Piezoresistive flexible sensor and preparation method thereof

Abstract

The invention provides a piezoresistive flexible sensor and a preparation method thereof. The piezoresistive flexible sensor comprises a flexible strain layer, a packaging structure and a flexible substrate layer. And an electrode layer with a preset size is arranged on the flexible strain layer. The flexible strained layer includes a polyimide. The package structure is under the flexible strained layer. The packaging structure is provided with a through hole corresponding to the preset size of the electrode layer, so as to form a cavity for accommodating the deformation of the flexible strain layer. The packaging structure comprises at least one substructure, and the at least one substructure comprises a first epoxy resin layer, a flexible packaging layer located below the first epoxy resin layer and a second epoxy resin layer located below the flexible packaging layer. The flexible encapsulation layer includes polyimide. The flexible substrate layer is under the package structure.

Description

Technical Field

[0001] This application relates to the field of flexible sensor technology, specifically to a piezoresistive flexible sensor and its fabrication method. Background Technology

[0002] Piezoresistive flexible sensors have become a research hotspot in flexible electronics, wearable devices, intelligent robots, and health monitoring technologies due to their advantages such as simple principle, rapid response, ease of fabrication, and convenient signal readout. Currently, piezoresistive flexible sensors typically use elastomers such as polydimethylsiloxane (PDMS) and polybutylene terephthalate (PBAT) as substrates and encapsulation materials, and incorporate nanoconductive materials such as carbon nanotubes (CNTs), graphene, or silver nanowires as sensing units within these elastomers. However, the contradiction between enhancing the device's stretchability and mechanical properties, as well as the inconsistencies in device performance caused by the agglomeration of nanoconductive materials, have become one of the technical bottlenecks restricting the performance improvement of piezoresistive flexible sensors. Summary of the Invention

[0003] In view of the above problems, this application provides a piezoresistive flexible sensor and its fabrication method.

[0004] According to a first aspect of this application, a piezoresistive flexible sensor is provided, comprising a flexible strain layer, an encapsulation structure, and a flexible substrate layer. An electrode layer of predetermined size is disposed on the flexible strain layer. The flexible strain layer comprises polyimide. The encapsulation structure is located below the flexible strain layer. The encapsulation structure has through-holes corresponding to the predetermined size of the electrode layer to form a cavity accommodating the deformation of the flexible strain layer. The encapsulation structure includes at least one substructure, the at least one substructure comprising a first epoxy resin layer, a flexible encapsulation layer located below the first epoxy resin layer, and a second epoxy resin layer located below the flexible encapsulation layer. The flexible encapsulation layer comprises polyimide. The flexible substrate layer is located below the encapsulation structure.

[0005] According to embodiments of this application, the thickness of the cavity in the encapsulation structure ranges from 276 μm to 300 μm.

[0006] According to embodiments of this application, the first epoxy resin layer and the second epoxy resin layer in the encapsulation structure have a coefficient of thermal expansion that matches that of the flexible encapsulation layer.

[0007] According to embodiments of this application, the piezoresistive flexible sensor further includes a support layer located beneath the flexible substrate layer. The flexible substrate layer comprises polyimide, and the support layer comprises a PET film.

[0008] According to the embodiments of this application, the pressure sensing range of the above-mentioned piezoresistive flexible sensor is less than or equal to 230 cm water column, the response time is less than or equal to 0.5 s, and the resolution is greater than or equal to 20 Pa.

[0009] According to a second aspect of this application, a method for fabricating a piezoresistive flexible sensor is provided. The method includes: forming an electrode layer with a predetermined size on a flexible strain layer, the flexible strain layer comprising polyimide; forming an encapsulation structure, wherein through-holes corresponding to the predetermined size of the electrode layer are formed in the encapsulation structure to form cavities accommodating deformation of the flexible strain layer, the encapsulation structure comprising at least one substructure, the at least one substructure comprising a first epoxy resin layer, a flexible encapsulation layer located below the first epoxy resin layer, and a second epoxy resin layer located below the flexible encapsulation layer; and bonding the flexible strain layer, the encapsulation structure, and the flexible substrate layer in a top-to-bottom order.

