High-sensitivity conductive composite hydrogel, flexible strain sensor as well as preparation method and application of flexible strain sensor

Abstract

The invention discloses a high-sensitivity conductive composite hydrogel, a flexible strain sensor and a preparation method and application of the high-sensitivity conductive composite hydrogel. The preparation method comprises the following steps: stirring a dispersion liquid containing carboxymethyl cellulose, acrylic acid, acrylamide, a cross-linking agent and an initiator to obtain a mixed solution, and carrying out vacuum defoaming treatment on the mixed solution to obtain a defoamed liquid; the conductive composite hydrogel prepared by the preparation method disclosed by the invention is based on the synergistic effect of a dual-network / composite network structure, so that the conductive composite hydrogel provided by the invention has excellent mechanical property and conductivity.

Description

Technical Field

[0001] This invention belongs to the fields of functional polymer materials, flexible electronic devices and intelligent sensing technology, specifically relating to a highly sensitive conductive composite hydrogel, a flexible strain sensor, its preparation method and its application. Background Technology

[0002] With the rapid development of wearable electronic devices, human-computer interfaces, and soft robots, traditional rigid sensors, due to their poor ductility, are unable to adapt to the large deformations of human skin and joints, and prolonged wear can easily cause discomfort or even inflammation, limiting their application in the field of bioelectronics. Flexible strain sensors, on the other hand, have become a research hotspot because they can adapt to the complex curves and large deformations of the human body.

[0003] Among them, conductive hydrogels are considered ideal materials for preparing next-generation flexible sensors due to their excellent biocompatibility, tunable mechanical properties, and skin-like soft and moist characteristics.

[0004] However, current conductive hydrogel sensor technology still faces many challenges. First, traditional single-network hydrogels have low mechanical strength and high brittleness, making them unable to withstand significant stretching. To improve conductivity, conductive fillers are usually introduced. These rigid fillers are prone to agglomeration in the matrix, and the conductive pathways are susceptible to irreversible breakage under large strain, causing signal distortion. Second, the preparation process is complex. The preparation of many high-performance hydrogels involves multi-step polymerization, organic solvent replacement, long-term dialysis, or complex post-processing, increasing production costs and time, which is not conducive to large-scale applications. Third, water retention is poor. Ordinary hydrogels readily evaporate moisture in open environments, leading to material hardening, decreased conductivity, and loss of sensing function. Therefore, it is necessary to develop a hydrogel sensor with a simple formulation, requiring no external conductive fillers, and possessing high strength, toughness, and intrinsic conductivity, along with an efficient preparation method. Summary of the Invention

[0005] In view of the shortcomings of the existing technology, the first objective of this invention is to provide a method for preparing a highly sensitive conductive composite hydrogel with excellent mechanical properties and stable conductivity.

[0006] A second objective of this invention is to provide a highly sensitive conductive composite hydrogel prepared by the above-described preparation method.

[0007] A third objective of this invention is to provide a flexible strain sensor.

[0008] The fourth objective of this invention is to provide an application of a flexible strain sensor.

[0009] To achieve the above objectives, the present invention adopts the following technical solution:

[0010] This invention discloses a method for preparing a highly sensitive conductive composite hydrogel, which involves stirring a dispersion containing carboxymethyl cellulose with acrylic acid, acrylamide, a crosslinking agent, and an initiator to obtain a mixed solution, subjecting the mixed solution to vacuum degassing to obtain a degassed liquid, and then subjecting the degassed liquid to a thermal polymerization reaction to obtain the final product.

