CN114396867A

CN114396867A - Alternating-current type hydrogel flexible strain sensor and preparation method thereof

Info

Publication number: CN114396867A
Application number: CN202210006109.0A
Authority: CN
Inventors: 张电波; 米立伟; 黄超; 王艳杰
Original assignee: Zhongyuan University of Technology
Current assignee: Zhongyuan University of Technology
Priority date: 2022-01-05
Filing date: 2022-01-05
Publication date: 2022-04-26

Abstract

The invention discloses an alternating current type hydrogel flexible strain sensor and a preparation method thereof, and relates to the field of strain sensors and preparation thereof. The skin-core structure imitating the nerve axon is adopted, the elastic skin layer provides excellent mechanical characteristics for the sensor, the contact between the hydrogel response medium of the core layer and the outside air is isolated, and the dehydration problem of the hydrogel response medium is avoided; hydrogel particles are used as a response medium, so that the response and the sensitivity of the hydrogel are higher than those of the whole hydrogel, and the flexible forming capability is obtained; the electrode and hydrogel particle act cooperatively to enhance the responsiveness of the sensor.

Description

Alternating-current type hydrogel flexible strain sensor and preparation method thereof

Technical Field

The invention relates to the field of strain sensors and preparation thereof, in particular to an alternating-current type hydrogel flexible strain sensor and a preparation method thereof.

Background

Along with the rapid improvement of the intelligent degree of the human society, the demands of the advanced industrial fields such as physiological signal monitoring, motion detection, human-computer interaction, intelligent robots and the like on high-performance flexible strain sensors are increasingly urgent. Compared with a strain sensor taking elastic high polymer materials such as silica gel, polyurethane and rubber as carriers, the strain sensor based on the hydrogel has a plurality of excellent unique performances such as high elasticity, high response range, various processing and forming modes, adjustable conductivity, strong self-healing capability, convenient material modification, strong multifunctionality and the like, and has huge market application prospect.

The hydrogel is composed of a crosslinked polymer network and an aqueous solution filled in the crosslinked polymer network, and in a practical application scene, the following three problems are often faced:

1. the hydrogel exposed in the atmosphere is easy to dehydrate, so that the response signal of the sensor is drifted, the elasticity is poor and the sensor completely fails.

2. The current technical scheme basically takes monolithic hydrogel as a response medium, the response rule basically follows a resistivity model, the responsiveness is low, and the detection capability on small strain is particularly weak.

3. In the technical scheme in the prior art, a direct current detection scheme with a response medium as an electronic conductor is generally adopted, and a large amount of movable ions exist in an aqueous solution component in hydrogel, so that the hydrogel response medium shows strong ionic conductivity, and if a direct current method is adopted for detection, ions in a sensor cannot form a complete loop in a circuit, and an electric double layer is formed at an electrode/hydrogel interface, so that huge baseline drift is introduced, and even consistency of a response signal and strain is damaged.

The above-mentioned outstanding problems greatly limit the practical application of hydrogel strain sensors, and how to solve these problems is becoming a hot spot of current research in this field.

Disclosure of Invention

The invention provides an alternating current type hydrogel flexible strain sensor imitating a nerve axon structure and a preparation method thereof, which are inspired by the stable transmission of nerve signals in neuron axons, and can solve the problems of easy dehydration of hydrogel materials, low responsiveness, baseline drift in a direct current test mode and the like. In order to solve the technical problems, the invention provides the following technical scheme:

the alternating current type hydrogel flexible strain sensor comprises an elastic cortex, two electrodes and a hydrogel particle response medium, wherein the two electrodes and the elastic cortex are combined to form a closed space, hydrogel particles are filled in the closed space, and the two electrodes adopt alternating current for power supply.

Preferably, the hydrogel microparticles comprise one or more of the following materials: polyacrylamide hydrogel, polyvinyl alcohol hydrogel, polyacrylic acid hydrogel, polyethylene oxide hydrogel, polyethylene glycol hydrogel, agar hydrogel, starch hydrogel, protein hydrogel, gelatin hydrogel, chitosan hydrogel, cellulose hydrogel, sodium alginate hydrogel, cyclodextrin hydrogel, double-network hydrogel and copolymer hydrogel.

Preferably, the hydrogel particles are pure ionic conductors or electron-ion mixed conductors.