[0010] According to an embodiment of this application, forming an electrode layer with a preset size on a flexible strain layer includes: preparing a metal mask containing through holes of a preset size using a laser cutting method; attaching the metal mask to the flexible strain layer; and forming an electrode layer with a preset size on the flexible strain layer through the through holes.

[0011] According to an embodiment of this application, forming an encapsulation structure includes: attaching a first epoxy resin layer and a second epoxy resin layer to the upper and lower surfaces of at least one flexible encapsulation layer to obtain at least one substructure; bonding the at least one substructure using a thermoforming bonding method to form an adhesive film; and laser-cutting through holes of a predetermined size in the adhesive film to form the encapsulation structure.

[0012] According to embodiments of this application, before bonding the flexible strain layer, the encapsulation structure, and the flexible substrate layer in a top-to-bottom order, the above preparation method further includes using polyimide double-sided adhesive to bond a PET film to the underside of the flexible substrate to form a support layer.

[0013] According to embodiments of this application, the thickness of the flexible strain layer includes 20 μm to 30 μm; the thickness of the flexible encapsulation layer includes 120 μm to 130 μm; and the thickness of the first epoxy resin layer and the second epoxy resin layer includes 5 μm to 15 μm.

[0014] According to an embodiment of this application, a piezoresistive flexible sensor is provided. This flexible sensor uses polyimide as both a flexible strain layer and a flexible encapsulation layer, enabling the sensor to possess excellent flexibility and bendability while also exhibiting good mechanical strength and resistance to chemical reagents. This not only improves the structural integrity and durability of the device under repeated deformation but also broadens its application scenarios. The encapsulation structure, employing a cavity-based double-layer epoxy resin and flexible encapsulation layer, can reduce the impact on the device's bending capability while providing rigid protection. Attached Figure Description

[0015] The above-mentioned contents, other objects, features and advantages of this application will become clearer from the following description of embodiments with reference to the accompanying drawings, in which:

[0016] Figure 1 The schematic diagram illustrates the structure of a piezoresistive flexible sensor according to an embodiment of this application;

[0017] Figure 2 The schematic diagram illustrates a tensile specimen of the flexible strain layer of a piezoresistive flexible sensor according to an embodiment of this application.

[0018] Figures 3-5 The finite element simulation diagrams of the flexible strain layer of the piezoresistive flexible sensor with electrode layers of different preset sizes in the embodiments of this application are shown schematically.

[0019] Figure 6 The resistance change rate curve of the piezoresistive flexible sensor according to an embodiment of this application is schematically shown;

[0020] Figure 7 The step curves of the piezoresistive flexible sensor according to an embodiment of this application under different pressures are illustrated schematically.

[0021] Figure 8 The illustration shows an image of a piezoresistive flexible sensor sensing pulse pressure and its response time according to an embodiment of this application.

[0022] Figure 9 The flowchart illustrating the fabrication method of the piezoresistive flexible sensor according to an embodiment of this application is shown in the illustration.

[0023] Figure 10 The schematic diagram illustrates the structure of the metal mask in the preparation method of this application embodiment;

[0024] Figure 11 The diagram illustrates the structural schematics of each stage in a method for fabricating a piezoresistive flexible sensor according to an embodiment of this application. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with specific embodiments and the accompanying drawings.

[0026] The endpoints and any values ​​of the ranges disclosed in this application are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this application.

[0027] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of this application. The terms “comprising,” “including,” etc., as used herein indicate the presence of the stated features, steps, operations, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, or components.

[0028] All terms used herein (including technical and scientific terms) have the meanings commonly understood by those skilled in the art, unless otherwise defined. It should be noted that the terms used herein are to be interpreted in a manner consistent with the context of this specification, and not in an idealized or overly rigid way.

[0029] In the description of this application, it should be understood that the terms "longitudinal", "length", "circumferential", "front", "rear", "left", "right", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the subsystem or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application.

