Dielectric near-zero polymer hydrogel, preparation method thereof and flexible capacitive sensor

Abstract

The application belongs to the technical field of polymer gels, and discloses a dielectric near-zero polymer hydrogel, a preparation method thereof and a flexible capacitive sensor. The preparation method comprises the following steps: mixing an alcohol solution of a photoinitiator and a crosslinking agent with a monomer solution, ultrasonic dispersion and removal of air bubbles to obtain a precursor solution; photo-curing the precursor solution in a mold to obtain a hydrogel matrix; soaking the hydrogel matrix in an inorganic salt solution with a concentration not less than 1M for a set time, and diffusing ions into the hydrogel matrix to make the hydrogel have a dielectric near-zero. The plasma oscillation is a long-range motion, which is extremely sensitive to the microstructure and morphological changes of the material. Especially in the ENZ state, a small deformation can cause an order of magnitude change in the negative dielectric constant, thereby greatly improving the sensitivity of the capacitive response.

Description

Technical Field

[0001] This invention belongs to the field of polymer gel technology, specifically relating to a near-zero dielectric polymer hydrogel, its preparation method, and a flexible capacitive sensor. Background Technology

[0002] The statements herein provide only background information in relation to this invention and do not necessarily constitute prior art.

[0003] Flexible capacitive sensors are generally sandwich structures consisting of two electrodes and a dielectric layer between them. The dielectric layer can be a hydrogel, which has shown broad application prospects in fields such as human-computer interaction, health monitoring, and soft robotics due to its advantages such as high drift stability and simple structure.

[0004] The dielectric constant can be modulated by introducing conductive fillers (such as MXene, AuNSs, etc.) into the hydrogel dielectric layer to improve the sensitivity of flexible capacitive sensors. However, the dielectric response of such materials mainly originates from dipole polarization, which is a short-range molecular motion. When the material undergoes macroscopic tensile strain, its contribution to the sensing signal is weak, resulting in low tensile sensitivity.

[0005] Furthermore, achieving effective dielectric modulation often requires the addition of high filler content, leading to filler agglomeration and sacrificing the flexibility, tensile strength, and other mechanical properties of the hydrogel. In recent years, negative dielectric metamaterials have offered new solutions for electronic components. However, typical methods for achieving near-zero dielectric (ENZ) dielectrics (such as constructing artificial periodic arrays or metal-dielectric stacks) are complex, and negative dielectric composites based on metal fillers require filler content exceeding the percolation threshold, also facing the problem of filler agglomeration and impaired material flexibility and mechanical properties, making direct application in flexible sensing difficult. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a near-zero dielectric polymer hydrogel, its preparation method, and a flexible capacitive sensor.

[0007] To achieve the above objectives, the present invention is implemented through the following technical solution:

[0008] In a first aspect, the present invention provides a method for preparing a near-zero dielectric polymer hydrogel, comprising the following steps: mixing a mixed alcohol solution of a photoinitiator and a crosslinking agent with a monomer solution, ultrasonically dispersing and removing bubbles to obtain a precursor solution;

[0009] The precursor solution was photocured in a mold to obtain a hydrogel matrix;

[0010] The hydrogel matrix is ​​immersed in an inorganic salt solution of not less than 1M for a set time. Ions diffuse into the interior of the hydrogel matrix, so that the inorganic salt concentration inside the hydrogel matrix reaches 1M or above, and the hydrogel exhibits near-zero dielectric.

[0011] Secondly, the present invention provides a near-zero dielectric polymer hydrogel, comprising a hydrogel matrix and salt ions distributed in the hydrogel matrix, wherein the salt is an inorganic salt.

[0012] Thirdly, the present invention provides a flexible capacitive sensor, including an upper electrode, a lower electrode, and a dielectric layer, wherein the dielectric layer is located between the upper electrode and the lower electrode, and the dielectric layer is prepared using a near-zero dielectric polymer hydrogel.

[0013] The beneficial effects achieved by one or more embodiments of the present invention described above are as follows:

[0014] This invention introduces the theory of negative dielectrics into the field of flexible sensing. It utilizes free and mobile salt ions in hydrogels as plasmonic excitons to replace metal fillers or artificial structures in traditional negative dielectric materials, thereby endowing hydrogels with negative dielectrics and near-zero (ENZ) properties.

