Carbon felt-hydrogel composite piezoelectric sensor and preparation method thereof

Abstract

The invention belongs to the technical field of flexible piezoelectric sensors, and relates to a carbon felt-hydrogel composite piezoelectric sensor and a preparation method thereof. The sensor comprises a piezoelectric sensing layer, a cross-scale dual-network hydrogel layer, an electrode layer, a leading-out wire and a packaging layer, the two cross-scale dual-network hydrogel layers are arranged on the upper side and the lower side of the piezoelectric sensing layer in a stacked mode to form a sandwich structure. The cross-scale dual-network hydrogel layer is an elastic hydrogel material formed by compounding a carbon fiber felt skeleton and a molecular-level network polymer, and the molecular-level network polymer penetrates through the interior and the surface of the carbon fiber felt skeleton and is formed by cross-linking and curing; the number of the electrode layers is two, and the two electrode layers are arranged on the outer sides of the two cross-scale dual-network hydrogel layers in a stacked mode respectively. The lead-out wire is electrically connected with the electrode layer; the packaging layer wraps the outer side of the electrode layer. According to the sensor, the mechanical property, the conductivity and the stability of the flexible piezoelectric sensor can be improved.

Description

Technical Field

[0001] This application relates to the field of flexible piezoelectric sensor technology, and in particular to a carbon felt-hydrogel composite piezoelectric sensor and its preparation method. Background Technology

[0002] This section is intended to provide background or context for the embodiments of the invention set forth in the claims. It should not be construed as an admission that the description herein is prior art.

[0003] With the rapid development of flexible electronics technology, flexible piezoelectric sensors have shown broad application prospects in health monitoring, human-computer interaction, and smart wearables due to their self-powered characteristics, simple structure, and fast dynamic response. Currently, most common flexible piezoelectric sensors use piezoelectric polymer films, such as polyvinylidene fluoride (PVDF), as the sensing layer, with metal foils or conductive polymer electrodes bonded to their upper and lower surfaces to form the basic sensing unit. However, this type of structure still has several inherent drawbacks in practical applications: First, the mechanical properties of a single polymer film are limited, making it prone to plastic deformation or cracking under repeated deformation, resulting in insufficient sensor durability; second, the interface compatibility between traditional rigid electrodes and flexible films is poor, easily introducing large contact resistance, affecting charge collection efficiency, and causing signal attenuation and decreased sensitivity; third, existing sensor structures are prone to interface peeling or performance drift under long-term cyclic loading, making it difficult to meet the needs of practical dynamic monitoring in terms of output stability and reliability. Although some research has attempted to improve performance by doping with inorganic piezoelectric particles or constructing composite structures, it is often difficult to simultaneously achieve good flexibility, high conductivity, and stable interface bonding.

[0004] In summary, existing flexible piezoelectric sensors suffer from common problems such as insufficient mechanical properties, unstable interface bonding, low charge transfer efficiency, and easy attenuation and poor reliability of output signals under long-term cyclic operation, which restricts their practical application in dynamic monitoring scenarios. Summary of the Invention

[0005] The present invention provides a carbon felt-hydrogel composite piezoelectric sensor and its preparation method, which at least solves the problems of insufficient mechanical properties, unstable interface bonding, low charge transfer efficiency, and easy attenuation and poor reliability of output signal under long-term cyclic operation in related technologies.

[0006] According to a first aspect of the present invention, a carbon felt-hydrogel composite piezoelectric sensor is provided, comprising: It includes: a piezoelectric sensing layer, a multi-scale dual-network hydrogel layer, an electrode layer, lead wires, and an encapsulation layer; the multi-scale dual-network hydrogel layer consists of two layers, which are stacked on the upper and lower sides of the piezoelectric sensing layer to form a sandwich structure; the multi-scale dual-network hydrogel layer is an elastic hydrogel material composed of a carbon fiber felt skeleton and a molecular-level network polymer, wherein the molecular-level network polymer penetrates the interior and surface of the carbon fiber felt skeleton and is formed by cross-linking and curing; The electrode layer comprises two layers, which are stacked on the outside of the two multi-scale dual-network hydrogel layers; the lead wire is electrically connected to the electrode layer; and the encapsulation layer covers the outside of the electrode layer.

[0007] According to a second aspect of the present invention, a method for fabricating a carbon felt-hydrogel composite piezoelectric sensor is provided, comprising: A molecular-level network polymer precursor solution was vacuum impregnated with carbon fiber felt to allow the solution to penetrate into the interior of the carbon fiber felt, and then cured and crosslinked to obtain the multi-scale dual-network hydrogel layer. A piezoelectric sensing layer is sandwiched between two of the aforementioned multi-scale dual-network hydrogel layers to form a sandwich structure; Electrode layers are provided on the upper and lower sides of the sandwich structure, and lead wires are electrically connected to the electrode layers. The assembled structure is then encapsulated to obtain the carbon felt-hydrogel composite piezoelectric sensor.

