A low hysteresis conductive polymer hydrogel and its preparation method and application

Abstract

This invention discloses a low-hysteresis conductive polymer hydrogel, its preparation method, and its applications. The preparation method includes: dissolving gelatin in water to form a gelatin solution, adding pyrrole monomer and stirring to disperse it, then adding an oxidizing agent to cause pyrrole to undergo oxidative polymerization near the gelatin molecular chains, followed by dialysis and drying to obtain polypyrrole-grafted gelatin; mixing the polypyrrole-grafted gelatin, acrylic monomer, crosslinking agent, and initiator to form a hydrogel precursor solution, and then polymerizing it via thermal initiation or photoinitiation to form a polypyrrole-grafted gelatin / polyacrylic acid composite conductive hydrogel. The polypyrrole is dispersed in the aqueous system through the gelatin backbone and forms a composite structure with the polyacrylic acid network, which helps reduce the aggregation of conductive components and migration and reconstruction during cyclic deformation, allowing the hydrogel to maintain a relatively stable conductive pathway during stretching, release, compression, or bending. The hydrogel can be used as a flexible detection element, exhibiting low electrical hysteresis and good repeatability of resistance response.

Description

Technical Field

[0001] This invention belongs to the field of flexible electronic materials technology, specifically relating to a low-hysteresis conductive polymer hydrogel, its preparation method, and its application. Background Technology

[0002] With the development of flexible electronics technology, fields such as flexible sensors, wearable devices, and human-computer interaction systems have placed higher demands on functional materials that are flexible, stretchable, and have stable electrical responses. Hydrogels have high water content, adjustable mechanical properties, and good biocompatibility, and can achieve interface matching with human tissues, skin surfaces, or flexible substrates to a certain extent. Therefore, they are often used to construct sensitive elements or conductive functional layers in flexible electronic devices.

[0003] To enable hydrogels to transmit electrical signals or respond to strain, conductive components are typically introduced into the hydrogel system. Commonly used conductive components in existing technologies include carbon nanotubes, graphene, metal nanoparticles, silver nanowires, and conductive polymers. Among these, conductive polymers such as polypyrrole and poly(3,4-ethylenedioxythiophene) are widely used in conductive hydrogel systems due to their flexibility and conductivity.

[0004] However, conductive polymers typically exhibit poor dispersion stability in aqueous systems and tend to aggregate when directly added to hydrogel precursor systems, leading to uneven distribution of conductive components within the hydrogel network. This localized aggregation of conductive components not only reduces the uniformity of the overall conductive structure of the material but also makes it prone to localized breakage, migration, or reconstruction of conductive pathways during deformation processes such as stretching, releasing, compression, or bending, thereby reducing the material's resistance response stability.

[0005] Especially in flexible strain sensing applications, hydrogel sensing elements need to undergo repeated deformations with human joints or flexible substrates. If the conductive network inside the hydrogel is unevenly distributed or there is a lack of stable interaction between the conductive phase and the hydrogel matrix, the conductive path is difficult to recover synchronously during cyclic loading and unloading, easily leading to electrical hysteresis, where the loading curve and unloading curve do not coincide. This phenomenon can cause differences in the output resistance signal under the same strain state, thus affecting the sensor's signal consistency, repeatability, and motion monitoring reliability.

[0006] In addition, although loading or combining polypyrrole onto a hydrophilic polymer framework can improve its aqueous dispersion, the hydrophilic polymer framework itself often still has problems such as insufficient gelation performance, limited flexible support capacity, and insufficient structural stability after deformation under stress. It is difficult to meet the requirements of flexible sensors for stretchability, deformation recovery capacity, and stability during repeated use on its own. Summary of the Invention

[0007] The purpose of this invention is to provide a low-hysteresis conductive polymer hydrogel, its preparation method and application, so as to improve the dispersibility of conductive polymers in hydrogel systems, reduce the problem of non-uniform conductive networks caused by the aggregation of conductive components, and improve the electrical response stability of hydrogels during cyclic deformation.

[0008] In a first aspect, the present invention provides a low-hysteresis conductive polymer hydrogel, comprising a polyacrylic acid network and polypyrrole-grafted gelatin dispersed in the polyacrylic acid network. The polypyrrole-grafted gelatin is dispersed in the polyacrylic acid network by initiating a polymerization reaction of the acrylic acid monomers after mixing the acrylic acid monomers and the polypyrrole-grafted gelatin. Therefore, the polyacrylic acid network is a network formed by the polymerization and crosslinking of acrylic acid monomers in the presence of a crosslinking agent and the polypyrrole-grafted gelatin.

[0009] The polypyrrole-grafted gelatin is obtained by adding pyrrole monomers to a gelatin solution and initiating an oxidative polymerization reaction of the pyrrole monomers. Therefore, the polypyrrole in the polypyrrole-grafted gelatin is formed in situ on the gelatin molecular chain. By grafting polypyrrole onto the gelatin backbone, the dispersibility of polypyrrole in the aqueous system can be improved, allowing the conductive components to form a more uniform distribution in the subsequent hydrogel network, thus facilitating the construction of a uniform and stable conductive network.

[0010] The polyacrylic acid network and the polypyrrole-grafted gelatin together form a composite network. This composite network can restrict the migration and reconstruction of conductive components during stretching, releasing, compression, or bending, enabling the conductive pathway to effectively recover after deformation, thereby reducing the difference in resistive response during loading and unloading.

