Bionic high-linearity fibrous flexible strain sensor and preparation method and application thereof

Abstract

The invention provides a bionic high-linearity fibrous flexible strain sensor and a preparation method and application thereof, and belongs to the technical field of strain sensors. According to the invention, the myelin sheath structure of the nerve fiber is used as the inspiration, the silicone tube is used for replacing the myelin sheath structure, and the conductive viscoelastic polymer / graphene conductive flexible film is used as the axon structure, so that the fibrous hollow tubular strain sensor packaged by the silicone tube is formed. Through the packaging effect of the outer silicone tube, the sensor can be endowed with good waterproof and dustproof performance, and environmental interference is avoided. Moreover, the bionic design copies the functional characteristics of the nerve myelin sheath in the aspects of signal protection and noise isolation, so that the stability and reliability of the sensor in a complex environment are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of strain sensors, and particularly relates to a bionic highly linear fibrous flexible strain sensor, a preparation method thereof, and an application thereof. Background Art

[0002] In recent years, stretchable flexible strain sensors have become a research hotspot in the field of flexible electronics, and their applications span wearable electronic products for human motion monitoring, physiological signal detection, and dynamic feedback in soft robots. From a morphological perspective, these sensors can be roughly divided into strip-shaped thin film sensors and fiber-shaped tubular sensors. The former is characterized by a relatively simple manufacturing process, easy encapsulation, and suitability for conventional scenarios. However, in an environment with limited space or complex geometry, its limitations will become apparent. In contrast, fiber-shaped tubular sensors are characterized by their linear shape, perform well in enclosed spaces, and have particular advantages in smart textiles. They can be seamlessly integrated with fabric fibers to create wearable devices with sensing or visualization functions. Despite these advantages, achieving reliable three-dimensional encapsulation for fiber-shaped tubular stretchable flexible strain sensors remains a major challenge.

[0003] Currently, most fiber-shaped tubular stretchable strain sensors are made by mixing conductive materials (such as metal nanowires, carbon-based materials, MXene, conductive polymers, etc.) with elastic polymers (such as silicone rubber, soft rubber, resin, etc.) through a wet spinning or dip coating process. However, the conductive layer of this type of sensor is exposed to the air, vulnerable to environmental interference such as electrostatic adsorption of dust, and cannot effectively isolate moisture and water, which limits its long-term reliability in practical applications. Although there have been research reports on conductive elastic fibers with hydrophobic and waterproof functions, relying solely on the hydrophobic properties of the conductive composite layer is not sufficient to fully protect the sensing layer, and the sensing layer may still experience performance degradation or even failure due to reasons such as electrostatic adsorption of dust, friction and wear, and external force damage.

[0004] To address this issue, some studies have adopted coaxial wet spinning or surface coating techniques to add an additional elastic material in the coaxial direction of the conductive fiber, enabling it to deform synchronously with the conductive fiber and effectively protecting the inner conductive fiber from external damage. For example, Wu, H.; Wang, L.; Lou, H.; Wan, J.; Pu, X. One-Step Coaxial Spinning of Core-Sheath Hydrogel Fibers for Stretchable Ionic Strain Sensors. Chemical Engineering Journal 2023, 458, 141393. https: / / doi.org / 10.1016 / j.cej.2023.141393. discloses a stretchable, conductive, and antifreeze hydrogel fiber prepared by continuous coaxial wet spinning. Its core layer is composed of a supramolecular hydrogel poly(acrylamide-co-N-acryloylphenylalanine) obtained by free radical polymerization of acrylamide and N-acryloylphenylalanine, and the outer layer is made of PVDF-HFP and polyurethane (PU). However, the coaxial wet spinning process has high technical requirements. During the spinning process, the liquid mixture must be transformed into a solid state while maintaining high efficiency, continuity, and uniformity, which poses a huge challenge. For example, Gao, J.; Fan, Y.; Zhang, Q.; Luo, L.; Hu, X.; Li, Y.; Song, J.; Jiang, H.; Gao, X.; Zheng, L.; Zhao, W.; Wang, Z.; Ai, W.; Wei, Y.; Lu, Q.; Xu, M.; Wang, Y.; Song, W.; Wang, X.; Huang, W. Ultra-Robust and Extensible Fibrous Mechanical Sensors for Wearable Smart Healthcare. Advanced Materials 2022, 34(20), 2107511. https: / / doi.org / 10.1002 / adma.202107511 discloses a highly robust and stretchable conductive fiber strain sensor prepared by electrospinning. Its central core fiber consists of carbon nanotubes embedded in a novel linear PU fiber as a conductive microfiber (CNTs / PU), and the surface of the conductive fiber is coated with Ecoflex material as an encapsulation layer. Although this method provides an excellent encapsulation outer layer for the sensor to achieve high strain resolution and a large detection range (from 0.0075% to 400%), the surface coating technology still poses a challenge for achieving a uniform coating thickness for tubular fiber sensors. Summary of the Invention

[0005] In view of this, the object of the present invention is to provide a bionic highly linear fibrous flexible strain sensor, a preparation method thereof and an application. The bionic highly linear fibrous flexible strain sensor provided by the present invention has a simple structure and is easy to prepare. On the basis of taking into account excellent linearity, stretchability and strain sensing, the sensor can be given good waterproof and dustproof properties to avoid environmental interference.