[0015] This negatively dielectric hydrogel serves as the dielectric layer of the sensor. Its sensing signal originates from two sources: firstly, changes in the hydrogel's geometry (thickness, area) under pressure or tension; and secondly, the significant influence of external deformation on the path and intensity of plasma oscillations within the hydrogel. This plasma oscillation is a long-range motion, extremely sensitive to changes in the material's microstructure and morphology. Especially in the ENZ state, even minute deformations can cause orders-of-magnitude changes in the negative dielectric constant, thereby greatly enhancing the capacitive response sensitivity.

[0016] By introducing a plasma oscillation mechanism that is extremely sensitive to deformation, the sensor's capacitance change no longer depends solely on geometric deformation, achieving a breakthrough in the theoretical sensitivity limit of traditional capacitive sensors. Its sensitivity to both pressure and tensile strain is improved by orders of magnitude, more closely resembling the multi-sensory capabilities of human skin.

[0017] When the polymer hydrogel of this invention is used as a dielectric layer, no rigid conductive filler (such as CNT or AgNW) needs to be added, completely avoiding problems such as material embrittlement and decreased flexibility caused by filler agglomeration. The material itself maintains the high tensile strength, flexibility, and biocompatibility of the hydrogel.

[0018] The preparation process of this invention does not require complex microstructure processing (such as photolithography) or expensive nanomaterials. The main raw materials are common hydrogel monomers and salts. The process is simple, low-cost, and suitable for large-scale production. Attached Figure Description

[0019] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0020] Figure 1 This is a schematic diagram of the mechanism of an embodiment of the present invention, wherein a is a schematic diagram of the structural changes of the gel under tensile and compressive strain; b is a schematic diagram of the structural changes of the gel under tensile strain and the dominance of internal plasmon oscillations; c is a comparison diagram of the capacitance changes of 1.5NaCl / PAAM and PAAM prepared in Example 2 after tensile strain, wherein k 0.01 k 0.5 The values ​​are: d, k, and e. The sensitivity of 1.5NaCl / PAAM at strains of 1% and 50%, respectively; d is a comparison of the capacitance changes of 1.5NaCl / PAAM and PAAM prepared in Example 2 after compressive strain; k is the sensitivity; e is a schematic diagram of the structural changes of gel under compressive strain and the dominance of internal interface polarization.

[0021] Figure 2 This is a schematic diagram of the 1.5NaCl / PAAM stretching capacitor, compression capacitor, and corresponding structural changes in Embodiment 2 of the present invention, wherein a is a diagram of the stretching capacitor change; b is a diagram of the compression capacitor change; c is a schematic diagram of the structural changes during the stretching process; and d is a schematic diagram of the structural changes during the compression process.

[0022] Figure 3 These are characterization diagrams of the tensile and compressive sensitivities of different NaCl / PAAM near-zero dielectric hydrogels in the embodiments of the present invention. Specifically, a is the tensile sensitivity characterization diagram of the 0.5 NaCl / PAAM near-zero dielectric hydrogel prepared in Comparative Example 1; b is the tensile sensitivity characterization diagram of the 1 NaCl / PAAM near-zero dielectric hydrogel in Example 1; c is the tensile sensitivity characterization diagram of the 1.5 NaCl / PAAM near-zero dielectric hydrogel in Example 2; d is the compressive sensitivity characterization diagram of the 0.5 NaCl / PAAM near-zero dielectric hydrogel; e is the compressive sensitivity characterization diagram of the 1 NaCl / PAAM near-zero dielectric hydrogel; and f is the compressive sensitivity characterization diagram of the 1.5 NaCl / PAAM near-zero dielectric hydrogel.

[0023] Figure 4These are the tensile and compressive sensitivity characterization graphs of PAAM gels immersed in KCl, CaCl2, and MgCl2 salt solutions, respectively, in embodiments of the present invention. Specifically, a) is the tensile sensitivity characterization graph of 1.5KCl / PAAM prepared by immersing PAAM gel in 1.5M KCl salt solution in Example 3; b) is the tensile sensitivity characterization graph of 1.5CaCl2 / PAAM prepared by immersing PAAM gel in 1.5M CaCl2 salt solution in Example 4; and c) is the tensile sensitivity characterization graph of PAAM gel immersed in 1.5M MgCl2 salt solution in Example 5. The tensile sensitivity characterization diagram of the prepared 1.5MgCl2 / PAAM is shown in Figure d; the compression sensitivity characterization diagram of 1.5KCl / PAAM prepared by soaking PAAM gel in 1.5M KCl salt solution in Example 3 is shown in Figure e; the compression sensitivity characterization diagram of 1.5CaCl2 / PAAM prepared by soaking PAAM gel in 1.5M CaCl2 salt solution in Example 4 is shown in Figure f; the compression sensitivity characterization diagram of 1.5MgCl2 / PAAM prepared by soaking PAAM gel in 1.5M MgCl2 salt solution in Example 5 is shown in Figure f.

