersions of different concentrations have different rheological properties (mainly manifested as different viscosities, about 1-10000 Pa s). The lower the solid content, the lower the viscosity, and the increased fluidity of the dispersion. This type of dispersion requires rapid solidification after deposition to prevent line breakage and shape distortion caused by surface tension during processing. The higher the solid content, the denser the polymer network, requiring a proper reduction in the cross-linking reaction rate to ensure uniform cross-linking inside and outside. The method of freeze-drying and cross-linking after freeze printing is usually difficult to achieve low solid content and complex overhanging structures, because such structures are prone to collapse due to ice crystal sublimation during the freeze-drying process, resulting in structural distortion. The present invention immerses the material in a cross-linking bath, and the adjustable cross-linking reaction can regulate the material modulus to achieve structural self-support and avoid collapse caused by freeze-drying. The synchronous crystal melting and cross-linking reaction, as well as the buoyancy support of the cross-linking bath (the ice crystal sacrificial process provides buoyancy support) can ensure the fidelity of the structural shape. In addition, the freeze-drying method has low process adjustability, resulting in a single mechanical property of the material. The present invention can adjust various process parameters such as cross-linking temperature, cross-linking agent content, cross-linking time, catalyst content, etc., thereby achieving wide-range adjustment of mechanical properties (Young's modulus).

[0021] As an embodiment of the present invention, in step S1, the deposition and solidification method is 3D printing; 3D printing includes one or more of ink direct writing, inkjet printing, electrofluidic printing, and meniscus guided printing.

[0022] As an embodiment of the present invention, in step S1, the layout of the freezing space is one of a flat plate, a special-shaped curved platform, a cavity and a rotating shaft.

[0023] As one embodiment of the present invention, in step S1, the solvent in the hydrogel precursor dispersion includes one or more of water, ethanol, and dimethyl sulfoxide. The solvent is preferably a combination of water and other solvents. During the deposition and solidification process, the solvent can form crystals or achieve crystallization-like solidification by further lowering the temperature.

[0024] In one embodiment of the present invention, in step S1, the deposition and solidification temperature is below the crystallization temperature of the hydrogel precursor dispersion. This temperature is typically between -5°C and -30°C. If the subsequent crosslinking step is not performed after low-temperature printing, the hydrogel structure will melt and collapse upon exposure to room temperature.

[0025] As an embodiment of the present invention, in step S1, during the deposition and solidification, hydrogel precursor dispersions with different compositions are deposited in sequence, or hydrogel precursor dispersions with different compositions are mixed and then deposited.

[0026] As one embodiment of the present invention, in step S2, the crosslinking bath is an aqueous solution comprising a crosslinking agent and an antifreeze agent. The crosslinking agent comprises one of glutaraldehyde and a calcium-containing compound. When the crosslinking agent is glutaraldehyde, the antifreeze agent is ethanol and / or a compound containing calcium ions. When the crosslinking agent is a calcium-containing compound, the antifreeze agent is ethanol.

[0027] The calcium ion compound includes one or more of calcium chloride, calcium hydrogen phosphate, and calcium carbonate. In the crosslinking bath, the molar concentration of glutaraldehyde is 0.0025-0.1 mol / L; the molar concentration of dilute hydrochloric acid is 0.01-0.5 mol / L; the mass fraction of calcium chloride is 0.5-10%; and the volume fraction of ethanol is 10-60%.

[0028] The hydrogel precursor includes polyvinyl alcohol, sodium alginate, chitosan, hyaluronic acid, and poly(N-isopropylacrylamide). Crosslinking is performed by a crosslinking agent or by light irradiation. The crosslinking agent and antifreeze agent used are preferably the following combination:

[0029] The hydrogel precursor is polyvinyl alcohol, the cross-linking agent is glutaraldehyde, and the antifreeze agent is a calcium ion compound and / or ethanol.

[0030] The hydrogel precursor is sodium alginate, the cross-linking agent is a calcium ion-containing compound, and the antifreeze agent is ethanol.

[0031] The hydrogel precursor is chitosan, the cross-linking agent is glutaraldehyde and / or a compound containing calcium ions, and the antifreeze agent is ethanol.