[0008] Beneficial effects of the embodiments of the present invention: The carbon felt-hydrogel composite piezoelectric sensor provided in this invention addresses the shortcomings of existing flexible piezoelectric sensors in terms of conductivity, mechanical properties, and reliability. Specifically, firstly, a sandwich structure is employed, with two multi-scale dual-network hydrogel layers stacked on the upper and lower sides of the piezoelectric sensing layer. This allows the piezoelectric sensing layer to achieve a more uniform deformation distribution and higher stress concentration under pressure through the elastic support and stress transmission of the hydrogel layers, thereby enhancing the charge generated by the piezoelectric effect and improving the sensor's output voltage and sensitivity. Secondly, the multi-scale dual-network hydrogel layer is composed of a carbon fiber felt skeleton and a molecular-level network polymer. The three-dimensional porous structure of the carbon fiber felt provides high-strength skeletal support for the hydrogel, enhancing overall mechanical properties and fatigue resistance, while the molecular-level network polymer penetrates the interior and surface of the carbon fiber felt and is cured through cross-linking. The dual-network structure not only improves the elasticity and resilience of the hydrogel but also utilizes the high conductivity of the carbon fiber felt to provide the hydrogel layer with excellent electron transport channels, reducing interfacial contact resistance and thus improving the transmission efficiency and stability of electrical signals. Furthermore, the two electrode layers, respectively disposed on the outside of the hydrogel layer and electrically connected to the lead wires, enable rapid and reliable collection of charge signals generated by the piezoelectric sensing layer, which are then output through the lead wires, ensuring the real-time performance and accuracy of signal detection. Finally, the encapsulation layer covers the outside of the electrode layers, providing mechanical protection, environmental isolation, and insulation for the sensor, enhancing its durability and long-term operational stability, allowing it to maintain reliable performance in complex environments. In summary, the sensor of this invention combines the advantages of high output performance, rapid response, excellent mechanical strength, and long-term stability.

[0009] Details of one or more embodiments of the present invention are set forth in the following drawings and description, so that other features, objects and advantages of the invention will be more readily understood. Attached Figure Description

[0010] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other embodiments can be obtained based on these drawings without creative effort.

[0011] Figure 1 This is a schematic diagram of the structure of a carbon felt-hydrogel composite piezoelectric sensor provided in an embodiment of the present invention.

[0012] Figure 2 This is a schematic flowchart illustrating a method for fabricating a carbon felt-hydrogel composite piezoelectric sensor according to an embodiment of the present invention.

[0013] Figure 3This is a schematic diagram of the linear fitting curve between the peak output voltage and the applied force of a PVA-carbon felt dual-network hydrogel piezoelectric sensor with different PVA concentrations, provided as an embodiment of the present invention.

[0014] Figure 4 This is a schematic diagram of the cyclic stability test curve of a PVA-carbon felt multi-scale dual-network hydrogel piezoelectric sensor provided in an embodiment of the present invention.

[0015] Figure 5 This is a schematic diagram of the dynamic response time test curve of a PVA-carbon felt multi-scale dual-network hydrogel piezoelectric sensor provided in an embodiment of the present invention.

[0016] In the figure: 1. Piezoelectric sensing layer; 2. Multiscale dual-network hydrogel layer; 3. Electrode layer; 4. Lead wire; 5. Encapsulation layer. Detailed Implementation

[0017] Embodiments of this embodiment will now be described in more detail with reference to the accompanying drawings. While some embodiments of this embodiment are shown in the drawings, it should be understood that this embodiment can be implemented in various forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of this embodiment. It should be understood that the accompanying drawings and embodiments are for illustrative purposes only and are not intended to limit the scope of protection of this embodiment.

[0018] The rapid development of flexible electronics technology has led to the emergence of flexible piezoelectric sensors, which, with their advantages of self-powered operation, simple structure, and fast dynamic response, have broad application prospects in fields such as health monitoring, human-computer interaction, and smart wearables. Currently, mainstream products use polyvinylidene fluoride (PVDF) piezoelectric polymer films as the sensing layer, with metal foils or conductive polymer electrodes bonded to the upper and lower surfaces to form the sensing unit. However, this type of structure suffers from weak mechanical properties, poor interfacial compatibility, and low stability and reliability. Although research has improved performance by doping with inorganic particles or constructing composite structures, it is difficult to simultaneously achieve flexibility, conductivity, and interfacial stability. In summary, the common problems of existing flexible piezoelectric sensors—poor mechanical properties, unstable interfacial bonding, inefficient charge transfer, and insufficient long-term reliability—limit their practical application in dynamic monitoring scenarios.

[0019] To address the aforementioned issues, this invention provides a carbon felt-hydrogel composite piezoelectric sensor and its fabrication method.

[0020] Figure 1 This is a schematic diagram of the structure of a carbon felt-hydrogel composite piezoelectric sensor provided in an embodiment of the present invention.

[0021] like Figure 1As shown, the piezoelectric sensor includes a piezoelectric sensing layer 1, a multi-scale dual-network hydrogel layer 2, an electrode layer 3, lead wires 4, and an encapsulation layer 5. In this embodiment, the multi-scale dual-network hydrogel layer 2 is provided in two layers, which are respectively stacked on the upper and lower sides of the piezoelectric sensing layer 1 to form a sandwich structure; the multi-scale dual-network hydrogel layer 2 is an elastic hydrogel material composed of a carbon fiber felt skeleton and a molecular-level network polymer, wherein the molecular-level network polymer penetrates the interior and surface of the carbon fiber felt skeleton and is formed by cross-linking and curing; The electrode layer 3 has two layers, which are stacked on the outside of the two cross-scale dual-network hydrogel layers 2 respectively; the lead wire 4 is electrically connected to the electrode layer 3; and the encapsulation layer 5 covers the outside of the electrode layer 3.

[0022] The carbon felt in this embodiment Hydrogel composite piezoelectric sensors, through a multi-layered sandwich structure, significantly improve the sensor's mechanical properties, conductivity, output sensitivity, and long-term stability based on the synergistic effect of each functional layer. For example... Figure 1 As shown, the sensor mainly includes a piezoelectric sensing layer 1, a multi-scale dual-network hydrogel layer 2, an electrode layer 3, lead wires 4, and an encapsulation layer 5.