Preferably, the ion conductivity of the hydrogel particles is controlled by the ion concentration and ion species in the hydrogel; or the like, or, alternatively,

the electronic conductivity of the hydrogel particles is controlled by adding an electronic conductive material to the hydrogel, wherein the electronic conductive material is one or more of the following materials: carbon black, carbon nanotubes, carbon fibers, graphene oxide, MXene, metal micro-nano materials, polyaniline or polypyrrole.

Preferably, the elastic skin layer is prepared from one or more of the following materials: silica gel, polyurethane, polyester, ethylene-vinyl acetate copolymer, polystyrene or styrene thermoplastic elastomer.

Preferably, the surfaces of the two electrodes on the side contacting the hydrogel particles have microstructures.

The invention also provides a preparation method of the alternating current type hydrogel flexible strain sensor, which comprises the following steps:

preparing a hydrogel microparticle response medium;

filling the hydrogel particles into a space surrounded by the elastic skin layer;

the space enclosed by the elastic skin is closed at both ends using two electrodes.

Preferably, the specific steps for preparing the hydrogel particles are as follows:

obtaining hydrogel;

breaking the hydrogel into microparticles;

adding hydrogel particles into aqueous solutions with different ion concentrations or aqueous dispersions with different electronic conductors, fully mixing, and screening with screens with different meshes to obtain hydrogel particle response media with different particle size ranges.

directly polymerizing by an emulsion method or a liquid drop method to obtain hydrogel particles;

Preferably, the specific steps of filling the hydrogel particles into the space enclosed by the elastic skin layer are as follows:

loading hydrogel microparticles into a syringe;

and injecting the hydrogel particles into a space surrounded by the elastic cortex by using a syringe.

The invention has the beneficial effects that:

1. adopts a skin-core structure imitating nerve axons. The sensor has the advantages that the sensor provides excellent mechanical properties, and meanwhile, the sensor is used as a blocking layer to isolate the contact between the hydrogel response medium of the core layer and the outside air, so that the dehydration problem of the hydrogel medium is avoided, the sensor has strong weather resistance, and can stably work for a long time.

2. Hydrogel microparticles are used as a response medium. The hydrogel particles have good injectability, can be flexibly filled in elastic shell layers with various sizes and shapes, and greatly expands the processability and application scenes of the sensor.

3. Compared with a monolithic hydrogel response medium, a large number of interfaces exist among hydrogel particles, and in sensor strain, the reconstruction of the particles and the rearrangement of the interfaces can introduce larger ion transmission impedance change, so that the limitation of a resistivity model is broken through, and the responsivity and the sensitivity of the sensor can be greatly improved.

4. If a whole block of hydrogel is used as a response medium, the hydrogel has high requirements on modulus, strength, ultimate deformation, fatigue performance and the like, and only several types of high-strength hydrogels with special structures can meet the requirements. By adopting the scheme provided by the invention, due to the existence of the limitation and protection of the elastic cortex, a plurality of aquagels with general mechanical properties can be used as response media to realize large-strain and high-sensitivity strain response after being crushed into particles, so that the selection range of the response media materials of the aquagel strain sensor is greatly expanded, the production cost of the aquagel strain sensor can be greatly reduced, and a larger operation space is provided for the multi-functionalization of the aquagel strain sensor.

5. The response behavior of the sensor can be conveniently regulated and controlled by adjusting the ionic conductivity and the electronic conductivity of the hydrogel particle response medium. The ionic conductance and electronic conductance regulation and control method is simple, has various schemes, and provides more possibilities for the diversified application of the flexible strain sensor.

6. The scheme adopts an alternating current method detection mode. Under the action of the AC excitation signal, the AC impedance value, the current peak value or the voltage peak value of the sensor and the like are taken as response signals. Under the alternating current mode, movable ions in the hydrogel do reciprocating motion, and a complete loop is formed at the interface of the hydrogel and the electrode in a double electric layer capacitive coupling mode with an electronic circuit, so that the signal drift problem under the direct current mode is solved. The sensor has excellent strain response consistency and reliability.

7. In the AC detection mode, the strain response of the sensor not only depends on the strain response of the hydrogel response medium, but also is influenced by the change of the configuration of the hydrogel/electrode interface. When tensile strain occurs, the transmission impedance is increased due to the fact that the transmission section of a current carrier in the hydrogel is reduced, the transmission distance and the interface are increased; the hydrogel/electrode interface area is reduced by necking down of the cortical structure, resulting in an increase in interfacial impedance. When compressive strain occurs, the transmission impedance is reduced due to the increase of the transmission section of the carriers in the hydrogel and the reduction of the transmission distance and the interface; the hydrogel/electrode interface area increases due to the reduced necking, resulting in a lower interface impedance. Therefore, in an alternating current mode, the hydrogel/electrode configuration of the skin-core structure strain sensor plays a positive feedback role in the response of strain, and the response characteristic of the sensor can be effectively enhanced. The strain response performance of the sensor can be regulated and controlled by designing the structure of the contact side of the electrode and the hydrogel particle response medium.

