Anisotropic organic hydrogel and preparation method and application thereof

Abstract

This invention discloses an anisotropic organic hydrogel, its preparation method, and its applications, belonging to the field of functional polymer materials technology. The preparation method disclosed in this invention includes the following steps: preparing a precursor solution using polyacrylamide, nanocellulose, MXene dispersion, and AlCl3•6H2O as raw materials; directionally freezing the precursor solution under a temperature gradient, causing ice crystals formed in the precursor solution to grow directionally along the temperature gradient direction, forming a layered structure, thus obtaining a directionally frozen sample; subjecting the directionally frozen sample to several freeze-thaw cycles to obtain a freeze-thawed hydrogel; and sequentially immersing the freeze-thawed hydrogel in ethylene glycol aqueous solutions of different volume fractions for gradient displacement to obtain anisotropic organic hydrogels. This method solves the technical problem that existing hydrogels cannot simultaneously possess excellent mechanical properties, conductivity, environmental stability, and sensing sensitivity.

Description

Technical Field

[0001] This invention belongs to the field of functional polymer materials technology, specifically relating to an anisotropic organic hydrogel, its preparation method, and its application. Background Technology

[0002] With the rapid development of flexible electronic devices and wearable technologies, conductive hydrogels, due to their unique flexibility, conductivity, and biocompatibility, have shown great potential in fields such as health monitoring, motion tracking, electronic skin, and human-computer interaction. However, existing conductive hydrogels still face many problems in practical applications, including insufficient mechanical properties, a contradiction between conductivity and mechanical properties, poor environmental stability, and low sensing sensitivity. For example, in the joints of wearable devices or in health monitoring sensors subjected to long-term pressure, the mechanical strength of existing ion-conductive hydrogels is often insufficient to maintain their structural integrity and functional stability, leading to easy device damage or performance degradation. Furthermore, it is often difficult to balance the mechanical properties and conductivity of existing ion-conductive hydrogels. For instance, while introducing a high-strength polymer matrix can improve mechanical properties, it significantly reduces the conductivity of the hydrogel, affecting its application in highly sensitive sensors. Ordinary hydrogels are prone to freezing and failure at low temperatures, causing electronic devices to malfunction in cold environments; in high-temperature or low-humidity environments, they are prone to water loss and cracking, affecting the long-term stability and lifespan of the device. Although introducing polyols or inorganic salts can improve freeze resistance, the effect is limited and may introduce new stability and durability challenges. Existing hydrogel sensors have low sensitivity, making it difficult to meet the requirements of high-precision flexible sensors, especially limiting their application in tracking minute deformations or complex motions.

[0003] In Chinese patent application CN109880201A, although the mechanical properties of hydrogels are effectively enhanced by introducing nanoparticles, this method usually reduces the conductivity of the hydrogels. In Chinese patent application CN110343245A, chemical cross-linking is used to improve the conductivity of hydrogels, but this leads to a decrease in the flexibility and environmental stability of the hydrogels. Summary of the Invention

[0004] The purpose of this invention is to provide an anisotropic organic hydrogel, its preparation method, and its application, in order to solve the technical problem that existing hydrogels cannot simultaneously possess excellent mechanical properties, conductivity, environmental stability, and sensing sensitivity.

[0005] To achieve the above objectives, the present invention employs the following technical solution: This invention discloses a method for preparing anisotropic organic hydrogels, comprising the following steps: A precursor solution was prepared using polyacrylamide, nanocellulose, MXene dispersion and AlCl3•6H2O as raw materials; The precursor solution is subjected to directional freezing in an environment with a temperature gradient, so that the ice crystals formed in the precursor solution grow directionally along the temperature gradient direction to form a layered structure, thus obtaining the directionally frozen sample. The sample after directional freezing was subjected to several freeze-thaw cycles to obtain a freeze-thawed hydrogel. The frozen-thawed hydrogel was sequentially immersed in ethylene glycol aqueous solutions of different volume fractions for gradient displacement to obtain anisotropic organic hydrogels.

[0006] Furthermore, the specific preparation process of the precursor solution includes the following steps: S1: Add polyacrylamide to deionized water, stir and heat to obtain PAAm solution; S2: Nanocellulose was added to PAAm solution and stirred. MXene dispersion and AlCl3•6H2O were added under inert conditions. Subsequently, ultrasonic treatment and magnetic stirring were performed to obtain the precursor solution.

[0007] Further, in S1, the ratio of polyacrylamide to deionized water is 8~12g:160~200mL; the stirring speed is 200~400rpm, the time is 1~2h; and the heating temperature is 55~65℃. The mass ratio of the nanocellulose in S2 to the polyacrylamide in S1 is 0.3~0.8g: 8~12g; the stirring speed is 1000~2000rpm; the mass ratio of the nanocellulose, MXene dispersion and AlCl3•6H2O is 0.3~0.8g: 1~3g: 0.3~0.8g; The concentration of the MXene dispersion is 3~8 mg / mL; the power of the ultrasonic treatment is 200~300 W and the frequency is 40~50 kHz; the speed of the magnetic stirring is 300~600 rpm and the time is 3~6 h.

[0008] Furthermore, the specific operation of directional cryopreservation molding of the precursor solution in an environment with a temperature gradient is as follows: The precursor solution is poured into a mold, and then a cold end and a hot end are formed at the bottom and top of the mold, respectively. The precursor solution is then subjected to directional freeze-forming in the direction of the temperature gradient between the cold end and the hot end.

[0009] Furthermore, the temperature of the cold end is -45~-35℃, and the temperature of the hot end is 20~30℃; the freezing rate of the directional freezing molding is 0.5~1.5mm / min, and the freezing time is 16~24h.

[0010] Furthermore, the specific procedure for subjecting the directionally frozen sample to several freeze-thaw cycles is as follows: The sample after directional freezing was thawed at room temperature for 2 hours, and then frozen at -20°C for 4 hours to complete one freeze-thaw cycle. The freeze-thaw cycle was repeated 3 to 6 times to obtain the freeze-thawed hydrogel.

