Flexible strain sensor and method of making and using same

Abstract

The application provides a flexible strain sensor and a preparation method and application thereof, and belongs to the technical field of flexible electronics. The flexible strain sensor comprises, from top to bottom, an upper flexible substrate, a sensing layer with a negative Poisson's ratio structure, and a lower flexible substrate. Metal electrodes are connected to both ends of the sensing layer for electrical connection. The negative Poisson's ratio structure is composed of two circular arc S units with the same structure and size in parallel. The two circular arc S units are symmetrically arranged in a whole mirror image with a horizontal straight line parallel to the length direction of the circular arc S units as the axis of symmetry, and the two ends of the two circular arc S units are respectively connected and communicated. The circular arc S unit is composed of continuously alternating convex outer arc segments and concave inner arc segments. The radius ratio of the convex outer arc to the concave inner arc is 1:3-3:1.

Description

Technical Field

[0001] This application relates to the field of flexible electronics technology, and in particular to a flexible strain sensor, its fabrication method, and its application. Background Technology

[0002] Flexible strain sensors are core components for enabling sensing functions in wearable smart devices, soft robots, and systems for dynamic monitoring of human health. Traditional metal- or semiconductor-based sensors generally suffer from inherent defects such as poor flexibility and narrow detection range, making them unsuitable for monitoring large deformations on flexible substrates such as human skin.

[0003] Currently, flexible strain sensors based on conductive polymer composite materials have become a research hotspot in the field of flexible sensing due to their excellent flexibility and stretchability. However, the research and development and industrialization of such sensors still face many challenges: First, there is a trade-off between the sensitivity and effective detection range of the sensor; high sensitivity often comes at the cost of a limited detection range, making it difficult to meet the dual requirements of wide range and high sensitivity. Second, existing fabrication processes mostly rely on complex micro-nano fabrication techniques, resulting in high production costs, low efficiency, and difficulty in ensuring the consistency of performance in batches of products, thus restricting large-scale applications. Summary of the Invention

[0004] In view of this, the main objective of this application is to provide a flexible strain sensor, its fabrication method, and its application, in order to at least partially solve at least one of the aforementioned technical problems.

[0005] To achieve the above objectives, the technical solution of this application is as follows.

[0006] In one aspect of this application, a flexible strain sensor is provided, comprising, from top to bottom: an upper flexible substrate, a sensing layer with a negative Poisson's ratio structure, and a lower flexible substrate, wherein metal electrodes are connected to both ends of the sensing layer for electrical connection.

[0007] The negative Poisson's ratio structure is composed of two S-shaped circular arc units of the same structure and size connected in parallel. The two S-shaped circular arc units are arranged in a mirror image symmetrically with a horizontal straight line parallel to their own length as the axis of symmetry, and the two ends of the two S-shaped circular arc units are connected respectively.

[0008] The S-shaped arc unit is composed of continuous alternating convex and concave arc segments connected end to end, with the radius ratio of the convex arc to the concave arc being 1:3 to 3:1.

[0009] In another aspect of this application, a method for fabricating the aforementioned flexible strain sensor is provided, comprising: pre-pressing and fixing metal electrodes to both ends of a sensing layer having a negative Poisson's ratio structure, and encapsulating the sensing layer with the pre-pressed metal electrodes between an upper flexible substrate and a lower flexible substrate by a hot pressing process to obtain a flexible strain sensor, wherein the temperature of the hot pressing process is 135~150℃, the pressure is 600~750Pa, and the time is 20~22s.

[0010] In another aspect of this application, an application of the aforementioned flexible strain sensor in human joint motion monitoring is provided.

[0011] According to embodiments of this application, a flexible strain sensor is provided, employing a negative Poisson's ratio structure composed of two parallel, mirror-symmetric S-shaped arc units as the sensing layer. This structural design collaboratively addresses the challenges of existing flexible strain sensors in simultaneously achieving high sensitivity, detection range, and operational stability. This negative Poisson's ratio structure generates a lateral expansion mechanical response under tensile loads, leveraging the structural mechanical amplification effect to enhance the effective deformation of the sensing layer. This allows the sensor to operate within a wide strain detection range of 5.2% to 30%, improving sensitivity while reducing linearity error, effectively balancing the dual performance requirements of detection accuracy and range. Simultaneously, this negative Poisson's ratio structure enables stress concentration control, reducing the mechanical hysteresis effect of the sensing layer during cyclic deformation and ensuring the long-term performance stability of the sensor. Furthermore, the fabrication process of this flexible strain sensor is simple and controllable, requiring no complex micro / nano fabrication equipment or processes, offering significant advantages in large-scale mass production and cost control.

[0012] This flexible strain sensor can conform well to human skin and is suitable for monitoring human joint movement. Compared with traditional metal and semiconductor strain sensors, this sensor has excellent flexibility and deformation adaptability, which can meet the detection needs of dynamic deformation of human joints. It does not require complicated wearable auxiliary devices, and its application scenarios are flexible, providing effective device support for wearable smart sensing, dynamic human health monitoring and other fields. Attached Figure Description

[0013] Figure 1 This is a schematic diagram illustrating the analysis of different negative Poisson's ratio structures and their expansion mechanisms in the embodiments of this application;

[0014] Figure 2 This is a flowchart of the fabrication method of the flexible strain sensor in Embodiment 1 of this application;

[0015] Figure 3 This is a schematic diagram illustrating the preparation of the thermoplastic polyurethane substrate film in Example 1 of this application;

[0016] Figure 4 This is a schematic diagram illustrating the preparation of the conductive composite material filament in Example 1 of this application;

[0017] Figure 5 This is a schematic diagram of the printing equipment used in Embodiment 1 of this application and a diagram of the printed product;

[0018] Figure 6 This is a flowchart of the thermo-press packaging process for the flexible strain sensor in Embodiment 1 of this application;

[0019] Figure 7 This is a finished product image of the flexible strain sensor in Embodiment 1 of this application;

[0020] Figure 8 The sensitivity calibration curve of the flexible strain sensor in Embodiment 1 of this application;

[0021] Figure 9 This is a graph showing the repeatability test results of the flexible strain sensor in Embodiment 1 of this application during five loading-unloading cycles;

[0022] Figure 10 This is a schematic diagram of the flexible strain sensor attached to the proximal interphalangeal joint of the finger in Embodiment 1 of this application for detecting bending angle, and the resistance response curves at different bending angles.

[0023] Figure 11 The fitted curve showing the mapping relationship between the flexible strain sensor and the bending angle of the proximal interphalangeal joint of the finger in Embodiment 1 of this application;

[0024] Figure 12 This is a schematic diagram of the sensing layer structure of the flexible strain sensor in Embodiments 1 to 5 of this application and its stress cloud diagram in finite element analysis under 30% tensile strain;

[0025] Figure 13 This is a schematic diagram of the sensing layer with different negative Poisson's ratio structures in Embodiment 1 and Comparative Examples 1 to 4 of this application.

[0026] Figure 14 The results are static simulations of different negative Poisson's ratio structures in Examples 1 and 5 of this application. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with specific embodiments.

