Manufacturing method of flexible piezoresistive sensor

Abstract

The invention discloses a manufacturing method of a flexible piezoresistive sensor, which comprises the steps of conductive composite material preparation, sensitive layer microstructure forming, sensor integrated assembly and the like, and is characterized in that conductive particles and an elastic polymer matrix are uniformly compounded to form an integrated sensitive layer structure; an interface problem caused by Young modulus mismatch in a traditional two-layer structure is eliminated from a material source, meanwhile, a micro geometric structure is elaborately designed and copied on the surface of the sensitive layer, and an externally applied pressure signal is ingeniously converted into effective local stress concentration. According to the structural design, the change rate of the contact area between the sensitive layer and the electrode can be amplified, so that the sensor can generate remarkable electric signal response to tiny pressure change.

Description

Technical Field

[0001] This invention relates to the field of flexible electronics technology, and more specifically to a method for fabricating a flexible piezoresistive sensor. Background Technology

[0002] Flexible pressure sensors convert external force signals into electrical signals and are core components in fields such as flexible electronic skin, health monitoring, and human-computer interaction. Based on different sensing mechanisms, they can be mainly classified into piezoresistive, capacitive, and piezoelectric types. Among them, flexible piezoresistive pressure sensors are considered an ideal choice for next-generation flexible pressure sensors due to their advantages such as simple structure, high sensitivity, fast response, and low manufacturing cost.

[0003] However, existing flexible piezoresistive sensors still face significant challenges. Most traditional sensors employ a two-layer structure model consisting of a low Young's modulus elastomer structure and a surface conductive layer. Under long-term cyclic stress, the significant Young's modulus mismatch between the elastomer deformation layer and the surface conductive layer can easily lead to cracks in the surface conductive layer or even its detachment from the deformation layer, severely affecting the long-term stability of the sensor.

[0004] Furthermore, simply changing the material composition or depositing a conductive layer is insufficient to further improve the sensor's sensitivity and sensing range. Therefore, developing a novel structural design that can simultaneously address interface stability and optimize sensing performance is of great significance.

[0005] Chinese patent literature discloses a gradient multilayer flexible piezoresistive sensor and its fabrication method [Application No.: 202211534611.5, Publication No.: CN115931186A]. This invention relates to a flexible piezoresistive sensor with a gradient multilayer structure, in which the sensitive layer is formed by stacking multiple composite material films with carbon nanotube content decreasing from top to bottom, forming an arch structure. This design aims to improve the sensor's sensitivity, linearity, and pressure response range simultaneously through gradient changes in modulus and resistance. However, this invention uses an integrally molded microstructure of conductive composite material, fundamentally avoiding the risk of interlayer delamination, and simultaneously achieving higher sensitivity through the micro-stress concentration effect, which is difficult to achieve with a gradient multilayer structure. Summary of the Invention

[0006] To address the problems existing in the prior art, the purpose of this invention is to provide a method for preparing a latent system dry continuous fiber epoxy composite material.

[0007] A method for fabricating a flexible piezoresistive sensor, characterized by comprising the following steps: S1. Preparation of conductive composite materials: Conductive particles with a volume fraction of 10% to 60% are uniformly dispersed in a liquid or slurry elastic polymer matrix to form a conductive composite slurry with curable properties. S2. Microstructure forming of sensitive layer: The conductive composite slurry obtained in step S1 is injected or coated into a pre-designed mold with a negative microstructure pattern, and then cured. After demolding, an integrated sensitive layer with a precisely replicated microstructure is obtained on one side. S3. Sensor integration and assembly: Place the microstructured side of the sensitive layer obtained in step S2 opposite to a flexible electrode layer, and fix and assemble it by physical alignment and pressing or by using a thin layer of conductive adhesive to form a layered sensor device.

[0008] Preferably, in step S1, the conductive particles are one or more of metal-based nanomaterials, carbon-based nanomaterials, and conductive polymers; The metal-based nanomaterials mentioned are metal nanowires or metal nanoparticles; The carbon-based nanomaterials mentioned are carbon nanotubes, graphene, or MXene nanosheets; The conductive polymer is PSS, polypyrrole, or polyaniline.

[0009] The above technical solutions enable efficient dispersion of conductive particles and the construction of a stable conductive network, thereby significantly improving the conductivity and sensitivity of the sensor.