[0024] Figure 5 The graphs show the cyclic capacitance changes of 1.5NaCl / PAAM under tensile strain and compressive strain in Example 2 of the present invention. Graph a shows the capacitance changes of 1.5NaCl / PAAM after 1000 cycles under tensile strain of 50% in Example 2, and graph b shows the capacitance changes of 1.5NaCl / PAAM after 1000 cycles under compressive strain of 30% in Example 2.

[0025] Figure 6 The graphs are: 1.5NaCl / PAAM in Example 2 of the present invention, under slight compression and slight stretching, and 2. a is the graph of capacitance change when a leaf is placed on 1.5NaCl / PAAM in Example 2 and the gel is slightly compressed; b is the graph of capacitance change when 1.5NaCl / PAAM is applied to the inside of the knee and the leg is slightly stretched from bent to straight.

[0026] Figure 7 This is a graph showing the capacitance change and sensitivity curves corresponding to the tensile and compressive strains of 1NaCl / AAc in Embodiment 6 of the present invention. In this graph, a is the capacitance change and sensitivity curve corresponding to the tensile strain of 100%, and b is the compressive stress and the corresponding capacitance change and sensitivity curve.

[0027] Figure 8 This is a graph showing the capacitance change and sensitivity curves corresponding to the tensile and compressive strains of 1NaCl / HEMA in Embodiment 7 of the present invention. In this graph, a is the capacitance change and sensitivity curve corresponding to the tensile strain of 100%, and b is the compressive stress and the corresponding capacitance change and sensitivity curve.

[0028] Figure 9 These are comparison graphs of tensile stress and compressive stress-strain curves of PAAM gel after immersion in NaCl solutions of different concentrations. Graph a shows the tensile stress-strain curves of a PAAM gel sample (a thin sheet 3cm long, 1.5cm wide, and 2mm thick) after immersion in deionized water, 0.5M NaCl solution, 1M NaCl solution, and 1.5M NaCl solution for 24 hours. Graph b shows the compressive stress-strain curves of a PAAM gel sample (cylindrical, 2cm high and 1.5cm in diameter) after immersion in deionized water, 0.5M NaCl solution, 1M NaCl solution, and 1.5M NaCl solution for 24 hours. Detailed Implementation

[0029] It should be noted that the following detailed description is illustrative and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0030] To address the technical problems mentioned in the background art, the present invention provides a method for preparing a near-zero dielectric hydrogel, comprising the following steps: mixing a mixed alcohol solution of a photoinitiator and a crosslinking agent with a monomer solution, ultrasonically dispersing and removing bubbles to obtain a precursor solution;

[0031] The precursor solution was photocured in a mold to obtain a hydrogel matrix;

[0032] The hydrogel matrix is ​​immersed in an inorganic salt solution of not less than 1M for a set time. Ions diffuse into the interior of the hydrogel matrix, so that the inorganic salt concentration inside the hydrogel matrix reaches 1M or above. At this time, the hydrogel exhibits a dielectric near zero, and thus the product is obtained.

[0033] In some embodiments, the concentration of the inorganic salt solution can be 1-9M, preferably 1-3M, and more preferably 1-1.7M.

[0034] In some embodiments, the monomer is selected from at least one of acrylamide (AM), N,N-dimethylacrylamide (DMAA), acrylic acid (AAc), hydroxyethyl methacrylate (HEMA), vinylpyrrolidone (VP), or 2-acrylamido-2-methylpropanesulfonic acid (AMPS).

[0035] Preferably, the monomer is acrylamide.

[0036] In some embodiments, the photoinitiator is selected from at least one of photoinitiator 2959 (Irgacure 2959), 2,2-dimethoxy-2-phenylacetophenone (Irgacure 651), 2-methyl-1-(4-methylthiophenyl)-2-morpholino-1-propanone (Irgacure 907), phenyl bis(2,4,6-trimethylbenzoyl)phosphine oxide (Irgacure 819), lithium phenyl (2,4,6-trimethylbenzoyl)phosphate (LAP), N-methylpyrrolidone, and 2,2'-azo(2-methyl-N-(2-hydroxyethyl)propionamide) (VA-086).