[0028] 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.

[0029] 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.

[0030] In the process of developing this application, it was discovered that, in order to overcome the performance bottleneck of balancing wide range and high sensitivity in flexible strain sensors, the mainstream existing technology optimizes sensing performance through microstructure design, and has successively developed various sensing substrate solutions based on crack structures, porous structures, and other microstructures. However, these microstructures generally suffer from poor structural stability, significant mechanical hysteresis effects, cumbersome fabrication processes, and low controllability, thus limiting their practical application effectiveness.

[0031] Negative Poisson's ratio structures (also known as tensile structures) are a type of structure with a unique mechanical response. When subjected to axial tension, they exhibit a unique deformation behavior of lateral expansion. Theoretically, the effective deformation of the sensing material can be enhanced through the mechanical amplification effect of the structure itself, thereby significantly improving the detection sensitivity of the sensor. This provides a new technical approach to resolving the inherent contradiction between sensitivity and detection range in flexible strain sensors. However, the application of this structure in flexible strain sensors is still in the initial exploratory stage, and the related technical system is not yet mature. How to design a reasonable negative Poisson's ratio structure suitable for flexible sensing scenarios and achieve a flexible strain sensor with high sensitivity, wide range, and high stability through a simple, controllable, and low-cost fabrication process remains a key technical challenge that urgently needs to be overcome in the field of flexible sensing.

[0032] Based on this, this application provides a flexible strain sensor, its fabrication method, and its application. It utilizes a pioneering dual-mirror symmetrical parallel circular arc S-shaped negative Poisson's ratio structure as the sensing layer, simultaneously achieving high sensitivity, wide detection range, and high operational stability through structural design. Furthermore, it is coupled with a simplified fused deposition modeling (FDM) fabrication process, eliminating reliance on complex micro / nano fabrication and enabling low-cost, scalable fabrication. The flexible strain sensor provided in this application can be widely used in wearable smart sensing scenarios such as human joint motion monitoring, providing core support for the industrialization of flexible electronics technology.

[0033] According to one embodiment of this application, a flexible strain sensor is proposed, comprising, from top to bottom: an upper flexible substrate, a sensing layer with a negative Poisson's ratio structure, and a lower flexible substrate, wherein metal electrodes are connected to both ends of the sensing layer for electrical connection.

[0034] The negative Poisson's ratio structure is composed of two parallel S-shaped arc units with identical structure and size. The two S-shaped arc units are arranged in a mirror image symmetrically with a horizontal straight line parallel to their own length as the axis of symmetry. The two ends of the two S-shaped arc units are connected respectively. The S-shaped arc unit is composed of continuous alternating convex arc segments and concave arc segments connected end to end. The ratio of the radii of the convex arc to the concave arc is 1:3 to 3:1.

[0035] According to embodiments of this application, the radius ratio of the convex arc to the concave arc can be, for example, 1:3, 1:2, 1:1, 2:1, 3:1, etc.

[0036] The flexible strain sensor provided in this application uses a negative Poisson's ratio structure constructed by parallel connection of two mirror-symmetric circular arc S-shaped units as the sensing layer. Through structural mechanics design, it achieves a comprehensive improvement in sensing performance, operational stability, and industrialization feasibility.

[0037] The S-shaped unit of the sensing layer is composed of continuously alternating convex and concave arc segments connected tangentially. The ratio of the radii of the convex to concave arcs is precisely controlled within the range of 1:3 to 3:1, which generates a unique negative Poisson's ratio effect of lateral synchronous expansion under axial tension. This deformation mode, through the structural mechanics amplification effect, significantly increases the actual deformation of the sensing layer, fundamentally improving the strain response sensitivity of the sensor. At the same time, the smoothly transitioning arc configuration allows the structural deformation to change gradually and uniformly with strain, ensuring high sensitivity and linearity error of the sensor, simultaneously achieving the dual performance requirements of high detection accuracy and wide detection range.

[0038] The symmetrical parallel design of the negative Poisson's ratio structure enables the control of stress concentration. The synchronous deformation of the two units allows the tensile load to be evenly distributed throughout the overall structure. The rounded transition without sharp corners avoids local stress overload and can strictly control the maximum stress of the structure within the allowable range of the material. This reduces plastic deformation and fatigue damage in cyclic deformation from the source and fully ensures the reliability of the sensor in long-term operation.

[0039] Furthermore, in order to explore the negative Poisson's ratio mechanism (expansion mechanism) of the negative Poisson's ratio structure in this application, a mathematical model of the structure was constructed.

[0040] Figure 1 This is a schematic diagram illustrating the analysis of different negative Poisson's ratio structures and their expansion mechanisms in the embodiments of this application. Among them, (a) is a schematic diagram of different negative Poisson's ratio structures in the unstretched state in the embodiments of this application; (b) is a schematic diagram of the expansion behavior of different negative Poisson's ratio structures in the embodiments of this application; and (c) is the expansion characteristic curve of different negative Poisson's ratio structures in the embodiments of this application.

[0041] like Figure 1 As shown in Figure (a), there are five negative Poisson ratio structures in this application, namely S13 In a negative Poisson's ratio structure, the ratio of the radii of the convex to the concave arcs is 1:3; S 12 In a negative Poisson's ratio structure, the ratio of the radii of the convex to the concave arcs is 1:2; S 11 In a negative Poisson's ratio structure, the ratio of the radii of the convex to the concave arcs is 1:1; S 21 In a negative Poisson's ratio structure, the ratio of the radii of the convex to the concave arcs is 2:1; S 31 In a negative Poisson's ratio structure, the ratio of the convex to the concave radius of the arc is 3:1. The S-shaped arc units of the five negative Poisson's ratio structures can be simplified into a typical concave honeycomb unit cell commonly used in ductile design. This concave honeycomb unit cell consists of four symmetrically arranged diagonal ribs and short vertical ribs on the left and right sides. Unlike traditional hexagonal honeycombs, the ribs of this unit cell are arranged inwards relative to the parallel direction, causing the honeycomb walls to contract inwards in the middle, forming a typical concave geometric profile. Each diagonal rib in the unit cell is connected to the top or bottom crossbeam at one end, and the other end merges with the adjacent diagonal rib at the node in the middle, forming a concave configuration similar to a "V". In the initial state, each diagonal rib is inclined towards the center of the unit cell, presenting an overall concave honeycomb shape. When the unit cell is subjected to... Figure 1 When stretched axially in (b), the diagonal members undergo bending deformation due to their limited bending stiffness, exhibiting a "rotational effect around the upper end point," causing simultaneous changes in the longitudinal length and transverse width of the unit cell. That is, in this negative Poisson's ratio structure, the diagonal members bend and open outwards during axial stretching, increasing the transverse dimension of the unit cell and thus exhibiting a typical negative Poisson's ratio effect. Parameters such as the diagonal member length *l*, the vertical member length *h*, and the inclination angle *-θ* in the unit cell collectively control the rotatable space and bending flexibility of the unit, determining the Poisson's ratio value of the overall honeycomb structure.

