Non-uniform paper-cutting topological piezoelectric-piezoresistive dual-mode flexible sensor and its applications

Abstract

This invention discloses a non-uniform paper-cut topological piezoelectric-piezoresistive dual-mode flexible sensor and its application. The method includes: pre-processing a flexible substrate film and a piezoelectric film; preparing a conductive sensitive layer on the surface of the flexible substrate film to obtain a flexible conductive composite film; patterning the flexible conductive composite film and the piezoelectric film according to a non-uniform density topological pattern; polarizing the cut piezoelectric film to obtain a piezoelectric functional layer; and aligning and stacking the two cut flexible conductive composite films and the piezoelectric functional layer to obtain a piezoelectric-piezoresistive dual-mode flexible sensor. The conductive sensitive layer forms a piezoresistive sensing channel for outputting a piezoresistive signal; the piezoelectric functional layer forms a piezoelectric sensing channel for outputting a piezoelectric signal. The non-uniform density topological pattern has different cutting densities along the stretching direction, which can release boundary constraint stress and form an effective deformation sensing area.

Description

Technical Field

[0001] This invention relates to the field of flexible electronics technology, specifically to a non-uniform paper-cutting topology piezoelectric-piezoresistive dual-mode flexible sensor and its applications. Background Technology

[0002] Flexible sensors can be attached to the surface of human skin, joints, or soft structures to sense mechanical signals such as strain, pressure, impact, and vibration. Most existing flexible strain sensors use a single sensing mechanism, either piezoresistive or piezoelectric. Piezoresistive sensors are suitable for static or low-frequency deformation detection, but they are easily affected by hysteresis and signal aliasing in high-frequency transient impacts and continuous rapid motion. Piezoelectric sensors are sensitive to transient impacts, but it is difficult to stably detect static forces and slow deformations.

[0003] Complex human movements often involve both low-frequency posture changes and high-frequency dynamic impacts. For example, in rope skipping, continuous bending of the legs or wrists generates low-frequency strain, while landing, tripping, or impact generates transient impacts. A single sensing mechanism cannot simultaneously acquire these signals, so a piezoelectric-piezoresistive dual-mode flexible sensor is needed to achieve synchronous monitoring and decoupling of dynamic and static signals.

[0004] Laser-induced graphene has advantages such as one-step forming, patternability, good conductivity, and porous structure sensitivity, making it suitable as a flexible piezoresistive sensing material and flexible electrode. Polyvinylidene fluoride (PVDF) films have good piezoelectric properties and are suitable as piezoelectric functional layers. However, the intrinsic ductility of thin film materials such as polyimide and PVDF is limited, and stress concentration, local tearing, and instability of conductive networks are prone to occur under large strain. Paper-cut topology can improve the ductility of films by unfolding through cuts, but traditional uniform density paper-cut structures are prone to generating non-deformable zones and stress concentrations at boundary constraints, limiting their long-term stable application in complex motion scenarios such as human joints.

[0005] While the concept of non-uniform paper-cutting structures has been proposed in existing technologies, they mainly focus on theoretical research and conventional materials such as PDMS and paper-based materials. There is a lack of systematic optimization for multilayer heterogeneous structure systems composed of LIG-PI composite films and PVDF piezoelectric films. In particular, how to improve the tensile properties, piezoresistive sensitivity, and piezoelectric sensitivity of devices by designing specific parameters of non-uniform density paper-cutting topology structures, and achieve synergistic optimization of the three, has not been disclosed in existing technologies.

[0006] Therefore, it is necessary to propose a flexible sensor for LIG-PVDF-LIG sandwich structures, which modulates stress distribution through non-uniform density topology and has both piezoresistive and piezoelectric dual-mode sensing capabilities, as well as its fabrication method and applications. Summary of the Invention

[0007] To overcome the limitations of existing technologies, such as the difficulty of simultaneously detecting static deformation and dynamic impact with a single sensing mechanism, the susceptibility of thin-film flexible sensors to boundary stress concentration and local failure under large strain, and the lack of system optimization parameters for LIG-PVDF-LIG multilayer heterogeneous systems in existing non-uniform paper-cutting structures, this invention provides a piezoelectric-piezoresistive dual-mode flexible sensor with a non-uniform density topology, its fabrication method, and its applications. This method combines finite element simulation and experiments to screen the optimal non-uniform density paper-cutting topology parameters for the LIG-PVDF-LIG system. The stress distribution of the device during stretching is controlled by the non-uniform density topological pattern, and the synchronous output and synergistic enhancement of piezoresistive and piezoelectric signals are achieved through the stacked integration of the LIG layer and the PVDF piezoelectric functional layer.

