Preparation method and application of lithium-aluminum-iron layered double hydroxide enhanced nano-conductive hydrogel strain sensor

Abstract

The application belongs to the field of cross and fusion of material science, flexible sensing technology and electronic information technology, and discloses a preparation method and application of a lithium aluminum iron layered double hydroxide reinforced nano conductive hydrogel strain sensor. A plurality of synergistic networks are constructed through an in-situ polymerization process, so that the strain sensor has excellent flexibility, high mechanical strength (tensile breaking strain ≥ 1875%, breaking strength ≥ 435 kPa) and excellent sensing performance (tensile sensitivity GF = 4.82, sensitivity S = 0.107 kPa-1 in a pressure range of 0-7.5 kPa), and the response is fast (tensile ≤ 150 ms, compression ≤ 180 ms) and long-term stable (signal stability rate ≥ 95% after 3000 cycles). The sensor can be assembled into a flexible array sensor and a foot pressure sensing intelligent insole, a matching APP can realize real-time visualization of foot pressure distribution, evaluate a motion state and provide health suggestions, and the application of the flexible wearable device in motion monitoring and disease prevention is expanded.

Description

Technical Field

[0001] This invention belongs to the interdisciplinary field of materials science and flexible electronic sensing technology, specifically relating to the preparation method and application of a lithium aluminum iron layered double hydroxide reinforced nano-conductive hydrogel strain sensor. Background Technology

[0002] With the advent of the era of intelligent electronics, flexible strain sensors, due to their excellent tensile and electrical properties, can convert external forces and deformations into recordable electrical signals such as resistance and voltage, showing broad application prospects in fields such as human motion detection, medical diagnosis, and human-computer interaction. Conductive hydrogels, as the core material of flexible strain sensors, combine flexibility, biocompatibility, and conductivity, making them an ideal candidate in this field.

[0003] Polyacrylamide (PAM) is often used as a polymer matrix for conductive hydrogels due to its simple preparation and high degree of functionalization. However, pure PAM hydrogels suffer from weak mechanical properties, limiting their practical applications. To address this issue, existing technologies typically introduce natural polysaccharides such as xanthan gum (XG) to form a double-network structure. The ordered double-helix chain structure and abundant hydroxyl and carboxyl groups of XG form hydrogen bonds, enhancing the mechanical stability of the hydrogel. Furthermore, sodium alginate (SA), a commonly used reinforcing agent, can further improve network connectivity by forming an "egg-box" structure with metal ions through the -COO- groups on its molecular chain.

[0004] Layered hydrogen hydroxides (LDHs) have attracted much attention in the field of nano-conductive hydrogels due to their tunable layered structure and biocompatibility. However, traditional LDHs suffer from insufficient intrinsic conductivity and easy aggregation, resulting in poor dispersibility in hydrogels and difficulty in fully exerting their conductive and reinforcing effects. Although conductivity can be improved by controlling the type and ratio of metal ions between LDH layers, and Li / Al / Fe-LDH shows excellent potential due to its high-layer charge density, achieving uniform dispersion of LDHs in the hydrogel network and synergistically improving the mechanical and sensing properties of the hydrogel remains a key challenge that has not yet been solved by current technologies.

[0005] Existing conductive hydrogel sensors also have the following drawbacks: 1. It is difficult to balance mechanical strength and tensile properties, and the fracture strain and tensile strength cannot meet the requirements of complex deformation scenarios. 2. Insufficient sensor sensitivity, limiting its ability to detect minute deformations; 3. Poor long-term stability; the signal is prone to drift after repeated stretching / compression cycles. 4. In practical wearable device applications, the level of integration is low, making it difficult to achieve functions such as pressure distribution visualization and real-time data transmission. Summary of the Invention

[0006] The purpose of this invention is to address the shortcomings of existing technologies by providing a nano-conductive hydrogel strain sensor (PXSL strain sensor) based on a synergistic enhancement of a polyacrylamide / xanthan gum semi-interpenetrating network using Li / Al / Fe-LDH and sodium alginate. By constructing a multi-synergistic network structure, the mechanical and sensing performance are simultaneously improved. The invention also provides a flexible array sensor based on this sensor, a smart insole for plantar pressure sensing, and related applications, solving problems such as low sensitivity, poor stability, and insufficient integration in existing devices. Technical solution