[0006] In order to achieve the above object of the invention, the present invention provides the following technical solutions:

[0007] The present invention provides a bionic highly linear fibrous flexible strain sensor, which includes an outer silicone tube and an annular conductive flexible film disposed on the inner wall of the outer silicone tube. Metal wires are respectively connected to both ends of the annular conductive flexible film, and the metal wires and the annular conductive flexible film are fixedly connected by encapsulation metal;

[0008] The composition of the annular conductive flexible film includes a viscoelastic polymer and graphene distributed in the viscoelastic polymer.

[0009] Preferably, in the annular conductive flexible film, the mass content of graphene is 20-60%.

[0010] Preferably, the material of the viscoelastic polymer includes SEBS, PDMS, SEPS or silicone;

[0011] The thickness of the annular conductive flexible film is 0.1-0.3 mm.

[0012] Preferably, the material of the metal wire is one or more of gold, silver and copper, and the material of the encapsulation metal is silver or tin.

[0013] Preferably, the length of the bionic highly linear fibrous flexible strain sensor is 50-2500 mm;

[0014] The inner diameter of the outer silicone tube is 1.0-2.5 mm, and the thickness is 0.9-1.1 mm.

[0015] The present invention provides a preparation method of the above bionic highly linear fibrous flexible strain sensor, including the following steps:

[0016] Mix a viscoelastic polymer, graphene and an organic solvent to obtain a polymer solution;

[0017] Inject the polymer solution into the silicone tube, and perform first curing to volatilize the organic solvent to obtain a silicone tube with an annular conductive flexible film attached to the inner wall;

[0018] Insert two metal wires with their ends covered with encapsulated metal paste into both ends of a silica gel tube whose inner wall is attached to an annular conductive flexible film, so that the metal paste contacts the annular conductive flexible film, and perform secondary curing to obtain a bionic highly linear fibrous flexible strain sensor.

[0019] Preferably, the temperature of the primary curing is 24-26 °C and the time is 1-2 h.

[0020] Preferably, the temperature of the secondary curing is 120 °C and the time is 20-30 min.

[0021] The present invention provides a connection method for the above-mentioned bionic highly linear fibrous flexible strain sensor, including the following steps:

[0022] Connect multiple bionic highly linear fibrous flexible strain sensors in series through metal wires, and an encapsulation device is provided at the connection of the metal wires.

[0023] The present invention provides the application of the above-mentioned bionic highly linear fibrous flexible strain sensor in the fields of human motion monitoring, building structure health monitoring or environmental monitoring of greenhouse greenhouses.

[0024] The present invention provides a bionic highly linear fibrous flexible strain sensor, including an outer silica gel tube and an annular conductive flexible film arranged on the inner wall of the outer silica gel tube. Both ends of the annular conductive flexible film are respectively connected with metal wires, and the metal wires and the annular conductive flexible film are fixedly connected through encapsulated metal; the composition of the annular conductive flexible film includes a viscoelastic polymer and graphene distributed in the viscoelastic polymer. Inspired by the myelin sheath structure of nerve fibers, the present invention uses a silica gel tube instead of the myelin sheath structure, and uses a conductive viscoelastic polymer / graphene conductive flexible film as the axon structure to form a fibrous hollow tubular strain sensor encapsulated by a silica gel tube. This sensor takes into account the superlinear response ability and good stretchability. Through the encapsulation of the outer silica gel tube, the sensor can be given good waterproof and dustproof performance to avoid environmental interference. Moreover, the bionic design of the present invention replicates the functional characteristics of nerve myelin in signal protection and noise isolation, thereby improving the stability and reliability of the sensor in a complex environment.

[0025] When the flexible strain sensor provided by the present invention undergoes bending or stretching, the resistance of the sensor will change. Utilizing this resistance change, human motion detection or building structure health monitoring can be achieved. Moreover, the conductive flexible film of the flexible strain sensor of the present invention contains graphene, and graphene has good temperature-sensitive characteristics. Therefore, the resistance of the flexible strain sensor provided by the present invention also has a highly linear response to temperature. The flexible strain sensor provided by the present invention imitates the stability and durability of the myelin sheath of nerve fibers and has broad application prospects in the fields of temperature monitoring, pressure detection, and strain monitoring.