[0023] In this embodiment, the piezoelectric sensing layer 1 is the core sensitive unit of the sensor, and its function is to convert externally applied mechanical pressure or deformation into electrical signals. In this embodiment, a polyvinylidene fluoride (PVDF) piezoelectric film can be used. PVDF is a polymer material with good flexibility and a high piezoelectric coefficient. When it is deformed by external force, the internal crystals will polarize, thereby inducing charges on the upper and lower surfaces of the film. The thickness of the piezoelectric sensing layer 1 can be from 40 micrometers to 80 micrometers. This thickness range ensures sufficient flexibility to adapt to complex deformations and generates a sufficiently strong piezoelectric signal. In an optional embodiment, the planar dimensions of the piezoelectric sensing layer 1 are a square of 15 mm × 15 mm to 18 mm × 18 mm to facilitate precise alignment and integration with other layers.

[0024] On the upper and lower sides of the piezoelectric sensing layer 1, two multi-scale dual-network hydrogel layers 2 are stacked respectively. These two hydrogel layers, together with the piezoelectric sensing layer 1 in the middle, form a stable "sandwich" structure. This structure can provide soft mechanical support and cushioning for the brittle piezoelectric film, preventing the piezoelectric sensing layer 1 from breaking under repeated impacts; at the same time, the highly elastic hydrogel layer can more effectively transmit and concentrate external pressure on the PVDF film in the middle, and its own deformation can also modulate the electric field distribution to a certain extent, thereby synergistically enhancing the overall electrical output of the sensor.

[0025] The cross-scale dual-network hydrogel of this invention differs from traditional homogeneous hydrogels. It is an elastic composite material formed by the interpenetration and composite of a macroscopic carbon fiber felt skeleton and a microscopic molecular-level polymer network. Specifically, carbon fiber felt (referred to as carbon felt) is a flexible skeleton material with a continuous three-dimensional porous network structure composed of randomly arranged carbon fibers. It has a high carbon content, excellent electronic conductivity, and its porous structure provides ample channels and space for impregnation with polymer solutions. The molecular-level network polymer can include water-soluble polymers such as polyvinyl alcohol (PVA), sodium alginate, or agarose. First, the molecular-level network polymer is prepared as a precursor aqueous solution. Through a vacuum impregnation process, the precursor aqueous solution is forcibly injected into every void of the carbon felt skeleton. Subsequently, through physical crosslinking (such as freeze-thaw cycles of PVA) or chemical / ionic crosslinking (such as calcium ion crosslinking of sodium alginate), the polymer chains in the solution entangle and solidify with each other on the carbon fiber surface and within the pores between the fibers, forming a second, molecular-scale crosslinked network. In the final composite material, the tough macroscopic network of carbon felt and the soft microscopic network of polymer are interlocked and work together to form a unique "cross-scale dual-network" structure. This structure endows the hydrogel layer with excellent comprehensive properties: the carbon felt network enhances the mechanical strength and toughness of the material, enabling it to withstand repeated deformations without easily being damaged; at the same time, the conductive carbon fiber network running through the entire hydrogel forms a highly efficient electron transport pathway, significantly improving the overall conductivity of the layer and ensuring low-loss transmission of electrical signals. The thickness of this hydrogel layer can range from 1 mm to 4 mm.

[0026] In this embodiment, an electrode layer 3 is further stacked on the outer side of the cross-scale dual-network hydrogel layer 2 on each side. The electrode layer 3 can be used to collect and conduct the charge generated by the piezoelectric effect. In this embodiment, conductive nonwoven fabric or similar materials can be used as electrode materials. For example, conductive nonwoven fabric has good conductivity, flexibility, and moisture permeability. Its porous fiber structure can form a tight and stable interfacial contact with the hydrogel layer, which can effectively reduce the contact resistance. At the same time, its softness ensures that the overall flexibility of the sensor is not compromised.

[0027] In this embodiment, the lead wire 4 is usually a flexible copper wire, which is reliably electrically connected to the two layers of conductive non-woven fabric electrodes by means of conductive adhesive or direct sewing, and is used to transmit the electrical signal generated by the sensor to the external measurement circuit or data acquisition system.

[0028] Finally, the encapsulation layer 5 covers the outermost part of the entire sensor, that is, it covers the outer surface of the two electrode layers 3 and the edges of the sensor. The encapsulation layer 5 is usually made of insulating tape or other flexible insulating film material. The encapsulation layer 5 can protect the delicate internal structural layers from damage caused by moisture, dust and mechanical friction in the environment, ensuring the long-term stability and reliability of the sensor under complex operating conditions, while also providing a certain degree of mechanical reinforcement.

[0029] When the carbon felt-hydrogel composite piezoelectric sensor of this embodiment is subjected to external pressure or mechanical deformation, the force is first transmitted through the outermost encapsulation layer 5 to the upper and lower electrode layers 3. Since the electrode layer 3 is made of soft conductive nonwoven fabric, it has a certain deformation capability and can transmit the pressure relatively evenly to the inner multi-scale dual-network hydrogel layer 2.

[0030] The multi-scale dual-network hydrogel layer 2, as a highly elastic material, can absorb some of the impact energy, thus providing excellent buffering and mechanical protection for the entire sensor. Secondly, its internal carbon felt forms a macroscopic three-dimensional conductive network, which, together with the interwoven and penetrating molecular-level polymer network of carbon fibers, forms a composite material that combines toughness and conductivity. When the hydrogel layer deforms under pressure, its internal carbon fiber network not only elastically deforms to maintain structural integrity, but more importantly, as a continuous and uniformly distributed conductive pathway, it ensures that charge can be efficiently and with low resistance laterally transferred to the closely contacting electrode layer 3 during deformation, avoiding signal loss due to poor local dissipation. Simultaneously, the deformation of the hydrogel layer can more effectively concentrate and transfer external pressure to the core piezoelectric sensing layer 1, increasing the effective strain of the piezoelectric material.