[0011] Furthermore, the specific procedure for gradient displacement by sequentially immersing the freeze-thawed hydrogel in ethylene glycol aqueous solutions of different volume fractions is as follows: The freeze-thawed hydrogel was first immersed in a 20% (v / v) ethylene glycol aqueous solution and shaken at 100-300 rpm for 2-3 hours. Then it was immersed in a 40% (v / v) ethylene glycol aqueous solution and shaken for 2-3 hours. Finally, it was transferred to a 60% (v / v) ethylene glycol aqueous solution and soaked for 24-48 hours.

[0012] The present invention also discloses an anisotropic organic hydrogel, which is prepared by the above preparation method. The anisotropic organic hydrogel has a porous structure oriented along the temperature gradient direction.

[0013] Furthermore, the anisotropic organic hydrogel exhibits a compressive stress of 0.141 MPa and a Young's modulus of 0.122 MPa in the longitudinal direction at 70% compressive strain, and after undergoing 3000 cycles of compression at 10% strain, the residual deformation is less than 5%. The anisotropic organic hydrogel has an electrical conductivity of 2.99 S / m.

[0014] The present invention also discloses the application of the above-mentioned anisotropic organic hydrogel in the preparation of strain sensors, characterized in that when the anisotropic organic hydrogel is used in the preparation of strain sensors, the sensitivity coefficient of the strain sensor prepared reaches 27.93.

[0015] Compared with the prior art, the present invention has the following beneficial effects: This invention discloses a method for preparing anisotropic organic hydrogels. First, the method uses PAAm (polyacrylamide), CNF (cellulose nanoparticles), MXene dispersion, and AlCl3•6H2O to prepare a precursor solution. The precursor solution is prepared through hydrogen bonding between PAAm / CNF / MXene and the presence of Al in AlCl3•6H2O. 3+The coordination bonds formed with hydroxyl groups construct a highly efficient and stable dual dynamic cross-linking network, which can improve the mechanical properties and conductivity of the hydrogel. Secondly, a synergistic effect occurs between the MXene dispersion and AlCl3•6H2O. Specifically, the MXene dispersion builds a highly efficient electron transport highway, while AlCl3•6H2O not only acts as a cross-linking agent to enhance the mechanical properties of the hydrogel, but also releases Al... 3+ and Cl - Furthermore, it provides ion-conducting channels. Through the synergistic effect of this electron-ion dual conductivity mechanism, the hydrogel achieves extremely high sensing sensitivity, with a sensitivity coefficient (GF) of up to 27.93. Secondly, during the preparation process, the precursor solution is placed in an environment with a temperature gradient for directional freeze-forming. Ice crystals grow directionally along the temperature gradient direction (single direction), forming a unique layered brick-and-mortar structure. This structure enables the hydrogel to have a high compressive modulus perpendicular to the ice crystal growth direction (longitudinal direction) and maintain flexibility in the parallel direction, effectively solving the problem of the incompatibility between toughness and strength in traditional hydrogels. Finally, the freeze-thawed hydrogel is sequentially immersed in ethylene glycol aqueous solutions of different volume fractions for gradient displacement. During this process, ethylene glycol molecules in the ethylene glycol aqueous solution form strong hydrogen bonds with water molecules, effectively inhibiting ice crystal formation. This allows the hydrogel to maintain excellent electrical and mechanical properties even at -25°C or lower temperatures. Simultaneously, the low volatility of the ethylene glycol aqueous solution prevents the hydrogel from losing water and cracking at high temperatures, achieving all-weather stability. In summary, this invention solves the technical problem that existing hydrogels cannot simultaneously possess excellent mechanical properties, high conductivity, good environmental stability, and high sensing sensitivity.

[0016] Furthermore, the precursor solution is prepared using a stepwise process in an inert environment, which effectively avoids the oxidative aggregation of MXene in the MXene dispersion and promotes the formation of Al in AlCl3•6H2O. 3+ The full coordination with the polymer chains of PAAm and CNF ensures the uniformity and stability of the dual dynamic crosslinking network, laying the foundation for high conductivity and high mechanical properties.

[0017] Furthermore, by further optimizing the proportions of each component in the precursor solution preparation process and controlling the reaction conditions, this invention achieves uniform dispersion of nanofillers in the matrix and effective construction of the permeation network, enabling the hydrogel to possess both high compressive modulus (0.122 MPa) and high electrical conductivity (2.99 S / m).

[0018] Furthermore, the present invention specifically defines the step of directional cryoforming of the precursor solution in an environment with a temperature gradient, which can construct a layered porous structure, endow the hydrogel with significant mechanical anisotropy, and enable it to maintain excellent fatigue resistance (residual deformation <5% after 3000 cycles) while withstanding high compressive stress (0.141MPa) in the longitudinal direction.

[0019] Furthermore, by limiting the specific number of freeze-thaw cycles and temperature control program (2 hours of thawing at room temperature / 4 hours of freezing at -20°C), the present invention promotes further crystallization of the polymer segments of PAAm and CNF and forms more physical connection points through 3 to 6 freeze-thaw cycles, thereby enhancing the toughness of the network and solving the problem of structural collapse of hydrogels under large strain.

[0020] Furthermore, this invention further defines the step of gradient replacement using ethylene glycol aqueous solutions of different volume fractions. By gradually replacing the ethylene glycol aqueous solutions with gradient volume fractions of 20%, 40%, and 60%, the strong hydrogen bonds formed between ethylene glycol molecules and water in the ethylene glycol aqueous solution are utilized to lower the freezing point. This not only achieves the non-freezing characteristic of the hydrogel at -25°C (conductivity retention >90%), but also utilizes the low volatility and oxygen-isolating properties of ethylene glycol to prevent high-temperature water loss and cracking, as well as MXene oxidation, ensuring the all-weather environmental stability of the device.

