A dual sensitive layer flexible piezoresistive sensor

Abstract

The application discloses a double-sensitive-layer flexible piezoresistive sensor, which comprises, from top to bottom, an upper electrode layer, an upper sensitive layer, a lower sensitive layer and a lower electrode layer; the upper sensitive layer and the lower sensitive layer are mainly made of a composite material of MWCNTs and SR; a sandpaper-like microstructure is arranged on the surface of the upper sensitive layer facing the lower sensitive layer; and a plurality of inclined protruding microstructures are arranged on the surface of the lower sensitive layer facing the upper sensitive layer. The inclined protruding microstructure of the lower sensitive layer is configured as a quadrangular pyramid, the inclination angle of the quadrangular pyramid is 70-130 DEG, and the inclination angle is the angle between the base and the side of the triangle section formed by passing through the vertex of the quadrangular pyramid and being perpendicular to one of the bases of the quadrangular pyramid. The application can simultaneously consider high sensitivity and wide range, realize sensitive detection when the piezoresistive sensor is subjected to inclined force, and finally realize significant improvement of the comprehensive performance of the sensor.

Description

Technical Field

[0001] This invention relates to the field of flexible pressure sensor technology, and in particular to a dual-sensor layer flexible piezoresistive sensor. Background Technology

[0002] Flexible pressure sensors can be classified into four types according to their working mechanism: capacitive, piezoelectric, piezoresistive, and triboelectric. Among them, piezoresistive sensors have become the focus of research due to their simple manufacturing process, low power consumption, and excellent stability.

[0003] The appropriate selection of active materials for piezoresistive sensors is crucial for achieving high performance. Optimizing material selection can significantly improve the performance of piezoresistive sensors, but it also presents challenges such as high manufacturing costs and the tendency for conductive materials to detach during long-term use. In addition to the appropriate selection of conductive materials, optimizing the design of the surface microstructure can distribute mechanical stress and improve the overall responsiveness of the piezoresistive sensor.

[0004] However, due to factors such as the viscoelasticity of the sensitive material and structure, and insufficient changes in the contact interface, the sensor may exhibit significant hysteresis during loading / unloading, affecting measurement accuracy and repeatability. Furthermore, existing flexible piezoresistive sensors often struggle to balance high sensitivity and wide measurement range, making it difficult to maintain effective resolution and stable output over a wide pressure range. Additionally, in multi-layered sensors, insufficient utilization of microstructure contact points may limit performance, weakening the amplitude of contact resistance changes and the effective signal output.

[0005] Furthermore, piezoresistive sensors are frequently used in robotic arms for flexible sensing. The force applied to a piezoresistive sensor is often not in the direct vertical direction, but rather at an angle. Based on force decomposition, the pressure at the angle can be decomposed into two components: one perpendicular to the surface of the piezoresistive sensor and the other parallel to it. However, current flexible piezoresistive sensors are often designed with only the sensitivity under direct vertical force in mind, neglecting the influence of the horizontal component on the piezoresistive sensor. Summary of the Invention

[0006] The terminology involved in this invention:

[0007] MWCNTs: Multi-walled carbon nanotubes.

[0008] SR: Silicone rubber.

[0009] MLP: Multilayer Perceptron.

[0010] To address the technical problems existing in the background art, this invention proposes a dual-sensor layer flexible piezoresistive sensor, which can effectively detect pressure signals over a wide pressure range and has high sensitivity.

[0011] This invention proposes a dual-sensor layer flexible piezoresistive sensor, comprising, from top to bottom, an upper electrode layer, an upper sensitive layer, a lower sensitive layer, and a lower electrode layer; the upper and lower sensitive layers are mainly composed of composite materials made of MWCNTs and SR; the surface of the upper sensitive layer facing the lower sensitive layer has a sandpaper-like microstructure; the surface of the lower sensitive layer facing the upper sensitive layer has multiple inclined protruding microstructures; the inclined protruding microstructures of the lower sensitive layer are configured as square pyramids, the tilt angle of the square pyramids is 70° to 130°, the tilt angle is the angle between the base and the side of the triangular cross section formed by the triangle passing through the vertex of the square pyramid and perpendicular to one of the base sides.

[0012] Furthermore, an SR substrate is disposed between the upper sensitive layer and the lower sensitive layer, and an intermediate electrode layer is disposed on the outer half of the SR substrate.

[0013] Furthermore, the outer surface of the upper electrode layer and / or the lower electrode layer is coated with an SR layer.

[0014] Furthermore, the tilting directions of the plurality of tilted protrusion microstructures may be the same or different.

[0015] Furthermore, the tilt angle of the square pyramid is 90°.

[0016] Furthermore, the base of the quadrangular pyramid is a parallelogram, with one side having a length of 2.5 to 3.5 mm and the other side having a length of 2 to 4 mm, and the height of the quadrangular pyramid being 3 to 5 mm.

[0017] Furthermore, the base of the square pyramid is a rhombus with a side length of 3mm and a height of 4mm.

[0018] Furthermore, the sandpaper-like microstructure of the upper sensitive layer is obtained by sandpaper transfer using a sandpaper template, wherein the sandpaper template has a mesh size of 80 to 150.

[0019] Furthermore, the sandpaper template has a mesh size of 120.

[0020] Furthermore, in the MWCNTs / SR composite solution used to prepare the upper and lower sensitive layers, the doping concentration of MWCNTs is 4%.

[0021] This invention designs a sandpaper-like microstructure in the upper sensitive layer and an inclined protruding microstructure in the lower sensitive layer. By precisely designing the hierarchical layout and microstructure of the key functional layers of the sensor, the deformation mode and stress distribution under pressure are actively controlled. This can simultaneously achieve high sensitivity and wide range, and realize sensitive detection of the piezoresistive sensor under inclined force, ultimately achieving a significant improvement in the overall performance of the sensor. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the structure of a dual-sensor layer flexible piezoresistive sensor proposed in this invention;

[0023] Figure 2 This is a schematic diagram of the preparation process of the MWCNTs / SR composite solution, the upper sensitive layer, the intermediate connecting layer, and the lower sensitive layer in an embodiment of the present invention;

[0024] Figure 3 The images shown are scanning electron microscope images of the upper and lower sensitive layers at different magnifications in the embodiments of the present invention, the elemental distribution diagrams of C, O, and Si of the sandpaper structure sample, and the X-ray diffraction patterns of SR, MWCNTs, and MWCNTs / SR.