[0032] The hydrogel precursor is hyaluronic acid, the cross-linking agent is a calcium ion-containing compound, and the antifreeze agent is ethanol.

[0033] The hydrogel precursor is poly (N-isopropylacrylamide), and cross-linking is initiated by external irradiation light after immersion in an antifreeze bath; the antifreeze agent is a calcium ion compound or ethanol.

[0034] The crosslinker physically or chemically crosslinks the hydrogel precursor, while the antifreeze lowers the freezing point of the crosslinking bath, allowing the submerged frozen structure to crosslink at low temperatures. During the low-temperature crosslinking process, the crosslinker and antifreeze simultaneously diffuse into the pre-melted layer on the surface of the ice crystals within the crystalline structure, allowing the crystalline structure to melt and undergo crosslinking simultaneously.

[0035] As one embodiment of the present invention, in step S2, the low-temperature crosslinking temperature is higher than the crystallization temperature of the crosslinking bath used and lower than the melting temperature of the crystalline structure. The low-temperature crosslinking time is 10 minutes to 72 hours.

[0036] As an embodiment of the present invention, in step S2, the polymer structure is further subjected to solvent post-treatment, which comprises immersing the polymer structure in a treatment bath (at room temperature) to obtain a hydrogel (having multiple material components and a three-dimensional structure).

[0037] Solvent post-treatment modifies the resulting hydrogel structure. This involves manipulating the hydrogel's aggregated state through the addition of salt ions or an acidic environment in the treatment bath, enhancing its mechanical, electrical, and other material properties. This step does not directly affect the hydrogel's formation; it primarily enhances its performance.

[0038] As an embodiment of the present invention, the post-treatment bath comprises one of deionized water, an acid solution, and a salt solution. The immersion time of the polymer structure is 1-72 hours.

[0039] Acid solutions include dilute sulfuric acid, dilute hydrochloric acid, and acetic acid; salts include sodium citrate, sodium chloride, ferric chloride, sodium sulfate, ammonium sulfate, and ammonium dihydrogen phosphate.

[0040] The present invention also provides a device and application of the hydrogel prepared by the preparation method as a soft device and a soft robot.

[0041] The hydrogel obtained by the present invention is customized into a three-dimensional structure from a variety of material components as needed. The overall size of the structure is at the millimeter level, and the material is all hydrogel.

[0042] The present invention does not rely on photochemical reactions or thixotropic properties between solute materials, but uses low-temperature phase changes of solvents to achieve structural molding, and is universally applicable to hydrogel precursor dispersions with different material components or different concentrations. The present invention immerses the frozen structure in a low-temperature cross-linking bath to achieve cross-linking. Compared with the melting cross-linking based on temperature control, it delays the kinetic process of the cross-linking reaction, allowing multiple cross-linking agent components and multiple cross-linking mechanisms to react simultaneously, thereby achieving high-fidelity manufacturing of heterogeneous hydrogel structures. The present invention can achieve precise control of the hydrogel molding and manufacturing process by changing the process parameters such as temperature and time of low-temperature printing and low-temperature cross-linking, and the structural properties of the manufactured hydrogel have a wide range of adjustability.

[0043] Compared with the traditional method (hierarchical porous microstructure), the present invention utilizes the surface preferential crystallization mechanism of the hydrogel precursor dispersion under supercooling state. The surface of the prepared hydrogel structure has a submicron self-assembled polymer shell, and the interior of the shell has a uniform porous micromorphology (mainly determined by the low-temperature printing temperature, and the cross-linking process is maintained as it is), with a local anchoring structure and a material eutectic layer at the heterogeneous interface of the multi-material structure.

[0044] Compared with the prior art, the present invention has the following beneficial effects:

[0045] (1) The present invention enables the creation of a three-dimensional, heterogeneous, complex structure composed of multiple hydrogel material components with different cross-linking mechanisms, without the need for the introduction of specific photo- or thermal-polymerizable groups or the doping of rheological modifiers. The resulting multi-material hydrogel possesses excellent mechanical properties and can withstand various deformations, such as stretching, torsion, and extrusion, without failure.