[0031] Subsequently, the pressure is transmitted to the piezoelectric sensing layer 1 at the core of the positional structure. In this embodiment, the piezoelectric sensing layer 1 is a PVDF film. PVDF is a polymer with significant piezoelectric properties, and its molecular chain contains a polar crystalline phase (such as the β phase). When the film is subjected to pressure perpendicular to its surface transmitted through the hydrogel layer, microscopic compression or bending deformation occurs. This mechanical deformation causes a relative displacement of the positive and negative charge centers in the internal lattice of the film, thereby generating a polarization phenomenon, that is, equal amounts of bound charges of opposite signs are induced on the upper and lower surfaces of the film. This process is the piezoelectric effect. At the instant the pressure continues to act, positive charges accumulate on the upper surface of the film, and negative charges accumulate on the lower surface (or vice versa, depending on the polarization direction), thereby establishing an instantaneous polarized electric field inside the film.

[0032] At this point, the multi-scale dual-network hydrogel layer 2, which is tightly bonded to the upper and lower surfaces of the piezoelectric film, can rapidly sense changes in the polarization electric field due to its excellent conductivity. Free charges (electrons or ions) in the hydrogel layer redistribute under the influence of the electric field to neutralize the bound charges induced on the surface of the piezoelectric film. This charge redistribution can be sensed and collected by the electrode layer 3, which is in close contact with it on the outside. The electrode layer 3, as the final charge collection end, effectively captures the charge transferred from the hydrogel layer and leads it to the external measurement circuit via the lead wire 4. At the instant of pressure application, the potential of the upper electrode (corresponding to the positively charged surface of the piezoelectric film) increases, and the potential of the lower electrode decreases, thereby generating a positive voltage peak signal in the external circuit.

[0033] When the external pressure decreases or disappears, the piezoelectric sensing layer 1 begins to elastically recover. Due to the reversibility of its piezoelectric effect, the recovery deformation leads to a weakening or even reversal of its internal polarization direction. This reduces the amount of bound charge induced on the upper and lower surfaces of the film or changes its polarity. Correspondingly, the free charges that have accumulated in the hydrogel layer and electrode layer 3 to neutralize these bound charges also flow in the opposite direction or dissipate. During this process, the external circuit exhibits a decrease in the upper electrode potential and an increase in the lower electrode potential, thereby generating a voltage peak signal (negative peak) with the opposite polarity to the pressurization process.

[0034] Therefore, under periodic or dynamic external pressure, the sensor, through the coordinated response of the piezoelectric sensing layer 1, the hydrogel layer, and the electrode layer 3, can output a series of alternating voltage signals that are synchronized with the frequency of pressure changes and whose amplitude is related to the magnitude of the pressure. The high elasticity and interfacial adhesion of the multi-scale dual-network hydrogel layer 2 ensure the high efficiency and uniformity of pressure transmission; its through-hole carbon fiber conductive network can improve the conductivity of the overall structure, acting as an efficient charge sensing and transmission medium, effectively reducing the signal attenuation caused by the large contact resistance between the electrode and the piezoelectric material interface in traditional sensors; the mechanical toughness provided by its dual-network structure ensures the signal stability and durability of the sensor under repeated deformation. Finally, the encapsulation layer 5 provides the necessary environmental protection and mechanical encapsulation for this precise charge generation and collection system, ensuring the reliable implementation of the working principle in complex application scenarios.

[0035] In summary, this embodiment creatively constructs a novel flexible piezoelectric sensor by integrating a conductive hydrogel with a unique cross-scale dual-network structure and a PVDF piezoelectric film into a sandwich structure in a specific manner. The sensor in this embodiment not only possesses the high elasticity and good biocompatibility of hydrogels and the excellent conductivity of carbon materials, but also amplifies the piezoelectric effect through structural design, achieving a comprehensive advantage of high output performance, fast response, excellent mechanical durability, and stable reliability. It is suitable for dynamic pressure sensing in wearable devices, health monitoring, human-computer interaction, and other fields.

[0036] Figure 2 This is a schematic flowchart illustrating a method for fabricating a carbon felt-hydrogel composite piezoelectric sensor according to an embodiment of the present invention. Figure 2 As shown, the method includes the following steps.

[0037] Step S201: The molecular-level network polymer precursor solution is vacuum impregnated with carbon fiber felt to allow the solution to penetrate into the interior of the carbon fiber felt, and then cured and crosslinked to obtain a multi-scale dual-network hydrogel layer.

[0038] Step S202: The piezoelectric sensing layer is sandwiched between two trans-scale dual-network hydrogel layers to form a sandwich structure.

[0039] Step S203: Electrode layers are set on the upper and lower sides of the sandwich structure respectively, and the lead wires are electrically connected to the electrode layers.

[0040] Step S204: The assembled and connected structure is encapsulated as a whole to obtain a carbon felt-hydrogel composite piezoelectric sensor.

[0041] First, a molecular-level network polymer precursor solution is prepared, which forms the basis for the formation of the microscopic polymer network. In this embodiment, the concentration of the molecular-level network polymer precursor solution can range from 2.5% to 15%. Within this concentration range, the polymer chains can be fully dissolved and have a suitable viscosity, which facilitates subsequent impregnation operations and ensures the formation of a network structure with sufficient crosslinking density after curing. Subsequently, pre-cut carbon fiber felt with a three-dimensional porous structure is used as a macroscopic reinforcing skeleton.