[0025] Figure 4 This is a graph showing the relationship between pressure and the sensitivity of the piezoresistive sensor when the upper sensitive layer, the lower sensitive layer, the upper and lower sensitive layers, and the upper sensitive layer, the intermediate connecting layer, and the lower sensitive layer are all subjected to pressure in an embodiment of the present invention.

[0026] Figure 5 This is a schematic diagram showing the change in contact resistance of the piezoresistive sensor when the lower sensitive layer at different tilt angles is compressed in an embodiment of the present invention.

[0027] Figure 6 The following are finite element simulation diagrams, schematic diagrams of piezoresistive sensing mechanisms, schematic diagrams of equivalent resistance of the sensing mechanism, and optical images of piezoresistive sensors under different pressure states for different tilt angles of the lower sensitive layer in embodiments of the present invention.

[0028] Figure 7 This is a graph showing the relationship between pressure and the sensitivity of the piezoresistive sensor when the present invention is transferred using sandpaper templates of different mesh sizes, when the lower sensitive layer is pressed at different tilt angles, when the size (L, W, H) of the quadrangular pyramid of the lower sensitive layer changes, and when the doping concentration of MWCNTs in the MWCNTs / SR composite solution changes.

[0029] Figure 8 The piezoresistive sensor of the present invention exhibits sensitivity and linearity, hysteresis characteristics, response and recovery times at 10, 30 and 100 kPa, step response curves in the 0-125 kPa range, minimum detection limit for pressure, response curves of the piezoresistive sensor at different temperatures, and response curves at 10, 20, 40 and 50 mm / min.

[0030] Figure 9The piezoresistive sensor of the present invention exhibits loading / unloading response curves at 5-125 kPa, as well as locally magnified curves of cyclic response at different pressures (2, 30, and 100 kPa), and response curves of the piezoresistive sensor at 2 kPa loading / unloading cycles of 2500, 30 kPa loading / unloading cycles of 2500, and 100 kPa loading / unloading cycles of 2500.

[0031] Figure 10 The piezoresistive sensor of the present invention provides response curves for detecting the whole, upper sensitive layer and lower sensitive layer in a normal piezoresistive sensor, and response curves for detecting damage to the upper sensitive layer, lower sensitive layer and upper and lower sensitive layers, in comparison with a normal piezoresistive sensor.

[0032] Figure 11 The waveforms of the resistance response signals of the piezoresistive sensor of the present invention are used to test four types of hardness materials.

[0033] Figure 12 The piezoresistive sensor of the present invention is shown in the resistance response signal waveforms when detecting the surface roughness of 600-grit sandpaper, 400-grit sandpaper, 240-grit sandpaper and 80-grit sandpaper, as well as when detecting five test objects.

[0034] Figure 13 This is a schematic diagram showing the tilt angle of the tilted quadrangular pyramid structure in the lower sensitive layer of the present invention.

[0035] The numbers in the figure are: 1-upper SR layer; 2-upper electrode layer; 3-upper sensitive layer; 4-SR substrate; 5-middle electrode layer; 6-lower sensitive layer; 7-lower electrode layer; 8-lower SR layer. Detailed Implementation

[0036] The technical solution of the present invention will now be described in detail with reference to the accompanying drawings and embodiments. The embodiments are implemented based on the technical solution of the present invention, providing detailed implementation methods and specific operating procedures. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them, and the scope of protection of the present invention is not limited to the following embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0037] Example 1, as Figure 1As shown, the present invention proposes a dual-sensor layer flexible piezoresistive sensor, comprising, from top to bottom, an upper electrode layer 2, an upper sensitive layer 3, a lower sensitive layer 6, and a lower electrode layer 7. The upper sensitive layer 3 and the lower sensitive layer 6 are mainly composed of composite materials made of MWCNTs and SR. The composite material, as a piezoresistive material, constructs a compressible conductive network. SR itself possesses ultra-low modulus, ultra-high elasticity, and large deformation tolerance, which is the basis for achieving "flexibility." The one-dimensional high aspect ratio structure of MWCNTs can form a conductive network with extremely low filler content, maximizing the preservation of the high elasticity and low modulus of the SR matrix. The upper electrode layer 2 and the lower electrode layer 7 are generally conductive materials such as metal sheets, for example, copper foil, which serve as electrode leads.

[0038] The surface of the upper sensitive layer 3 facing the lower sensitive layer 6 has a sandpaper-like microstructure, which is characterized by random micro-protrusions of varying heights and sizes. The surface of the lower sensitive layer 6 facing the upper sensitive layer 3 has multiple inclined protruding microstructures.

[0039] When the piezoresistive sensor is compressed, the upper sensitive layer 3 and the lower sensitive layer 6 form a conductive path. At low pressures, only the higher protrusions of the upper sensitive layer 3 contact the tilted protrusions of the lower sensitive layer 6. As the pressure increases, more lower protrusions gradually participate in the contact, increasing the effective contact area and enabling continuous resistance variation over a wide measurement range. The tilted protrusions of the lower sensitive layer 6 bend and deform under pressure, exhibiting greater deformation capacity than existing technologies. Furthermore, the bending deformation of the tilted protrusions of the lower sensitive layer 6 increases the contact area with the upper sensitive layer 3, further amplifying the resistance variation. Therefore, as the pressure increases, the number of contact points and the contact area between the upper sensitive layer 3 and the lower sensitive layer 6 increase simultaneously, causing changes in both contact resistance and volume resistance, thus improving overall sensitivity. The compressibility of the tilted protrusions of the lower sensitive layer 6, combined with the elasticity of the SR material, allows the piezoresistive sensor to respond quickly to changes in external pressure, meeting dynamic detection requirements. The mechanical properties of MWCNTs and the elastic recovery capability of SR material reduce the hysteresis caused by the viscoelasticity of pressure-sensitive materials, thereby improving measurement accuracy and repeatability.