[0026] The present invention provides a preparation method for the above-mentioned bionic highly linear fibrous flexible strain sensor. The present invention adopts a one-step injection molding method to inject a polymer solution containing a viscoelastic polymer and graphene into the interior of a silicone tube. Through the volatilization of the solvent, the conductive flexible film is uniformly attached to the inner wall of the silicone tube, thereby realizing the preparation of the fibrous flexible conductive structure of the sensor. Compared with the existing coaxial wet spinning technology, the present invention avoids the operation of high continuity of the surface coating, greatly reduces the operation difficulty, has a simple and economical process, has repeatability, and is easy to realize industrial mass production. Brief Description of the Drawings

[0027] Figure 1 is a schematic structural diagram of the bionic highly linear fibrous flexible strain sensor;

[0028] Figure 2 is the connection method of the bionic highly linear fibrous flexible strain sensor;

[0029] Figure 3 is a schematic structural diagram of the myelin sheath in nerve fibers;

[0030] Figure 4 is the morphological characterization diagram of the bionic highly linear fibrous flexible strain sensor;

[0031] Figure 5 is a schematic diagram of the sensing mechanism and a diagram of the generation of wrinkles of the bionic highly linear fibrous flexible strain sensor;

[0032] Figure 6 is the mechanical property of the bionic highly linear fibrous flexible strain sensor;

[0033] Figure 7 is the stress-strain curve of the medical silicone tube, SEBS / graphene composite conductive film, and bionic highly linear fibrous flexible strain sensor under 100% strain cyclic loading and unloading;

[0034] Figure 8 is the strain sensing performance of the bionic highly linear fibrous flexible strain sensor;

[0035] Figure 9Response time of the bionic highly linear fibrous flexible strain sensor under different strains;

[0036] Figure 10 Resistance response results of the bionic highly linear fibrous flexible strain sensor at different temperatures;

[0037] Figure 11 Resistance response results of the bionic highly linear fibrous flexible strain sensor under radial pressure. Detailed implementation manners

[0038] The present invention provides a bionic highly linear fibrous flexible strain sensor, including an outer silicone tube, and an annular conductive flexible film disposed on the inner wall of the outer silicone tube. Both ends of the annular conductive flexible film are respectively connected with metal wires, and the metal wires and the annular conductive flexible film are fixedly connected through encapsulation metals.

[0039] Unless otherwise specified, the raw materials used in the present invention are commercially available.

[0040] In the present invention, the silicone tube is preferably a medical silicone rubber tube. In the present invention, the inner diameter of the silicone tube is preferably 1.0 - 2.5 mm, more preferably 1.5 mm; the outer diameter of the silicone tube is preferably 2.0 - 3.5 mm, more preferably 2.5 mm, and the thickness is preferably 0.9 - 1.1 mm, more preferably 1 mm.

[0041] In the present invention, the composition of the annular conductive flexible film includes a viscoelastic polymer and graphene distributed in the viscoelastic polymer. In the present invention, the material of the viscoelastic polymer preferably includes SEBS (styrene - ethylene - butene - styrene block copolymer), PDMS (polydimethylsiloxane), SEPS (styrene - ethylene - propylene - styrene block copolymer) or silicone.

[0042] In the present invention, the thickness of the graphene is preferably ≤1 nm, the D50 particle size is preferably 5 - 8 μm, the bulk density is preferably 0.01 - 0.02 g / mL, the specific surface area is preferably 50 - 200 m 2 / g, and the average diameter - thickness ratio is preferably 15000. In the present invention, in the annular conductive flexible film, the mass content of graphene is preferably 20 - 60%, specifically it can be 20%, 30%, 40%, 50% or 60%, more preferably 40%. In the present invention, the thickness of the annular conductive flexible film is preferably 0.1 - 0.3 mm, more preferably 0.1 - 0.2 mm. In the present invention, the annular conductive flexible film has a hollow structure and is uniformly attached to the inner wall of the silicone tube.

[0043] In the present invention, metal wires are respectively connected to both ends of the annular conductive flexible film. In the present invention, the number of the metal wires is preferably two. One metal wire is fixedly connected to one end of the annular conductive flexible film through a packaging metal and extends to the outside of the silica gel tube; the other metal wire is fixedly connected to the other end of the annular conductive flexible film through a packaging metal and extends to the outside of the silica gel tube; the two metal wires are not connected in the hollow structure inside the silica gel tube.

[0044] In the present invention, the material of the metal wire is preferably one or more of gold, silver, and copper, more preferably copper, and the material of the packaging metal is preferably silver or tin, more preferably silver.