[0021] This invention also discloses anisotropic organic hydrogels prepared using the above-described method. According to relevant experimental results, the layered structure formed by the directional freezing technique of this invention gives the hydrogel an extremely high elastic modulus in the longitudinal direction. The anisotropic organic hydrogel exhibits a 70% compressive stress of 0.141 MPa and a Young's modulus of 0.122 MPa, and after 3000 cycles of compression at 10% strain, the residual deformation is less than 5%, demonstrating excellent fatigue resistance. The MXene nanosheets in the MXene dispersion construct a highly efficient electron transport highway, while AlCl3•6H2O not only acts as a crosslinking agent to enhance mechanical properties, but also releases Al³⁺ from its dissociation. + and Cl - It also provides ion-conducting channels, and the synergistic effect of the dual conductivity mechanisms gives the hydrogel extremely high sensitivity (GF 27.93) and a conductivity of 2.99 S / m. Immersion in an ethylene glycol aqueous solution allows ethylene glycol molecules to form strong hydrogen bonds with water molecules, inhibiting ice crystal formation and preventing the hydrogel from freezing at -25°C or even lower temperatures. Simultaneously, the low volatility of the ethylene glycol aqueous solution prevents water loss at high temperatures, achieving all-weather stability. The ethylene glycol aqueous solution also isolates oxygen and slows down the oxidation of MXene. The anisotropic organic hydrogel prepared by this invention retains more than 90% of its conductivity at -25°C.

[0022] This invention also discloses the application of the above-mentioned anisotropic organic hydrogel in the fabrication of strain sensors. In application, due to its unique layered brick-and-mortar structure and electron-ion dual conductivity synergistic mechanism, it maintains excellent conductivity stability (conductivity retention rate >90%) and mechanical integrity in extreme environments of -25℃ low temperature and 60℃ high temperature. At the same time, in human-computer interaction experiments, this anisotropic hydrogel can not only accurately identify weak signals such as human joint movement and voice vibration, but also use its mechanical anisotropy to simulate skin characteristics, providing tactile feedback for intelligent robots. It has extremely high application value in all-weather wearable devices and biomimetic tactile systems. Attached Figure Description

[0023] Figure 1 This is a flowchart illustrating the preparation process of the anisotropic organic hydrogel of the present invention. Figure 2 SEM (scanning electron microscope) images of different organic hydrogels prepared in Example 1 and Comparative Example 1 of this invention; Wherein: (a) - SEM image of the organic hydrogel in Comparative Example 1 at 50 μm magnification; (b) - SEM image of the organic hydrogel in Comparative Example 1 at 10 μm magnification; (c) - SEM image of the organic hydrogel in Comparative Example 1 at 5 μm magnification; (d) - SEM image of the anisotropic organic hydrogel prepared in Example 1 at 50 μm magnification; (e) - SEM image of the anisotropic organic hydrogel prepared in Example 1 at 10 μm magnification; (f) - SEM image of the anisotropic organic hydrogel prepared in Example 1 at 5 μm magnification; Figure 3 The sensing performance test results of the anisotropic organic hydrogel prepared in Example 1 of the present invention after being frozen at -25°C for 72 h. Figure 4 The sensing performance test results of the anisotropic organic hydrogel prepared in Example 1 of the present invention after being placed in a high temperature environment of 60°C for 12 hours; Figure 5 Stress-strain curves of different organic hydrogels prepared in Example 1 and Comparative Example 1; Wherein: a-Example 1; b-Comparative Example 1; Figure 6 A comparison diagram of stress and Young's modulus of different organic hydrogels prepared in Example 1 and Comparative Example 1; Figure 7 Stress-strain curves of different organic hydrogels prepared in Example 1 and Comparative Example 2; Wherein: a-Example 1; b-Comparative Example 2; Figure 8A comparison of Young's modulus and stress curves of different organic hydrogels prepared in Example 1 and Comparative Example 2; Figure 9 The conductivity of the different organic hydrogels prepared in Example 1 and Comparative Example 2; Figure 10 The sensitivity of the anisotropic organic hydrogel prepared in Example 1 changes with compressive strain; Figure 11 Cyclic testing experiments were conducted on the anisotropic organic hydrogel prepared in Example 1. Figure 12 The anisotropic organic hydrogel prepared in Example 1 was applied to the detection of various aspects of human movement and even sound. Among them: (a) - finger pressing; (b) - elbow joint; (c) - throat vibration when repeating the word ("hello") four times in a row with varying volume; (d) - wrist joint; (e) - finger joint; (f) - knee joint. Detailed Implementation

[0024] To enable those skilled in the art to understand the features and effects of the present invention, the terms and expressions used in the specification and claims are explained and defined in general below. Unless otherwise specified, all technical and scientific terms used herein have the ordinary meaning understood by those skilled in the art regarding the present invention, and in case of conflict, the definitions in this specification shall prevail.

[0025] The theories or mechanisms described and disclosed herein, whether right or wrong, should not in any way limit the scope of the invention, that is, the contents of the invention can be implemented without being limited by any particular theory or mechanism.

[0026] In this document, all features defined by numerical ranges or percentage ranges, such as numerical values, quantities, contents, and concentrations, are for the sake of brevity and convenience only. Accordingly, descriptions of numerical ranges or percentage ranges should be considered as covering and specifically disclosing all possible sub-ranges and individual numerical values ​​(including integers and fractions) within those ranges.

[0027] In this article, unless otherwise specified, “contains,” “includes,” “containing,” “has,” or similar terms cover the meanings of “composed of” and “mainly composed of,” for example, “A contains a” covers the meanings of “A contains a and others” and “A contains only a.”

[0028] For the sake of brevity, not all possible combinations of the technical features in each implementation scheme or embodiment are described herein. Therefore, as long as there is no contradiction in the combination of these technical features, the technical features in each implementation scheme or embodiment can be combined arbitrarily, and all possible combinations should be considered within the scope of this specification.

[0029] The technical terms used in this invention are explained as follows: PAAm: Polyacrylamide; CNF: Nanocellulose; EG: Ethylene glycol; MXene: Two-dimensional transition metal carbide / nitride (Ti3C2T) X ); AlCl3•6H2O: Aluminum trichloride hexahydrate; GF: Sensor sensitivity.

[0030] This invention discloses a method for preparing anisotropic organic hydrogels, wherein the raw materials used in the preparation process include: Polyacrylamide (PAAm): analytical grade; Nanocellulose (CNF): purity > 99%; MXene dispersion: concentration 5 mg / mL; Aluminum chloride hexahydrate (AlCl3•6H2O): analytical grade; Ethylene glycol (EG) aqueous solution; Deionized water.