[0011] The conductive composite hydrogel prepared in this invention is based on the synergistic effect of a dual-network / composite-network structure. Acrylamide (AAm) forms flexible polyacrylamide (PAAm) chains through free radical polymerization, constituting the elastic backbone of the hydrogel. Carboxymethyl cellulose (CMC) serves as the natural polymer backbone, interpenetrating with the synthetic polymer (PAAm-PAAc) to form a composite network. A large number of hydrogen bonds exist within this network as sacrificial bonds. During stretching, these weak hydrogen bonds break first and dissipate energy, effectively preventing material fracture caused by stress concentration. This endows the hydrogel with extremely high ductility and toughness. Furthermore, the polyacrylic acid (PAAc) generated after the polymerization of acrylic acid (AAc) has carboxylic acid groups (-COOH) that dissociate in solution (-COO). - + H + CMC provides free ion carriers, imparting conductivity to the hydrogel, while simultaneously releasing free Na+. + These free ions, acting as carriers, provide stable intrinsic conductivity, thereby enabling the conductive composite hydrogel provided by this invention to possess excellent mechanical and electrical properties.

[0012] In a preferred embodiment, the process of obtaining the carboxymethyl cellulose dispersion is as follows: dissolving carboxymethyl cellulose in deionized water and stirring at 65℃~75℃ for 1.5~2.5h.

[0013] In actual operation, the dispersion containing carboxymethyl cellulose is cooled to room temperature, and then acrylic acid, acrylamide, crosslinking agent and initiator are added in sequence.

[0014] In a preferred embodiment, the crosslinking agent is N,N'-methylenebisacrylamide.

[0015] In a preferred embodiment, the initiator is ammonium persulfate.

[0016] In a preferred embodiment, the mass fraction of carboxymethyl cellulose in the mixed solution is 0.1% to 3.0%, preferably 2% to 2.4%.

[0017] Experiments have shown that optimal performance is achieved when the mass fraction of carboxymethyl cellulose (CMC) is controlled within this range. If the CMC content is too high, the viscosity will be too large, resulting in an overly viscous dispersion that is difficult to stir and degas. In addition, excessive carboxymethyl content will lead to excessive cross-linking, which will restrict the sliding of polymer chains and cause the ultimate strain elongation to decrease, failing to achieve the best stretching effect. If the CMC content is too low, the lack of reinforcement from the natural skeleton will cause the hydrogel to exhibit the brittleness typical of synthetic gels, with low fracture strength, and its conductivity as a strain sensor will decrease.

[0018] In a preferred embodiment, the mass ratio of acrylic acid to acrylamide in the mixed solution is 2~4:5~7.

[0019] In a preferred embodiment, the mass ratio of acrylic acid to crosslinking agent in the mixed solution is 2~4g:10~100mg.

[0020] In a preferred embodiment, the mass ratio of acrylic acid to initiator in the mixed solution is 2-4 g: 10 mg-100 mg.

[0021] In a preferred embodiment, the volume-to-mass ratio of the deionized water to the mixed solution is 3-4 mL: 1 g.

[0022] In a preferred embodiment, the stirring is carried out at room temperature for 15-30 minutes.

[0023] In a preferred embodiment, the mixed solution is subjected to vacuum degassing in a mold until no visible bubbles remain.

[0024] In a preferred embodiment, during the vacuum degassing process, the gas pressure is controlled to drop to -0.08 to -0.12 MPa. Vacuum degassing is crucial in this invention. Without it, the tiny bubbles remaining in the mixture will become pores within the gel after thermal polymerization. During high-ratio stretching, these pores become stress concentration points, inducing rapid crack propagation and causing premature gel fracture. Furthermore, the presence of bubbles disrupts the continuity of conductive pathways. Under pressure or stretching, bubble deformation causes irregular fluctuations in local resistance, resulting in noise in the resistance response signal and reducing the sensor's sensitivity and linearity.

[0025] In a preferred embodiment, the temperature of the thermal polymerization reaction is 60℃~70℃, and the time of the thermal polymerization reaction is 1.0~2.0h.

[0026] The present invention also provides a highly sensitive conductive composite hydrogel prepared by the above preparation method.

[0027] The present invention also provides a flexible strain sensor comprising the highly sensitive conductive composite hydrogel.

[0028] In a preferred embodiment, the flexible strain sensor further includes a sensing element, a conductive connector, and an insulating encapsulation layer. The conductive connector is connected to both ends of the sensing element and is used to output an electrical signal.

[0029] The encapsulation layer covers the junction of the sensing element and the conductive connector.