[0042] like Figure 1 As shown in (c), the in-plane Poisson's ratio of the two-dimensional honeycomb structure under stress can be expressed as the following equation (1).

[0043] Equation (1)

[0044] From equation (1), it can be seen that the sign and magnitude of Poisson's ratio V are jointly determined by the combined relationship between h / l+sinθ and sinθ. Figure 1 As shown in (b), θ < 0 and sinθ < 0 at all times. If h / l > -sinθ, then the overall Poisson's ratio V is negative, and the structure forms a typical negative Poisson's ratio deformation mode, such as... Figure 1 As shown in (c), it can be observed that under a given h / l condition, the overall Poisson's ratio V is always negative and increases negatively with the increase of -θ.

[0045] As -θ increases (i.e., the concave angle becomes larger), the rotational unfolding effect of the concave diagonal bar becomes more significant, resulting in a stronger "unfolding angle" and lateral traction at the nodes between the ribs. This further increases the absolute value of V and exhibits a more pronounced lateral expansion effect. Therefore, since only the radius ratio R2 is in S 13 To S 31 As the structures change, the corresponding angle -θ increases, S 31 The structure has a larger concave angle of the diagonal bar, and its Poisson's ratio V has the largest negative value. It exhibits a stronger negative Poisson's ratio effect during tension, and thus, when S... 31 When the structure is used as a sensing layer, it undergoes greater lateral expansion during deformation, which ultimately leads to a greater rate of change in resistance.

[0046] Because negative Poisson's ratio deformation directly promotes sensor performance, the concave honeycomb expands laterally synchronously under tension, which can significantly increase the deformation degree of the conductive network inside the sensitive layer, amplify the resistive response, and thus improve the sensor's sensitivity. Therefore, the negative Poisson's ratio structure of this application not only exhibits a significant negative Poisson's ratio effect, but also achieves good sensing response enhancement capabilities.

[0047] According to embodiments of this application, the thickness of the sensing layer is 100~200μm, for example, it can be 100μm, 120μm, 150μm, 180μm, 200μm, etc.; the thickness of the upper flexible substrate and the lower flexible substrate is 50~200μm, for example, it can be 50μm, 80μm, 100μm, 150μm, 180μm, 200μm, etc.; the metal electrode has a diameter of 40~60μm, for example, it can be 40μm, 45μm, 50μm, 55μm, 60μm, etc.

[0048] According to embodiments of this application, controlling the thickness of the sensing layer to 100-200 μm ensures the mechanical stability of the negative Poisson's ratio structure during tensile deformation, avoiding the problems of easy breakage due to excessive thinness and excessive rigidity due to excessive thickness. It also adapts to the strain response sensitivity and linearity of resistance change. Setting the thickness of the upper and lower flexible substrates to 50-200 μm allows the flexible substrates to possess both sufficient support strength and good flexibility. This effectively encapsulates and protects the internal sensing layer while ensuring that the sensor as a whole adheres tightly to complex surfaces such as curved surfaces and human skin, without affecting deformation transmission, thus improving detection accuracy in practical use.

[0049] According to embodiments of this application, the upper and lower flexible substrates are thermoplastic polyurethane (TPU) films. Thermoplastic polyurethane films possess excellent flexibility, stretchability, and conformability, allowing them to adhere tightly to flexible curved surfaces such as human skin without hindering deformation transmission, while providing reliable encapsulation and protection for the internal sensing layer. The sensing layer is a conductive composite material comprising carbon black (CB) and thermoplastic polyurethane. TPU ensures structural flexibility and stretchability, while carbon black provides a stable conductive path, allowing the sensor to maintain a sensitive and stable resistance response even under large deformations. The metal electrode is made of any one of copper, gold, or silver. These materials have high conductivity, low contact resistance, and good chemical stability, enabling them to bond firmly with the sensing layer, effectively transmit electrical signals, and improve detection accuracy.

[0050] According to the embodiments of this application, the mass percentage of carbon black and thermoplastic polyurethane is 85~90:15~10. For example, the mass percentage of carbon black and thermoplastic polyurethane can be 85:15, 88:15, 88:12, 90:15, 90:10, etc. By adjusting the ratio of the two, the feasibility of the preparation process and the reliability of the device structure can be taken into account while ensuring the sensing performance of the sensing layer. The particle size of carbon black is less than 30μm.

[0051] Carbon black, as a conductive filler, needs to be controlled at a level near the percolation threshold of the conductive composite material to ensure both conductivity and processing characteristics. When the carbon black content is below 10 wt%, it is difficult to form a continuous and stable conductive network in the TPU matrix, resulting in high and fluctuating resistivity of the conductive composite material. This reduces the sensitivity and repeatability of the sensing layer, failing to meet the requirements for flexible sensors. When the carbon black content is above 15 wt%, it reduces the melt flowability of the conductive composite material, leading to problems such as nozzle clogging and uneven filament output during 3D printing of the sensing layer. It also reduces the flexibility and mechanical properties of the composite conductive material, affecting the adhesion and lifespan of the sensor.

[0052] According to the embodiments of this application, the preferred mass percentage of carbon black and thermoplastic polyurethane is 88:12. When the carbon black content in the composite conductive material is around 12wt%, good conductivity, melt processability and mechanical flexibility can be achieved simultaneously, which is the optimal ratio.

[0053] According to embodiments of this application, the performance of the flexible strain sensor satisfies at least one of the following conditions.

[0054] (1) Within the strain detection range of 5% to 30%, the sensitivity (GF) of the flexible strain sensor is ≥3.84 and the linearity error is ≤4.65%.

[0055] It can maintain good detection accuracy under a wide range of deformation, which not only solves the problem of the difficulty in balancing the sensitivity and detection range of traditional flexible sensors, but also ensures that the resistance signal changes uniformly and stably with strain, thus greatly improving the measurement accuracy and data reliability in actual monitoring.

[0056] (2) The response time of the flexible strain sensor is ≤0.34s.

[0057] The short response time enables flexible strain sensors to capture rapid deformation signals in real time without significant lag, meeting the requirements for dynamic real-time monitoring.

[0058] (3) After 300 load-unload cycle tests, the repeatability error of the flexible strain sensor is less than 5%.

[0059] This indicates that the flexible strain sensor of this application has a highly stable signal output during long-term repeated stretching and deformation. The conductive network and structure are not prone to fatigue, drift or failure, and have excellent durability and reliability, which can meet the practical application requirements of long-term, high-frequency dynamic monitoring.

[0060] According to another embodiment of this application, a method for fabricating the above-mentioned flexible strain sensor is provided, comprising: pre-pressing and fixing metal electrodes to both ends of a sensing layer having a negative Poisson's ratio structure, and encapsulating the sensing layer with pre-pressed metal electrodes between an upper flexible substrate and a lower flexible substrate by a hot pressing process to obtain a flexible strain sensor, wherein the temperature of the hot pressing process is 135~150℃, the pressure is 600~750Pa, and the time is 20~22s.