[0030] Similarly, to simplify this application and aid in understanding one or more of the various disclosed aspects, in the above description of exemplary embodiments of this application, various features of this application are sometimes grouped together into a single embodiment, figure, or description thereof. The use of terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicates that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of this application. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0031] With the rapid development of technologies such as flexible electronics, wearable devices, intelligent robots, and health monitoring, there is an urgent need for flexible sensors that can conformally fit human skin, soft surfaces, or complex curved surfaces, and accurately sense external mechanical signals such as pressure and strain in real time. Piezoresistive flexible sensors have become a research hotspot in these technological fields due to their simple principle, rapid response, ease of fabrication, and convenient signal readout. Currently, piezoresistive flexible sensors typically use elastomers such as polydimethylsiloxane (PDMS) and polybutylene terephthalate (PBAT) as substrates and encapsulation materials. Although elastomer materials have good ductility and flexibility, their inherently low glass transition temperature, poor mechanical strength, and poor environmental stability (such as easy swelling and poor aging resistance) limit the long-term reliability of elastomer materials under complex working conditions. Piezoresistive flexible sensors often incorporate nanoconductive materials such as carbon nanotubes (CNTs), graphene, or silver nanowires into the aforementioned elastomer materials as sensing units. However, composite nanoconductive materials present the following challenges: First, nanoconductive materials tend to agglomerate in polymer matrices, making it difficult to control the uniformity of their distribution, which leads to reduced performance consistency of sensors, significant hysteresis in sensing signals, and low linearity. Second, the nonlinear mechanical behavior of elastomers under large deformations further exacerbates the nonlinearity of the sensor's electrical response, increasing the difficulty of calibrating and interpreting sensing signals.

[0032] In view of this, embodiments of this application provide a piezoresistive flexible sensor and its fabrication method. The flexible strain layer using polyimide possesses high mechanical strength, low creep, and excellent fatigue resistance, ensuring high durability and long-term measurement stability of the sensor under repeated deformation. Simultaneously, the thinner flexible strain layer improves the sensor's low resolution, low latency, and high response performance. The encapsulation structure employs multiple substructures of epoxy resin and polyimide films, providing superior insulation, airtightness, and watertightness protection for the sensor. This effectively isolates the sensor from environmental moisture and impurities, improving its working life and reliability in complex and humid environments.

[0033] Figure 1 A schematic diagram of the structure of a piezoresistive flexible sensor according to an embodiment of this application is shown.

[0034] like Figure 1 As shown, the piezoresistive flexible sensor in this embodiment may include a flexible strain layer 1, an encapsulation structure 2, and a flexible substrate layer 3.

[0035] An electrode layer 11 of a predetermined size can be disposed on the flexible strain layer 1. The flexible strain layer 1 may include polyimide (PI) or the like. The thickness of the flexible strain layer 1 may include 20 μm to 30 μm, such as 20 μm, 25 μm, 30 μm, etc. A thinner flexible strain layer 1 can improve the low resolution, low latency, and high response performance of the sensor.

[0036] In a specific embodiment, the electrode layer 11 may include a Cr / Au metal, which can be deposited at a ratio of 15 nm / 150 nm. Au metal is chemically stable and not easily oxidized, which can reduce the impact on the resistance of the electrode layer 11 during the high-temperature packaging process of the sensor. However, Au metal is highly inert and does not easily form chemical bonds with polyimide. Therefore, the introduction of Cr metal provides a large number of uniform nucleation sites for Au metal during the bonding process between polyimide and Cr metal. This helps Au atoms form a thin film with good crystallinity and continuity and low resistance, enhancing the adhesion and stability of Au metal.

[0037] The preset size and shape of the electrode layer 11 can be set according to specific application requirements, and this application does not impose specific limitations here. For example, the electrode layer 11 can be set as a rectangle, and the preset size can be 3 mm*3 mm, 4 mm*4 mm or 5 mm*5 mm, etc.

[0038] The encapsulation structure 2 is located below the flexible strain layer 1. The encapsulation structure 2 has through-holes corresponding to a preset size of the electrode layer 11 to form a cavity to accommodate the deformation of the flexible strain layer 1. The encapsulation structure 2 may include at least one substructure, which includes a first epoxy resin layer 21, a flexible encapsulation layer 22 located below the first epoxy resin layer 21, and a second epoxy resin layer 23 located below the flexible encapsulation layer 22. The flexible encapsulation layer 22 includes polyimide.