[0042] Subsequently, the prepared polymer precursor solution and the cut carbon felt are placed together in a vacuum chamber. Depending on the type of polymer precursor solution, thorough vacuum impregnation is performed for 1 to 3 hours at temperatures ranging from room temperature to 100 °C (the specific temperature depends on the type of polymer; for example, agarose solutions require higher temperatures to maintain fluidity). During this process, the vacuum environment eliminates air from the voids in the carbon felt, creating negative pressure conditions that drive the polymer precursor solution to actively and deeply penetrate every tiny pore and fiber interlacing point of the carbon felt skeleton, ensuring complete wetting of the carbon fiber surface. In this step, the molecular-level network and the macroscopic network of the carbon felt can be interconnected at the microscale, forming a tight bond.

[0043] After impregnation, the sample is removed and briefly placed on a filter screen to remove excess solution adhering to the carbon fiber felt surface, thereby controlling the thickness and shape consistency of the final hydrogel layer. Subsequently, the fully impregnated sample undergoes a curing and cross-linking treatment to transform the polymer solution impregnated within the carbon felt from a liquid state into an elastic solid gel network. In practical applications, different curing and cross-linking methods need to be selected based on the type of molecular-level network polymerization chosen. For example, if the polymer is polyvinyl alcohol (PVA), a freeze-thaw cycle treatment is used. Specifically, the sample is sealed and placed in a low-temperature environment of -15°C to -50°C for 4 to 20 hours to allow water molecules in the solution to crystallize, forcing the PVA molecular chains to approach each other and form physical cross-linking points; then, it is thawed at room temperature for 4 to 12 hours, where the ice crystals melt but the cross-linking points are retained. Repeating this freeze-thaw process 3 to 5 times gradually increases the cross-linking density, obtaining a PVA-carbon felt composite hydrogel with excellent mechanical properties. If the polymer is sodium alginate or agarose, a room temperature standing treatment is used. For sodium alginate, the precursor solution typically already contains cross-linking ions (such as calcium ions). After standing at room temperature for 1 to 2 days, the ionic cross-linking reaction proceeds slowly, eventually forming a stable gel. For agarose, after impregnation, the solution is cooled to room temperature and allowed to stand, forming a thermally reversible gel through the refolding of hydrogen bonds between molecular chains. Thus, the multi-scale dual-network hydrogel layer is prepared. Its internal carbon fiber network provides conductivity and skeletal strength, while the in-situ formed polymer hydrogel network provides elasticity and interfacial bonding.

[0044] After obtaining the hydrogel layer, the sensor can be assembled. The prepared piezoelectric sensing layer is sandwiched between two multi-scale dual-network hydrogel layers, and gentle pressure is applied to ensure that the interfaces are tightly adhered, thus forming a stable sandwich structure.

[0045] Next, electrode layers are respectively applied to the upper and lower outer surfaces of the sandwich structure. Then, the lead wires are firmly electrically connected to the upper and lower conductive nonwoven fabric electrodes by means of conductive silver paste spot welding, sewing, or crimping, ensuring low-loss output of electrical signals.

[0046] Finally, the entire multi-layered structure, after assembly and connection, is encapsulated. In practical applications, flexible insulating tape or elastic encapsulating adhesive can be used to completely cover the sensor except for the lead wires, forming an encapsulation layer. It is important to ensure that there is no relative sliding between the layers and that the seal is good during encapsulation to protect the internal structure from environmental interference and to give the sensor final mechanical integrity and durability. Through these steps, a high-performance carbon felt-hydrogel composite piezoelectric sensor can be obtained.

[0047] The carbon felt-hydrogel composite piezoelectric sensor and its preparation method provided in the embodiments of the present invention will be described in detail below with reference to specific examples.

[0048] Example 1: This embodiment provides a specific method for fabricating a multi-scale dual-network hydrogel piezoelectric sensor based on a composite of polyvinyl alcohol (PVA) and carbon fiber felt.

[0049] First, a PVA precursor solution was prepared. 10 grams of PVA powder (degree of alcoholysis ≥ 99%) was weighed and added to 90 grams of deionized water. The mixture was placed in a constant-temperature water bath with a magnetic stirrer at 80–120°C (95°C in this example) and stirred continuously at 500 rpm for 3 hours until the PVA was completely dissolved, yielding a clear, viscous PVA aqueous solution with a mass fraction of 10%. Subsequently, a piece of commercial carbon fiber felt with a thickness of approximately 2 mm and an area of ​​approximately 20 mm × 20 mm was cut into squares with a size of 15 mm × 15 mm to serve as a macroscopic reinforcing skeleton.

[0050] Next, vacuum impregnation was performed to construct a multi-scale dual-network structure. The prepared PVA precursor solution and the cut carbon fiber felt were placed together in a tray of a vacuum drying oven. At room temperature (approximately 25°C), the vacuum pump was turned on, and the pressure inside the oven was reduced to -0.1 MPa, and this vacuum state was maintained for 2 hours. This process utilizes the negative pressure driving effect to allow the PVA solution to fully penetrate and completely fill the three-dimensional porous network inside the carbon fiber felt, ensuring that the polymer molecules achieve close microscale contact and wetting between the carbon fiber surface and the pores between the fibers.

[0051] After impregnation, carefully remove the saturated impregnated sample with tweezers, spread it flat on a 200-mesh nylon filter screen, and let it stand at room temperature for 8 minutes to drain and remove excess PVA solution adhering to the surface of the carbon fiber felt, thereby controlling the shape and thickness uniformity of the final hydrogel layer.

