Flexible piezoresistive sensor with structure-adjustable matching layer and application thereof

Abstract

The invention discloses a flexible piezoresistive sensor with a structure-adjustable matching layer and an application of the flexible piezoresistive sensor. The flexible piezoresistive sensor comprises a packaging layer and a sensing layer; the sensing layer comprises a sensitive layer and a matching layer; and the matching layer is constructed into a preset micro-pattern supporting network through a programmable electrostatic direct-writing printing process. And gradient distribution in which the content of the conductive filler is gradually reduced from the sensitive layer to the matching layer is formed in the thickness direction of the sensing layer. Through matching layer geometric microstructure design and material gradient distribution, the mechanical response and sensitivity of the sensor are significantly improved. The matching layer effectively guides stress redistribution on the microscopic scale, stress concentration is avoided, the stress uniformity of the sensitive layer is improved, and therefore balance and efficient response of piezoresistive signals are achieved. By adjusting the structural morphology and thickness of the matching layer, the rigidity and sensitivity balance of the sensor can be accurately adjusted on the premise of not changing the material composition of the sensitive layer, and the mechanical strength and long-term stability are improved.

Description

Technical Field

[0001] The present invention belongs to the field of intelligent sensor technology, and relates to a flexible piezoresistive sensor with a structurally adjustable matching layer and its application, and in particular to a flexible piezoresistive sensor with a sensing layer composed of a sensitive layer and a matching layer. The sensor can effectively change the mechanical properties by changing the matching layer structure, and regulate the sensor sensitivity by adjusting the thickness of the matching layer and the sensitive layer. Background Art

[0002] In recent years, with the rapid development of smart healthcare and wearable technologies, flexible pressure sensors, as a key component of flexible electronic products, have shown broad application prospects in health monitoring, smart wearables, motion behavior recognition, and human-computer interaction. In particular, in scenarios such as personalized healthcare, elderly health management, and rehabilitation assistance, flexible pressure sensors have become an important functional module for such terminal devices due to their good softness, lightness, and high conformability. However, faced with complex and diverse application requirements, existing flexible pressure sensors have significant technical bottlenecks in sensitivity regulation, mechanical property control, and structural construction, especially in the dimensions of material system construction, structural parameter optimization, and manufacturing process integration.

[0003] Currently, research on improving the performance of flexible pressure sensors focuses primarily on optimizing material formulations, innovating structural designs, and upgrading processing techniques. First, rational material formulation design is the foundation for achieving high-performance sensing. Carbon-based conductive materials (such as graphene and carbon nanotubes) and polymer materials are widely used due to their excellent conductivity, flexibility, and film-forming properties. However, precisely controlling the ratios and synergistic effects of these components to achieve a balance between conductivity, flexibility, and environmental stability remains a key challenge in current research. Second, device structural design has a crucial impact on sensor performance. Constructing multilayer composite structures, such as sensitive and matching layers, effectively modulates stress transfer pathways and interfacial response behavior, significantly improving sensor sensitivity and mechanical adaptability. The matching layer pattern (such as grid, ring, or fiber wall) and its number of layers enable directional response adjustment under varying load conditions. Third, regarding manufacturing technology, traditional processes such as solution casting, spin coating, or evaporation have limitations in microstructure construction and achieving uniformity over large areas. In comparison, 3D printing technology, especially direct writing printing and electrostatic direct writing printing, has become an effective means of manufacturing flexible sensors because of its advantages such as precise pattern control, programmable process parameters, and customizable structure.

[0004] Based on this background, the present invention proposes a flexible piezoresistive sensor with a structurally adjustable matching layer. Relying on an innovative material ratio scheme, an adjustable structural design, and a 3D printing manufacturing strategy compatible with a variety of flexible materials, a multi-scale composite structure composed of a sensitive layer and a matching layer is constructed. By adjusting the number, thickness, and microstructure of the sensitive and matching layers, the sensor's sensitivity and mechanical properties can be effectively and comprehensively controlled. This solution offers simple processing, flexible structure, and highly versatile materials, making it suitable for a variety of wearable health monitoring and human-machine interface applications. Summary of the Invention

[0005] The purpose of the present invention is to overcome the deficiencies of the above-mentioned prior art and to provide a flexible piezoresistive sensor with a structurally adjustable matching layer and its application. The mechanical response and sensitivity of the sensor are significantly improved by the unique geometric microstructure design and material gradient distribution of the matching layer. The matching layer effectively guides stress redistribution on a microscopic scale, avoids stress concentration, and improves the force uniformity of the sensitive layer, thereby achieving a balanced and efficient response of the piezoresistive signal. By adjusting the structural morphology and thickness of the matching layer, the stiffness and sensitivity balance of the sensor can be accurately adjusted without changing the material composition of the sensitive layer, thereby improving the mechanical strength and long-term stability. This design overcomes the performance compromise between sensitivity and structural strength of traditional sensors, and has the advantages of simple process, strong controllability, high repeatability and low cost. It is widely used in flexible electronic fields such as electronic skin, intelligent rehabilitation and human-computer interaction.

[0006] The present invention is implemented as follows: In a first aspect, the present invention provides a flexible piezoresistive sensor with a structurally adjustable matching layer, comprising a packaging layer, a sensing layer, leads, and copper foil; wherein the sensor packaging layer comprises a top packaging layer and a bottom packaging layer, the sensing layer comprises a sensitive layer and a matching layer; the sensitive layer comprises a sensitive layer matrix material and a conductive filler; and the matching layer comprises a matching layer matrix material and a conductive filler. The matching layer is constructed into a preset micro-pattern support network through a programmable electrostatic direct writing printing process; the micro-pattern support network is composed of regularly arranged fibrous supports; and the fibrous supports are a three-dimensional multi-layer structure.

[0007] Preferably, a gradient distribution of the conductive filler content is formed along the thickness direction of the sensing layer, with the content gradually decreasing from the sensitive layer to the matching layer. More preferably, the conductive filler comprises a mixture of graphene (G) and polyaniline (PANI); the weight content of the graphene (G) in the matching layer is less than 0.15%, and the weight content of the polyaniline (PANI) is less than 0.15%.

[0008] Preferably, the micro-pattern support network adopts at least one of a parallel fiber wall structure, a concentric ring structure or a grid cross structure.

