Porous sandwich type dual-network cross-linked hydrogel sensor and preparation method thereof

Abstract

The invention discloses a porous sandwich type dual-network cross-linked hydrogel sensor and a preparation method thereof, the sensor is prepared from a pre-gel solution, and the pre-gel solution contains an ionic conductive material and a heat-sensitive material. The preparation method comprises the following steps: proportionally preparing methacrylamidated gelatin and sodium alginate in deionized water, magnetically stirring to fully dissolve the components to form a homogeneous pre-gel solution, and determining the viscosity of the pre-gel solution through viscosity measurement; starting an ultrasonic device to carry out defoaming treatment on the pre-gel solution; the pre-gel solution is placed in a spray gun, the spraying air pressure, the nozzle size and the spraying distance are adjusted and controlled, the pre-gel solution is rapidly cooled and atomized in the spraying process, multiple holes are generated, and in-situ locking is achieved; placing the sprayed sample under the condition of 5-20 DEG C for standing and cross-linking; and sequentially soaking the cross-linked sample in a moisturizing displacement solution and an anti-swelling displacement solution, and standing and cross-linking at 5-20 DEG C for more than 1.5 hours after the displacement is finished, so as to form a second cross-linked network.

Description

Technical Field

[0001] The present invention relates to the field of highly sensitive hydrogel-based flexible sensors, and particularly to a preparation method of a porous sandwich-type double-network crosslinked hydrogel sensor. Background Art

[0002] Hydrogel-based flexible sensors have excellent deformation following ability and are widely used in fields such as smart wearable devices, motion monitoring, and human-computer interaction. Since hydrogel sensors prepared by the one-pot method generally have a thickness of millimeters, when monitoring motion signals, they are prone to blocking the deformation of the body, resulting in response delay and signal loss, leading to low sensitivity and inability to achieve high-fidelity sensing. Currently, the methods for improving the sensitivity of hydrogel sensors and the existing problems are as follows: 1) Using composite materials such as conductive fillers and ionic liquids to modify the hydrogel. The results show that while the hydrogel improves the conductivity and sensitivity, the mechanical properties decrease due to the composite materials occupying the crosslinking sites, thereby affecting the flexibility and service life of the sensor; 2) Adopting a dynamic crosslinking mechanism to improve the gel strength and response sensitivity, but this method makes the preparation process more complex, and the high requirements for the preparation environment of dynamic crosslinking may lead to unstable performance; 3) Based on intelligent algorithms and external energy field assistance to improve the sensitivity and signal-to-noise ratio of sensing signals, but this method relies on precise external field control and complex signal processing algorithms, increasing the power consumption and response delay of the sensor, restricting it in applications with low power consumption or high real-time requirements.

[0003] In addition, during the long-term detection process, due to evaporation or infiltration, the water content inside the hydrogel changes accordingly, resulting in resistance drift of the ion-conductive hydrogel. To maintain the stable conductivity of the gel, researchers have proposed various moisture retention and anti-swelling strategies. For example, Singh et al. (“Human immune organoids to decode B cell response in healthy donors and patients with lymphoma”, Nat. Mater. 2024) added the highly water-retaining polymer polyethylene glycol, Schroeder et al. (“Supramolecular Ionogels Tougher than Metals”, Adv. Mater. 2023, 35, 2301383.) used ionic liquid replacement, while Huang et al. (“Control nucleation for strong and tough crystalline hydrogels with high water content”, Nature Communications, 2024, 15, 7777) controlled the crosslinking density to inhibit the change of water content.

[0004] At present, methods such as sacrificing templates or phase separation have been proven to introduce micro-nano porous structures inside hydrogels, which can enhance the response to external minute deformations. The porous structure effectively optimizes the ion transport path, thereby improving the sensing sensitivity. Coordinately regulating the saturated vapor pressure and structural stiffness of the gel can effectively inhibit water loss and swelling. However, existing preparation methods have defects such as uncontrollable pore size, poor surface quality, and high gel thickness dependence, and lack a water-phase stable coordination regulation mechanism. Therefore, new efficient preparation methods must be explored to meet the requirements of sensor sensitivity and stability. Summary of the Invention

[0005] The purpose of the present invention is to overcome the deficiencies in the prior art and provide a porous sandwich-type double-network crosslinked hydrogel sensor and a preparation method thereof, in which the porous structure inside the hydrogel is uniformly distributed and the parameters are controllable, and the surface quality is good and the thickness resolution can reach the micron level.