[0008] To achieve the above objectives, the present invention provides the following technical solution: a non-uniform paper-cutting topology piezoelectric-piezoresistive dual-mode flexible sensor, comprising:

[0009] A flexible substrate on which a non-uniform density paper-cutting topology is formed, the non-uniform density paper-cutting topology including a cross-shaped or rhomboid structure;

[0010] A laser-induced graphene (LIG) conductive layer is formed on the flexible substrate;

[0011] A polarized polyvinylidene fluoride (PVDF) piezoelectric layer is disposed above the laser-induced graphene conductive layer;

[0012] Another laser-induced graphene conductive layer is disposed above the polarized polyvinylidene fluoride piezoelectric layer to form a LIG-PVDF-LIG sandwich stacked structure.

[0013] The paper-cutting topology includes a central cutting area and a boundary cutting area, with a non-uniform density distribution. The cut density of the boundary cutting area is higher than that of the central cutting area. The overall paper-cutting topology density is higher than that of the central cutting area.

[0014] Furthermore, the slit spacing of the central cutting area is 300-600μm, and the slit spacing of the boundary cutting area is 200-350μm; the stress of the sensor under 120% tensile strain is not higher than 6MPa, the piezoresistive sensitivity GF is not lower than 6, and the piezoelectric sensitivity is not lower than 300mV / N.

[0015] Furthermore, the laser-induced graphene conductive layer is formed in situ by laser direct writing technology, and the processing parameters of the laser direct writing technology are: pulse frequency 100-150kHz, pulse width 3-6μs, scanning speed 15-25mm / s, and filling line spacing 0.005-0.02mm.

[0016] Furthermore, both LIG conductive layers are positioned facing the PVDF layer, serving as both piezoresistive sensitive layers and piezoelectric electrodes; at least one layer is provided with lead-out electrodes located in the non-cutting region of the paper-cutting topology.

[0017] This invention also provides a method for preparing the above-mentioned non-uniform paper-cutting topological piezoelectric-piezoresistive dual-mode flexible sensor, comprising the following steps:

[0018] S1. Select a polyimide film with a thickness of 50-150μm as a flexible substrate, and clean and dry it;

[0019] S2. A laser-induced graphene conductive layer was prepared on the surface of a polyimide film using laser direct writing technology to obtain a LIG / PI composite film.

[0020] S3. According to the preset non-uniform density paper-cutting topology, the LIG / PI composite film obtained in step S2 is cut with a laser. The slit spacing of the central cutting area of ​​the paper-cutting pattern is 300-600μm, and the slit spacing of the boundary cutting area is 200-350μm.

[0021] S4. Select a polyvinylidene fluoride film with a thickness of 20-80μm, cut it according to the same paper-cutting pattern as in step S3, and perform polarization treatment. The polarization voltage is 8-15kV and the polarization time is 30-120s.

[0022] S5. Align and stack the two cut LIG / PI composite films with the polarized PVDF film, with the LIG layer facing the PVDF layer, and fix them by hot pressing or bonding to form a LIG-PVDF-LIG sandwich structure.

[0023] S6. Lead out electrodes on the LIG layer and encapsulate them with an elastomer encapsulation material.

[0024] This invention also provides the application of the above-mentioned non-uniform paper-cutting topology piezoelectric-piezoresistive dual-mode flexible sensor in rope skipping exercise monitoring and complex dynamic and static composite signal monitoring and rehabilitation assessment of human joints, including the following steps: encapsulating the sensor in an elastic wristband or leg sleeve;

[0025] Low-frequency deformation signals are acquired through the piezoresistive channel of the sensor, and high-frequency impact signals are acquired through the piezoelectric channel of the sensor. The dual-channel signals are acquired synchronously and transmitted to the data processing terminal.