[0030] In a further preferred embodiment, the sensing element is elongated or dumbbell-shaped;

[0031] The conductive connector is selected from one of copper wire, metal foil or conductive fabric;

[0032] The insulating encapsulation layer is a medical breathable tape or an elastomer film.

[0033] The flexible strain sensor provided by this invention can be attached to irregular curved surfaces and generate a resistance signal that monotonically changes with strain within a tensile strain range of 0-400%.

[0034] The present invention also provides an application of a flexible strain sensor, which is applied to one of human joint motion monitoring, gesture recognition, pulse monitoring, and human-computer interaction interfaces.

[0035] Beneficial effects

[0036] The conductive composite hydrogel prepared in this invention is based on the synergistic effect of a dual-network / composite-network structure. Acrylamide (AAm) forms flexible polyacrylamide (PAAm) chains through free radical polymerization, constituting the elastic backbone of the hydrogel. Carboxymethyl cellulose (CMC) serves as the natural polymer backbone, interpenetrating with the synthetic polymer (PAAm-PAAc) to form a composite network. A large number of hydrogen bonds exist within this network as sacrificial bonds. During stretching, these weak hydrogen bonds break first and dissipate energy, effectively preventing material fracture caused by stress concentration. This endows the hydrogel with extremely high ductility and toughness. Furthermore, the polyacrylic acid (PAAc) generated after the polymerization of acrylic acid (AAc) has carboxylic acid groups (-COOH) that dissociate in solution (-COO). - + H + CMC provides free ion carriers, imparting conductivity to the hydrogel, while simultaneously releasing free Na+. + These free ions, acting as carriers, provide stable intrinsic conductivity, thereby enabling the conductive composite hydrogel provided by this invention to possess excellent mechanical and electrical properties.

[0037] The flexible strain sensor based on the conductive composite hydrogel of the present invention has at least the following advantages:

[0038] 1. It exhibits excellent linear resistance response over a wide strain range of 0-400%, and its sensitivity (GF value) increases monotonically with strain, reaching a maximum of 2.31, allowing for accurate deformation calculation without complex algorithms.

[0039] It has an extremely fast response time (approximately 127ms response time and approximately 109ms recovery time) and can capture high-frequency minute vibrations (such as pulse and voice) in real time.

[0040] 3. Excellent fatigue resistance; the hysteresis curve remains stable after multiple tensile tests at 300% strain, allowing for repeated use.

[0041] The preparation method of the present invention is simple, requiring no complex freeze-thaw cycles or organic solvent replacement, and the formulation is simple with low production cost. Attached Figure Description

[0042] Figure 1 This is a process flow diagram illustrating the fabrication and application of a highly sensitive conductive composite hydrogel flexible strain sensor according to the present invention.

[0043] Figure 2 The graph shows the change in the mass loss ratio of the hydrogels prepared in Examples 1-4 after being placed in a natural environment over time.

[0044] Figure 3 The diagram shows a comparison of the ultimate strain and fracture strength of the hydrogels prepared in Examples 1-4.

[0045] Figure 4 The stress-strain curve of the hydrogel prepared in Example 3 after 10 cycles of tensile stress-strain at 400% strain is shown.

[0046] Figure 5 The strain sensor sensitivity curve of the hydrogel prepared in Example 3 is shown.

[0047] Figure 6 The relative resistance changes of the hydrogel prepared in Example 3 under stepped strain and under different strains are shown in (a), (b), (c), (d), and (d) the resistance change rate response curves under low strain, (c), (d), and (e) the resistance change rate response curves under high strain.

[0048] Figure 7 Human strain sensing attempts were performed by attaching the hydrogel prepared in Example 3 to the human fingers, wrists, elbows and knees, wherein (a) was attached to the human fingers, (b) to the wrists, (c) to the elbows and (d) to the knees. Detailed Implementation

[0049] The following examples further illustrate the above-described content of the present invention, but it should not be construed as limiting the scope of the subject matter of the present invention to the following examples. All technologies implemented based on the above-described content of the present invention fall within the scope of the present invention.