[0045] In the present invention, the length of the bionic highly linear fibrous flexible strain sensor is preferably 50 - 2500 mm, and specifically can be 50 mm, 100 mm, 200 mm, 500 mm, 800 mm, 1000 mm, 1500, 2000 or 2500 mm.

[0046] As a specific embodiment of the present invention, the structural schematic diagram of the bionic highly linear fibrous flexible strain sensor is as Figure 1 shown.

[0047] The present invention provides a preparation method of the above-mentioned bionic highly linear fibrous flexible strain sensor, including the following steps:

[0048] Mix a viscoelastic polymer, graphene, and an organic solvent to obtain a polymer solution;

[0049] Inject the polymer solution into the inside of the silica gel tube, and perform first curing to volatilize the organic solvent to obtain a silica gel tube with an annular conductive flexible film attached to the inner wall;

[0050] Insert two metal wires with the ends covered with packaging metal paste into both ends of the silica gel tube with an annular conductive flexible film attached to the inner wall, make the metal paste contact the annular conductive flexible film, and perform second curing to obtain a bionic highly linear fibrous flexible strain sensor.

[0051] The present invention mixes a viscoelastic polymer, graphene, and an organic solvent to obtain a polymer solution. In the present invention, the organic solvent is preferably one or more of toluene, xylene, and cyclohexane, more preferably toluene. In the present invention, the above-mentioned organic solvent has high volatility in the air, and the polymer solution injected into the silica gel tube can quickly solidify at room temperature to form an annular conductive flexible film adhered to the inner side wall of the silica gel tube.

[0052] In the present invention, the mass ratio of the total mass of the viscoelastic polymer and graphene to the mass of the organic solvent is preferably 1:6 to 8, more preferably 1:6. In the present invention, the mixing method is preferably stirring, the stirring is preferably magnetic stirring, and the time is preferably 24 h.

[0053] After obtaining the polymer solution, in the present invention, the polymer solution is injected into the interior of the silica gel tube, and the organic solvent is volatilized by first curing to obtain a silica gel tube with an annular conductive flexible film attached to the inner wall. In the present invention, a syringe is preferably used for the injection, and after the injection is completed, the silica gel tube is preferably placed horizontally without bending.

[0054] In the present invention, the polymer solution preferably fills the interior of the silica gel tube. In the present invention, the temperature of the first curing is preferably room temperature, specifically preferably 24 to 26 °C, more preferably 25 °C, and the time is preferably 1 to 2 h, more preferably 1 h.

[0055] After obtaining the silica gel tube with an annular conductive flexible film attached to the inner wall, in the present invention, two metal wires with the ends covered and encapsulated with metal paste are respectively inserted into both ends of the silica gel tube with an annular conductive flexible film attached to the inner wall, so that the metal paste contacts the annular conductive flexible film, and second curing is carried out to obtain a bionic highly linear fibrous flexible strain sensor. In the present invention, the encapsulated metal paste is preferably stretchable conductive silver paste. In the present invention, the covering length of the encapsulated metal paste is preferably 0.7 to 0.8 cm, more preferably 0.75 cm.

[0056] In the present invention, the temperature of the second curing is preferably 90 to 120 °C, more preferably 100 to 120 °C; the time is preferably 20 to 30 min, more preferably 25 min. In the present invention, the encapsulated metal paste is converted into a solid by curing and sintering to realize the connection between the metal wire and the annular conductive flexible film.

[0057] The present invention provides a connection method for the above-mentioned bionic highly linear fibrous flexible strain sensor, including the following steps:

[0058] A plurality of bionic highly linear fibrous flexible strain sensors are connected by metal wires in a series manner, and an encapsulation device is provided at the connection of the metal wires.

[0059] In the present invention, the encapsulation device is preferably a heat shrinkable tube. In the present invention, the number of connected bionic highly linear fibrous flexible strain sensors is preferably 2 to 5.

[0060] As a specific embodiment of the present invention, the connection method of the bionic highly linear fibrous flexible strain sensor is as Figure 2 shown.

[0061] The present invention provides the application of the above-mentioned bionic highly linear fibrous flexible strain sensor in the fields of human motion monitoring, building structure health monitoring or environmental monitoring of greenhouse greenhouses.