[0046] (2) The hydrogel structures obtained by the present invention possess self-supporting geometric features including overhangs, thin walls, hollow cores, and high aspect ratios, as well as high precision and shape fidelity. These structures can accurately implement the designed functions of soft devices, including sensing and actuation. This invention provides a general technical approach for the integrated manufacturing of soft devices and soft robots. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Other features, objects and advantages of the present invention will become more apparent upon reading the detailed description of non-limiting embodiments with reference to the following drawings:

[0048] Figure 1 This is a photo of the three-dimensional crystal structure prepared in Example 3 of the present invention;

[0049] Figure 2 This is a photograph of the three-dimensional hydrogel structure prepared in Example 3 of the present invention;

[0050] Figure 3 3D hydrogel structures prepared in Examples 1 and 3 of the present invention are stress-strain curves;

[0051] Figure 4 3. The stress-strain curves of the three-dimensional hydrogel structures prepared in Example 3 and Comparative Example 1 of the present invention;

[0052] Figure 5 This is a photograph of the hydrogel structure prepared in Example 3 of the present invention attached to the joint of the back of the hand as a soft strain sensor (dilute hydrochloric acid 0.1 mol / L);

[0053] Figure 6 This is a microscopic image of the hydrogel structure obtained in Example 3, where a is the outer surface printed at low temperature, b is the cross-sectional edge printed at low temperature, c is the outer surface printed at room temperature, and d is the cross-sectional edge printed at room temperature.

[0054] Figure 7 This is a multi-material interface diagram of the three-dimensional hydrogel structure prepared in Example 3;

[0055] Figure 8 This is a sample cross-sectional view of the hydrogel structure prepared in Comparative Example 1;

[0056] Figure 9 Schematic diagrams of room temperature printing in comparative example 2 (Figure a) and frozen printing in example 2 (Figure b);

[0057] Figure 10 These are pictures of the sample in Example 1 (12.5% ​​wt polyvinyl alcohol portion, 0.1 mol / L dilute hydrochloric acid), where a is the hydrogel in Example 1, b is the hydrogel obtained by freeze-printing and freeze-drying, and c is the hydrogel obtained by freeze-printing, freeze-drying, and cross-linking. DETAILED DESCRIPTION

[0058] The present invention will be described in detail below with reference to specific embodiments. The following examples will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

[0059] Example 1

[0060] (1) Polyvinyl alcohol (PVA) and sodium alginate (SA) solids were prepared into 12.5% ​​wt and 4% wt aqueous dispersions, respectively;

[0061] (2) extruding the aqueous dispersion obtained in step (1) onto a frozen plate at -20 degrees Celsius as needed using an ink direct writing device, and solidifying it to form a multi-material crystalline structure;

[0062] (3) Glutaraldehyde at a molar concentration of 0.1 mol / L, dilute hydrochloric acid at molar concentrations of (0.032, 0.1, and 0.32 mol / L), and calcium chloride at a mass fraction of 10% by weight were added to a 30% by volume aqueous ethanol solution to prepare a crosslinking bath;

[0063] (4) immersing the crystalline structure obtained in step (2) in the crosslinking bath obtained in step (3) at -10 degrees Celsius for 12 hours to produce a polymer structure;

[0064] (5) placing the hydrogel structure obtained in step (4) in deionized water for 24 hours to obtain a hydrogel structure.

[0065] The stress-strain curves of the multi-material three-dimensional hydrogel structure (12.5% ​​wt polyvinyl alcohol part, dilute hydrochloric acid 0.1 mol / L) are shown in Figure 2. Figure 3 shown.

[0066] like Figure 10 As shown (all are partial hydrogels prepared with 0.1 mol / L dilute hydrochloric acid and 12.5% ​​wt polyvinyl alcohol), the hydrogel of Example 1 is shown as 10a, and the hydrogel obtained according to the freeze printing and freeze drying steps of Comparative Example 1 is shown as 10b, which is further cross-linked as shown in 10c.