[0050] Example 1

[0051] 1. Measure 30 ml of deionized water, then add 3 g of acrylic acid (AAc), 6 g of acrylamide (AAM), 3 mg of N,N'-methylenebisacrylamide (MBA), and 30 mg of ammonium persulfate (APS) in sequence, and stir well.

[0052] 2. Slowly pour the above solution into the mold, and then place the mold in a constant temperature drying oven preheated to 65°C for 1.5 hours.

[0053] 3. After the reaction is complete, remove the mold and allow it to cool naturally to room temperature. Peel the hydrogel sample from the mold to obtain a complete conductive hydrogel sample, labeled CMC0.

[0054] Example 2

[0055] 1. Add 0.6g of carboxymethyl cellulose (CMC) to 30 ml of deionized water. After initial mixing, transfer the beaker to a water bath preheated to 70°C and stir continuously for 2 hours until fully mixed.

[0056] 2. After the above solution has cooled to room temperature, add 3g of acrylic acid (AAc), 6g of acrylamide (AAM), 3mg of N,N'-methylenebisacrylamide (MBA), and 30mg of ammonium persulfate (APS) in sequence and stir until homogeneous.

[0057] 3. Pour the mixed solution into the mold, place it in a vacuum instrument to remove air bubbles, and after the visible air bubbles are eliminated, open the vent valve to remove the sample.

[0058] 4. Place the degassed sample along with the mold into a constant temperature drying oven preheated to 65°C and react for 1.5 hours.

[0059] 5. After the reaction is complete, remove the mold and allow it to cool naturally to room temperature. Peel the hydrogel sample from the mold to obtain a complete conductive hydrogel sample, labeled CMC6.

[0060] Example 3

[0061] 1. Add 0.9g of carboxymethyl cellulose (CMC) to 30 ml of deionized water. After initial mixing, transfer the beaker to a water bath preheated to 70°C and stir continuously for 2 hours until fully mixed.

[0062] 2. After the above solution has cooled to room temperature, add 3g of acrylic acid (AAc), 6g of acrylamide (AAM), 3mg of N,N'-methylenebisacrylamide (MBA), and 30mg of ammonium persulfate (APS) in sequence and stir until homogeneous.

[0063] 3. Pour the mixed solution into the mold, place it in a vacuum instrument to remove air bubbles, and after the visible air bubbles are eliminated, open the vent valve to remove the sample.

[0064] 4. Place the degassed sample along with the mold into a constant temperature drying oven preheated to 65°C and react for 1.5 hours.

[0065] 5. After the reaction is complete, remove the mold and allow it to cool naturally to room temperature. Peel the hydrogel sample from the mold to obtain a complete conductive hydrogel sample, labeled CMC9.

[0066] Example 4

[0067] 1. Add 1.2g of carboxymethyl cellulose (CMC) to 30 ml of deionized water. After initial mixing, transfer the beaker to a water bath preheated to 70°C and stir continuously for 2 hours until fully mixed.

[0068] 2. After the above solution has cooled to room temperature, add 3g of acrylic acid (AAc), 6g of acrylamide (AAM), 3mg of N,N'-methylenebisacrylamide (MBA), and 30mg of ammonium persulfate (APS) in sequence and stir until homogeneous.

[0069] 3. Pour the mixed solution into the mold, place it in a vacuum instrument to remove air bubbles, and after the visible air bubbles are eliminated, open the vent valve to remove the sample.

[0070] 4. Place the degassed sample along with the mold into a constant temperature drying oven preheated to 65°C and react for 1.5 hours.

[0071] 5. After the reaction is complete, remove the mold and allow it to cool naturally to room temperature. Peel the hydrogel sample from the mold to obtain a complete conductive hydrogel sample, labeled CMC12.