[0031] This invention provides a method for preparing anisotropic organic hydrogels, such as... Figure 1 As shown, the specific steps include: S1. Preparation of precursor solution: Polyacrylamide (PAAm) was added to deionized water at room temperature, stirred, and then heated to obtain a PAAm solution; the ratio of polyacrylamide to deionized water was 8~12g:160~200mL; the stirring speed was 200~400rpm, the stirring time was 1~2h; and the heating temperature was 55~65℃. Subsequently, nanocellulose (CNF) was added to PAAm solution and stirred. After cooling to room temperature, MXene dispersion and AlCl3•6H2O were added under inert conditions. Then, ultrasonic treatment and magnetic stirring were performed sequentially to obtain the precursor solution. The mass ratio of nanocellulose to polyacrylamide is 0.3~0.8g:8~12g; the stirring speed is 1000~2000rpm; the mass ratio of nanocellulose, MXene dispersion, and AlCl3•6H2O is 0.3~0.8g:1~3g:0.3~0.8g; the concentration of the MXene dispersion is 3~8mg / mL; the ultrasonic treatment power is 200~300W, and the frequency is 40~50kHz; the magnetic stirring speed is 300~600rpm, and the time is 3~6h. S2, directional freezing treatment: The precursor solution is poured into a mold (copper mold). The bottom of the mold is cooled by a copper plate cooled by liquid nitrogen to form a cold end, while the top of the mold is kept warm to form a hot end, creating a temperature gradient that gradually increases from bottom to top. The precursor solution is then directionally frozen and molded within this temperature gradient to form a layered structure, resulting in a directionally frozen sample. The temperature of the cold end is -45 to -35°C, and the temperature of the hot end is 20 to 30°C. The freezing rate for directionally frozen molding is 0.5 to 1.5 mm / min, and the freezing time is 16 to 24 hours. The preferred temperature is -40℃ at the cold end and 25℃ at the hot end; the freezing rate for directional freezing is 1mm / min and the freezing time is 12h, because under the above conditions, the ice crystal growth is most uniform and the layered structure is most obvious. S3. Molding and Post-processing: The directionally frozen sample was thawed at room temperature for 2 hours, then frozen at -20°C for 4 hours to complete one freeze-thaw cycle. This freeze-thaw cycle was repeated 3-6 times to obtain a freeze-thawed hydrogel, which promotes the formation of hydrogen bonds between CNF, PAAm, and MXene, as well as the formation of Al in AlCl3•6H2O. 3+ Coordination complexation with the polymer chains of PAAm and CNF forms a double physical crosslinking network; preferably 5 times, because under this condition the crosslinking network is most stable and has the best mechanical properties; The freeze-thawed hydrogel was first immersed in a 20% (v / v) ethylene glycol aqueous solution and agitated at 100-300 rpm for 2-3 hours. Then it was immersed in a 40% (v / v) ethylene glycol aqueous solution and agitated at the same speed for 2-3 hours. Finally, it was immersed in a 60% (v / v) ethylene glycol aqueous solution for 24-48 hours for gradient displacement. Gradient displacement was chosen because gradually increasing the volume fraction of ethylene glycol can avoid damage to the hydrogel structure and improve freeze resistance. After removal, the surface liquid was absorbed with filter paper to obtain a black elastic hydrogel (anisotropic organic hydrogel).

[0032] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0033] The following examples use instruments and equipment conventional in the art. Experimental methods in the following examples, unless otherwise specified, are generally performed under conventional conditions or as recommended by the manufacturer. All raw materials used in the following examples are conventional commercially available products with specifications conventional in the art. In this specification and the following examples, unless otherwise specified, "%" refers to weight percentage, "parts" refers to parts by weight, and "ratio" refers to weight proportion.

[0034] Example 1 A method for preparing anisotropic organic hydrogel includes the following steps: Step 1: Add polyacrylamide (PAAm) to deionized water at room temperature, stir and then heat to obtain PAAm solution; the ratio of polyacrylamide to deionized water is 10g:180mL; the stirring speed is 300rpm and the time is 1h; the heating temperature is 60℃. Step 2: Subsequently, nanocellulose (CNF) was added to the PAAm solution and stirred. After cooling to room temperature, MXene dispersion and AlCl3•6H2O were added under inert conditions. Then, ultrasonic treatment and magnetic stirring were performed sequentially to obtain the precursor solution. The mass ratio of nanocellulose to polyacrylamide was 0.5g:10g; the stirring speed was 1000rpm; the mass ratio of nanocellulose, MXene dispersion, and AlCl3•6H2O was 0.5g:2g:0.5g; the concentration of the MXene dispersion was 5mg / mL; the ultrasonic treatment power was 200W and the frequency was 40kHz; the magnetic stirring speed was 500rpm and the time was 4h. Step 3: Pour the precursor solution into a mold (copper mold). The bottom of the mold is cooled by a copper plate cooled with liquid nitrogen to form a cold end, while the top of the mold is kept warm to form a hot end, creating a temperature gradient that gradually increases from bottom to top. Within this temperature gradient, the precursor solution is directionally frozen to form a layered structure, resulting in a directionally frozen sample. The temperature of the cold end is -40℃, and the temperature of the hot end is 25℃. The freezing rate for directionally frozen molding is 1 mm / min, and the freezing time is 12 hours. Step 4: Thaw the directionally frozen sample at room temperature for 2 hours, then freeze it at -20°C for 4 hours to complete one freeze-thaw cycle. Repeat the above freeze-thaw cycle 5 times to obtain the freeze-thawed hydrogel. Step 5: Immerse the freeze-thawed hydrogel in a 20% (v / v) ethylene glycol aqueous solution and shake at 100 rpm for 2 hours. Then immerse it in a 40% (v / v) ethylene glycol aqueous solution and shake at the same speed for 2 hours. Finally, transfer it to a 60% (v / v) ethylene glycol aqueous solution and soak for 24 hours to perform gradient displacement, thus obtaining a black elastic hydrogel (anisotropic organic hydrogel).