[0009] Preferably, the distance between adjacent fibers in the micro-pattern support network is greater than or equal to 200 μm.

[0010] Preferably, the fibrous scaffold has 10-30 layers, and each layer has a thickness of 5 μm.

[0011] Preferably, the sensor is prepared by the following method: (1) Ink preparation: At room temperature, graphene (G) and polyaniline (PANI) are dispersed in a solvent system, and then polyethylene oxide (PEO) is added to form a uniformly dispersed sensitive layer composite ink; Using the same method, graphene (G) and polyaniline (PANI) were dispersed in an organic solvent, and then the matrix material was added to prepare a matching layer composite ink. (2) Sensing layer printing: A sensitive layer is prepared on a pretreated substrate using a programmable direct writing printing process using a sensitive layer composite ink, and then a matching layer having a preset micro-pattern support network structure is printed on top of the sensitive layer using a matching layer composite ink using a programmable electrostatic direct writing technique; (3) Pretreatment of the sensing layer and preparation of the encapsulation layer ink: The printed sensing layer is cut to a predetermined size, surface treated, and ink for the encapsulation layer is prepared; (4) Bottom encapsulation layer printing: The bottom encapsulation layer is printed using encapsulation layer ink using electrostatic direct writing technology, and is semi-cured in a vacuum oven or at room temperature; (5) Top encapsulation layer printing and packaging: The surface-treated sensing layer is adhered to the bottom packaging layer, so that the sensitive layer is bonded to the bottom packaging layer. Then, the top packaging layer is printed on the upper surface of the matching layer using electrostatic direct writing technology using the packaging layer ink, and is cured in a vacuum oven or at room temperature to form a complete flexible piezoresistive sensor.

[0012] More preferably, in the sensitive layer, the matrix material of the sensitive layer is polyethylene oxide (PEO) with a mass concentration of 7.4 wt %, the mass concentration of graphene (G) is 0.5 wt %, and the mass concentration of polyaniline (PANI) is 1.6 wt %; In the matching layer, the matching layer matrix material is polycaprolactone (PCL) with a mass concentration of 22-24 wt%, polyaniline (PANI) with a mass concentration of 0.01-0.15 wt%, and graphene (G) with a mass concentration of 0.01-0.15 wt%.

[0013] More preferably, the sensitive layer is constructed using a programmable direct write printing process, and the printing parameters are: a fan is set above the printing platform, the printing needle specification is 20 G, the solution flow rate is 0.5-2.5 ml / h, the working distance is 0.1-2 mm, and the platform movement speed is 10-90 mm / s; The matching layer is printed using electrostatic direct writing technology, and its printing parameters are: no fan is set above the printing platform, the extrusion rate is 0.1-0.4 ml / h, the working distance is 1.5-3.5 mm, the platform movement speed is 40-70 mm / s, and the operating voltage is 1.9-2.5 kV.

[0014] In a second aspect, the present invention provides an application of the sensor in at least one of a wearable device, an electronic skin, and a human-computer interaction interface.

[0015] In combination with the above technical solutions and technologies for solving the problems, the advantages and positive effects of the technical solutions to be protected by the present invention are as follows: The matching layer in this invention uses programmable electrostatic direct writing to construct a three-dimensional micropattern support network, with fiber spacing ≥200μm and 10-30 layers. This micropattern network forms a multi-level stress buffer structure through regularly arranged fiber supports, overcoming the "high sensitivity but low mechanical strength" trade-off between conventional sensors.

[0016] The present invention forms a gradient-decreasing distribution of conductive fillers in the thickness direction of the sensing layer, and the high filler concentration in the sensitive layer forms a continuous conductive network, ensuring high responsiveness of the piezoresistive effect; the low filler concentration in the matching layer ensures mechanical properties and structural integrity, achieving a "high sensitivity-high rigidity" functional transition, and solving the defect that homogeneous materials cannot take into account both electrical and mechanical properties.

[0017] The present invention provides a flexible piezoresistive sensing layer. By conducting in-depth research on the sensing performance of different printable polymer composite inks, the polymer material with the best sensing performance under the same conductive filler ratio was discovered and determined. In addition, the present invention discussed in detail the influence of the content of each ink component on the printing effect, and then established the optimal printing parameters. The influence of different structures on sensor performance was systematically studied, providing a detailed process technology route and design ideas for the development of controllable printable flexible pressure sensors, and optimizing the balance between printing process and sensor performance.

[0018] The sensing layer material of the present invention does not need to be replaced; the mechanical properties and sensitivity of the sensor can be quickly customized by adjusting the structure and number of layers. Specifically, by changing the structure of the matching layer and the number of sensitive layers, the mechanical response performance of the sensor can be effectively controlled. Simultaneously, the sensitivity of the sensing layer can be adjusted by varying the thickness of the matching layer, further improving the adaptability of the sensor. Furthermore, the flexible piezoresistive sensor with a structurally adjustable matching layer prepared by the present invention can rapidly respond to external pressure signals and is suitable for various application scenarios, such as smart healthcare, wearable devices, and robotics.

[0019] The flexible piezoresistive sensor of the present invention utilizes a fully printed manufacturing process, with all materials (including packaging and sensing materials) being manufactured using printing technology. This manufacturing method significantly reduces material waste, lowers production costs, and enables precise customization of the three-dimensional structure and sensing performance as required. Furthermore, all of the sensor's materials are flexible, enabling excellent surface adaptability and making it particularly suitable for lamination to irregular surfaces, greatly facilitating sensor integration. While maintaining its flexibility, the sensor can maintain the natural deformation of its main structure without incurring significant deformation constraints, resulting in excellent integration and making it particularly suitable for applications such as wearable devices and soft robotics. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the technical solution of the present invention, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0021] Figure 1 3D printing system with an additional fan assembly provided in an embodiment of the present invention.

[0022] Figure 2 yes Figure 1 Schematic diagram of the structure of the extrusion system with an additional fan assembly, where (a) is a stereoscopic view, (b) is a top view, (c) is a main view, (d) is a side view, and (e) is a cross-sectional view.

[0023] Figure 3 This is a flow chart of a method for preparing a flexible piezoresistive sensor provided by an embodiment of the present invention.

[0024] Figure 4 It is a schematic diagram of the preparation process of the flexible piezoresistive sensor provided by an embodiment of the present invention.