[0007] The method for preparing a lithium-aluminum-iron layered double hydroxide-reinforced nano-conductive hydrogel strain sensor provided by this invention specifically includes the following steps: Step 1: Mix lithium chloride, aluminum chloride, ferric chloride hexahydrate and urea in a certain proportion and dissolve them in deionized water. Stir mechanically until the system is completely homogeneous to obtain lithium aluminum iron layered double hydroxide (Li / Al / Fe-LDH) precursor solution. Step 2: Transfer the above Li / Al / Fe-LDH precursor solution to a reflux reflux device, control the constant temperature for reflux reaction, use a red laser pointer to irradiate the solution to monitor the reaction process in real time, terminate the reaction when the Tyndall effect of the solution is about to disappear, and obtain a uniformly dispersed Li / Al / Fe-LDH solution. Step 3: Measure the freshly prepared Li / Al / Fe-LDH solution and mix it with deionized water to make up the set total volume. Slowly add acrylamide monomer, xanthan gum powder and sodium alginate powder in sequence. Stir magnetically until all components are completely dissolved. Ultrasonically treat under ice-water bath conditions to remove air bubbles and obtain a homogeneous and stable mixture. Step 4: Add the photoinitiator 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylphenylacetone and the crosslinking agent N,N-methylenebisacrylamide to the mixture obtained in Step 3. After magnetic stirring until completely mixed, perform vacuum degassing to obtain the PXSL nano-conductive hydrogel precursor solution. Step 5: Rapidly inject the PXSL hydrogel precursor solution into a polytetrafluoroethylene (PTFE) mold, seal the mold with a glass slide to isolate it from air, and carry out an in-situ polymerization reaction under ultraviolet light irradiation to obtain PXSL nano-conductive hydrogel with Li / Al / Fe-LDH synergistic enhancement. Step 6: Mix the A and B components of polydimethylsiloxane (PDMS) at a fixed volume ratio, stir mechanically until homogeneous, and then degas using ultrasonication to obtain the PDMS precursor solution. Step 7: Inject the PDMS precursor liquid into a polytetrafluoroethylene mold and place it in a vacuum oven for high-temperature curing to obtain a PDMS film with flexibility and encapsulation properties. Step 8: Cut the PDMS film to the set size and assemble it with the PXSL nano-conductive hydrogel to form a sandwich structure of "upper PDMS film - middle PXSL hydrogel - lower PDMS film". Fix copper wires at both ends of the structure to establish a conductive path, and finally obtain the PXSL nano-conductive hydrogel strain sensor.

[0008] Preferably, in step 1, the mass-to-volume ratio of lithium chloride, aluminum chloride, ferric chloride hexahydrate, urea, and deionized water is 1:3:0.7:14.4:100 (g:g:g:g:mL), and the mechanical stirring time is 15 minutes; in step 2, the reflux reaction temperature is controlled at 97°C, and the total reaction time is 2 hours. Preferably, in step 3, the total volume of Li / Al / Fe-LDH solution and deionized water is 20 mL, the mass ratio of acrylamide, xanthan gum and sodium alginate is 3:0.15:0.03; the magnetic stirring time is 6 hours, the ice-water bath temperature is 0℃, the ultrasonic frequency is 10 kHz, and the ultrasonic treatment time is 15 minutes. Preferably, in step 4, the ratio of photoinitiator 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylphenylacetone, crosslinking agent N,N-methylenebisacrylamide, and acrylamide is 9:2.4:3 (mg:mg:g); the magnetic stirring time is 30 minutes, and the vacuum degassing time is 1 hour; As a preferred embodiment, in step 5, the ultraviolet light power is 20W, the wavelength is 365nm, and the irradiation polymerization time is 30 minutes; the resulting PXSL nano-conductive hydrogel has a tensile fracture strain of not less than 1875%, a tensile fracture strength of not less than 435kPa, and a tensile sensitivity GF of 4.82 in the strain range of 0-500%. Preferably, in step 6, the volume ratio of PDMS A adhesive to B adhesive is 10:1, the mechanical stirring time is 2 hours, and the ultrasonic degassing frequency is 10kHz for 30 minutes; in step 7, the high-temperature curing temperature is 50℃ and the curing time is 12 hours. Preferably, the PDMS film in step 8 is a square of 3cm×3cm; the sensor has a sensitivity of 0.107kPa-¹ in the pressure range of 0-7.5kPa, a signal stability retention rate of not less than 95% after 3000 consecutive stretching / compression cycles, a stretching response time of ≤150ms, and a compression response time of ≤180ms. This invention further provides a flexible array sensor based on the aforementioned PXSL nano-conductive hydrogel strain sensor. This array sensor consists of 4×4 independent sensing units, each of which is a square structure with a side length of 1cm. During assembly, the sensing units are interconnected via pre-set conductive lines, and the entire assembly is encapsulated and protected using PDMS before being stably connected to a custom printed circuit board (PCB). A data acquisition channel is established between the PCB and an Arduino microcontroller via wires. The microcontroller and computer complete signal transmission and interface. Combined with a MATLAB visualization program, the changes in the electrical signals of each sensing unit can be mapped in real time to a pressure distribution image, achieving precise pressure location and quantitative display of pressure magnitude. Each sensing unit operates independently without cross-interference, making it suitable for complex curved surfaces and multi-point pressure monitoring scenarios.