[0037] In some embodiments, the crosslinking agent is selected from at least one of N,N-methylenebis(acrylamide) (MBAA), divinylbenzene (DVB), citric acid (CA), glutaraldehyde, ethylenediamine, ethylene glycol dimethacrylate (EGDMA), and poly(melamine-formaldehyde).

[0038] Preferably, the crosslinking agent is N,N-methylenebis(acrylamide).

[0039] In some embodiments, the inorganic salt is NaCl, KCl, CaCl2, or MgCl2.

[0040] Preferably, the inorganic salt is NaCl.

[0041] Secondly, the present invention provides a near-zero dielectric hydrogel, comprising a hydrogel matrix and inorganic salt ions distributed in the hydrogel matrix, wherein the concentration of inorganic salts in the hydrogel matrix reaches 1M or above.

[0042] The inorganic salt ions can undergo plasma oscillation in the hydrogel network under the action of an alternating electric field, causing the hydrogel to exhibit a negative permittivity and an absolute value of the real part close to zero at a specific frequency (i.e., ENZ characteristic).

[0043] Thirdly, the present invention provides a flexible capacitive sensor, including an upper electrode, a lower electrode, and a dielectric layer, wherein the dielectric layer is located between the upper electrode and the lower electrode, and the dielectric layer is prepared using the near-zero dielectric hydrogel.

[0044] Flexible sensors based on near-zero dielectric hydrogels exhibit unprecedentedly high sensitivity (8451 kPa and 2884 kPa) under micro-tensile strain (~1%) to large-tensile strain (~50%). Simultaneously, they achieve a high sensitivity of up to 8457 kPa under compression (~1%-30%). -1The sensitivity is significantly improved. When near-zero dielectric materials are applied to ion-conducting hydrogel sensors, the sensing signal may originate not only from the macroscopic deformation of the hydrogel network but also from the directional migration of microscopic mobile ions under external electric field stimulation. Free ions, acting as plasmon resonances, generate negative dielectric properties, which, together with the positive dielectric effect generated by interface polarization, endow the hydrogel with near-zero dielectric properties. This greatly enhances the sensor's sensitivity.

[0045] The present invention will be further described below with reference to the embodiments.

[0046] Example 1

[0047] The preparation method of near-zero dielectric hydrogel includes the following steps:

[0048] Preparation of precursor solution: 0.00325g photoinitiator (I2959) and 0.0021875g crosslinking agent (N,N′-methylenebisacrylamide, MABA) were sonicated for 10min to completely dissolve in 1ml anhydrous ethanol to obtain a mixed ethanol solution of photoinitiator and crosslinking agent.

[0049] Take 10 μL of a mixed ethanol solution of photoinitiator and crosslinking agent and add it to 2 g of aqueous solution of 50% acrylamide monomer. Then stir at 500 rpm for 10 min and ultrasonically disperse to remove bubbles to obtain the precursor solution.

[0050] UV curing molding: The precursor solution is injected into a mold using a 1mm thick silicone pad. A groove is cut into the pad, and a Pet release film is attached to the bottom of the groove. After the precursor solution is injected, the Pet release film is placed on the surface of the precursor solution and pressed with a glass plate. Finally, it is cured under 365nm UV light for 1.5 hours to obtain a polyacrylamide (PAAM) hydrogel matrix.

[0051] Ion loading: The cured polyacrylamide hydrogel matrix was immersed in a 1M NaCl solution for 24 hours to allow salt ions to fully diffuse into the hydrogel network, ultimately yielding a 1NaCl / PAAM near-zero dielectric hydrogel. After wiping the salt solution off the gel surface with clean paper, electrodes were attached for testing.

[0052] Example 2

[0053] The preparation of the polyacrylamide hydrogel matrix is ​​the same as in Example 1, except that:

[0054] The cured polyacrylamide hydrogel matrix was immersed in a 1.5M NaCl solution for 24 hours to allow salt ions to diffuse fully into the hydrogel network, ultimately yielding a 1.5NaCl / PAAM near-zero dielectric hydrogel.