[0035] Example 2 A method for preparing anisotropic organic hydrogel includes the following steps: Step 1: Add polyacrylamide (PAAm) to deionized water at room temperature, stir, and then heat to obtain a PAAm solution; the ratio of polyacrylamide to deionized water is 8g:160mL; the stirring speed is 300rpm and the time is 1h; the heating temperature is 55℃. Step 2: Subsequently, nanocellulose (CNF) was added to the PAAm solution and stirred. After cooling to room temperature, MXene dispersion and AlCl3•6H2O were added under inert conditions. Then, ultrasonic treatment and magnetic stirring were performed sequentially to obtain the precursor solution. The mass ratio of nanocellulose to polyacrylamide was 0.3g:8g; the stirring speed was 1000rpm; the mass ratio of nanocellulose, MXene dispersion, and AlCl3•6H2O was 0.3g:3g:0.3g; the concentration of the MXene dispersion was 3mg / mL; the ultrasonic treatment power was 200W and the frequency was 40kHz; the magnetic stirring speed was 300rpm and the time was 6h. Step 3: Pour the precursor solution into a mold (copper mold). The bottom of the mold is cooled by a copper plate cooled with liquid nitrogen to form a cold end, while the top of the mold is kept warm to form a hot end, creating a temperature gradient that gradually increases from bottom to top. Within this temperature gradient, the precursor solution is directionally frozen to form a layered structure, resulting in a directionally frozen sample. The temperature of the cold end is -45℃, and the temperature of the hot end is 20℃. The freezing rate for directional freezing is 0.5 mm / min, and the freezing time is 24 hours. Step 4: Thaw the directionally frozen sample at room temperature for 2 hours, then freeze it at -20°C for 4 hours to complete one freeze-thaw cycle. Repeat the above freeze-thaw cycle 6 times to obtain the freeze-thawed hydrogel. Step 5: Immerse the freeze-thawed hydrogel in a 20% (v / v) ethylene glycol aqueous solution and shake at 100 rpm for 2 hours. Then immerse it in a 40% (v / v) ethylene glycol aqueous solution and shake at the same speed for 2 hours. Finally, transfer it to a 60% (v / v) ethylene glycol aqueous solution and soak for 24 hours to perform gradient displacement, thus obtaining a black elastic hydrogel (anisotropic organic hydrogel).

[0036] Example 3 A method for preparing anisotropic organic hydrogel includes the following steps: Step 1: Add polyacrylamide (PAAm) to deionized water at room temperature, stir, and then heat to obtain PAAm solution; the ratio of polyacrylamide to deionized water is 12g:200mL; the stirring speed is 400rpm, the time is 2h; the heating temperature is 65℃. Step 2: Subsequently, nanocellulose (CNF) was added to the PAAm solution and stirred. After cooling to room temperature, MXene dispersion and AlCl3•6H2O were added under inert conditions. Then, ultrasonic treatment and magnetic stirring were performed sequentially to obtain the precursor solution. The mass ratio of nanocellulose to polyacrylamide was 0.8 g:12 g; the stirring speed was 2000 rpm; the mass ratio of nanocellulose, MXene dispersion, and AlCl3•6H2O was 0.8 g:3 g:0.8 g; the concentration of the MXene dispersion was 8 mg / mL; the ultrasonic treatment power was 300 W and the frequency was 50 kHz; the magnetic stirring speed was 600 rpm and the time was 3 h. Step 3: Pour the precursor solution into a mold (copper mold). The bottom of the mold is cooled by a copper plate cooled with liquid nitrogen to form a cold end, while the top of the mold is kept warm to form a hot end, creating a temperature gradient that gradually increases from bottom to top. Within this temperature gradient, the precursor solution is directionally frozen to form a layered structure, resulting in a directionally frozen sample. The temperature of the cold end is -35℃, and the temperature of the hot end is 30℃. The freezing rate for directional freezing is 1.5 mm / min, and the freezing time is 16 hours. Step 4: Thaw the directionally frozen sample at room temperature for 2 hours, then freeze it at -20°C for 4 hours to complete one freeze-thaw cycle. Repeat the above freeze-thaw cycle 5 times to obtain the freeze-thawed hydrogel. Step 5: Immerse the freeze-thawed hydrogel in a 20% (v / v) ethylene glycol aqueous solution and shake at 100 rpm for 2 hours. Then immerse it in a 40% (v / v) ethylene glycol aqueous solution and shake at the same speed for 2 hours. Finally, transfer it to a 60% (v / v) ethylene glycol aqueous solution and soak for 24 hours to perform gradient displacement, thus obtaining a black elastic hydrogel (anisotropic organic hydrogel).

[0037] Example 4 A method for preparing anisotropic organic hydrogel includes the following steps: Step 1: Add polyacrylamide (PAAm) to deionized water at room temperature, stir, and then heat to obtain a PAAm solution; the ratio of polyacrylamide to deionized water is 10g:180mL; the stirring speed is 200rpm, the time is 1.5h; and the heating temperature is 60℃. Step 2: Subsequently, nanocellulose (CNF) was added to the PAAm solution and stirred. After cooling to room temperature, MXene dispersion and AlCl3•6H2O were added under inert conditions. Then, ultrasonic treatment and magnetic stirring were performed sequentially to obtain the precursor solution. The mass ratio of nanocellulose to polyacrylamide was 0.5g:10g; the stirring speed was 1500rpm; the mass ratio of nanocellulose, MXene dispersion, and AlCl3•6H2O was 0.5g:1g:0.5g; the concentration of the MXene dispersion was 5mg / mL; the ultrasonic treatment power was 300W and the frequency was 50kHz; the magnetic stirring speed was 400rpm and the time was 5h. Step 3: Pour the precursor solution into a mold (copper mold). The bottom of the mold is cooled by a copper plate cooled with liquid nitrogen to form a cold end, while the top of the mold is kept warm to form a hot end, creating a temperature gradient that gradually increases from bottom to top. Within this temperature gradient, the precursor solution is directionally frozen to form a layered structure, resulting in a directionally frozen sample. The temperature of the cold end is -35℃, and the temperature of the hot end is 30℃. The freezing rate for directional freezing is 1.5 mm / min, and the freezing time is 24 hours to ensure complete directional crystallization within the large-sized sample. Step 4: Thaw the directionally frozen sample at room temperature for 2 hours, then freeze it at -20°C for 4 hours to complete one freeze-thaw cycle. Repeat the above freeze-thaw cycle 5 times to obtain the freeze-thawed hydrogel. Step 5: Immerse the freeze-thawed hydrogel in a 20% (v / v) ethylene glycol aqueous solution and shake at 100 rpm for 2 hours. Then immerse it in a 40% (v / v) ethylene glycol aqueous solution and shake at the same speed for 2 hours. Finally, transfer it to a 60% (v / v) ethylene glycol aqueous solution and soak for 24 hours to perform gradient displacement, thus obtaining a black elastic hydrogel (anisotropic organic hydrogel).

