A flexible composite stress sensing element and a preparation method and application thereof

Abstract

The application discloses a flexible composite stress sensing element and a preparation method and application thereof, and is a sandwich layered hydrogel composite structure, wherein pure nylon fabric is used as a framework, and conductive hydrogel is loaded; the conductive hydrogel is obtained by mixing and heating polymerization of A component and B component, the A component contains L-tryptophan and water, and the B component contains acrylamide, ammonium persulfate, N, N'-methylene bisacrylamide, potassium chloride and water; L-tryptophan and ammonium persulfate constitute a redox initiation system, L-tryptophan is combined at the end of a polyacrylamide chain, and adhesion to the skin is realized by means of hydrogen bond, hydrophobic and pi-pi interaction; potassium chloride provides ion conduction; the pure nylon fabric plays a mechanical reinforcing role in the hydrogel; the element has strong self-adhesion, high conductive stability, excellent mechanical properties and sensing sensitivity; after integration with a Bluetooth wireless transmission module, precise acquisition and real-time visual display of motion signals can be realized, and the long-term motion monitoring demand of the human body can be met.

Description

Technical Field

[0001] This invention belongs to the field of flexible electronic materials and wearable sensor technology, specifically relating to a flexible composite stress sensing element, its preparation method, and its application. Background Technology

[0002] In recent years, with the rapid development of flexible electronics technology and the demand for wireless health monitoring, conductive hydrogel-based skin interface materials have shown broad application prospects in wearable physiological signal monitoring devices. Conductive hydrogels are considered one of the most ideal skin-electronic interface materials due to their excellent biocompatibility, stretchability, and tissue-like mechanical properties.

[0003] Currently, the main methods for preparing conductive hydrogels include the following: First, the physical blending method based on conductive polymers (such as PEDOT:PSS, polyaniline) and polymeric materials (such as polyacrylamide, polyvinyl alcohol); second, the composite filling method that imparts conductivity to hydrogels by introducing metal nanoparticles, carbon-based materials (such as carbon nanotubes, graphene) or ionic salts (such as lithium chloride, sodium chloride); and third, the construction of dual-network or multi-network structures using in-situ polymerization or template methods to improve mechanical properties and conductive stability.

[0004] Despite significant progress in the laboratory-level preparation methods described above, the integration of conductive hydrogels as skin interface layers with Bluetooth wireless transmission modules into wearable physiological signal monitoring devices still faces the following key challenges: 1. The contradiction between interfacial adhesion and signal fidelity: Existing conductive hydrogels either lack sufficient adhesion, making them prone to detachment in dynamic motion scenarios; or while they have strong adhesion, their modulus does not match that of the skin, leading to signal distortion during deformation. Achieving a balance between high adhesion and high signal fidelity is a pressing technical challenge that needs to be addressed. 2. The stringent requirements for interface stability in Bluetooth wireless transmission are difficult to meet: Wireless transmission systems require sensing elements to have low resistance drift and stable signals during dynamic movement. Existing hydrogel sensors often exhibit resistance drift after long-term cyclic stress, making it difficult to meet the signal stability requirements of wireless transmission. 3. Poor compatibility between preparation methods and wearable integration processes: The preparation processes of many high-performance conductive hydrogels are complex and demanding, making them difficult to be compatible with the mass production and system integration of wearable devices; 4. Insufficient stability under long-term use: Hydrogel sensors with a surface conductive layer are prone to conductive layer peeling and cracking during repeated bending and stretching, resulting in signal attenuation. Summary of the Invention

[0005] Technical problems to be solved: In view of the technical problems existing in conductive hydrogels and flexible wearable sensors, such as poor skin adhesion, weak adhesion, insufficient interface stability, poor mechanical fatigue resistance, poor conductivity stability, low sensing accuracy, poor compatibility in fabrication and device integration, and unstable wireless signal transmission, this application provides a flexible composite stress sensing element, its fabrication method and application.