[0011] Preferably, the mass ratio of the polypyrrole-grafted gelatin to the acrylic monomer is (1-5):20; and the mass ratio of the pyrrole monomer to the gelatin is 1:(5-10).

[0012] Preferably, the polypyrrole grafted gelatin contains 4.2 wt% to 6.7 wt% polypyrrole.

[0013] Secondly, the present invention provides a preparation method for preparing the aforementioned low-hysteresis conductive polymer hydrogel. The preparation method includes:

[0014] S1: Prepare a pyrrole-gelatin mixed solution, and add an oxidant to the pyrrole-gelatin mixed solution to initiate the oxidative polymerization reaction of pyrrole monomers, so that the formed polypyrrole is grafted onto the gelatin backbone to obtain polypyrrole-grafted gelatin.

[0015] S2: Mix the polypyrrole-grafted gelatin, acrylic monomer, crosslinking agent and initiator to initiate the polymerization reaction of acrylic monomer to form a polyacrylic acid network, and disperse the polypyrrole-grafted gelatin in the polyacrylic acid network to obtain a low-hysteresis conductive polymer hydrogel.

[0016] Preferably, the polymerization reaction of the acrylic monomer is a thermally initiated polymerization reaction; the reaction temperature of the thermally initiated polymerization reaction is 45℃~65℃, and the reaction time is 10min~30min. The initiator is preferably ammonium persulfate.

[0017] Preferably, the initiator is a water-soluble photoinitiator; the acrylic monomer polymerization reaction is a photoinitiated polymerization reaction with a reaction time of 5 to 20 minutes. The photoinitiator is preferably lithium phenyl(2,4,6-trimethylbenzoyl)phosphonate and / or 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylphenylacetone.

[0018] Preferably, the mass fraction of the gelatin solution is 0.5wt% to 1.5wt%.

[0019] Preferably, the oxidant includes ferric chloride hexahydrate; the molar ratio of ferric chloride hexahydrate to pyrrole monomer is (2.5-3.5):2, preferably 3:2.

[0020] Preferably, the oxidative polymerization reaction of the pyrrole monomer is carried out under an inert atmosphere and at a temperature of 0°C to 6°C for 16 to 24 hours.

[0021] Preferably, after the oxidative polymerization reaction of the pyrrole monomer is completed, the resulting product is subjected to dialysis separation to obtain the polypyrrole grafted gelatin.

[0022] Preferably, the crosslinking agent is N,N'-methylenebisacrylamide.

[0023] Thirdly, the present invention provides the application of the aforementioned low-hysteresis conductive polymer hydrogel in a flexible strain sensor. The low-hysteresis conductive polymer hydrogel serves as a flexible sensing element in the flexible strain sensor.

[0024] Fourthly, the present invention provides a flexible strain sensor, which includes a flexible detection element and electrodes connected to both ends of the flexible detection element; the flexible detection element is made of the aforementioned low-hysteresis conductive polymer hydrogel.

[0025] Preferably, the flexible strain sensor further includes a flexible substrate. The flexible sensing element is fixed on the substrate.

[0026] The beneficial effects of this invention are as follows:

[0027] 1. This invention disperses polypyrrole-grafted gelatin in a polyacrylic acid network, so that the polypyrrole, gelatin skeleton and polyacrylic acid network form a synergistic composite structure. While inhibiting the aggregation of conductive components and improving the conductivity uniformity of conductive hydrogel, it overcomes the problems of poor individual gelling properties, insufficient flexible support and insufficient structural stability after deformation under stress that exist in polypyrrole-grafted gelatin itself.

[0028] 2. In the hydrogel provided by the present invention, hydrogen bonds and other interactions are formed between the polypyrrole-grafted gelatin and the polyacrylic acid network, making it difficult for the conductive components to migrate and reconstruct during stretching, release or bending, effectively reducing the electrical hysteresis during loading and unloading; when applied to flexible strain sensors, it is beneficial to improve the consistency, repeatability and motion monitoring reliability of the sensor's resistance response.

[0029] 3. This invention improves the repeatability of the resistive response and the signal stability of the hydrogel by enabling the polypyrrole-grafted gelatin to deform in synergy with the flexible polyacrylic acid network, allowing the conductive pathway to effectively recover after stretching, releasing, compressing or bending.

[0030] 4. This invention overcomes the problems of easy agglomeration and poor dispersion uniformity of polypyrrole when it is directly added to an aqueous system by in-situ grafting polypyrrole onto a gelatin skeleton, so that the polypyrrole can be stably distributed by means of the aqueous dispersibility of the gelatin skeleton, thereby improving the dispersion stability of conductive components.

[0031] 5. This invention achieves a balance between conductivity sensitivity, low hysteresis response, and cycle stability by adjusting the ratio of polypyrrole-grafted gelatin to acrylic monomers, thereby matching the conductive network density with the flexibility of the polyacrylic acid network. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of electrical hysteresis during a cyclic loading-unloading process.

[0033] Figure 2 This is a schematic diagram illustrating the preparation of polypyrrole-grafted gelatin in Example 1 of the present invention.

[0034] Figure 3 This is a schematic diagram illustrating the preparation of the Gel-PPy / PAA hydrogel in Example 1 of the present invention.

[0035] Figure 4 The images show a comparison of the microstructure of the products prepared in Example 1 and Comparative Example 1 of this invention (part a is a scanning electron microscope image of the Gel-PPy / PAA hydrogel prepared in Example 1; part b is a scanning electron microscope image of the PPy / PAA hydrogel prepared in Comparative Example 1).