[0039] In a specific embodiment, the encapsulation structure 2 may include at least one substructure. The number of substructures can be designed according to the thickness requirements of the cavity, and is not specifically limited in this application. The first epoxy resin layer, the flexible encapsulation layer, and the second epoxy resin layer in the substructure can be formed by thermocompression bonding.

[0040] The flexible substrate layer 3 is located below the encapsulation structure 2. The flexible substrate layer may include polyimide.

[0041] In specific embodiments, the thickness of the flexible substrate 3 may include 40 μm to 60 μm, such as 40 μm, 50 μm, 60 μm, etc.

[0042] According to an embodiment of this application, a piezoresistive flexible sensor is provided. This flexible sensor uses polyimide as both a flexible strain layer and a flexible encapsulation layer, enabling the sensor to possess excellent flexibility and bendability while also exhibiting good mechanical strength and resistance to chemical reagents. This not only improves the structural integrity and durability of the device under repeated deformation but also broadens its application scenarios. The encapsulation structure, employing a cavity-based double-layer epoxy resin and flexible encapsulation layer, can reduce the impact on the device's bending capability while providing rigid protection.

[0043] Figure 2 The schematic diagram illustrates a tensile specimen of a flexible strain layer of a piezoresistive flexible sensor according to an embodiment of this application. Figures 3-5 The finite element simulation diagrams of the flexible strain layer of the piezoresistive flexible sensor with electrode layers of different preset sizes in the embodiments of this application are shown schematically. It should be noted that... Figures 2-5 The thickness of the flexible strain layer in each layer is 25 μm. Figures 3-5 The flexible strain layer in the figure is a simulation diagram under the maximum pressure in the demand pressure range. Figure 3 Finite element simulation diagrams characterizing the flexible strain layer under the condition that the electrode layer has a preset size of 5 mm * 5 mm are presented. Figure 4 Finite element simulation diagrams characterizing the flexible strain layer under the condition that the electrode layer has a preset size of 4 mm * 4 mm are presented. Figure 5 Finite element simulation diagrams characterizing the flexible strain layer under the condition that the electrode layer has a preset size of 3 mm * 3 mm are presented. Figures 3-5 The x-axis and y-axis in the diagram represent the plane in which the flexible strain layer is located, and the z-axis represents the direction perpendicular to the plane in which the flexible strain layer is located.

[0044] like Figure 2 As shown, tensile tests were conducted on a 25 μm thick flexible strain layer using internationally sized tensile specimens. Figure 2 The dimensions are in mm. The stress-strain curve of the flexible strain layer can be obtained through tensile testing, and the elastic modulus E = 2.4 GPa, Poisson's ratio ν = 0.37, and bulk modulus K = 3.0769 × 10⁻⁶ mm are calculated. 9 Pa, shear modulus G = 8.7591 × 10 8 Pa, density ρ = 1420 kg / m³ 3 The tensile ultimate strength σμ = 153.67 MPa.

[0045] The above parameters are input as material properties of the flexible strain layer into simulation software, such as ANASYS. Electrode layers of different preset sizes are used as strain regions, and an application perpendicular to the surface (e.g.) Figures 3-5 A uniform pressure along the z-axis can yield the following: Figures 3-5Finite element simulation diagram.

[0046] like Figures 3-5 As shown, under the condition of a uniform pressure of 200 cm water column applied to the electrode layer, the offset distance Δz of the center point on the flexible strain layer along the z-axis can be obtained as follows: Δz 5*5 =276 μm, Δz 4*4 =289 μm and Δz 3*3 =300 μm. Further comparison of the resistance change rate of the electrode layer under the three conditions reveals that the resistance change rate is higher when the preset size of the electrode layer is 5mm*5mm.