[0006] Purpose of the invention: To address the aforementioned technical problems, the purpose of this invention is to provide a flexible composite stress sensing element, its preparation method, and its application. This element possesses strong self-adhesion, high conductivity stability, excellent mechanical properties, and sensing sensitivity. The process is simple and easy to operate, enabling convenient wear, accurate acquisition of motion signals, real-time Bluetooth wireless transmission, and data visualization, thus meeting the needs of long-term human motion monitoring.

[0007] Technical Solution: A flexible composite stress sensing element, wherein the sensing element is a sandwich-like layered hydrogel composite structure, which is prepared by loading conductive hydrogel onto a composite fiber fabric as a skeleton; the conductive hydrogel is obtained by heating and polymerizing a mixture of component A and component B, wherein, by weight, component A consists of 0.025~0.035 parts L-tryptophan and 2.4~3.6 parts distilled water; and component B consists of 1.6~2.4 parts acrylamide, 0.018~0.022 parts ammonium persulfate, 0.018~0.022 parts N,N'-methylenebisacrylamide, 0.16~0.24 parts potassium chloride, and 3.2~4.8 parts distilled water.

[0008] Furthermore, the composite fiber fabric is made of pure nylon, which has high mechanical deformation properties that match the hydrogel.

[0009] Furthermore, the heating polymerization reaction process conditions are as follows: after mixing components A and B, the mixture is kept at 60~70℃ for 50~80 minutes to complete the molding.

[0010] A method for fabricating a flexible composite stress sensing element includes the following steps: The first step, pre-cutting: cut the composite fiber fabric into 95 mm × 95 mm sizes and lay it flat in a polytetrafluoroethylene mold; The second step is material preparation: Weigh all the raw materials of component A and component B separately, and stir them separately until completely dissolved; The third step is mixing and pouring: Component A and component B are thoroughly stirred and mixed to obtain a mixture. The mixture is then slowly poured into a mold with composite fiber fabric laid on it, so that the mixture completely soaks the composite fiber fabric. The fourth step is isothermal polymerization: the mold is transferred to a constant temperature chamber and kept at 60~70℃ for 50~80 minutes. After the reaction is completed, the flexible composite stress sensing element is obtained.

[0011] This application also discloses the application of a flexible composite stress sensing element in a flexible human motion sensor.

[0012] Furthermore, the flexible human motion sensor consists of three main parts: a flexible composite stress sensing element, an integrated circuit, and a terminal program.

[0013] Furthermore, the integrated circuit comprises an ESP32 chip, terminals, copper wires, a 3KΩ fixed resistor, and a power supply.

[0014] Furthermore, the integrated circuit adopts a voltage divider circuit design, is powered by 3.3V, and uses copper wires and terminals to connect the chip, fixed resistor, and sensing element, forming a complete signal acquisition circuit to realize the acquisition of stress sensing resistance value signal.

[0015] Furthermore, the terminal program consists of three parts: the ESP32 device program, the web page program, and the mobile program.

[0016] Furthermore, the flexible human motion sensor establishes wireless communication via Bluetooth Low Energy (BLE) to achieve real-time acquisition, transmission, calculation, and visualization of the resistance values ​​of the flexible composite stress sensing element.