[0025] Figure 5 Schematic diagram of the top view of the matching layer structure, where (a) is M22, (b) is M32, (c) is M42, and (d) is M52.

[0026] Figure 6 This is a top-down schematic diagram of the flexible piezoresistive sensing layer structure, where (a) is M1, (b) is M2, (c) is M3, (d) is M4, and (e) is M5.

[0027] Figure 7 (a)- Figure 7 (b) are surface morphologies of the sensing layers of Example 1 and Example 5, respectively.

[0028] Figure 8 is the effect of the number of sensitive layers on the mechanical properties of the sensing layer, where (a) is stress-strain and (b) is fracture strain.

[0029] Figure 9 is the effect of the matching layer structure on the mechanical properties of the sensing layer, where (a) is stress-strain and (b) is modulus.

[0030] Figure 10 The influence of the number of grid structure layers on the mechanical properties of the flexible piezoresistive sensor, where (a) is the pressure of 0-50kPa and (b) is the pressure of 0-7kPa.

[0031] Figure 11 These are the results of muscle movement monitoring using a flexible piezoresistive sensor, where (a) is a rapid 45° twist of the sternocleidomastoid muscle, (b) is a slow 45° twist of the sternocleidomastoid muscle, (c) is bending the little finger, (d) is bending all four fingers, (e) is a light fist clench and a strong fist clench, and (f) is the different angles of the forearm raised from the plane. DETAILED DESCRIPTION

[0032] The technical solutions in the embodiments of the present invention will be described clearly and completely below with reference to the accompanying drawings in the embodiments of the present invention.

[0033] This embodiment provides a flexible piezoresistive sensor with a structurally adjustable matching layer, comprising a packaging layer, a sensing layer, leads, and copper foil; The encapsulation layer includes a top encapsulation layer and a bottom encapsulation layer; The sensing layer includes a sensitive layer and a matching layer; the sensitive layer includes a sensitive layer matrix material and a conductive filler; the matching layer includes a matching layer matrix material and a conductive filler; In one embodiment, the conductive filler includes a mixture of graphene (G) and polyaniline (PANI); the mass content of graphene (G) in the matching layer is less than 0.15%, and the mass content of polyaniline (PANI) is less than 0.15%.

[0034] The matching layer is constructed into a preset micro-pattern support network through a programmable electrostatic direct-write printing process; the micro-pattern support network is composed of regularly arranged fibrous supports; the fibrous supports are a three-dimensional multi-layer structure, and the fiber walls formed by the tightly stacked layers can provide directional support and stress dispersion, forming a mechanical adjustment structure between the sensitive layer and the external load. On the one hand, the three-dimensional pattern structure of the present invention guides stress redistribution at the microscale, reduces the stress concentration effect, and makes the sensitive layer more uniformly stressed, thereby achieving a balanced response of the piezoresistive network; on the other hand, by changing the structural morphology and thickness of the matching layer, the overall stiffness and sensitivity of the device can be precisely adjusted without changing the components of the sensitive layer. In particular, when the number of matching layers is moderate, it can not only enhance the sensitivity, but also significantly improve the structural strength of the sensing layer, which is beneficial to the long-term stable operation of the device.

[0035] In one embodiment, a gradient distribution of conductive filler content is formed across the thickness of the sensing layer, with the content gradually decreasing from the sensitive layer to the matching layer. In terms of material distribution, the functional material content of the sensitive and matching layers is set at a gradient, clearly defining the functional division of different regions within the sensing layer. The sensitive layer is based on a flexible polymer (PEO) matrix, with a high proportion of graphene (G) and polyaniline (PANI) conductive fillers uniformly dispersed within it, forming a continuous conductive network that generates a significant piezoresistive effect under external force. In contrast, the matching layer is based on a mechanically stronger polymer (such as PCL) matrix, with a lower proportion of graphene and polyaniline (e.g., a mass fraction of each no more than 0.15%, and the total filler content is much lower than that of the sensitive layer). It primarily provides a supporting framework and provides a certain degree of conductivity. This creates a gradient distribution of conductive filler content across the thickness of the sensing layer, with the content gradually decreasing from the sensitive layer to the matching layer. In other words, the conductive component content is relatively high in the sensitive layer, while the conductive component content is relatively low in the matching layer. This material conductivity gradient arrangement creates a functional zoning of the sensing layer from bottom to top, transitioning from a high-sensitivity region to a high-rigidity support region: the lower sensitive layer is highly sensitive to pressure changes, while the upper matching layer provides structural support and stress buffering. The two layers are well bonded at the interface, and if necessary, material compatibility can be used to allow some components to penetrate and fuse at the interface, further enhancing the gradient transition effect and interfacial bonding. This material distribution not only ensures that the sensitive and matching layers perform optimally in their respective regions, but also imparts a gradient of mechanical and electrical properties across the thickness of the sensor, improving the device's response linearity and stability under varying stress levels.

[0036] In one embodiment, the micropatterned support network utilizes at least one of a parallel fiber wall structure, a concentric ring structure, or a grid-like cross structure. Matching layers of varying geometry exhibit distinct mechanical properties: for example, parallel fiber walls provide directional support and stress dispersion, concentric ring structures create symmetrical support in the central region, and grid structures achieve uniform stress conduction in all directions due to the even distribution of longitudinal and transverse fibers.

[0037] In one embodiment, the spacing between adjacent fibers in the micropatterned support network is greater than or equal to 200 μm. The fibrous scaffold has 10-30 layers, each 5 μm thick. By selecting appropriate microstructure morphology and parameters (such as fiber diameter, spacing, and number of layers), a minimum fiber spacing of approximately 200 μm can be achieved, and a printing layer thickness accuracy of 5 μm can be achieved.

[0038] At the same time, this embodiment also provides a method for preparing the sensor, which specifically includes: (1) Ink preparation: At room temperature, graphene (G) and polyaniline (PANI) are dispersed in a solvent system, and then the sensitive layer matrix material is added to form a uniformly dispersed sensitive layer composite ink; In one embodiment, in the sensitive layer, the matrix material of the sensitive layer is polyethylene oxide (PEO) with a mass concentration of 7.4 wt%; the mass concentration of graphene (G) is 0.5 wt%; and the mass concentration of polyaniline (PANI) is 1.6 wt%.