[0062] Inspired by the myelin sheath structure of nerve fibers, the present invention uses a silicone tube to replace the myelin sheath structure, and uses a conductive viscoelastic polymer / graphene conductive flexible film as the axon structure to form a fibrous hollow tubular strain sensor encapsulated by a silicone tube. Among them, the schematic diagram of the myelin sheath structure in nerve fibers is as Figure 3 shown. The cylindrical structure of the nerve myelin sheath consists of an outer myelin sheath layer and an inner axon. Specifically, the outer myelin sheath is composed of lipids and proteins, which wraps around the axon and plays the role of electrical insulation and enhancing signal transmission, while the inner axon is responsible for conducting nerve electrical signals. The protrusions of neuron cells are divided into axons and dendrites. The dendritic membrane protrusions form numerous dendritic spines, which together with the ends of the axons of other neurons form synapse structures to receive information from the axons of other neurons. The axon is the only efferent structure of neurons, with lengths ranging from dozens of micrometers to several centimeters. Therefore, the myelin sheath structure of nerve fibers is an important protective structure for axons, not only acting as an electrical insulation shell, but also capable of increasing the transmission speed of axon electrical signals. The myelin sheath structure can also promote axon regeneration under injury conditions and is expected to guide the anastomosis between axons. Inspired by this, the present invention directly uses a commercially available silicone tube to replace the myelin sheath structure, and uses a conductive viscoelastic polymer / graphene conductive flexible film as the axon structure to form a fibrous hollow tubular sensor encapsulated by a silicone tube. The bionic design of the sensor replicates the functional characteristics of nerve myelin sheaths in signal protection and noise isolation, thereby improving the stability and reliability of the sensor in complex environments.

[0063] The following examples are used to elaborate in detail on the bionic highly linear fibrous flexible strain sensor provided by the present invention, its preparation method and application, but they should not be construed as limiting the protection scope of the present invention.

[0064] The graphene used in the examples was purchased from Shenzhen Suiheng Graphene Technology Co., Ltd., and SEBS was obtained in particulate form from the TUFTEC series (H1401, Asahi Kasei, Japan). The transparent medical silicone rubber tube was supplied by Huaian Lierde Rubber and Plastic Products Co., Ltd. The conductive silver paste (JY-12) was purchased from Shanghai Julong Electronic Technology Co., Ltd. The toluene organic solvent (AR) was purchased from Tianjin Xintong Fine Chemical Co., Ltd., China.

[0065] Example 1

[0066] The preparation of the bionic highly linear fibrous flexible strain sensor adopts the following steps:

[0067] Prepare a conductive polymer solution by mixing SEBS and a certain amount of graphene in five different weight ratios, with the mass concentrations of graphene being 20%, 30%, 40%, 50%, and 60% respectively. Take 4 g of each mixture and place it in a glass container with 26 g of toluene as the organic solvent. Stir magnetically at room temperature for 24 hours to ensure complete mixing and dissolution, obtaining the polymer solution.

[0068] Then transfer the obtained polymer solution into a syringe and inject it into a medical silicone rubber tube with an inner diameter of 1.5 mm, a thickness of 1 mm, and a length of 5 cm. After filling, let it cure at room temperature for 1 hour. After curing, insert copper wires into both ends of the medical silicone rubber tube using stretchable conductive silver paste, and then cure the silver paste at 120 °C for 20 min to sinter the silver paste, obtaining a bionic highly linear fibrous flexible strain sensor.

[0069] For the connection of two bionic highly linear fibrous flexible strain sensors, first connect both ends of the wire, and then encapsulate the connector with a heat shrinkable tube.

[0070] Use a super-depth-of-field microscope (VH - 5000, KEYENCE) to observe the cross-sectional geometry of the strain sensor. The morphology characterization diagram of the bionic highly linear fibrous flexible strain sensor with a graphene doping amount of 40% is as Figure 4 shown. Figure 4 In it, a is the optical image of the cross-section morphology of the sensor, and b is the 3D super-depth-of-field image showing the wrinkled SEBS / graphene film (left) and the 3D structure of the wrinkles (right) on the silicone rubber tube. It can be seen from Figure 4 that the thickness of the conductive flexible film on the inner layer of the silicone rubber tube is 0.1 mm and is evenly distributed on the inner wall of the silicone rubber tube.

[0071] The schematic diagram of the sensing mechanism and the diagram of wrinkle generation of the bionic highly linear fibrous flexible strain sensor obtained in Example 1 are as Figure 5 shown. In the present invention, the elastic material of the strain sensor is SEBS in the outer silicone rubber tube and the flexible film, and the conductive material is graphene in the flexible film; during the bending and stretching process of the sensor, the flexible film will be stretched. Because the stretching of the film will make the conductive path longer, the difficulty of conduction increases, and the resistance increases. When it returns to the original length, due to the inconsistent elastic moduli of the outer silicone rubber tube and the inner flexible film, the inner flexible film will generate wrinkles, and the generated wrinkles will promote the resistance response of the sensor device during bending and stretching.