[0026] The data processing terminal calculates the motion frequency based on the change period of the piezoresistive channel signal amplitude, counts the number of times the signal drops based on the number of signal spikes in the piezoelectric channel, and determines an error when the piezoresistive channel shows a dropping characteristic but the piezoelectric channel does not show a corresponding impact spike.

[0027] Furthermore, the data processing terminal includes a dual-channel synchronous acquisition module; the dual-channel synchronous acquisition module includes: a piezoresistive signal conditioning circuit connected to the piezoresistive channel, used to convert the resistance change of the piezoresistive sensor into a voltage signal; a piezoelectric signal conditioning circuit connected to the piezoelectric channel, including a negative pressure generator and a charge amplifier, used to convert the charge signal generated by the piezoelectric sensor into a voltage signal; the output terminals of the piezoresistive signal conditioning circuit and the piezoelectric signal conditioning circuit are respectively connected to two ADC channels of the microcontroller, and the microcontroller samples in a dual ADC regular synchronous mode to achieve zero-phase delay acquisition.

[0028] Furthermore, the specific method for the data processing terminal to determine errors is as follows: within a preset time window, when the amplitude of the piezoresistive channel signal drops from its peak value and falls below the first threshold, it is determined to be a grounding state; if the piezoelectric channel does not detect an impact spike exceeding the second threshold after the grounding state occurs, it is determined to be an error.

[0029] This invention provides a non-uniform paper-cutting topological piezoelectric-piezoresistive dual-mode flexible sensor and its applications. It offers the following advantages:

[0030] 1. This invention combines finite element simulation and experiments to systematically screen the optimal non-uniform density paper-cutting topology parameters (boundary cut spacing 300-600μm, center cut spacing 200-350μm) for LIG-PVDF-LIG multilayer heterogeneous systems. This parameter combination reduces the stress of the device to no more than 6MPa under 120% ultimate tensile stress, effectively alleviating the boundary stress concentration and local tearing problems of traditional uniform paper-cutting structures under large strain. At the same time, a stable and uniform core sensing area is formed through the central low-density cutting area.

[0031] 2. Compared with traditional uniform density paper-cutting structures, the non-uniform density topology of this invention, under the same tensile strain, increases the piezoresistive sensitivity GF to no less than 6, while simultaneously achieving a piezoelectric sensitivity of no less than 300 mV / N. This synergistic enhancement effect of "dual structural improvement" has not been reported in the prior art.

[0032] 3. This invention uses LIG as both a piezoresistive sensing layer and a piezoelectric electrode, forming a sandwich stacked structure with the PVDF piezoelectric functional layer. This reduces the number of additional electrode layers and improves the device's structural integration. The three-dimensional porous network structure of LIG is beneficial for both improving piezoresistive sensitivity and the collection efficiency of PVDF piezoelectric charges.

[0033] 4. This invention monitors low-frequency static deformation (such as joint bending, take-off / landing state) through a piezoresistive channel and captures high-frequency dynamic impact (such as landing impact, tripping events) through a piezoelectric channel, thus achieving accurate monitoring in complex dynamic and static coupled motion scenarios.

[0034] 5. This invention attaches a sensor to the wrist or ankle of the human body and achieves rope skipping counting, frequency calculation, and automatic error determination through dual-channel signal processing. Compared with existing rope skipping counting devices (which only count or measure speed), this invention is the first to realize an automatic error determination function based on piezoelectric-piezoresistive dual-channel collaboration, and can also be extended to the field of complex dynamic and static composite signal monitoring and rehabilitation assessment of human joints. Attached Figure Description

[0035] Figure 1 This is a flowchart of the preparation method of the present invention;

[0036] Figure 2 This is a schematic diagram of the sensor structure with a uniform density MCK structure in Comparative Embodiment 1 of the present invention;

[0037] Figure 3 This is a design diagram of a non-uniform density paper-cutting structure with high density at the boundaries and low density at the center in this invention:

[0038] Figure 4 This serves as a control group for the non-uniform density paper-cutting structure design of the present invention;

[0039] Figure 5 This is a stress distribution diagram of the uniform density structure of the present invention under tension;

[0040] Figure 6 This is a comparison diagram of the stress distribution of the non-uniform density structures in Embodiments 2 and 3 of the present invention under tension.