[0038] Example 5 Step 1: Add polyacrylamide (PAAm) to deionized water at room temperature, stir and then heat to obtain PAAm solution; the ratio of polyacrylamide to deionized water is 10g:180mL; the stirring speed is 300rpm and the time is 1h; the heating temperature is 60℃. Step 2: Subsequently, nanocellulose (CNF) was added to the PAAm solution and stirred. After cooling to room temperature, MXene dispersion and AlCl3•6H2O were added under inert conditions. Then, ultrasonic treatment and magnetic stirring were performed sequentially to obtain the precursor solution. The mass ratio of nanocellulose to polyacrylamide was 0.5g:10g; the stirring speed was 1000rpm; the mass ratio of nanocellulose, MXene dispersion, and AlCl3•6H2O was 0.5g:2g:0.5g; the concentration of the MXene dispersion was 5mg / mL; the ultrasonic treatment power was 200W and the frequency was 40kHz; the magnetic stirring speed was 500rpm and the time was 4h. Step 3: Pour the precursor solution into a mold (copper mold). The bottom of the mold is cooled by a copper plate cooled with liquid nitrogen to form a cold end, while the top of the mold is kept warm to form a hot end, creating a temperature gradient that gradually increases from bottom to top. Within this temperature gradient, the precursor solution is directionally frozen to form a layered structure, resulting in a directionally frozen sample. The temperature of the cold end is -40℃, and the temperature of the hot end is 25℃. The freezing rate for directionally frozen molding is 1 mm / min, and the freezing time is 12 hours. Step 4: Thaw the directionally frozen sample at room temperature for 2 hours, then freeze it at -20°C for 4 hours to complete one freeze-thaw cycle. Repeat the above freeze-thaw cycle 3 times to obtain the freeze-thawed hydrogel. Step 5: Immerse the freeze-thawed hydrogel in a 20% (v / v) ethylene glycol mixed aqueous solution and shake at 300 rpm for 3 hours. Then immerse it in a 40% (v / v) ethylene glycol mixed aqueous solution and shake at the same speed for 3 hours. Finally, transfer it to a 60% (v / v) ethylene glycol mixed aqueous solution and soak for 48 hours to perform gradient displacement, thus obtaining a black elastic hydrogel (anisotropic organic hydrogel).

[0039] Comparative Example 1 Compared with Example 1, this comparative example did not add MXene dispersion, but the remaining steps and parameters were the same as in Example 1, resulting in an organic hydrogel.

[0040] Comparative Example 2 Compared with Example 1, this comparative example does not add AlCl3•6H2O, and does not immerse the freeze-thawed hydrogel in ethylene glycol aqueous solutions of different volume fractions. The remaining steps and parameters are the same as in Example 1, resulting in an organic hydrogel.

[0041] Table 1 presents a comparative data on the performance of the anisotropic organic hydrogels prepared in Examples 1 through 5. As can be seen from Table 1, Example 2, using the lowest concentration of raw materials, exhibited the highest porosity (96%) and the largest interlayer spacing (80 μm), but also the lowest mechanical strength (longitudinal compressive modulus 24 kPa). This indicates that at lower raw material concentrations, the network structure formed within the hydrogel is relatively loose, resulting in higher porosity and lower mechanical strength. Example 3, using the highest concentration combination, reduced the porosity to 88% and the interlayer spacing to 20 μm, but significantly increased the longitudinal modulus. This demonstrates that increasing the raw material concentration helps form a denser and more robust hydrogel network structure, thereby improving mechanical strength.

[0042] Example 2 underwent 6 freeze-thaw cycles and still formed a stable hydrogel at low polymer concentrations, without becoming brittle at -40°C. This indicates that increasing the number of freeze-thaw cycles helps enhance the stability and freeze resistance of the hydrogel network. Example 5 reduced the number of freeze-thaw cycles to 3 compared to Example 1. It is speculated that its mechanical properties (such as longitudinal compressive modulus of 85 kPa) are slightly lower than those of Example 1 (90 kPa), but still remain at a high level. This shows that the number of freeze-thaw cycles is one of the important factors affecting the mechanical properties of hydrogels, but not the only determining factor.

[0043] All examples employed a gradient displacement of ethylene glycol aqueous solution with volume fractions ranging from 20% to 40% and then to 60%. This treatment method exhibited high conductivity (longitudinal conductivity 5 S / m) at high MXene content (as in Example 3), while effectively improving the freeze resistance of the hydrogel (as in Example 2, it did not become brittle at -40°C).

[0044] It can be seen that anisotropic organic hydrogels can be successfully prepared at the endpoints of each parameter range, and no examples failed. By adjusting different parameters, the mechanical strength, conductivity, and porosity of the hydrogel can be controlled. The directional freeze-molding combined with freeze-thaw cycle treatment strategy successfully constructed layered structures under different parameter combinations, demonstrating the reliability and stability of this preparation method.

[0045] Table 1. Performance comparison data of anisotropic organic hydrogels prepared in different embodiments

[0046] Figure 2The images shown are SEM images of different organic hydrogels prepared in Example 1 and Comparative Example 1. The microstructure of the anisotropic organic hydrogels is a three-dimensional porous cross-linked structure. This structure not only improves the channels for the directional movement of ions, but also enhances the mechanical properties of the hydrogel to a certain extent. Figure 1 (d)~ Figure 1 (f)).

[0047] The organic hydrogel prepared in Comparative Example 1 ( Figure 1 (a) ~ Figure 1 (c) Compared to the hydrogel with added MXene dispersion, i.e. the anisotropic organic hydrogel prepared in Example 1 ( Figure 1 (d)~ Figure 1 (f) The presence of a tighter cross-linked state and a smaller pore size network structure demonstrates that the increased degree of cross-linking of the hydrogel is beneficial to the improvement of the hydrogel's mechanical properties.