[0009] This invention also provides a smart insole for plantar pressure sensing based on the aforementioned PXSL nano-conductive hydrogel strain sensor. The PXSL hydrogel sensor array is precisely embedded into the insole corresponding to key pressure areas of the foot. A complete conductive circuit is constructed using copper wires, and the entire insole is sealed using PET film as the encapsulation substrate and EVA as the adhesive layer, ensuring both wearing comfort and structural stability. The sensor array is sequentially connected to an AD converter, a microcontroller, a Bluetooth module, a voltage regulator module, and a lithium battery via wires to complete hardware integration and debugging. A dedicated APP is developed to receive plantar pressure data transmitted from the microcontroller in real time via the Bluetooth module, visually displaying the plantar pressure distribution. Based on the pressure data, the user's exercise status is accurately assessed, and personalized health suggestions are automatically generated, enabling foot health monitoring and disease prevention and early warning. It has the advantages of being lightweight, wearable, providing real-time data, and cost-effective, and can be widely applied in scenarios such as daily exercise monitoring, rehabilitation training assistance, and prevention of chronic foot diseases.

[0010] 1. This invention innovatively constructs a polyacrylamide / xanthan gum semi-interpenetrating network structure synergistically reinforced by Li / Al / Fe-LDH and sodium alginate. Through multi-component hydrogen bonding, coordination bonds and molecular chain entanglement, it fundamentally solves the problem of the difficulty in balancing mechanical strength and tensile properties of traditional hydrogels. The resulting PXSL nano-conductive hydrogel has a tensile fracture strain ≥1875% and a tensile fracture strength ≥435kPa, which significantly improves its mechanical properties.

[0011] 2. Freshly prepared Li / Al / Fe-LDH solutions with Tyndall effect are used in the polymerization process, which allows Li / Al / Fe-LDH nanosheets to be uniformly dispersed in the polymer matrix without agglomeration, forming a continuous and stable conductive pathway. This not only endows the hydrogel with excellent conductivity but also further enhances the network crosslinking density, achieving simultaneous optimization of mechanical and sensing properties.

[0012] 3. The fabricated strain sensor has the advantages of high sensitivity, fast response, and long-cycle stability. The tensile sensitivity GF=4.82 in the strain range of 0-500%, the sensitivity S=0.107kPa-¹ in the pressure range of 0-7.5kPa, the tensile response ≤150ms, the compression response ≤180ms, and the signal stability retention rate ≥95% after 3000 tensile / compression cycles. It can accurately identify minute deformations and complex stress changes.

[0013] 4. A sandwich encapsulation structure is constructed using PDMS film. The high flexibility and encapsulation properties of PDMS allow for synchronous deformation with the hydrogel sensing layer. At the same time, it effectively blocks environmental interference and reduces the loss of water from the hydrogel, significantly improving the sensor's environmental adaptability and long-term service life, and meeting the usage requirements of actual wearable scenarios.