[0017] Explanation of Principles: 1. Polymerization and Self-Adhesion Principle: L-tryptophan and ammonium persulfate form a redox initiation system, which initiates free radical polymerization of acrylamide under heating conditions; L-tryptophan is covalently bonded to the end of the polyacrylamide molecular chain, and through hydrogen bonds, hydrophobic interactions, and multiple non-covalent interactions such as π-π stacking, the hydrogel achieves reversible self-adhesion to the skin without the need for additional adhesives, avoiding the skin irritation and residue problems caused by traditional adhesives; 2. Ionic Conductivity Principle: Potassium chloride in the formula completely dissociates in aqueous solution, generating freely mobile K+ ions. + Cl -The conductive layer is uniformly distributed within the three-dimensional network of the hydrogel, forming an ion-conducting pathway. This conductive layer penetrates the entire structure, avoiding the problems of wear, cracking, and detachment common in traditional surface conductive layers, thus ensuring long-term conductive stability. 3. Mechanical reinforcement principle: Pure nylon fiber fabric acts as the skeleton, deforming flexibly with the hydrogel during low strain stages, without restricting wear flexibility. During high strain stages, the fiber skeleton shares the tensile load, preventing hydrogel breakage. Under cyclic stress, the fabric preferentially bears the stress, delaying hydrogel fatigue damage and improving cyclic stability and deformation recovery capability. 4. Signal acquisition and transmission principle: The sensing element of this invention can be integrated into a flexible human motion sensor, including an integrated circuit and a terminal program. The integrated circuit uses a voltage divider circuit, with the ESP32 chip acquiring the voltage divider analog voltage signal through the ADC module. Based on the voltage divider principle, the real-time resistance value of the sensing element is calculated, and data is transmitted externally via BLE Bluetooth at a fixed frequency. The terminal program consists of three parts: the ESP32 device program, the Web page program, and the mobile program. The Web page and mobile program receive data and plot real-time change curves, realizing wireless acquisition and data visualization of human motion signals.

[0018] Beneficial effects: 1. Excellent adhesion and bonding performance: The hydrogel has a high match with the Young's modulus of human skin, and the adhesion strength can reach 1147 Pa. In dynamic scenarios such as flexion and extension movements of human elbow joints, fingers, and throat vibrations, the sensing element can deform synchronously with the skin without detachment, lifting, or delamination. It adheres firmly and leaves no residue on the skin after peeling. This effect is attributed to the triple self-adhesion mechanism provided by L-tryptophan at the end of the polyacrylamide chain: hydrogen bonds, hydrophobic interactions, and π-π stacking. 2. Excellent mechanical and fatigue resistance properties: The hydrogel has a tensile strength of 33 kPa, an elongation at break of 60%, and a Young's modulus of approximately 55 kPa. After 200 cycles of tensile testing, the stress retention rate is still above 90%, and the hysteresis loop does not increase significantly. It has excellent fatigue resistance and long-term mechanical stability, which can meet the requirements of long-term continuous monitoring. This effect is attributed to the mechanical reinforcement mechanism of the pure nylon fabric skeleton. 3. Excellent conductivity and sensing properties: The hydrogel has a conductivity ≥0.78 S / m, and the internal ion conduction pathway is stable; the resistance and strain have a linear correlation coefficient R. 2 With a strain sensitivity of approximately 94.94% and a strain sensitivity GF of 2.75, the sensor exhibits excellent linearity and sensitivity, enabling it to accurately distinguish minute deformation signals such as different bending angles of joints and weak vocal vibrations of the throat. 4. Excellent process and integration effect: The preparation process is simple, the conditions are mild, the operation is convenient, the equipment requirements are low, and it is easy to mass produce; the sensor circuit structure is simple and the operation is stable. It relies on BLE Bluetooth to realize wireless transmission and is compatible with both mobile terminals and computer web terminals. The data is visualized in real time, truly realizing pick-up and use, wearable portability, and strong integration compatibility with wearable devices. 5. Excellent long-term stability: The hydrogel itself is conductive, and repeated bending and stretching will not cause the conductive layer to peel off or crack. There is no significant signal attenuation after long-term use. Attached Figure Description