[0006] The purpose of the present invention is achieved through the following technical solutions:

[0007] A porous sandwich-type double-network crosslinked hydrogel sensor, which is prepared from a pre-gel solution containing:

[0008] An ion-conductive material, sodium alginate is selected;

[0009] A thermosensitive material, methacrylamide gelatin is selected.

[0010] Furthermore, this sensor can be used to detect various surface deformation signals of the human body, realize human-machine interaction and machine tactile feedback, and has the functions of adaptive adhesion and adjustable thickness.

[0011] The present invention also provides a preparation method for a porous sandwich-type double-network crosslinked hydrogel sensor, including:

[0012] S1. Dissolve methacrylamide gelatin and sodium alginate in deionized water according to a ratio, and stir magnetically at 40-70 °C to fully dissolve each component to form a homogeneous pre-gel solution, and determine the viscosity of the pre-gel solution through viscosity measurement;

[0013] S2. Start an ultrasonic device to defoam the pre-gel solution;

[0014] S3. Place the pre-gel solution in a spray gun, adjust the spraying air pressure, nozzle size and spraying distance, so that the pre-gel solution cools down and atomizes rapidly during spraying, generates pores and realizes in-situ locking, and at the same time uses the instantaneous fluidity of the pre-gel to repair surface defects;

[0015] S4. Place the sprayed sample at 5-20 °C and let it stand for crosslinking for more than 1.5 h;

[0016] S5. Immerse the crosslinked porous hydrogel in the anti-swelling replacement solution and the moisture-retaining replacement solution in sequence, and regulate the moisture replacement time in sequence. After each replacement, let it stand for crosslinking at 5-20 °C for more than 1.5 h to finally form a second crosslinked network.

[0017] Furthermore, in step S3, the spraying atomization process is adopted, so that the pre-gel solution rapidly crosslinks and encapsulates air during the spraying process due to the sudden temperature drop, forming a porous interlayer structure with a micron-level thickness.

[0018] Furthermore, in step S5,

[0019] The saturated vapor pressure inside the pre-gel solution is regulated by the moisture-retaining replacement solution to form a moisture-retaining layer on the surface of the pre-gel solution;

[0020] The anti-swelling replacement solution containing calcium chloride forms a second crosslinked network with sodium alginate in the form of "core-shell crosslinking" to enhance the anti-swelling performance and structural stability of the pre-gel solution.

[0021] Furthermore, the instantaneous fluidity of the pre-gel solution after spraying enables the surface of the gel to self-repair defects. Therefore, the prepared hydrogel sensor presents a continuous, flat and non-porous distributed outer surface layer, while the inside is a uniformly distributed and controllable porous interlayer structure.

[0022] Furthermore, in step S1, heating magnetic stirring is used to inhibit the spontaneous crosslinking and degradation of methacrylamide gelatin molecules.

[0023] Compared with the prior art, the beneficial effects brought by the technical solution of the present invention are:

[0024] 1. Rapid in-situ crosslinking and porous locking: The present invention adopts a one-step spraying atomization method to rapidly gel the pre-gel solution under sudden temperature drop. Due to the rapid crosslinking of temperature-sensitive methacrylamide gelatin in a short time and encapsulating air, a porous interlayer structure with a micron-level thickness is formed, thus effectively locking the pores and ensuring that the sensor has high sensitivity and rapid deformation following performance when responding to external forces.

[0025] 2. Construction of a double-network crosslinked structure: In addition to the first crosslinked network, a second crosslinked network is formed by sodium alginate and calcium chloride by immersing in the anti-swelling replacement solution (containing calcium chloride). This double-network structure not only improves the mechanical strength and anti-swelling ability of the sensor, but also maintains the internal moisture stability in dry and wet environments, effectively reducing the resistance drift caused by swelling or moisture loss, thereby ensuring the signal stability during long-term detection.

[0026] 3. Fine thickness and porous distribution regulation: By adjusting the spraying air pressure, nozzle size, spraying distance, and spraying times, the present invention achieves a preparation effect with a thickness resolution reaching the micrometer level (such as 163 μm), and can precisely control the porous size and number density. The fine thickness control and uniform porous structure enable the sensor to generate a larger deformation when subjected to an external force, thereby increasing the resistance change rate (such as the maximum sensitivity can reach 18.9), ensuring high-fidelity and rapid response of the sensing signal.