[0047] Combination Figures 2-5 The thickness of the cavity in the encapsulation structure can range from 276 μm to 300 μm. Because the first epoxy resin layer, the flexible encapsulation layer, and the second epoxy resin layer in the encapsulation structure have through-holes corresponding to the dimensions of the electrode layers, a cavity can be formed inside the encapsulation structure. The thickness of the cavity is the same as the overall thickness of the encapsulation structure.

[0048] According to embodiments of this application, the cavity of the encapsulation structure can provide space for the flexible strain layer to deform. Through simulation analysis, it is found that when the thickness of the cavity, or the thickness of the encapsulation structure, is between 276 μm and 300 μm, the overall thickness of the flexible sensor can achieve a high resistance change rate under relatively low conditions.

[0049] In a specific embodiment, the first epoxy resin layer and the second epoxy resin layer in the encapsulation structure may have a coefficient of thermal expansion that matches that of the flexible encapsulation layer.

[0050] The flexible encapsulation layer may include polyimide, and its thickness may include 120 μm to 130 μm, for example, 120 μm, 125 μm, 130 μm, etc. The first epoxy resin layer and the second epoxy resin layer may include thermosetting epoxy resin used for bonding polyimide, and their thickness may include 5 μm to 15 μm, for example, 5 μm, 10 μm, 15 μm, etc.

[0051] According to embodiments of this application, epoxy resin and polyimide with matching coefficients of thermal expansion can form an encapsulating film with hermetic or watertight properties, suitable for electronic packaging. This encapsulating film provides superior insulation, hermetic and watertight protection for the device, effectively isolating it from environmental moisture and impurities, thus improving the sensor's lifespan and reliability in complex and humid environments.

[0052] In a specific embodiment, the piezoresistive flexible sensor may further include a support layer located beneath the flexible substrate layer. The support layer may include a PET film. The thickness of the support layer may include 40 μm to 60 μm, for example, 40 μm, 50 μm, 60 μm, etc. The support layer and the flexible substrate layer can be bonded together using polyimide double-sided adhesive.

[0053] According to the embodiments of this application, a support layer of PET film bonded below the flexible substrate layer can further provide better mechanical support for the flexible sensor, increase the structural integrity and durability of the device under repeated deformation, and improve the service life of the device.

[0054] Continue to refer to Figure 1 In this embodiment, the flexible strain layer 1 of the flexible sensor also includes electrode leads 12 extending from both ends of the electrode layer 11. The width of the electrode leads 12 can be 0.3 mm, and the spacing between the two electrode leads 12 after convergence can be 0.2 mm. The electrode leads 12 can be positioned using 4P anisotropic conductive film (ACF) leads and connected to the 4P interface of the flexible printed circuit board (FPC) line to output the resistance data on the flexible strain layer through the FPC line. The other end of the FPC line can be connected to a ribbon cable adapter board and connected to various required test instruments via DuPont wires.

[0055] Figure 6 The diagram schematically illustrates the resistance change rate curve of a piezoresistive flexible sensor according to an embodiment of this application. It should be noted that... Figure 6 The diagram illustrates the rate of change of resistance when subjected to pressures ranging from 0 to 230 cmH2O. Here, sensitivity S = (ΔR / R0) / ΔP, ΔR = R P -R0, where R P R0 represents the sensor resistance at pressure P, R0 represents the initial resistance of the sensor, and ΔP represents the change in relative pressure.

[0056] like Figure 6 As shown, the pressure sensing range of the piezoresistive flexible sensor in this embodiment can be less than or equal to 230 cm water column, and the resolution can be greater than or equal to 20 Pa. That is, under a small pressure of 20 Pa in the cavity, the sensor's resistance can change significantly, indicating that the flexible sensor in this embodiment has good resolution. The sensitivity of the flexible sensor is mainly divided into two stages: the first stage is within the range of 0~50 cmH2O, and the coefficient of determination R of the fitted sensor is... 2 The value is 0.9344, and the sensitivity is 1.79 × 10⁻⁶. －3cmH2O －1 The second stage, at 50 cmH2O to 230 cmH2O, had a fitting coefficient of determination R0. 2 The value is 0.9924, and the sensitivity is 0.443 × 10⁻⁴. －3 cmH2O －1 In the first stage, the flexible strain layer is relatively relaxed due to its lower tension, resulting in relatively high sensitivity. In the second stage, as the pressure increases, the tension of the flexible strain layer increases, limiting its deformation and thus reducing sensitivity.

