Moisture-stable adherent conductive hydrogel and methods of making and using the same

Abstract

The present application relates to a kind of wet stable adhesion type conductive hydrogel and its preparation method and application method, belong to functional polymer materials and flexible electronic technical field.The method is aimed at solving the problem of poor adhesion and electrochemical stability of traditional conductive hydrogel in low temperature freezing, humid environment.First, tannic acid coated cellulose nanocrystals TA@CNCs are pre-assembled with part of acrylamide monomer to form an interface layer, then lithium chloride, the rest of the monomer is added to construct the main precursor solution, then a parallel direct current field is applied on the substrate to make TA@CNCs directional arrangement, finally, gradient polymerization process is combined by ultraviolet light curing and heat curing.The hydrogel prepared by the method has excellent anti-freezing, wet adhesion, high conductivity and mechanical properties, mainly used as wearable strain sensor, suitable for health monitoring and human-computer interaction in extreme environment.

Description

Technical Field

[0001] This invention relates to the fields of functional polymer materials and flexible electronics, and more specifically, to a moisture-resistant, stable, adhesive conductive hydrogel, its preparation method, and its application method. Background Technology

[0002] Hydrogels, due to their high water content, good biocompatibility, and flexibility, have shown great potential in fields such as flexible electronics and wearable sensing. When used as strain sensors, their core function is to convert mechanical deformation into measurable electrical signals, requiring materials to possess good conductivity, mechanical properties, and stable integration with the detection interface. However, in practical applications, especially in complex environments, hydrogel-based sensors still face several key technological challenges.

[0003] First, water has a high freezing point, causing traditional hydrogels to freeze easily at low temperatures. Once ice crystals form inside, the material loses its flexibility drastically, becoming brittle and fragile. Simultaneously, impaired ion transport leads to a significant decrease or even loss of electrical conductivity. This severely limits its application in cold climates or special low-temperature environments (such as cold chain monitoring and high-altitude operations). While introducing organic solvents or high-concentration salts can lower the freezing point, the former may raise biosafety concerns and hinder ion migration, while the latter may interfere with polymer network formation, negatively impacting mechanical properties.

[0004] Secondly, to achieve reliable adhesion to the skin or device substrate, hydrogels need to possess sufficient adhesive strength. Many studies have attempted to improve this performance by introducing biomimetic adhesive molecules such as tannins. However, during actual wear, skin sweat secretion or changes in environmental humidity can form a hydration layer, severely weakening the wet adhesive forces based on interactions such as hydrogen bonds. This leads to sensor displacement or detachment, and unstable signal acquisition. Developing durable and robust adhesive interfaces that can withstand moisture interference remains a challenge.

[0005] Furthermore, the long-term electrical signal stability of the sensor depends on the reliability of the electrode-gel interface. When hydrogels are combined with metal electrodes for long-term physiological monitoring, electrolyte-rich sweat or tissue fluid continuously permeates to the interface. This complex bodily fluid environment can cause electrochemical corrosion of the metal electrodes and lead to unnecessary redox side reactions at the interface. In addition, skin metabolites such as proteins and oils gradually accumulate at the interface, potentially physically blocking effective electrical contact and causing a slow decline in sensitivity. Ensuring that this interface maintains stable electrochemical properties and long-term resistance to biofouling under dynamic, humid physiological conditions is crucial for extending the sensor's lifespan, and its achievement is quite challenging.

[0006] The aforementioned shortcomings in low-temperature adaptability, reliable adhesion in wet conditions, and long-term stability of the electrode-gel interface collectively restrict the widespread practical application of hydrogel-based flexible sensors. Summary of the Invention

[0007] One objective of this invention is to address the limitations of traditional ion-conductive hydrogels, which are prone to freezing and loss of flexibility at low temperatures, and exhibit insufficient interfacial adhesion in humid environments, thus restricting their practical application in wearable sensing. Existing antifreeze strategies, such as adding organic solvents, may compromise biocompatibility and ion conductivity, while conventional adhesion enhancement methods are prone to failure in wet conditions. Therefore, there is an urgent need for a preparation method that can simultaneously impart excellent antifreeze properties, stable adhesion in wet conditions, and high conductivity to the hydrogel.

[0008] To achieve the above objectives, the present invention provides a method for preparing a moisture-resistant, stable, and adhesive conductive hydrogel, comprising the following steps: S1. A tannic acid-coated cellulose nanocrystal TA@CNCs dispersion is provided, wherein the mass ratio of tannic acid to cellulose nanocrystals is (0.05-0.15):1; the total amount of acrylamide monomer is set to 100 parts by weight, and 10-30 parts by weight of acrylamide monomer, 0.5-2 parts by weight of the first part of the initiator, and 0.1-1.0 parts by weight of the reversible crosslinking agent containing boric acid groups are mixed with 30-150 parts by weight of the TA@CNCs dispersion in a buffer solution with a pH of 7.5-9.0, and stirred at 15-40°C in the dark for 2-12 hours to obtain a pre-assembled composite dispersion; wherein the dry weight of cellulose nanocrystals in the TA@CNCs dispersion accounts for 0.1-2.0 parts by weight of the total mass of acrylamide monomer; S2. To the pre-assembled composite dispersion obtained in step S1, add 200-300 parts by weight of lithium chloride, 350-500 parts by weight of deionized water, the remaining acrylamide monomer, 0.05-0.10 parts by weight of crosslinking agent N,N'-methylenebisacrylamide, and 0.5-2 parts by weight of the second initiator, stir until homogeneous, and obtain the final precursor solution. S3. Apply the final precursor liquid obtained in step S2 to the surface of the target substrate, and then, while the final precursor liquid is in a flowing state, apply a DC electric field for a duration of 10-120 seconds in a direction parallel to the substrate surface. S4. After completing the electric field treatment in step S3, the precursor liquid attached to the substrate surface is immediately cured: firstly, the first stage of curing is carried out under ultraviolet light irradiation at 25-40℃ and wavelength 365-405 nm for 10-30 minutes; then the temperature is raised to 45-70℃ for the second stage of thermal curing for 1-3 hours, so that the monomers are polymerized, thereby forming a composite conductive hydrogel in situ on the substrate.

[0009] When coating hydrogel precursor solutions onto irregular surfaces or performing patterned processing, the solvent evaporation can easily cause a coffee ring effect, leading to uneven filler distribution and inconsistent performance. Simultaneously, the prepolymer solution exhibits poor storage stability, making it difficult to match the polymerization rate with the processing procedure, thus affecting the yield and efficiency of large-scale production. A process optimization scheme is needed to improve the rheological properties of the precursor solution and enhance the stability of the processing window. Preferably, in the preparation method of this invention, when constructing the main conductive precursor solution, 0.5-3.0 parts by weight of agarose or gellan gum are added as a physical gelling agent; and the second initiator is composed of a thermal initiator ammonium persulfate and a photoinitiator phenyl-2,4,6-trimethylbenzoyl lithium phosphine sulfate in a mass ratio of (5-1):1; in step S3, after applying the final precursor solution to the target substrate surface, it is first allowed to stand at 30-40°C for 1-5 minutes to allow the precursor solution to form a pregel coating under the action of the physical gelling agent, and then a directional shear field or DC electric field is applied to the pregel coating.

[0010] Preferably, the reversible crosslinking agent containing boric acid groups in this invention is 3-acrylamidophenylboronic acid.

[0011] The present invention provides a composite hydrogel prepared by the above-described preparation method.

[0012] Reliably integrating high-performance hydrogel materials with electrodes and substrates into functional sensors is a major challenge. Simple lamination or physical bonding can easily lead to delamination of the electrode-gel interface and unstable contact resistance during dynamic use. An integration method is needed that can achieve in-situ integrated bonding and encapsulation of electrodes, activation layers, and hydrogels to construct a robust conductivity-impedance matching interface, ensuring long-term stability and reliability of the sensor signal. This invention provides a method for integrating a composite hydrogel prepared by the aforementioned method into a strain sensor, comprising the following electrode integration steps: The following electrode integration steps are included: E1. A sensor electrode pattern is pre-set on the surface of the target substrate. The electrode pattern includes at least one set of electrode pairs for strain sensing. An interface transition layer containing a pre-assembled composite dispersion is coated or printed in the electrode area of ​​the electrode pattern and the pre-set hydrogel coverage area, and pre-cured at 40-60°C for 30-120 seconds to form an adhesive activation layer. E2. The prepared final precursor liquid is coated or injected onto the substrate surface treated in step E1, so that the final precursor liquid covers the electrode area and contacts the adhesive activation layer; then, steps S3 and S4 are immediately executed to complete the external field-induced orientation and gradient polymerization molding in situ on the substrate, thereby integrally molding a strain sensing unit with electrode leads.