[0039] Using the same method, graphene (G) and polyaniline (PANI) were dispersed in an organic solvent, and then the matching layer matrix material was added to prepare a composite ink for matching layer printing. In one embodiment, in the matching layer, the matching layer matrix material adopts a polymer material such as polycaprolactone (PCL), polyvinyl pyrrolidone (PVP), thermoplastic polyurethane (TPU), polylactic acid (PLA) and polyurethane (PU), and the concentration can be adjusted to a printable concentration according to the different matching layer matrix materials. PCL is preferably used, and the concentration of PCL is 22-24 wt%; the mass concentration of polyaniline (PANI) is 0.01-0.15 wt%, and the mass concentration of graphene (G) is 0.01-0.15 wt%.

[0040] In one embodiment, the solvent of the ink in the sensitive layer is a mixed system of water and ethanol, and the volume ratio can be 4:6, 5:5, or 3:7, preferably 3:7.

[0041] Specifically, the matching layer ink contains conductive fillers G and PANI, both at concentrations of 0.01-0.15 wt%. G has a purity of >98%, and PANI has a conductivity of 7.5 S / cm and a molecular weight of 5-6 W. Both G and PANI can be replaced with other conductive fillers, such as carbon-based conductive materials or metal conductive nanoparticles.

[0042] The organic solvent of the ink in the matching layer is selected from one or more of glacial acetic acid, chloroform, dichloromethane or dimethylformamide, and preferably a good solvent corresponding to the polymer material.

[0043] (2) Sensing layer printing: A sensitive layer is prepared on a pretreated substrate using a programmable direct writing printing process using a sensitive layer composite ink, and then a matching layer with a preset micro-pattern support network structure is printed on top of the sensitive layer using a programmable electrostatic direct writing technology; the printed sensing layer can also be dried as a whole in a drying dish for 0.5 hours.

[0044] In one embodiment, the sensitive layer is constructed using a programmable direct write printing process, and its printing parameters are: a fan is set above the printing platform, the printing needle specification is 20 G, the solution flow rate is 0.5-2.5 ml / h (preferably 2.0-2.5 ml / h), the working distance is 0.1-2 mm (preferably 0.1-0.5 mm), the platform movement speed is 10-90 mm / s (preferably 40-50 mm / s), and the number of layers can preferably be 6-14 layers.

[0045] In one embodiment, the matching layer is printed by electrostatic direct writing technology, and its printing parameters are: no fan is set above the printing platform, the extrusion rate is 0.1-0.4 ml / h (preferably 0.2-0.3 ml / h), the working distance is 1.5-3.5 mm (preferably 2.5-3 mm), the platform movement speed is 40-70 mm / s, the operating voltage is 1.9-2.5 kV, and the number of printed layers can preferably be 10-40 layers.

[0046] The thickness of the sensitive and matching layers can be adjusted by controlling the printing parameters, thereby regulating the mechanical properties and sensitivity of the sensor. Different micropattern structures can be used to adjust the mechanical properties of the sensing layer. The thickness of the matching layer can be precisely controlled by the number of printed layers, with a minimum resolution of 5 μm and a minimum spacing of 200 μm between adjacent fibers.

[0047] (3) Pretreatment of the sensing layer and preparation of the encapsulation layer ink: The printed sensing layer is cut to a predetermined size, surface treated, and ink for the encapsulation layer is prepared; In one embodiment, the encapsulation layer material is selected from polydimethylsiloxane (PDMS) or Ecoflex, preferably PDMS. The encapsulation layer ink is prepared by mixing the encapsulation layer material base and curing agent in a suitable ratio, stirring at room temperature for 10 minutes, and then removing air bubbles in a vacuum oven before use. The Ecoflex or PDMS base and curing agent ratios are 1:1 and 10:1, respectively.

[0048] In one embodiment, the surface treatment of the sensing layer includes adhering leads to both ends of the sensitive layer with conductive silver glue, with a drying time of 10-30 minutes; then, the leads and both ends of the sensing film are coated with copper foil to enhance electrical contact stability.

[0049] (4) Bottom encapsulation layer printing: The bottom encapsulation layer is printed using encapsulation layer ink using electrostatic direct writing technology, and is semi-cured in a vacuum oven or at room temperature; In one embodiment, the bottom encapsulation layer is stacked layer by layer according to a programmed pattern using electrostatic direct writing technology. The printing parameters include: working distance of 3-5 mm, operating voltage of 2.6-4.5 kV, extrusion rate of 2.0-4.0 ml / h, and platform movement speed of 40-70 mm / s. After printing 1-4 layers, the layer is left at room temperature for 5 hours for semi-curing or heated in a vacuum oven at 60-80°C (preferably 60°C) for 20-40 minutes for semi-curing.

[0050] (5) Top encapsulation layer printing and packaging: The surface-treated sensing layer is adhered to the bottom packaging layer, so that the sensitive layer is bonded to the bottom packaging layer. Then, the top packaging layer is printed on the upper surface of the matching layer using electrostatic direct writing technology using the packaging layer ink, and is cured in a vacuum oven or at room temperature to form a complete flexible piezoresistive sensor.

[0051] In one embodiment, the top encapsulation layer printing and encapsulation steps are as follows: first, the sensing layer pre-treated in step (4) is adhered to the bottom encapsulation layer that is semi-cured and has a certain viscosity in step (4); secondly, the top encapsulation layer is printed on the bottom encapsulation layer adhered with the sensing layer, and the printing parameters are: working distance of 3-5 mm, working voltage of 2.6-4.5 kV, extrusion rate of 2.0-4.0 ml / h, and platform movement speed of 40-70 mm / s; after printing 1-4 layers, the top encapsulation layer is placed at room temperature for more than 10 hours for curing or heated in a vacuum oven at 60-80°C (preferably 60°C) for 2-5 hours for curing.

[0052] This embodiment provides a sensor that is adaptable to different application scenarios, and its sensitivity and mechanical properties can be customized, quickly prepared, and adjusted according to application requirements.

[0053] Using the above-mentioned flexible piezoresistive sensor with a structurally adjustable matching layer, the tensile modulus of its sensing layer is significantly increased compared to the sensing layer with only a sensitive layer structure (without a matching layer). When the matching layer structures are a grid, parallel fiber walls, and concentric ring structures, the tensile modulus increases by 1.9, 2.1, and 2.7 times, respectively.