[0067] Example 2

[0068] (1) preparing polyvinyl alcohol solid into 1% wt, 2% wt and 5% wt aqueous dispersions respectively;

[0069] (2) spraying the aqueous dispersion obtained in step (1) onto a frozen rotating shaft at -10 degrees Celsius as required by an inkjet printing device, and solidifying the solution in sequence to form a crystalline structure;

[0070] (3) Glutaraldehyde and dilute hydrochloric acid were added to a 20% by volume ethanol aqueous solution at a molar concentration of 0.5 mol / L to prepare a crosslinking bath;

[0071] (4) immersing the crystalline structure obtained in step (2) in the crosslinking bath obtained in step (3) at -5 degrees Celsius for 24 hours to produce a polymer structure;

[0072] (5) The hydrogel structure obtained in step (4) was placed in a 1.5 mol / L saturated sodium citrate solution for 2 hours, and then washed with deionized water to obtain a hydrogel structure.

[0073] Example 3

[0074] (1) Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonic acid) (PEDOT:PSS) particles and polyvinyl alcohol (PVA) particles were prepared into 7% wt aqueous dispersions and mixed at a ratio of 2:3;

[0075] (2) preparing polyvinyl alcohol particles into a 10% wt aqueous dispersion;

[0076] (3) The aqueous dispersion obtained in steps (1) and (2) is sequentially deposited on a special-shaped frozen surface at -30 degrees Celsius by means of an ink direct writing device, and solidified to form a crystalline structure, such as Figure 1 As shown;

[0077] (4) Glutaraldehyde and dilute hydrochloric acid at a molar concentration of 0.1 mol / L and calcium chloride at a mass fraction of 20% by weight were added to deionized water to prepare a crosslinking bath;

[0078] (5) immersing the crystalline structure obtained in step (3) in a crosslinking bath obtained in step (3) at -10 degrees Celsius for 8 hours to produce a polymer structure;

[0079] (6) The hydrogel structure obtained in step (5) was placed in 5 mol / L dilute sulfuric acid for 1 hour, and then washed with deionized water to obtain a hydrogel structure.

[0080] The obtained multi-material three-dimensional hydrogel structure photos are as follows Figure 2 The stress-strain curve of the multi-material three-dimensional hydrogel structure is shown in Figure 3 shown.

[0081] like Figure 4 As shown, the mechanical properties (elongation and mechanical modulus) of the hydrogel prepared by the method of the present invention (10% wt polyvinyl alcohol portion) vary with different hydrochloric acid contents, showing a wide range of adjustability. The hydrogel dispersion was cryoprinted, freeze-dried, and further cross-linked according to the steps of Comparative Example 1 for comparison. The hydrogel prepared by the method of the present invention (Example 3) showed an approximately fourfold increase in elongation at break compared to the comparative example, and also exhibited low modulus properties (Young's modulus reduced by one order of magnitude), which can better provide self-supporting structures for soft robotic applications.

[0082] The obtained hydrogel structure (dilute hydrochloric acid 0.1 mol / L) is attached to the joint of the back of the hand as a soft strain sensor. Figure 5 shown.

[0083] The hydrogel structure obtained by the method of the present invention is as follows Figure 6 a(outer surface), Figure 6 As shown in b (cross-section edge), the surface phase transition leads to the formation of a polymer shell. However, when the aqueous dispersion of this example is printed at room temperature and then cross-linked at low temperature, a hierarchical porous structure is presented (6c, d).

[0084] The hydrogel structure obtained by the method of the present invention is as follows Figure 7As shown, the multi-material interface is clear, with anchoring structures between layers. Clear heterogeneous material interfaces ensure that the structure precisely performs the specific tasks of the soft device according to the designed material and functional zoning. It also ensures that stress and strain are rationally distributed at the interface, avoiding localized stress concentrations caused by unclear interfaces between materials and reducing structural fragility or failure risk.

[0085] Comparative Example 1: Freeze-printing-freeze-drying-cross-linking

[0086] (1) Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonic acid) (PEDOT:PSS) particles and polyvinyl alcohol particles were prepared into 7% wt aqueous dispersions and mixed at a ratio of 1:3;

[0087] (2) Prepare a 10% wt aqueous dispersion of polyvinyl alcohol particles.