[0072] During the bending and stretching process of the sensor, according to the definition formula of the conductor resistance (as shown in Equation I), it can be known that when the conductive path becomes longer, the resistance increases significantly.

[0073] R = ρL / S Equation I;

[0074] In Formula I: ρ is the resistivity, L is the length, and S is the cross-sectional area.

[0075] Test Example 1

[0076] Figure 6 It is for the mechanical properties of the bionic highly linear fibrous flexible strain sensor. Among them, a is the stress-strain curve of the SEBS / graphene hybrid conductive film with different graphene composition ratios. b is the stress-strain curve of the pure medical silicone tube. c is the statistical analysis of the elastic modulus of the SEBS / graphene hybrid conductive film with different composition ratios and the silicone tube. d is the initial resistance value of the SEBS / graphene hybrid conductive film with different composition ratios. e is the maximum effective tensile strain of the sensor prepared with the SEBS / graphene hybrid conductive film with different composition ratios.

[0077] (1) First, the present invention studied the mechanical properties of conductive composite films with different ratios. Conductive hybrid solutions with graphene mass concentrations of 20%, 30%, 40%, 50%, and 60% were prepared with reference to Example 1, and conductive composite films with the same geometry (width 5 mm, thickness 0.1 mm) were obtained. Using a universal tensile testing machine, repeated experiments were carried out on multiple conductive composite film samples with different ratios, and the corresponding stress-strain curves were obtained as shown in Figure 6 a in. It can be observed that as the graphene content increases, the elongation at break of the film decreases and the tensile strength required increases.

[0078] (2) After analyzing the conductive composite film, mechanical tests were carried out on commercially available medical silicone tubes (diameter 1.5 mm). Similarly, tensile tests were carried out on 5 samples, and the obtained results are shown in Figure 6 b in. The results show that the average elongation at break of the medical silicone tube is greater than 300%, and the average tensile strength is about 7 MPa.

[0079] (3) The present invention compared the elastic moduli of the SEBS / graphene hybrid conductive films with different composition ratios and the silicone tube, and the obtained results are shown in Figure 6 c in. The results show that the elastic moduli of the conductive composite films are all greater than that of the medical silicone tube. Among them, the elastic modulus of the conductive composite film containing 20% graphene is about 5 times that of the medical silicone tube, and the elastic modulus of the conductive composite film containing 60% graphene is about 35 times that of the medical silicone tube. The sensor composed of these two materials with a large difference in elastic modulus forms a rigid-flexible coupling state, which is beneficial to improving the electrical performance of the sensor.

[0080] (4) The present invention studied the electrical properties of the sensors composed of conductive composite films with different ratios. Figure 6The d in [Figure] shows the initial resistance per centimeter of sensors composed of conductive composite films with different ratios. Obviously, as the graphene content increases, the initial resistance of the conductive composite film decreases, but the trend of increasing conductivity weakens. More specifically, when the graphene ratio increases from 30% to 40%, the initial resistance of the sensor decreases from 0.6133 kΩ / cm to 0.1196 kΩ / cm, a decrease of more than 5 times. However, when the graphene ratio increases from 40% to 50%, the change in the initial resistance is not obvious. Especially when the graphene ratio reaches 60%, the initial resistance is almost the same as that at 50%. Without a doubt, the higher the graphene ratio, the better the conductivity of the SEBS / graphene conductive composite film, but the maximum allowable elongation rate of the effective electrical properties is lower.

[0081] The maximum effective tensile strain of sensors prepared with SEBS / graphene hybrid conductive films with different composition ratios is as Figure 6 shown in e of [Figure]. When the graphene content is 40%, the effective electrical tensile limit of the sensor reaches the maximum. Therefore, considering the above factors comprehensively, the present invention selects a conductive SEBS / graphene composite film with a graphene ratio of 40% and a medical silicone tube to construct a bionic highly linear fibrous flexible strain sensor for subsequent related tests.

[0082] (5) To further illustrate the positive effect of the elastic modulus difference between the conductive composite film and the silicone tube on the overall mechanical properties of the sensor, the stress-strain curves of a medical silicone tube, a SEBS / graphene composite conductive film (graphene mass content is 40%), and a bionic highly linear fibrous flexible strain sensor (graphene mass content in the conductive flexible film is 40%) under 100% strain cyclic loading and unloading were respectively tested as Figure 7 shown. Figure 7 In [Figure], a is the mechanical hysteresis of the medical silicone tube at 100% strain; b is the mechanical hysteresis of the SEBS / graphene composite conductive film containing 40% graphene at 100% strain; c is the mechanical hysteresis of the bionic highly linear fibrous flexible strain sensor at 100% strain.