[0013] Sensors require substrates with specific properties, such as hydrophobic silicone. Adhesive layers with a single formulation struggle to achieve strong bonding on all substrates, potentially leading to problems like uncontrolled penetration, poor wetting, or circuit corrosion. A universal adhesive layer solution is needed that can dynamically adjust the formulation and process based on the substrate surface characteristics. Preferably, in the method of this invention, the interface transition layer in step E1 is prepared and applied as follows: Based on a pre-assembled composite dispersion, when the substrate is a low-surface-energy hydrophobic substrate, 0.1%-2.0% by mass of a nonionic surfactant and / or a siloxane-containing wetting agent is added to the pre-assembled composite dispersion to prepare a composite adhesive liquid, supplemented by online plasma or corona pretreatment of the substrate surface; the prepared composite adhesive liquid is then patterned or selectively coated onto a predetermined area of ​​the substrate with a dry film thickness of 1-10 μm using spraying, slot coating, or microgravure printing processes, followed by pre-curing.

[0014] Preferably, in the method of the present invention, the electrode pairs in the electrode pattern are nested in a meandering or interdigitated structure, and the ratio of the line width to the spacing of the electrodes is 1:1 to 1:5.

[0015] Preferably, in the method of the present invention, in step E2, the coating or injection is carried out by screen printing or pneumatic dispensing to form a patterned hydrogel sensing unit with a preset thickness and boundary.

[0016] Besides its use in resistive strain sensing, the dielectric properties of hydrogel materials can also be used to sense pressure. To expand its application scenarios, it is necessary to clarify how to utilize the dielectric properties of this hydrogel to construct a capacitive pressure sensor. Through structural design, the physical changes caused by pressure can be converted into measurable capacitance changes, achieving multi-dimensional mechanical signal sensing. This invention provides the application of the composite hydrogel prepared by the aforementioned method in the fabrication of a capacitive pressure sensor. The composite hydrogel serves as the dielectric layer, sandwiched between two layers of flexible conductive electrodes, at least one of which is transparent. Utilizing the characteristic that the dielectric constant of the composite hydrogel changes synchronously with its thickness under pressure, pressure is detected by measuring the capacitance change.

[0017] The present invention has at least the following beneficial effects: 1. This invention achieves excellent antifreeze properties and low-temperature adaptability. By introducing a high concentration of lithium chloride (LiCl) as an antifreeze agent into the polymer network, and combining it with a pre-assembled and externally induced ordered nanocomposite structure, the freezing point of free water is significantly reduced and ice crystal growth is inhibited. The prepared hydrogel maintains good flexibility (can be bent 180° without cracking) and considerable electrical conductivity (remaining at 10.7% of that at room temperature) even at extreme low temperatures of -80℃, solving the core problem of low-temperature embrittlement and inactivation of traditional hydrogels, and greatly expanding its application range in cold environments.

[0018] 2. This invention exhibits superior wet adhesion performance and interfacial stability. Tannic acid-coated cellulose nanocrystals (TA@CNCs) are dynamically pre-assembled and oriented in the interfacial region using a DC electric field, constructing a robust adhesive interface with multiple molecular interactions (hydrogen bonds of catechol groups, π-π stacking, etc.) and microscopic physical interlocking. This hydrogel demonstrates high and stable adhesion strength to various substrates (including hydrophobic PTFE and pigskin simulating moist skin) in humid environments, with a wet peel strength reaching 165-285 N / m, effectively solving the problem of sensor displacement and detachment in sweat or high-humidity environments.

[0019] 3. This invention exhibits high conductivity and long-term electrochemical stability. The optimized formulation (high concentration of LiCl providing charge carriers) and structure (external field-induced formation of orderly ion transport pathways) endow the hydrogel with high ionic conductivity (reaching 5.82 S / m at 25°C). Simultaneously, the gradient polymerization molding process forms a dense and stable surface layer and overall network. Combined with integrated electrode technology, an impedance-matched, permeation-resistant, and stable electrode-gel interface is constructed. After 168 hours of long-term operation in simulated sweat, the sensor resistance change rate was only +12.5%, and the sensitivity decay rate was -8.5%, demonstrating excellent long-term electrical signal stability and resistance to environmental interference.

[0020] 4. This invention possesses excellent mechanical properties, toughness, and anisotropy. Through dynamic pre-assembly, external field-induced orientation, and the introduction of a reversible crosslinking agent containing boric acid groups, a multi-level synergistic reinforcement and energy dissipation mechanism is constructed. The resulting hydrogel exhibits high tensile strength (0.156 MPa), ultra-high elongation at break (1380%), and high toughness (1.52 MJ / m). 3 This ensures the material's flexibility and durability. Furthermore, external field induction endows the material with a controllable anisotropic structure, laying the foundation for the development of directional sensitive sensors.

[0021] 5. This invention possesses highly efficient integration and patterning processing capabilities. By introducing physical gelling agents (such as agarose) and a photo-thermal composite initiation system, and optimizing the pre-gelling and settling steps, the rheological properties and storage stability of the precursor liquid are significantly improved, the coffee ring effect during the coating process is effectively suppressed, and a uniform distribution of hydrogel components and thickness is achieved, providing a feasible process window for large-scale production.

[0022] Other advantages, objectives and features of the present invention will become apparent in part from the following description, and in part from those skilled in the art through study and practice of the invention. Attached Figure Description

[0023] Figure 1Fourier transform infrared spectrum of tannic acid-coated cellulose nanocrystals (TA@CNCs) of the present invention; Figure 2 This is a SEM image of the composite conductive hydrogel of the present invention. Detailed Implementation

[0024] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments, so that those skilled in the art can implement it based on the description.

[0025] It should be noted that, unless otherwise specified, the experimental methods described in the following implementation plan are conventional methods, and the reagents and materials mentioned are commercially available. All performance tests were conducted in a standard laboratory environment (temperature 25±2℃, relative humidity 50±5%).

[0026] The main raw materials of this invention are as follows: all chemical reagents are analytical grade or higher and have not been further purified before use. Acrylamide (AAm); anhydrous lithium chloride (LiCl); tannic acid (TA, food grade); cellulose nanocrystals (CNCs, 5wt% aqueous dispersion); N,N'-methylenebisacrylamide (MBA); sodium persulfate (SPS); 3-acrylamidophenylboronic acid (AAPBA); tris(hydroxymethyl)aminomethane (Tris, biochemical reagent); hydrochloric acid (HCl); deionized water (laboratory-made, resistivity ≥18.2 MΩ·cm). Agarose (low gel temperature type, gel temperature 35-37℃); gellan gum (high acyl type, gel temperature 30-40℃); ammonium persulfate (APS); lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP, photoinitiator); sodium chloride (NaCl); anhydrous calcium chloride (CaCl2). Medical-grade silicone elastomer (Sylgard 184, Dow Corning, USA, base adhesive to curing agent mass ratio 10:1); flexible printed circuit (FPC) substrate (polyimide film, 50 μm thick, 18 μm copper plating on one side); silver paste conductive ink (curing temperature 120-150℃); poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate dispersion (PEDOT:PSS); polydimethylsiloxane prepolymer (PDMS); isopropanol (IPA) and nitrogen.

[0027] The main equipment is as follows: precision electronic balance (accuracy 0.1 mg); magnetic stirrer; precision pH meter; constant temperature water bath; vacuum drying oven; ultraviolet light curing system (UVA, main wavelength 365 nm, light intensity 15 mW / cm²). 2DC high-voltage power supply (0-30kV adjustable); electric tensile testing machine (maximum load 100 N); digital bridge (frequency range 4 Hz-8 MHz); scanning electron microscope (accelerating voltage 5-15 kV); Fourier transform infrared spectrometer (4000-400 cm⁻¹). -1 Low-temperature constant temperature chamber (-80℃ to 150℃); plasma cleaner (30 W power); screen printing machine; hot press (temperature range RT-300℃); laser cutting machine; 3D profilometer; electrochemical workstation (frequency range 10 μHz-1 MHz); environmental test chamber (temperature -70℃~150℃, humidity 20-98%RH).