[0088] (3) depositing the aqueous dispersions obtained in steps (1) and (2) on a frozen plate at -15°C as needed using an ink direct writing device, and solidifying them to form a crystalline structure;

[0089] (4) freeze-drying the crystalline structure obtained in step (3) at -80°C and 10 Pascal pressure for 48 hours to prepare a freeze-dried sample;

[0090] (5) The partially crystalline structure obtained in step (2) was immersed in the cross-linking bath obtained in Example 3 for 8 hours to prepare a polymer structure.

[0091] like Figure 8 As shown, the cross-section of the sample directly freeze-dried after low-temperature printing has a dense interior, collapsed pores, and an uneven surface.

[0092] Comparative Example 2: Normal Temperature Printing

[0093] The preparation method of this comparative example is basically the same as that of Example 2, except that a 5 wt % aqueous dispersion of polyvinyl alcohol is used and printing is performed at room temperature.

[0094] like Figure 9 As shown in (Figure a and Figure b are Example 2), the hydrogel structure cannot be formed.

[0095] Comparative Example 3: Freeze Printing-Room Temperature Cross-linking

[0096] The preparation method of this comparative example is basically the same as that of Example 1, except that the cross-linking step is carried out at room temperature (25° C.).

[0097] The crystalline structure of this comparative example melts at room temperature, collapsing and distorting under the influence of gravity and surface tension. The resulting cross-linked product exhibits a high Young's modulus and strong mechanical brittleness. The uneven degree of cross-linking reaction causes the product to warp due to internal stress.

[0098] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art may make various variations or modifications within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A method for preparing a three-dimensional hydrogel, characterized in that: The steps include: S1. Low temperature printing The hydrogel precursor dispersion is deposited and solidified as needed in a freezing space to form a crystalline structure; S2, low temperature cross-linking The crystalline structure is immersed in a cross-linking bath and cross-linked at low temperature to obtain a polymer structure, i.e., a three-dimensional hydrogel; In step S1, the deposition and solidification temperature is lower than the crystallization temperature of the hydrogel precursor dispersion; In step S2, the cross-linking bath is an aqueous solution including a cross-linking agent and an antifreeze agent; the temperature of the low-temperature cross-linking is higher than the crystallization temperature of the cross-linking bath used and lower than the melting temperature of the crystalline structure; During the low-temperature cross-linking process, the cross-linking agent and the antifreeze agent simultaneously diffuse into the pre-melted layer on the surface of the ice crystals inside the crystalline structure, realizing the simultaneous melting of the crystalline structure and the cross-linking reaction.

2. The method for preparing a three-dimensional hydrogel according to claim 1, wherein: In step S1, the hydrogel precursor includes one or more of polyvinyl alcohol, sodium alginate, poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonic acid), chitosan, hyaluronic acid, and poly(N-isopropylacrylamide).

3. The method for preparing a three-dimensional hydrogel according to claim 1, wherein: In step S1 , the hydrogel precursor dispersions are hydrogel precursor dispersions of different compositions; the hydrogel precursor dispersions of different compositions include dispersions of different hydrogel precursor materials and / or dispersions of different hydrogel precursor contents.

4. The method for preparing a three-dimensional hydrogel according to claim 1, wherein: In step S1, the deposition and solidification method is 3D printing; 3D printing includes one or more of ink direct writing, inkjet printing, electrofluidic printing, and meniscus guided printing.

5. The method for preparing a three-dimensional hydrogel according to claim 1, wherein: In step S1 , the solvent in the hydrogel precursor dispersion includes one or more of water, ethanol, and dimethyl sulfoxide.

6. The method for preparing a three-dimensional hydrogel according to claim 1, wherein: In step S2, the polymer structure is further subjected to solvent post-treatment; The solvent post-treatment step is: immersing the polymer structure in a post-treatment bath to obtain a hydrogel; the post-treatment bath comprises one of deionized water, acid solution, and salt solution.

7. Use of the hydrogel obtained by the preparation method according to claim 1 in soft devices and soft robots.

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