[0040] In Example 2, based on Example 1, an SR substrate 4 is further disposed between the upper sensitive layer 3 and the lower sensitive layer 6. An intermediate electrode layer 5 is partially surrounded by the outer side of the SR substrate 4, forming an intermediate connection layer. The intermediate electrode layer 5 is a thin metal sheet, typically copper foil, adhered to the SR substrate 4 as an electrode lead-out, thus forming a three-electrode interface together with the upper electrode layer 2 and the lower electrode layer 7. This interface can be used for layered testing of self-damage detection in piezoresistive sensors.

[0041] In this embodiment, the intermediate connecting layer is disposed between the upper sensitive layer 3 and the lower sensitive layer 6. For example... Figure 4 As shown, Figure 4 In the diagram, a) shows the relationship between pressure and the sensitivity of the piezoresistive sensor when the upper sensitive layer is under pressure, represented by a blue line; b) shows the relationship between pressure and the sensitivity of the piezoresistive sensor when the lower sensitive layer is under pressure, represented by a purple line; c) shows the relationship between pressure and the sensitivity of the piezoresistive sensor when both the upper and lower sensitive layers are under pressure in this embodiment of the invention, represented by a green line. It can be seen that the piezoresistive sensor has a higher sensitivity when both the upper and lower sensitive layers are under pressure compared to when only one upper or lower sensitive layer is under pressure; d) shows the relationship between pressure and the sensitivity of the piezoresistive sensor when the upper sensitive layer, the intermediate connecting layer, and the lower sensitive layer are under pressure, represented by a red line. It can be observed that the piezoresistive sensor has the highest sensitivity after adding the intermediate connecting layer, proving that the introduction of the intermediate connecting layer has an improving effect on performance. Under pressure, the sandpaper-like microstructure of the upper sensitive layer 3 and the inclined protruding microstructure of the lower sensitive layer 6 come into contact with the intermediate connecting layer. The sandpaper-like microstructure of the upper sensitive layer 3 has protrusions of randomly distributed heights. Under pressure, the higher protrusions contact the intermediate connecting layer first, and as the pressure increases, the lower protrusions gradually participate in the contact, forming a multi-layered contact interface. This causes the number of effective contact points to increase in a gradient with pressure. The inclined protruding microstructure of the lower sensitive layer 6 undergoes bending deformation under pressure, flexing along the inclined direction, increasing the contact area with the intermediate connecting layer, and simultaneously altering the distribution of conductive paths at the contact interface. Through this synergistic effect, the utilization rate of effective contact points is improved, and the magnitude of resistance change is enhanced.

[0042] In a preferred embodiment, the outer surface of the upper electrode layer 2 is coated with an upper SR layer 1 to improve the stability of the electrode contact.

[0043] In a preferred embodiment, the outer surface of the lower electrode layer 7 is coated with a lower SR layer 8 to improve the stability of the electrode contact.

[0044] In a specific embodiment, the above structure is prepared as follows:

[0045] Figure 2 Figure a shows a schematic diagram of the preparation process of the MWCNTs / SR composite solution provided in this embodiment. MWCNTs are added to the solvent and stirred at 400 r / min, maintaining the solution at 25°C for 30 min. Then, SR is added and stirred at 800 r / min, maintaining the temperature at 25°C for 90 min. This prepares the MWCNTs / SR composite solution. Scanning electron microscopy images of the sample surface show that MWCNTs are uniformly dispersed in the SR.

[0046] Figure 2Figure b shows a schematic diagram of the preparation process of the upper sensitive layer 3 provided in this embodiment. First, sandpaper is cut into a suitable size and attached to the groove mold. After cleaning and drying, MWCNTs / SR composite solution is poured in, and the mixture is placed in an oven and heated to 40°C for 3 hours. Then, the mold is removed, and copper foil and SR are attached to the other surface of the upper sensitive layer 3 away from the lower sensitive layer 6, thereby obtaining an upper sensitive layer 3 with a sandpaper-like microstructure.

[0047] Figure 2 Figure c shows a schematic diagram of the preparation process of the intermediate connecting layer provided in this embodiment. SR is poured into a prepared mold, heated to 40°C and cured for 3 hours, and then demolded to obtain SR substrate 4. A copper foil is adhered to half of SR substrate 4 to form the intermediate connecting layer.

[0048] Figure 2 Figure d shows a schematic diagram of the preparation process of the lower sensitive layer 6 provided in this embodiment; the MWCNTs / SR composite solution is poured into a 3D-printed mold with inclined raised grooves. Then, the same steps as for the upper sensitive layer 3 are followed to obtain the lower sensitive layer 6.

[0049] The upper electrode layer 2, upper sensitive layer 3, intermediate connecting layer, lower sensitive layer 6, and lower electrode layer 7 are encapsulated with a PU film to obtain the final piezoresistive sensor.

[0050] It should be noted that the specific manufacturing methods described above are illustrative and not intended to limit the invention. Those skilled in the art can adjust or replace the manufacturing process according to actual needs. For example, the sandpaper-like microstructure of the upper sensitive layer 3 can be obtained not only through sandpaper template transfer but also through other microstructure forming methods such as laser etching, chemical etching, and 3D printing; the SR substrate 4 of the intermediate connecting layer can be formed not only through mold casting and curing but also through pressing, spraying, etc.; the tilted protruding microstructure of the lower sensitive layer 6 can be formed not only through 3D printing molds but also through micro-injection molding, photolithography, etc.; the electrode material can be other metals besides copper foil, or conductive materials such as silver paste, conductive polymers, and carbon-based conductive films; the encapsulation method can be other flexible encapsulation materials such as PDMS besides PU film encapsulation.

