Three-gradient microstructure flexible pressure sensor and preparation method and application thereof

Abstract

The invention discloses a three-gradient microstructure flexible pressure sensor and a preparation method and application thereof, and aims to provide a sensor which is clear in structural design and can realize wide linear range, high sensitivity and high stability, a preparation method of the sensor, and application of the sensor in micro-pressure signal detection and plant physiological monitoring. The sensor comprises a sensing layer, an electrode layer and a hollow supporting layer, the sensing layer comprises an elastomer substrate with a three-gradient microstructure and a conductive film layer covering the surface of the three-gradient microstructure; the elastomer substrate is prepared by the following steps: spin-coating a flexible precursor solution on the surface of a mold, carrying out vacuum defoaming, heating and curing, and stripping to obtain the elastomer substrate with a three-gradient convex structure; the flexible precursor solution is composed of a polydimethylsiloxane solution and a single-layer MXene dispersion liquid; and the mass content of the MXene in the elastomer solid matrix is 2.5%-8%. The sensor is excellent in comprehensive sensing performance and has high stability and reliability.

Description

Technical Field

[0001] This invention relates to the fields of flexible electronics and micro / nano sensing technology, specifically to a flexible pressure sensor with a discontinuous three-level gradient microstructure and its fabrication method, as well as its application in wide-range, high-sensitivity pressure detection and biological object recognition. Background Technology

[0002] Flexible pressure sensors have wide applications in human-computer interaction, health monitoring, and robotic tactile sensing. To improve sensitivity and detection range, existing technologies often employ biomimetic multi-level microstructure designs. For example, patent document CN 120538732A, entitled "A Flexible Pressure Sensor with Multi-Level Hemispherical Structure and Its Preparation Method," discloses a sensor with a multi-level hemispherical structure. Its lower friction layer is formed by a multi-level hemispherical structure constructed from the surface of a flexible thin film. While this improves sensitivity to some extent, its effective pressure sensing range remains relatively narrow (e.g., 0-4 kPa), and its sensitivity is low across various pressure ranges, making it difficult to meet the requirements for wide-range, high-precision detection. Patent document CN 120403929A, entitled "A Skin-Inspired Multi-Level Micro / Nano Structure Flexible Wearable Pressure Sensor and Its Preparation Method and Application," discloses a multi-level micro / nano composite structure based on transfer from natural or rough templates such as sandpaper. Although this type of method can produce biomimetic microstructures, their structural morphology is random and cannot be precisely controlled, resulting in large batch-to-batch variations in sensor performance and poor repeatability.

[0003] Therefore, existing multi-level structure sensors generally suffer from problems such as ambiguous structural gradient definition, complex fabrication processes, difficulty in balancing sensitivity and linear range, and insufficient long-term stability. It is necessary to propose a gradient structure sensor with a well-defined structure, controllable fabrication process, and the ability to achieve a wide linear range, high sensitivity, and high stability, along with its fabrication method. Summary of the Invention

[0004] The purpose of this invention is to overcome the problems of narrow linear response range, limited sensitivity, and poor repeatability and stability caused by inaccurate structural gradients, complex processes, and unoptimized materials in existing multi-gradient pressure sensors. Instead, this invention provides a flexible pressure sensor with a non-continuous three-level gradient microstructure and its application, which has a well-defined structural design and can achieve a wide linear range, high sensitivity, and high stability.

[0005] Another objective of this invention is to provide a method for fabricating a flexible pressure sensor with a discontinuous three-level gradient microstructure that is process-controllable, has a wide linear range, high sensitivity, and high stability.

[0006] To solve the above-mentioned technical problems, the present invention provides the following technical solution: A three-gradient microstructure flexible pressure sensor includes a sensing layer, an electrode layer, and a hollow support layer; The sensing layer includes an elastomer substrate with a three-gradient microstructure and a conductive thin film layer covering the surface of the three-gradient microstructure. The three-gradient microstructure comprises a plurality of first, second, and third microstructure units with successively decreasing heights. The plurality of first, second, and third microstructure units are arranged in a matrix-like staggered manner, forming a discontinuous three-level gradient. The elastomer substrate is prepared by spin-coating a flexible precursor solution onto the surface of a three-gradient microstructure mold, followed by vacuum degassing, heat curing, and then peeling to obtain an elastomer substrate with a three-gradient protrusion structure. The flexible precursor solution is composed of a polydimethylsiloxane solution and a monolayer MXene dispersion. The mass content of MXene in the elastomer solid matrix is ​​2.5%-8%. The electrode layer includes a flexible insulating substrate and interdigitated electrodes disposed on the flexible insulating substrate; The hollow support layer is disposed between the sensing layer and the electrode layer, and the cavity in its middle allows the three-gradient microstructure region of the sensing layer to be positioned opposite to the interdigitated electrode region of the electrode layer.

