Flexible pressure sensor and preparation method thereof

Abstract

The invention relates to the technical field of flexible electronics, and discloses a flexible pressure sensor and a preparation method thereof.The flexible pressure sensor comprises a conductive layer, a first electrode and a second electrode, the first electrode and the second electrode are arranged on the two sides of the conductive layer, and the conductive layer comprises a first conductive layer and a second conductive layer which are arranged in a stacked mode; wherein the first conducting layer is a plane conducting layer, a microstructure is arranged on the surface of one side of the second conducting layer, the microstructure comprises a microstructure unit with gradient in geometrical characteristics, and the side, provided with the microstructure, of the second conducting layer is in contact with the first conducting layer. And in the process of increasing the pressure from small to large, the microstructures of different levels sequentially participate in contact and deformation according to a height sequence. The mechanism fundamentally solves the problem that the contact area of a single structure is too fast to saturate, so that the linear response range of the flexible pressure sensor can be greatly widened while the initial high sensitivity is maintained.

Description

Technical Field

[0001] This invention relates to the field of flexible electronics technology, and in particular to a flexible pressure sensor and its fabrication method. Background Technology

[0002] With the rapid development of the Internet of Things (IoT), artificial intelligence (AI), and 5G communication technologies, electronic devices are evolving towards intelligence, portability, and wearability. Against this backdrop, flexible electronics technology has emerged, enabling electronic devices to attach to irregular surfaces and even conform well to human tissue, greatly expanding the application boundaries of electronic products. Among these, flexible pressure sensors, as key components for realizing human-computer interaction, health monitoring, and intelligent sensing, have received widespread attention from academia and industry. Traditional pressure sensors, such as those based on rigid silicon, noble metals, or metal oxide semiconductors, while mature in specific fields, suffer from inherent rigidity, fragility, complex manufacturing processes, and high costs, making them difficult to meet the demands of emerging application scenarios. For example, in applications such as electronic skin (E-skin), health monitoring wristbands, smart gloves, and soft robots, sensors must possess excellent flexibility, tensile strength, lightweight, and biocompatibility to accurately capture human movement or complex interactions with the environment in real time.

[0003] To overcome the limitations of traditional sensors, novel microstructure-based flexible pressure sensors have emerged. These sensors typically use polymers (such as PDMS and PU) as flexible substrates and achieve sensitive responses to applied pressure by constructing specific surface microstructures and combining them with conductive active materials. Their working mechanism is generally as follows: when pressure is applied, the microstructures on the flexible substrate deform, causing changes in the contact area, contact resistance, or tunneling resistance between conductive materials, thereby converting the pressure signal into a measurable electrical signal (such as changes in resistance, capacitance, or current). Compared with traditional sensors, microstructure-based flexible pressure sensors exhibit significant advantages, including but not limited to: high sensitivity, wide linear response range, low detection limit, fast response and recovery time, excellent mechanical flexibility and stretchability, low power consumption, and low-cost mass production potential. These superior properties make them highly promising for applications in wearable health monitoring (pulse, respiration, gait analysis), smart prostheses, electronic skin, human-computer interfaces, and soft robot tactile sensing, and have become one of the most important research hotspots in the field of flexible electronic materials and devices.

[0004] While existing microstructure-based flexible pressure sensors have made significant progress in some aspects, they generally suffer from a series of problems, such as the difficulty in simultaneously achieving high sensitivity and a wide linear range, poor signal hysteresis and long-term stability, and complex fabrication processes and high costs. These shortcomings severely restrict the widespread commercial application of high-performance flexible pressure sensors in fields such as electronic skin, medical health, and intelligent robotics.

[0005] Therefore, there is an urgent need in this field to develop a new type of flexible pressure sensor that not only possesses excellent performance in terms of high sensitivity, wide measurement range and fast response, but also overcomes the problems of signal hysteresis and long-term instability that are common in existing technologies. Furthermore, its structural design should be more ingenious, its fabrication method should be simpler, and its cost should be lower to meet the needs of large-scale industrialization. Summary of the Invention

[0006] In view of the shortcomings of the prior art, the purpose of this invention is to provide a flexible pressure sensor and its manufacturing method, which aims to solve the problem that it is difficult to achieve both high sensitivity and wide linear measurement range in the existing flexible pressure sensors.