[0014] 5. Based on this sensor, flexible array sensors and foot pressure sensing smart insoles can be easily integrated. With supporting hardware and APP, pressure distribution visualization, real-time data transmission, motion status assessment and health warning functions can be realized. It has a high degree of integration, is lightweight and easy to wear, and is economical in cost, successfully expanding the practical application of flexible sensing technology in human motion monitoring, rehabilitation assistance, disease prevention and other fields. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the fabrication process of PXSL nano-conductive hydrogel in this invention; Figure 2 This is a sandwich structure diagram of the PXSL pressure sensor in this invention; Figure 3 SEM images of different hydrogels obtained in Comparative Example 1, Comparative Example 2, Comparative Example 3 and Example 1 of this invention; Figure 4 The PXSL obtained in Example 1 of this invention 10.00% TEM image and elemental distribution map of the nano-conductive hydrogel; Figure 5 The infrared spectra of different hydrogels obtained in Comparative Example 1, Comparative Example 3 and Example 1 of this invention are shown. Figure 6 The PXL obtained in Example 1 of this invention 10.00% XRD pattern of the hydrogel; Figure 7 The tensile stress-strain curves of hydrogels with different components prepared in Comparative Examples 1, 2, 3, 4, Example 1, and 6 of this invention, and the PXSL prepared in Example 1 are shown. 10.00%The pressure sensor's various compressive mechanical properties are shown in the graphs; where (a) is the tensile stress-strain curve of hydrogels with different components, and (b) is the PXSL prepared in Example 1. 10.00% Compressive stress diagrams of the strain sensor under 0-80% strain, (c) is the PXSL prepared in Example 1. 10.00% Compression stress diagrams of the strain sensor at different compression rates under 80% strain, (d) is the PXSL prepared in Example 1. 10.00% Compressive stress diagrams of a strain sensor at different residence times under 60% strain; Figure 8 The PXSL prepared in Example 1 of this invention 10.00% Test diagrams of various electrical performance parameters of the strain sensor; Figure 9 The PXSL prepared in Example 2 of this invention and placed for seven days 10.00% Electrical performance test diagram of strain sensor; Figure 10 This is a diagram showing the structural composition and signal transmission framework of the flexible array sensor based on the PXSL strain sensor in this invention. Figure 11 This is a diagram showing the overall connection frame of the smart insole based on the PXSL strain sensor for three-point pressure sensing on the sole of the foot in this invention. Figure 12 This is a real-time foot pressure distribution monitoring diagram from the APP of the smart insole based on the PXSL strain sensor and three-point pressure sensing on the sole of the foot in this invention. Detailed Implementation

[0016] The present invention will be further described in detail below with reference to specific embodiments. The following embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Experimental operations and reagent use not described in detail in the present invention are all conventional technical means in the field.

[0017] Example 1 A lithium aluminum iron layered double hydroxide reinforced nano-conductive hydrogel (PXSL) 10.00% The fabrication method of the strain sensor, with specific steps as follows: (1) Add 1g lithium chloride, 14.4g urea and 0.7g ferric chloride hexahydrate to 100mL deionized water, slowly add 3g aluminum chloride in a fume hood and mechanically stir for 15 minutes until completely dissolved to obtain Li / Al / Fe-LDH precursor solution. (2) Place the precursor solution in a reflux apparatus and reflux at 97°C for 2 hours. Monitor the reaction process with a red laser pointer. Stop the reaction when the Tyndall effect is about to disappear to obtain a uniformly dispersed Li / Al / Fe-LDH solution. (3) Measure 8 mL of the above Li / Al / Fe-LDH solution and mix it with 12 mL of deionized water to a total volume of 20 mL. Then add 3 g of acrylamide monomer, 0.15 g of xanthan gum powder and 0.03 g of sodium alginate powder in sequence. Stir magnetically for 6 hours until completely dissolved. Sonicate at 10 kHz for 15 minutes in an ice-water bath at 0 °C to remove air bubbles and obtain a homogeneous mixture. (4) Add 9 mg of photoinitiator 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylphenylacetone and 2.4 mg of crosslinking agent N,N-methylenebisacrylamide to the mixture, stir magnetically for 30 minutes, and degas under vacuum for 1 hour to obtain PXSL10.00% hydrogel precursor solution. (5) The precursor liquid was quickly transferred to the PTFE mold, sealed with a glass slide to isolate it from the air, and irradiated under 20W, 365nm ultraviolet light for 30 minutes to carry out in-situ polymerization to obtain cylindrical PXSL10.00% nano-conductive hydrogel. (6) Mix PDMS A gel and B gel at a volume ratio of 10:1, stir mechanically for 2 hours, and degas by sonication at 10kHz for 30 minutes to obtain PDMS precursor solution. (7) Inject the PDMS precursor liquid into the PTFE mold and cure it in a vacuum oven at 50°C for 12 hours to obtain a PDMS film. (8) Cut the PDMS film into 3cm×3cm squares and combine them with PXSL. 10.00% The hydrogel was assembled into a sandwich structure, with copper wires fixed at both ends to produce PXSL. 10.00% Nanoconductive hydrogel strain sensor.