[0007] Each of the first microstructure units is composed of a first protrusion of height h1 arranged in a 2×2 array; each of the second microstructure units is composed of four second protrusions of height h2 arranged in a 2×2 array; each of the third microstructure units is composed of four third protrusions of height h3 arranged in a 2×2 array; the first, second, and third protrusions are all conical, with the same diameter at the base of the cone, and satisfying h1 > h2 > h3.

[0008] Multiple first, second, and third microstructural units are arranged in a matrix-like staggered pattern on the elastomer substrate: in the first row direction, the first, second, and third microstructural units are arranged in a sequential cyclic pattern; in the adjacent second row direction, the second, third, and first microstructural units are arranged in a sequential cyclic pattern; in the adjacent third row direction, the third, first, and second microstructural units are arranged in a sequential cyclic pattern; and so on, so that the first, second, and third microstructural units of different heights are arranged in a stepped cyclic pattern in both the row and column directions.

[0009] A method for fabricating a three-gradient microstructure flexible pressure sensor includes the following steps: S1. Preparation of a three-gradient microstructure mold: Using a laser engraving device, a metal substrate is engraved with different laser repetitions in different areas to form a microstructure mold with several pits arranged in a matrix and having a non-continuous three-level gradient height difference. S2. Preparation of an elastomer substrate with a three-gradient microstructure: A flexible precursor solution is spin-coated onto the surface of the microstructure mold, and after vacuum degassing, heating and curing, it is peeled off to obtain an elastomer substrate with a three-gradient microstructure; the preparation of the flexible precursor solution includes the following steps: the main agent of polydimethylsiloxane and its curing agent are thoroughly mixed at a mass ratio of 10:1 to obtain a polydimethylsiloxane solution, and then a monolayer MXene dispersion is added and mixed evenly to obtain a flexible precursor solution; wherein, the mass content of MXene in the elastomer solid matrix is ​​2.5% to 8%; S3. Preparation of conductive film for sensing layer: A metal conductive thin film layer is deposited on the microstructure surface of the elastomer substrate using magnetron sputtering process to obtain the sensing layer; S4. Preparation of interdigitated electrode layer: A metal layer is deposited on a flexible insulating substrate by magnetron sputtering, and then an interdigitated electrode pattern is formed by laser etching to obtain the electrode layer; S5. Packaging and Integration: Using a hollow support layer with a cavity, the sensing layer and the electrode layer are aligned with the microstructure surface and the interdigitated electrodes, and then packaged through a hot pressing process to lead out wires, thereby obtaining the three-gradient microstructure flexible pressure sensor.

[0010] In step S1, the several pits formed by laser engraving are circular; the parameters of the laser engraving are: speed 2000-3000 mm / s, power 12-15W, and scanning frequency 40-60kHz.

[0011] The MXene content in the elastomeric solid matrix is ​​5% by mass.

[0012] In step S2, the spin coating speed is 900-3000 rpm, the spin coating time is 10-30 s, the curing temperature is 70-90℃, and the curing time is 20-40 min.

[0013] In steps S3 and S4, the target material used in the magnetron sputtering is selected from gold, silver, or platinum; the process parameters for the magnetron sputtering are: vacuum degree below 6.6 × 10⁻⁶. -4 Pa, argon working pressure 0.5-2.0 Pa, sputtering power 10-30 W.

[0014] Application of a three-gradient microstructure flexible pressure sensor in micro-pressure signal detection and plant physiological monitoring.