[0072] Performance testing:

[0073] (1) Water retention test of different gels:

[0074] The water retention capacity of the CMC hydrogels prepared in Examples 1-4 of this invention was investigated. Figure 2 As shown, the initial mass of the hydrogels prepared in Examples 1-4 is denoted as W0. They were placed in an indoor natural environment, and their weight was recorded as W every 24 hours. t Calculate the mass loss rate (W0 - W) t(W0×100%). After 6 days, the CMC0 sample hydrogel showed the greatest mass loss at 66.7%, indicating severe shrinkage and hardening. The CMC6 sample hydrogel showed a mass loss of 64.7%, the CMC12 sample hydrogel showed a mass loss of 62.9%, and the CMC9 sample hydrogel showed the smallest mass loss at 59.9%. This demonstrates that the introduction of CMC significantly improves the water retention capacity of the hydrogel, with the optimal CMC content being 0.9g.

[0075] (2) Mechanical property testing of different gels:

[0076] The mechanical properties of the CMC hydrogels prepared in Examples 1-4 of this invention were investigated. Figure 3 As shown, the hydrogels prepared in Examples 1-4 were cut into standard dumbbell-shaped strips with a thickness of 3 mm, a width of 3.5 mm, and a gauge length of 20 mm. Uniaxial tensile tests were performed using a universal tensile testing machine equipped with a 10 N sensor, with a tensile rate set to 50 mm / min. It can be seen that the CMC0 sample hydrogel exhibited a fracture strength of 0.45 MPa and an ultimate strain of 679%, demonstrating typical brittleness of synthetic hydrogels. The CMC6 sample hydrogel showed a fracture strength of 0.76 MPa and an ultimate strain of 786%, while the CMC12 sample hydrogel showed a fracture strength of 0.92 MPa and an ultimate strain of 810%. The CMC9 sample hydrogel showed a fracture strength of 0.74 MPa and an ultimate strain of 952%. This indicates that with the addition of CMC, both strength and ductility begin to improve, with the optimal CMC content being 0.9 g.

[0077] (3) Fatigue resistance test:

[0078] The fatigue resistance of the CMC9 hydrogel prepared in Example 3 of this invention was investigated. Figure 4 As shown, the hydrogel prepared in Example 3 was subjected to cyclic loading-unloading tests on a universal tensile testing machine. The maximum strain was set to 300%, and 10 consecutive cycles were performed with a tensile / recovery rate of 50 mm / min. A significant hysteresis loop was observed between the first loading and unloading curves, indicating that some weak hydrogen bonds and physical entanglements were broken during the initial tensile test, dissipating energy. The subsequent curves for the 2nd to 10th cycles largely overlapped, and the area of ​​the hysteresis loop decreased and tended to stabilize.

[0079] (4) Strain sensing performance test:

[0080] The strain sensing performance of the CMC9 hydrogel prepared in Example 3 of this invention was investigated. The hydrogel prepared in Example 3 was cut into strips of 40 mm x 10 mm x 3 mm, and both ends were fixed with dovetail clips. The dovetail clips were then fixed to a stretching machine, and resistance testing machines were connected to both ends of the dovetail clips. The stretching was performed at a constant rate, and the resistance changes were recorded simultaneously. Figure 5 As shown, the relative resistance change rate monotonically increases with increasing strain. In the strain range of 0-60%, the GF value is approximately 0.74; in the strain range of 60-100%, the GF value is 0.96; in the strain range of 100-200%, the GF value is 1.53; and in the strain range of 200-400%, the GF value is 2.31, demonstrating the excellent sensitivity of the hydrogel sensor. Figure 6 As shown in Figure a, the stepped strain test demonstrates that the resistance signal closely follows the strain change, with a rapid response and no obvious overshoot or relaxation. Figure 6 As shown in b, 6c, and 6d, the real-time relative resistance (ΔR / R0) signal can be stably observed under low strain (1-5%), medium strain (15-70%), and high strain (60-300%). Furthermore, during cyclic observations, the relative resistance variation shows no significant drift, and the corresponding relative resistance signal changes can be accurately identified, especially under the 1-5% strain condition. Figure 5 and Figure 6 It can be seen that the sensor has a high sensitivity response and a wide operating range, and the signal is stable.