[0061] According to embodiments of this application, the temperature of the hot-pressing process can be, for example, 135°C, 140°C, 145°C, 150°C, etc.; the pressure can be, for example, 600 Pa, 650 Pa, 700 Pa, 750 Pa, etc.; and the time can be, for example, 20 s, 21 s, 22 s, etc. The above-mentioned hot-pressing process parameters allow the upper and lower flexible substrates to melt appropriately, which can ensure that the upper and lower flexible substrates are tightly bonded to the sensing layer and electrodes, without causing structural deformation or electrode damage due to excessive temperature or pressure. Stable encapsulation can be completed in a short time (20~22s), ensuring device consistency and structural reliability.

[0062] According to the preparation method of this application, a simple process of electrode pre-pressing and overall hot-pressing encapsulation is adopted, which does not require complex micro-nano processing, has low equipment requirements, and is controllable in operation. It can effectively reduce production costs and improve preparation efficiency, while better protecting the negative Poisson's ratio structure of the sensing layer, ensuring stable performance of batch products, and is suitable for large-scale production.

[0063] According to an embodiment of this application, the sensing layer is prepared by fused deposition modeling (FDM): a conductive composite material containing carbon black and thermoplastic polyurethane is printed to obtain a sensing layer with a negative Poisson's ratio structure; wherein, the printing process parameters include: nozzle temperature of 230~250℃, printing platform temperature of 50~55℃, and printing speed of 10~20mm / s.

[0064] According to embodiments of this application, the nozzle temperature can be, for example, 230°C, 235°C, 240°C, 245°C, 250°C, etc., the printing platform temperature can be, for example, 50°C, 51°C, 52°C, 53°C, 54°C, 55°C, etc., and the printing speed can be, for example, 10mm / s, 12mm / s, 15mm / s, 18mm / s, 20mm / s, etc. This fused deposition modeling process and its supporting parameters can ensure that the conductive composite material containing carbon black and thermoplastic polyurethane is fully melted and smoothly extruded, which can accurately print a complete and regular negative Poisson's ratio structure, avoiding problems such as nozzle blockage, filament breakage, and deformation, and also ensure that the conductive network inside the sensing layer is uniform and stable.

[0065] According to the embodiments of this application, the upper and lower flexible substrates can be prepared by a coating film-forming method. The viscosity of the water-based thermoplastic polyurethane (TPU) adhesive is adjusted to 10~15 Pa·s, and after vacuum degassing, it is coated onto release paper and cured at room temperature to form a thermoplastic polyurethane film with a thickness of 50-200 μm.

[0066] According to an embodiment of this application, the preparation method of the conductive composite material includes: mixing carbon black and thermoplastic polyurethane in a mass percentage to obtain a mixed powder; repeatedly melting and granulating the mixed powder three or more times at a melting temperature of 180°C to 200°C and an extrusion pressure of 100 to 120 MPa to obtain composite particles; and extruding the composite particles at a heating temperature of 180°C to 200°C and a screw speed of 30 to 50 rpm to obtain the conductive composite material.

[0067] According to the embodiments of this application, conductive composite filaments with a diameter of 1.75±0.05mm can be produced after extrusion molding.

[0068] According to the embodiments of this application, the preparation method, through melting, extrusion, and multiple granulation processes, enables carbon black to be more uniformly dispersed and more tightly bound in the thermoplastic polyurethane matrix, effectively avoiding the agglomeration of conductive fillers and ensuring the continuous and stable conductive network of the conductive composite material. At the same time, by controlling the melting temperature, extrusion pressure, and screw speed, the conductive composite material can possess both good melt flowability and mechanical flexibility, adapting to the subsequent melt deposition molding process for preparing the sensing layer, ensuring uniform and consistent performance of the sensing layer, and providing a stable and reliable material basis for the sensor.

[0069] According to embodiments of this application, pre-pressing and fixing the metal electrodes to both ends of a sensing layer with a negative Poisson's ratio structure includes: bending the metal electrodes into an irregular shape, and then fixing the bent metal electrodes to both ends of the sensing layer using a local hot-pressing process. Bending the metal electrodes into an irregular shape can increase the contact area and bonding force between the metal electrodes and the sensing layer, effectively preventing the metal electrodes from falling off, loosening, or making poor contact during repeated stretching and deformation. For example, the metal electrodes can be bent into an S-shape.

[0070] The local hot-pressing process involves a temperature of 135℃~150℃, a pressure of 700~800Pa, and a time of 20~22s. This range of local hot-pressing parameters allows for moderate melting and bonding at the interface between the metal electrode and the sensing layer. This ensures the metal electrode is firmly embedded and the contact resistance is stable, while preventing deformation of the negative Poisson's ratio microstructure or damage to the metal electrode due to excessive temperature or pressure, thus achieving reliable fixation of the metal electrode.

[0071] According to another embodiment of this application, an application of the above-described flexible strain sensor in human joint motion monitoring is provided.

[0072] According to the embodiments of this application, the flexible strain sensor of this application has good flexibility, fit and stable sensing performance. It can be closely attached to the surface of human skin and can monitor the movement state of human joints in real time and non-invasively. It has fast signal response and strong cycle stability. It can be widely used for dynamic motion detection of joints such as fingers, wrists, elbows and knees, and provides efficient and reliable sensing support for wearable health monitoring, sports rehabilitation and intelligent interaction scenarios. Its practicality and adaptability are outstanding.

[0073] According to embodiments of this application, the application of the flexible strain sensor in human joint motion monitoring can specifically be a non-invasive monitoring of the flexion angle of the proximal interphalangeal joint (PIP) of the finger. In use, the flexible strain sensor of this application is attached to the skin surface on the dorsal side of the PIP, with the sensor's main conductive direction along the finger axis. During joint flexion, the sensor can synchronously generate a stable resistance change signal. By acquiring this signal and processing the data, a cubic polynomial mapping model between the relative rate of change of resistance ((R0-R) / R0) and the joint flexion angle can be established. The goodness of fit of this model is R... 2 The value is not less than 0.999, thus enabling quantitative inversion of the joint bending angle.

[0074] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with specific embodiments and accompanying drawings. Unless otherwise specified, specific techniques or conditions in the embodiments are conventional methods, which can be performed according to the techniques or conditions described in the literature in this field or according to the product instructions. It should be noted that, unless otherwise specified, the methods provided in this application are conventional methods, and the reactants and reagents can be obtained from publicly available commercial sources unless otherwise specified.

[0075] Example 1:

[0076] This embodiment provides a flexible strain sensor.

[0077] Figure 2 This is a flowchart of the fabrication method of the flexible strain sensor in Embodiment 1 of this application.

[0078] like Figure 2 As shown, the fabrication method of this flexible strain sensor is as follows.

[0079] 1. Preparation of thermoplastic polyurethane film

[0080] Figure 3 This is a schematic diagram of the preparation of the thermoplastic polyurethane substrate film in Example 1 of this application. Wherein (a) is a schematic diagram of the preparation process of the thermoplastic polyurethane flexible film; (b) is a process flow diagram of the preparation of the thermoplastic polyurethane flexible film.