[0007] In a first aspect, the present invention provides a flexible pressure sensor, the flexible pressure sensor comprising: a conductive layer and a first electrode and a second electrode disposed on both sides of the conductive layer, the conductive layer comprising a first conductive layer and a second conductive layer stacked thereon;

[0008] Wherein, the first conductive layer is a planar conductive layer, and a microstructure is provided on one side surface of the second conductive layer. The microstructure includes microstructure units with gradients in geometric features, and the side of the second conductive layer with the microstructure is in contact with the first conductive layer.

[0009] Preferably, the geometry of the microstructure unit is one or more of the following: prism, pyramid, cone, hemisphere, and wave shape.

[0010] Preferably, the microstructure comprises three prism structures with a gradient in height, the heights of which are 0.1 mm, 0.075 mm and 0.05 mm, respectively.

[0011] Preferably, the material of the first conductive layer includes a first flexible polymer matrix and a first conductive filler, and the material of the second conductive layer includes a second flexible polymer matrix and a second conductive filler.

[0012] Preferably, the first flexible polymer matrix and the second flexible polymer matrix are each independently selected from one or more of polydimethylsiloxane, polyurethane, Ecoflex silicone, and hydrogel.

[0013] Preferably, the first conductive filler and the second conductive filler are selected from one or more of carbon nanotubes, graphene, carbon black, silver nanowires, silver nanosheets, and conductive polymers.

[0014] Preferably, the first conductive filler accounts for 3%-10% of the mass of the first conductive layer, and the second conductive filler accounts for 3%-10% of the mass of the second conductive layer.

[0015] Preferably, the first conductive layer comprises polydimethylsiloxane, carbon nanotubes, and graphene, and the second conductive layer comprises polydimethylsiloxane, carbon nanotubes, and graphene.

[0016] Preferably, in the first conductive layer, the mass ratio of carbon nanotubes to graphene is (2-10):1, and in the second conductive layer, the mass ratio of carbon nanotubes to graphene is (2-10):1.

[0017] A second aspect of the present invention provides a method for fabricating the above-described flexible pressure sensor, the method comprising the following steps:

[0018] A mixed solution containing a flexible polymer matrix and conductive fillers is prepared;

[0019] Molds with microstructures are prepared using 3D printing technology;

[0020] A first conductive layer is obtained by preparing a film using the mixed solution;

[0021] The mixed solution is poured into the mold with the microstructure to obtain the second conductive layer;

[0022] The side of the second conductive layer with the microstructure is attached to the first conductive layer to obtain a conductive layer;

[0023] A first electrode and a second electrode are fabricated on both sides of the conductive layer to obtain the flexible pressure sensor.

[0024] Preferably, the step of fabricating a mold with a microstructure using 3D printing technology specifically includes:

[0025] Modeling of molds with microstructures using computer-aided design methods;

[0026] The mold was prepared using 3D printing technology according to the model described above.

[0027] Preferably, the conductive filler accounts for 6% of the mass of the mixed solution.

[0028] The present invention has the following beneficial effects:

[0029] This invention proposes a flexible pressure sensor and its fabrication method. Unlike existing technologies that commonly employ single-sized pyramid, micro-cylinder, or hemispherical structures, or porous / wrinkled structures with random and uncontrollable morphology, the flexible pressure sensor provided by this invention does not use a single microstructure. Instead, it creatively designs an array of at least two or more microstructures arranged in an ordered manner, exhibiting clear gradients in key geometric features (especially height). This constitutes a "hierarchical" or "multi-level" concept. This hierarchical structure directly leads to a "sequential contact" working mechanism. As the pressure increases, the microstructures at different levels participate in contact and deformation sequentially according to their height. This mechanism fundamentally solves the problem of rapid saturation of the contact area in a single structure, enabling the flexible pressure sensor to maintain high initial sensitivity while significantly widening its linear response range. This is an effect that existing single-structure designs cannot achieve. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the structure of a flexible pressure sensor provided in an embodiment of the present invention.

[0031] Figure 2 The image shows a finite element simulation diagram of the flexible pressure sensor provided in an embodiment of the present invention.