[0051] To further analyze the microstructure of the sensitive layer, the morphology of the upper sensitive layer 3 and the lower sensitive layer 6 was observed using scanning electron microscopy (SEM). Figure 3In the image, (a) clearly shows the sandpaper-like microstructure of the upper sensitive layer 3, indicating successful transfer of the sandpaper surface microstructure onto the upper sensitive layer 3. (b) and (c) show further magnified SEM images of the upper sensitive layer 3 sample, demonstrating that MWCNTs are dispersed and embedded within the SR matrix. (d) shows a scanning electron microscope image of the lower sensitive layer 6, revealing the morphology of the lower sensitive layer 6 sample and further demonstrating that MWCNTs are uniformly dispersed within the sensitive layer. Furthermore, Figure 3 Figure e shows the overall distribution of elements C, O, and Si in the sample, while figures f, g, and h show the distribution of elements C, O, and Si, respectively. The uniform distribution of C, O, and Si in the upper sensitive layer 3 and the lower sensitive layer 6 can be observed. Figure 3 Figure i shows the X-ray diffraction patterns of SR, MWCNTs, and MWCNTs / SR. The diffraction angle on the horizontal axis refers to the angle between the incident ray and the diffracted ray during actual scanning by the diffractometer. This figure further characterizes the crystal structure and its changes in the upper sensitive layer 3 and the lower sensitive layer 6. A diffraction peak of the MWCNT crystal plane was observed at a diffraction angle of 26.0°. However, the characteristic peaks of MWCNTs in the MWCNTs / SR composite material almost disappeared. This indicates that MWCNTs are uniformly dispersed in the SR matrix in the upper and lower sensitive layers, and a voltage-sensitive material with high conductivity was successfully fabricated.

[0052] In a preferred embodiment, the tilted protrusion microstructure of the lower sensitive layer 6 is configured as a square pyramid, the tilt angle θ of which is 70° to 130°. Figure 13 As shown, the inclination angle θ is the angle between the base and the side of the triangular cross section formed by the triangle passing through the vertex of the quadrangular pyramid and perpendicular to one of the base sides.

[0053] Some existing piezoresistive sensors incorporate a pyramidal (square pyramid structure) microstructure in their sensitive layer. The sides and base of the pyramid are symmetrically arranged, with the apex located directly above the center of the base. The corresponding triangular cross-section is an isosceles triangle with an acute angle θ. Under pressure, these pyramids primarily exhibit vertical compressive deformation. Under inclined forces, they tend to undergo axial compression, with stress distribution relatively concentrated along the compression path from the apex to the base. Due to their symmetrical structure, they are not sensitive to horizontal forces, and under oblique forces, unilateral stress concentration is more likely to occur. Contact is mainly concentrated at local edges or corners, easily manifesting as localized crushing or slippage, and the contact area variation is not continuous.

[0054] This invention differs from existing technologies by employing an inclined quadrangular pyramid structure in the lower sensitive layer 6. The apex of this pyramid is offset vertically above the center of the base, causing the entire pyramid to tilt to one side. Under pressure, because the apex is off-center from the center of the base, the line of pressure does not coincide with the structural axis, generating a bending moment and causing the structure to bend and deform. Figure 5As shown, under the same pressure conditions, the tilted quadrangular pyramid structure exhibits more significant deformation than the regular quadrangular pyramid structure. This indicates that the tip displacement caused by bending deformation is greater than that caused by compression deformation. This deformation mechanism results in a larger variation in the contact area between the lower sensitive layer 6 and the intermediate connecting layer, leading to a greater change in contact resistance. This can effectively improve the sensitivity of the piezoresistive sensor. Moreover, when the regular quadrangular pyramid structure is compressed, the contact area expands symmetrically with increasing pressure, resulting in the tip being flattened and the sides being pressed together. After the pressure reaches a certain level, this type of piezoresistive sensor enters the saturation region and becomes insensitive to higher pressures. However, in this application, because the tilted quadrangular pyramid is set with a specific tilt angle range, when compressed, the tip first contacts the intermediate connecting layer, and then the structure undergoes bending deformation as the pressure increases, with the contact area continuously expanding to the entire side, resulting in a higher saturation pressure point. This allows it to maintain an effective response over a wider range. Furthermore, the vertical compression deformation mode of a regular square pyramid structure is relatively simple, while the tilted square pyramid structure of this application can simultaneously generate bending and torsional deformation under pressure. By combining with a sandpaper-like microstructure, it can enrich the resistance changes at the contact interface, more fully activating the random micro-protrusions of different heights on the sandpaper surface. This is beneficial for maintaining the continuity of resistance changes over a wide pressure range, and compared to the compression deformation of a regular pyramid, the tilted square pyramid has a faster deformation recovery capability. The intermediate connecting layer, as the intermediate interface, simultaneously receives contact pressure from both the upper sensitive layer 3 and the lower sensitive layer 6. The deformation mechanisms of the two contact interfaces do not interfere with each other, which helps simplify the contact state and improve signal stability.

[0055] In this embodiment, the piezoresistive sensor, when subjected to tilting force, achieves an effective response to tilting pressure through the synergy of the sandpaper-like microstructure, the tilted pyramidal structure, and the intermediate connecting layer. Specifically, under the action of the horizontal component force, when the direction of the horizontal component force is consistent with the tilting direction, the tilted pyramidal structure exhibits more significant bending deformation and a larger change in contact resistance, enabling a direct response to the horizontal component force. The random multi-level protrusions of the sandpaper-like microstructure, when the tilting angle of the tilting pressure is large (i.e., the vertical component force is small), allow the lower protrusions to participate in contact, compensating for the sensitivity corresponding to the smaller vertical component force. The SR substrate of the intermediate connecting layer is elastic, which can mitigate and redistribute the pressure, making the force on the lower sensitive layer more uniform. The synergistic effect of these three elements allows the piezoresistive sensor of this invention to maintain high sensitivity and stable response under tilting pressure.