[0054] The flexible piezoresistive sensor with a structurally adjustable matching layer prepared by the above method has a tensile modulus and a fracture strain of its sensing layer that increases with the number of sensitive layers. When the number of sensitive layers is 6, 8, and 10, respectively, the tensile modulus of the sensing layer increases to 18.8, 70.6, and 70.0 MPa, respectively.

[0055] The above-mentioned flexible piezoresistive sensor with a structurally adjustable matching layer has a sensitivity that can be adjusted to adapt to different application scenarios, including as a component of wearable, smart medical, human-computer interaction systems and other equipment. The sensitivity of the sensor can be adjusted according to the number of matching layers. When the number of grid matching layers increases from 0 to 10 layers, the sensitivity of the sensor can be increased by 24.4 times. In summary, through the unique geometric microstructure design of the matching layer and the gradient distribution of the material content of the sensitive layer and the matching layer, the present invention effectively defines and strengthens the originality of the technical solution of the flexible pressure sensor. Compared with the sensing layer that usually only uses a single structure or uniform material distribution in the prior art, the present invention introduces an adjustable micro-pattern matching layer in the structure and forms a functional gradient from sensitive to supporting on the material. This design enables the sensor to significantly improve the mechanical stability and structural adjustability while ensuring high sensitivity, avoiding the defects of stress concentration and performance compromise, fully reflecting the innovation and limitation of the technical solution of the present invention, and is conducive to enhancing the uniqueness of the solution within the scope of patent protection.

[0056] In order to more clearly illustrate the technical problems, technical solutions and beneficial effects to be solved by this application, the present application is further described in detail below in conjunction with specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the technical solutions of the present invention and are not intended to limit the present invention.

[0057] The present invention screens out the most suitable polymer matrix material for flexible piezoresistive sensors at the same ratio of graphene to polyaniline by comparing the sensitive properties of the sensing layers printed using a variety of different polymer-based conductive inks. Using 3D printing technology, a sensing layer with a composite structure of a sensitive layer and a matching layer is constructed and applied to a flexible piezoresistive sensor. By precisely controlling the material composition, printing parameters, and film structure design of the sensing layer and the matching layer, precise regulation of the sensor sensitivity and mechanical properties can be achieved. The following examples will demonstrate that the technical solution proposed in the present invention can effectively prepare flexible piezoresistive sensors with customizable performance to meet the needs of various application scenarios, such as wearable devices, smart medical care, and human-computer interaction interfaces.

[0058] The preliminary work of Example 1 mainly studied the scalability of the sensitive layer ink formula. Figure 1 As shown, the present invention fabricated a flexible pressure sensor 4 using a laboratory-made 3D printing device (including a syringe 1, a microinjection pump 2, an extrusion system with an accessory fan assembly 3, a receiving platform 4, and a high-voltage power supply 5). The sensitive layer printing inks were formulated using PEO, TPU, PCL, and PVP as polymer materials, respectively, with graphene and polyaniline as conductive functional materials. The ratio of the conductive materials remained constant, and the polymer materials were all used at concentrations suitable for printing. The number of layers and structure of the prepared sensitive layer were consistent. According to the experimental data in Table 1, with the exception of the PEO-based sensitive film, which exhibited significant pressure sensitivity, the other TPU, PCL, and PVP-based sensitive films showed no pressure response when external force was applied, demonstrating the irreplaceable nature of PEO.

[0059] Furthermore, all embodiments of the present invention employ an extrusion system 3 with an additional fan assembly ( Figure 2 Middle (a) - Figure 2 In (e), the sensitive layer is prepared by using a needle 11, a fan assembly 12, a fan mold 1201, and a fan 1202. Otherwise, the sensitivity of the prepared sensitive layer will be greatly reduced. The fan assembly is not used in preparing the encapsulation layer.

[0060] Table 1 Properties of sensitive layers prepared based on different polymer substrates Example 1 Materials: PEO, PCL, G, PANI, and PDMS were purchased from Huagao Fine Chemical Co., Ltd., Sigma-Aldrich, Suzhou Carbonfeng Graphene Technology Co., Ltd., Kuer Technology Co., Ltd., and Dow Corning, respectively.

[0061] method: like Figure 3 and Figure 4As shown, an embodiment of the present invention provides a method for preparing a flexible pressure sensor 4, comprising the following steps: S1 Ink Preparation: Prepare inks for printing the sensitive layer 4101 and matching layer 4102, respectively. Measure 3 ml of deionized water and 7 ml of ethanol into a wide-mouth bottle. Weigh 0.05 g of G and 0.15 g of PANI into the bottle and magnetically stir for 4 hours to ensure that the G and PANI are fully dispersed and form a suspension. Weigh 0.7 g of PEO and add it to the suspension. Magnetic stir at room temperature for 10 hours to obtain the ink for printing the sensitive layer. Separately, measure 10 ml of glacial acetic acid and place it in a wide-mouth bottle. Add 0.0005 g of G and 0.0005 g of PANI and magnetic stir for 4 hours to form a suspension. Weigh 0.3113 g of PCL and add it to the suspension. Magnetic stir at 50°C for 4 hours to obtain the ink for printing the matching layer.

[0062] S2 Sensing layer printing: Based on the ink prepared in step S1, the sensitive layer 4101 of the sensing layer 41 is first printed using direct writing technology (with an additional fan component). Then, a grid structure matching layer 4102 is electrostatically written (without a fan component) on the sensitive layer 4101. A gradient distribution of the conductive filler content is formed in the thickness direction of the sensing layer, with the conductive filler content gradually decreasing from the sensitive layer to the matching layer.

[0063] S3 Sensing layer pretreatment and encapsulation layer ink preparation: cutting the sensing layer into the target shape and preparing the printing ink for the encapsulation layer 42; S4 bottom encapsulation layer printing: using electrostatic direct writing technology and the encapsulation layer ink prepared in step S3 to print the bottom encapsulation layer 4201, and heating it in a vacuum oven for a certain period of time to semi-curing treatment; S5: Printing and packaging of the top encapsulation layer: The sensing layer 41 pre-treated in step S3 is adhered to the bottom encapsulation layer 4201 prepared in step S4, and a top encapsulation layer 4202 is printed thereon by electrostatic direct writing and cured in a vacuum oven.