[0036] Figure 5The electrical sensing performance test graphs of the Gel-PPy / PAA hydrogel prepared in Example 3 of the present invention are shown in part a (the relative resistance change curves of the hydrogel under different strain conditions, part b (the relative resistance change curves of the hydrogel under different tensile speeds), part c (the relative resistance change curves in the cyclic loading / unloading test), and part d (the relative resistance change curves in a single tensile loading-unloading cycle).

[0037] Figure 6 The mechanical properties of the hydrogels prepared in Examples 1-5 and Comparative Example 2 are compared, and the deformation recovery performance test diagram of the hydrogel prepared in Example 3 is shown (part a is the stress-strain curve, part b is the Young's modulus and toughness test results, part c is the actual picture of the hydrogel prepared in Example 3 during the stretching and release process, and part d is the actual picture of the hydrogel prepared in Example 3 during the compression and release process).

[0038] Figure 7 The ATR-FTIR characterization diagrams of the hydrogels prepared in Example 3 and Comparative Example 2 of the present invention are shown (part a is the comparative ATR-FTIR spectrum of the hydrogels prepared in Example 3 and Comparative Example 2, and part b is the ATR-FTIR spectrum of the hydrogel prepared in Example 3 after different cycles of stretching). Detailed Implementation

[0039] The present invention will be further described below with reference to specific embodiments. It should be understood that the following embodiments are used to illustrate the technical solutions of the present invention and should not be construed as limiting the scope of protection of the present invention. Adjustments can be made to the specific raw material dosage, reaction time, reaction temperature, initiation method, and testing conditions without departing from the technical concept of the present invention, and these adjustments can be made according to actual needs.

[0040] Example 1

[0041] A method for preparing a low-hysteresis conductive polymer hydrogel includes the following steps:

[0042] Step 1: Preparation of polypyrrole-grafted gelatin

[0043] Step 1-1. Prepare the gelatin solution.

[0044] Weigh out gelatin and add it to deionized water to fully dissolve the gelatin, obtaining a gelatin solution. The mass fraction of the gelatin solution is 0.5 wt% to 1.5 wt%. In some embodiments, 500 mg of gelatin is added to 100 g of deionized water to prepare a gelatin solution with a mass fraction of approximately 0.5 wt%.

[0045] Steps 1-2. Prepare a pyrrole-gelatin mixed solution

[0046] Pyrrole monomer is added to the gelatin solution. The mass ratio of pyrrole monomer to gelatin is 1:(5-10). For example, when the amount of gelatin is 500 mg, the amount of pyrrole monomer can be 25 mg to 50 mg (40 mg in this example). After adding the pyrrole monomer, the mixture is stirred for 1 hour to ensure that the pyrrole monomer is fully dispersed in the gelatin solution.

[0047] Steps 1-3. Pyrrole oxidative polymerization

[0048] An aqueous solution of ferric chloride hexahydrate is prepared and gradually added to the pyrrole-gelatin mixed solution obtained in steps 1-2 to initiate the oxidative polymerization reaction of pyrrole monomers. The oxidative polymerization reaction is carried out under an inert atmosphere and at low temperature. In some embodiments, the inert atmosphere is specifically a nitrogen atmosphere, the reaction temperature is 4°C, and the reaction time is 16 to 24 hours.

[0049] In some embodiments, the molar ratio of ferric chloride hexahydrate to pyrrole monomer is 3:2.

[0050] During the reaction in steps 1-3, pyrrole monomers undergo in-situ oxidative polymerization near the gelatin molecular chains, causing the formed polypyrrole to be grafted or bonded to the gelatin backbone, resulting in polypyrrole-grafted gelatin. Because gelatin has good aqueous dispersibility, after polypyrrole is grafted onto the gelatin backbone, the polypyrrole disperses along with the backbone in the aqueous system, reducing the tendency for aggregation when polypyrrole is directly added to the aqueous system. This structure is beneficial for the subsequent formation of a uniformly distributed conductive phase within the hydrogel network.

[0051] Steps 1-4. Product dialysis separation

[0052] After the oxidative polymerization reaction in steps 1-3 is completed, the reaction product is dialyzed to remove unreacted pyrrole monomers, oxidants, and low-molecular-weight byproducts. The dialyzing time is 48 to 96 hours. After dialyzing, the obtained product is freeze-dried to obtain polypyrrole-grafted gelatin, denoted as Gel-PPy.

[0053] The Gel-PPy obtained in this embodiment can be used for the preparation of conductive polymer hydrogels in subsequent steps. Since polypyrrole is loaded onto the gelatin backbone in a grafted state, Gel-PPy has good dispersibility in water and can form a uniform precursor solution when mixed with acrylic monomers, crosslinking agents and initiators, thus providing a basis for the construction of a uniform conductive network in the hydrogel.

[0054] Step 2. Thermal initiation for preparation of conductive polymer hydrogel

[0055] Step 2-1. Mix the Gel-PPy obtained in Step 1, acrylic monomer (AA), crosslinking agent, initiator, and deionized water to obtain the hydrogel precursor solution. The crosslinking agent is N,N′-methylenebisacrylamide (MBAA); the initiator is ammonium persulfate (APS).