[0028] Example 1: Preparation and Properties of Moisture-Resistant Stable Adhesive Conductive Hydrogel 1. Prepare the raw materials.

[0029] 2. Preparation of tannic acid-coated cellulose nanocrystals (TA@CNCs) Accurately weigh 1.21 g of Tris into a 50 mL beaker, add 10 mL of deionized water, and stir magnetically until completely dissolved to obtain a 1 M Tris-HCl buffer (pH=8.5, solution A). Take 30 g of CNCs dispersion (5 wt%, equivalent to 1.5 g dry weight CNCs) into a 100 mL three-necked flask, add 20 mL of deionized water to dilute, and stir magnetically at room temperature for 30 minutes until uniformly dispersed (solution B). Add solution A dropwise to solution B to adjust the pH to 8.5, then add 0.15 g of tannic acid (TA to CNCs mass ratio of 0.1:1). Stir the mixture magnetically at room temperature for 6 hours, keeping it away from light. After the reaction is complete, a uniform pale yellow TA@CNCs dispersion is obtained, in which the CNCs concentration is 3.6 wt%, and the TA@CNCs to CNCs dry weight ratio is 1:1.

[0030] The TA@CNCs dispersion was freeze-dried, and then the freeze-dried sample and TA were separately mixed with potassium bromide and compressed into tablets using a tablet press. Fourier transform infrared spectroscopy was then used for detection, with a scanning range of 400-4000 cm⁻¹. -1 The result is as follows Figure 1 As shown, Fourier transform infrared spectroscopy confirmed the success of the surface modification of cellulose nanocrystals. The modified TA@CNCs showed significantly more new stretching vibration peaks compared to the unmodified CNCs, including aromatic groups at 1539 cm⁻¹. -1 and 1602 cm -1 Stretching vibration peak.

[0031] 3. Preparation of moisture-resistant, stable, adhesive conductive hydrogels 3.1 Dynamic pre-assembled adhesion interface layer (step S1) The total mass of acrylamide monomer was set at 3.0 g (100 parts by weight), and the total mass of sodium persulfate (SPS) initiator was 30 mg (1 part by weight). The following steps were performed: 20 parts by weight (0.6 g) of acrylamide monomer were placed in a 50 mL amber glass bottle. 0.5 parts by weight (15 mg) of sodium persulfate (SPS) were added as the first initiator. 0.5 parts by weight (15 mg) of 3-acrylamidophenylboronic acid (AAPBA) was added as a reversible crosslinking agent containing boric acid groups. 100 parts by weight (3.0 g) of TA@CNCs dispersion (where the dry weight of CNCs was 0.36 parts by weight, or 10.8 mg, accounting for 0.36% of the total mass of AAM) was added. 5 mL of Tris-HCl buffer solution at pH 8.5 was added. The mixture was magnetically stirred in a 25°C water bath in the dark at 300 rpm for 6 hours. During the reaction, the system gradually changed from pale yellow to light brown, and the viscosity increased slightly. After the reaction was completed, a uniform pre-assembled complex dispersion was obtained.

[0032] 3.2 Constructing the main conductive precursor solution (step S2) To the pre-assembled complex dispersion described above, the following components were added in sequence: lithium chloride: 250 parts by weight (7.5 g); deionized water: 400 parts by weight (12.0 mL); remaining acrylamide monomer: 80 parts by weight (2.4 g); crosslinking agent N,N'-methylenebisacrylamide: 0.08 parts by weight (2.4 mg). 0.5 parts by weight (15 mg) of sodium persulfate was added as the second initiator (i.e., 15 mg of the first initiator, 15 mg of the second initiator, and a total of 30 mg of initiator). After each addition, the mixture was magnetically stirred for 5 minutes until completely dissolved. A clear and transparent final precursor solution was obtained, with a total volume of approximately 20 mL.

[0033] 3.3 External field-induced orientation of interfacial nanostructures (step S3) Take two glass slides, each 5 cm × 2 cm, as substrates, place them parallel to each other with a 2 mm gap. Place a 1 mm thick PTFE gasket between the two glass slides as a spacer. Use a syringe to inject the final precursor solution into the gap between the two glass slides, forming a uniform liquid film. Immediately connect the assembled device to a DC high-voltage power supply. Connect the positive electrode to the ITO coating on the outer surface of one glass slide, and the negative electrode to the ITO coating on the outer surface of the other glass slide. Apply a DC electric field with a strength of 500 V / mm (i.e., 1000 V between the two glass slides) for 60 seconds. The direction of the electric field should be parallel to the glass slide surfaces.

[0034] 3.4 In-situ gradient aggregation and combination (step S4) After the electric field treatment was completed, the device was immediately transferred to the ultraviolet curing system. It was then irradiated with 365nm ultraviolet light at 30°C with a light intensity of 15 mW / cm². 2 The first stage of curing was performed by irradiation for 20 minutes. The device was then transferred to a forced-air drying oven for the second stage of thermal curing at 60°C for 2 hours. After curing, the two glass plates were carefully separated to obtain a composite conductive hydrogel film with a thickness of approximately 1 mm, denoted as PLCT-ES (where ES represents electric field shear).

[0035] The microstructure of the composite conductive hydrogel film was observed using scanning electron microscopy. The sample was quenched in liquid nitrogen, freeze-dried for 48 hours, and then sputter-coated with gold before observation. The results are as follows: Figure 2 As shown, from left to right, the SEM images of hydrogels at sizes of 500 μm, 50 μm, and 250 μm are shown. The microstructure of the hydrogels shows a porous structure with certain orientation of the pores and smooth and continuous pore walls.

[0036] 4. To verify the necessity and effectiveness of the various technical means in this embodiment, the following four comparative examples are set up: Comparative Example 1: Traditional one-pot hydrogel (DP-1) Comparative Example 1 did not undergo a pre-assembly step; all raw materials were mixed and polymerized at once. Specific steps: 3.0 g AAM, 7.5 g LiCl, 12.0 mL deionized water, 2.4 mg MBA, 0.6 g SPS, and a mixture containing 0.15 g TA and 10.8 mg CNCs (simulating TA@CNCs composition) were added to the reaction vessel at once. After magnetic stirring for 30 minutes, the mixture was poured into a mold and thermally polymerized at 60°C for 2 hours.

[0037] Comparative Example 2: Orientation without external field induction (DP-2) Except for omitting step S3 (DC electric field treatment), the rest is exactly the same as in Example 1. After the precursor liquid is injected into the mold, it is directly subjected to UV and heat curing.

[0038] Comparative Example 3: Gradient-free Aggregation (DP-3) Single thermal polymerization is used, omitting the UV curing step. Specific steps: After completing steps S1 and S2, perform electric field treatment in step S3, and then directly perform thermal polymerization at 60℃ for 2 hours, without performing stepwise curing of UV followed by thermal polymerization.

[0039] Comparative Example 4: Boric Acid-Free Dynamic Crosslinking Agent (DP-4) In step S1, 3-acrylamidophenylboronic acid (AAPBA) is not added, and the rest is exactly the same as in Example 1.

[0040] 5. Performance Testing and Result Analysis 5.1 Mechanical property testing Hydrogel samples were cut into dumbbell shapes (gauge length 20 mm, width 4 mm, thickness 1 mm) using an electric tensile testing machine at a tensile speed of 100 mm / min. Five parallel samples were tested in each group, and the average value was taken. The tests were conducted under standard laboratory conditions.