[0048] Figure 3 The results show the sensing performance of the anisotropic organic hydrogel prepared in Example 1 of this invention after being frozen at -25°C for 72 h. Figure 4 The results show the sensing performance of the anisotropic organic hydrogel prepared in Example 1 of this invention after being placed at a high temperature of 60°C for 12 hours; from Figure 3 and Figure 4 It can be seen that when pressed by a finger, the hydrogel can still generate different electrical signals in response to different external pressures, proving that it still has sensing properties.

[0049] Figure 5 The stress-strain curves of the different organic hydrogels prepared in Example 1 and Comparative Example 1 are shown. Figure 6 This is a comparison chart of the stress and Young's modulus of different organic hydrogels prepared in Example 1 and Comparative Example 1; from Figure 5 and Figure 6 It can be seen that the organic hydrogel prepared in Comparative Example 1 has a maximum stress of 0.0601 MPa under 70% compressive deformation and a Young's modulus of 0.064 MPa; while the anisotropic organic hydrogel prepared in Example 1 has a compressive stress of 0.141 MPa under 70% deformation, which is 2.27 times higher than the maximum stress. The addition of MXene dispersion makes the crosslinking network of the final anisotropic organic hydrogel more compact, thereby improving its mechanical properties.

[0050] Figure 7The figures show the stress-strain curves of different organic hydrogels prepared in Example 1 and Comparative Example 2. As can be seen from the figures, the anisotropic organic hydrogel prepared in Example 1, compared to the organic hydrogel prepared in Comparative Example 2, exhibits a significantly higher Young's modulus (0.122 MPa) and compressive stress (0.141 MPa) at 70% compressive strain. This indicates that treatment with ethylene glycol aqueous solutions of different volume fractions and a precursor solution containing AlCl3•6H2O significantly enhances the mechanical properties of the anisotropic organic hydrogel. This is mainly due to the further enrichment of AlCl3 through soaking. 3+ Crosslinking with PVA, CNF, and MXene, while the hydroxyl groups in the ethylene glycol aqueous solution can regulate the hydrogen bond network inside the anisotropic organic hydrogel, making the hydrogen bond distribution more uniform, increasing the intermolecular forces, indirectly optimizing the internal network structure of the anisotropic organic hydrogel, thereby improving its mechanical properties.

[0051] Figure 8 The graph shows a comparison of Young's modulus and stress curves of different organic hydrogels prepared in Example 1 and Comparative Example 2. As can be seen from the graph, the Young's modulus of the anisotropic organic hydrogel prepared in Example 1 (approximately 0.12 MPa) is higher than that of Comparative Example 2 (approximately 0.09 MPa), indicating that the anisotropic organic hydrogel prepared in Example 1 has stronger rigidity. The stress of the anisotropic organic hydrogel prepared in Example 1 (approximately 0.14 MPa) is also significantly higher than that of Comparative Example 2 (approximately 0.12 MPa), indicating that the anisotropic organic hydrogel prepared in Example 1 has better resistance to deformation. This is because the AlCl3•6H2O catalyst in Example 1... 3+ The anisotropic organic hydrogel in Example 1 exhibits superior mechanical properties due to its coordination with polymer chains to construct a dual dynamic crosslinking network and its structure optimized by ethylene glycol gradient displacement.

[0052] The electrical conductivity of anisotropic organic hydrogels was studied using a four-probe experimental setup. Figure 9 The conductivity of the different organic hydrogels prepared in Example 1 and Comparative Example 2 is shown in the figure. It can be seen that the conductivity increased by 1.35 times after soaking in the precursor solution containing AlCl3•6H2O and the ethylene glycol aqueous solution. This is mainly because the Al in Example 1 was dissociated by AlCl3•6H2O. 3+ Coordination with polymer chains (acrylamide amide groups and nanocellulose hydroxyl groups) significantly increases the ion concentration in the system; at the same time, gradient displacement of the ethylene glycol aqueous solution optimizes the hydrogel pore structure, further promotes ion transport, and ultimately enhances conductivity.

[0053] In this invention, the anisotropic organic hydrogel can be used to prepare strain sensors. The anisotropic organic hydrogel prepared in Example 1 of this invention is used as a strain sensor to obtain a flexible hydrogel sensor, and application tests are performed.

[0054] The assembled anisotropic organic hydrogel sample was compressed using an electronic universal testing machine (UTM2103) (default compression rate 50 mm / min), and the changes in electrical signals during compression were recorded using an electrochemical workstation (CS310H). The test voltage was fixed at 1V, and the compressive strain sensitivity coefficient was calculated. A flexible hydrogel sensor (32 mm in diameter × 5 mm in thickness) was attached to a human joint using polyurethane (PU) tape, and the changes in electrical signals generated during joint movement were recorded using the electrochemical workstation (CS310H). All human motion tests in this paper were conducted on volunteers. Before being placed close to the skin, the sample was wrapped with PU tape to ensure that the flexible hydrogel sensor did not come into direct contact with the skin.

[0055] Figure 10 The figure shows the change in sensitivity of the anisotropic organic hydrogel prepared in Example 1 with compressive strain. As can be seen from the figure, the maximum sensitivity of the anisotropic organic hydrogel in the strain ranges of 0%~10%, 10%~40%, 40%~50%, and 50%~70% are 2.62, 4.10, 7.74, and 27.93, respectively, and the linear regression coefficients are 0.955, 0.997, 0.964, and 0.971, respectively.

[0056] Figure 11 The anisotropic organic hydrogel prepared in Example 1 was applied to the detection of various human movements and even sound, verifying the compression cycle stability of the anisotropic organic hydrogel. As can be seen from the figure, after 3000 compression cycles with 10% strain, the current change of the anisotropic organic hydrogel prepared in Example 1 is almost stable, as can be seen from the cyclic magnification diagrams near 800s and 2800s.