[0056] In this embodiment, the hydrogel precursor solution was prepared according to the following ratio: AA 1g, MBAA 2mg, APS 80mg, Gel-PPy 50mg, and deionized water 4mL. After mixing the above components, the mixture was stirred thoroughly to ensure that Gel-PPy was uniformly dispersed in the system and that the acrylic monomer, MBAA, and APS formed a homogeneous mixture.

[0057] Step 2-2. Place the hydrogel precursor solution in a mold and initiate a thermal polymerization reaction at approximately 50°C for 10 to 30 minutes. During the reaction, AA polymerizes under the crosslinking action of MBAA to form a polyacrylic acid network (PAA). Simultaneously, Gel-PPy is dispersed in the polyacrylic acid network and together with the polyacrylic acid network forms a Gel-PPy / PAA hydrogel, denoted as PGP-1.

[0058] In this embodiment, the introduction of polypyrrole increases the steric hindrance effect in the polymerization reaction, making polymerization less likely to occur, thereby enabling thermally initiated polymerization at 50°C.

[0059] In the Gel-PPy / PAA hydrogel, PAA provides the material with formability, flexibility, and stretchability, while Gel-PPy provides electrical conductivity. The gelatin backbone in Gel-PPy helps maintain the dispersion of polypyrrole in the hydrogel system and reduces polypyrrole aggregation. The interaction between Gel-PPy and PAA helps limit the migration and reconstruction of conductive components during cyclic deformation. Therefore, the resulting hydrogel can maintain a relatively stable conductive pathway during stretching and release, exhibiting low electrical hysteresis.

[0060] Comparative Example 1

[0061] A method for preparing a polyacrylic acid hydrogel includes the following steps:

[0062] (1) Acrylic acid monomer (AA), pyrrole monomer, crosslinking agent, initiator and deionized water are mixed to obtain hydrogel precursor solution. Among them, the crosslinking agent is N,N′-methylenebisacrylamide, denoted as MBAA; the initiator is ammonium persulfate, denoted as APS.

[0063] The hydrogel precursor solution for this comparative example was prepared according to the following ratio: AA 1g, pyrrole monomer 40mg, MBAA 2mg, APS 200mg, and deionized water 4mL. The above components were mixed and stirred thoroughly to form a homogeneous mixture of acrylic monomer, MBAA, and APS.

[0064] (2) The hydrogel precursor liquid was placed in a mold and subjected to thermally initiated polymerization at approximately 50°C for 1 hour. During the reaction, pyrrole monomers polymerized to polypyrrole, and AA polymerized under the crosslinking action of MBAA to form PPy / PAA hydrogel.

[0065] To verify the various properties of the Gel-PPy / PAA hydrogel prepared in Example 1, the following experiments were conducted:

[0066] Experimental Example 1: Determination of Polypyrrole Grafting Rate in Gel-PPy

[0067] To verify the polypyrrole grafting rate in Gel-PPy prepared in step 1 of Example 1, the following measurements were performed:

[0068] (1) Weigh a certain amount of Gel-PPy sample, for example, 1 g, add the Gel-PPy sample to a 15 mol / L hydrochloric acid solution, and carry out a hydrolysis reaction at about 110 °C for about 12 h. During the hydrolysis process, the gelatin backbone is hydrolyzed into soluble amino acids, while the polypyrrole grafted or bound to the gelatin backbone is insoluble in the acidic system and is retained as an insoluble substance.

[0069] (2) After hydrolysis, the insoluble matter is collected and thoroughly washed to remove hydrochloric acid, soluble amino acids, and other soluble impurities. The washed insoluble matter is then dried and weighed. The mass percentage of polypyrrole in Gel-PPy is calculated based on the ratio between the mass of the insoluble matter and the mass of the Gel-PPy sample, thus obtaining the grafting rate.

[0070] After multiple measurements, the grafting rate of Gel-PPy obtained in step 1 of Example 1 ranged from 4.2 wt% to 6.7 wt%. Within this grafting rate range, polypyrrole can stably distribute as a conductive functional component on the gelatin backbone, enabling Gel-PPy to possess both conductive properties and aqueous phase dispersion capabilities. This grafting rate range also helps avoid the problems of insufficient network conductivity due to excessively low polypyrrole content, or decreased dispersibility and localized aggregation of the conductive phase due to excessively high polypyrrole content.

[0071] Experimental Example 2: Characterization using scanning electron microscopy

[0072] To further analyze the conductivity uniformity of the Gel-PPy / PAA hydrogel prepared in Example 1, scanning electron microscopy was used to characterize the Gel-PPy / PAA hydrogel prepared in Example 1 and the PPy / PAA hydrogel prepared in Comparative Example 1.

[0073] Before testing, the hydrogel sample can be freeze-dried to maintain its porous network morphology. The microstructure of the hydrogel cross-section or surface can then be observed. Characterization results are as follows: Figure 4 As shown, the Gel-PPy / PAA composite conductive hydrogel in part a exhibits a relatively uniform porous network structure with no obvious agglomeration of conductive components; while in part b, the PPy / PAA hydrogel shows significant agglomeration of conductive components. This result indicates that, compared to the direct composite of polypyrrole and polyacrylic acid, polypyrrole-grafted gelatin maintains better dispersibility in the hydrogel system, resulting in a more uniform distribution of conductive components within the PAA network. Because the uniform distribution of conductive components in the hydrogel network reduces abrupt changes in local conductive pathways, the resulting Gel-PPy / PAA composite conductive hydrogel maintains a more stable conductive path during stretching, release, and other deformation processes, thus helping to reduce electrical hysteresis and improve the repeatability of the resistance response.