[0057] In a specific embodiment, since the resistance change rate of the sensor is small, a filtering and amplification module can be added to the data transmission circuit so that the resistance change trend of the sensor can be clearly displayed on the oscilloscope.

[0058] Figure 7 The step curves of the piezoresistive flexible sensor according to an embodiment of this application are schematically shown under different pressures. Figure 8 The illustration shows an image of a piezoresistive flexible sensor according to an embodiment of this application sensing pulse pressure and its response time. Figure 7 The step curves of the rate of change of resistance with time were characterized at pressures of 33 cmH2O, 72 cmH2O and 115 cmH2O, respectively. Figure 8 The response image is characterized when the flexible sensor of this embodiment is subjected to a pressure of 18 kPa at a fixed frequency in the cavity.

[0059] like Figure 7 As shown, within the measurement range of the device, the step change of the flexible sensor in this embodiment exhibits good repeatability and stability. Figure 8 As shown, the flexible sensor in this embodiment has a response time of less than or equal to 0.5 s and a fast recovery time of 0.45 s under a pulse pressure of 180 cmH2O, with an average response time of 27.7 ms / kPa, exhibiting superior response characteristics.

[0060] Figure 9 The flowchart illustrating the fabrication method of the piezoresistive flexible sensor according to an embodiment of this application is shown.

[0061] like Figure 9 As shown, the preparation method of this embodiment may include operations S910 to S930.

[0062] In operation S910, an electrode layer with a preset size is formed on a flexible strain layer, the flexible strain layer comprising polyimide.

[0063] In operation S920, a package structure is formed, and through holes corresponding to the preset size of the electrode layer are opened in the package structure to form a cavity to accommodate the deformation of the flexible strain layer.

[0064] In a specific embodiment, the encapsulation structure may include at least one substructure. The at least one substructure may include a first epoxy resin layer, a flexible encapsulation layer located below the first epoxy resin layer, and a second epoxy resin layer located below the flexible encapsulation layer.

[0065] When operating the S930, the flexible strain layer, encapsulation structure, and flexible substrate are bonded in a top-down order.

[0066] According to the embodiments of this application, the preparation method of this embodiment avoids the agglomeration phenomenon that is easy to occur when coating nano-conductive materials using the solution method, and can form a firmly attached thin film device. Moreover, the preparation method is simple and easy to operate, which can expand the application range of flexible sensors.

[0067] The following is a schematic diagram of the structure obtained during the fabrication of the piezoresistive flexible sensor according to the embodiments of this application, combined with... Figures 10-11 The preparation method of the embodiments of this application will be described below.

[0068] Figure 10 The schematic diagram illustrates the structure of the metal mask in the preparation method of this application embodiment. Figure 11 The diagram illustrates the structural schematics of each stage in a method for fabricating a piezoresistive flexible sensor according to an embodiment of this application.

[0069] like Figure 10 As shown, a metal mask containing multiple through-holes of preset sizes can be fabricated using laser cutting. The metal mask can include stainless steel, etc., and its thickness can be 0.3 mm. To improve fabrication efficiency, an array of 5×4 through-holes can be cut into the metal mask. The through-holes can be cut according to preset sizes; in this embodiment, a preset size of 5 mm × 5 mm is used as an example, and other sizes of through-holes will not be described further. Electrode leads with a width of 0.3 mm extend from both ends of the through-holes, and the spacing between the converged electrode leads can be 0.2 mm. The cutting tolerance can be controlled within ±0.05 mm.

[0070] like Figure 11 As shown, a metal mask is attached to the flexible strain layer. The thickness of the flexible strain layer can range from 20 μm to 30 μm, for example, 25 μm. The metal mask and the flexible strain layer are fixed together on a circular glass plate to maintain a flat surface. On the metal disk of the electron beam evaporator, Cr / Au metal can be deposited at a ratio of 15 mm / 150 mm to form an electrode layer of predetermined size on the flexible strain layer through through-holes.