[0015] The plant physiological monitoring includes: using the sensor to acquire dynamic pressure response signals of different types of plant leaves under stress, and using a machine learning model to identify and classify plant leaf types based on the waveform characteristics of the response signals.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. Precise and controllable structure, significantly optimized performance: The three-gradient microstructure used in the sensor structure of this invention realizes a discontinuous, clearly partitioned three-gradient microstructure with a clear structural gradient. This structure enables the sensor to exhibit three-stage sequential contact under pressure, thereby achieving a piecewise high linear response over a wide total pressure range of 0-50 kPa through its discontinuous three-gradient structure, and achieving a high response of up to 131 kPa in the low-pressure range (0-0.4 kPa). -1 The improved sensitivity effectively solves the problem that existing gradient structures struggle to maintain high linearity and high sensitivity across different segments while expanding the pressure sensitivity range.

[0017] 2. Material-Structure Co-design: In the sensor of this invention, the elastomer is prepared by mixing PDMS and Mxene and then spin-coating. The optimal doping concentration of Mxene is 5 wt%, which enables the PDMS / Mxene composite material to form a continuous conductive network to improve electrical response while maintaining a moderate Young's modulus (about 2.5 MPa). This ensures that the microstructure can effectively deform under pressure, avoiding both low sensor sensitivity due to insufficient filler and a sharp increase in Young's modulus and structural embrittlement due to excessive filler. This lays the foundation for high sensitivity and high mechanical durability at the material level.

[0018] 3. Simple process and good consistency: The preparation method of the present invention can achieve three-level gradient in one step by simply adjusting the number of markings through laser engraving. It does not require complex templates or multiple overlays, which greatly simplifies the process, makes the process controllable, and helps to improve the consistency and yield of the finished product.

[0019] 4. Excellent overall sensing performance with high stability and reliability: The sensor fabrication method of this invention achieves precise fabrication of a discontinuous, clearly partitioned three-gradient microstructure by directly controlling the depth of the mold pit through the number of laser markings. The elastomer is prepared by mixing PDMS and MXene and then spin-coating. A metal conductive layer is prepared by sputtering, thereby forming a PDMS-MXene / Ag composite microstructure and constructing a highly efficient three-dimensional conductive network. The resulting sensor has a wide pressure sensing range of 0-50 kPa, with outstanding sensitivity in the low-pressure range. In rigorous long-term reliability testing, after 2000 consecutive load-unload cycles, the signal output attenuation rate is less than 5%, demonstrating its excellent mechanical fatigue resistance and electrical stability.

[0020] 5. Pioneering high-precision biological object recognition applications: The sensor of this invention can clearly capture the unique dynamic pressure "fingerprint" signals of different plant leaves. Combined with the random forest algorithm, the recognition accuracy for six types of leaves reaches 98.1%, significantly outperforming models such as CNN and MLP. It realizes a complete solution from hardware perception to intelligent recognition, and has broad application prospects in precision agriculture, virtual reality interaction and other fields. Attached Figure Description

[0021] Figure 1 The diagram shown is a schematic representation of the three-gradient microstructure flexible pressure sensor of the present invention. Figure 2 The diagram shown is a schematic of a three-gradient microconical array. Figure 3 The image shown is a cross-sectional view of a three-gradient microconical elastic body. Figure 4 The diagram shows the fabrication process of a three-gradient microstructure flexible pressure sensor. Figure 5 The figure shows the stress-strain curves of PDMS / MXene elastomers with different MXene contents; Figure 6 The figure shows the sensor sensitivity at different MXene contents; Figure 7 The image shows the stability test of the sensor under 2000 cycles of pressure testing; Figure 8 The image shows a comparison of the dynamic response signal waveforms generated by the sensor for six different types of blades. Figure 9 The bar chart shown is a comparison of the accuracy of different classification algorithms in identifying six types of leaves. Figure 10 The diagram shown is a confusion matrix for identifying different plant leaves. Detailed Implementation

[0022] The present invention will be further described below with reference to the accompanying drawings and embodiments, but the scope of protection of the present invention is not limited to the following embodiments.

[0023] A schematic diagram of a three-gradient microstructure flexible pressure sensor according to the present invention is shown below. Figures 1-3 As shown, it includes a sensing layer 1, an electrode layer 2, and a hollow support layer 3.