[0019] Figure 1 This is a physical image of the flexible composite stress sensing element prepared according to Embodiment 1 of this application, wherein A is a cross-sectional view, B is a front top view, and C is a bottom surface top view. Figure 2 The figures show the tensile properties of the hydrogel membrane prepared in Example 1 of this application, where A is the stress-strain diagram of uniaxial tension; B is the cycle number-stress curve of 200 cycles of tension; and C is the stress-strain curve of 200 cycles of tension. Figure 3 This is a schematic diagram illustrating the application scenario of the hydrogel of this application attached to the human elbow joint, where A is the extended state diagram and B is the flexed state diagram; Figure 4 The graph shows the strain sensitivity test results of the hydrogel in this application. Figure 5 This is a schematic diagram of the bending test results of the hydrogel of this application, where A is the result of the finger bending test; B is the result of the elbow joint bending test; and C is the result of the throat vibration test. Figure 6 This is a schematic diagram of the circuit design for the flexible human motion sensor of this application; Figure 7 This is a multi-angle structural schematic diagram of the flexible human motion sensor of this application, wherein A is the front view of the sensor, B is the upper surface view of the sensor, C is the sensor chip welding schematic diagram, and D is the rear view of the sensor. Figure 8 This is a schematic diagram illustrating the wearing of the flexible human motion sensor of this application; Figure 9 This is a schematic diagram of the mobile terminal interface for data acquisition of the flexible human motion sensor of this application. The left image shows the mobile terminal data acquisition interface when the hydrogel is stretched, and the right image shows the mobile terminal data acquisition interface when it is static. Figure 10 This is a schematic diagram of the web interface for data acquisition of the flexible human motion sensor of this application. Detailed Implementation

[0020] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. The following embodiments are only for illustrating the invention and are intended to limit its scope. Unless otherwise specified, the experimental techniques and methods involved in the embodiments are common techniques in plant genetic engineering and molecular biology, and are performed under conventional conditions.

[0021] Explanation of English abbreviations / professional codes in this application: ESP32: A system-on-a-chip with integrated Wi-Fi and Bluetooth functionality, used for signal acquisition, processing, and wireless transmission; BLE: Bluetooth Low Energy, a short-range wireless communication technology used for data transmission between devices and terminals; ADC: Analog-to-Digital Converter, which converts analog voltage signals into digital signals.

[0022] Example 1: A method for preparing a flexible composite stress sensing element. 140 mg L-tryptophan and 12 mL distilled water are added to a beaker and stirred to form a solution to obtain component A. Then, 8 g acrylamide, 0.08 g ammonium persulfate, 0.08 g N,N'-methylenebisacrylamide, 0.8 g potassium chloride, and 16 mL distilled water are added to another beaker and stirred to obtain an aqueous solution to obtain component B.

[0023] The composite fiber fabric was cut into 95 mm × 95 mm pieces and laid in a polytetrafluoroethylene mold (100 mm long × 100 mm wide × 4 mm high). Components A and B were thoroughly mixed (stirring for 1 minute), and then slowly poured to impregnate the fabric. The remaining solution was then poured evenly into the mold. The mixture was then transferred to a 60°C incubator and kept at that temperature for 60 minutes to obtain a fabric-supported conductive hydrogel film, i.e., a flexible composite stress sensing element. This was sealed and stored for later use. Figure 1 As shown, the composite fiber fabric is located in the center of the hydrogel membrane, forming a sandwich-like layered composite structure. The surface of this composite structure in contact with the mold is flat, and the wrinkles on the upper surface give it superior adhesion properties. The adhesion strength was tested to be 1147 Pa.

[0024] Figure 2 The tensile properties test results are for the hydrogel membrane prepared in Example 1 of this invention. Figure 2 The tensile stress-strain curve of the hydrogel membrane shown in Figure A is illustrated. As can be seen from the figure, the hydrogel membrane of this invention has a tensile strength of 33 kPa, an elongation at break of 60%, and a Young's modulus of approximately 55 kPa, indicating that this hydrogel can be well applied to skin adhesion testing. Figure 2 China B and Figure 2 As described in section C, the cycle number-stress curve and stress-strain curve of the hydrogel cyclic tensile test are respectively. The test results show that the maximum stress of the hydrogel remains stable during 200 cycles of tensile testing, and the stress retention rate after the cycle is over 90%. The hysteresis loop area does not increase significantly, demonstrating excellent fatigue resistance and mechanical stability, which can meet the sensing requirements for several hours of use.