[0081] like Figure 3 As shown, a milky white water-based modified thermoplastic polyurethane (viscosity 15 Pa·s, solid content 50%) was filtered through a 0.45 μm filter membrane, and 1.5 wt% deionized water was added to adjust the viscosity to 12 Pa·s. Vacuum degassing was performed at -0.08 MPa for 8 min to obtain a water-based modified thermoplastic polyurethane adhesive. A 150 mm × 150 mm release paper was wiped with deionized water, dried with a lint-free cloth, and allowed to stand for 5 min. The solution was then degassed under vacuum at -0.08 MPa for 80 mm·s. -1 The water-based modified thermoplastic polyurethane adhesive was unidirectionally coated onto the release paper at a speed of [unclear]. The wet film thickness obtained by coating was 200 μm. The wet film was left to level horizontally for 4 min and cured at room temperature of 25°C for 3.5 h. It was then slowly peeled off along the edge to obtain a thermoplastic polyurethane (TPU) base film with a thickness of 100 μm (Young's modulus 0.8 MPa, ultimate tensile strength 1.15 MPa).

[0082] 2. Preparation of conductive composite materials

[0083] Figure 4 This is a schematic diagram of the preparation of the conductive composite material filament in Example 1 of this application.

[0084] like Figure 4As shown, thermoplastic polyurethane (TPU) raw material powder (density 1.33 g / cm³) is used. 3 TPU and conductive carbon black (CBPs) particles (particle size less than 30 μm) were weighed at a mass ratio of 88:12 and then mixed in a planetary mixer for 72 h to obtain a mixed powder. The mixed powder was added to a twin-screw granulator and melted and granulated three or more times at a melting temperature of 190℃ and an extrusion pressure of 110 MPa to obtain TPU / CBPs composite particles. The composite particles were fed into a wire extruder and extruded at a melting temperature of 190℃, a screw speed of 40 rpm, and an extrusion rate of 0.9 kg / h. After cooling in water at 25℃ and traction winding, TPU / CBPs conductive composite filaments with a diameter of 1.75 mm (conductivity up to 7.70 S / m) were obtained.

[0085] 3. Fabrication of the sensing layer

[0086] Drawing a negative Poisson's ratio structure S in mechanical design software 31 This negative Poisson's ratio structure consists of two parallel S-shaped arc units of identical structure and dimensions. The two S-shaped arc units are arranged in a mirror-symmetrical manner with a horizontal straight line parallel to their own length as their axis of symmetry. The two ends of the two S-shaped arc units are connected respectively. Each S-shaped arc unit is composed of continuously alternating convex and concave arc segments connected end-to-end. The ratio of the radius (R1) of the convex arc to the radius (R2) of the concave arc is 3:1. Specific parameters are R1 = 5.4 mm, R2 = 1.8 mm, lateral length 30 mm, and arc width 1.5 mm. The mechanical design file is then exported.

[0087] Install a 0.2 mm nozzle on the Bambu X1E printer and level the polyethyleneimine substrate to achieve a nozzle spacing of 0.1 mm. Set the printing parameters as follows: filament diameter 1.75 mm, nozzle temperature 248℃, platform temperature 55℃, printing speed 20 mm / s, layer height 80 μm, infill density 100%, fan speed 70%, and raster angle 45°. After importing the mechanical design file slices, start printing. After printing, cool to 25℃ and hold at that temperature for 10 minutes to obtain the sensing layer.

[0088] Figure 5 This is a schematic diagram of the printing device used in Embodiment 1 of this application and a diagram of the printed product. (a) is a schematic diagram of the Bambu X1E printer; (b) is a diagram of the printed sensor layer.

[0089] like Figure 5 As shown, the printed sensing layer has a thickness of approximately 0.2 mm and a smooth, defect-free surface.

[0090] 4. Hot-press packaging

[0091] Figure 6 This is a flowchart of the thermo-press packaging process for the flexible strain sensor in Embodiment 1 of this application.

[0092] like Figure 6 As shown, (1) a copper wire with a diameter of 50 μm and a length of 10 mm was cut as a metal electrode, and the electrode area of ​​the sensing layer connected to the electrode was placed on the polyethylene terephthalate (PET) release paper of the acrylic plate with the electrode area facing upward; (2) the copper wire was aligned with the electrode area and fixed with polyimide (PI) tape; (3) the point was hot-pressed with a 200℃ hot iron for 8 s; (4) a TPU film was covered on one side of the sensing layer, and hot-pressed at 140℃ and 750 Pa for 20 s to complete the single-sided encapsulation; (5) after flipping, the other side was covered with a TPU film and hot-pressed with the same parameters; (6) the sensor was left to stand at room temperature for 10 minutes to cool; (7) after peeling off the release paper, it was trimmed to a size of 30 mm × 18 mm × 0.3 mm; (8) a flexible strain sensor was obtained.

[0093] Figure 7 This is a finished product image of the flexible strain sensor in Embodiment 1 of this application. Among them, (a) is a schematic diagram of the flexible strain sensor being trimmed; (b) is a finished product image of the flexible strain sensor after thermo-press packaging; and (c) is a diagram of the flexible strain sensor in bending / torsion state.

[0094] like Figure 7 As shown, the flexible strain sensor of this application can be cut to fit the size requirements of different application scenarios; after thermo-press packaging, the TPU substrate completely covers the sensing layer of the flexible strain sensor, the metal electrodes are firmly connected, and the device has good consistency; it still maintains structural integrity under large deformation and no delamination occurs, which provides structural protection for its dynamic monitoring such as human joint movement.

[0095] The performance of the flexible strain sensor in Embodiment 1 of this application was tested. The test items, methods and results are as follows.

[0096] (1) Sensitivity test: 0~30% strain was applied using a Zwick / RoellzwickiLine universal testing machine, and the resistance change was detected using a Crenova MS8233D digital multimeter.

[0097] Figure 8 This is the sensitivity calibration curve of the flexible strain sensor in Embodiment 1 of this application.

[0098] like Figure 8 As shown, the sensitivity (GF) of the flexible strain sensor is 4.014 within the 5-15% strain range.

[0099] (2) Stability test: Under 30% strain conditions, the resistance of the flexible strain sensor fluctuates by less than 5% after 300 cycles of stretching, and there is no delamination between the layers of the sensor.

[0100] (3) Dynamic response test: To further evaluate the performance of the sensor under dynamic loading, periodic tensile tests were conducted, and its response under dynamic loading was recorded. The response time of the flexible strain sensor was measured to be 0.34 seconds, and the hysteresis parameter was 12.34%.

[0101] (4) Environmental adaptability test: The resistance of the flexible strain sensor changes steadily within the temperature range of 20~62℃; the relative resistance of the flexible strain sensor changes only 0.06% in the environment with humidity of 40%~100%, and the environmental stability is good.

[0102] (5) Repeatability test: Within the strain detection range of 0~30%, the same sensor is subjected to five load-unload cycle tests (Test1~Test5) under the same loading conditions to test its repeatability and verify the repeatability of the sensor.

[0103] Figure 9 This is a graph showing the repeatability test results of the flexible strain sensor in Embodiment 1 of this application during five loading-unloading cycles.