[0041] Table 1 Mechanical properties of hydrogels The results show that Example 1 exhibits the best overall mechanical properties, with tensile strength (0.156 MPa), elongation at break (1380%), and toughness (1.52 MJ / m). 3 The performance of TA@CNCs was significantly higher than all comparative examples, indicating that the material possesses both excellent flexibility and strength. This superior performance stems from the synergistic construction of a multi-level structure: First, the dynamic pre-assembly step (S1) pre-entangles TA@CNCs with some polymer chains, serving as effective reinforcement points and energy dissipation centers in the final network. Comparative example 1 (traditional one-pot method) lacked pre-assembly, resulting in a disordered structure and numerous defects, thus exhibiting relatively weaker performance. Second, the external field-induced orientation step (S3) orients TA@CNCs in the interface region, forming a biomimetic ordered structure, thereby significantly improving the tensile properties and toughness of the material. The performance degradation observed in comparative example 2 (without external field) confirms the crucial role of this step. Furthermore, the gradient polymerization molding step (S4), through a curing method that combines UV radiation followed by heat, forms a gradient network that cross-links from the surface inwards, avoiding the internal stress and fragile interfaces that may result from single rapid curing. The performance degradation of comparative example 3 (single thermal polymerization) confirms this point. Finally, the introduction of a reversible crosslinking agent containing boric acid groups helps to form dynamic covalent bonds in the polymer network, which can be reversibly broken and recombined under external force, significantly improving the toughness and damage tolerance of the material, compared to the decrease in toughness observed in Comparative Example 4 (without this crosslinking agent).

[0042] 5.2 Conductivity Test The AC impedance of the hydrogel was tested using a digital bridge at a standard environment of 25°C and a frequency of 1 kHz. The sample was cut into 10 mm × 10 mm × 1 mm cubes, and parallel silver foil electrodes were attached using a standard method. Conductivity was calculated using the formula σ = L / (R × W × T), where L = 5 mm (electrode spacing), R is the measured impedance modulus, W = 10 mm, and T = 1 mm. Each set of data was based on at least three independent samples. For low-temperature conductivity testing, the sample was treated at -80°C and then rapidly transferred to a test fixture connected to a low-temperature probe station (the sample temperature recovery during the process did not exceed 5°C), and the measurement was completed within 3 minutes.

[0043] Table 2. Conductivity and electrochemical stability of hydrogels Example 1 exhibited the highest ionic conductivity (5.82 S / m) at 25°C, maintained a respectable conductivity of 0.62 S / m even at an extreme low temperature of -80°C, and retained a conductivity retention of up to 95.2% after 1000 stretching cycles. This was mainly due to the synergistic effect of the following factors: the addition of a high concentration of lithium chloride (LiCl) provided a high density of free Li. + and Cl - This forms the material basis for achieving high ionic conductivity. The introduction of TA@CNCs and their directional arrangement under an external field constructs more regular ion transport microchannels in the polymer matrix, reducing ion migration resistance. This is demonstrated by comparing the conductivity differences between Example 1 and Comparative Example 2 (without an external field) at room temperature and low temperature. Furthermore, the dense surface layer formed by gradient polymerization effectively reduces leakage and volatilization of the internal electrolyte during long-term use or deformation, thus resulting in better cycle stability compared to Comparative Example 3. Comparative Example 1, lacking pre-assembly and external field induction, exhibits disordered distribution and easy agglomeration of conductive fillers, leading to a low-efficiency conductive network.

[0044] 5.3 Adhesion performance test The wet adhesion strength of hydrogels to various substrates was evaluated using a 180° peel test. Sample bonding and wet treatment were performed according to a standard procedure. Hydrogel samples (25 mm × 25 mm × 1 mm) were bonded to the test substrates, a pressure of 5 N was applied and held for 30 seconds, and then peeled off at a speed of 100 mm / min. Test substrates included: glass, polytetrafluoroethylene (PTFE), polyethylene terephthalate (PET), and pigskin (simulating human skin). Each data point for each substrate was based on three successful peel tests.

[0045] Table 3 Wet adhesion strength of hydrogels (unit: N / m) Example 1 demonstrates the strong adhesion of the hydrogel to various substrates in a humid environment, particularly on hydrophobic PTFE surfaces (180 N / m) and pigskin surfaces simulating moist skin (165 N / m). This is primarily due to the abundant catechol groups provided by the TA@CNCs introduced in the pre-assembly step. These groups can form strong hydrogen bonds and π-π stacking interactions with various material surfaces. A DC electric field induces the enrichment and oriented arrangement of these catechol-rich TA@CNCs in the interface region close to the substrate, greatly enhancing the physical interlocking effect. The uniform and robust overall structure formed by gradient polymerization ensures sufficient mechanical strength at the adhesion interface under stress, preventing internal damage. Comparative Example 1 exhibits weaker adhesion due to the lack of such an ordered interface design; while data from Comparative Examples 2, 3, and 4 show the significant roles of external field induction and gradient polymerization in enhancing and maintaining adhesion.

[0046] 5.4 Freeze-resistant and low-temperature performance test Hydrogel samples were stored in a -80°C chamber for 4 hours, and their flexibility and conductivity were tested immediately after removal. Low-temperature bending properties were determined using a standard method. The conductivity retention rate was the ratio of the conductivity at -80°C to the initial conductivity at 25°C. Freeze-thaw cycle test: The sample was frozen at -80°C for 4 hours, and then thawed in a standard environment at 25°C for 2 hours, which constituted one cycle. After 10 cycles, the tensile strength was tested and the retention rate was calculated. Each set of data was based on at least 3 parallel samples.

[0047] Table 4 Low-temperature properties of hydrogels The results showed that Example 1 could maintain a 180° bend without cracking at -80°C, and its electrical conductivity remained at 10.7% of that at room temperature. After 10 freeze-thaw cycles, its tensile strength retention rate was as high as 92.5%. This excellent antifreeze performance is primarily due to the introduction of a high concentration of LiCl in the formulation, whose strong ion hydration significantly lowers the freezing point of free water in the system. More importantly, the well-organized and uniform three-dimensional polymer network constructed by pre-assembly and external field induction greatly restricts the formation and growth of large-sized ice crystals, confining water within nanodomains. At the same time, the dense surface layer formed by gradient polymerization further acts as a physical barrier, protecting the internal structure. Comparative Example 1, due to its disordered network structure, has relatively weaker water binding ability and antifreeze performance; other comparative examples also confirmed the positive contributions of external field-induced structural orientation and gradient curing process to improving low-temperature stability.

[0048] In summary, Example 1 provides a complete method for preparing a moisture-resistant, stable, and adhesive conductive hydrogel. Through a synergistic process of dynamic pre-assembly, external field-induced orientation, and gradient polymerization, it successfully solves the problems of poor adhesion, low electrochemical stability, and low-temperature brittleness of traditional conductive hydrogels in humid environments. Compared to the simple one-pot method often used in the preparation of antifreeze conductive hydrogels (Comparative Example 1), this example, through innovative process design, achieves a more than three-fold increase in wet adhesion strength compared to traditional methods; maintains good flexibility and conductivity at -80℃; exhibits excellent stability under complex environments (humidity, temperature changes, mechanical deformation); and achieves structural tunability through external field induction, laying the foundation for the fabrication of anisotropic sensors.

[0049] Example 2: Optimizing hydrogel processing performance by introducing a physical gelling agent and a composite initiator system 1. Prepare the raw materials.

[0050] 2. Preparation of optimized moisture-resistant and stable adhesive conductive hydrogels 2.1 Basic steps (same as steps S1 and part S2 in Example 1). First, prepare the pre-assembled complex dispersion according to the method in Example 1.

[0051] 2.2 Constructing a main conductive precursor solution containing a physical gelling agent (optimization of step S2) Based on a total acrylamide monomer mass of 3.0 g (100 parts by weight): Take 1.5 parts by weight (45 mg) of agarose in a 50 mL beaker, add 200 parts by weight (6.0 mL) of deionized water, heat and stir in an 80°C water bath for 30 minutes until completely dissolved, and obtain a clear agarose solution. Cool to 50°C for later use.

[0052] In the pre-assembled complex dispersion, lithium chloride: 250 parts by weight (7.5 g), the remaining deionized water: 200 parts by weight (6.0 mL), the remaining acrylamide monomer: 80 parts by weight (2.4 g), and crosslinking agent MBA: 0.08 parts by weight (2.4 mg) were added sequentially. After each addition, the mixture was magnetically stirred for 5 minutes until completely dissolved.

[0053] The prepared 50°C agarose solution was slowly added to the above mixture while stirring, and the temperature was kept above 50°C to prevent the agarose from gelling prematurely.

[0054] A composite initiator system was added, comprising a photoinitiator and a thermal initiator, wherein the photoinitiator LAP was 0.1 parts by weight (3 mg), and the thermal initiator APS was 0.4 parts by weight (12 mg), with a mass ratio of LAP to APS of 1:4. The total mass of the initiator in the second part was 0.5 parts by weight (15 mg), of which LAP accounted for 20% and APS accounted for 80%.