[0032] Figure 3 The waveform of resistance change of the flexible pressure sensor provided in the embodiment of the present invention. Detailed Implementation

[0033] This invention provides a flexible pressure sensor and its fabrication method. To make the objectives, technical solutions, and effects of this invention clearer and more explicit, the invention is further described in detail below. It should be understood that the specific embodiments described herein are only for explaining the invention and are not intended to limit the invention.

[0034] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of this invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0035] This invention provides a flexible pressure sensor, which includes a conductive layer and a first electrode and a second electrode disposed on both sides of the conductive layer. The conductive layer includes a first conductive layer and a second conductive layer stacked together.

[0036] Wherein, the first conductive layer is a planar conductive layer, and a microstructure is provided on one side surface of the second conductive layer. The microstructure includes microstructure units with gradients in geometric features, and the side of the second conductive layer with the microstructure is in contact with the first conductive layer.

[0037] In some specific implementation methods, such as Figure 1 As shown, in this embodiment, the microstructure includes a first microstructure unit and a second microstructure unit. The first microstructure unit on one side surface of the second conductive layer includes three prism structures with a height gradient, wherein the height of the first prism structure is 0.1 mm, the height of the second prism structure is 0.075 mm, and the height of the third prism structure is 0.05 mm. The first prism structure, the second prism structure, and the third prism structure are arranged sequentially along the first direction.

[0038] A second microstructure unit is disposed adjacent to the first microstructure unit. The second microstructure unit is located in a second direction from the first microstructure unit, and the height of the first prism structure of the second microstructure unit is 0.1 mm, the height of the second prism structure is 0.075 mm, and the height of the third prism structure is 0.05 mm. The first, second, and third prism structures are arranged sequentially along the second direction, where the first and second directions are two opposite directions on the same straight line. That is, the first and second microstructure units are symmetrically arranged, and the first and second microstructure units are arranged cyclically.

[0039] The flexible pressure sensor provided in this invention has the following advantages: 1. High sensitivity: The sharp geometry of the prism structure can generate significant stress concentration and efficient deformation under initial pressure, thus endowing the sensor with extremely high sensitivity. 2. Ultra-wide linear range: As the pressure continues to increase, prism structures of different levels and heights will sequentially and in stages contact and deform. This sequential contact mechanism effectively avoids the problem of rapid saturation of the contact area under a single structure through gradient compensation of the contact area, thereby greatly expanding the linear detection range of the sensor.

[0040] It is worth noting that a prism, with a line at its apex, initially contacts a plane, resulting in a "line contact." Compared to the extremes of "point contact" and "surface contact," the "line contact" approach combines the advantages of both while overcoming their disadvantages, producing the following unexpected technical effects:

[0041] 1. Superior Balance Between Sensitivity and Linearity: Point contact exhibits the strongest stress concentration and extremely high initial sensitivity. However, its drawback is that once the tip begins to deform, the contact area expands rapidly in a non-linear manner, causing the signal to quickly enter the non-linear region or even the saturation region. Surface contact has a larger initial contact area and a weaker stress concentration effect, resulting in relatively lower initial sensitivity. The line contact in this embodiment of the invention effectively achieves a balance between sensitivity and linearity. Its stress concentration effect is stronger than that of "surface contact," ensuring high initial sensitivity. Simultaneously, in the initial stage of continued pressure increase, the contact area preferentially extends linearly along the length of the prism before expanding in the width direction. This "linear extension" stage provides a more predictable and smoother response range for the flexible pressure sensor, thus achieving better linearity and a wider high-sensitivity range than "point contact." 2. Superior Mechanical Durability and Lifespan: The stress of "point contact" is highly concentrated at an infinitesimally small point. Under long-term, repeated pressure cycles, this point is highly susceptible to mechanical fatigue, wear, or permanent plastic deformation, leading to sensor performance degradation and shortened lifespan. This invention distributes pressure along a single line, significantly reducing peak stress. This greatly slows down the material fatigue process, making the microstructure less prone to damage, thus giving the sensor a longer cycle life and higher operational stability. 3. Potential anisotropic sensing capability: Arrays composed of points or surfaces typically have isotropic responses, meaning the response is essentially the same regardless of the direction from which pressure is applied. However, prismatic arrays have inherent directionality. When subjected to pressure with a shear force component, the shear force along the prism direction and the shear force perpendicular to the prism direction have completely different effects on the contact line length and width. This means that the flexible pressure sensor provided by this invention can not only detect pressure (Z-axis) but also has the potential to detect the direction of shear force (XY plane).