[0025] Comparative Example 1: Hydrogels were prepared using the method of Example 1, except that no composite fiber fabric was added.

[0026] In Comparative Example 2, component A (0.02 g ammonium persulfate and 3 mL distilled water) was added to a beaker and stirred to form a solution. Then, component B (2 g acrylamide, 0.02 g N,N'-methylenebisacrylamide, 0.2 g potassium chloride, 0.2 g N,N,N,N'-tetramethylethylenediamine and 4 mL distilled water) was added to another beaker and stirred until completely dissolved. Components A and B were then mixed and poured into a mold. Polymerization was completed in a 60°C incubator for 10 minutes.

[0027] Comparative Example 1, lacking a composite fiber fabric skeleton for support, exhibited poor mechanical properties and cyclic tensile stability; the sample crumbled directly upon placement in the instrument, making data unmeasurable. Comparative Example 2, employing N,N,N,N'-tetramethylethylenediamine and ammonium persulfate to form a redox initiation system, lacked L-tryptophan components, thus failing to rely on L-tryptophan to construct hydrogen bonds, hydrophobic interactions, and π-π stacking interactions; the resulting hydrogel exhibited almost no adhesion and fatigue resistance.

[0028] In Example 2, the weight parts of L-tryptophan in component A were adjusted to 0.020 parts, 0.025 parts, 0.030 parts, 0.035 parts and 0.040 parts respectively, and sensor element samples were prepared accordingly. The adhesion strength, breaking strength and conductivity were tested.

[0029] The test results show that when the L-tryptophan concentration is between 0.025 and 0.035 parts, the hydrogel has both high adhesive strength and mechanical properties; when the concentration is below 0.025 parts, the hydrogel polymerization is incomplete; and when the concentration is above 0.035 parts, L-tryptophan cannot be dissolved.

[0030] In Example 3, the reaction temperature and reaction time were adjusted, and the reaction conditions were set as follows: 60℃ / 50min, 60℃ / 80min, 65℃ / 65min, 70℃ / 50min, and 70℃ / 80min, respectively, to prepare sensor element samples and test their fracture strength and conductivity.

[0031] Test results show that qualified products with complete structure and uniform conductivity can be obtained under reaction conditions within the temperature range of 60~70℃ and the time range of 50~80min. When the temperature or time range is exceeded, for example, when the reaction is carried out at 55℃ or 80℃, performance degradation occurs due to incomplete polymerization or excessive moisture evaporation.

[0032] The feeding order of this invention is not particularly limited, and the mixing order of components A and B can be interchanged without affecting the polymerization reaction and the performance of the finished product. The reaction temperature and time can be adjusted arbitrarily within the range of 60~70℃ and 50~80 min, and qualified products can be prepared in any way.

[0033] Example 3: Application of flexible composite stress sensing element in flexible human motion sensor. The preparation method of the flexible composite stress sensing element includes the following steps: Step 1, pre-cutting: Cut the composite fiber fabric into 95 mm × 95 mm size and lay it flat inside the polytetrafluoroethylene mold (100 mm long × 100 mm wide × 4 mm high). Step 2, material preparation: Weigh component A separately: Add 0.03 parts L-tryptophan and 3 parts distilled water to a beaker and stir until completely dissolved; Add 2 parts acrylamide, 0.02 parts ammonium persulfate, 0.02 parts N,N'-methylenebisacrylamide, 0.2 parts potassium chloride and 4 parts distilled water to another beaker and stir until completely dissolved. The third step is mixing and pouring: Component A and component B are thoroughly stirred and mixed to obtain a mixture. The mixture is then poured drop by drop into a mold on which the composite fiber fabric is laid, so that the mixture completely soaks the composite fiber fabric. The fourth step is isothermal polymerization: the mold is transferred to a constant temperature chamber and kept at 60~70℃ for 50~80 minutes. After the reaction is completed, the flexible composite stress sensing element is obtained.