[0104] like Figure 9 As shown, within the strain detection range of 0-30%, the response curves of the relative change rate of resistance with strain change in the five independent tests are basically consistent, and the test data deviation at each strain node is small, demonstrating good repeatability, stability, and reliability.

[0105] The flexible strain sensor in Embodiment 1 of this application is applied to finger joint bending monitoring, and the specific application method is as follows.

[0106] A flexible strain sensor was attached to the dorsal side of the proximal interphalangeal joint (PIP) of the left index finger of the subject using a polyurethane (PU) film, with the conductive direction along the finger axis. The joint was then subjected to bending movements of 50°, 60°, 70°, 80°, 90°, 100°, and 110° sequentially, and the resistance was recorded in real time using an LCR meter. The relative rate of change of resistance was fitted to the bending angle using a cubic polynomial, and the goodness of fit R0 was determined. 2 =0.99946, thus reversing the joint bending angle.

[0107] Figure 10This diagram illustrates the flexible strain sensor attached to the proximal interphalangeal joint of the finger in Embodiment 1 of this application for detecting bending angles, and shows the resistance response curves at different bending angles. Specifically, (a) shows the resistance change rate curves when the finger sequentially completes 25 cycles of bending motions at 50°, 60°, 70°, 80°, 90°, 100°, and 110° along the illustrated direction; (b) shows the real-time resistance change rate curve of the flexible strain sensor when the index finger is freely bent at any angle; and (c) shows the characteristic resistance change rate signal output by the sensor when the finger is in four typical gripping states: fully extended, naturally relaxed, attempting to grasp, and tightly gripping.

[0108] like Figure 10 As shown, in (a), during the stepped bending cycle test from 50° to 110°, the relative rate of change of resistance (ΔR / R0) increases regularly and stepwise with the increase of the joint bending angle. The signal differentiation at different angles is high and the repeatability is good, which can clearly reflect the quantitative bending state of the joint. In (b), when the finger is freely bent at any angle, the flexible strain sensor can capture the continuous resistance change in the dynamic bending process in real time. The signal response is sensitive and there is no obvious lag, which can accurately track the real-time motion state of the joint. In (c), under four typical gripping states of fully extended (0°), naturally relaxed (≈70°), attempting to grasp (≈90°), and tightly gripped (≈105°), the flexible strain sensor outputs characteristic and distinguishable resistance change signals, which can more intuitively and accurately indicate different degrees of bending of the joint, verifying its application potential in human finger joint motion monitoring, gesture recognition, rehabilitation training and other scenarios.

[0109] Figure 11 This is a fitted curve showing the mapping relationship between the flexible strain sensor and the bending angle of the proximal interphalangeal joint of the finger in Embodiment 1 of this application.

[0110] like Figure 11 As shown, experiments were conducted to test the resistance changes at different bending angles (50°, 60°, 70°, 80°, 90°, 100°, 110°), and the experimental data were fitted and analyzed. The fitting results revealed a highly cubic linear relationship between the resistance change of the flexible strain sensor and the bending angle of the PIP joint. This indicates that cubic polynomial fitting can better capture the relationship between resistance change and joint angle, especially at larger angle changes, where the resistance change curve exhibits obvious nonlinear characteristics, and the trend of the fitted resistance signal is smoother and more stable. The resistance change of the flexible strain sensor increases (negatively) with increasing bending angle, indicating that the flexible strain sensor of this application has the ability to detect and quantify applied strain.

[0111] Example 2

[0112] This embodiment provides a flexible strain sensor. The difference between this flexible strain sensor and Embodiment 1 is that the sensor layer contains a negative Poisson's ratio structure S. 13 The negative Poisson's ratio structure S 13 The ratio of the radius (R1) of the convex arc to the radius (R2) of the concave arc is 1:3.

[0113] Example 3

[0114] This embodiment provides a flexible strain sensor. The difference between this flexible strain sensor and Embodiment 1 is that the sensor layer contains a negative Poisson's ratio structure S. 12 The negative Poisson's ratio structure S 12 The ratio of the radius (R1) of the convex arc to the radius (R2) of the concave arc is 1:2.

[0115] Example 4

[0116] This embodiment provides a flexible strain sensor. The difference between this flexible strain sensor and Embodiment 1 is that the sensor layer contains a negative Poisson's ratio structure S. 11 The negative Poisson's ratio structure S 11 The ratio of the radius (R1) of the convex arc to the radius (R2) of the concave arc is 1:1.

[0117] Example 5

[0118] This embodiment provides a flexible strain sensor. The difference between this flexible strain sensor and Embodiment 1 is that the sensor layer contains a negative Poisson's ratio structure S. 21 The negative Poisson's ratio structure S 21 The ratio of the radius (R1) of the convex arc to the radius (R2) of the concave arc is 2:1.

[0119] The performance of the flexible strain sensors in Examples 1 to 5 was tested.

[0120] Figure 12 These are schematic diagrams of the sensing layer structure of the flexible strain sensors in Embodiments 1 to 5 of this application, and their stress cloud diagrams obtained through finite element analysis at 30% tensile strain. Specifically, (a) is a schematic diagram of the sensing layer with different negative Poisson's ratio structures; (b) is a stress cloud diagram obtained through finite element analysis of the sensing layer with different negative Poisson's ratio structures at 30% strain; (c) is a curve showing the relationship between X-direction strain and Y-direction strain for the sensing layer with different negative Poisson's ratio structures; and (d) is a curve showing the relationship between X-direction strain and stress for the sensing layer with different negative Poisson's ratio structures.

[0121] like Figure 12As shown in Figure (b), the finite element analysis stress cloud diagram results of sensing layers with different negative Poisson's ratio structures under 30% strain are displayed. From the local distribution of each negative Poisson's ratio structure, it can be seen that stress concentration mainly occurs in the concave areas and joints of the structure. Under a unified stress scale, the smaller the proportion of the red high-stress area, the lower the failure risk of the structure. Figure (c) shows the X-strain and Y-strain relationship curves of different negative Poisson's ratio structures. The results show that all negative Poisson's ratio structures exhibit typical negative Poisson's ratio behavior (synchronous expansion in the Y direction when stretched in the X direction) across the entire strain range. Among them, the negative Poisson's ratio structure S in Example 1... 31 Under the same X-direction strain, the Y-direction strain is the largest, the negative Poisson's ratio effect is the most significant, and the tensile expansion characteristics are optimal. The X-direction strain-stress relationship curves of each negative Poisson's ratio structure in (d) reflect the mechanical load-bearing and stress concentration characteristics of the negative Poisson's ratio structure. As the size of the concave arc R2 decreases and the ratio of the convex arc R1 to the concave arc R2 increases, the stress concentration of the negative Poisson's ratio structure gradually intensifies; the negative Poisson's ratio structure S in Example 1... 31 The maximum stress at 30% strain is 180.86 MPa, which is lower than the allowable stress of the CB / TPU composite material, meeting the structural safety requirements. Although the negative Poisson's ratio structure S... 31 The peak stress of this structure is relatively higher than that of other structures, but its overall stress distribution is uniform with no extreme local overloads, and its negative Poisson's ratio effect is far superior to that of other structures. This can significantly improve the strain sensitivity and detection range of the sensor. Therefore, this application ultimately selected the negative Poisson's ratio structure S. 31 The optimal configuration for the sensing layer.