[0018] Example 2 A lithium aluminum iron layered double hydroxide-reinforced conductive nanogel (PXSL) that has been left at room temperature for 7 days 10.00% The fabrication method of the strain sensor is as follows: (1) Dissolve 1g lithium chloride, 14.4g urea and 0.7g ferric chloride hexahydrate in 100mL deionized water, slowly add 3g aluminum chloride, and mechanically stir for 15 minutes until completely dissolved to obtain Li / Al / Fe-LDH precursor solution. (2) The precursor solution was refluxed at 97°C for 2 hours, and the Tyndall effect was monitored in real time. After the reaction was completed, a uniformly dispersed Li / Al / Fe-LDH solution was obtained. (3) Take 8 mL of Li / Al / Fe-LDH solution and mix it with 12 mL of deionized water to make 20 mL. Add 3 g of acrylamide, 0.15 g of xanthan gum and 0.03 g of sodium alginate. Stir magnetically for 6 hours and sonicate at 0 °C and 10 kHz for 15 minutes to remove bubbles to obtain a mixture. (4) Add 9 mg of photoinitiator and 2.4 mg of crosslinking agent, stir for 30 minutes, and degas under vacuum for 1 hour to obtain PXSL10.00% hydrogel precursor solution; (5) The precursor liquid was injected into the PTFE mold and irradiated with 20W, 365nm ultraviolet light for 30 minutes to obtain PXSL10.00% nano-conductive hydrogel by in-situ polymerization. (6) PDMS A and B are mixed at a volume ratio of 10:1, stirred for 2 hours, degassed by ultrasonication at 10kHz for 30 minutes, and cured at 50℃ for 12 hours to obtain PDMS film. (7) Mix a 3cm×3cm PDMS film with PXSL 10.00% Hydrogels are assembled into a sandwich structure to fix copper wires to obtain a strain sensor; (8) Place the obtained sensor in a ventilated environment at 25°C for 7 days for later use.

[0019] Comparative Example 1 (1) Take 20 mL of deionized water and put it in a beaker. Add 3 g of acrylamide monomer to it and stir magnetically for 6 hours until completely dissolved. (2) Remove air bubbles by sonication at 10kHz for 15 minutes in an ice-water bath at 0℃ to obtain a transparent and homogeneous solution; (3) Add 9 mg of photoinitiator 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylphenylacetone and 2.4 mg of crosslinking agent N,N-methylenebisacrylamide to the solution, stir magnetically for 30 minutes, and degas under vacuum for 1 hour to obtain pure PAM hydrogel precursor solution. (4) The precursor liquid was quickly transferred to the PTFE mold, sealed with a glass slide to isolate it from the air, and irradiated under 20W, 365nm ultraviolet light for 30 minutes to carry out in-situ polymerization to obtain a cylindrical pure PAM hydrogel.