[0042] In some specific embodiments, the conductive layer is made of polydimethylsiloxane, multi-walled carbon nanotubes (MWCNTs) and graphene nanosheets (GNPs), with the mass percentage of MWCNTs and GNPs being 6% and the mass ratio of MWCNTs to GNPs being 5:1.

[0043] This mass ratio represents the optimal balance point obtained through extensive comparative experiments. When the mass fraction is below 6%, the conductive filler struggles to form effective conductive pathways within the PDMS matrix, leading to a sharp decline in the material's conductivity. Conversely, when the mass fraction exceeds 6%, excessive conductive filler disrupts the polymer chain structure of PDMS, causing the cured material to become brittle and lose its flexibility and ductility. Therefore, a 6% ratio strikes a balance between excellent conductivity and outstanding mechanical flexibility.

[0044] This synergistic ratio produces a conductivity enhancement effect greater than the sum of its parts ("1+1>2"). One-dimensional MWCNTs have a large aspect ratio, which excels at building long-range "conductive bridges" in the PDMS matrix; while two-dimensional GNPs can fill the gaps between MWCNTs, forming "surface-to-line" contacts, effectively reducing contact resistance. The 5:1 ratio optimizes these two effects, resulting in the best conductivity performance.

[0045] Furthermore, the first conductive layer and the second conductive layer in this embodiment of the invention are made of the same material, and their interface is completely bonded. This integrated design ensures excellent mechanical integrity and connection strength, eliminating the risk of failure such as air leakage or delamination caused by poor adhesion of cavities or encapsulation layers, making the sensor more robust and reliable overall.

[0046] In summary, the unique structural design combined with a stable material system enables the flexible pressure sensor provided by this invention to maintain stable sensing performance and rapid response recovery under repeated compression, demonstrating excellent durability and low signal hysteresis.

[0047] This invention also provides a method for fabricating the above-described flexible pressure sensor, the method comprising the following steps:

[0048] A mixed solution containing a flexible polymer matrix and conductive fillers is prepared;

[0049] Molds with microstructures are prepared using 3D printing technology;

[0050] A first conductive layer is obtained by preparing a film using the mixed solution;

[0051] The mixed solution is poured into the mold with the microstructure to obtain the second conductive layer;

[0052] The side of the second conductive layer with the microstructure is attached to the first conductive layer to obtain a conductive layer;

[0053] A first electrode and a second electrode are fabricated on both sides of the conductive layer to obtain the flexible pressure sensor.

[0054] In some embodiments, the step of fabricating a mold with a microstructure using 3D printing technology specifically includes:

[0055] Modeling of molds with microstructures using computer-aided design methods;

[0056] The mold was prepared using 3D printing technology according to the model described above.

[0057] The preparation method provided in this invention is as follows: digital design → 3D printing mold → casting molding. This is a digital and integrated process of "additive manufacturing". While the method appears simple, it actually has significant technical advantages in terms of efficiency, cost, and flexibility.

[0058] Specifically, 3D printing solutions enable convenient and efficient integrated design and manufacturing. By simply modifying a few parameters (such as prism height, spacing, and aspect ratio) in Python code or CAD software, a completely new mold can be obtained within hours. This capability makes it possible to quickly and cost-effectively customize flexible pressure sensors for specific applications (such as simulating specific pulse waveforms or adapting to specific robot fingertip curvatures).

[0059] Moreover, high-resolution commercial photopolymerization (SLA / DLP) 3D printers are now widely available, with low equipment and resin material costs. A single 3D printed master mold can be quickly used to create multiple sub-molds or directly for thousands of castings, significantly reducing unit production costs and paving the way for product commercialization.

[0060] The following detailed description uses specific examples.