[0081] (5) Human body strain sensor test:

[0082] The strain sensing performance of the CMC9 hydrogel prepared in Example 3 of this invention was further investigated. The hydrogel prepared in Example 3 was attached to different parts of a volunteer's body using medical breathable tape. Copper wires were connected to both ends of the gel and then connected to a resistance detector to monitor the relative resistance change signal in real time. Figure 7 As shown in (a), the sensor can clearly record the relative resistance change of a finger from extension to flexion, with good waveform repeatability, and can be used for gesture recognition. Figure 7 (b) and Figure 7 As shown in (c), the sensor exhibits a stable response for moderate-amplitude joint movements. Figure 7 As shown in (d), during the large deformation movement from extension to squatting, the sensor did not break or detach, and the resistance signal showed a large-amplitude periodic change with the knee flexion angle, verifying its reliability in large strain monitoring. Figure 7 It is known that this strain sensor is sensitive to deformation and can detect subtle human motion signals, making it applicable to the field of strain sensors.

[0083] The above embodiments of the present invention are merely examples for clearly illustrating the present invention and are not intended to limit the implementation of the present invention. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively describe all possible implementations here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A method for preparing a highly sensitive conductive composite hydrogel, characterized in that: A carboxymethyl cellulose dispersion is stirred with acrylic acid, acrylamide, a crosslinking agent and an initiator to obtain a mixed solution. The mixed solution is then subjected to vacuum degassing to obtain a degassed liquid, which is then subjected to a thermal polymerization reaction to obtain the final product.

2. The method for preparing a highly sensitive conductive composite hydrogel according to claim 1, characterized in that: The process of obtaining the dispersion containing carboxymethyl cellulose is as follows: carboxymethyl cellulose is dissolved in deionized water and stirred at 65℃~75℃ for 1.5~2.5h.

3. The method for preparing a highly sensitive conductive composite hydrogel according to claim 1, characterized in that: The crosslinking agent is N,N'-methylenebisacrylamide; The initiator is ammonium persulfate.

4. The method for preparing a highly sensitive conductive composite hydrogel according to claim 1, characterized in that: In the mixed solution, the mass fraction of carboxymethyl cellulose is 0.1% to 3.0%; In the mixed solution, the mass ratio of acrylic acid to acrylamide is 2~4:5~7; In the mixed solution, the mass ratio of acrylic acid to crosslinking agent is 2~4g:10~100mg; In the mixed solution, the mass ratio of acrylic acid to initiator is 2~4g:10mg~100mg; The volume-to-mass ratio of the deionized water to the mixed solution is 3-4 mL: 1 g; The stirring is carried out at room temperature for 15-30 minutes.

5. The method for preparing a highly sensitive conductive composite hydrogel according to claim 1, characterized in that: The mixed solution is placed in a mold and subjected to vacuum degassing until no visible bubbles remain. During the vacuum degassing process, the gas pressure is controlled to drop to -0.08 ~ -0.12 MPa.

6. The method for preparing a highly sensitive conductive composite hydrogel according to claim 1, characterized in that: The temperature of the thermal polymerization reaction is 60℃~70℃, and the reaction time is 1.0~2.0h.

7. The highly sensitive conductive composite hydrogel prepared by the preparation method according to any one of claims 1-6.

8. A flexible strain sensor, characterized in that: The flexible strain sensor comprises a highly sensitive conductive composite hydrogel prepared by the preparation method according to any one of claims 1-6.

9. A flexible strain sensor according to claim 8, characterized in that: The flexible strain sensor also includes a sensing element, a conductive connector, and an insulating encapsulation layer. The conductive connector is connected to both ends of the sensing element and is used to output an electrical signal. The edge encapsulation layer covers the junction of the sensing element and the conductive connector; The sensing element is elongated or dumbbell-shaped; The conductive connector is selected from one of copper wire, metal foil or conductive fabric; The insulating encapsulation layer is a medical breathable tape or an elastomer film.

10. The application of a flexible strain sensor according to any one of claims 8-9, characterized in that: The flexible strain sensor is applied to one of the following: human joint motion monitoring, gesture recognition, pulse monitoring, and human-computer interaction interface.

Images