[0024] Sensing layer 1 includes an elastomer substrate with a three-gradient microstructure 1-1 and a conductive thin film layer covering the surface of the three-gradient microstructure. The three-gradient microstructure comprises a plurality of first microstructure units 1-1-1, second microstructure units 1-1-2, and third microstructure units 1-1-3 with successively decreasing heights. The plurality of first microstructure units 1-1-1, second microstructure units 1-1-2, and third microstructure units 1-1-3 are arranged in a matrix-like staggered pattern, forming a discontinuous three-level gradient. The elastomer substrate with the three-gradient microstructure is prepared by the following method: a flexible precursor solution is spin-coated onto the surface of a three-gradient microstructure mold, and after vacuum degassing, heating and curing, it is peeled off to obtain an elastomer substrate with a three-gradient protrusion structure. The flexible precursor solution is composed of a polydimethylsiloxane (PDMS) solution and a monolayer MXene dispersion; wherein the PDMS solution is composed of a main agent and a curing agent in a mass ratio of 10:1; the mass content of MXene in the elastomer solid matrix is ​​2.5% to 8%. Electrode layer 2 includes a flexible insulating substrate 2-1 and interdigitated electrodes 2-2 disposed on the flexible insulating substrate; A hollow support layer 3 is disposed between the sensing layer 1 and the electrode layer 2, and the cavity in the middle of the support layer allows the three-gradient microstructure region of the sensing layer 1 to be positioned opposite to the interdigitated electrode 2-2 region of the electrode layer.

[0025] In this embodiment, a schematic diagram of the surface morphology of the three-gradient microstructure is shown below. Figure 3 As shown, each of the first microstructure units 1-1-1 is composed of a 2×2 array of first protrusions with a height of h1; each of the second microstructure units 1-1-2 is composed of a 2×2 array of four second protrusions with a height of h2; and each of the third microstructure units 1-1-3 is composed of a 2×2 array of four third protrusions with a height of h3. The first, second, and third protrusions are all conical, with the same base diameter of 40-50 μm. The array spacing is 35-50 μm, and the order h1 > h2 > h3 is satisfied.

[0026] Multiple first microstructural units 1-1-1, second microstructural units 1-1-2, and third microstructural units 1-1-3 are arranged in a matrix-like staggered pattern on the elastomer substrate: in the first row direction, the first microstructural units 1-1-1, second microstructural units 1-1-2, and third microstructural units 1-1-3 are arranged in a sequential cyclic pattern; in the adjacent second row direction, the second microstructural units 1-1-2, third microstructural units 1-1-2, and first microstructural units 1-1-1 are arranged in a sequential cyclic pattern; in the adjacent third row direction, the third microstructural units 1-1-3, first microstructural units 1-1-1, and second microstructural units 1-1-2 are arranged in a sequential cyclic pattern; and so on, so that the first microstructural units 1-1-1, second microstructural units 1-1-2, and third microstructural units 1-1-3 of different heights present a stepped cyclic distribution in both the row and column directions.

[0027] In this embodiment, the hollow support layer 3 is a thermoplastic polyurethane film, and the flexible insulating substrate 2-1 is made of polyimide (PI) material.

[0028] The fabrication process of the three-gradient microstructure flexible pressure sensor of this invention is shown in the following flowchart. Figure 4 As shown, it includes the following steps: S1. Preparation of a three-gradient microstructure mold: Using a laser engraving device, a metal substrate is engraved with different laser repetitions in different areas to form a microstructure mold with several pits arranged in a matrix and having a non-continuous three-level gradient height difference. S2. Preparation of an elastomer substrate with a three-gradient microstructure: The flexible precursor solution is spin-coated onto the surface of the microstructure mold, and after vacuum degassing, heating and curing, it is peeled off to obtain an elastomer substrate with a three-gradient microstructure; the preparation of the flexible precursor solution includes the following steps: the main agent and curing agent of polydimethylsiloxane are fully mixed in a mass ratio of 10:1 to obtain a polydimethylsiloxane (PDMS) solution, and then a single-layer MXene dispersion is added and mixed evenly to obtain a flexible precursor solution; wherein, the mass content of MXene in the elastomer solid matrix is ​​2.5% to 8%; wherein, the single-layer MXene dispersion is a product of Lilenco Electronic Components Co., Ltd.

[0029] S3. Preparation of conductive film for sensing layer: A metal conductive film layer is deposited on the microstructure surface of the elastomer by magnetron sputtering to obtain the sensing layer. S4. Preparation of interdigitated electrode layer: A metal layer is deposited on a flexible insulating substrate by magnetron sputtering, and then an interdigitated electrode pattern is formed by laser etching to obtain the electrode layer; S5. Packaging and Integration: Using a hollow support layer with a cavity, the hollow support layer being a thermoplastic polyurethane film, the sensing layer and the electrode layer are aligned with the microstructure surface facing the interdigitated electrodes, and then packaged through a hot-pressing process to lead out wires, thereby obtaining the three-gradient microstructure flexible pressure sensor.