[0027] 4. Surface self-healing and excellent adhesion: After spraying, the solution still maintains instantaneous fluidity, can automatically repair surface defects, and forms a continuous cross-linked surface layer without pores after curing. The excellent surface quality not only ensures good adhesion between the sensor and human skin or other detection objects, but also avoids motion artifacts caused by surface unevenness, thereby improving the sensing accuracy.

[0028] 5. Template-free dependence and substrate universality: After spraying, the pre-gel solution can complete cross-linking within 5 s. This rapid liquid-solid conversion process can avoid the influence of template interface characteristics on the solution morphology, that is, without considering the selective requirements of the substrate surface wettability, roughness, and chemical properties, and can be stably formed on heterogeneous interfaces such as metals, polymers, and biological tissues.

[0029] 6. Comprehensive electrochemical stability and low-power consumption application: The internal porous structure reduces the density of the gel matrix, and at the same time, the double-network cross-linking ensures the continuity of the ion transport channels; in addition, the initial resistance is stabilized by using a moisture replacement treatment. This structural design ensures the stability of the sensor resistance change rate and response period during continuous stretching-unloading cycle tests while maintaining a high conductivity (about 4.8 mS / cm), which helps to achieve real-time monitoring and low-power consumption detection.

[0030] In summary, the present invention uses the spraying method to achieve second-level cross-linking and porous in-situ locking, combines the double-network structure design and fine process parameter regulation, effectively solves the problems of response delay, signal loss, and resistance drift existing in traditional hydrogel sensors, and thus realizes high-sensitivity, high-stability, low-power consumption, and excellent surface quality flexible sensing applications. Brief Description of the Drawings

[0031] Figure 1 It is a schematic diagram of the preparation method and sensing application of a porous sandwich-type double-network cross-linked hydrogel sensor of the present invention;

[0032] Figures 2a to 2f It is a schematic diagram of the preparation mechanism of the hydrogel sensor; Figure 2a It is a thermal infrared image of the pre-gel solution before and after spraying; Figure 2b It is the process of solution fluidity and color change; Figure 2c It is the preparation of hydrogel by sol-gel method;Figure 2d Schematic diagram of the porous in-situ locking mechanism; Figure 2e Moisture treatment process; Figure 2f Anti-swelling treatment process.

[0033] Figures 3a to 3e Schematic diagram of the structural characterization of the hydrogel sensor: Figure 3a SEM image of the inside of the gel; Figure 3b SEM images of the surface and cross-section of the gel; Figure 3c AFM image; Figure 3d Porous optical image; Figure 3e Confocal laser scanning microscopy image of a single spray.

[0034] Figures 4a to 4c Test chart for adhesion and sensing performance; Figure 4a Test chart for adhesion effect in different medium environments; Figure 4b Test chart for strain sensitivity; Figure 4c Test chart for tensile-unloading cycle.

[0035] Figures 5a to 5d Schematic diagrams of using porous hydrogels as human-machine interfaces and machine touch, respectively; Figure 5a Schematic diagram of synchronized human-machine actions. Figure 5b Schematic diagram of the mapping relationship between the resistance change rate and the bending angle of the mechanical finger; Figure 5c Schematic diagram of the interactive control manipulator grasping an object; Figure 5d Schematic diagram of machine touch.

[0036] Figure 5e For Figure 5d Schematic diagram of the mechanical strength borne by the hydrogel sensors on each finger during the machine touch process in Specific implementation manners

[0037] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0038] To clarify the content of this application more clearly, multiple specific embodiments are provided below, and relevant details are listed. It should be noted that those skilled in the art should understand that even if some specific details are omitted, this application can still be implemented. In the embodiments, common methods, tools, instruments, and equipment that are already familiar to those skilled in the art are not elaborated too much, aiming to highlight the core idea of this application.

[0039] The "Examples" and "Test Examples" provided in this application are only "demonstration examples". It should be understood that the terms used are only for describing specific implementation manners, aiming to clarify the content of this application, and do not mean to limit the disclosed technical scope.

[0040] In the description of the drawings, the term abbreviation Gel represents the mass fraction of methacrylamide gelatin, and Pre represents the selected spraying air pressure. It is only for the convenience of describing the content of the present invention, rather than referring to specific solution preparation parameters and spraying conditions. Therefore, it cannot be understood as a limitation to the present invention.

[0041] The present invention discloses a preparation method and sensing application of a porous sandwich-type double-network crosslinked hydrogel sensor.