[0056] In a preferred embodiment, the tilting directions of the multiple tilted protrusions may be the same or different. The tilting directions of the multiple tilted protrusions can be set to the same direction to enhance the response sensitivity to horizontal forces in a specific direction; alternatively, they can be set to a random distribution or an array-like regular variation to achieve a balanced response to horizontal forces in multiple directions, improving the sensor's adaptability in complex force scenarios. For example, when the sensor is mainly used to detect horizontal forces in a specific direction (such as the main force direction during robotic gripping), the tilting directions of the multiple tilted pyramids can be set to the same direction to enhance the response sensitivity to horizontal forces in that direction. When the sensor needs to be used in complex force scenarios where the direction of the horizontal forces is uncertain, the tilting directions of the multiple tilted pyramids can be set to a random distribution or an array-like regular variation, so that horizontal forces in different directions can encounter tilted pyramids with matching directions, thereby achieving a balanced response to horizontal forces in multiple directions. All of the above designs are within the scope of protection of this application.

[0057] Therefore, in this embodiment, the surface of the upper sensitive layer 3 facing the lower sensitive layer 6 is provided with a sandpaper-like microstructure, and the surface of the lower sensitive layer 6 facing the upper sensitive layer 3 is provided with multiple inclined protruding microstructures. An intermediate connecting layer is provided between the upper sensitive layer 3 and the lower sensitive layer 6. This structure has the following advantages:

[0058] Firstly, there is the synergy between gradient contact and progressive contact. The protrusions of varying heights on the surface of the sandpaper-like microstructure of the upper sensitive layer 3 form gradient contact with the intermediate connecting layer under pressure. That is, at lower pressures, only the higher protrusions make contact; as pressure increases, more lower protrusions gradually participate, resulting in a gradient increase in the number of contact points. The inclined quadrangular pyramid of the lower sensitive layer 6 bends under pressure, forming progressive contact with the intermediate connecting layer. That is, the tip makes contact first; as pressure increases, the degree of bending increases, and the contact area gradually expands laterally, resulting in a continuous increase in the contact area of ​​each individual contact point. These two contact modes increase with the intermediate connecting layer as a common interface, ensuring that the overall contact state increases both the number of contact points and the area of ​​each individual contact point as pressure increases. The sandpaper-like microstructure, the intermediate connecting layer, and the inclined quadrangular pyramid structure complement each other, jointly achieving continuous resistance variation over a wide measurement range.

[0059] Secondly, the intermediate connecting layer prevents direct contact between the sandpaper-like microstructure and the tilted pyramid structure. This avoids potential failures due to excessive compression or lack of contact between protrusions, as well as uncontrollable deformation paths and unstable contact resistance caused by random contact point distribution. Furthermore, the tilt direction of the tilted pyramid and the positional distribution of the protrusions in the sandpaper-like microstructure affect the stress state under pressure. A mismatch in direction could lead to excessive stress and damage to some parts of the tilted pyramid, while others might experience insufficient stress. The intermediate connecting layer isolates the two, making the upper sensitive layer 3 and the lower sensitive layer 6 relatively independent, avoiding mutual interference caused by direct contact between heterogeneous microstructures and improving signal stability and repeatability.

[0060] Third, when subjected to tilted forces, the tilted pyramidal structure of the piezoresistive sensor is more sensitive to tilted forces than existing technologies due to its inherent directionality. The tilted force is more easily converted into bending and compression coupling deformation along its tilt direction, and the horizontal and vertical components of the force are more effectively converted into structural deformation. The contact area is not just a sudden contact at a certain point or on a certain edge, but is more likely to gradually expand along the tilt direction. The resistance change is also easier to amplify and stably read.

[0061] The intermediate connecting layer contains an SR substrate. When pressure is transmitted from top to bottom, it first passes through a sandpaper-like microstructure, where it is dispersed into multiple contact points acting on the intermediate connecting layer. Thanks to the elasticity of the SR substrate, the intermediate connecting layer buffers and redistributes the pressure transmitted from the upper sensitive layer 3, then transfers it to the inclined quadrangular pyramid structure of the lower sensitive layer 6, making the stress on the lower sensitive layer 6 more uniform and avoiding localized stress concentration. This pressure transmission path demonstrates the synergistic effect of the upper sensitive layer 3, the intermediate connecting layer, and the lower sensitive layer 6.

[0062] Fourth, the intermediate connecting layer, upper sensitive layer, and lower sensitive layer can be processed as independent structures and then assembled, which facilitates mass production.

[0063] To investigate the influence mechanism of tilt angle on sensing performance, a quadrangular pyramid structure model with different tilt angles was established using the finite element simulation software COMSOL Multiphysics. Figure 6 Figure a presents finite element simulation results for different tilt angles (70°, 90°, 110°, and 130°), clearly showing that under the same pressure, the quadrangular pyramid structures with different tilt angles exhibit different stress distributions and deformation degrees. With increasing pressure, the tilted quadrangular pyramid structure undergoes bending deformation, increasing the compressibility of the lower sensitive layer 6. Under the same pressure, the quadrangular pyramid structure with a 130° tilt angle exhibits the maximum deformation, indicating that the larger the tilt angle, the easier the structure is to compress.

[0064] However, the actual performance of a sensor depends not only on the deformation capability of a single structure in its free state, but also on the coupled influence of multiple factors such as encapsulation conditions and interface contact states. Specifically, to improve the stability of piezoresistive sensors, a PU film is typically used to encapsulate a three-layer structure, which applies pre-stress to the internal tilted pyramidal structure. Under this condition, the larger the tilt angle, the greater the initial deformation of the structure under pre-stress, and the larger the initial contact area with the intermediate connecting layer, resulting in a smaller variable range of contact resistance under actual pressure. Therefore, the relationship between tilt angle and sensitivity is not monotonic, but rather an optimal range determined by the competition between deformation and the initial contact state.