[0030] In step S1, the several pits formed by laser engraving are circular with a diameter of 40-50μm and an array spacing of 35-50μm. The laser engraving parameters are: speed 2000-3000 mm / s, power 13-15W, and scanning frequency 40-60kHz.

[0031] The preferred mass content of MXene in the elastomeric solid matrix is ​​5%.

[0032] In step S2, the spin coating speed is 900-3000 rpm, the spin coating time is 10-30 s, the curing temperature is 70-90℃, and the curing time is 20-40 min.

[0033] In steps S3 and S4, the target material used in the magnetron sputtering is selected from gold, silver, or platinum; the process parameters for the magnetron sputtering are: vacuum degree below 6.6 × 10⁻⁶. -4 Pa, argon working pressure 0.5-2.0 Pa, sputtering power 10-30W.

[0034] The three-gradient microstructure flexible pressure sensor prepared by this invention can be applied in micro-pressure signal detection and plant physiological monitoring. The plant physiological monitoring includes: using the sensor to acquire dynamic pressure response signals of different plant species' leaves under stress, and identifying and classifying plant leaf species based on the waveform characteristics of the response signals using a machine learning model. Example

[0035] Step 1: Fabrication of a three-gradient microstructure mold 304 stainless steel plate was selected as the substrate, ultrasonically cleaned with ethanol and air-dried. Laser engraving was then performed on the stainless steel surface using a laser marking machine.

[0036] Pattern design: Draw a circular array with a diameter of 50μm and an array spacing of 50μm.

[0037] Laser parameters: speed 2200mm / s, power 13.5W, scanning frequency 50kHz.

[0038] Three-gradient construction: The depth of the microstructure is controlled by adjusting the number of markings, which are 8, 6, and 4 times respectively, to form an array of pits with three depths: deep, medium, and shallow.

[0039] Post-processing: The sculpted mold is placed in an ultrasonic cleaner and then cleaned a second time with ethanol to remove surface residues, thus obtaining a three-gradient microstructure mold.

[0040] Step 2: Preparation of PDMS / MXene elastomer substrate A PDMS solution was prepared by mixing the PDMS main agent and curing agent at a mass ratio of 10:1. Different amounts of monolayer MXene aqueous dispersion (5 mg / ml) were added to the PDMS solution, and the mixture was stirred for 5 minutes and allowed to stand for 40 minutes to degas, preparing flexible precursor solutions with MXene mass fractions of 0%, 2.5%, 5%, and 10%. The monolayer MXene aqueous dispersion was purchased from Lilenko Electronic Components Co., Ltd. Each flexible precursor solution was spin-coated onto four three-gradient microstructure molds prepared in step 1 at a spin-coating speed of 1500 rpm for 20 seconds. After vacuum degassing, the solutions were cured at 80℃ for 30 minutes, and after peeling, a series of PDMS / MXene elastomer substrates were obtained.

[0041] Step 3: Prepare the conductive thin film layer of the sensing layer. The series of PDMS / MXene elastomer substrates prepared in step 2 were used to form a sensing layer by magnetron sputtering a silver layer on their microstructure surface. The sputtering parameters were: vacuum degree 6.6 × 10⁻⁶. -4 Pa, argon flow rate 25 sccm, power 20 W, time 30 min.

[0042] Step 4: Fabrication of interdigitated electrode layer Silver was sputtered onto a polyimide (PI) film and laser etched (frequency 30 kHz, power 6 W, speed 2000 mm / s) to create an interdigitated electrode layer with an interdigitated finger width and spacing of 200 μm.

[0043] Step 5: Packaging A thermoplastic polyurethane (TPU) sheet with square holes is used as a hollow support layer. The sensing layer (microstructure facing down) and the interdigitated electrode area of ​​the electrode layer are aligned, and the three are bonded together using a hot-pressing method. A PVC film is then attached to the back of the microstructure for protective encapsulation, and leads are soldered to obtain the sensor. The hot-pressing temperature is 110℃, the pressure is 0.5 MPa, and the time is 30 s.