[0074] Experiment Example 3: Electrical Hysteresis and Cyclic Stability Test

[0075] A Gel-PPy / PAA composite conductive hydrogel sample was prepared into a strip structure suitable for tensile testing. Both ends of the hydrogel sample were fixed between two clamps of the tensile testing apparatus, and the hydrogel sample was connected to a digital source meter. The digital source meter can be a Keithley 2450 SourceMeter. The resistance change of the hydrogel sample during the tensile process was monitored and recorded in real time using the digital source meter.

[0076] During the testing process, the hydrogel samples were subjected to cyclic tensile loading and unloading, and strain and resistance changes were recorded simultaneously. The relative resistance change ΔR / R0 was calculated based on the test data, where R0 is the initial resistance and ΔR is the resistance change during deformation. The electrical response stability and electrical hysteresis behavior of the hydrogel during cyclic loading and unloading were evaluated by plotting the ΔR / R0-strain relationship curve.

[0077] When testing under different tensile speed conditions, the tensile speed can be set to 30 mm / min, 60 mm / min, 120 mm / min, and 240 mm / min under 100% strain. The test results are as follows: Figure 5As shown in section a, the hydrogel maintains a stable resistance response under different stretching speeds, indicating that its sensing signal is not easily affected by significant changes in stretching speed. This property is beneficial for using hydrogels as flexible strain sensors to detect human motion at different speeds.

[0078] When conducting tests under different strain conditions, the strain can be set to 10%, 50%, 100%, and 200%. The test results are as follows: Figure 5 As shown in section a, the Gel-PPy / PAA composite conductive hydrogel can produce a stable and repeatable resistance response under different strain conditions. As the strain increases, the conductive path inside the hydrogel changes accordingly, and the resistance shows a detectable change; after release, the conductive path can be restored, and the resistance signal can return to near the initial state.

[0079] In long-term cyclic stability testing, over 500 cyclic load-unload tests can be performed under 50% strain conditions, for example, 550 cyclic load-unload tests. Test results are as follows... Figure 5 As shown in section c, the ΔR / R0 signal of the hydrogel remains stable after multiple cycles, and the resistance baseline does not show significant drift. This result indicates that the Gel-PPy / PAA composite conductive hydrogel can maintain a stable conductive path after repeated deformation and has good cycling stability.

[0080] In the single load-unload test, the test results are as follows: Figure 5 As shown in section d, the resistance-strain curves of the hydrogel largely overlap during the loading and unloading processes, indicating that the hydrogel exhibits low electrical hysteresis. This low electrical hysteresis is related to the uniform dispersion of Gel-PPy and the synergistic composite network of Gel-PPy / PAA. The uniform dispersion of Gel-PPy reduces abrupt changes in conductive pathways caused by local aggregation of conductive components; the interaction between Gel-PPy and PAA restricts the migration and reconstruction of conductive components during cyclic deformation, allowing the conductive pathways to deform synchronously with the hydrogel network and recover after release. Therefore, the hydrogel can output a resistance signal with good repeatability.

[0081] Example 2

[0082] A method for preparing a low-hysteresis conductive polymer hydrogel is disclosed. The difference between this embodiment and Example 1 lies only in the amount of Gel-PPy used in step 2-1 when preparing the hydrogel precursor solution. In this embodiment, the amount of Gel-PPy used is 100 mg. The hydrogel precursor solution is prepared according to the following ratio: AA 1 g, MBAA 2 mg, APS 80 mg, Gel-PPy 100 mg, and deionized water 4 mL. After mixing the above components, the mixture is stirred thoroughly to ensure uniform dispersion of Gel-PPy in the system and to form a homogeneous mixture of acrylic monomer, MBAA, and APS. The Gel-PPy / PAA hydrogel obtained in this embodiment is designated as PGP-2.

[0083] Example 3

[0084] A method for preparing a low-hysteresis conductive polymer hydrogel is disclosed. The difference between this embodiment and Example 1 lies only in the amount of Gel-PPy used in step 2-1 when preparing the hydrogel precursor solution. In this embodiment, the amount of Gel-PPy used is 150 mg. The hydrogel precursor solution is prepared according to the following ratio: AA 1 g, MBAA 2 mg, APS 80 mg, Gel-PPy 150 mg, and deionized water 4 mL. After mixing the above components, the mixture is stirred thoroughly to ensure uniform dispersion of Gel-PPy in the system and to form a homogeneous mixture of acrylic monomer, MBAA, and APS. The Gel-PPy / PAA hydrogel obtained in this embodiment is designated as PGP-3.

[0085] Example 4

[0086] A method for preparing a low-hysteresis conductive polymer hydrogel is disclosed. The difference between this embodiment and Example 1 lies only in the amount of Gel-PPy used in step 2-1 when preparing the hydrogel precursor solution. In this embodiment, the amount of Gel-PPy used is 200 mg. The hydrogel precursor solution is prepared according to the following ratio: AA 1 g, MBAA 2 mg, APS 80 mg, Gel-PPy 200 mg, and deionized water 4 mL. After mixing the above components, the mixture is stirred thoroughly to ensure uniform dispersion of Gel-PPy in the system and to form a homogeneous mixture of acrylic monomer, MBAA, and APS. The Gel-PPy / PAA hydrogel obtained in this embodiment is designated as PGP-4.