[0122] Comparative Example 1

[0123] This comparative example provides a flexible strain sensor. The difference between this flexible strain sensor and Example 1 is that the sensor layer has a honeycomb negative Poisson's ratio structure. This honeycomb negative Poisson's ratio structure is a cell structure arranged with concave hexagons. The sensor layer has a length of 30 mm and a thickness of 200 μm.

[0124] Comparative Example 2

[0125] This comparative example provides a flexible strain sensor. The difference between this flexible strain sensor and Example 1 is that the sensor layer has an arrow-shaped negative Poisson's ratio structure, which is formed by periodically arranging arrow-shaped hinged units. The sensor layer has a length of 30 mm and a thickness of 200 μm.

[0126] Comparative Example 3

[0127] This comparative example provides a flexible strain sensor. The difference between this flexible strain sensor and Example 1 is that the sensor layer has a perforated plate type negative Poisson's ratio structure. This perforated plate type negative Poisson's ratio structure is an elliptical and / or elongated hole structure arranged at a specific angle. The sensor layer has a length of 30 mm and a thickness of 200 μm.

[0128] Comparative Example 4

[0129] This comparative example provides a flexible strain sensor. The difference between this flexible strain sensor and Example 1 is that the sensor layer has a double-star negative Poisson's ratio structure, which is formed by two centrally symmetrical star-shaped rigid units connected by a hinge point. The sensor layer has a length of 30 mm and a thickness of 200 μm.

[0130] Comparative Example 5

[0131] This comparative example provides a flexible strain sensor. The difference between this flexible strain sensor and Example 1 is that the sensor layer has a non-tensile structure, the sensor layer length is 30 mm, and the thickness is 200 μm.

[0132] Simulation analysis was performed on sensing layers with different negative Poisson's ratio structures in Example 1 and Comparative Examples 1-5. The tensile mechanical response and deformation characteristics of the negative Poisson's ratio structure were obtained by linear static analysis with a sliding constraint at one end and a strain condition of 0-30% applied at the other end, and the mesh size was set to 0.1 mm.

[0133] Figure 13 These are schematic diagrams of sensing layers with different negative Poisson's ratio structures in Embodiment 1 and Comparative Examples 1-4 of this application. Specifically, (a) shows the honeycomb negative Poisson's ratio structure in Comparative Example 1; (b) shows the arrow-shaped negative Poisson's ratio structure in Comparative Example 2; (c) shows the arc-shaped S-shaped negative Poisson's ratio structure in Embodiment 1; (d) shows the perforated plate negative Poisson's ratio structure in Comparative Example 3; and (e) shows the double-star negative Poisson's ratio structure in Comparative Example 4.

[0134] Figure 14 The static simulation results are for different negative Poisson's ratio structures in Examples 1 and 5 of this application. Among them, (a) is the Poisson's ratio-displacement curve of different negative Poisson's ratio structures in Examples 1 and 5; (b) is the stress-strain curve of different negative Poisson's ratio structures in Examples 1 and 4; and (c) is the strain energy-strain curve of different negative Poisson's ratio structures in Examples 1 and 4.

[0135] like Figure 13 and Figure 14As shown, the Poisson's ratio of the non-tensile control group remains positive throughout; all structures with negative Poisson's ratio exhibit a negative Poisson's ratio across the entire displacement range, but it increases slightly with increasing displacement. Figure 14 As shown in (a), for the same size, the honeycomb and perforated plate negative Poisson's ratio structures have the smallest Poisson's ratio and the most significant tensile expansion characteristics. Among them, the Poisson's ratio of the perforated plate negative Poisson's ratio structure is basically stable at around -0.6 with small fluctuations, and the structure has good stability, which can meet the requirements of uniform deformation in wearable applications of flexible sensors. The initial Poisson's ratio of the honeycomb negative Poisson's ratio structure is about -0.8, but the change range with displacement is large, and the stability is insufficient. The Poisson's ratio of the arrow-shaped negative Poisson's ratio structure and the double star structure are close to 0 or tend to be positive, and the negative Poisson's ratio effect is weak, which is not suitable as a sensor layer configuration guided by structural design. The Poisson's ratio of the negative Poisson's ratio structure of this application is in a reasonable range, and it has both a good negative Poisson's ratio effect and structural stability, providing a structural basis for the high sensitivity sensing of the sensor.

[0136] Besides Poisson's ratio, stress level is also a critical factor. Excessive stress during sensor structure deformation can lead to premature material failure or permanent deformation, affecting its lifespan. Therefore, the selected structure needs to undergo significant deformation at relatively low stress levels to ensure flexibility and durability. Figure 14 As shown in (b), the maximum stress of the negative Poisson's ratio structure in this application is lower than that of other structures, and the stress concentration is less. It can maintain a lower stress level under higher deformation requirements, and has good flexibility and durability, making it suitable for large strain dynamic monitoring scenarios. Compared with other structures, the perforated plate type negative Poisson's ratio structure has a relatively moderate stress level and a relatively gentle trend, indicating that it has good mechanical stability under different strain conditions, but its flexibility is weaker than that of the negative Poisson's ratio structure in this application. The stress level of the honeycomb type negative Poisson's ratio structure increases exponentially, and the stress under large strain far exceeds the material bearing limit, which can easily lead to structural tearing and fatigue failure, making it unsuitable for flexible applications. Although the arrow-shaped and double-star type negative Poisson's ratio structures have low stress levels, their negative Poisson's ratio characteristics are insufficient when combined with Poisson's ratio analysis.

[0137] Furthermore, the control of strain energy is also crucial. A reasonable strain energy distribution can ensure that the sensor absorbs energy uniformly under different tensile conditions, avoiding local stress concentration, thereby guaranteeing the stability and repeatability of the measurement. Figure 14As shown in (c), the strain energy of the negative Poisson's ratio structure in this application is relatively low, meaning that the structure absorbs less energy under stress, which can effectively reduce the possibility of structural failure. The strain energy of the perforated plate type negative Poisson's ratio structure is slightly higher than that of the negative Poisson's ratio structure in this application, but it is still within a reasonable range, although its energy absorption characteristics are weaker than those of the negative Poisson's ratio structure in this application. The honeycomb type and arrow type negative Poisson's ratio structures have the highest strain energy, which will accumulate a large amount of energy during deformation, significantly increasing the possibility of structural failure and making them unsuitable for flexible applications. Although the strain energy of the binary star type negative Poisson's ratio structure is low, its negative Poisson's ratio characteristics are insufficient and cannot meet the sensing requirements.

[0138] Based on the above analysis, the negative Poisson's ratio structure of this application has better structural flexibility and scalability, and can take into account flexible deformation, stability and energy absorption performance, providing better comprehensive mechanical properties in practical applications.

[0139] In summary, the flexible strain sensor, its fabrication method, and its application presented in this application have at least the following beneficial effects.