[0044] Step 6, Performance Testing The obtained series of sensors were tested using conventional methods, and the test results are as follows: Figures 5-7 As shown.

[0045] Performance tests show that the sensor sample containing 5 wt% PDMS / MXene elastomer exhibits the best performance, with a moderate Young's modulus (2.5 MPa) and the best microstructure deformation capability (e.g., Figure 5(As shown). Pressure testing was performed on the obtained sensor: the sensitivity curve exhibited a three-stage linear response in the 0-50 kPa range, with a sensitivity of 131 kPa in the low-pressure range (0-0.4 kPa). -1 (like Figure 6 (As shown). After 2000 cycles of testing, the signal attenuation is <5% (e.g. Figure 7 As shown in the figure, it exhibits excellent mechanical fatigue resistance and electrical stability.

[0046] Example 2: Application of Plant Leaf Recognition The sensor was fixed to the mechanical testing platform, and gentle loading tests were performed on six kinds of leaves: golden pothos (0), natans leaf (1), rose multiflora (2), saxifrage (3), phytophyte leaf (4), and camphor tree (5). Dynamic pressure signals were collected, and the dynamic signal diagram is shown in the figure. Figure 8 As shown, the leaves of the six plant species all produced response waveforms with significantly different morphological, amplitude, and temporal characteristics, which fully demonstrates the sensor's ability to capture subtle mechanical differences. Each waveform can be regarded as the unique "mechanical fingerprint" of the leaf.

[0047] To verify the superiority of the Random Forest (RF) algorithm, eight comparative models—Convolutional Neural Network (CNN), Long Short-Term Memory Network (LSTM), PCA, MLP, PNet, KNN, and GRU—were trained simultaneously. Figure 9 As shown, the RF model achieved the highest recognition accuracy of 98.1%, which is significantly better than other models (such as CNN: 83.21%, MLP: 87.38%, KNN: 94.76%). Figure 10 The confusion matrix shows that the optimized RF model has extremely high recognition accuracy for all six types of leaves, with an overall accuracy of 98.1%, which is higher than the 97.16% recognition accuracy of the flexible pressure sensor based on a single pyramid microstructure in the literature (Highly efficient recognition of similar objects based on ionic robotic tactile sensors[J]. Science Bulletin, 2024, 69(13): 2089-2098.). This demonstrates the superiority of this sensor.

[0048] Therefore, the three-gradient microstructure flexible pressure sensor prepared by this invention can be applied in micro-pressure signal detection and plant physiological monitoring. The sensor is used to acquire dynamic pressure response signals of different plant leaf species under stress, and based on the waveform characteristics of the response signals, a machine learning model is used to identify and classify the plant leaf species.

[0049] The sensor structure of this invention employs a three-gradient microstructure, achieving a discontinuous, clearly partitioned three-gradient microstructure with a well-defined structural gradient. This structure enables the sensor to exhibit three-stage sequential contact under pressure, achieving a wide pressure sensing range of 0-50 kPa, and reaching up to 131 kPa in the low-pressure range (0-0.4 kPa). -1 The sensitivity is excellent. After 2000 consecutive load-unload cycles, the signal output attenuation rate is less than 5%, demonstrating excellent mechanical durability and electrical stability.

[0050] The above description is only a preferred embodiment of the present invention. It should be noted that, for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A three-gradient microstructure flexible pressure sensor, characterized in that, It includes a sensing layer, an electrode layer, and a hollow support layer; The sensing layer includes an elastomer substrate with a three-gradient microstructure and a conductive thin film layer covering the surface of the three-gradient microstructure. The three-gradient microstructure comprises a plurality of first, second, and third microstructure units with successively decreasing heights. The plurality of first, second, and third microstructure units are arranged in a matrix-like staggered manner, forming a discontinuous three-level gradient. The elastomer substrate is prepared by spin-coating a flexible precursor solution onto the surface of a three-gradient microstructure mold, followed by vacuum degassing, heat curing, and then peeling to obtain an elastomer substrate with a three-gradient protrusion structure. The flexible precursor solution is composed of a polydimethylsiloxane solution and a monolayer MXene dispersion. The mass content of MXene in the elastomer solid matrix is ​​2.5%-8%. The electrode layer includes a flexible insulating substrate and interdigitated electrodes disposed on the flexible insulating substrate; The hollow support layer is disposed between the sensing layer and the electrode layer, and the cavity in its middle allows the three-gradient microstructure region of the sensing layer to be positioned opposite to the interdigitated electrode region of the electrode layer.