[0010] Specifically, by selecting one or more of metal-based nanomaterials, carbon-based nanomaterials, or conductive polymers as conductive particles, the conductivity can be flexibly controlled according to the actual application requirements. Metal nanowires and nanoparticles can provide continuous conductive paths, carbon nanotubes, graphene, and MXene nanosheets can improve the percolation threshold and mechanical flexibility of composite materials, while conductive polymers can enhance the overall uniformity and interfacial bonding of composite materials. The synergistic effect of multiple conductive particles not only ensures that the composite material can produce significant resistance changes under small strains, but also improves the stability and fatigue resistance of the device.

[0011] In practical applications, this solution can significantly expand the sensor's operating range and improve its response to both low and high pressures, while maintaining short response and recovery times. This design is particularly suitable for scenarios requiring long-term stable operation, such as flexible electronic skin, health monitoring, and human-computer interaction. It can improve the sensor's lifespan and reliability, and is of great significance for enhancing the overall performance of flexible piezoresistive sensors.

[0012] Preferably, in step S1, the elastic polymer matrix is ​​polydimethylsiloxane, polyurethane, polyvinyl alcohol, polyethylene terephthalate, or polyimide.

[0013] The above technical solution enables precise control of the mechanical properties and flexibility of the sensitive layer substrate, thereby achieving both high sensitivity and good durability.

[0014] Specifically, polydimethylsiloxane possesses excellent flexibility and biocompatibility, making it suitable for wearable sensing applications; polyurethane combines good mechanical strength and elasticity, maintaining stable performance under large strain; polyvinyl alcohol exhibits good film-forming properties and hydrophilicity, making it suitable for humidity-controlled sensing applications; and polyethylene terephthalate and polyimide possess high thermal and dimensional stability, meeting the requirements for use in high-temperature or complex environments. By rationally selecting or combining these elastic polymers, the Young's modulus, flexibility, and fatigue resistance of the sensor can be adjusted according to application requirements.

[0015] In practical applications, this solution can improve the structural stability of the sensitive layer under cyclic loading, reduce the risk of cracks or performance degradation during long-term use, and thus ensure the reliability and repeatability of the sensor's output signal. This design is particularly suitable for flexible wearable devices, electronic skin, and long-term pressure monitoring scenarios, and is of great significance for improving the service life and overall performance of flexible piezoresistive sensors.

[0016] Preferably, in step S2, the negative microstructure pattern structure is a single micro-protrusion structure, a composite micro-protrusion structure, a three-dimensional porous structure, or a biomimetic structure; the single micro-protrusion structure is a pyramid structure, a hemispherical structure, or a semi-columnar structure.

[0017] Through the above technical solution, it is possible to achieve precise control of the microstructure of the sensitive layer surface, thereby significantly improving the stress concentration effect and resistance change amplitude of the sensor and enhancing the overall sensitivity.

[0018] Specifically, single micro-protrusion structures such as pyramids, hemispherical or semi-cylindrical structures can generate significant local stress concentration under external force, causing the contact area to increase nonlinearly with pressure, thereby obtaining a higher resistance response; composite micro-protrusion structures and three-dimensional porous structures can further increase the effective contact area, increase the number of conductive paths, and enhance the ability to distinguish small pressure signals; taking into account both high sensitivity and fast recovery performance, they are suitable for scenarios with long-term cyclic loading.

[0019] In practical applications, this scheme can effectively broaden the linear operating range of the sensor, reduce hysteresis and signal noise, and improve the accuracy and stability of measurements. This design is particularly suitable for high-resolution tactile sensing, electronic skin, and flexible human-computer interaction systems, and is of great significance for improving the overall performance of flexible piezoresistive sensors.

[0020] Preferably, the curing process in step S2 adopts a segmented heating method: First stage: Curing at 70℃~85℃ for 15~30 minutes; Second stage: Raise the temperature to 100℃~120℃ and maintain it for 40~60 minutes.

[0021] The above technical solutions enable precise control of the curing process of conductive composite materials, thereby obtaining a dense, uniform, and structurally stable sensitive layer, and improving the performance consistency and reliability of the sensor.