[0055] The mixture was stirred at 50°C for another 15 minutes to obtain a homogeneous final precursor solution. This precursor solution was a low-viscosity sol (approximately 200 mPa·s) at 50°C, but its viscosity increased sharply to over 5000 mPa·s when cooled to 35°C.

[0056] 2.3 Formation of pre-gel coating and field treatment (optimization of step S3) The same glass slide apparatus as in Example 1 (5 cm × 2 cm, 1 mm spacing) was used. The final precursor solution was injected into the gap between the two glass slides using a syringe preheated to 50°C.

[0057] The key improvement step is to immediately place the device on a 35°C constant-temperature platform and let it stand for 3 minutes. During this period, the temperature of the precursor solution drops from 50°C to 35°C, the agarose network forms, and the system transforms from a fluid sol into a flexible pregel.

[0058] A DC electric field was applied to the pregel with the same parameters as in Example 1: electric field strength 500 V / mm, duration 60 seconds. Under a microscope, it could be observed that the pregel underwent local shear deformation rather than overall flow under the action of the electric field. TA@CNCs could still be reoriented along the direction of the electric field, but the orientation speed was slow (it took about 30 seconds to reach a stable orientation state).

[0059] 2.4 Gradient polymerization molding (step S4, same as in Example 1) Immediately after electric field treatment, UV curing was performed: irradiation with 365nm UV light at 35℃, with a light intensity of 15 mW / cm². 2 The curing time was 20 minutes. Infrared thermal imaging revealed that due to the low thermal conductivity of the pre-gelled material and the exothermic nature of photopolymerization, the sample center temperature rose to 45°C within the first 10 minutes, but then stabilized at around 40°C. Heating promoted the decomposition and polymerization of LAP. The apparatus was then transferred to a forced-air drying oven and cured at 60°C for 2 hours. The resulting optimized hydrogel is designated PLCT-ES-PG (PG stands for Physical Gel).

[0060] 3. Comparative Example Settings. To verify the effect of Example 2, the following comparative example was set: Comparative Example 5: No physical gelling agent was used, only the composite initiator (DP-5) was used. Except for the absence of agarose and the addition of a single dose of deionized water (400 parts by weight, 12.0 mL), the procedure was identical to Example 2. That is, there was no settling step; an electric field was applied directly after the precursor solution was injected.

[0061] Comparative Example 6: Single Initiator System (DP-6) 1.0 part by weight (30 mg) of a single thermal initiator APS was used, without the addition of LAP. All other conditions, including the physical gelling agent and the settling process, were exactly the same as in Example 2.

[0062] Comparative Example 7: No static pregelation step (DP-7) After the precursor liquid was injected, it was not allowed to stand. An electric field was immediately applied at 50°C (at which point the system was still in a sol state), followed by UV and thermal curing. Other conditions were exactly the same as in Example 2.

[0063] Comparative Example 8: Effect of Physical Gel Type (DP-8) Use gellan gum instead of agarose, adding 1.5 parts by weight (45 mg). Dissolution method: Disperse gellan gum in deionized water, heat to 90°C to dissolve, and cool to 40°C before use. Set the gelation temperature to 30°C and let stand for 3 minutes.

[0064] 4. Performance Testing and Result Analysis 4.1 Processing performance and coating uniformity test Test 1: Evaluation of resistance to coffee ring effect. Precursor solutions from Examples 2 and Comparative Examples 5-8 were applied to 5 cm × 5 cm hydrophobic PET films using a dispensing machine (needle inner diameter 0.33 mm, dispensing height 1 mm, pressure 0.2 MPa) with 1 cm diameter and 5 cm volume (1 cm diameter, 50 μL volume) respectively. The gel distribution at the edges and center was observed during drying and curing. After curing, the thickness was measured at at least five points at equal intervals along the sample diameter using a three-dimensional profilometer, and the ratio of the average thickness at two edge points to the thickness at the center point was calculated. Surface roughness Ra was calculated according to ISO 4287 standard on the same measurement profile.

[0065] Table 5 Evaluation of Coating Uniformity Test results show that Example 2 exhibits excellent coating uniformity, with an edge-to-center thickness ratio of only 1.05 ± 0.03 after curing, and the lowest surface roughness (Ra = 0.52 μm). This fully demonstrates that the introduced physical gelling agent (agarose) and the static pre-gelling step play a crucial role in suppressing the coffee ring effect. This is because after the precursor liquid is coated, the temperature drops below the gel point of agarose during the static process, and the rapidly formed three-dimensional physical network locks the spatial distribution of solid components such as TA@CNCs, effectively resisting capillary flow from the center to the edge caused by solvent evaporation, thereby achieving a uniform distribution of components and thickness. Comparative Example 5 (without physical gelling agent) shows non-uniformity due to the lack of this physical locking mechanism. Data from Comparative Example 7 (without static step) indicates that even with the presence of a physical gelling agent, the optimal uniformity cannot be achieved if the static step is omitted, preventing the physical network from fully forming.

[0066] Test 2: Prepolymer stability test. The prepared precursor solutions were stored at 25°C and 4°C in sealed glass bottles. Every hour, a sample was taken out and its viscosity was measured using a rotational viscometer (LV-3 rotor, 60 rpm) at a constant temperature of 25°C for 12 hours or until gelation. Viscosity tests under each storage condition were performed on three independently prepared precursor solution samples.

[0067] Table 6. Storage stability of precursor solutions (viscosity change rate, %) The precursor solution in Example 2, after being stored at 4°C for 8 hours, showed a viscosity increase of only 10%, exhibiting a good operating window and meeting the requirements for the storage period of the prepolymer solution in actual production. This improved stability stems from the combined effect of the physical gel network and the chemical initiation system: the physical gel network initially forms at the storage temperature (especially 4°C), playing a stabilizing role in the prestructure and limiting the movement of molecular chains and premature chemical crosslinking; simultaneously, the design of the photo / thermal composite initiator, especially the photoinitiator LAP, is very stable under light-protected storage conditions, further delaying the polymerization reaction. In contrast, the viscosity of Comparative Example 5 (without a physical gelling agent) increased and gelled in a short time, indicating that the chemical initiation system alone cannot effectively inhibit the reaction progress during storage, resulting in a short operating window. The stability of Comparative Example 6 (single thermal initiator) was also poor, indicating that the photoinitiator component in the composite initiator, which is stable at low temperatures, is crucial for extending the storage period.

[0068] 4.2 Material property testing Test 3: Mechanical Properties and Recovery. Tensile properties were tested using the same method as in Example 1, with the addition of a cyclic compression test: A cylindrical specimen (10 mm in diameter, 5 mm in height) was compressed to 50% strain at a speed of 5 mm / min on a universal testing machine, held for 10 seconds, and then unloaded. This was repeated 100 times. The thickness difference between the specimen after the 100th unloading cycle and before the 1st loading cycle was calculated, and the ratio to the original thickness was the permanent deformation rate. Self-recovery rate test: The fractured specimens were joined end-to-end and placed in a standard environment for 24 hours. The tensile strength was then tested again, and the ratio to the original strength was the self-recovery rate. Each set of data was based on three parallel specimens.

[0069] Table 7 Mechanical and recovery properties of hydrogels Mechanical tests showed that Example 2 outperformed Example 1 and other comparative examples in tensile strength, elongation at break, and especially in permanent deformation after cyclic compression (8.5%) and 24-hour self-recovery rate (92.3%). This improvement can be attributed to two aspects: firstly, the physical gel network, as a reversible physical crosslinking point, can effectively dissipate energy through destruction and recombination during material deformation, reducing the plastic deformation of the polymer backbone covalent network and thus improving elastic recovery; secondly, the UV-thermal composite initiation system achieved a gentler and more uniform gradient polymerization, reducing local stress concentration and network defects caused by excessively fast polymerization rates or uneven temperatures, resulting in a more uniform and robust final chemical network. Comparative Example 6 (single thermal initiator) had the lowest self-recovery rate, confirming that UV curing plays an irreplaceable role in forming a more elastic initial network interface layer at lower temperatures.