8. The alternating current type hydrogel flexible strain sensor can stably work under a lower potential excitation signal, and the energy consumption is very low. In addition, the sensor has natural matching advantages with novel wearable energy supply devices such as friction nano-generators and piezoelectric nano-generators which provide alternating-current energy, and can be directly matched for use without complex rectifying circuits. The method has great application potential in the field of wearable electronic equipment.

Drawings

FIG. 1 is a schematic view of an AC hydrogel flexible strain sensor in example 1;

FIG. 2 is a graph showing the strain response characteristics of the AC hydrogel flexible strain sensor in example 2;

FIG. 3 is a graph showing the strain response characteristics of the AC hydrogel flexible strain sensor in example 3;

fig. 4 is a diagram of the detection of human breath by the ac hydrogel flexible strain sensor in example 4 with the wearable friction nano-generator providing energy.

Detailed Description

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

Example 1

The embodiment provides an alternating current type hydrogel flexible strain sensor, as shown in fig. 1, which includes an elastic cortex 1, two electrodes 3 and hydrogel particles 2, where the two electrodes 3 and the elastic cortex 1 are combined to form a closed space, the closed space is filled with the hydrogel particles 2, and the two electrodes 3 are supplied with alternating current.

In this embodiment, the hydrogel microparticles 2 are used as a response medium, and one or more of the following materials of the hydrogel microparticles 2: polyacrylamide hydrogel, polyvinyl alcohol hydrogel, polyacrylic acid hydrogel, polyethylene oxide hydrogel, polyethylene glycol hydrogel, agar hydrogel, starch hydrogel, protein hydrogel, gelatin hydrogel, chitosan hydrogel, cellulose hydrogel, sodium alginate hydrogel, cyclodextrin hydrogel, double-network hydrogel and copolymer hydrogel. The hydrogel particles 2 are prepared by crushing with a blade crusher, grinding on a rough surface, plane crushing, screen crushing, extruding and crushing with an injector to obtain a whole hydrogel from top to bottom, or by in-situ polymerization, emulsion polymerization, electrostatic spinning, droplet forming and other bottom-to-top methods.

In this embodiment, the particle size of the hydrogel particle 2 may be controlled by controlling the preparation conditions or by passing through a screen at a later stage. The electronic conductivity of the hydrogel particles 2 can be controlled by adding various electronic conductive fillers such as carbon black, carbon nanotubes, carbon fibers, graphene oxide, MXene, metal micro-nano materials, polyaniline, polypyrrole and the like in the hydrogel preparation process or in the final particle aggregate; the ion conductivity can also be controlled by adjusting the ion concentration, ion species, and the like in the fine particles.

In this embodiment, the sensor structure is a skin-core structure simulating neurites. Elastic cortex 1 provides good mechanical properties for the sensor, and simultaneously as the barrier layer, isolated sandwich layer aquogel response medium and outside air's contact solve the dehydration problem of aquogel, guarantee the long-term stable work of sensor. The elastic skin layer 1 can be made of high polymer materials with good elasticity, such as silica gel, rubber, polyurethane, polyester, ethylene-vinyl acetate copolymer, polypropylene ethylene, styrene thermoplastic elastomer and the like. The elastic skin layer has good tightness and adjustable thickness, and can adjust the shape, size and structure according to application scenes.

In this embodiment, the hydrogel microparticle 2 aggregate can be filled into the elastic structure of the skin layer by injection. The elastic structure of the skin layer is formed by integrally forming the elastic skin layer 1 into a cylindrical shape or other shape suitable for accommodating the hydrogel particles 2. Because the hydrogel is rich in elasticity and the hydrogen bond effect among the particles, the hydrogel particles have the tendency of self-agglomeration, thereby avoiding the occurrence of bubbles and faults and forming a continuous response medium. In the strain, the hydrogel particles respond to the shape of the medium to realize continuous deformation through rearrangement and deformation of the particles, and the alternating current impedance, current and voltage of the hydrogel particles make stable response to the strain of the sensor under the action of alternating current excitation due to the change of internal electronic and ionic conductive networks.