[0042] Example 1

[0043] The preparation method of a porous sandwich-type double-network crosslinked hydrogel sensor provided in this example is as follows:

[0044] Refer to Figure 1 , take 2 g of methacrylamide gelatin and 1 g of sodium alginate in 7 mL of deionized water, stir magnetically at 55 °C for 2 hours to fully dissolve each component to form a homogeneous pre-gel solution, and calibrate its viscosity with a viscometer. The viscosity value is used as a reference for the selection of spraying air pressure and nozzle size. After ultrasonic defoaming of the pre-gel solution for 5 minutes, pour it into the gun cavity, control the spraying air pressure at 2 kPa, the spraying distance at 5 cm, and the carrier substrate is selected as polyethylene terephthalate plastic. After spraying, let it stand and crosslink at 10 °C for 24 hours, and then soak it in a 5 wt% GaCl2 solution for 0.5 hours. After taking it out, let it stand at 25 °C for 12 hours to form a second crosslinked network. To provide a suitable water environment for ion transport, soak the hydrogel in ethylene glycol for 2 minutes to improve the moisture retention.

[0045] Example 2

[0046] A bulk hydrogel sensor prepared based on the sol-gel method has the same pre-gel solution preparation as in Example 1.

[0047] The core mechanism of the porous sandwich-type double-network crosslinked hydrogel disclosed in this example is that the thermosensitive pre-gel solution realizes second-level crosslinking and encapsulates air through spraying and atomization with temperature loss. As Figure 2a shown, an infrared thermal imager is used to characterize the temperature distribution of the solution at three moments (before spraying, after spraying, 1.5 h after spraying). The results show that the spraying process significantly reduces the solution temperature, and the temperature change is less different after spraying is completed. This temperature reduction method effectively avoids the floating and escape of pores. As Figure 2cAs shown, spraying causes partial evaporation of water in the solution, resulting in the re-aggregation and solidification crystallization of methylacrylamide gelatin molecules, making the solution after spraying appear as an opaque milky white. After spraying is completed, the solution completely loses its fluidity within 5 s, and the milky white color fades after 1.5 h, and it has a certain light transmittance.

[0048] Figure 2d The following is an explanation of the origin of porous in-situ locking. The polypeptide nature of gelatin enables it to spontaneously crosslink at temperatures below human body temperature. This process does not require the intervention of crosslinking agents and initiators, so it can reach the gel state in a short time. After cooling, gelatin molecules recombine into a triple helix structure and form a crosslinked network under the action of hydrogen bonds and van der Waals forces. Since the solution after spraying loses its fluidity in seconds, the combined effect of structural constraints and reduced buoyancy limits the position of the pores in the gel.

[0049] After the porous deforms, responsive ions transmit signals between the gel networks. This process relies on a suitable water environment, and the water loss caused by evaporation in the gel will affect the stability of the resistance. To avoid water loss, as Figure 2e shown, the porous hydrogel is immersed in ethylene glycol for 2 minutes. While ethylene glycol displaces some of the water in the hydrogel, it forms an ethylene glycol film on the surface of the gel. The ethylene glycol incorporated into the gel reduces the saturated vapor pressure of the mixture, and the ethylene glycol film on the surface not only slows down the internal water evaporation but also forms hydrogen bonds with water vapor in the environment to obtain moisture absorption properties. Contrary to the internal water evaporation, the swelling of the hydrogel due to water absorption in a humid environment will also cause resistance drift, thereby reducing the sensing accuracy. To achieve application expansion in in-vivo, underwater and other working conditions, the porous hydrogel adopts a double-network crosslinking form to overcome swelling. The first crosslinked network formed by methylacrylamide gelatin aims to fix the position of the pores through rapid gelation, and then sodium alginate and calcium chloride form a second network through ionic crosslinking to enhance the gel stiffness. In addition, the existing pores also play a promoting role in counteracting swelling because the internal air pressure in the pores and the surface tension of the surrounding solvent can synergistically regulate the gel network.