[0057] Figure 12 The anisotropic organic hydrogel prepared in Example 1 is applied to the detection of various aspects of human movement and even sound. As shown in the figure, the anisotropic organic hydrogel is adhered to the elbow joint of the human body, such as... Figure 12 (b) Wrist joint, such as Figure 12 (d) Finger joints, such as Figure 12 (e) Knee joint, such as Figure 12 (f) and when pressing with the fingers, such as Figure 12 (a). Figure 12(a) The relative current change signal of anisotropic organic hydrogel is shown when the finger is pressed. The curve shows multiple peaks corresponding to the pressing action, which can detect the mechanical stimulation of finger pressing. Figure 12 (b) is the sensing signal of the elbow joint repetitive flexion and extension movement. The curve has obvious peaks, which match the elbow joint movement cycle and can detect the elbow joint movement state. Figure 12 (d) The relative current change corresponding to the repetitive motion of the wrist joint is consistent with the wrist joint motion cycle, and the wrist joint motion can be sensed. Figure 12 (e) is the signal when the finger joint is bent and extended. The curve shows multiple peaks, which correspond to the finger joint movement and can detect finger joint movement. Figure 12 (f) shows the relative current change during repetitive flexion and extension of the knee joint, matched to the knee joint movement cycle, which can detect knee joint movement. It can be seen that different degrees of flexion or pressure result in different electrical signal changes. When the degree of flexion increases, the current also increases; when the human joint returns to its original state, the current also returns to its original state, indicating that hydrogel has good practicality in human motion detection. Figure 12 (c) is the vibration signal when the throat repeats "hello" four times in a row. The curve contains multiple small peaks, reflecting the throat vibration at different volume levels, realizing sound-related vibration detection. The hydrogel can distinguish four tiny electrical signals, verifying its application in fields such as sound detection and speech recognition.

[0058] The above content is only for illustrating the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made to the technical solution based on the technical concept proposed in this invention shall fall within the scope of protection of the claims of this invention.

Claims

1. A method for preparing anisotropic organic hydrogels, characterized in that, Includes the following steps: A precursor solution was prepared using polyacrylamide, nanocellulose, MXene dispersion and AlCl3•6H2O as raw materials; The precursor solution is subjected to directional freezing in an environment with a temperature gradient, so that the ice crystals formed in the precursor solution grow directionally along the temperature gradient direction to form a layered structure, thus obtaining the directionally frozen sample. The sample after directional freezing was subjected to several freeze-thaw cycles to obtain a freeze-thawed hydrogel. The frozen-thawed hydrogel was sequentially immersed in ethylene glycol aqueous solutions of different volume fractions for gradient displacement to obtain anisotropic organic hydrogels.

2. The method for preparing anisotropic organic hydrogel according to claim 1, characterized in that, The specific preparation process of the precursor solution includes the following steps: S1: Add polyacrylamide to deionized water, stir and heat to obtain PAAm solution; S2: Nanocellulose was added to PAAm solution and stirred. MXene dispersion and AlCl3•6H2O were added under inert conditions. Subsequently, ultrasonic treatment and magnetic stirring were performed to obtain the precursor solution.

3. The method for preparing anisotropic organic hydrogel according to claim 2, characterized in that, In S1, the ratio of polyacrylamide to deionized water is 8~12g:160~200mL; the stirring speed is 200~400rpm, the stirring time is 1~2h; and the heating temperature is 55~65℃. The mass ratio of the nanocellulose in S2 to the polyacrylamide in S1 is 0.3~0.8g: 8~12g; the stirring speed is 1000~2000rpm; the mass ratio of the nanocellulose, MXene dispersion and AlCl3•6H2O is 0.3~0.8g: 1~3g: 0.3~0.8g; The concentration of the MXene dispersion is 3~8 mg / mL; the power of the ultrasonic treatment is 200~300 W and the frequency is 40~50 kHz; the speed of the magnetic stirring is 300~600 rpm and the time is 3~6 h.

4. The method for preparing anisotropic organic hydrogel according to claim 1, characterized in that, The specific steps for directional cryopreservation molding of the precursor solution in an environment with a temperature gradient are as follows: The precursor solution is poured into a mold, and then a cold end and a hot end are formed at the bottom and top of the mold, respectively. The precursor solution is then subjected to directional freeze-forming in the direction of the temperature gradient between the cold end and the hot end.

5. The method for preparing anisotropic organic hydrogel according to claim 4, characterized in that, The temperature of the cold end is -45~-35℃, and the temperature of the hot end is 20~30℃; the freezing rate of the directional freezing molding is 0.5~1.5mm / min, and the freezing time is 16~24h.

6. The method for preparing anisotropic organic hydrogel according to claim 1, characterized in that, The specific procedure for subjecting the directionally frozen sample to several freeze-thaw cycles is as follows: The sample after directional freezing was thawed at room temperature for 2 hours, and then frozen at -20°C for 4 hours to complete one freeze-thaw cycle. The freeze-thaw cycle was repeated 3 to 6 times to obtain the freeze-thawed hydrogel.

7. The method for preparing anisotropic organic hydrogel according to claim 1, characterized in that, The specific procedure for gradient displacement by sequentially immersing the freeze-thawed hydrogel in ethylene glycol aqueous solutions of different volume fractions is as follows: The freeze-thawed hydrogel was first immersed in a 20% (v / v) ethylene glycol aqueous solution and shaken at 100-300 rpm for 2-3 hours. Then it was immersed in a 40% (v / v) ethylene glycol aqueous solution and shaken for 2-3 hours. Finally, it was transferred to a 60% (v / v) ethylene glycol aqueous solution and soaked for 24-48 hours.

8. An anisotropic organic hydrogel, characterized in that, The anisotropic organic hydrogel, prepared by any one of claims 1 to 7, has a porous structure oriented along the temperature gradient direction.

9. An anisotropic organic hydrogel according to claim 8, characterized in that, The anisotropic organic hydrogel has a compressive stress of 0.141 MPa and a Young's modulus of 0.122 MPa in the longitudinal direction at 70% compressive strain, and after undergoing 3000 cycles of compression at 10% strain, the residual deformation is less than 5%. The anisotropic organic hydrogel has an electrical conductivity of 2.99 S / m.

10. The application of the anisotropic organic hydrogel according to claim 8 or 9 in the fabrication of strain sensors, characterized in that, When the anisotropic organic hydrogel is used in the fabrication of strain sensors, the resulting strain sensors have a sensitivity coefficient of 27.93.

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