[0064] In step S1, the conductive fillers in the sensitive layer ink are G and PANI, with concentrations of 0.5 wt% and 1.6 wt%, respectively. The solvent is a mixture of deionized water and ethanol, with a volume ratio of 3:7. The polymer material is PEO, with a concentration of 7.4 wt%. The conductive fillers in the matching layer ink are G and PANI, both with a concentration of 0.15 wt%. The solvent is acetic acid. The polymer material is PCL, with a concentration of 24 wt%.

[0065] In step S2, matching layer 4102 was printed on top of sensitive layer 4101 and placed in a drying dish to dry for 30 minutes. The printing parameters for sensitive layer 4101 were: solution flow rate 2.5 ml / h, working distance 0.1 mm, and platform movement speed 50 mm / s. The printing parameters for matching layer 4102 were: flow rate 0.3 ml / h, voltage 2.1 kV, working distance 2 mm, and platform movement speed 60 mm / s. The minimum area of ​​a complete grid in the matching layer was 500 × 500 μm. 2 , the number of matching layers is 10.

[0066] In step S3, the sensing layer is cut into 11 × 25 mm 2 A rectangular shape was formed, and two equal-length leads (4103) were attached 5 mm from each end of the sensitive layer using conductive silver glue. The entire structure was then placed in a desiccating dish to dry for 10 minutes. After complete drying, the leads were wrapped with copper foil (4104). The encapsulation layer ink was PDMS, with a base and curing agent weight ratio of 10:1.

[0067] In step S4, a certain amount of the PDMS liquid prepared in step S3 was drawn into syringe 1 and extruded through peristaltic pump 2 and a catheter at a rate of 3.0 ml / h. The printing working distance was set to 3 mm, the operating voltage was 2.6 kV, and the movement speed of receiving platform 4 was set to 50 mm / s. The two printed PDMS layers were then placed in a vacuum oven and semi-cured at 80°C for 20 minutes to prepare the bottom encapsulation layer 4201.

[0068] In step S5, the sensor layer 41 processed in S3 is adhered to the semi-cured bottom encapsulation layer 4201 in S4. A top encapsulation layer 4202 is then printed on top. Three layers are printed using the following printing parameters: a working distance of 3 mm, an operating voltage of 2.6 kV, and a receiving platform 4 movement speed of 50 mm / s. Finally, the printed sensor is heated in a vacuum oven at 60°C for 2 hours and then cooled at room temperature for 5 hours.

[0069] Comparative Example 1 In step S2 of Example 1, the number of grid structure matching layers is 20, and other conditions remain unchanged.

[0070] Comparative Example 2 In step S2 of Example 1, the number of grid structure matching layers is 30, and other conditions remain unchanged.

[0071] Comparative Example 3 In step S2 of Example 1, the number of grid structure matching layers is 40, and other conditions remain unchanged.

[0072] Example 2 The grid structure of the matching layer in step S2 ( Figure 5 (d) M52) is changed to a concentric ring structure ( Figure 5 (c) M42), the rest of the steps remain the same.

[0073] Example 3 The grid structure of the matching layer in step S2 ( Figure 5 (d) M52) is changed to a hollow parallel fiber wall structure ( Figure 5 (b) M32), the rest of the steps remain the same.

[0074] Example 4 The grid structure of the matching layer in step S2 ( Figure 5 (d) M52) is changed to a parallel fiber wall structure ( Figure 5 (a) M22), the rest of the steps remain the same.

[0075] Example 5 In step S2, no matching layer is set, and only the sensitive layer is printed as the sensing layer. The other steps remain unchanged.

[0076] Furthermore, the sensitive layers in Examples 1-5 are all prepared by steps S1 and S2 in Example 1. In Examples 1-4, matching layers of different structures are respectively constructed on the sensitive layers. The schematic diagram of the top view of the formed sensing layer structure is shown in FIG. Figure 6 Middle (a) - Figure 6 Middle (e).

[0077] Furthermore, PCL in step S1 provided in Example 1 of the present invention can be replaced by other printable polymers such as TPU, PVP, PU, ​​etc.; PANI can be replaced by other conductive polymers; G can be replaced by other conductive (nano) micro-sheets, wires, and particles.

[0078] Furthermore, the structures of the sensitive layer and the matching layer in step S2 provided in Example 1 of the present invention can be customized as needed and are not limited to grid, concentric ring, and parallel fiber wall structures. The structure and thickness can be customized as needed.

[0079] Furthermore, the printing parameters of steps S1, 2, 4, and 5 provided in Example 1 of the present invention can be adjusted according to actual conditions.

[0080] Furthermore, in step S3 provided in embodiment 1 of the present invention, the target shape and size of the sensing layer to be cut can be adjusted as needed.

[0081] Furthermore, the number of printed layers of the sensitive layer, matching layer, and encapsulation layer provided in Example 1 of the present invention can be adjusted according to demand.

[0082] The performance tests and application studies of the sensors prepared in the above examples or comparative examples were conducted, and the results are as follows: Surface topography testing The surface morphology of the sensing layer prepared in Example 1 and Example 5 was observed using a field emission scanning electron microscope. Figure 7 As shown in (a), the surface is rough and has pores. The large picture in the upper right corner shows the high-resolution microstructure of the sensitive layer. The rod-shaped structure in the picture is PANI, and the multilayer structure material is G. A 40-layer grid structure matching layer is printed on the sensitive layer. Figure 7 Middle (b) shows the precise stacking of the grid structure, and the high-resolution image in the upper right corner shows that the surface of the matching layer is relatively smooth.

[0083] Mechanical properties testing The mechanical properties of the sensors prepared in the above examples or comparative examples were tested using a universal material testing machine. The upper and lower ends of the sensors were fixed with the fixtures provided by the instrument and stretched at a rate of 5 mm / s until they broke. The stress-strain curve of the sensors was recorded. The results are shown in Figure 2. Figure 8 Middle (a) - Figure 8 As shown in (b), the greater the slope of the linear segment of the stress-strain curve, the higher the modulus; conversely, the smaller the slope, the lower the modulus. It can be seen that as the number of sensitive layers increases from 6 to 10 and 14, the elongation at break and modulus of the sensing layer gradually increase. The flexibility of the sensing layer decreases, but its resistance to deformation increases. Therefore, in device fabrication and soft interface applications, the flexibility and resistance of the sensing layer can be controlled by adjusting the number of sensitive layers.