[0034] The flexible human motion sensor consists of an integrated circuit, a terminal program, and a flexible composite stress sensing element. The integrated circuit comprises an ESP32 chip, terminals, copper wires, a 3KΩ fixed resistor, and a power supply. The integrated circuit adopts a voltage divider circuit design and is powered by 3.3V. The copper wires and terminals electrically connect the chip, the fixed resistor, and the sensing element, forming a complete signal acquisition circuit to acquire the resistance value of the sensing element. The terminal program consists of an ESP32 device terminal, a web page terminal, and a mobile terminal. The flexible human motion sensor establishes wireless communication via BLE Bluetooth to achieve real-time acquisition, transmission, and visualization of the resistance value of the flexible composite stress sensing element.

[0035] The embodiments described above are only some, not all, of the embodiments of the present invention. The detailed description of the embodiments of the present invention is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments. The scope of protection of the present invention is determined by the scope claimed in the claims. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A flexible composite stress sensing element, characterized in that, The sensing element is a sandwich-like layered hydrogel composite structure, which is prepared by loading conductive hydrogel onto a composite fiber fabric as a skeleton. The conductive hydrogel is obtained by mixing component A and component B and then heating and polymerizing them. By weight, component A consists of 0.025~0.035 parts L-tryptophan and 2.4~3.6 parts distilled water; component B consists of 1.6~2.4 parts acrylamide, 0.018~0.022 parts ammonium persulfate, 0.018~0.022 parts N,N'-methylenebisacrylamide, 0.16~0.24 parts potassium chloride, and 3.2~4.8 parts distilled water.

2. The flexible composite stress sensing element according to claim 1, characterized in that, The composite fiber fabric is a pure nylon fabric.

3. The flexible composite stress sensing element according to claim 1, characterized in that, The heating polymerization conditions are as follows: after mixing components A and B, the mixture is kept at 60-70°C for 50-80 minutes to complete the molding process.

4. A method for preparing a flexible composite stress sensing element as described in any one of claims 1-3, characterized in that, Includes the following steps: The first step, pre-cutting: cut the composite fiber fabric into 95 mm × 95 mm sizes and lay it flat in a polytetrafluoroethylene mold; The second step is material preparation: Weigh all the raw materials of component A and component B separately, and stir them separately until completely dissolved; The third step is mixing and pouring: Component A and component B are thoroughly stirred and mixed to obtain a mixture. The mixture is then slowly poured into a mold with composite fiber fabric laid on it, so that the mixture completely soaks the composite fiber fabric. The fourth step is isothermal polymerization: the mold is transferred to a constant temperature chamber and kept at 60~70℃ for 50~80 minutes. After the reaction is completed, a flexible composite stress sensing membrane is obtained.

5. The application of a flexible composite stress sensing element as described in any one of claims 1-3 in a flexible human motion sensor, characterized in that: The flexible human motion sensor consists of an integrated circuit, a terminal program, and a flexible composite stress sensing membrane.

6. The application according to claim 5, characterized in that, The integrated circuit consists of an ESP32 chip, terminals, copper wires, a 3KΩ fixed resistor, and a power supply.

7. The application according to claim 6, characterized in that, The integrated circuit adopts a voltage divider circuit design, is powered by 3.3V, and uses copper wires and terminals to connect the chip, fixed resistor, and sensing membrane to form a complete signal acquisition circuit, realizing the acquisition of resistance value signals of stress sensing.

8. The application according to claim 5, characterized in that, The terminal program consists of an ESP32 device terminal, a web page terminal, and a mobile terminal.

9. The application according to claim 8, characterized in that, The flexible human motion sensor establishes wireless communication via BLE Bluetooth to achieve real-time acquisition, transmission, and visualization of the resistance value of the flexible composite stress sensor.

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