[0020] Comparative Example 2 (1) Take 20 mL of deionized water and put it in a beaker. Add 3 g of acrylamide monomer and 0.15 g of xanthan gum powder in sequence. Stir magnetically for 6 hours until completely dissolved. (2) Remove air bubbles by sonication at 10kHz for 15 minutes in an ice-water bath at 0℃ to obtain a transparent viscous solution; (3) Add 9 mg of photoinitiator 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylphenylacetone and 2.4 mg of crosslinking agent N,N-methylenebisacrylamide to the solution, stir magnetically for 30 minutes, and degas under vacuum for 1 hour to obtain PX hydrogel precursor solution. (4) The precursor liquid was quickly transferred to the PTFE mold, sealed with a glass slide to isolate it from the air, and irradiated under 20W, 365nm ultraviolet light for 30 minutes to carry out in-situ polymerization to obtain a cylindrical PX hydrogel.

[0021] Comparative Example 3 (1) Take 20 mL of deionized water and put it in a beaker. Add 3 g of acrylamide monomer, 0.15 g of xanthan gum powder and 0.03 g of sodium alginate powder in sequence. Stir magnetically for 6 hours until completely dissolved. (2) Remove air bubbles by sonication at 10kHz for 15 minutes in an ice-water bath at 0℃ to obtain a transparent viscous solution; (3) Add 9 mg of photoinitiator 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylphenylacetone and 2.4 mg of crosslinking agent N,N-methylenebisacrylamide to the solution, stir magnetically for 30 minutes, and degas under vacuum for 1 hour to obtain PXS hydrogel precursor solution. (4) The precursor liquid was quickly transferred to the PTFE mold, sealed with a glass slide to isolate it from the air, and irradiated under 20W, 365nm ultraviolet light for 30 minutes to carry out in-situ polymerization to obtain a cylindrical PXS hydrogel.

[0022] Comparative Example 4 (1) Dissolve 1g lithium chloride, 14.4g urea and 0.7g ferric chloride hexahydrate in 100mL deionized water, slowly add 3g aluminum chloride, and mechanically stir for 15 minutes until completely dissolved to obtain Li / Al / Fe-LDH precursor solution. (2) The precursor solution was refluxed at 97°C for 2 hours, and the Tyndall effect was monitored in real time. After the reaction was completed, a uniformly dispersed Li / Al / Fe-LDH solution was obtained. (3) Measure 2.4 mL of the above Li / Al / Fe-LDH solution and mix it with 17.6 mL of deionized water to a total volume of 20 mL. Then add 3 g of acrylamide monomer, 0.15 g of xanthan gum powder and 0.03 g of sodium alginate powder in sequence. Stir magnetically for 6 hours until completely dissolved. Sonicate at 10 kHz for 15 minutes in an ice-water bath at 0 °C to remove bubbles and obtain a homogeneous mixture. (4) Add 9 mg of photoinitiator and 2.4 mg of crosslinking agent to the mixture, stir magnetically for 30 minutes, and degas under vacuum for 1 hour to obtain PXSL. 3.00% Hydrogel precursor solution; (5) The precursor solution was quickly transferred to the PTFE mold, sealed with a glass slide to isolate it from air, and irradiated under 20W, 365nm ultraviolet light for 30 minutes to carry out in-situ polymerization, resulting in cylindrical PXSL. 3.00% Nano-conductive hydrogel; (6) PDMS A and B are mixed at a volume ratio of 10:1, stirred for 2 hours, degassed by ultrasonication at 10kHz for 30 minutes, and cured at 50℃ for 12 hours to obtain PDMS film. (7) Mix a 3cm×3cm PDMS film with PXSL 3.00%A PXSL3.00% nano-conductive hydrogel strain sensor was fabricated by assembling the hydrogel into a sandwich structure and fixing copper wires.