[0050] As Figure 3a shown, before SEM observation, the hydrogel is freeze-dried to sublimate water while maintaining the three-dimensional crosslinked network structure. The observed cross-section is obtained by brittle fracture with liquid nitrogen. The results show that the porous structure is wrapped by a dense GelMA network. In this test example, the spatial occupancy ratios of the porous structure and the crosslinked network are approximately the same (the porosity is 54%). This spatial distribution can utilize the physical properties of large molecular gaps and easy compressibility inside the pores to achieve a highly sensitive deformation response, and can also complete the ionic transmission of the response signal through the interconnected crosslinked network. Figure 3b It shows that the surface of the hydrogel is a fully crosslinked layer with a thickness of about 20 μm, and the absence of porous distribution gives it good surface flatness. The surface quality of the hydrogel is further characterized by an atomic force microscope, asFigure 3c As shown, the distance between the peaks and valleys on the surface of the hydrogel does not exceed 54 nm. The reason for the absence of pores on the surface is that the solution does not completely lose its fluidity after spraying, thus providing a buffer time for the pores at the solution-air interface to break due to surface tension and for the solution to repair the pores. Figure 3d is the optical image of the porous hydrogel sensor, which has good uniform distribution characteristics and pore consistency. As Figure 3e shown, the thickness of the gel film prepared by single spraying is only 163 μm, providing a fine resolution for the regulation of the gel thickness.

[0051] Good adhesion between the flexible sensor and the detected object can effectively reduce the generation of motion artifacts. As Figure 4a shown, the hydrogel sensor acts as a bonding medium between the glass and the metal substrate and can lift heavy objects in different liquid environments. When traditional hydrogels come into contact with the wetting interface, the adhesion force decreases due to the presence of the lubricating layer, while the gelatin methacrylamide network can maintain good adhesion performance due to its high surface energy and a large number of hydrophilic groups (amino acid residues, methacrylate groups). In the ion-conductive circuit, the electric double layer generated at the hydrogel / electrode interface can be equivalent to a contact model in which a resistor and a capacitor are connected in parallel. Therefore, an AC test voltage of 10 kHz is used to avoid signal errors caused by interface impedance. The gauge factor (GF) is defined as the ratio of the resistance change rate to the elongation rate to characterize the strain sensitivity. The two ends of the ACH with dimensions of 60 mm × 15 mm × 1 mm are clamped on a tensile testing machine, as Figure 4b shown, and its resistance change rates reach 0.9, 2.7, and 18.9 in different strain ranges respectively. As Figure 4c shown, after 500 stretching-unloading cycle tests at 30% strain, the ACH still maintains a stable resistance change rate and response period.

[0052] In this test example, porous hydrogels are used to capture human motion signals, and then delay-free and coordinated mechanical terminal control is realized, demonstrating its high sensitivity. In the control circuit, the porous hydrogel is connected in series with a 5 kΩ voltage-dividing resistor and then connected to a 3.3 V DC voltage source. The development board pins convert the 5-channel analog signals into digital signals in real time and transmit them to the manipulator control board through Bluetooth. The manipulator control board performs pulse width modulation on the digital signals, and finally realizes the motion following of the mechanical fingers. As Figure 5a shown, when performing multi-finger coordinated actions, the manipulator can complete the deformation mapping of the human hand actions. Figure 5b It shows that as the finger bends, the resistance change rate of the hydrogel sensor increases after being stretched. According to the dynamic data detected by the serial port, the mapping relationship between the resistance change and the rotation angle of the mechanical finger is further verified. As Figure 5cAs shown, controlling the mechanical fingers to grasp objects with different shapes and ensuring stable resistance values in a certain temporary state provide guarantees for the reliability of human-machine interaction. For example Figure 5d As shown, a hydrogel sensor is attached to the end of the mechanical finger to simulate the tactile sense of the human body. When contacting an object, it is compressed to generate a resistance change. The force on each finger can be monitored through the resistance signal, which is beneficial for assisting in completing various morphological perception tasks such as tactile imaging. Figure 5e It is Figure 5d a schematic diagram of the mechanical strength borne by the hydrogel sensors on each finger during the machine tactile process in

[0053] In summary

[0054] The present invention prepares a porous sandwich-type double-network hydrogel with a micron-level thickness and high sensitivity by a one-step spraying method. The second-level crosslinking is realized by atomizing and cooling the thermosensitive solution to complete the in-situ locking of the pores, while the instantaneous fluidity of the pre-gel can repair surface defects. The porous hydrogel contains a double-crosslinking network to enhance its anti-swelling property, self-healing property, and mechanical properties, and can maintain the same frequency and hysteresis-free deformation following. In the detection of body surface movements, the porous hydrogel sensor can make different responses for different individuals. Making it into a sensing glove demonstrates its application value in human-machine interaction and machine touch. The beneficial effects of the present invention are as follows: The porous sandwich-type hydrogel sensor has a high surface quality, exhibits excellent deformation following ability when responding to external forces, and can maintain the stability of internal moisture in dry and wet environments, thereby reducing motion artifacts and faithfully reflecting the measured signal.