[0055] Example 3

[0056] The preparation of the polyacrylamide hydrogel matrix is ​​the same as in Example 1, except that:

[0057] The cured polyacrylamide hydrogel matrix was immersed in a 1.5M KCl solution for 24 hours to allow salt ions to diffuse fully into the hydrogel network, ultimately yielding a 1.5KCl / PAAM near-zero dielectric hydrogel.

[0058] Example 4

[0059] The preparation of the polyacrylamide hydrogel matrix is ​​the same as in Example 1, except that:

[0060] The cured polyacrylamide hydrogel matrix was immersed in a 1.5M CaCl2 solution for 24 hours to allow salt ions to diffuse fully into the hydrogel network, ultimately yielding a 1.5CaCl2 / PAAM near-zero dielectric hydrogel.

[0061] Example 5

[0062] The preparation of the polyacrylamide hydrogel matrix is ​​the same as in Example 1, except that:

[0063] The cured polyacrylamide hydrogel matrix was immersed in a 1.5M MgCl2 solution for 24 hours to allow salt ions to diffuse fully into the hydrogel network, ultimately yielding a 1.5MgCl2 / PAAM near-zero dielectric hydrogel.

[0064] Example 6

[0065] The preparation method of near-zero dielectric hydrogel includes the following steps:

[0066] Preparation of precursor solution: 0.00333g photoinitiator (Irgacure 819) and 0.0021915g crosslinking agent (divinylbenzene, DVB) were sonicated for 10min to completely dissolve in 1ml anhydrous ethanol to obtain a mixed ethanol solution of photoinitiator and crosslinking agent.

[0067] Take 10 μL of a mixed ethanol solution of photoinitiator and crosslinking agent and add it to 2 g of 50% (w / w) aqueous solution of acrylic acid (AAc) monomer. Then stir at 500 rpm for 10 min and ultrasonically disperse to remove bubbles to obtain the precursor solution.

[0068] UV curing molding: The precursor solution is injected into a mold and cured under 365nm UV light for 1.5 hours to obtain a polyacrylamide (PAAM) hydrogel matrix.

[0069] Ion loading: The cured polyacrylamide hydrogel matrix was immersed in a 1M NaCl solution for 24 hours to allow salt ions to diffuse fully into the hydrogel network, ultimately resulting in a 1NaCl / AAc near-zero dielectric hydrogel.

[0070] Example 7

[0071] The preparation method of near-zero dielectric hydrogel includes the following steps:

[0072] Preparation of precursor solution: 0.00333g photoinitiator (LAP) and 0.0021915g crosslinking agent (divinylbenzene, DVB) were sonicated for 10min to completely dissolve in 1ml anhydrous ethanol to obtain a mixed ethanol solution of photoinitiator and crosslinking agent.

[0073] Take 10 μL of a mixed ethanol solution of photoinitiator and crosslinking agent and add it to 2 g of an aqueous solution of 50% hydroxyethyl methacrylate (HEMA) monomer. Then stir at 500 rpm for 10 min and ultrasonically disperse to remove bubbles to obtain the precursor solution.

[0074] UV curing molding: The precursor solution is injected into a mold and cured under 365nm UV light for 1.5 hours to obtain a polyacrylamide (PAAM) hydrogel matrix.

[0075] Ion loading: The cured polyacrylamide hydrogel matrix was immersed in a 1M NaCl solution for 24 hours to allow salt ions to diffuse fully into the hydrogel network, ultimately obtaining a 1NaCl / HEMA near-zero dielectric hydrogel.

[0076] Comparative Example 1

[0077] The preparation of the polyacrylamide hydrogel matrix is ​​the same as in Example 1, except that:

[0078] The cured polyacrylamide hydrogel matrix was immersed in a 0.5M NaCl solution for 24 hours to allow salt ions to fully diffuse into the hydrogel network, ultimately yielding a 0.5NaCl / PAAM hydrogel.

[0079] The dielectric constant of the prepared negative dielectric hydrogel was analyzed. In traditional negative dielectric composite materials, the internal conductive metal filler is a solid plasma, and free electrons generate collective oscillations under the action of an external electric field. The dispersion relationship of the dielectric constant is described by the Drude model.

[0080] (1)

[0081] (2)

[0082] (3)

[0083] Among them, Γ D It is the damping constant. n eff It is the effective concentration of electrons. m eff It is the effective weight of electrons. e It is the electron charge (1.6 × 10⁻⁶). -19 C), ε0 is the vacuum permittivity (8.85 × 10⁻⁶). -12 F m -1 ).