[0023] Comparative Example 5 (1) Dissolve 1g lithium chloride, 14.4g urea and 0.7g ferric chloride hexahydrate in 100mL deionized water, slowly add 3g aluminum chloride, and mechanically stir for 15 minutes until completely dissolved to obtain Li / Al / Fe-LDH precursor solution. (2) The precursor solution was refluxed at 97°C for 2 hours, and the Tyndall effect was monitored in real time. After the reaction was completed, a uniformly dispersed Li / Al / Fe-LDH solution was obtained. (3) Measure 16 mL of the above Li / Al / Fe-LDH solution and mix it with 4 mL of deionized water to a total volume of 20 mL. Then add 3 g of acrylamide monomer, 0.15 g of xanthan gum powder and 0.03 g of sodium alginate powder in sequence. Stir magnetically for 6 hours until completely dissolved. Sonicate at 10 kHz for 15 minutes in an ice-water bath at 0 °C to remove air bubbles and obtain a homogeneous mixture. (4) Add 9 mg of photoinitiator and 2.4 mg of crosslinking agent to the mixture, stir magnetically for 30 minutes, and degas under vacuum for 1 hour to obtain PXSL. 20.00% Hydrogel precursor solution; (5) The precursor solution was quickly transferred to the PTFE mold, sealed with a glass slide to isolate it from air, and irradiated under 20W, 365nm ultraviolet light for 30 minutes to carry out in-situ polymerization, resulting in cylindrical PXSL. 20.00% Nano-conductive hydrogel; (6) PDMS A and B are mixed at a volume ratio of 10:1, stirred for 2 hours, degassed by ultrasonication at 10kHz for 30 minutes, and cured at 50℃ for 12 hours to obtain PDMS film. (7) Assemble a 3cm×3cm PDMS film with PXSL 20.00% hydrogel to form a sandwich structure, fix copper wires, and obtain PXSL. 20.00% Nanoconductive hydrogel strain sensor.

[0024] This invention systematically characterized and tested the hydrogels and sensors prepared in Examples 1, 2, and Comparative Examples 1–6. The tests included microstructure, chemical structure, crystal structure, mechanical properties, electrical sensing performance, and environmental stability. All tests employed conventional techniques in the field. The results are as follows: 1. Microstructure: SEM and TEM tests showed that the pure PAM hydrogel had rough pore walls and large pores; after the introduction of xanthan gum and sodium alginate, the pore structure gradually became denser; after the addition of Li / Al / Fe-LDH, the pore density of the hydrogel was significantly increased, and the Li / Al / Fe-LDH nanosheets were uniformly dispersed in the polymer network without obvious agglomeration, forming a stable conductive and reinforcing structure.

[0025] 2. Chemistry and Crystal Structure: FT-IR tests confirmed the formation of hydrogen bonds and metal coordination bonds between PAM, XG, SA and Li / Al / Fe-LDH. XRD tests showed that the Li / Al / Fe-LDH crystal structure was fully embedded in the hydrogel network, proving the successful construction of a multi-component synergistic network.

[0026] 3. Mechanical properties: Tensile tests show that pure PAM hydrogel has low fracture strength and limited strain; after synergistic reinforcement with XG, SA and Li / Al / Fe-LDH, PXSL10.00% hydrogel has a fracture strength of 435 kPa and a fracture strain ≥1875%, balancing high strength and high ductility; Compression tests show that the hydrogel has excellent elasticity and outstanding creep resistance.

[0027] 4. Electrical sensing performance: The PXSL10.00% sensor has a strain range of 0–500% with GF=4.82 and a pressure range of 0–7.5kPa with S=0.107kPa⁻¹. The tensile response is ≤150ms and the compressive response is ≤180ms. After 3000 cycles, the signal stability is ≥95%. The sensitivity, response speed and cycle stability are significantly better than the comparative sample.

[0028] 5. Environmental stability: The sensor placed at room temperature for 7 days in Example 2 showed no significant degradation in mechanical and electrical properties, proving that PDMS packaging can effectively ensure the long-term stable use of the sensor and meet the needs of actual wearable scenarios.

[0029] Test results fully demonstrate that this invention successfully solves the problems of weak mechanical properties, low sensitivity, and poor stability of traditional conductive hydrogels by synergistic enhancement of Li / Al / Fe-LDH and sodium alginate and in-situ polymerization to construct a multi-network structure. The sensor produced has excellent performance and strong integration, and can be widely used in the field of flexible wearable health monitoring.