[0041] Figure 7 This is a comparison diagram of stress-strain curves of six groups of non-uniform density paper-cutting structures in Embodiments 2 and 3 of the present invention;

[0042] Figure 8 This is a piezoresistive mode sensitivity diagram of the LPL-MCK dual-mode ultra-stretch flexible strain sensor of the present invention.

[0043] Figure 9 This is a piezoelectric mode sensitivity diagram of the LPL-MCK dual-mode ultra-stretch flexible strain sensor of the present invention;

[0044] Figure 10 This is a sensitivity comparison diagram of the dual-mode flexible strain sensor paper-cutting structure for six groups of non-uniform density paper-cutting structures in Embodiments 2 and 3 of the present invention.

[0045] Figure 11 This is a schematic diagram of the overall framework of the monitoring system for the non-uniform paper-cutting topology piezoelectric-piezoresistive dual-mode flexible sensor in this invention.

[0046] Figure 12 This is a diagram showing the voltage-resistance signal analysis when the jump rope enters the stable phase in this invention.

[0047] Figure 13 This is a diagram showing the piezoelectric resistance signal analysis of a sudden error in rope skipping in this invention. Detailed Implementation

[0048] The technical solutions in 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.

[0049] Example 1

[0050] A method for fabricating a non-uniform paper-cutting topological piezoelectric-piezoresistive dual-mode flexible sensor includes the following steps:

[0051] (1) Material pretreatment: Select a 50 μm thick polyimide (PI) film and a 28 μm thick polyvinylidene fluoride (PVDF) film, clean them three times with anhydrous ethanol using ultrasonic cleaning, and then dry them in an oven for 30 min for later use.

[0052] (2) Laser-induced preparation of LIG conductive layer: The PI film was fixed on an ultraviolet laser cutter, and the laser pulse frequency was set to 120kHz, the pulse width to 4.5μs, the scanning speed to 20mm / s, and the filling line spacing to 0.01mm. A fluffy and porous laser-induced graphene (LIG) conductive sensitive layer was generated in situ on the surface of the PI film, resulting in a LIG / PI composite film.

[0053] (3) Non-uniform density paper-cutting pattern cutting: A center-cut (MCK) paper-cutting topology pattern with "high density at the boundary and low density at the center" is designed. The pattern is divided into a boundary cutting area and a center cutting area along the stretching direction, so that the cut density in the boundary area of ​​the flexible substrate is higher than that in the center area. The specific parameters are: the longitudinal spacing of the cuts in the boundary cutting area is about 260 μm (40% shorter than the standard size), and the longitudinal spacing of the cuts in the center cutting area is about 350 μm (20% shorter than the standard size). Two LIG / PI composite films are cut according to this pattern using a laser, and conductive silver paste is applied to the non-cutting area to lead out copper wire electrodes to obtain the piezoresistive sensing layer (L-MCK).

[0054] (4) Piezoelectric layer polarization and cutting: The 28μm thick PVDF film was laser-cut using the same non-uniform density paper cutting pattern. The cut PVDF was then placed in a polarizer and polarized with a 10kV DC high voltage for 60s at room temperature to activate its piezoelectric properties and obtain the piezoelectric functional layer (P-MCK).

[0055] (5) Sandwich structure assembly: Insulating double-sided adhesive was applied to both ends of the polarized P-MCK and the two L-MCKs with pre-connected lead wires. Then, strictly following the sandwich structure sequence of "L-MCK (piezoresistive layer) / P-MCK (piezoelectric layer) / L-MCK (piezoresistive layer)", precise alignment and stacking were performed from bottom to top. After encapsulation, a piezoelectric-piezoresistive dual-mode flexible sensor (LPL-MCK) with a non-uniform density topology was successfully fabricated.

[0056] Comparative Example 1 (Uniform Density Paper Cutting Structure): To highlight the advantages of the non-uniform density topology structure of the present invention, a sensor with a uniform density MCK structure was prepared as a comparative example. Except for the use of a uniform pattern with equal cut spacing in steps (3) and (4), the other materials, laser parameters, and polarization conditions were exactly the same as in Example 1.