[0071] According to embodiments of this application, Au metal is deposited on an ultrathin polyimide layer as the flexible strain layer of the sensor. This gives the flexible strain layer high mechanical strength, low creep, and excellent fatigue resistance, ensuring high durability and long-term measurement stability of the sensor under repeated deformation. Simultaneously, the ultrathin flexible strain layer improves the sensor's low resolution, low latency, and high response performance. Electron beam evaporation and metal mask patterning processes are used to fabricate the electrode layer on the ultrathin flexible strain layer, forming a firmly adhered, uniform, dense, low-sheet-resistance, and highly consistent metal thin film, improving the performance consistency and reliability of the sensing unit. High-precision laser-cut masks further enhance the accuracy and consistency of the electrode pattern.

[0072] After the electrode layer is formed, the metal mask on the surface of the flexible strain layer can be removed.

[0073] Continue to refer to Figure 11 Two epoxy resin layers can be bonded to both sides of a flexible encapsulation layer to form a substructure of adhesive film. At least one substructure is then thermo-bonded at 180 °C using a laminator to form the adhesive film. Through-holes of a predetermined size (e.g., 5 mm × 5 mm) can be cut into the adhesive film using laser cutting to form the encapsulation structure. The thickness of the flexible encapsulation layer can range from 120 μm to 130 μm, for example, 125 μm. The thickness of the first and second epoxy resin layers can range from 5 μm to 15 μm, for example, 10 μm. Thus, an encapsulation structure with a thickness of 145 μm can be formed. Therefore, using an encapsulation structure with two substructures, a cavity with a thickness of 290 μm can be formed. Subsequently, the encapsulation structure can be positioned below the flexible strain layer so that the through-holes of the encapsulation structure are aligned with the electrode layer.

[0074] Continue to refer to Figure 11 The flexible substrate layer and the support layer can be bonded together using polyimide double-sided adhesive. The flexible substrate layer can include polyimide with a thickness of 40 μm to 60 μm, such as 50 μm. The support layer can include a PET film with a thickness of 40 μm to 60 μm, such as 50 μm.

[0075] With the flexible substrate layer attached beneath the encapsulation structure, the flexible strain layer, encapsulation structure, flexible substrate layer, and support layer can be bonded in a top-to-bottom order to form the piezoresistive flexible sensor of this embodiment. The bonding pressure can be 10 kPa, the bonding temperature can be 180 ℃, and the bonding time can be 40 min.

[0076] Finally, one end of the 4P ACF silver wire can be positioned with the 2P leads at both ends of the electrode layer on the flexible strain layer, and then hot-pressed for 2-3 minutes using a soldering iron to achieve bonding. The 4P interface end of the FPC wire can be hot-pressed to the other end of the ACF silver wire in the same way, and the sensor's resistance data can be output through the FPC wire. The end of the FPC wire can be connected to a ribbon cable adapter board, and connected to various testing instruments via DuPont wires.

[0077] According to embodiments of this application, flexible sensors are fabricated using polyimide, a high-performance polymer material with excellent thermal stability, high mechanical strength, excellent chemical inertness, and an extremely low coefficient of thermal expansion. This gives the sensor excellent flexibility and bendability, ensuring its structural integrity and durability under repeated deformation. Furthermore, polyimide's ability to withstand various chemical solvents further broadens its application scenarios. In terms of encapsulation and protection, epoxy resin, as a thermosetting polymer, exhibits higher hardness, stronger adhesion, better wear resistance, and impermeability after curing compared to silicone rubbers such as PDMS. This provides more robust and durable physical and chemical protection for the core sensitive element. Simultaneously, through modification, epoxy resin systems with a certain degree of flexibility can be obtained, providing rigid protection without affecting the overall bending capability of the sensor. The combination of polyimide and epoxy resin gives the sensor both flexibility and toughness, showing great application potential in high-end human-computer interaction, precision tactile sensing, and long-term implantable medical devices.