[0070] Test 4: Electrochemical Stability. A standard sensor (10 mm × 40 mm × 1 mm) was fabricated from hydrogel, connected to electrodes, and immersed in artificial sweat (standard formulation: 5 g / L NaCl, 1 g / L lactic acid, 1 g / L urea, pH 4.7). It was continuously operated at 37°C for 72 hours, with baseline resistance and response sensitivity measured every 12 hours. Before measurement, the sensor was removed from the sweat, gently rinsed with deionized water, and dried with nitrogen. After standing for 30 minutes under standard laboratory conditions, resistance was measured and strain calibrated (calibration strain 5%, tensile speed 10 mm / min). Sensitivity GF = (ΔR / R0) / ε.

[0071] Long-term testing in artificial sweat: each data point (initial resistance, rate of change, sensitivity decay rate) is based on continuous monitoring results from three independently packaged sensor samples.

[0072] Table 8 Long-term electrochemical stability of artificial sweat Tests conducted over 72 hours in a simulated sweat environment showed that Example 2 exhibited the best baseline resistance change rate (+8.5%) and sensitivity decay rate (-6.3%). This is attributed to the comprehensive optimization of the interface and bulk structure in its preparation process: First, the pre-gelation step ensures a stable morphology of the hydrogel precursor solution in the initial stage of contact with the electrode, resulting in a denser electrode-gel interface with fewer defects after curing, thus reducing sites for electrolyte penetration and interfacial side reactions. Second, the mild composite-initiated polymerization process reduces microcracks and residual stress within the material, defects that are prone to developing into pathways for performance degradation under long-term electrolyte immersion and electrical signal loading. Finally, the presence of a physical gel network enhances the material's resistance to swelling stress, maintaining overall structural stability. Comparative Example 6 exhibited the worst stability; the intense and rapid polymerization reaction caused by single thermal initiation more easily leads to microphase separation and structural inhomogeneity, thus exposing more problems during long-term use.

[0073] The process design in Example 2 produced a significant synergistic effect. The physical gelling agent mainly solved the rheological control and stability problems during processing; the composite initiator optimized the polymerization kinetics and improved the quality of the final network structure.

[0074] Example 3: Fabrication and Performance of a Strain Sensor Based on a Silicone Substrate 1. Prepare the raw materials.

[0075] 2. Fabrication of silicone substrate strain sensors 2.1 Pretreatment of silicone substrate and preparation of electrodes 2.1.1 Preparation of silicone substrate. Sylgard 184 base adhesive and curing agent were mixed at a mass ratio of 10:1 and magnetically stirred for 10 minutes. The mixture was placed in a vacuum drying oven and degassed at -0.1 MPa for 30 minutes until no bubbles remained. The degassed silicone was poured onto a clean glass plate and smoothed with a spatula into a uniform film with a thickness of 0.5 mm. It was cured in an oven at 80℃ for 2 hours to obtain a silicone sheet with dimensions of 10 cm × 10 cm. The silicone sheet was then cut into strips of 4 cm × 2 cm using a laser cutter for later use.

[0076] 2.1.2 Electrode pattern design and fabrication. An interdigitated electrode pattern (IDA) was designed with the following parameters: interdigitation index: 20 pairs; interdigitation length: 15 mm; interdigitation width: 200 μm (line width); interdigitation spacing: 400 μm (line width: spacing = 1:2); total electrode length: 40 mm; width: 10 mm.

[0077] A comparison of two electrode fabrication methods: 1. Screen printing silver paste method (used in Example 3): 300 mesh stainless steel screen is used, and the latex thickness is 30 μm.

[0078] Silver paste was screen-printed onto a silicone substrate to form the designed interdigitated pattern. It was then cured at 120°C for 30 minutes to form silver electrodes approximately 15 μm thick. The electrode resistance was measured with a multimeter to ensure that the resistance of each interdigitated pair was <5 Ω.

[0079] 2. Transfer method (used for comparative example): Silver electrodes are printed on PET film and then transferred to silicone through PDMS transition layer.

[0080] 2.2 Preparation and Coating of Adaptive Adhesive Activation Layer Substrate surface characteristics analysis and activation layer adaptation were performed. The surface characteristics of the silicone substrate were measured: contact angle (water): 115° ± 3° (high hydrophobicity); surface energy: approximately 22 mN / m (low surface energy); surface roughness Ra: 0.8 ± 0.2 μm (micro-roughness). The activation layer was prepared and coated according to a scheme for low surface energy hydrophobic substrates. Using the pre-assembled composite dispersion from step S1 of Example 1 as the base liquid (100g, containing 3.6g TA@CNCs, 0.108g dry weight of CNCs, and approximately 15wt% acrylamide prepolymer), 1.5% of the total mass of the nonionic surfactant Triton X-100 (1.5g) and 0.3% of the siloxane-containing wetting agent Dow Corning DC-57 (0.3g) were added, and the mixture was magnetically stirred for 30 minutes to dissolve. Subsequently, 0.8% hydroxyethyl cellulose (HEC, 0.8 g) was added as a thickener, and stirring was continued for 60 minutes to ensure full hydration, at which point the system was homogeneous and transparent. The pH was then adjusted from 8.5 to 7.2 with 1 M hydrochloric acid to approach neutrality and accommodate potential circuit compatibility, followed by equilibration and stirring for another 10 minutes. The final activated solution was a pale yellow, transparent, viscous liquid, prepared as a composite adhesive with a solid content of approximately 8.5%. The viscosity, measured at 25°C using a viscometer (rotor LV-3, 60 rpm), was 3500 ± 200 mPa·s, meeting the requirements of the 1000-5000 mPa·s range.

[0081] Before coating, the silicone substrate with printed electrodes underwent online plasma pretreatment: the substrate was placed in a plasma cleaner, oxygen was introduced (flow rate 20 sccm, chamber pressure 50 Pa), and RF power of 30 W was applied for 90 seconds. After treatment, the sample was transferred to the coating station within 5 seconds to prevent surface performance degradation. The activation layer was coated using a microgravure printing process. The prepared composite adhesive liquid was patterned and coated on the substrate at 25°C using a custom gravure roller (cell depth 25 μm, line width 150 LPI, pattern matching the electrode pattern and extending 0.5 mm on each side) at a speed of 0.5 m / min and a pressure of 0.2 MPa. After the obtained wet film was pre-cured on a hot plate at 50°C for 90 seconds, a translucent milky white activation layer with a dry film thickness of approximately 5 μm (wet film approximately 25 μm) and a thickness uniformity CV value of less than 8% was formed. This layer bonded well to the silicone substrate and could not be wiped off with fingers.

[0082] After the activation layer was prepared, the hydrogel precursor solution was coated and integrated into the substrate. The hydrogel precursor solution was the final precursor solution prepared using the optimized formulation of Example 2. Before coating, a custom silicone mold (1.0 mm thick, 22 mm × 8 mm window size, with positioning pins on the back and a positioning accuracy of ±0.1 mm) was aligned and fixed onto the prepared silicone substrate. Subsequently, a pneumatic dispensing system (0.33 mm needle inner diameter) was used to precisely coat the substrate in a zigzag path (1.0 mm line spacing) at a pressure of 0.15 MPa and a speed of 10 mm / s, controlling the dispensing amount to 280 ± 5 mg per sensor (corresponding to a hydrogel volume of approximately 250 μL). After coating, the device was immediately placed on a 35°C hot stage and left to stand for 4 minutes to allow the precursor solution to form a pre-gel coating (at this point, the storage modulus G' is approximately 85 ± 10 Pa, and the device can move without liquid flow).

[0083] Subsequently, external field induction and gradient polymerization were performed. First, a custom fixture (two parallel platinum plates, 10 mm apart and 30 mm long) was used to press on both sides of the sensor, ensuring that the electric field direction was parallel to the substrate surface. A voltage of 500 V (corresponding to a field strength of 500 V / mm) was applied for 80 seconds to induce the directional alignment of TA@CNCs (the process current decreased from the initial 15 μA to 8 μA). Immediately after the external field treatment, gradient polymerization was performed: the sample was first irradiated with ultraviolet light at 365 nm wavelength and 18 mW / cm² intensity at 35 °C for 25 minutes (the transmittance of the silicone substrate at this wavelength is 92%, which does not affect curing), completing the first stage of curing; then the sample was transferred to an oven for the second stage of thermal curing at 60 °C for 2 hours. After curing, the silicone mold was carefully removed, finally obtaining an integrated sensor, the structure of which is as follows: a silicone substrate with a thickness of 0.5 mm, an activation layer with a thickness of about 5 μm, a hydrogel sensing layer with a thickness of 1.0 mm, and the silver interdigitated electrodes are completely encapsulated within it.