[0087] Example 5

[0088] A method for preparing a low-hysteresis conductive polymer hydrogel is disclosed. The difference between this embodiment and Example 1 lies only in the amount of Gel-PPy used in step 2-1 when preparing the hydrogel precursor solution. In this embodiment, the amount of Gel-PPy used is 250 mg. The hydrogel precursor solution is prepared according to the following ratio: AA 1 g, MBAA 2 mg, APS 80 mg, Gel-PPy 250 mg, and deionized water 4 mL. After mixing the above components, the mixture is stirred thoroughly to ensure uniform dispersion of Gel-PPy in the system and to form a homogeneous mixture of acrylic monomer, MBAA, and APS. The Gel-PPy / PAA hydrogel obtained in this embodiment is designated as PGP-5.

[0089] In Examples 1-5, the amounts of AA, MBAA, APS, and water were kept constant, while the amount of Gel-PPy was gradually increased. By adjusting the ratio of Gel-PPy to AA, the content of conductive functional components, the density of the conductive network, and the composite network structure in the hydrogel can be controlled, thereby regulating the electrical response and hysteresis behavior of the hydrogel. When the amount of Gel-PPy is low, the content of conductive functional components in the hydrogel is low, and the density of the conductive network is relatively low; as the amount of Gel-PPy increases, the conductive network gradually strengthens. Since the polypyrrole in Gel-PPy is grafted onto the gelatin backbone, Gel-PPy can be dispersed relatively uniformly in the aqueous system, thus facilitating the formation of a continuous and uniform conductive network within the above-mentioned dosage range. This conductive network can undergo synergistic deformation with the PAA network during cyclic deformation and recover after release, thereby improving the repeatability of the resistance response.

[0090] Comparative Example 2

[0091] A method for preparing a polyacrylic acid hydrogel includes the following steps:

[0092] (1) The acrylic monomer (AA), crosslinking agent, initiator and deionized water are mixed to obtain the hydrogel precursor solution. The crosslinking agent is N,N′-methylenebisacrylamide, denoted as MBAA; the initiator is ammonium persulfate, denoted as APS.

[0093] The hydrogel precursor solution for this comparative example was prepared according to the following ratio: AA 1g, MBAA 2mg, APS 40mg, and deionized water 4mL. The above components were mixed and stirred thoroughly to form a homogeneous mixture of acrylic monomer, MBAA, and APS.

[0094] (2) The hydrogel precursor liquid is placed in a mold and subjected to thermally initiated polymerization at approximately 50°C for 10 to 30 minutes. During the reaction, AA polymerizes under the crosslinking action of MBAA to form a polyacrylic acid network, denoted as PAA.

[0095] Based on Comparative Example 2 and Examples 1-5, the following experimental examples are further provided:

[0096] Experiment Example 4: Tensile Mechanical Property Testing

[0097] To analyze the performance differences between Comparative Example 2 and Examples 1-5, tensile mechanical property tests were conducted, and the Young's modulus and toughness of each sample were calculated based on the test results. The results are as follows: Figure 6 As shown (Comparative Example 2 corresponds to PAA hydrogel, Examples 1-5 correspond to PGP-1 to PGP-5 respectively).

[0098] Figure 6 Part a presents the stress-strain curves for Comparative Example 2 and Examples 1-5. From... Figure 6 As can be seen from Part a, the PAA sample prepared in Comparative Example 2 exhibits lower fracture stress, indicating that the mechanical strength of PAA hydrogel alone is limited. Examples 1-5 all showed higher stress response and greater tensile deformation capacity after the addition of Gel-PPy, indicating that the composite network formed by Gel-PPy and PAA can improve the mechanical properties of the hydrogel. Among them, the PGP-3 sample prepared in Example 3 has higher fracture stress and greater fracture strain, indicating that at this Gel-PPy content, the Gel-PPy / PAA hydrogel has a good balance between strength and ductility.

[0099] Figure 6 Part b presents the Young's modulus and toughness test results for Comparative Example 2 and Examples 1-5. From... Figure 6 As shown in Part b, compared with the PAA sample prepared in Comparative Example 2, the Young's modulus and toughness of the PGP-1 to PGP-5 samples prepared in Examples 1-5 are generally improved, indicating that the introduction of Gel-PPy can enhance the PAA hydrogel network and improve the material's resistance to deformation and ability to absorb deformation energy. With increasing Gel-PPy content, the Young's modulus and toughness of the hydrogel show a trend of first increasing and then decreasing, indicating that the Gel-PPy content affects the mechanical properties of the composite network structure; an appropriate amount of Gel-PPy is beneficial for forming a stable composite network, while excessive content may affect network uniformity or flexibility, thus causing changes in mechanical properties.

[0100] The tensile recovery and compression recovery tests were performed on the Gel-PPy / PAA composite conductive hydrogel (i.e., the PGP-3 sample) prepared in Example 3. The results are as follows: Figure 6 Parts c and d are shown. Figure 6 Part c shows a photograph of the hydrogel stretching and release process; from Figure 6As can be seen from part c, the hydrogel can undergo large deformation when stretched by external force, and can recover to near its initial state after the external force is released, indicating that the hydrogel has good tensile deformation capacity and deformation recovery capacity. Figure 6 Part d is a photograph of the hydrogel compression and release process; from Figure 6 As can be seen from part d, the hydrogel can still maintain its overall structure after compression and recover its shape after the external force is released, indicating that the hydrogel has good compression recovery ability.