[0052] Then, physical cross-linking and curing are performed to form a stable hydrogel. The drained sample is placed in a self-sealing bag and sealed for freeze-thaw cycling. The specific steps are as follows: the sealed sample is placed in a -20 °C freezer for 12 hours; then removed and placed at room temperature (approximately 25 °C) to allow it to thaw completely for 12 hours. This constitutes one complete freeze-thaw cycle. The above freezing and thawing process is repeated a total of 3 times. During the cycle, the PVA molecular chains form strong physical cross-linking points through repeated crystallization and decrystallization processes, thereby constructing a tough polymer network in situ within the carbon fiber felt skeleton. Ultimately, a PVA-carbon felt multi-scale dual-network hydrogel with interpenetrating and synergistically reinforced PVA molecular networks and carbon fiber felt macroscopic networks is obtained. This hydrogel exhibits good elasticity and flexibility, and possesses excellent electrical conductivity due to the permeating carbon fiber network.

[0053] Finally, the piezoelectric sensor was integrated and packaged. A commercially available polarized PVDF piezoelectric film with a thickness of 50 μm and a size of 15 mm × 15 mm was used as the core sensing layer. Two pieces of the prepared PVA-carbon felt hydrogel were cut to a thickness of approximately 2 mm and an area of ​​the same as the PVDF film (15 mm × 15 mm). A sandwich structure was used for assembly: the PVDF film was placed in the middle, and a PVA-carbon felt hydrogel layer was attached to each of its upper and lower surfaces. Slight pressure was applied to ensure tight adhesion between the layers and to eliminate interfacial air bubbles. Subsequently, a flexible conductive nonwoven fabric of similar size was attached to the outside of the upper and lower hydrogel layers as electrode layers. Two thin tin-plated copper wires were firmly bonded to the upper and lower conductive nonwoven fabrics using a small amount of conductive silver paste to form electrical leads. After the silver paste has cured, the entire multilayer assembly (except for the lead wires) is completely encapsulated with polyimide insulating tape to obtain the PVA-carbon felt multi-scale dual-network hydrogel piezoelectric sensor.

[0054] The sensor structure fabricated in this embodiment is robust and flexible. Its internal double-network hydrogel layer not only effectively protects and increases the strain of the PVDF film, but also provides an efficient charge collection and transport pathway. Preliminary tests show that the sensor is sensitive to dynamic pressure and has a stable output signal, exhibiting excellent overall sensing performance.

[0055] Example 2 This embodiment provides a specific method for preparing a multi-scale dual-network hydrogel piezoelectric sensor based on sodium alginate and carbon fiber felt through ionic cross-linking.

[0056] First, prepare an ion-crosslinked sodium alginate precursor solution. This process includes: Weigh 0.15 g of sodium pyrophosphate and dissolve it in 10 mL of deionized water. Stir for 5 minutes to obtain a clear sodium pyrophosphate aqueous solution. Add 0.3 g of calcium sulfate dihydrate (CaSO4·2H2O) powder to 10 mL of deionized water and stir for 10 minutes to obtain a uniform calcium sulfate dihydrate suspension.

[0057] Weigh 2 grams of sodium alginate powder and add it to 98 grams of deionized water. Stir magnetically at 400 rpm for 2 hours at room temperature until completely dissolved to obtain a homogeneous, viscous sodium alginate aqueous solution with a mass fraction of 2%.

[0058] Under continuous stirring, an aqueous solution of sodium pyrophosphate was slowly added to an aqueous solution of sodium alginate, and the mixture was stirred for 10 minutes to ensure thorough homogenization. Subsequently, under rapid stirring, a suspension of calcium sulfate dihydrate was slowly added dropwise to the mixture, and stirring was continued for 10 minutes. This utilized the slow-release effect of sodium pyrophosphate to allow calcium ions (Ca) to be released. 2+The alginate precursor undergoes a slow and uniform ionic cross-linking reaction with the guluronic acid (G) units on the sodium alginate molecular chain, ultimately yielding a homogeneous sodium alginate precursor solution. Simultaneously, a piece of carbon fiber felt approximately 2 mm thick is cut into 15 mm × 15 mm squares for later use.

[0059] Next, vacuum impregnation is performed. The precursor solution and the cut carbon felt are placed in a vacuum chamber and impregnated at a vacuum of -0.1 MPa for 2 hours at room temperature to allow the solution to fully penetrate the pores of the carbon felt.

[0060] After impregnation, the sample was removed and left to stand on a filter screen for 8 minutes to remove excess solution from the surface. The sample was then placed in a resealable bag and sealed, and left to stand at room temperature (approximately 25°C) for 36 hours. During this period, calcium ions continuously cross-link with sodium alginate, forming a stable "eggbox" structure gel network in situ within the carbon fiber felt skeleton, thus producing a sodium alginate-carbon felt multi-scale dual-network hydrogel.

[0061] Finally, sensor integration was performed. Following the same sandwich structure assembly and packaging steps as in Example 1: a 50 μm thick PVDF film was sandwiched between two sodium alginate-carbon felt hydrogels with a thickness of approximately 2 mm prepared by the above method; conductive nonwoven electrodes were attached to the outside of the hydrogels and wires were led out; finally, polyimide tape was used for overall encapsulation to obtain the sodium alginate-carbon felt composite piezoelectric sensor.

[0062] Example 3: This embodiment provides a specific method for preparing a multi-scale dual-network hydrogel piezoelectric sensor based on the thermally reversible gelation composite of agarose and carbon fiber felt.

[0063] First, prepare the agarose precursor solution (agarose concentration of 1%~5%, 3% is used as an example in this embodiment). Weigh 1.5 g of agarose powder and add it to 48.5 g of deionized water to achieve a target mass fraction of 3%. Place the mixture in an oil bath at 120 °C and continuously stir magnetically at 300 rpm for 2 hours until the agarose is completely dissolved, obtaining a clear and homogeneous hot melt gel. Simultaneously, cut a piece of carbon fiber felt approximately 2 mm thick into 15 mm × 15 mm squares for later use.