[0084] Unlike traditional negative dielectric materials, the near-zero dielectric hydrogel prepared in this invention is not composed of electrons, but rather of larger, more massive ions that move within the liquid, such as Na+. + Cl - This also results in the damping coefficient of ion reciprocating motion being much higher than that of free electrons within the negatively charged hydrogel of this invention. For the negatively charged hydrogel of this invention, ε′ decreases with increasing frequency, exhibiting a significant Debye relaxation phenomenon, which can be explained using the Debye model:

[0085] (4)

[0086] (5)

[0087] Where, ε ∞ It is the dielectric constant at the optical frequency, ε s It is the dielectric constant under static or low-frequency conditions. ω ( ω = 2π f ) is the angular frequency of the electric field, and τ is the relaxation time.

[0088] Furthermore, when the salt concentration (such as NaCl) exceeds the perosmosis threshold, the negative dielectric hydrogel of this invention does not exhibit a negative dielectric constant across the entire frequency range; instead, it changes from positive to negative with increasing frequency. Within the negative dielectric hydrogel of this invention, plasma oscillations are no longer caused by electrons, but by ions with larger mass and volume. At this point, the movement of these ions under the influence of an alternating electric field undergoes Debye relaxation, affecting the dielectric constant and frequency dispersion. Therefore, the Debye-Drude model can be used to fit the dielectric constant of 1NaCl / PAAM. ε ′ – f curve.

[0089] (6)

[0090] The sensitivity under tension is (GF = ΔC / C0·ε), and the sensitivity under compression is GF = ΔC / C0·dP. Where C0 is the initial capacitance without external pressure, ΔC is the capacitance change, ε is the strain of the hydrogel, and P is the stress of the hydrogel.

[0091] During testing, the hydrogels prepared in the examples and comparative examples were cut into thin sheets with a length of 3 cm, a width of 1.5 cm, and a thickness of 2 mm.

[0092] Figure 1 In the diagram, 'a' represents the structural changes of the gel under tensile and compressive strains. The light blue line represents the hydrogel, the dark blue line represents the polyacrylamide chain, and the gray and yellow spheres represent Na₂O₃ and Na₂O₃, respectively. + Cl - b shows a schematic diagram of the structural changes of the gel under tensile strain and the interfacial polarization and plasmon oscillations present inside, with plasmon oscillations being dominant; c compares the capacitance changes of 1.5NaCl / PAAM and PAAM after tensile strain, where k 0.01 =8451.06、k 0.5 =2884.16 represents the sensitivity of 1.5NaCl / PAAM at strains of 1% and 50%, respectively. e is a schematic diagram of the structural changes of the gel under compressive strain and the interfacial polarization and plasmon oscillations present inside, in which interfacial polarization is dominant; d is a comparison of the capacitance changes of 1.5NaCl / PAAM and PAAM after compressive strain, with a sensitivity k=8457.24 kPa. -1 .

[0093] Figure 2 This is a schematic diagram of the stretching capacitance, compressive capacitance, and corresponding structural changes of 1.5NaCl / PAAM in Embodiment 2 of the present invention. Figure a shows the stretching capacitance change; figure b shows the compressive capacitance change; figure c shows the structural changes during stretching; and figure d shows the structural changes during compression. From figures a and c, the sensitivity of 1.5NaCl / PAAM during stretching can be divided into three stages: initial strain (0-0.3), mid-strain (0.3-0.7), and strain equilibrium (0.7-1). At the initial strain, the orientation of polymer chains and the internal pore structure of the gel increase during stretching. At this time, the ion movement within the gel also undergoes orientation, and plasma oscillation is enhanced, resulting in a significant change in the negative capacitance from its initial near-zero point during stretching. During the mid-strain stage, while the pore size increases radially, its longitudinal size decreases, and the distance between polymer chains decreases. At this time, the positive dielectric effect interface polarization between ions and polymer chains increases, and the increase in negative capacitance weakens. In the strain equilibrium stage, the positive and negative dielectric effects within the gel reach equilibrium, and the negative capacitance is at its maximum.

[0094] As shown in b and d, compressive stress causes the collapse of the polyacrylamide network structure inside the near-zero dielectric hydrogel of xNaCl / PAAM, leading to contact between polymer chains within the gel and thus generating more interfaces. This results in a significant increase in interfacial polarization, while the reduction in internal porosity leads to a decrease in the ion diffusion coefficient. Consequently, the negative dielectric response induced by plasma oscillation weakens, while the positive capacitance increases exponentially under the synergistic effect of positive and negative dielectrics. The results indicate that the near-zero dielectric effect caused by the synergistic effect of positive and negative dielectrics enables the 1.5NaCl / PAAM hydrogel sensor to exhibit excellent performance. This mechanism also applies to KCl, MgCl2, and CaCl2 (e.g., ...). Figure 4 Inorganic salts have universal applicability.