[0061] Example

[0062] The fabrication of a flexible pressure sensor includes the following steps:

[0063] Step 1: Design and fabrication of hierarchical microstructure templates

[0064] 1. 3D Modeling Design: Using Python and its scientific computing libraries such as mesh, a parametric 3D model is generated programmatically. The core of the design is a three-level cyclic prism structure, which consists of an array of prisms with three different heights. The lowest level prism has a height of 0.05mm; the second level prism has a height of 0.075mm; and the highest level prism has a height of 0.1mm.

[0065] This gradient height design, ranging from 0.1 mm to 0.05 mm, is the cornerstone of achieving the key effects of this invention. When a small pressure is applied, the highest (0.1 mm) prism makes contact first, its sharp structure providing extremely high initial sensitivity. As the pressure increases, when the deformation of the highest-order prism reaches a certain level, the second-order (0.075 mm) prism begins to make contact, providing a new increase in contact area. Similarly, as the pressure further increases, the lowest-order (0.05 mm) prism also participates in contact. This sequential contact mechanism effectively compensates for the nonlinearity caused by the decrease in the rate of change of contact area when a single structure increases pressure, thereby greatly widening the linear detection range of the sensor and effectively suppressing premature signal saturation.

[0066] 2. Mold Generation and Printing: The model designed above is the positive mold. To facilitate subsequent casting and molding, a corresponding negative mold needs to be generated in the software based on this positive mold. Subsequently, the generated negative mold is exported as an STL (Standard Triangle Language) format file, and a high-precision 3D printer is used to materialize it into a solid mold.

[0067] Negative mold design: Using a negative mold for subsequent operations allows for the direct fabrication of conductive films with hierarchical prism structures in a single process via casting. The process is simple and has good repeatability.

[0068] 3D printing technology: Molds are made using 3D printing technology, which has the advantages of low cost, high speed and high customizability. It can quickly iterate and optimize microstructure design, greatly shortening the research and development cycle.

[0069] Step 2: Preparation of conductive layer material

[0070] 1. Material composition and ratio: Polydimethylsiloxane (PDMS) is used as the flexible substrate, and multi-walled carbon nanotubes (MWCNTs) and graphene nanosheets (GNPs) are used as conductive fillers. The conductive fillers account for 6% of the mass of the conductive layer material, and the mass ratio of multi-walled carbon nanotubes (MWCNTs) to graphene nanosheets (GNPs) is 5:1.

[0071] 2. Mixing and dispersing: Mix the weighed PDMS prepolymer with the above conductive filler, first perform thorough mechanical stirring, and then place it in an ultrasonic cleaner for ultrasonic treatment for 1 hour.

[0072] Ultrasonic processing utilizes the high-energy shock waves generated by its cavitation effect to effectively break up agglomerates of conductive fillers in viscous PDMS, achieving uniform nanoscale dispersion within the matrix. A uniform conductive network is a prerequisite for stable sensor performance and signal consistency.

[0073] 3. Add curing agent: Add the curing agent (crosslinking agent) of PDMS to the uniformly dispersed mixture, and ensure that the mass ratio of PDMS prepolymer to curing agent is 15:1 to obtain the final mixed liquid.

[0074] A 15:1 ratio is another crucial balancing factor. It ensures that the cured conductive layer material: 1) maintains good elasticity and resilience, which is fundamental for the repeated operation of flexible pressure sensors; 2) possesses moderate surface adhesion, facilitating subsequent stacking and bonding with planar films; and 3) guarantees excellent biocompatibility, expanding its applications in wearable devices. If the curing agent ratio is too low, the curing time will be too long, and the finished product surface will be too sticky, affecting the demolding effect of the microstructure; if the ratio is too high, the material will become too hard and brittle, losing its flexibility.

[0075] Step 3: Molding and Packaging of Flexible Pressure Sensors

[0076] 1. Thin Film Preparation: The prepared final liquid mixture is divided into two portions. One portion is uniformly coated onto a flat substrate to form a planar conductive thin film using a precision doctor blade with a thickness of 500 μm. The other portion is uniformly added to the prepared 3D printing negative mold using a dropper, utilizing the self-leveling properties of the liquid to fill the mold.

[0077] 2. Curing: Place the coated planar film and the cast structured film together in an 80℃ constant temperature oven and heat to cure for 2 hours.

[0078] 80℃ is a commonly used curing temperature for PDMS, which ensures complete cross-linking within a reasonable time, resulting in stable mechanical and electrical properties.