[0057] Example 2 describes a method for fabricating a non-uniform paper-cutting topological piezoelectric-piezoresistive dual-mode flexible sensor. The difference from Example 1 lies in the non-uniform cutting pattern: three sets of biomimetic topological structures with "high density at the boundary and low density at the center" were designed, specifically:

[0058] Figure 3 (a) The distance between the two boundary points is shortened by 20%, while the distance in the middle remains unchanged (MCK-P20-M0).

[0059] Figure 3 (b) The distance between the two boundary points is shortened by 40%, while the distance in the middle remains unchanged (MCK-P40-M0).

[0060] Figure 3 (c) The distance between the two boundary points is shortened by 40% and the distance in the middle is shortened by 20% (MCK-P40-M20).

[0061] The remaining assembly steps are the same as in Example 1.

[0062] Example 3 describes the fabrication method of a non-uniform paper-cut topological piezoelectric-piezoresistive dual-mode flexible sensor. The difference from Example 1 is that the non-uniform cutting pattern uses a control group with a "low density at the boundary, high density at the center" structure arranged in opposite directions. Figure 3 The parameter configuration is as follows:

[0063] Figure 4 (a) The two ends of the boundary remain the same size, and the center distance is shortened by 20% (MCK-P0-M20);

[0064] Figure 4 (b) The two ends of the boundary remain the same size, and the center distance is shortened by 40% (MCK-P0-M40);

[0065] Figure 4(c) The distance between the two ends is reduced by 20% and the distance between the centers is reduced by 40% (MCK-P20-M40).

[0066] The remaining assembly steps are the same as in Example 1.

[0067] Simulation Result Analysis:

[0068] Simulation results intuitively reveal the fundamental differences in mechanical response between the two arrangement logics. In the biomimetic topology structure with "high density at the boundary and low density at the center," the region near the fixed boundary can first undergo in-plane buckling and unfolding under external force, effectively absorbing and dissipating the concentrated stress generated by the boundary constraints. This stress release greatly improves the stress environment of the central core region, allowing the cut in the sparse central region to unfold fully and uniformly, significantly expanding the area of ​​the sensor's "effective uniform deformation zone." Conversely, as shown in the control group with "low density at the boundary and high density at the center," due to the relatively sparse boundary, its equivalent stiffness is high, and it hardly unfolds effectively during the stretching process, forming a significant "deformation-free dead zone" at the boundary. This not only forces the overall tensile strain to concentrate sharply in the central dense region, resulting in extremely uneven structural deformation, but also easily triggers severe stress concentration at the interface of abrupt changes in mesh density, thus creating a hidden danger of local tearing. Therefore, simulation analysis strongly confirms that the "high density at the boundary and low density at the center" topology design can successfully break the homogeneous boundary constraints of the uniform density paper-cutting structure from a physical mechanism perspective, and achieve reasonable redistribution of stress and uniform deformation.

[0069] The uniaxial tensile electromechanical properties of the sensors prepared in Examples 1, 2, and 3 and Comparative Example 1 were tested.

[0070] Mechanical stability test: When the tensile strain reaches 120%, Comparative Example 1 (uniform structure) has a severe stress concentration due to the "deformation dead zone" at the boundary, and its stress soars to about 8.0 MPa, which easily induces micro-tears at the edge; while Example 1 of the present invention (non-uniform structure) effectively absorbs the constraint stress through the high-density grid at the boundary and guides the uniform expansion of the central region. Under the ultimate tensile stress of 120%, its stress is significantly reduced to about 5.5 MPa, which greatly improves the tear resistance.

[0071] Electrical sensitivity test: In the tensile strain range of 0-20%, the piezoresistive sensitivity coefficient (GF) of Comparative Example 1 is only 0.28; while Example 1 of the present invention benefits from the efficient concentration and uniform amplification of the strain in the core sensing area, with a GF as high as 6.22, the sensitivity is about 22 times higher than that of the traditional uniform structure, and the linear fitting correlation coefficient R2 reaches 0.998.

[0072] Dual-mode decoupling and dynamic testing: The piezoelectric channel sensitivity of Example 1 can reach up to 309.14mV / N, and the piezoelectric charge recovery time is only 160ms, which is significantly faster than the piezoresistive response. It can keenly capture broadband transient mechanical impacts of 10-150Hz, and achieve perfect physical layer decoupling of complex human movements (such as low-frequency joint bending and high-frequency landing impact when jumping rope).