[0022] Specifically, the segmented heating method allows for initial cross-linking and shaping of the material at low temperatures in the first stage, avoiding stress concentration and bubble formation caused by rapid heating. The second stage, with increased temperature for thorough curing, improves the degree of cross-linking and mechanical strength, ensuring the complete replication and stable maintenance of the microstructure morphology on the sensitive layer surface. This segmented curing strategy balances the processability and final performance of the material.

[0023] In practical applications, this solution can significantly reduce performance fluctuations between sensor batches, ensure sensitivity stability and fatigue resistance during long-term cyclic testing, and extend device lifespan. It is particularly suitable for flexible electronics, medical monitoring, and industrial testing scenarios where stability and consistency are highly required.

[0024] The sensor has a layered structure, comprising an upper electrode layer, a sensitive layer, and a lower electrode layer, or a structure consisting of a sensitive layer and a single electrode layer. The sensitive layer is an integral structure, with the elastic polymer matrix as the continuous phase, and the conductive particles uniformly dispersed therein to form a conductive network. At least one surface of the sensitive layer has a micro-geometric structure for stress concentration, formed by replication through a mold. The electrode layer is a flexible conductive film or a patterned conductive circuit.

[0025] Compared with the prior art, the present invention has the following advantages: 1. This invention eliminates the interface problems caused by Young's modulus mismatch in traditional two-layer structures by uniformly compounding conductive particles with an elastic polymer matrix to form an integrated sensitive layer structure, fundamentally eliminating such problems at the material level. This homogeneous composite material system can undergo coordinated deformation under external pressure, effectively avoiding the risk of cracking or peeling of the surface conductive layer, and providing a structural basis for the sensor to maintain stable performance output under long-term, cyclic pressure loads.

[0026] 2. This invention cleverly transforms externally applied pressure signals into effective localized stress concentrations by meticulously designing and replicating the microscopic geometry on the surface of the sensitive layer. This structural design amplifies the rate of change of the contact area between the sensitive layer and the electrode, enabling the sensor to generate significant electrical signal responses even to minute pressure changes. Therefore, this method provides an effective path to improve sensor sensitivity without requiring complex processes or expensive materials. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the cross-section of the piezoresistive sensor of the present invention; Figure 2 This is a schematic diagram of the piezoresistive sensor of the present invention; Figure 3 This is a schematic diagram of the contact points of the present invention; Figure 4 This is a schematic diagram of a single contact of a piezoresistive sensor. Detailed Implementation

[0028] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] The present invention will be described in detail below through multiple embodiments and comparative examples. These embodiments are intended to further illustrate the technical solutions and beneficial effects of the present invention, and are not intended to limit the scope of protection of the present invention. Unless otherwise specified, all parts mentioned in the present invention are parts by weight, and the process equipment mentioned is conventional equipment in the art.

[0030] Example 1: A method for fabricating a flexible piezoresistive sensor based on a pyramid structure 1) Preparation of conductive composite materials Weigh out polydimethylsiloxane (PDMS) prepolymer and curing agent, mix them evenly at a mass ratio of 10:1, and slowly add 20% by volume of multi-walled carbon nanotubes (MWCNTs). Disperse the mixture ultrasonically for 30 minutes to obtain a uniformly dispersed conductive composite slurry. Place the slurry in a vacuum drying oven for 20 minutes to degas and remove air bubbles.

[0031] 2) Formation of the microstructure of the sensitive layer The aforementioned conductive composite paste was injected into a silicone mold with a pyramidal array on its surface, ensuring the paste fully filled the mold cavity. The mold was then cured at 70°C for 20 minutes, followed by a temperature increase to 110°C and a holding time of 50 minutes to complete the two-stage curing process. After cooling, the mold was demolded, yielding an integral sensitive layer with a pyramidal microstructure on its surface.

[0032] 3) Sensor integration and assembly Flexible copper foil is used as the electrode. The sensitive layer pyramid face is aligned with the electrode and then gently pressed together. Conductive adhesive is used to fix it along the edge, thus forming a layered structure of upper electrode layer / sensitive layer / lower electrode layer.

[0033] 4) Performance Testing The fabricated sensor was placed on a standard test platform, and a static pressure was applied within the range of 0–50 kPa. The resistance change was recorded. The loading was repeated 1000 times to examine the cyclic stability. The test results are shown in Table 1. Table 1

[0034] After 1000 cycles of cyclic loading, the fluctuation of ΔR / R0 is less than 3%, indicating that the device has excellent repeatability and stability.