[0084] The effect of the matching layer structure on the mechanical properties of the sensing layer is shown in Figure 9 Middle (a) - Figure 9 Figure (b) shows the sensing layers M2, M4, and M5, whose matching layer structures consist of fiber walls M22, rings M42, and meshes M52. Their moduli are 2.1, 2.7, and 1.9 times greater, respectively, than those of the sensing layer M1 in the composite structure without a matching layer. However, the modulus of the sensing layer with a hollow fiber wall structure is similar to that of the sensing layer with only the sensitive layer. The uneven distribution of the hollow fiber wall structure leads to stress concentration in certain sections of the membrane, making it more susceptible to fracture. Therefore, when designing the matching layer structure, full consideration should be given to structures that can evenly distribute stress.

[0085] Sensitivity test In addition to the matching layer structure, the number of matching layers also plays a key role in the performance of the sensor. Therefore, the present invention uses a weight to apply static pressure to test the response performance of the composite structure sensor in the above embodiment 1 and its comparative examples 2-4 under different pressures, and compares the response curves. Figure 10 The sensitivity of each sensor was obtained by linear fitting the data from 0 to 5 kPa in (a). The results are shown in Figure 10 (b). All dotted lines in the figure are linear fitting curves for different sensors, and the slope of the dotted line (S1-S4) is the sensitivity of the corresponding sensor. Since the sensor with 40 matching layers has a R 2 (about 0.7) shows that the fitting is not of reference value, so S5 is not marked in the figure. The sensitivity data shows that as the number of matching layers increases from 0 to 40, the sensor sensitivity first increases and then decreases. The highest S2 is 4.64 kPa -1 (R 2 = 0.901), the sensitivity of S3 and S4 are 0.91 kPa respectively -1 (R 2 = 0.909), 0.16 kPa -1 (R 2 = 0.916), the sensitivity of the 40-layer sensor was the worst. When the number of matching layers reached 10, the sensor's elasticity and strength increased, and deformation under the same external pressure also increased. However, when the number of matching layers was too large (≥ 30), the sensor's ability to resist deformation was too strong. Even with the same external force, deformation decreased with increasing layer thickness, resulting in a decrease in sensitivity compared to devices without matching layers. Therefore, for the flexible piezoresistive sensor with adjustable matching layers designed in this invention, if the need for improved sensitivity outweighs the need for enhanced mechanical performance, the number of matching layers should not exceed 30.

[0086] Muscle movement monitoring The application performance of a flexible piezoresistive sensor prepared by the present invention was investigated by detecting human muscle movement. Using the preparation method of Example 1 above, a flexible piezoresistive sensor (SMS) with 10 sensitive layers and 10 grid structure matching layers was prepared. First, it was attached to the sternocleidomastoid muscle (SCM) on the neck, and an electrochemical workstation was used to measure the relative resistance change of the sensor SMS when the head and neck were twisted. After twisting the head and neck to a certain position at a relatively uniform rate and a relatively fixed angle, the head and neck did not stay at this position or stayed for a few seconds and then returned to the initial position, and this cycle was repeated several times. When the twisting angle was about 45°, the test group in which the head and neck did not stay at this angle was recorded as fast, and the relative resistance change of the device caused was as follows: Figure 11 As shown in (a). The test group in which the head and neck are twisted to 45 degrees and remain in this position for a few seconds is recorded as slow, causing the relative resistance change of the device to be as shown in Figure 11 As shown in (b). Since the force and angle of the head and neck twist are slightly different, whether the head and neck stay at the 45° position or not, Figure 11 Middle (a), Figure 11The results of the eight repeated experiments in (b) all show that the relative resistance changes of the SMS sensor are not completely consistent, which also shows that the SMS is highly sensitive and can sense the force differences of the SCM caused by different head and neck twist angles. The difference is that when the head and neck stay at the 45° position and then return to the initial position, the relative resistance change curve has a "double peak" phenomenon ( Figure 11 (b)). The first peak is the stress response peak to the SCM. As the neck rotates to a 45° position, the change in SCM force instantly causes the SMS to deform. The spacing and potential barriers of the conductive material within the sensing material increase, creating microcracks. A very small number of conductive pathways are established, but they are non-dominant. Overall, the device resistance increases, resulting in a response peak. When the neck remains in this position, the stress decreases slightly over time due to the stress relaxation properties of the polymer-based flexible pressure sensor, causing a decrease in the relative resistance change. The moment the neck returns to its initial position, the stress is suddenly unloaded, and the newly established conductive pathway within the small portion of the sensing membrane material quickly disconnects, causing a rapid increase in resistance. This results in a second peak (recovery peak). Due to the hysteresis properties of the polymer material, the microcracks created within the polymer after the previous stress recovery more slowly than the conductive pathways break. Therefore, after the second peak appears, the resistance gradually decreases as the microcracks recover and returns to its initial value.

[0087] The SMS sensor was attached to the abductor digiti minimi (ADM) muscle of the palm to monitor the movement of the little finger, in order to investigate the potential application of the SMS sensor in the fields of prosthesis and human-machine interface. Figure 11 As shown in Figure (c), each time the pinky bends, the ADM contracts and increases force, causing the sensor's resistance to increase. As the pinky recovers, the AMD relaxes and the force decreases, causing the resistance of the sensor SMS to return to its initial value. The greater the pinky bend angle, the greater the force generated by the ADM, resulting in a larger peak in the curve. The smaller the pinky bend angle, the smaller the force generated by the ADM, the smaller the sensor deformation, and the smaller the peak in the curve.

[0088] Figure 11 Middle (d) shows the results of the SMS sensor monitoring the flexion of the remaining four fingers. The SMS sensor was placed at the Jianshi point, 3 inches above the transverse wrist crease, between the flexor carpi radialis tendon and the palmaris longus tendon. The four fingers, excluding the thumb, were repeatedly flexed. The temporal change in the sensor's relative resistance reflects the motion of the four fingers and the changes in muscle force induced by flexion. A greater change in relative resistance indicates a greater degree of flexion and a greater change in the strength of the flexor carpi radialis and palmaris longus tendons.