[0083] It can be seen that for the pure medical silicone tube, after the first loading-unloading cycle, its stress-strain curve changed significantly. But by the 20th and 30th cycles, the stress-strain curves were basically the same. This phenomenon was also observed in the SEBS / graphene composite conductive film and the prepared sensor ( Figure 7b and c) in. Interestingly, both the medical silicone tube and the fibrous strain sensor exhibit highly similar mechanical hysteresis behavior, and the resilience hysteresis performance is significantly better than that of the SEBS / graphene composite conductive film. That is to say, the silicone tube enhances the resilience characteristics of the SEBS / graphene composite conductive film. This improvement can be attributed to the formation of a hollow conductive layer along the inner wall of the silicone tube during the volatile molding process of the SEBS / graphene composite conductive film. When the two ends of the silicone tube are sealed with silver paste to form solid electrode terminals, the hollow conductive film inside forms a mechanically supported structure fixed at both ends and adheres to the inner wall of the silicone tube. Obviously, the difference in elastic modulus and mechanical hysteresis between the silicone tube and the conductive film results in the formation of surface wrinkles on the conductive film. This wrinkle morphology will significantly improve the linearity of the output signal of the conductive film under tensile strain.

[0084] Test Example 2

[0085] Prepare a bionic highly linear fibrous flexible strain sensor with an inner diameter of 1.5 mm and a length of 5 cm in the manner of Example 1. The strain sensing performance of the obtained flexible strain sensor is as Figure 8 shown. Figure 8 In it, a is the sensitivity curve of the flexible strain sensor within a 100% strain range. b is the current-voltage (I-V) characteristic of the flexible strain sensor under different tensile strains. c is the relative resistance change of the flexible strain sensor under cyclic tensile of different tensile strains. d is the average peak curve of the relative resistance change rate, derived from the cyclic tensile at different tensile strain levels shown in c. e is the cyclic response of the flexible strain sensor at a 15% tensile strain, with a frequency range of 0.3 to 2.4 Hz. f is the result of the 2500-cycle stability test of the flexible strain sensor at a 90% tensile strain. The inset in f is an enlarged view of the output signal response during cycles 1,220 to 1,255. g is a comparison of the linearity and strain working range between the flexible strain sensor of the present invention and previously reported sensors. The operation in the "U"-shaped region in the 0% to 300% strain range indicates that the reported sensors are not linear throughout the strain range, while the operation in the "J"-shaped region in the 0% to 200% strain range indicates that the reported sensors are linear throughout the strain range.

[0086] As can be seen from Figure 8 a in, the strain factor (GF) of the flexible strain sensor of the present invention is 8.60 within the strain range of 0 to 100%, and a linearity of 0.9958 is achieved throughout the strain range. Compared with other reported sensors, the flexible strain sensor of the present invention exhibits competitive performance.

[0087] Stability and durability are measured by the number of cycles in which the relative resistance change of the sensor does not drift or change significantly during testing, and they are also important indicators for the long-term application of the sensor in real life. To demonstrate the stability and durability of the fibrous sensor, the sensor was stretched to strain levels of 10%, 20%, 30%, 40%, and 50%, and the voltammetry curves under these different strain conditions were tested, as shown in Figure 8 b in

[0088] . It can be seen that the voltammetry curves under different strain levels all exhibit a high degree of linearity, indicating that the resistance of the sensor remains highly stable under different stretching states. Figure 8 To further illustrate the high stability of the sensor under different strain conditions, the present invention placed the sensor under five strain levels (20%, 40%, 60%, 80%, and 100%) and performed five reciprocating motions and stretches, and the results are plotted in Figure 8 c in

[0089] . It can be seen that the consistent peak heights at each strain level reflect the stable characteristics of the sensor. Then, the present invention extracted the peaks from the five cycles at each strain level and plotted the relationship between the average relative resistance change rate and the strain level, and the obtained results are shown in Figure 8 d in

[0090] . The results show that the linearity of the curve after linear fitting is 0.9937, further verifying the high linear input-output signal characteristics of the sensor. Figure 8 g in

[0091] Table 1 Comparison of the flexible strain sensor of the present invention with other sensors

[0092]

[0093] In Table 1:

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[0103] [6] Wang, F.; Chen, J.; Cui, X.; Liu, X.; Chang, X.; Zhu, Y. Wearable Ionogel-Based Fibers for Strain Sensors with Ultrawide Linear Response and Temperature Sensors Insensitive to Strain. ACS Appl Mater Interfaces 2022, 14(26), 30268–30278.

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[0112]

[10] Fan, Y.; Shen, Z.; Zhou, X.; Dan, Z.; Zhou, L.; Ren, W.; Tang, T.; Bao, S.; Nan, C.; Shen, Y. Highly Sensitive Strain Sensor from Topological-Structure Modulated Dielectric Elastic Nanocomposites. Adv Mater Technol 2022, 7, 2101190.