[0055] The porous sandwich-type double-network crosslinked hydrogel prepared by the spraying method in the present invention has a porous sandwich structure, and the introduction of uniformly distributed pores inside the gel does not sacrifice the surface quality. This hydrogel sensor has the advantages of self-adaptive adhesion, high sensitivity (the maximum sensitivity is 18.9), high conductivity (4.8 mS / cm), high thickness resolution (163 μm), etc. Compared with the sol-gel method, the spraying method shortens the gelation time to several seconds, which is beneficial for the in-situ locking of the porous structure. Through the replacement of the moisturizing solution and the formation of the second crosslinking network, the hydrogel sensor can maintain the stability of the matrix moisture in dry or humid environments, avoiding the influence of resistance drift on the signal-to-noise ratio and sensitivity during long-term detection. The porous hydrogel sensor can maintain the same frequency and hysteresis-free response of strain input and signal output, and maintain a stable resistance change rate and response period after repeated stretching-unloading cycles.

[0056] Although the embodiments of the present invention have been disclosed as above, they are not limited to the applications listed in the specification and the embodiments. It can be fully applied to various fields suitable for the present invention. For those skilled in the art, additional modifications can be easily made. Therefore, without departing from the general concept defined by the claims and the equivalent scope, the present invention is not limited to the specific details and the examples shown and described herein.

[0057] The present invention is not limited to the embodiments described above. The above description of the specific embodiments is intended to describe and illustrate the technical solutions of the present invention. The above specific embodiments are merely illustrative and not restrictive. Without departing from the spirit of the present invention and the scope protected by the claims, those of ordinary skill in the art can also make many specific transformations in various forms under the inspiration of the present invention, and these all fall within the protection scope of the present invention.

Claims

1. A porous sandwich - type double - network cross - linked hydrogel sensor, characterized in that, The sensor is prepared from a pre-gel solution, which contains: An ion-conductive material, sodium alginate, is selected. A thermosensitive material, methacrylamide gelatin, is selected.

2. A porous sandwich - type double - network cross - linked hydrogel sensor, characterized in that, This sensor can be used to detect various surface deformation signals of the human body, achieve human-machine interaction and machine tactile feedback, and has functions of adaptive adhesion and adjustable thickness.

3. The preparation method of the porous sandwich-type double-network crosslinked hydrogel sensor according to claim 1, wherein, It includes: S1. Methacrylamide gelatin and sodium alginate are proportionally configured in deionized water and magnetically stirred at 40-70 °C to fully dissolve each component to form a homogeneous pre-gel solution, and the viscosity of the pre-gel solution is determined by viscosity measurement. S2. Start the ultrasonic device to defoam the pre-gel solution. S3. Place the pre-gel solution in a spray gun, adjust the spraying air pressure, nozzle size and spraying distance, so that the pre-gel solution rapidly cools down and atomizes during spraying, generates pores and realizes in-situ locking, and at the same time uses the instantaneous fluidity of the pre-gel to repair surface defects. S4. Place the sprayed sample at 5-20 °C and let it stand for crosslinking for more than 1.5 h. S5. Immerse the crosslinked porous hydrogel in an anti-swelling replacement solution and a moisture-retaining replacement solution in sequence, and adjust the moisture replacement time in sequence. After each replacement, let it stand for crosslinking at 5-20 °C for more than 1.5 h to finally form a second crosslinking network.

4. The preparation method according to claim 3, characterized in that, In step S3, a spraying and atomization process is adopted, so that due to the sudden temperature drop during spraying, the thermosensitive material is rapidly crosslinked and air is encapsulated to form a porous sandwich structure with a micron-level thickness.

5. The preparation method according to claim 3, characterized in that, In step S5, The saturated vapor pressure inside the pre-gel solution is regulated by the moisture-retaining replacement solution to form a moisture-retaining layer on the surface of the pre-gel solution. An anti-swelling replacement solution containing calcium chloride is used to form a second crosslinking network with sodium alginate in a core-shell crosslinking form to enhance the anti-swelling performance and structural stability of the pre-gel solution.

6. The preparation method according to claim 3, characterized in that, The instantaneous fluidity of the pre-gel solution after spraying enables the gel surface to self-repair defects, so the prepared hydrogel sensor presents a continuous, flat and non-obviously porous outer layer, while the inside is a uniformly distributed and controllable porous sandwich structure.

7. The preparation method according to claim 3, characterized in that, In step S1, heating and magnetic stirring are used to inhibit the spontaneous crosslinking and degradation of methacrylamide gelatin molecules.

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