[0035] Example 2: A method for fabricating a flexible piezoresistive sensor based on a three-dimensional porous structure 1) Preparation of conductive composite materials Weigh out the polyurethane (PU) prepolymer and chain extender, mix them evenly according to the formula ratio, add 30% by volume of silver nanowires (AgNWs), stir magnetically for 60 minutes, and add an appropriate amount of surfactant to prevent nanowire agglomeration. Vacuum degassing for 20 minutes to obtain a uniform and stable conductive composite slurry.

[0036] 2) Formation of the microstructure of the sensitive layer The conductive composite slurry is poured into a three-dimensional porous template to allow it to fully penetrate. It is then heated at 80°C for 25 minutes to complete the first stage of curing; the temperature is then increased to 120°C and maintained for 40 minutes to complete the second stage of curing, forming a three-dimensional porous conductive network structure. After cooling, it is demolded to obtain a sensitive layer with interconnected pores.

[0037] 3) Sensor integration and assembly A flexible polyimide circuit is used as the upper and lower electrodes. The sensitive layer is sandwiched between the upper and lower electrodes and a complete layered structure is formed by physical pressing. Then, it is encapsulated.

[0038] 4) Performance Testing Pressure was applied in the range of 0 to 100 kPa, and the resistance change and response characteristics were recorded. The test results are shown in Table 2. Table 2

[0039] The sensor exhibits a wide linear operating range, fast response characteristics, and stable performance after 2000 cycles, demonstrating excellent reliability and durability.

[0040] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for fabricating a flexible piezoresistive sensor, characterized in that, Includes the following steps: S1. Preparation of conductive composite materials: Conductive particles with a volume fraction of 10% to 60% are uniformly dispersed in a liquid or slurry elastic polymer matrix to form a conductive composite slurry with curable properties. S2. Microstructure forming of sensitive layer: The conductive composite slurry obtained in step S1 is injected or coated into a pre-designed mold with a negative microstructure pattern, and then cured. After demolding, an integrated sensitive layer with a precisely replicated microstructure is obtained on one side. S3. Sensor integration and assembly: Place the microstructured side of the sensitive layer obtained in step S2 opposite to a flexible electrode layer, and fix and assemble it by physical alignment and pressing or by using a thin layer of conductive adhesive to form a layered sensor device.

2. The manufacturing method according to claim 1, characterized in that, In step S1, the conductive particles are one or more of metal-based nanomaterials, carbon-based nanomaterials, and conductive polymers. The metal-based nanomaterials mentioned are metal nanowires or metal nanoparticles; The carbon-based nanomaterials mentioned are carbon nanotubes, graphene, or MXene nanosheets; The conductive polymer is PSS, polypyrrole, or polyaniline.

3. The manufacturing method according to claim 1, characterized in that, In step S1, the elastic polymer matrix is ​​polydimethylsiloxane, polyurethane, polyvinyl alcohol, polyethylene terephthalate, or polyimide.

4. The manufacturing method according to claim 1, characterized in that, In step S2, the negative microstructure pattern structure is a single micro-convex structure, a composite micro-convex structure, a three-dimensional porous structure, or a biomimetic structure; the single micro-convex structure is a pyramid structure, a hemispherical structure, or a semi-columnar structure.

5. The manufacturing method according to claim 1, characterized in that, The curing process in step S2 adopts a segmented heating method: First stage: Curing at 70℃~85℃ for 15~30 minutes; Second stage: Raise the temperature to 100℃~120℃ and maintain it for 40~60 minutes.

6. A flexible piezoresistive sensor manufactured using the method described in claims 1 to 6, characterized in that, The sensor has a layered structure, comprising an upper electrode layer, a sensitive layer, and a lower electrode layer, or a structure consisting of a sensitive layer and a single electrode layer. The sensitive layer is an integral structure, with the elastic polymer matrix as the continuous phase, and the conductive particles uniformly dispersed therein to form a conductive network. At least one surface of the sensitive layer has a micro-geometric structure for stress concentration, formed by replication through a mold. The electrode layer is a flexible conductive film or a patterned conductive circuit.

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