[0064] Subsequently, hot vacuum impregnation was performed. To maintain the fluidity of the agarose solution, the precursor solution and the cut carbon felt were placed together in a vacuum drying oven preheated to 90 °C. At this temperature, the vacuum pump was started, and the pressure was maintained at -0.1 MPa for 2 hours of vacuum impregnation. The high temperature reduced the solution viscosity, and combined with the vacuum negative pressure drive, ensured that the agarose hot melt could deeply and uniformly penetrate into all the pores of the carbon fiber felt.

[0065] After impregnation, the sample was quickly removed and placed on a filter screen for approximately 8 minutes under a preheated support to remove excess hot melt adhesive from the surface. Subsequently, the sample was sealed in a resealable bag and placed flat at room temperature (approximately 25 °C) for 48 hours. During this period, as the temperature slowly decreased below room temperature, the agarose molecular chains impregnated in the carbon fiber network underwent self-assembly and refolding through hydrogen bonding, forming a stable three-dimensional thermally reversible gel network. This resulted in the in-situ construction of an agarose-carbon felt multi-scale dual-network hydrogel within the macroscopic framework of the carbon fiber felt.

[0066] Finally, following the same sandwich structure assembly, electrode connection, and overall encapsulation steps as in Example 1, the agarose-carbon felt hydrogel (approximately 2 mm thick) prepared above was integrated with a PVDF piezoelectric film (50 μm thick) and a conductive nonwoven fabric electrode, and encapsulation was completed to obtain an agarose-carbon felt composite piezoelectric sensor.

[0067] This embodiment utilizes the properties of agarose thermally reversible gel to prepare a composite hydrogel with a stable dual-network structure through high-temperature impregnation and room-temperature curing processes, and verifies its feasibility as a highly efficient elastic conductive layer in flexible piezoelectric sensors.

[0068] Example 4: This embodiment examines the impact of preparing a series of hydrogel layers with different polymer concentrations on the final performance of the piezoelectric sensor, thereby verifying the universality and performance stability of the sensor structure design of this invention for different material parameters.

[0069] First, following a similar hydrogel matrix preparation process as in Example 1, PVA precursor solutions with mass fractions of 5%, 7.5%, 10%, 12.5%, and 15% were prepared. Specifically, the corresponding masses of PVA powder were weighed and mixed with deionized water, then placed in a 95°C constant temperature water bath and stirred continuously for 3 hours until completely dissolved, yielding five homogeneous and transparent solutions of different concentrations. Simultaneously, multiple sheets of carbon fiber felt approximately 2 mm thick were uniformly cut into 15 mm × 15 mm squares to serve as a reinforcing skeleton.

[0070] Subsequently, each PVA solution of different concentrations underwent further processing. The corresponding concentration of PVA precursor solution was placed together with a pre-cut carbon felt in a vacuum chamber and thoroughly impregnated under vacuum at room temperature, maintaining a vacuum level of -0.1 MPa for 2 hours to ensure effective penetration of solutions of varying viscosities into the carbon felt. After impregnation, the samples were removed and allowed to stand on a filter screen for 8 minutes to remove excess solution from the surface.

[0071] Next, all impregnated samples were placed in individual self-sealing bags and subjected to a uniform freeze-thaw crosslinking process. The specific steps were as follows: the samples were frozen at -20 °C for 12 hours, then thawed at room temperature (approximately 25 °C) for 12 hours; this constituted one cycle, and this process was repeated three times for all samples. Through this step, five PVA-carbon felt cross-scale dual-network hydrogels with different PVA concentrations were prepared. These hydrogels all exhibited good elasticity and structural integrity, and intuitively, their mechanical strength increased slightly with increasing PVA concentration.

[0072] Then, following the standard fabrication steps of the piezoelectric sensor in this embodiment, the five hydrogels of different concentrations (all cut to a thickness of approximately 2 mm) prepared above were integrated and assembled with a PVDF film of 50 μm thickness and a conductive nonwoven fabric electrode. The specific assembly and encapsulation process was the same as in Example 1, resulting in five piezoelectric sensor samples with the same structure but different PVA concentrations in the hydrogel layer, labeled C5, C7.5, C10, C12.5, and C15, respectively.

[0073] Finally, the sensing performance of the fabricated series of sensors was tested systematically. Under standard test conditions, dynamic pressure was applied to the sensors, and their output voltage signals were recorded. The test results show that all samples can generate stable and repeatable voltage signals under pressure.

[0074] Figure 3 This is a schematic diagram of the linear fitting curve between the peak output voltage and the applied force of a PVA-carbon felt dual-network hydrogel piezoelectric sensor with different PVA concentrations, provided as an embodiment of the present invention.

[0075] like Figure 3 As shown, the sensor exhibits good linear output characteristics at all concentrations, with the output voltage steadily increasing with increasing external force, reaching a maximum output voltage of approximately 400mV. Different PVA concentrations have minimal impact on the sensor's output performance, indicating that the sensor possesses stable electrical response characteristics within the experimental concentration range and is suitable for accurate detection of force signals over a wide range.

[0076] Figure 4 This is a schematic diagram of the cyclic stability test curve of a PVA-carbon felt multi-scale dual-network hydrogel piezoelectric sensor provided in an embodiment of the present invention.

[0077] like Figure 4 As shown, during thousands of continuous tests, the sensor output voltage remained stable without significant attenuation or drift, indicating that the sensor possesses excellent cyclic stability and can operate stably for extended periods, making it suitable for applications requiring repeated force monitoring. Figure 5This is a schematic diagram of the dynamic response time test curve of a PVA-carbon felt multi-scale dual-network hydrogel piezoelectric sensor provided in an embodiment of the present invention.