[0078] Those skilled in the art will understand that the features described in the various embodiments of this application can be combined and / or combined in various ways, even if such combinations or combinations are not explicitly described in this application. In particular, the features described in the various embodiments of this application can be combined and / or combined in various ways without departing from the spirit and teachings of this application. All such combinations and / or combinations fall within the scope of this application.

[0079] The embodiments of this application have been described above. However, these embodiments are merely illustrative and not intended to limit the scope of this application. Although various embodiments have been described above, this does not mean that the measures in the various embodiments cannot be used advantageously in combination. Without departing from the scope of this application, those skilled in the art can make various substitutions and modifications, all of which should fall within the scope of this application.

Claims

1. A piezoresistive flexible sensor, characterized in that, include: A flexible strain layer, wherein an electrode layer of a predetermined size is disposed on the flexible strain layer; wherein the flexible strain layer comprises polyimide; An encapsulation structure located below the flexible strain layer has through holes corresponding to the preset size of the electrode layer to form a cavity to accommodate the deformation of the flexible strain layer; the encapsulation structure includes at least one substructure, the at least one substructure including a first epoxy resin layer, a flexible encapsulation layer located below the first epoxy resin layer and a second epoxy resin layer located below the flexible encapsulation layer, the flexible encapsulation layer including polyimide; A flexible substrate layer located beneath the encapsulation structure.

2. The piezoresistive flexible sensor according to claim 1, characterized in that, The thickness of the cavity in the encapsulation structure ranges from 276 μm to 300 μm.

3. The piezoresistive flexible sensor according to claim 2, characterized in that, The first epoxy resin layer and the second epoxy resin layer in the encapsulation structure have a coefficient of thermal expansion that matches that of the flexible encapsulation layer.

4. The piezoresistive flexible sensor according to claim 1, characterized in that, The piezoresistive flexible sensor also includes a support layer located below the flexible substrate layer; The flexible substrate layer comprises polyimide, and the support layer comprises a PET film.

5. The piezoresistive flexible sensor according to any one of claims 1 to 4, characterized in that, The pressure sensing range of the piezoresistive flexible sensor is less than or equal to 230 cm water column, the response time is less than or equal to 0.5 s, and the resolution is greater than or equal to 20 Pa.

6. A method for fabricating a piezoresistive flexible sensor, characterized in that, include: An electrode layer of predetermined size is formed on a flexible strain layer; wherein the flexible strain layer comprises polyimide; An encapsulation structure is formed, and through holes corresponding to the preset size of the electrode layer are opened on the encapsulation structure to form a cavity to accommodate the deformation of the flexible strain layer; the encapsulation structure includes at least one substructure, the at least one substructure including a first epoxy resin layer, a flexible encapsulation layer located below the first epoxy resin layer, and a second epoxy resin layer located below the flexible encapsulation layer; The flexible strain layer, the encapsulation structure, and the flexible substrate are bonded in a top-to-bottom order.

7. The preparation method according to claim 6, characterized in that, The process of forming an electrode layer with a predetermined size on the flexible strain layer includes: A metal mask containing through holes of the preset size is prepared by laser cutting. The metal mask is attached to the flexible strain layer, and the electrode layer with the preset size is formed on the flexible strain layer through the through holes.

8. The preparation method according to claim 6, characterized in that, The formation of the encapsulation structure includes: The first epoxy resin layer and the second epoxy resin layer are attached to the upper and lower surfaces of at least one of the flexible encapsulation layers to obtain at least one of the substructures; At least one substructure is bonded together by thermo-press bonding to form an adhesive film; The preset size through holes are laser-cut into the adhesive film to form the encapsulation structure.

9. The preparation method according to claim 6, characterized in that, Before bonding the flexible strain layer, the encapsulation structure, and the flexible substrate layer in a top-to-bottom order, the fabrication method further includes: A PET film is bonded to the underside of the flexible substrate using polyimide double-sided adhesive to form a support layer.

10. The preparation method according to claim 5, characterized in that, in, The thickness of the flexible strain layer is 20 μm to 30 μm; The thickness of the flexible encapsulation layer includes 120 μm to 130 μm; The thickness of the first epoxy resin layer and the second epoxy resin layer ranges from 5 μm to 15 μm.

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