[0065] Figure 6 Figures b and c show schematic diagrams of the piezoresistive sensing mechanism and the equivalent resistance of the sensing mechanism for the lower sensitive layer at different tilt angles, respectively. The equivalent resistance mainly consists of the volume resistance Rf and the contact resistances Rc and Re. Under external pressure, the tip of the tilted pyramid structure and the highest layer of the sandpaper-like microstructure first come into contact with the copper foil of the intermediate connecting layer, causing the tilted pyramid structure to deform and resulting in a decrease in the contact resistances Rc and Re. As the pressure further increases, the tilted pyramid structure undergoes greater bending deformation, and the protrusions at different height levels in the sandpaper-like microstructure are compressed and deformed, leading to a further decrease in the contact resistances Rc and Re. Figure 6 Figure d shows optical images of a piezoresistive sensor compressed under different pressure levels. It can be observed that the shape of the piezoresistive sensor changes to varying degrees as the pressure increases, which is consistent with the previous finite element analysis results, verifying the rationality of the sensing mechanism analysis.

[0066] Figure 7 Figure b shows the change in relative resistance of the piezoresistive sensor as the tilt angles (70°, 90°, 110°, and 130°) of the tilted quadrangular pyramid structure change. The results show that the piezoresistive sensor has the highest sensitivity when the tilt angle is 90°. The determination of this optimal tilt angle stems from the balance between the pre-pressure of the packaging and the structural deformation capability. While a tilt angle that is too small (70°) is less affected by the pre-pressure, the structural deformation capability is insufficient. Conversely, a tilt angle that is too large (110°, 130°) provides stronger bending deformation capability, but the initial contact area is too large under pre-pressure, resulting in a reduced variable range of contact resistance under actual pressure. Therefore, this invention sets the optimal tilt angle to 90°, which is the optimal value under this mechanism.

[0067] In a preferred embodiment, the sandpaper-like microstructure of the upper sensitive layer 3 is obtained by sandpaper transfer using a sandpaper template with a mesh size of 80 to 150. Specifically, the surface micro-protrusions of sandpaper with different mesh sizes (80, 120, and 150) are not uniform in size and height. As the sandpaper mesh size increases, the roughness decreases, the number of micro-protrusions gradually increases, and the corresponding size gradually decreases. Figure 7 Figure a shows the relationship between the pressure on the upper sensitive layer and the sensitivity of the piezoresistive sensor when using sandpaper templates of different mesh sizes for transfer printing according to this invention. Experimental results show that, due to the larger surface protrusions of 80-mesh sandpaper, the number of protrusions per area is smaller, resulting in fewer conductive loops formed by the increased contact area under pressure compared to 120-mesh sandpaper. Therefore, under the same pressure, its variable resistance range is smaller. Furthermore, the surface structure of 150-mesh sandpaper is finer, which limits the variability of the contact area between the micro-protrusions when pressure is applied, thus reducing sensitivity. Therefore, the piezoresistive sensor exhibits the highest sensitivity when the mesh size is 120.

[0068] In a preferred embodiment, the base of the pyramid is a parallelogram. The length (L) of one side is 2.5 to 3.5 mm, the length (W) of the other side is 2 to 4 mm, and the height (H) of the pyramid is 3 to 5 mm. Figure 7 Figure c shows the relationship between the length L of one side of the inclined quadrangular pyramid structure and the sensitivity of the piezoresistive sensor when the length W of the other side is equal. When the length L is between 2.5 and 3.5 mm, the resistance of the piezoresistive sensor shows a relatively sensitive change with small pressure variations. However, when the length L is 3 mm, the relative change in resistance of the piezoresistive sensor is the largest, and the sensitivity is the highest. Similarly, Figure 7 The relationship between the length W of the other side of the base of the inclined pyramid structure and the sensitivity of the piezoresistive sensor was compared when the base length L was set to 3 mm. By comparing different widths (2 to 4 mm), the optimal length W of the other side of the base was found to be 3 mm. The influence of the height H of the inclined pyramid structure on the performance of the piezoresistive sensor is as follows. Figure 7 As shown in Figure e, the optimal height H is 4 mm. At different heights, the distribution of MWCNTs at the top of the tilted pyramid structure varies, thus affecting the performance of the piezoresistive sensor. Ultimately, it was determined that the piezoresistive sensor achieves optimal sensitivity when the base of the pyramid is a rhombus with sides L and W both 3 mm long, and the height H is 4 mm.

[0069] Based on a defined sandpaper mesh size and an inclined quadrangular pyramid structure, the effect of different MWCNT doping ratios on the sensitivity of the piezoresistive sensor in the MWCNTs / SR composite solution is as follows: Figure 7As shown in Figure f, when the MWCNT content is low, MWCNTs are sparsely dispersed in the SR matrix, making it difficult to form a continuous conductive network. With increasing content, MWCNTs are uniformly dispersed in the matrix, and the volume resistivity of the sensitive layer decreases. When the MWCNT content further increases, because the MWCNTs have already formed a dense network, the additional MWCNTs are unlikely to significantly increase the effective conductive path; instead, local aggregation may occur, reducing the performance of the piezoresistive sensor. Experimental results show that the sensitivity of the piezoresistive sensor first increases and then decreases with increasing MWCNT doping concentration. The piezoresistive sensor exhibits optimal sensitivity when the MWCNT doping concentration is 4%.