In this embodiment, the ac hydrogel flexible strain sensor further includes a lead 4, and both of the electrodes 3 are connected to an external circuit through the lead. After the hydrogel particle aggregate is filled into the elastic structure of the skin layer, electrodes 3 are inserted into two ends of the skin-core tubular structure, and the electrodes 3 are connected with an external circuit through leads 4. The electrode 3 may be made of metal, carbon black, carbon nanotube, carbon fiber, graphite, graphene oxide, MXene, polyaniline, polypyrrole, inorganic semiconductor, or other electronic conductors, and the elastic skin structure may be processed into a shape of column, cone, square, fiber, film, or the like according to the skin structure. The electrodes 3 are in good contact with the hydrogel particles 2, and can effectively seal two ends of the elastic structure of the cortex layer, so that the hydrogel response medium is prevented from being communicated with the outside. The side of the electrode 3 in contact with the hydrogel particles 2 may be related to different microstructures to modulate the strain response behavior of the sensor.

In this embodiment, the sensor uses an ac detection mode. Under the action of the AC excitation signal, the AC impedance, current, voltage and the like of the sensor are taken as response signals. Due to the structural characteristics of the hydrogel, the aqueous solution in the hydrogel inevitably has ion conductivity, and if a direct current test mode is adopted, the problems of response signal drift, inconsistent signals in the process of strain application recovery and the like are caused due to concentration polarization of conductive ions at the hydrogel/electrode interface. In an alternating current mode, conductive ions do reciprocating motion in a hydrogel response medium, a complete circuit loop is formed by hydrogel/electrode interface capacitance and an external electronic circuit, and signals such as alternating current impedance, current and voltage have very stable response to the strain of the sensor. The disadvantages of the dc test mode are avoided.

In this embodiment, the ac excitation signal may be provided by an LCR bridge, an electrochemical workstation, a friction nanogenerator, a piezoelectric nanogenerator, or the like. The peak value range of the alternating current potential is 0.005V-1.2V, the sensor fails due to the electrolysis of the aqueous solution when the potential is too high, and the detection accuracy of the sensor is reduced because the detection signal strength is too weak when the potential is too low. The frequency range of the alternating current excitation signal is 0.1Hz-1MHz, response signal drift is caused by concentration polarization of conductive ions at a hydrogel/electrode interface when the frequency is too low, the frequency is too high, interface response is dominant, interference of wireless signals is increased, and reliability of the sensor is affected.

Example 2

In this embodiment, the specific steps for preparing hydrogel particles using cellulose hydrogel as the hydrogel material are as follows:

s101, placing the large cellulose hydrogel into a rotary blade type stirrer to be stirred, and crushing for 1 min.

S102, adding ultrapure water into the crushed cellulose hydrogel for dispersing, and then sieving the cellulose hydrogel with a 20-mesh sieve to filter out larger particles; and (4) sieving the mixed solution below the filter screen with a 80-mesh sieve again, and taking hydrogel particles on the filter screen.

S103, respectively adding the obtained hydrogel particles into aqueous solutions of sodium chloride with different concentrations, and standing for 24 hours to ensure that the ion concentrations in the hydrogel particles are fully balanced. The sodium chloride concentrations were 0, 0.154, 0.5, 1M, respectively.

And S104, passing the mixed solution through a 80-mesh filter screen, and obtaining hydrogel particles with different ion concentrations on the filter screen. Respectively loading into a syringe and sealing.

S105, filling the hydrogel particles into a space surrounded by the elastic cortex: and injecting the hydrogel particles into a space surrounded by the elastic cortex by using a syringe. The elastic skin layer in this embodiment is a highly elastic rubber tube (3.2 mm outer diameter, 1.6mm inner diameter).

In this embodiment, the electrode may be a stainless steel electrode having a conical top end. After being filled with the cellulose hydrogel particles, the electrodes were inserted into both ends of the rubber tube and fastened externally with a ribbon. The distance between the fastening silk ribbons is 28mm, and the distance between the top ends of the electrodes on the two sides is selected to be 12 mm.

In this embodiment, the step of connecting the lead to the electrode is also included, and the connection mode is common in the art. Additionally, the leads may be metal leads.