[0084] 3. Scale settings To systematically evaluate the effectiveness of Example 3, four control experimental groups were set up, namely Comparative Examples 9 to 12. These comparative examples adopted different interface processing or integration strategies under strict control of single variables, aiming to provide a comparison with the complete process of Example 3.

[0085] Comparative Example 9 (denoted as DP-9) employs a conventional adhesive integration method to simulate commonly used technical solutions. The specific implementation process is as follows: First, a composite conductive hydrogel sheet with dimensions identical to the target sensor of Example 3 (approximately 40 mm long and 10 mm wide) and a thickness of 1.0 mm was prepared according to the complete method of Example 1. Then, commercial double-sided conductive tape (3M 9703) was taken and cut into a shape with the same area as the hydrogel sheet. The conductive tape was smoothly applied to a blank silicone substrate (with the same dimensions as in Example 3) that had been cleaned and dried with isopropanol, and slight pressure was applied to ensure adhesion to the substrate without air bubbles. Next, the prepared hydrogel sheet was accurately aligned and adhered to the other side of the conductive tape, and uniform pressure was applied by manual rolling to ensure full contact between the hydrogel and the tape. Finally, the entire assembly was placed in an environment of room temperature (25±2℃) and relative humidity of 50±5% for 24 hours to cure, obtaining the final sensor sample.

[0086] Comparative Example 10 (denoted as DP-10) aimed to investigate the crucial role of plasma pretreatment in the bonding effect of the activation layer. Its implementation process was almost identical to that of Example 3, the only difference being the omission of the plasma treatment steps in the online plasma pretreatment and activation layer coating. In this comparative example, the silicone substrate with the silver cross-finger electrodes was cleaned three times with a lint-free cloth moistened with isopropyl alcohol to remove visible surface contaminants before coating the activation layer, and then dried in a 50°C oven for 5 minutes. Without plasma treatment, the surface energy of the substrate did not undergo any active change. All subsequent steps, including the preparation and coating of the activation layer according to the same formulation as in Example 3, the application of the hydrogel precursor solution, external field induction, and gradient polymerization, followed the operating procedures and parameters of Example 3.

[0087] Comparative Example 11 (denoted as DP-11) was used to verify the necessity of specific formulation components (wetting agent and thickener) added for the hydrophobic substrate. This comparative example prepared a simplified activation solution whose base liquid directly used the pre-assembled composite dispersion from step S1 of Example 1, without adding any nonionic surfactants (such as Triton X-100), siloxane-containing wetting agents (such as DC-57), or thickeners (such as HEC). Therefore, the activation solution remained in its original state, with a viscosity of approximately 85 mPa·s and a pH of approximately 8.5 measured at 25°C. The silica substrate was first subjected to online plasma pretreatment according to the same parameters as in Example 3. After pretreatment, the unmodified pre-assembled dispersion was coated onto a predetermined area of ​​the substrate using the same microgravure printing process. Immediately after coating, it was pre-cured on a hot plate at 50°C for 90 seconds to form an activation layer. Subsequent hydrogel coating, field treatment, and polymerization curing steps were completely consistent with Example 3.

[0088] Comparative Example 12 (denoted as DP-12) was used to investigate the effect of completely omitting the key structural layer of the activation layer. This comparative example completely skipped the entire preparation and coating steps of the activation layer (i.e., step E1 of Example 3). First, the silicone substrate with the printed electrodes was subjected to online plasma pretreatment according to the exact same parameters as in Example 3. After pretreatment, without coating any interface transition layer, the hydrogel precursor liquid prepared according to the same formulation and method as in Example 3 was directly and precisely coated onto the plasma-treated silicone substrate and electrode surface using the same mold positioning and pneumatic dispensing process. The subsequent static pregelation, DC electric field-induced orientation, and UV-thermal gradient polymerization curing steps were all strictly performed according to the operating conditions and parameters of Example 3.

[0089] 4. Performance Testing and Result Analysis 4.1 Interface Integration Performance Testing Test 1: Adhesion reliability of different integration methods was quantified by a 180° peel strength test. The test used a universal testing machine to peel the hydrogel layer of the sensor sample from the substrate at a constant rate of 100 mm / min. Peel strength was measured under each test condition (dry, after water immersion, and after bending), with each data point based on test results from three sensor samples.

[0090] Table 9. Interfacial peel strength (N / m) Example 3 achieved a peel strength of 320±18 N / m under initial dry conditions, significantly higher than all comparative examples. After immersion in deionized water for 30 minutes, its strength remained at 285±16 N / m, demonstrating excellent water resistance. In contrast, Comparative Example 9, using conventional conductive tape, experienced a sharp decrease in strength to 65±8 N / m in the wet state, indicating significant adhesive failure. After undergoing 50 bending cycles (bending radius 5 mm) of mechanical fatigue testing, Example 3 maintained a high strength (305±17 N / m), while Comparative Examples 9 and 12 decreased to 120 N / m and 65 N / m, respectively. These data indicate that the plasma treatment + wetting agent modified activation layer strategy employed in Example 3 is key to achieving a strong, tough, dynamic fatigue-resistant, and water-interference-resistant interfacial bond.

[0091] Test 2: The samples were placed in an environmental test chamber and subjected to 100 cycles. Each cycle included three stages: -20℃ (1 hour), 25℃ (1 hour), and 60℃ with 90% relative humidity (1 hour), with a heating / cooling rate of approximately 5℃ / min. After the cycles, the samples were placed in a standard environment for 2 hours, followed by peel strength and contact resistance tests. Interface delamination was determined by visual inspection and observation with a 10x magnifying glass. The peel strength retention rate and contact resistance change rate after cycling were based on the test results of three sensor samples that underwent complete thermal and humidity cycling.

[0092] Table 10 Interface Integrity After Thermal and Humidity Cycling Example 3 exhibited a peel strength retention rate of 92.5 ± 3.2%, with no delamination observed at the interface. Simultaneously, the contact resistance between the electrode and the gel increased by only 8.5 ± 1.5%. Comparative Example 9, however, showed severe delamination, with a strength retention rate of only 35.2% and a contact resistance surge of 85.2%. Comparative Examples 10 (without plasma pretreatment) and 11 (simple activation layer) also showed varying degrees of edge or localized delamination, as well as greater resistance changes. This confirms that a complete pretreatment and a suitable activation layer are crucial for resisting interfacial stress caused by alternating temperature and humidity.

[0093] 4.2 Electrical and Sensing Performance Testing Test 3: Basic electrical performance. Initial resistance R0 was measured using a digital multimeter. Resistance uniformity was calculated as the coefficient of variation (CV%) of the initial resistances of the five sensors in the same batch. Electrode-gel contact resistance was calculated by measuring the voltage drop between the electrode leads and the gel interior using the two-point method in the four-probe method under a small current (1 mA).

[0094] Table 11 Electrical Performance Data Test 4: Strain Sensing Performance. Tensile strain was applied using a linear displacement platform, and the change in resistance was measured. The sensor sample was fixed at both ends to the platform fixture with an initial gauge length of 20 mm. Displacement was monitored in real time using a laser displacement sensor to calculate the true strain (ε = ΔL / L0). Resistance was acquired in real time using a digital bridge (sampling rate 10 Hz). Sensitivity GF was obtained by linearly fitting the slope of the strain-resistance change curve within the corresponding strain range. Response time refers to the time required from the initial application of strain (strain rate 1% / s) to the resistance change reaching 90% of the steady-state value under that step strain; recovery time refers to the time required from the initial unloading of strain to the resistance recovering to within 110% of its initial value.

[0095] Table 12 Strain sensing performance (strain range: 0-30%) The sensor of Example 3 exhibits low initial resistance (128 ± 6 Ω) and excellent inter-sheet uniformity (resistance CV value 5.2%). Its electrode-gel contact resistance is only 2.5 ± 0.3 Ω, significantly lower than the tape impedance of Comparative Example 9 (8.5 Ω) and the higher contact resistance (12.5 Ω) resulting from the lack of an activation layer in Comparative Example 12. This is attributed to the robust and impedance-matched interface formed by the activation layer.