[0070] This embodiment employs a packaging structure consisting of an upper sensitive layer 3, a lower sensitive layer 6, an intermediate connecting layer, and a three-level electrode layer. Addressing technical challenges such as the difficulty in balancing high sensitivity and wide measurement range, large hysteresis, insufficient utilization of contact points, and lack of long-term reliability, it ultimately achieves the expected technical results. The sensitivity and linearity of the piezoresistive sensor across the entire measurement range of 0-125 kPa are as follows: Figure 8 As shown in Figure a, based on the compressibility of the sandpaper-like microstructure and the tilted protrusion microstructure, and the synergistic effect of the intermediate connecting layer, the piezoresistive sensor can maintain an effective response in the pressure range of 0-125 kPa; in the low-pressure range (0-2.5 kPa), the sensitivity reaches 17.33% / kPa, and the linearity is 0.9444; it also maintains measurable sensitivity in the pressure ranges of 2.5-25 kPa and 25-125 kPa, thus realizing wide-range pressure detection.

[0071] This application tested the hysteresis of the piezoresistive sensor through loading and unloading pressure tests, and the results are as follows: Figure 8 As shown in Figure b, the piezoresistive sensor corresponding to the vertical dashed line exhibits the largest hysteresis value, at 2.175, with a maximum hysteresis error of approximately 2.74%. This is primarily attributed to the inherent mechanical properties of MWCNTs and the recoverability of the SR itself, which reduces the hysteresis of the piezoresistive sensor. The response and recovery characteristics of the piezoresistive sensor are shown in Figure b. Figure 8 As shown in Figure c, the response and recovery times of the piezoresistive sensor are approximately 60 ms under different pressure conditions (10, 30, 100 kPa). This is attributed to the compressibility of the tilted protrusion microstructure and the elasticity of the SR, enabling the piezoresistive sensor to respond quickly to changes in external pressure. To determine the response stability of the piezoresistive sensor to continuously applied and unloaded pressures, pressure was continuously applied to the piezoresistive sensor within the range of 0-125 kPa, with a 5-second pause time for each force gradient. The step response curves are shown below. Figure 8 As shown in Figure d, this indicates that the piezoresistive sensor exhibits a stable resistance response to gradient-changing forces. Figure 8Figure e shows the lowest detection limit of the piezoresistive sensor of the present invention for pressure. Testing determined that the minimum detection limit of the piezoresistive sensor is approximately 6.4 Pa. This result indicates that the piezoresistive sensor of this application is relatively sensitive to slight pressure. Figure 8 The figure describes the temperature-resistance characteristics of the piezoresistive sensor in the range of 25-90 °C, indicating that the resistance of the piezoresistive sensor changes very little with increasing temperature. Figure 8 Figure g shows the response curves of the piezoresistive sensor of the present invention at 10, 20, 40, and 50 mm / min, indicating that the piezoresistive sensor has good response stability at different loading rates under a pressure of 10 kPa. Therefore, the piezoresistive sensor of this application exhibits good sensitivity in pressure detection while maintaining fast response and a wide range.

[0072] Figure 9 Figure a shows the loading / unloading response curves of the piezoresistive sensor of the present invention at 5-125 kPa. The experimental results show that the piezoresistive sensor has good dynamic response and stability. To further confirm the durability and reliability of the piezoresistive sensor, 2500 repeatable experiments were conducted on the piezoresistive sensor at different pressures (2 kPa, 30 kPa and 100 kPa). Figure 9 Figure b shows a partially magnified curve of the cyclic response of the piezoresistive sensor of the present invention under different pressures (2, 30 and 100 kPa) and the response curves of the piezoresistive sensor under 2 kPa loading / unloading for 2500 cycles, 30 kPa loading / unloading for 2500 cycles and 100 kPa loading / unloading for 2500 cycles. It shows that the curves change relatively smoothly during the cycle and there is no obvious drift, indicating that the piezoresistive sensor of this application has good durability and long-term stability.

[0073] Since the intermediate electrode layer 5 of the intermediate connection layer can be led out independently to form a three-electrode interface with the upper electrode layer 2 and the lower electrode layer 7, when the data displayed by the piezoresistive sensor is abnormal, the specific location of the damage to the piezoresistive sensor can be detected through self-damage testing, thereby improving reliability and reducing maintenance costs. When abnormal data is found during overall testing, segmented testing using different interface combinations can be used to determine whether the upper sensitive layer 3 or the lower sensitive layer 6 is damaged. Figure 10 Figure a shows the response curves of the normal piezoresistive sensor of the present invention, detecting the entire sensor body (connecting the upper electrode layer 2 and the lower electrode layer 7), the upper sensitive layer (connecting the upper electrode layer 2 and the middle electrode layer 5), and the lower sensitive layer (connecting the middle electrode layer 5 and the lower electrode layer 7). The response curves are shown in red, purple, and blue, respectively. Figure 10As shown in Figure b, when testing the piezoresistive sensor as a whole, the orange response curve deviates significantly from the normal red curve, indicating that the piezoresistive sensor is damaged. Further testing of the upper sensitive layer 3 by connecting the upper electrode layer 2 and the middle electrode layer 5 reveals that the green response curve deviates from the normal purple curve corresponding to the upper sensitive layer 3, indicating damage to the upper sensitive layer 3. Connecting the middle electrode layer 5 and the lower electrode layer 7, testing the lower sensitive layer 6 shows that the pink curve essentially overlaps with the normal blue curve corresponding to the lower sensitive layer 6, indicating that the lower sensitive layer 6 is normal. Therefore, the specific damaged structure of the piezoresistive sensor can be determined, such as... Figure 10 As shown in Figure c, the damage to the lower sensitive layer 6 can be determined by comparison, and as shown in Figure c. Figure 10 As shown in d, it can be determined that both the upper and lower sensitive layers are damaged, thereby enabling early warning, location maintenance, and reducing the risk of sudden failure and the frequency of blind replacement.

[0074] Furthermore, piezoresistive sensors, in addition to pressure sensing, can also identify the hardness of materials. Under the same displacement compression, when the same pressure F1 is applied to the materials, the harder material A and the softer material B will exert different pressures on the piezoresistive sensor. Therefore, under the same displacement compression, hardness testing of materials with different hardnesses, such as sponge, cardboard, tissue paper, and rubber, can obtain... Figure 11 The resistance response signal waveforms corresponding to the four materials shown are used to perform feature extraction, hardness recognition testing and training in combination with the MLP model. This yields training and testing datasets, which can be used to classify silicone rubber blocks of different hardness.