2. The three-gradient microstructure flexible pressure sensor according to claim 1, characterized in that, Each of the first microstructure units is composed of a first protrusion of height h1 arranged in a 2×2 array; each of the second microstructure units is composed of four second protrusions of height h2 arranged in a 2×2 array; each of the third microstructure units is composed of four third protrusions of height h3 arranged in a 2×2 array; the first, second, and third protrusions are all conical, with the same diameter at the base of the cone, and satisfying h1 > h2 > h3.

3. The three-gradient microstructure flexible pressure sensor according to claim 2, characterized in that, Multiple first, second, and third microstructural units are arranged in a matrix-like staggered pattern on the elastomer substrate: in the first row direction, the first, second, and third microstructural units are arranged in a sequential cyclic pattern; in the adjacent second row direction, the second, third, and first microstructural units are arranged in a sequential cyclic pattern; in the adjacent third row direction, the third, first, and second microstructural units are arranged in a sequential cyclic pattern; and so on, so that the first, second, and third microstructural units of different heights are arranged in a stepped cyclic pattern in both the row and column directions.

4. A method for fabricating a three-gradient microstructure flexible pressure sensor according to any one of claims 1-3, characterized in that, Includes the following steps: S1. Preparation of a three-gradient microstructure mold: Using a laser engraving device, a metal substrate is engraved with different laser repetitions in different areas to form a microstructure mold with several pits arranged in a matrix and having a non-continuous three-level gradient height difference. S2. Preparation of an elastomer substrate with a three-gradient microstructure: A flexible precursor solution is spin-coated onto the surface of the microstructure mold, and after vacuum degassing, heating and curing, it is peeled off to obtain an elastomer substrate with a three-gradient microstructure; the preparation of the flexible precursor solution includes the following steps: the main agent of polydimethylsiloxane and its curing agent are thoroughly mixed at a mass ratio of 10:1 to obtain a polydimethylsiloxane solution, and then a monolayer MXene dispersion is added and mixed evenly to obtain a flexible precursor solution; wherein, the mass content of MXene in the elastomer solid matrix is ​​2.5% to 8%; S3. Preparation of conductive film for sensing layer: A metal conductive thin film layer is deposited on the microstructure surface of the elastomer substrate using magnetron sputtering process to obtain the sensing layer; S4. Preparation of interdigitated electrode layer: A metal layer is deposited on a flexible insulating substrate by magnetron sputtering, and then an interdigitated electrode pattern is formed by laser etching to obtain the electrode layer; S5. Packaging and Integration: Using a hollow support layer with a cavity, the sensing layer and the electrode layer are aligned with the microstructure surface and the interdigitated electrodes, and then packaged by hot pressing process to lead out the wires, thus obtaining the three-gradient microstructure flexible pressure sensor.

5. The preparation method according to claim 4, characterized in that, In step S1, the several pits formed by laser engraving are circular; the parameters of the laser engraving are: speed 2000-3000 mm / s, power 12-15 W, and scanning frequency 40-60 kHz.

6. The preparation method according to claim 4, characterized in that, The MXene content in the elastomeric solid matrix is ​​5% by mass.

7. The preparation method according to claim 4, characterized in that, In step S2, the spin coating speed is 900-3000 rpm, the spin coating time is 10-30 s, the curing temperature is 70-90℃, and the curing time is 20-40 min.

8. The preparation method according to claim 4, characterized in that, In steps S3 and S4, the target material used in the magnetron sputtering is selected from gold, silver, or platinum; the process parameters for the magnetron sputtering are: vacuum degree below 6.6 × 10⁻⁶. -4 Pa, argon working pressure 0.5-2.0 Pa, sputtering power 10-30 W.

9. The application of a three-gradient microstructure flexible pressure sensor according to any one of claims 1-3 in micro-pressure signal detection and plant physiological monitoring.

10. The application according to claim 9, characterized in that, The plant physiological monitoring includes: using the sensor to acquire dynamic pressure response signals of different types of plant leaves under stress, and using a machine learning model to identify and classify plant leaf types based on the waveform characteristics of the response signals.

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