[0140] 1. This application effectively alleviates the technical bottleneck of the mutual constraint between sensitivity and detection range in traditional flexible strain sensors by designing a sensing layer with a specific negative Poisson's ratio structure, thereby improving sensing performance. It can generate a unique mechanical response of lateral expansion under axial tension, and enhance the actual deformation of the sensing material through the structural mechanical amplification effect. This allows the sensor to maintain a sensitivity (GF) of no less than 3.84 and a linearity error of no more than 4.65% within a relatively wide strain detection range of 5% to 30%, effectively balancing the dual requirements of high sensitivity and wide range. This improves the performance imbalance problem of existing microstructure sensors. At the same time, the optimized structure has controllable stress concentration, and combined with CB / TPU conductive composite material, it significantly reduces the mechanical hysteresis effect. After 300 load-unload cycle tests, the performance is stable with a repeatability error of less than 5%, and the structural stability is superior to traditional cracked and porous sensors.

[0141] 2. This application employs a simple, highly controllable, and low-cost fabrication system that balances production efficiency and batch consistency, providing a feasible path for the large-scale industrialization of flexible strain sensors. The flexible substrate can be prepared using a blade coating method, a simple and easy-to-operate process. The conductive composite material undergoes a melt-blending and multiple granulation process to improve the dispersion uniformity of TPU and carbon black particles, followed by extrusion molding to obtain composite filaments of uniform specifications. The sensing layer is formed using mature FDM 3D printing technology, offering highly controllable process parameters and accurately replicating a preset negative Poisson's ratio structure, avoiding the high cost and low efficiency issues associated with complex micro-nano fabrication. The encapsulation process utilizes a thermoforming process for integrated molding, coupled with a local pre-pressing design for the metal electrodes, ensuring a reliable connection between the metal electrodes and the sensing layer, improving device consistency, and effectively solving the problem of large performance differences in batch production using existing processes.

[0142] 3. The flexible strain sensor prepared in this application has strong practicality and broad application prospects, especially suitable for the field of human joint motion monitoring. The sensor adopts a "sandwich" structure including an upper flexible substrate, a sensing layer with a negative Poisson's ratio structure, and a lower flexible substrate. The upper and lower flexible substrates give the device flexibility and skin-adhesiveness, allowing it to adhere closely to the human skin surface and achieve non-invasive monitoring. When applied to the monitoring of the flexion angle of the proximal interphalangeal joint (PIP) of the finger, the goodness of fit R of the cubic polynomial mapping model established based on the resistance change signal is [value missing]. 2 With a strain gauge of at least 0.999, it can accurately capture dynamic joint deformation; the device response time is no more than 0.34 s, enabling real-time feedback of motion status. Compared with traditional metal and semiconductor sensors, the flexible strain sensor of this application exhibits superior flexibility, adapting to dynamic deformation of the human body, requiring no complex wearable devices, and is easy to use. This flexible strain sensor can not only be used for finger joint monitoring but can also be extended to joint motion monitoring in multiple locations such as the elbow and knee joints, providing a core sensing unit for cutting-edge fields such as wearable smart devices and dynamic human health monitoring, and powerfully promoting the practical application and implementation of flexible electronics technology.

[0143] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of this application. It should be understood that the above descriptions are merely specific embodiments of this application and are not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A flexible strain sensor, characterized in that, The flexible strain sensor comprises, from top to bottom: an upper flexible substrate, a sensing layer with a negative Poisson's ratio structure, and a lower flexible substrate. Metal electrodes are connected to both ends of the sensing layer for electrical connection. The negative Poisson's ratio structure is composed of two parallel S-shaped arc units of identical structure and size. The two S-shaped arc units are arranged in a mirror-symmetrical manner with a horizontal line parallel to their own length as their axis of symmetry, and their ends are connected. The S-shaped arc unit is composed of continuously alternating convex arc segments and concave arc segments connected end to end, and the ratio of the radii of the convex arc to the concave arc is 1:3 to 3:

1.

2. The flexible strain sensor according to claim 1, characterized in that, The thickness of the sensing layer is 100~200μm; The thickness of the upper flexible substrate and the lower flexible substrate is 50~200μm; The metal electrode has a diameter of 40~60μm.

3. The flexible strain sensor according to claim 1, characterized in that, The upper flexible substrate and the lower flexible substrate are thermoplastic polyurethane films; The material of the sensing layer is a conductive composite material containing carbon black and thermoplastic polyurethane. The metal electrode is made of any one of copper, gold, or silver.

4. The flexible strain sensor according to claim 3, characterized in that, The mass percentage of carbon black and thermoplastic polyurethane is 85~90:15~10, and the particle size of the carbon black is less than 30μm.

5. The flexible strain sensor according to any one of claims 1 to 4, characterized in that, The performance of the flexible strain sensor meets at least one of the following conditions: Within the strain detection range of 5% to 30%, the sensitivity of the flexible strain sensor is ≥3.84, and the linearity error is ≤4.65%. The response time of the flexible strain sensor is ≤0.34s; After 300 load-unload cycle tests, the repeatability error of the flexible strain sensor is less than 5%.

6. A method for fabricating a flexible strain sensor as described in any one of claims 1 to 5, characterized in that, The preparation method includes: A flexible strain sensor is fabricated by pre-pressing and fixing metal electrodes to both ends of a sensing layer with a negative Poisson's ratio structure, and then encapsulating the pre-pressed sensing layer with metal electrodes between an upper flexible substrate and a lower flexible substrate using a hot-pressing process. The hot pressing process is carried out at a temperature of 135~150℃, a pressure of 600~750Pa, and a time of 20~22s.

7. The preparation method according to claim 6, characterized in that, The sensing layer is prepared using a fused deposition modeling (FDM) process: a conductive composite material containing carbon black and thermoplastic polyurethane is printed to obtain the sensing layer with the negative Poisson's ratio structure; wherein, The printing process parameters include: nozzle temperature of 230~250℃, printing platform temperature of 50~55℃, and printing speed of 10~20mm / s.

8. The preparation method according to claim 7, characterized in that, The method for preparing the conductive composite material includes: Carbon black and thermoplastic polyurethane are mixed in a certain mass percentage to obtain a mixed powder; The mixed powder is repeatedly melted and granulated three or more times at a melting temperature of 180℃~200℃ and an extrusion pressure of 100~120MPa to obtain composite particles. The composite particles are extruded at a heating temperature of 180℃~200℃ and a screw speed of 30~50 rpm to obtain the conductive composite material.

9. The preparation method according to claim 6, characterized in that, The step of pre-pressing and fixing the metal electrodes to both ends of the sensing layer with a negative Poisson's ratio structure includes: The metal electrode is bent into an irregular shape, and then the bent metal electrode is fixed to both ends of the sensing layer by a local hot-pressing process; wherein, The local hot pressing process is carried out at a temperature of 135℃~150℃, a pressure of 700~800Pa, and a time of 20~22s.

10. The application of a flexible strain sensor according to any one of claims 1 to 5 in human joint motion monitoring.

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