[0096] The strain sensing performance was calibrated using a linear displacement platform. Within the strain range of 0-30% (Table 4), Example 3 exhibited high sensitivity (GF: 0.85 for 0-10% strain and 1.92 for 10-30% strain), with response and recovery times of 280 ms and 320 ms, respectively. In contrast, Comparative Example 9 showed a decline in all performance characteristics due to tape interface slippage under strain.

[0097] Test 5: Humidity Effect. At 25°C, the ambient humidity was gradually increased from 30%RH to 90%RH, and the change in resistance was measured. The sensor was placed in a programmable environmental chamber, and the resistance was measured after stabilizing for 30 minutes after each humidity setpoint change. The rate of change in resistance is a percentage of the resistance value relative to the initial humidity (30%RH). The test was based on continuous monitoring data from at least three sensor samples in a humidity-controlled environmental chamber.

[0098] Table 13 Humidity Sensitivity When the ambient humidity increased from 30%RH to 90%RH, the resistance change in Example 3 was +5.8%, the smallest change. After the humidity returned to 30%RH, the resistance recovery rate reached 94.5%, indicating that the activation layer effectively buffered the interference of the hydrogel's moisture absorption and expansion on the interfacial electrical contact.

[0099] Test 6: Long-term test in a sweat environment. Artificial sweat (pH 4.7, 37℃) was prepared, and the sensor portion was immersed (excluding the electrode area), and the system operated continuously for 168 hours (7 days). Partial immersion meant the hydrogel sensing area was submerged approximately 2 mm below the liquid surface, while the electrode leads and solder joints were sealed with waterproof silicone and kept above the liquid surface. Samples were removed every 24 hours, cleaned and dried according to the method in Test 4, and resistance and sensitivity were measured. Data are based on continuous monitoring results of three sensor samples immersed in artificial sweat.

[0100] Table 14 Long-term performance in sweat environment Example 3 remained fully functional throughout the test, with only a 12.5% ​​increase in resistance and an 8.5% decrease in sensitivity after 168 hours. Microscopic observation confirmed that there was no obvious corrosion or delamination at the interface, and the electrodes were intact. In contrast, Comparative Example 9 completely failed after 72 hours due to delamination, and Comparative Example 10 showed a 38.5% change in resistance after 168 hours, with corrosion spots appearing at the electrode edges. This fully demonstrates that the integrated system constructed in Example 3 has outstanding advantages in resisting the erosion of complex bodily fluid environments and ensuring the long-term stable operation of the sensor.

[0101] In summary, the system's performance testing and comparative analysis clearly demonstrate that the technical solution of Example 3 is superior to traditional or partially simplified integration methods in terms of interface bonding strength, electrical stability, sensing performance, and environmental tolerance, laying a solid foundation for its practical application in the field of wearable sensing.

[0102] Although embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. It can be applied to various fields suitable for the present invention. Further modifications can be readily implemented by those skilled in the art.

Claims

1. A method for preparing a moisture-resistant, stable, adhesive conductive hydrogel, characterized in that, Includes the following steps: S1. Provides a dispersion of tannic acid-coated cellulose nanocrystals TA@CNCs, wherein the mass ratio of tannic acid to cellulose nanocrystals is (0.05-0.15):1; The total amount of acrylamide monomer is set at 100 parts by weight. 10-30 parts by weight of the acrylamide monomer, 0.5-2 parts by weight of the first-part initiator, and 0.1-1.0 parts by weight of the reversible crosslinking agent containing boric acid groups are mixed with 30-150 parts by weight of the TA@CNCs dispersion in a buffer solution with a pH of 7.5-9.

0. The mixture is stirred and reacted at 15-40°C in the dark for 2-12 hours to obtain a pre-assembled composite dispersion. The dry weight of the cellulose nanocrystals in the TA@CNCs dispersion accounts for 0.1-2.0 parts by weight of the total mass of the acrylamide monomer. S2. To the pre-assembled composite dispersion obtained in step S1, add 200-300 parts by weight of lithium chloride, 350-500 parts by weight of deionized water, the remaining acrylamide monomer, 0.05-0.10 parts by weight of crosslinking agent N,N'-methylenebisacrylamide, and 0.5-2 parts by weight of the second initiator, stir until homogeneous, and obtain the final precursor solution. S3. Apply the final precursor liquid obtained in step S2 to the surface of the target substrate, and then, while the final precursor liquid is in a flowing state, apply a DC electric field for a duration of 10-120 seconds in a direction parallel to the substrate surface. S4. After completing the electric field treatment in step S3, the precursor liquid attached to the substrate surface is immediately cured: firstly, the first stage of curing is carried out under ultraviolet light irradiation at 25-40℃ and wavelength 365-405 nm for 10-30 minutes; then the temperature is raised to 45-70℃ for the second stage of thermal curing for 1-3 hours, so that the monomers are polymerized, thereby forming a composite conductive hydrogel in situ on the substrate.

2. The preparation method according to claim 1, characterized in that, In step S2, when constructing the main conductive precursor liquid, 0.5-3.0 parts by weight of agarose or gellan gum are added as a physical gelling agent; and the second part of the initiator is composed of thermal initiator ammonium persulfate and photoinitiator phenyl-2,4,6-trimethylbenzoyl lithium phosphinate in a mass ratio of (5-1):1; in step S3, after the final precursor liquid is applied to the surface of the target substrate, it is first allowed to stand at 30-40°C for 1-5 minutes to allow the precursor liquid to form a pre-gel coating under the action of the physical gelling agent, and then a DC electric field is applied to the pre-gel coating.

3. The preparation method according to claim 1 or 2, characterized in that, The reversible crosslinking agent containing boric acid groups is 3-acrylamidophenylboronic acid.

4. A composite hydrogel prepared by the preparation method according to claim 1 or 2.

5. A method for integrating a composite hydrogel prepared by the method described in claim 1 or 2 into a strain sensor, characterized in that, The following electrode integration steps are included: E1. A sensor electrode pattern is pre-set on the surface of the target substrate. The electrode pattern includes at least one set of electrode pairs for strain sensing. An interface transition layer containing a pre-assembled composite dispersion is coated or printed in the electrode area of ​​the electrode pattern and the pre-set hydrogel coverage area, and pre-cured at 40-60°C for 30-120 seconds to form an adhesive activation layer. E2. The prepared final precursor solution is coated or injected onto the substrate surface after step E1 treatment, so that the final precursor solution covers the electrode area and contacts the adhesive activation layer. Subsequently, steps S3 and S4 are immediately executed to complete the external field-induced orientation and gradient polymerization molding in situ on the substrate, thereby integrally molding a strain sensing unit with electrode leads.

6. The method for applying a strain sensor as described in claim 5, characterized in that, In step E1, the interface transition layer is prepared and applied in the following manner: Based on the pre-assembled composite dispersion, when the substrate is a low surface energy hydrophobic substrate, 0.1%-2.0% of a nonionic surfactant and / or a siloxane-containing wetting agent is added to the pre-assembled composite dispersion to prepare a composite adhesive liquid, and the substrate surface is pretreated by online plasma or corona treatment; the prepared composite adhesive liquid is applied to the preset area of ​​the substrate in a patterned or selective manner with a dry film thickness of 1-10 μm by spraying, slot coating or microgravure printing process, and then pre-cured.

7. The method for applying a strain sensor as described in any one of claims 5 or 6, characterized in that, The electrode pairs in the electrode pattern have a nested, meandering or interdigitated structure, and the ratio of the line width to the spacing of the electrodes is 1:1 to 1:

5.

8. The method for applying a strain sensor as described in any one of claims 5 or 6, characterized in that, In step E2, the coating or injection method is screen printing or pneumatic dispensing to form a patterned hydrogel sensing unit with a preset thickness and boundary.

9. The application of the composite hydrogel prepared by the preparation method according to claim 1 or 2 in the preparation of a capacitive pressure sensor, characterized in that, A composite hydrogel is used as a dielectric layer and sandwiched between two flexible conductive electrodes, at least one of which is transparent. The pressure is detected by measuring the change in capacitance, taking advantage of the property that the dielectric constant of the composite hydrogel changes synchronously with its thickness under pressure.

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