[0101] The above results demonstrate that the combined introduction of Gel-PPy and PAA networks can provide a continuous flexible network for the composite hydrogel; simultaneously, Gel-PPy can be dispersed within the PAA network as a conductive functional component. By forming a composite network through Gel-PPy and PAA, the resulting hydrogel possesses both excellent mechanical deformation capacity and a foundation for constructing a conductive network, which is beneficial for maintaining stable conductive pathways during cyclic deformation and reducing electrical hysteresis caused by the migration or reconstruction of conductive components.

[0102] Experiment Example 5: Tensile Mechanical Property Testing

[0103] The Gel-PPy / PAA composite conductive hydrogel prepared in Example 3 was used as the test sample, denoted as PGP-3; the polyacrylic acid network prepared in Comparative Example 2 was used as the control sample, denoted as PAA. ATR-FTIR tests were performed on the PGP-3 and PAA samples, and the results are as follows: Figure 7 As shown in part a. Figure 7 Part a shows the ATR-FTIR spectra of the PGP-3 and PAA samples, with the left side showing the 4000 cm⁻¹ spectra. -1 Up to 1000cm -1 The spectrum is within the range, with the right side at 3600 cm⁻¹. -1 Up to 3200cm -1 A magnified view of a portion of the area.

[0104] from Figure 7 As can be seen from part a, the PGP-3 sample and the PAA sample at 3600 cm⁻¹ -1 Up to 3200cm -1 A broad absorption peak is present throughout the range, which can be attributed to stretching vibration absorptions caused by hydrogen bonding interactions of hydroxyl, amino, or carboxyl groups. Compared to the PAA sample, the absorption peak position of the PGP-3 sample shifts in this region, indicating that hydrogen bonding interactions are formed between Gel-PPy and PAA after Gel-PPy is introduced into the PAA network. Therefore, it can be concluded that Gel-PPy is not simply physically dispersed within the PAA network, but rather interacts with the PAA network through hydrogen bonding, thus forming a Gel-PPy / PAA synergistic complex network.

[0105] Furthermore, the PGP-3 samples were subjected to cyclic tensile treatment, and ATR-FTIR tests were performed on the PGP-3 samples after the 1st, 5th, and 20th cycles of tensile treatment, respectively. The results are as follows. Figure 7 As shown in part b. Figure 7 Part b shows the ATR-FTIR spectra of the PGP-3 sample after different cycles of stretching; the left side is the 4000 cm⁻¹ spectrum. -1 Up to 1000cm -1 The spectrum is within the range, with the right side at 3600 cm⁻¹. -1 Up to 3200cm -1 A magnified view of a portion of the area.

[0106] from Figure 7 As can be seen in part b, after the 1st, 5th, and 20th cycles of tensile testing, the PGP-3 sample at 3600 cm⁻¹... -1 Up to 3200cm -1 The broad absorption peaks within the range still exist, and their positions do not significantly disappear or shift drastically, indicating that the hydrogen bond interaction between Gel-PPy and PAA remains stable during cyclic stretching. This result demonstrates that the Gel-PPy / PAA synergistic composite network is not easily disrupted during cyclic deformation.

[0107] The ATR-FTIR test results show that there is a stable hydrogen bond interaction between Gel-PPy and PAA. This hydrogen bond interaction helps to limit the migration and reconstruction of the conductive component of Gel-PPy during cyclic deformation, enabling the conductive phase to deform in tandem with the PAA network and recover after release. This reduces the electrical hysteresis caused by the evolution of the conductive pathway and improves the consistency of the resistive response of the hydrogel during the loading-unloading process.

[0108] Example 6

[0109] A method for preparing a low-hysteresis conductive polymer hydrogel. The difference between this embodiment and Example 1 is that the Gel-PPy / PAA hydrogel is not generated by thermally initiated polymerization, but by photoinitiated polymerization.

[0110] The preparation method provided in this embodiment is as follows:

[0111] Step 1: Preparation of polypyrrole-grafted gelatin, the process is the same as step 1 in Example 1.

[0112] Step 2: Photoinitiation preparation of conductive polymer hydrogel

[0113] Step 2-1. Mix the Gel-PPy obtained in Step 1, acrylic monomer (AA), crosslinking agent, water-soluble photoinitiator, and deionized water to obtain a hydrogel precursor solution. The crosslinking agent is N,N'-methylenebisacrylamide (MBAA); the water-soluble photoinitiator is lithium phenyl(2,4,6-trimethylbenzoyl)phosphonate (LAP) or 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylphenylacetone (Irgacure 2959).

[0114] In this embodiment, the hydrogel precursor solution was prepared according to the following ratio: AA 1g, MBAA 2mg, Gel-PPy 50mg, deionized water 4mL, and an appropriate amount of water-soluble photoinitiator (selected in this embodiment as 20mg-50mg, for example 30mg). After mixing the above components, the mixture was stirred thoroughly to ensure that Gel-PPy was uniformly dispersed in the system and that the acrylic monomer, MBAA, and water-soluble photoinitiator formed a homogeneous mixture. In some other embodiments, the amount of Gel-PPy can be other values, such as 100mg, 150mg, 200mg, or 250mg.

[0115] Step 2-2. Place the hydrogel precursor liquid in a mold and carry out a photoinitiated polymerization reaction under light irradiation for 5 to 20 minutes to form a Gel-PPy / PAA composite conductive hydrogel.