[0073] Application Experiment:

[0074] To verify the transient response and decoupling capability of the non-uniform paper-cutting topology piezoelectric-piezoresistive dual-mode flexible sensor and its acquisition system to real rope skipping motion, actual human wearing tests were conducted. By acquiring the raw quantized signals from a standstill to continuous rope skipping, the waveform characteristics under typical conditions such as motion initiation and error were analyzed to calibrate the trigger threshold of the core algorithm. When entering a uniform and stable rope skipping state, the measured waveform of the dual-mode sensor is as follows: Figure 12 As shown, firstly, the number of peaks in the piezoelectric and piezoresistive signals exhibits a one-to-one mapping relationship. During a steady jump, a single wrist flexion and extension force is inevitably accompanied by an impact or vibration of the rope. The measured waveforms show that the high-frequency spikes of the piezoelectric channel and the low-frequency peaks of the piezoresistive channel maintain a stable synchronous response on the time axis. This absolute consistency in the number of cycles proves that the system possesses extremely high acquisition fidelity and step counting reliability under continuous dynamic conditions.

[0075] Secondly, in terms of in-depth characterization of motion features, the two signals exhibit distinctly different physical measurement limitations and advantages. Due to its physical mechanism, the piezoelectric signal is only sensitive to transient high-frequency strain rates. Therefore, the piezoelectric waveform appears as a series of extremely narrow impact peaks, which can only serve as "time triggers" to accurately mark the instant the rope-swinging action occurs; the peak size is greatly affected by the rope rotation angle and high-frequency mechanical vibration, and cannot truly reflect the magnitude of the subject's rope-swinging force.

[0076] Conversely, the piezoresistive signal, due to its direct response to the continuous structural strain of the flexible substrate, exhibits a smooth, sinusoidal waveform. During the stationary period, the relative amplitude of the piezoresistive signal peaks directly characterizes the force with which the subject swings the rope. The more forceful the rope swing, the greater the deformation of the local muscles and bones in the wrist, the stronger the tensile strain on the sensor, and the significantly higher the amplitude of the corresponding piezoresistive peak. This characteristic allows the piezoresistive channel to transcend simple frequency statistics, endowing the system with the ability to assess the intensity of local human motion. In summary, the mechanism of "piezoelectric calibration of the moment of action and piezoresistive quantification of the force amplitude" fully highlights the core advantages of dual-mode collaborative monitoring in complex motion monitoring.

[0077] In actual rope skipping exercises, test subjects inevitably experience sudden errors such as tripping or collisions. These abnormal movements generate loud mechanical noise, severely interfering with the accuracy of step counting. Figure 13 The measured waveforms, including two typical failure conditions, are presented, clearly demonstrating the dual-mode sensor's ability to capture abnormal strains.

[0078] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A non-uniform paper-cutting topology piezoelectric-piezoresistive dual-mode flexible sensor, characterized in that, include: A flexible substrate on which a non-uniform density paper-cutting topology is formed, the non-uniform density paper-cutting topology including a cross-shaped or rhomboid structure; A laser-induced graphene (LIG) conductive layer is formed on the flexible substrate; A polarized polyvinylidene fluoride (PVDF) piezoelectric layer is disposed above the laser-induced graphene conductive layer; Another laser-induced graphene conductive layer is disposed above the polarized polyvinylidene fluoride piezoelectric layer to form a LIG-PVDF-LIG sandwich stacked structure. The paper-cutting topology includes a central cutting area and a boundary cutting area, which have a non-uniform density distribution. The cut density of the boundary cutting area is higher than that of the central cutting area, and the paper-cutting topology density is higher than that of the central cutting area.

2. The non-uniform paper-cutting topology piezoelectric-piezoresistive dual-mode flexible sensor according to claim 1, characterized in that, The slit spacing in the central cutting area is 300-600μm, and the slit spacing in the boundary cutting area is 200-350μm; the stress of the sensor under 120% tensile strain is not higher than 6MPa, the piezoresistive sensitivity GF is not lower than 6, and the piezoelectric sensitivity is not lower than 300mV / N.