[0095] Figure 3 These are characterization diagrams of the tensile and compressive sensitivities of different NaCl / PAAM near-zero dielectric hydrogels in the embodiments of the present invention. Specifically, a) is the tensile sensitivity characterization diagram of the 0.5 NaCl / PAAM near-zero dielectric hydrogel prepared in Comparative Example 1, where the gel did not exhibit ENZ and the capacitance change was small; k1=1.9 and k2=0.76 represent the sensitivities at 1% and 50% tensile strain, respectively. b) is the tensile sensitivity characterization diagram of the 1 NaCl / PAAM near-zero dielectric hydrogel prepared in Example 1, where the gel exhibited ENZ and the capacitance change was large; k1=4818.43 and k2=1883.69 represent the sensitivities at 1% and 50% tensile strain, respectively. c) is the tensile sensitivity characterization diagram of the 1.5 NaCl / PAAM near-zero dielectric hydrogel, where the gel exhibited ENZ and the capacitance change was large; k1=8451.06 and k2=2884.16 represent the sensitivities at 1% and 50% tensile strain, respectively.

[0096] Figure 3 The figure shows the compression sensitivity characterization of a 0.5 NaCl / PAAM hydrogel with near-zero dielectric strength (d). At this point, the gel does not exhibit ENZ, the capacitance change is small, and k1 = 0.026 kPa. -1 k2 = 0.16 kPa -1 ε represents the sensitivity; e is the compression sensitivity characterization graph of a 1NaCl / PAAM hydrogel with near-zero dielectric strength, at which point the gel exhibits the ENZ phenomenon, with a large and linear change in capacitance, k = 7437.21 kPa. -1 The value is denoted as sensitivity; f is the compression sensitivity characterization graph of a 1.5NaCl / PAAM hydrogel with near-zero dielectric, where the gel exhibits ENZ, with a large and linear change in capacitance, k = 8457.24 kPa. -1 Sensitivity.

[0097] Figure 4These are the tensile and compressive sensitivity characterization graphs of PAAM gels immersed in KCl, CaCl2, and MgCl2 salt solutions, respectively, in embodiments of the present invention. Specifically, a) is the tensile sensitivity characterization graph of 1.5KCl / PAAM prepared by immersing PAAM gel in 1.5M KCl salt solution in Example 3; b) is the tensile sensitivity characterization graph of 1.5CaCl2 / PAAM prepared by immersing PAAM gel in 1.5M CaCl2 salt solution in Example 4; and c) is the tensile sensitivity characterization graph of 1.5MgCl2 / PAAM prepared by immersing PAAM gel in 1.5M MgCl2 salt solution in Example 5. The image shows the tensile sensitivity characterization diagram; d is the compression sensitivity characterization diagram of 1.5KCl / PAAM prepared by soaking PAAM gel in 1.5M KCl salt solution in Example 3; e is the compression sensitivity characterization diagram of 1.5CaCl2 / PAAM prepared by soaking PAAM gel in 1.5M CaCl2 salt solution in Example 4; f is the compression sensitivity characterization diagram of 1.5MgCl2 / PAAM prepared by soaking PAAM gel in 1.5M MgCl2 salt solution in Example 5. All of these images show high strain sensitivity due to the ENZ phenomenon observed in the gel after soaking in 1.5M salt solution.

[0098] Depend on Figure 3 and Figure 4 It can be seen that the near-zero dielectric gel obtained by soaking in NaCl not only exhibits higher sensitivity compared to KCl, MgCl2, and CaCl2 at a concentration of 1.5M, but also demonstrates better stability after 1000 cycles at tensile strain of 50% and compressive strain of 30% (e.g., Figure 5 (As shown in a and b).

[0099] A leaf was placed on the 1.5NaCl / PAAM hydrogel sheet prepared in Example 2, and the capacitance change when it was slightly compressed is shown in the figure. Figure 6 As shown in Figure a; then, a 1.5NaCl / PAAM hydrogel sheet was vertically adhered to the inside of the knee (at an angle of approximately 90° between the calf and thigh). The capacitance change of the gel under slight stretching when the leg was bent from 90° to extended to 180° is shown in the figure. Figure 6 As shown in Figure b, it can be seen that 1.5NaCl / PAAM has high sensitivity to minute compression and minute stretching.