[0116] Compared to Example 1, this embodiment uses photo-initiated polymerization to replace thermally initiated polymerization, making it suitable for applications sensitive to fabrication temperature. After forming the PAA network via photoinitiation, Gel-PPy is also dispersed within the PAA network, forming a composite hydrogel with a uniform conductive network. This method expands the applicability of hydrogel fabrication processes, facilitating the selection of thermal or photoinitiation methods based on device fabrication conditions.

[0117] Example 7

[0118] A flexible strain sensor is disclosed for human motion monitoring. The flexible strain sensor includes a flexible substrate, a flexible sensing element, and electrodes. The flexible sensing element is prepared using a Gel-PPy / PAA composite conductive hydrogel prepared according to any one of Examples 1-6. Both the flexible sensing element and the electrodes are fixed to the flexible substrate. The two electrodes are respectively fixed at both ends of the flexible sensing element. The resistance value changes accordingly when the length of the flexible sensing element changes.

[0119] The fabrication process of the flexible strain sensor is as follows:

[0120] (1) Cut the Gel-PPy / PAA composite conductive hydrogel prepared in any one of the examples 1-6 into strips, sheets or other shapes suitable for attachment to obtain a flexible detection element.

[0121] (2) Connect electrodes to both ends of the flexible sensing element to form a flexible strain sensor. The electrodes can be metal wires, conductive tape, conductive fabric or flexible electrode sheets.

[0122] During use, the flexible strain sensor is attached to human joints, such as the knee, elbow, finger, or wrist joints, taking advantage of the self-adhesive properties of the Gel-PPy / PAA composite conductive hydrogel. The sensor is then connected to a resistance testing device. When the human joint bends or extends, the hydrogel sensor stretches or releases with the joint movement, and the internal conductive network changes with the deformation, thereby generating a corresponding resistance change signal.

[0123] Taking knee joint motion monitoring as an example, a flexible strain sensor is attached to the knee. When the knee is bent, the hydrogel sensor is stretched, and its resistance changes; when the knee is straightened, the hydrogel sensor is released and returns to its original position, and the resistance signal recovers accordingly. Due to the low electrical hysteresis and good cyclic stability of the Gel-PPy / PAA composite conductive hydrogel, the sensor can generate a stable and repeatable electrical signal response to different degrees of knee flexion and can maintain a relatively stable signal output at different flexion speeds.

[0124] In this embodiment, the flexible strain sensor can be used to detect minute deformations and larger joint movements. Because hydrogels are flexible and stretchable, they can adhere well to human skin or flexible substrates; and because they have a uniform conductive network and low electrical hysteresis, they can improve the stability, repeatability, and reliability of the sensing signal.

Claims

1. A low-hysteresis conductive polymer hydrogel; characterized in that, The invention includes a polyacrylic acid network and a polypyrrole-grafted gelatin dispersed in the polyacrylic acid network; the polypyrrole-grafted gelatin is dispersed in the polyacrylic acid network by initiating a polymerization reaction of the acrylic acid monomer after mixing the acrylic acid monomer and the polypyrrole-grafted gelatin. The polypyrrole-grafted gelatin is obtained by adding pyrrole monomers to the gelatin solution and initiating an oxidative polymerization reaction of the pyrrole monomers.

2. The low-hysteresis conductive polymer hydrogel according to claim 1, characterized in that, The mass ratio of the polypyrrole-grafted gelatin to the acrylic monomer is (1-5):20; the mass ratio of the pyrrole monomer to the gelatin is 1:(5-10).

3. The low-hysteresis conductive polymer hydrogel according to claim 1, characterized in that, The polypyrrole grafted gelatin contains 4.2 wt% to 6.7 wt% polypyrrole.

4. A preparation method, characterized in that, A method for preparing a low-hysteresis conductive polymer hydrogel as described in any one of claims 1-3; the preparation method includes: Prepare a pyrrole-gelatin mixed solution, and add an oxidant to the pyrrole-gelatin mixed solution to initiate the oxidative polymerization reaction of pyrrole monomers, so that the formed polypyrrole is grafted onto the gelatin backbone to obtain polypyrrole-grafted gelatin. The polypyrrole-grafted gelatin, acrylic monomer, crosslinking agent and initiator are mixed to initiate the polymerization reaction of acrylic monomer to form a polyacrylic acid network, and the polypyrrole-grafted gelatin is dispersed in the polyacrylic acid network to obtain a low-hysteresis conductive polymer hydrogel.

5. The preparation method according to claim 4, characterized in that, The polymerization reaction of the acrylic monomer is a thermally initiated polymerization reaction; the reaction temperature of the thermally initiated polymerization reaction is 45℃~65℃, and the reaction time is 10min~30min.

6. The preparation method according to claim 4, characterized in that, The initiator is a water-soluble photoinitiator; the acrylic monomer polymerization reaction is a photoinitiated polymerization reaction with a reaction time of 5 to 20 minutes.

7. The preparation method according to claim 4, characterized in that, The mass fraction of the gelatin solution is 0.5wt% to 1.5wt%.

8. The preparation method according to claim 4, characterized in that, The oxidant includes ferric chloride hexahydrate; the molar ratio of ferric chloride hexahydrate to pyrrole monomer is (2.5-3.5):

2.

9. The preparation method according to claim 8, characterized in that, The reaction conditions for the oxidative polymerization of the pyrrole monomer are: reaction at an inert atmosphere and at a temperature of 0℃~6℃ for 16h~24h.

10. The application of a low-hysteresis conductive polymer hydrogel as described in any one of claims 1-3 in a flexible strain sensor.

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