Claims

1. A method for preparing a lithium-aluminum-iron layered double hydroxide-reinforced nano-conductive hydrogel strain sensor, characterized in that, Includes the following steps: (1) Lithium chloride, aluminum chloride, ferric chloride hexahydrate and urea were mixed and dissolved in deionized water and stirred evenly to obtain lithium aluminum iron layered double hydroxide precursor solution. (2) The precursor solution was placed in a reflux device and reacted at a constant temperature. The Tyndall effect was monitored to obtain a uniformly dispersed lithium aluminum iron layered double hydroxide solution. (3) Take lithium aluminum iron layered double hydroxide solution and deionized water, add acrylamide, xanthan gum and sodium alginate, stir to dissolve, and then use ice water bath to ultrasonically remove bubbles to obtain a mixed solution; (4) Add photoinitiator 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylphenylacetone and crosslinking agent N,N-methylenebisacrylamide to the mixture, stir and degas under vacuum to obtain hydrogel precursor solution; (5) The precursor liquid was injected into a polytetrafluoroethylene mold and polymerized in situ by ultraviolet light irradiation to obtain PXSL nano-conductive hydrogel. (6) Mix polydimethylsiloxane A glue and B glue, stir, degas by ultrasonication and then cure to obtain PDMS film; (7) The PDMS film and PXSL nano-conductive hydrogel were assembled into a sandwich structure, and copper wires were fixed to obtain a strain sensor.

2. The preparation method according to claim 1, characterized in that, In step (1), the mass-to-volume ratio of lithium chloride, aluminum chloride, ferric chloride hexahydrate, urea and deionized water is 1:3:0.7:14.4:100 (g:g:g:g:mL), and the mechanical stirring time is 15 minutes.

3. The preparation method according to claim 1 or 2, characterized in that, In step (2), the reflux reaction temperature is controlled at 97°C, the total reaction time is 2 hours, the solution is irradiated with a red laser pen to monitor the Tyndall effect, and the reaction is terminated when the Tyndall effect is about to disappear.

4. The preparation method according to claim 1, characterized in that, In step (3), the total volume of the lithium aluminum iron layered double hydroxide solution and deionized water is 20 mL, and the mass ratio of acrylamide, xanthan gum and sodium alginate is 3:0.15:0.

03.

5. The preparation method according to claim 1 or 4, characterized in that, In step (3), the magnetic stirring time is 6 hours, the ice water bath temperature is 0℃, the ultrasonic frequency is 10kHz, and the ultrasonic treatment time is 15 minutes.

6. The preparation method according to claim 1, characterized in that, In step (4), the ratio of photoinitiator 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylphenylacetone, crosslinking agent N,N-methylenebisacrylamide and acrylamide is 9:2.4:3 (mg:mg:g), the magnetic stirring time is 30 minutes, and the vacuum degassing time is 1 hour.

7. The preparation method according to claim 1, characterized in that, In step (5), the ultraviolet light power is 20W, the wavelength is 365nm, and the irradiation polymerization time is 30 minutes; the tensile fracture strain of the obtained PXSL nano-conductive hydrogel is not less than 1875%, and the tensile fracture strength is not less than 435kPa.

8. The preparation method according to claim 1, characterized in that, In step (6), the volume ratio of polydimethylsiloxane A glue to B glue is 10:1, the mechanical stirring time is 2 hours, and the ultrasonic degassing frequency is 10kHz for 30 minutes; in step (7), the curing temperature of the PDMS film is 50℃ and the curing time is 12 hours.

9. The preparation method according to claim 1, characterized in that, In step (7), the PDMS film is a 3cm × 3cm square; the strain sensor has a tensile sensitivity GF = 4.82 in the strain range of 0-500% and a sensitivity S = 0.107kPa in the pressure range of 0-7.5kPa. -1 The tensile response time is ≤150ms, the compression response time is ≤180ms, and the signal stability retention rate is not less than 95% after 3000 tensile / compression cycles.

10. A lithium-aluminum-iron layered double hydroxide-reinforced nano-conductive hydrogel strain sensor prepared by any one of the preparation methods described in claims 1-9, characterized in that, The sensor has a sandwich structure consisting of an upper PDMS film, a middle PXSL nano-conductive hydrogel, and a lower PDMS film. Copper wires are fixed at both ends to form a conductive path, and it can be assembled into a flexible array sensor or a foot pressure sensing smart insole.

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