[0100] Figure 7 This is a graph showing the capacitance change and sensitivity curves corresponding to tensile and compressive strains of 1NaCl / AAc in Example 6 of this invention. In graph a, the capacitance change and sensitivity curves are shown at 100% tensile strain, with sensitivity k1 = 3510.05 and k2 = 974.74. In graph b, the capacitance change and sensitivity curves are shown at compressive stress, with sensitivity k1 = 7681.75 kPa. -1 ;

[0101] Figure 8 This is a graph showing the capacitance change and sensitivity curves corresponding to the tensile and compressive strains of 1NaCl / HEMA in Example 7 of the present invention. Here, a is the capacitance change and sensitivity curve corresponding to 100% tensile strain, with sensitivity k1=2435.59 and k2=1652.62.

[0102] b is the graph showing the compressive stress, the corresponding capacitance change, and the sensitivity curve. The sensitivity is k1 = 1607.16 kPa. -1 k2 = 4130.09 kPa -1 .

[0103] Figure 9 In the figure, 'a' represents the tensile stress-strain curves of the PAAM gel sample (a thin sheet 3 cm long, 1.5 cm wide, and 2 mm thick) in Example 1 after being immersed in deionized water, 0.5 M NaCl solution, 1 M NaCl solution, and 1.5 M NaCl solution for 24 h. The best mechanical properties were obtained by immersing the sample in 1 M NaCl solution, with a tensile fracture strain of up to 820% and a corresponding tensile fracture stress of 213.86 kPa.

[0104] Figure 9 Figure b shows the compressive stress-strain curves of PAAM gel samples (the samples are cylinders, 2 cm high and 1.5 cm in diameter) after being immersed in deionized water, 0.5 M NaCl solution, 1 M NaCl solution and 1.5 M NaCl solution for 24 h. The best mechanical properties are those of the 1 NaCl / PAAM hydrogel obtained by immersion in 1 M NaCl solution, with a maximum compressive strain of 80% and a corresponding compressive stress of 75.62 kPa.

[0105] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a near-zero dielectric hydrogel, characterized in that: The process includes the following steps: mixing a mixed alcoholic solution of photoinitiator and crosslinking agent with a monomer solution, ultrasonically dispersing and removing air bubbles to obtain a precursor solution; The precursor solution was photocured in a mold to obtain a hydrogel matrix; The hydrogel matrix is ​​immersed in an inorganic salt solution of not less than 1M for a set time, allowing ions to diffuse into the interior of the hydrogel matrix, so that the inorganic salt concentration inside the hydrogel matrix reaches 1M or above, thus obtaining the product. The crosslinking agent is selected from at least one of N,N-methylenebis(acrylamide), divinylbenzene, and ethylene glycol dimethacrylate; The inorganic salt is NaCl, KCl, CaCl2, or MgCl2; The monomer is acrylamide.

2. The method for preparing a near-zero dielectric hydrogel according to claim 1, characterized in that: The photoinitiator is selected from at least one of photoinitiator 2959, 2,2-dimethoxy-2-phenylacetophenone, 2-methyl-1-(4-methylthiophenyl)-2-morpholino-1-propanone, phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, phenyl(2,4,6-trimethylbenzoyl)lithium phosphate, and 2,2'-azo(2-methyl-N-(2-hydroxyethyl)propionamide).

3. The method for preparing a near-zero dielectric hydrogel according to claim 1, characterized in that: The crosslinking agent is N,N-methylenebis(acrylamide).

4. The method for preparing a near-zero dielectric hydrogel according to claim 1, characterized in that: The inorganic salt is NaCl.

5. A near-zero dielectric hydrogel, characterized in that: Prepared by any one of the preparation methods described in claims 1-4, comprising a hydrogel matrix and salt ions distributed in the hydrogel matrix, wherein the concentration of inorganic salts in the hydrogel matrix reaches 1M or above.

6. A flexible capacitive sensor, characterized in that: It includes an upper electrode, a lower electrode, and a dielectric layer, with the dielectric layer located between the upper and lower electrodes. The dielectric layer is prepared using the near-zero dielectric hydrogel described in claim 5.

Images