In a preferred embodiment, the power supply is an electrochemical workstation, the excitation signal is a sine wave with a frequency of 1000Hz, a bias voltage of 0V and an alternating current potential of 0.2V, and the response signal is the alternating current impedance of the sensor. As shown in fig. 2, it is a characteristic of the strain response performance of the ac flexible strain sensor prepared in this example. a is the stress-strain curve of the sensor; b is the strain response curve of the hydrogel particle device with different ion concentrations; c is the strain impedance response of the 0.05M device at different pull rates; d. e, f are the results of repeated tests of 0.05M devices at different strains.

The test results show that the strain sensor has very low residual strain (1.96% at 400% strain). The response can reach 630% under 400% tensile strain, which is far beyond the response level reported in the literature for bulk hydrogel strain sensors. The impedance strain response is basically consistent in the stretching recovery process, and different strain rates have no influence on the responsiveness, which shows that the response of the sensor to the strain has high reliability. Under the condition that other conditions are not changed, the responsivity of the sensor manufactured by the hydrogel particles with different ion concentrations is obviously different, which shows that the response index of the sensor can be conveniently regulated and controlled through the conductivity of the hydrogel particle response medium.

Example 3

The embodiment provides a preparation method of an alternating current type hydrogel flexible strain sensor, which takes polyacrylic acid-acrylamide copolymerized hydrogel as a hydrogel material to prepare hydrogel particles, and comprises the following specific steps:

s301, adding 3g of acrylic acid monomer into 10g of deionized water, adding 1.37g of sodium hydroxide while stirring, adding 3g of acrylamide monomer after dissolution, then adding 0.23g of potassium persulfate, pouring the obtained solution into a grinding tool, placing the grinding tool in an oven at 50 ℃, and reacting for 6 hours to obtain the block polyacrylic acid-acrylamide copolymerized hydrogel.

S302, placing the polyacrylic acid-acrylamide copolymerized hydrogel into deionized water to be soaked for 24 hours, and removing unreacted monomers and ions.

S303, rubbing the block polyacrylic acid-acrylamide copolymerized hydrogel on the surface of a 20-mesh sieve to obtain polyacrylic acid-acrylamide copolymerized hydrogel particles below the sieve.

S304, adding the obtained hydrogel particles into a 0.5M lithium chloride aqueous solution, and standing for 24 hours to ensure that the ion concentration in the hydrogel particles is fully balanced. Then passing through an 50/80/150/200/350/400 sieve, taking hydrogel particles between 50-80 meshes, 150-200 meshes and 350-400 meshes as research objects, and respectively filling the hydrogel particles into syringes for sealing.

S305, filling the hydrogel particles with different particle sizes into a space surrounded by the elastic skin layer: and injecting the hydrogel particles into a space surrounded by the elastic cortex by using a syringe. The elastic skin layer in this embodiment is a silicone tube (outer diameter 1.5mm, inner diameter 1 mm).

In this embodiment, the electrode may be a cylindrical graphite electrode having a diameter of 1 mm. After the cellulose hydrogel particles are filled, the electrodes are plugged into two ends of the silicone tube and are fastened externally by ribbons. The distance between the fastening silk ribbons is 20mm, and the distance between the top ends of the electrodes on the two sides is selected to be 14 mm.

In a preferred embodiment, the power supply is an LCR bridge, the excitation signal is a sine wave with a frequency of 200Hz, a bias voltage of 0V and an alternating current potential of 0.05V, and the response signal is the alternating current impedance of the sensor. As shown in fig. 3, a strain response performance characterization of the ac type flexible strain sensor prepared in this example is shown. The response curves of flexible strain sensors prepared from hydrogel microparticles having different particle sizes are shown. The test result shows that the response characteristic of the strain sensor can be regulated and controlled by the size of hydrogel particles. The responsivity of the sensor with the grain size range of 50-80 meshes under 200% tensile strain can reach 46.9, and the impedance strain response is basically consistent in the tensile recovery process, which shows that the response of the sensor to strain has high reliability.

Example 4

The embodiment provides a preparation method of an alternating current type hydrogel flexible strain sensor, which takes protein hydrogel as a hydrogel material to prepare hydrogel particles, and comprises the following specific steps:

s401, taking 200g of egg protein, fully stirring, pouring into a mold, sealing and placing in a 90 ℃ oven for 30min to obtain the block protein hydrogel.

S402, crushing the block protein hydrogel in a ceramic mortar, and then soaking in an aqueous solution containing 0.1M of sodium sulfate and 0.05% wt of multi-walled carbon nanotubes for 24 hours.