[0079] 3. Stacking and Encapsulation: The two cured materials are peeled from the substrate and mold, and the prismatic surfaces of the structured thin film and the flat surfaces of the planar thin film are stacked face to face. Finally, conductive silver paste is applied to the outer surfaces of the two thin films, and copper electrodes are attached to the silver paste to complete the electrical connection. The final sensor structure is as follows. Figure 1 As shown.

[0080] Working principle and effect verification

[0081] The working principle of the flexible pressure sensor prepared in this embodiment of the invention is dominated by the change in its internal contact resistance. For example... Figure 2 As shown in the finite element simulation, when the flexible pressure sensor is subjected to external pressure, the contact area between the graded prism and the planar thin film increases with increasing pressure. Figure 3 The actual resistance change waveform shown also verifies this; when pressure is applied, the resistance value of the flexible pressure sensor decreases significantly. This is because the increased contact area provides more conductive paths for electrons, thereby reducing the total resistance of the flexible pressure sensor. The entire process conforms to the design expectation of the piezoresistive effect, achieving an effective conversion of pressure signals into resistance signals.

[0082] It should be understood that the application of the present invention is not limited to the examples above. Those skilled in the art can make improvements or modifications based on the above description, and all such improvements and modifications should fall within the protection scope of the appended claims.

Claims

1. A flexible pressure sensor, characterized in that, The flexible pressure sensor includes: a conductive layer and a first electrode and a second electrode disposed on both sides of the conductive layer, wherein the conductive layer includes a first conductive layer and a second conductive layer stacked together. Wherein, the first conductive layer is a planar conductive layer, and a microstructure is provided on one side surface of the second conductive layer. The microstructure includes microstructure units with gradients in geometric features, and the side of the second conductive layer with the microstructure is in contact with the first conductive layer.

2. The flexible pressure sensor according to claim 1, characterized in that, The geometry of the microstructure unit is one or more of the following: prism, pyramid, cone, hemisphere, and wave shape.

3. The flexible pressure sensor according to claim 1, characterized in that, The microstructure includes three prism structures with gradients in height, the heights of which are 0.1 mm, 0.075 mm, and 0.05 mm, respectively.

4. The flexible pressure sensor according to claim 1, characterized in that, The first conductive layer is made of a first flexible polymer matrix and a first conductive filler, and the second conductive layer is made of a second flexible polymer matrix and a second conductive filler.

5. The flexible pressure sensor according to claim 4, characterized in that, The first flexible polymer matrix and the second flexible polymer matrix are each independently selected from one or more of polydimethylsiloxane, polyurethane, Ecoflex silicone, and hydrogel.

6. The flexible pressure sensor according to claim 4, characterized in that, The first conductive filler and the second conductive filler are selected from one or more of carbon nanotubes, graphene, carbon black, silver nanowires, silver nanosheets, and conductive polymers.

7. The flexible pressure sensor according to claim 4, characterized in that, The first conductive filler accounts for 3%-10% of the mass of the first conductive layer, and the second conductive filler accounts for 3%-10% of the mass of the second conductive layer.

8. The flexible pressure sensor according to claim 4, characterized in that, The first conductive layer comprises polydimethylsiloxane, carbon nanotubes, and graphene, and the second conductive layer comprises polydimethylsiloxane, carbon nanotubes, and graphene.

9. The flexible pressure sensor according to claim 8, characterized in that, In the first conductive layer, the mass ratio of carbon nanotubes to graphene is (2-10):1, and in the second conductive layer, the mass ratio of carbon nanotubes to graphene is (2-10):

1.

10. A method for manufacturing the flexible pressure sensor according to claim 1, characterized in that, The preparation method includes the following steps: A mixed solution containing a flexible polymer matrix and conductive fillers is prepared; Molds with microstructures are prepared using 3D printing technology; A first conductive layer is obtained by preparing a film using the mixed solution; The mixed solution is poured into the mold with the microstructure to obtain the second conductive layer; The side of the second conductive layer with the microstructure is attached to the first conductive layer to obtain a conductive layer; A first electrode and a second electrode are fabricated on both sides of the conductive layer to obtain the flexible pressure sensor.

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