[0089] The SMS sensor can also be used to monitor changes in muscle strength caused by clenching a fist, allowing for a comprehensive assessment of grip strength or finger movement. The SMS sensor is placed on the flexor carpi radialis (FCR) muscle to monitor fluctuations in the FCR caused by both natural and forced clenching of the fingers. Figure 11 Middle (e) shows that when you clench your fist lightly, the smaller the grip force, the smaller the FCR fluctuation, the less obvious the contraction, and the smaller the change in SMS relative resistance; when you clench your fist hard, the greater the grip force, the greater the FCR fluctuation, the tighter the contraction, resulting in a larger change in SMS relative resistance. When you clench your fist, the contraction pressure of the FCR breaks some of the conductive paths in the sensor and creates tiny cracks, which increases the SMS resistance. When you release your fingers, the FCR relaxes, the force applied to the SMS disappears, and the resistance returns to its initial value. When you clench your fist hard (strong) and hold it for about 1 s, the curve of the change in relative resistance over time also shows a "double peak", and its principle is the same as Figure 11 The principle of producing the "double peaks" in (b) is the same. Figure 11 The comparison of the two groups of curves in (e) shows that the sensor SMS can better identify the size of the grip force.

[0090] The contraction and relaxation of the biceps can also cause changes in the resistance of the sensor. By attaching the SMS sensor to the biceps brachii of the upper arm, changes in biceps strength can be monitored. The contraction amplitude of the biceps is controlled by the angle at which the forearm is raised from a flat surface. The smaller the angle of the forearm is raised, the smaller the contraction amplitude of the biceps is caused. The angles of the raised forearm are 30°, 60°, and 90° respectively. The monitoring results are as follows: Figure 11 As shown in (f), when the arm is raised 30°, the sensor's relative resistance change is minimal (<7%). When the arm is raised 90°, the relative resistance change increases, by approximately 22.5%. In summary, within the 30-90° arm elevation range, the relative resistance change increases with increasing arm elevation angle or biceps contraction amplitude.

[0091] The above muscle movement detection is only intended to illustrate one of the potential applications of the flexible piezoresistive sensor proposed in the present invention, and does not limit the application of the sensor proposed in the present invention.

[0092] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications are also considered to be within the scope of protection of the present invention.

Claims

1. A flexible piezoresistive sensor with a structurally adjustable matching layer, comprising a packaging layer, a sensing layer, leads, and copper foil; wherein: The sensor packaging layer includes a top packaging layer and a bottom packaging layer, and the sensing layer includes a sensitive layer and a matching layer; the sensitive layer includes a sensitive layer matrix material and a conductive filler; the matching layer includes a matching layer matrix material and a conductive filler; It is characterized in that the matching layer is constructed into a preset micro-pattern support network through a programmable electrostatic direct writing printing process; the micro-pattern support network is composed of regularly arranged fibrous supports; and the fibrous supports are a three-dimensional multi-layer structure.

2. The sensor according to claim 1, characterized in that A gradient distribution of the conductive filler content is formed in the thickness direction of the sensing layer, with the content gradually decreasing from the sensitive layer to the matching layer.

3. The sensor according to claim 2, characterized in that: The conductive filler includes a mixture of graphene (G) and polyaniline (PANI); the mass content of graphene (G) in the matching layer is less than 0.15%, and the mass content of polyaniline (PANI) is less than 0.15%.

4. The sensor according to claim 1, characterized in that The micro-pattern support network adopts at least one of a parallel fiber wall structure, a concentric ring structure or a grid cross structure.

5. The sensor according to claim 1, characterized in that: The distance between adjacent fibers in the micro-pattern support network is greater than or equal to 200 μm.

6. The sensor according to claim 1, characterized in that: The number of layers of the fibrous scaffold is 10-30, and the thickness of each layer is 5 μm.

7. The sensor according to claim 1, characterized in that: The sensor is prepared by the following method: (1) Ink preparation: At room temperature, graphene (G) and polyaniline (PANI) are dispersed in a solvent system, and then polyethylene oxide (PEO) is added to form a uniformly dispersed sensitive layer composite ink; Using the same method, graphene (G) and polyaniline (PANI) were dispersed in an organic solvent, and then the matrix material was added to prepare a matching layer composite ink. (2) Sensing layer printing: A sensitive layer is prepared on a pretreated substrate using a programmable direct writing printing process using a sensitive layer composite ink, and then a matching layer having a preset micro-pattern support network structure is printed on top of the sensitive layer using a matching layer composite ink using a programmable electrostatic direct writing technique; (3) Pretreatment of the sensing layer and preparation of the encapsulation layer ink: The printed sensing layer is cut to a predetermined size, surface treated, and ink for the encapsulation layer is prepared; (4) Bottom encapsulation layer printing: The bottom encapsulation layer is printed using encapsulation layer ink using electrostatic direct writing technology, and is semi-cured in a vacuum oven or at room temperature; (5) Top encapsulation layer printing and packaging: The surface-treated sensing layer is adhered to the bottom packaging layer, so that the sensitive layer is bonded to the bottom packaging layer. Then, the top packaging layer is printed on the upper surface of the matching layer using electrostatic direct writing technology using the packaging layer ink, and is cured in a vacuum oven or at room temperature to form a complete flexible piezoresistive sensor.

8. The sensor according to claim 7, characterized in that: In the sensitive layer, the matrix material of the sensitive layer is polyethylene oxide (PEO) with a mass concentration of 7.4 wt%, the mass concentration of graphene (G) is 0.5 wt%, and the mass concentration of polyaniline (PANI) is 1.6 wt%. In the matching layer, the matching layer matrix material is polycaprolactone (PCL) with a mass concentration of 22-24 wt%, polyaniline (PANI) with a mass concentration of 0.01-0.15 wt%, and graphene (G) with a mass concentration of 0.01-0.15 wt%.

9. The sensor according to claim 7, characterized in that: The sensitive layer was constructed using a programmable direct-write printing process with the following printing parameters: a fan was placed above the printing platform, the printing needle was 20 G, the solution flow rate was 0.5-2.5 ml / h, the working distance was 0.1-2 mm, and the platform movement speed was 10-90 mm / s. The matching layer is printed using electrostatic direct writing technology, and its printing parameters are: no fan is set above the printing platform, the extrusion rate is 0.1-0.4 ml / h, the working distance is 1.5-3.5 mm, the platform movement speed is 40-70 mm / s, and the operating voltage is 1.9-2.5 kV.

10. Application of the sensor according to any one of claims 1 to 9 in at least one of a wearable device, an electronic skin, and a human-computer interaction interface.

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