[0075] The piezoresistive sensor of this application can also be used for surface roughness discrimination, as it can generate different amplitude resistance waveforms by sliding friction on different object surfaces. Figure 12 Figures a, b, c, and d show the signal waveforms output by the piezoresistive sensor when detecting the surface roughness of 600-grit, 400-grit, 240-grit, and 80-grit sandpaper, respectively. Figure 12 Figure e shows the resistance response signal waveforms of five test objects—pen holder, cardboard, mouse pad, letter paper, and plastic box—detected by the piezoresistive sensor of this invention. By testing and distinguishing objects with different grits of sandpaper and different textures, it was found that objects with different surface roughness produce different resistance response signal waveforms, demonstrating the application potential of the piezoresistive sensor in wearable electronics and human-computer interaction.

[0076] It should be noted that the tilted protruding microstructure of the lower sensitive layer in this invention is not limited to the square pyramid structure described in the embodiments. Those skilled in the art can adjust or replace the geometry of the tilted protruding microstructure according to actual needs. For example, the top of the tilted protruding microstructure can be a pointed top, a flat top, a dome, or other shapes; the tilting direction of the tilted protruding microstructure can be unidirectional or multidirectional; the cross-sectional shape of the tilted protruding microstructure can be triangular, trapezoidal, semi-circular, or other polygonal; the arrangement of the tilted protruding microstructure can be an array arrangement, a random arrangement, or other regular or irregular arrangements; any equivalent structural replacement that can achieve bending deformation of the tilted protruding structure under pressure, increase the contact area, and improve the performance of the piezoresistive sensor falls within the protection scope of this invention.

[0077] Similarly, the sandpaper-like microstructure of the upper sensitive layer in this invention is not limited to a specific morphology formed by sandpaper template transfer. Those skilled in the art can adaptively adjust the morphology and forming method of the microstructure according to actual needs. For example, the morphology of the microstructure can be randomly distributed protrusions, depressions, ripples, or other irregular shapes of different heights; the size distribution of the microstructure can be controlled by selecting templates with different roughness or adjusting process parameters; any equivalent structural replacement that can achieve gradient contact, expand the range of contact area variation, and improve the sensitivity and range of the piezoresistive sensor under pressure falls within the protection scope of this invention.

[0078] The term "an embodiment" or "embodiment" as used in this invention refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the invention. In the description of this invention, it should be understood that the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or apparatus that includes a series of steps or units is not limited to the steps or units listed, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to these processes, methods, products, or apparatuses.

[0079] This specification provides method operation steps as shown in the embodiments or flowcharts, but based on conventional or non-inventive labor, more or fewer operation steps may be included. The order of steps listed in the embodiments is merely one of many possible execution orders and does not represent the only possible execution order. In actual system or server product execution, the method can be executed in the order shown in the embodiments or drawings, or in parallel (e.g., in a parallel processor or multi-threaded processing environment), or the execution order of steps without timing restrictions can be adjusted.

[0080] The preferred embodiments of the present invention have been described in detail above. It should be understood that those skilled in the art can make numerous modifications and variations based on the concept of the present invention without creative effort. Therefore, all technical solutions that can be obtained by those skilled in the art based on the concept of the present invention through logical analysis, reasoning, or limited experimentation on the basis of existing technology should be within the scope of protection defined by the claims.

Claims

1. A flexible piezoresistive sensor with dual sensitive layers, characterized in that, The structure comprises, from top to bottom, an upper electrode layer, an upper sensitive layer, a lower sensitive layer, and a lower electrode layer. The upper and lower sensitive layers are composed of composite materials made of MWCNTs and SR. The surface of the upper sensitive layer facing the lower sensitive layer has a sandpaper-like microstructure. The surface of the lower sensitive layer facing the upper sensitive layer has multiple inclined protruding microstructures. The inclined protruding microstructures of the lower sensitive layer are configured as square pyramids with an inclination angle of 70° to 130°. The inclination angle is the angle between the base and the side of the triangle formed by the triangle passing through the vertex of the square pyramid and perpendicular to one of the base sides. An SR substrate is also disposed between the upper and lower sensitive layers, and the outer half of the SR substrate partially surrounds the intermediate electrode layer.

2. The dual-sensor layer flexible piezoresistive sensor according to claim 1, characterized in that, The outer surface of the upper electrode layer and / or the lower electrode layer is coated with an SR layer.

3. The dual-sensor layer flexible piezoresistive sensor according to claim 1, characterized in that, The tilting directions of the multiple tilted protrusion microstructures may be the same or different.

4. The dual-sensor layer flexible piezoresistive sensor according to claim 3, characterized in that, The tilt angle of the square pyramid is 90°.

5. The dual-sensor layer flexible piezoresistive sensor according to claim 3, characterized in that, The base of the square pyramid is a parallelogram, with one side having a length of 2.5 to 3.5 mm and the other side having a length of 2 to 4 mm. The height of the square pyramid is 3 to 5 mm.

6. The dual-sensor layer flexible piezoresistive sensor according to claim 5, characterized in that, The base of the square pyramid is a rhombus with a side length of 3mm and a height of 4mm.

7. The dual-sensor layer flexible piezoresistive sensor according to claim 2, characterized in that, The sandpaper-like microstructure of the upper sensitive layer is obtained by sandpaper transfer using a sandpaper template, wherein the sandpaper template has a mesh size of 80 to 150.

8. The dual-sensor layer flexible piezoresistive sensor according to claim 7, characterized in that, The sandpaper template has a mesh size of 120.

9. The dual-sensor layer flexible piezoresistive sensor according to claim 1, characterized in that, In the MWCNTs / SR composite solution used to prepare the upper and lower sensitive layers, the doping concentration of MWCNTs is 4%.

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