And S403, sieving the obtained mixed liquid by using a sieve, taking hydrogel particles among 80-150 meshes of the sieve as research objects, and respectively filling the hydrogel particles into an injector for sealing.

S404, filling the protein hydrogel particles into a space surrounded by an elastic skin layer: and injecting the hydrogel particles into a space surrounded by the elastic cortex by using a syringe. The elastic skin layer in this example is a polyurethane tube with a square cross section (wall thickness 0.4mm, cross section outer side length 1.6mm, outer side width 1.2 mm).

In this embodiment, the electrode may be a PDMS embedded carbon fiber composite electrode. The PDMS is a spline with a square section (the outer side length of the section is 0.8mm, and the outer side width is 0.4mm), the polyurethane tube core layer is matched in shape, and carbon fiber bundles penetrating through two ends are embedded inside the polyurethane tube core layer. The carbon fiber bundle on one side of the electrode is exposed by 1mm to be used as a contact electrode, and the exposed length on the other side of the electrode is not limited to be used as an external lead.

In the implementation, after the protein hydrogel particles are filled into the polyurethane tube by the syringe, the electrodes are plugged into two ends of the silicone tube to form the flexible strain sensor.

In a preferred embodiment, the power supply adopts a wearable friction nano generator, the working frequency is 5Hz, and the output excitation signal is an alternating current pulse signal with alternating positive and negative polarities. After the flexible strain sensor is connected with the friction nano generator in series, the strain of the sensor is detected through a current peak value in a detection circuit.

As shown in fig. 4, the ac flexible strain sensor prepared in this embodiment detects human respiration when a wearable friction nano-generator provides energy. The test result shows that the flexible strain sensor can work stably under the support of the wearable friction nano generator, the problems of baseline drift and the like do not exist, shallow breathing and deep breathing signals have obvious discrimination, the flexible strain sensor is proved to be capable of accurately detecting actions such as human breathing, and the unique advantages are shown in the aspects of human health and motion detection.

Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes, modifications, substitutions and alterations can be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.

Claims

1. The alternating current type hydrogel flexible strain sensor is characterized by comprising an elastic cortex, two electrodes and hydrogel particles, wherein the two electrodes and the elastic cortex are combined to form a closed space, the closed space is filled with the hydrogel particles, and the two electrodes are supplied with alternating current.

2. The alternating current hydrogel flexible strain sensor of claim 1, wherein the hydrogel microparticles comprise one or more of the following materials: polyacrylamide hydrogel, polyvinyl alcohol hydrogel, polyacrylic acid hydrogel, polyethylene oxide hydrogel, polyethylene glycol hydrogel, agar hydrogel, starch hydrogel, protein hydrogel, gelatin hydrogel, chitosan hydrogel, cellulose hydrogel, sodium alginate hydrogel, cyclodextrin hydrogel, double-network hydrogel and copolymer hydrogel.

3. The alternating current hydrogel flexible strain sensor of claim 2, wherein the hydrogel microparticles are pure ionic conductors or electron-ion mixed conductors.

4. The alternating current hydrogel flexible strain sensor of claim 3, wherein the ionic conductivity properties of the hydrogel particles are controlled by the ionic concentration and ionic species in the hydrogel; or the like, or, alternatively,

the electronic conductivity of the hydrogel particles is controlled by adding an electronic conductive material in the hydrogel or among the hydrogel particles, wherein the electronic conductive material is one or more of the following materials: carbon black, carbon nanotubes, carbon fibers, graphene oxide, MXene, metal micro-nano materials, polyaniline or polypyrrole.

5. The alternating current hydrogel flexible strain sensor of claim 1, wherein the elastic skin layer is made of one or more of the following materials: silica gel, polyurethane, polyester, ethylene-vinyl acetate copolymer, polystyrene or styrene thermoplastic elastomer.

6. The alternating current hydrogel flexible strain sensor of claim 1, wherein the surfaces of the two electrodes on the side in contact with the hydrogel particles have microstructures.

7. A method for preparing the alternating current hydrogel flexible strain sensor according to any one of claims 1 to 6, comprising the following steps:

preparing a hydrogel microparticle response medium;

8. The method according to claim 7, wherein the hydrogel microparticles are prepared by the following steps:

obtaining hydrogel;

breaking the hydrogel into microparticles;

9. The method according to claim 7, wherein the hydrogel microparticles are prepared by the following steps:

10. The method according to claim 7, wherein the step of filling the hydrogel microparticles into the space surrounded by the elastic skin layer comprises:

loading hydrogel microparticles into a syringe;