[0078] like Figure 5 As shown, the voltage signal of the sensor changes rapidly after being subjected to force, with the time from the initial state to reaching the peak response being approximately 40 ms, exhibiting fast dynamic response characteristics. This response speed meets the requirements for real-time force signal monitoring and is suitable for dynamic force detection scenarios requiring high timeliness.

[0079] This embodiment demonstrates, through detailed comparative experimental data, that the piezoelectric sensor based on PVA-carbon felt multi-scale dual-network hydrogel has comprehensive advantages such as stable output performance, good linearity, high durability, and rapid response. Moreover, these advantages can be stably maintained within a certain range of material parameters (such as PVA concentration), fully demonstrating the practicality and reliability of the technical solution in this embodiment.

[0080] It should be noted that the term "comprising" and its variations used in the embodiments of the present invention are open-ended, meaning "including but not limited to". The term "based on" means "at least partially based on". The term "one embodiment" means "at least one embodiment"; the term "another embodiment" means "at least one additional embodiment"; the term "some embodiments" means "at least some embodiments". The modifications of "one" and "multiple" mentioned in the embodiments of the present invention are illustrative and not restrictive. Those skilled in the art should understand that, unless explicitly indicated otherwise in the context, they should be understood as "one or more".

[0081] The steps described in the method embodiments provided by this invention can be performed in different orders and / or in parallel. Furthermore, the method embodiments may include additional steps and / or omit the steps shown. The scope of protection of this invention is not limited in this respect.

[0082] The term "embodiment" in this specification refers to a specific feature, structure, or characteristic described in connection with an embodiment that may be included in at least one embodiment of the invention. The appearance of this phrase in various places in the specification does not necessarily imply the same embodiment, nor does it imply independence or alternativeity from other embodiments. The various embodiments in this specification are described in a related manner, with reference to each other for similar or identical parts. In particular, for apparatus, device, and system embodiments, since they are substantially similar to method embodiments, the description is relatively simple, and relevant details are referred to in the description of the method embodiments.

[0083] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of patent protection. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the appended claims.

Claims

1. A carbon felt-hydrogel composite piezoelectric sensor, characterized in that, include: The device comprises a piezoelectric sensing layer, a multi-scale dual-network hydrogel layer, an electrode layer, lead wires, and an encapsulation layer. The multi-scale dual-network hydrogel layer consists of two layers, stacked on the upper and lower sides of the piezoelectric sensing layer to form a sandwich structure. The multi-scale dual-network hydrogel layer is an elastic hydrogel material composed of a carbon fiber felt skeleton and a molecular-level network polymer, wherein the molecular-level network polymer penetrates the interior and surface of the carbon fiber felt skeleton and is formed through cross-linking and curing. The electrode layer comprises two layers, which are stacked on the outside of the two multi-scale dual-network hydrogel layers; the lead wire is electrically connected to the electrode layer; and the encapsulation layer covers the outside of the electrode layer.

2. The carbon felt-hydrogel composite piezoelectric sensor according to claim 1, characterized in that, The molecular-level network polymer is a water-soluble polymer.

3. The carbon felt-hydrogel composite piezoelectric sensor according to claim 2, characterized in that, The water-soluble polymer includes polyvinyl alcohol, agarose, or sodium alginate.

4. The carbon felt-hydrogel composite piezoelectric sensor according to claim 1, characterized in that, The piezoelectric sensing layer includes a polyvinylidene fluoride piezoelectric film with a thickness of 40~80 μm; the thickness of the multi-scale dual-network hydrogel layer is 1~4 mm.

5. The carbon felt-hydrogel composite piezoelectric sensor according to claim 1, characterized in that, The electrode layer includes conductive nonwoven fabric electrodes.

6. A method for preparing a carbon felt-hydrogel composite piezoelectric sensor, characterized in that, include: A molecular-level network polymer precursor solution was vacuum impregnated with carbon fiber felt to allow the solution to penetrate into the interior of the carbon fiber felt, and then cured and crosslinked to obtain the multi-scale dual-network hydrogel layer. A piezoelectric sensing layer is sandwiched between two of the aforementioned multi-scale dual-network hydrogel layers to form a sandwich structure; Electrode layers are provided on the upper and lower sides of the sandwich structure, and lead wires are electrically connected to the electrode layers. The assembled structure is then encapsulated to obtain the carbon felt-hydrogel composite piezoelectric sensor.

7. The method according to claim 6, characterized in that, The concentration of the molecular-level network polymer precursor solution is 2.5% to 15%; the vacuum impregnation treatment is performed at a temperature of room temperature to 100°C for 1 to 3 hours.

8. The preparation method according to claim 7, characterized in that, The molecular-level network polymer is polyvinyl alcohol, and the curing and crosslinking treatment is a freeze-thaw cycle treatment. The freeze-thaw cycle treatment includes: freezing the impregnated carbon fiber felt in an environment of -15 ℃ to -50 ℃ for 4 to 20 hours, and then thawing it at room temperature for 4 to 12 hours, with the number of cycles being 3 to 5.

9. The preparation method according to claim 7, characterized in that, The molecular-level network polymer is sodium alginate or agarose, and the curing and crosslinking treatment is a room temperature standing treatment for 1 to 2 days.

10. The preparation method according to claim 9, characterized in that, The molecular-level network polymer is sodium alginate, and the preparation of the molecular-level network polymer precursor solution includes: Sodium alginate was mixed with deionized water and stirred to obtain a homogeneous solution; Add the sodium pyrophosphate solution to the sodium alginate solution and mix thoroughly; Calcium sulfate suspension was added to the mixture and stirred to carry out an ionic cross-linking reaction, resulting in a cross-linked sodium alginate precursor solution.

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