3. The non-uniform paper-cutting topology piezoelectric-piezoresistive dual-mode flexible sensor according to claim 1, characterized in that, The laser-induced graphene conductive layer is formed in situ by laser direct writing technology. The processing parameters of the laser direct writing technology are: pulse frequency 100-150kHz, pulse width 3-6μs, scanning speed 15-25mm / s, and filling line spacing 0.005-0.02mm.

4. The non-uniform paper-cutting topology piezoelectric-piezoresistive dual-mode flexible sensor according to claim 1, characterized in that, Both LIG conductive layers are positioned facing the PVDF layer and serve as both piezoresistive sensitive layers and piezoelectric electrodes; at least one layer is provided with lead-out electrodes located in the non-cutting region of the paper-cutting topology.

5. The method for fabricating a non-uniform paper-cutting topological piezoelectric-piezoresistive dual-mode flexible sensor according to any one of claims 1-4, characterized in that, Includes the following steps: S1. Select a polyimide film with a thickness of 50-150μm as a flexible substrate, and clean and dry it; S2. A laser-induced graphene conductive layer was prepared on the surface of a polyimide film using laser direct writing technology to obtain a LIG / PI composite film. S3. According to the preset non-uniform density paper-cutting topology, the LIG / PI composite film obtained in step S2 is cut with a laser. The slit spacing of the central cutting area of ​​the paper-cutting pattern is 300-600μm, and the slit spacing of the boundary cutting area is 200-350μm. S4. Select a polyvinylidene fluoride film with a thickness of 20-80μm, cut it according to the same paper-cutting pattern as in step S3, and perform polarization treatment. The polarization voltage is 8-15kV and the polarization time is 30-120s. S5. Align and stack the two cut LIG / PI composite films with the polarized PVDF film, with the LIG layer facing the PVDF layer, and fix them by hot pressing or bonding to form a LIG-PVDF-LIG sandwich structure. S6. Lead out electrodes on the LIG layer and encapsulate them with an elastomer encapsulation material.

6. The application of the non-uniform paper-cutting topology piezoelectric-piezoresistive dual-mode flexible sensor according to claim 1, characterized in that, It is mainly used in rope skipping exercise monitoring and complex dynamic and static signal monitoring and rehabilitation assessment of human joints, including the following steps: The sensor is encapsulated in an elastic wristband or leg warmer. Low-frequency deformation signals are acquired through the piezoresistive channel of the sensor, and high-frequency impact signals are acquired through the piezoelectric channel of the sensor. The dual-channel signals are acquired synchronously and transmitted to the data processing terminal. The data processing terminal calculates the motion frequency based on the change period of the piezoresistive channel signal amplitude, counts the number of times the signal drops based on the number of signal spikes in the piezoelectric channel, and determines an error when the piezoresistive channel shows a dropping characteristic but the piezoelectric channel does not show a corresponding impact spike.

7. The application of the non-uniform paper-cutting topology piezoelectric-piezoresistive dual-mode flexible sensor according to claim 6, characterized in that, The data processing terminal includes a dual-channel synchronous acquisition module; the dual-channel synchronous acquisition module includes: a piezoresistive signal conditioning circuit connected to the piezoresistive channel, used to convert the resistance change of the piezoresistive sensor into a voltage signal; and a piezoelectric signal conditioning circuit connected to the piezoelectric channel, including a negative pressure generator and a charge amplifier, used to convert the charge signal generated by the piezoelectric sensor into a voltage signal; the output terminals of the piezoresistive signal conditioning circuit and the piezoelectric signal conditioning circuit are respectively connected to two ADC channels of the microcontroller, and the microcontroller samples in a dual ADC regular synchronous mode to achieve zero-phase delay acquisition.

8. The application of the non-uniform paper-cutting topology piezoelectric-piezoresistive dual-mode flexible sensor according to claim 6, characterized in that, The specific method for the data processing terminal to determine errors is as follows: within a preset time window, when the amplitude of the piezoresistive channel signal drops from its peak value and falls below the first threshold, it is determined to be a grounding state; if the piezoelectric channel does not detect an impact peak exceeding the second threshold after the grounding state occurs, it is determined to be an error.

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