[0114] It can be seen that the sensor in the strain range of 0 - 300% is not linear throughout the strain range, while the sensor in the strain range of 0 - 200% is linear throughout the strain range. In fact, most stretchable flexible sensors do not exhibit a linear response output throughout the strain range. Piecewise fitting is a commonly used method for dealing with the relationship between the input signal and the output signal of the sensor. However, this full-range non-linearity poses a huge challenge in practical applications because it reduces the measurement accuracy and complicates the usability of the sensor. Therefore, it highlights the better linearity of the present invention in the full-range strain.

[0115] The response of the sensor to the applied excitation frequency is also an important performance parameter. Mechanical signals with different frequencies and different waveforms are generated by a vibrator system including a vibrator (SA-JZ002, Shi'ao Technology Co., Ltd., Wuxi, China), a power amplifier (SP-PA003, Shi'ao Technology Co., Ltd., Wuxi, China), and a signal generator (DG1022Z, RIGOL, China). For mechanical sensors, the response frequency bandwidth is crucial, which determines the actual application range of the sensor. Generally, due to the viscoelasticity of soft materials, the frequency response bandwidth of sensors mainly made of soft materials is much lower than that of sensors made of rigid materials. In addition, the thicker the soft strain sensor, the narrower its frequency response bandwidth. The present invention first evaluated the cyclic response of the sensor under low-frequency signals. The obtained results are as Figure 8As shown in e in [reference], when the sensor is subjected to a loading-unloading cycle with a strain of 15% at an interval of 0.3 Hz and a tensile frequency ranging from 0.3 Hz to 2.4 Hz, the peak value of the relative resistance change rate remains at the same level, which is sufficient for most application scenarios (such as wearable devices and structural health monitoring).

[0116] The response time refers to the time required for the output signal of the sensor to jump and reach a steady state when the external excitation is loaded and unloaded, and it is also the basis for whether the sensor can perform real-time monitoring. The response times of the flexible strain sensor of the present invention under different strains are as Figure 9 shown. It can be seen that when the flexible strain sensor of the present invention is subjected to a strain of 0.15%, the response time of the sensor is 0.035 seconds. Generally speaking, the flexible strain sensor of the present invention is comparable to other reported sensors in terms of strain range and linearity, and its fast response characteristics exhibited are crucial for applications in real-time monitoring.

[0117] Test Example 3

[0118] Due to the temperature-sensitive characteristics of the graphene material, the response of the flexible strain sensor of the present invention to environmental factors such as temperature cannot be ignored. The present invention explored the relative resistance change rate of the sensor at different temperatures, and the resistance response results of the sensor at different temperatures are as Figure 10 shown. Figure 10 The temperature in [figure] was measured by a thermal imager (UTi260A, Uni-Trend Technology (China) Co., Ltd.). From Figure 10 it can be seen that as the environmental temperature increases from 15 °C to 100 °C, the output resistance change rate increases to about 0.15, showing a monotonically increasing trend. The correlation coefficient obtained by using the polynomial second-order fitting method is as high as 0.9914, indicating that the sensor of the present invention can not only be used as a sensor for responding to strain, but also as a sensor for responding to temperature.

[0119] Test Example 4

[0120] For the pressure test of the sensor, the present invention used a dynamometer (HP-20N, Yueqing Handepai Instrument Co., Ltd., China) and an electric stepper motor displacement stage to apply an axial compressive force to the sensor. Utilizing the response ability of the sensor to vertical pressure, an increasingly large vertical pressure was applied and the output resistance of the sensor was recorded. The resistance response results of the sensor under radial pressure are as Figure 11 shown. It can be seen that as the applied force increases from 0 to 8 N, the output resistance change rate increases to approximately 0.045, showing a monotonically increasing trend. The correlation coefficient obtained by using the polynomial second-order fitting method is as high as 0.999, indicating that the sensor can not only be used as a strain sensor for responding to axial strain, but also as a pressure sensor for responding to radial pressure.

[0121] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A bionic high linear fiber-shaped flexible strain sensor, characterized in that: It includes an outer silicone tube, and an annular conductive flexible film arranged on the inner wall of the outer silicone tube, wherein both ends of the annular conductive flexible film are respectively connected with metal wires, and the metal wires and the annular conductive flexible film are connected and fixed by encapsulating metal; The ring-shaped conductive flexible film comprises a viscoelastic polymer and graphene distributed in the viscoelastic polymer.

2. The bionic high linear fiber-shaped flexible strain sensor according to claim 1, characterized in that: In the annular conductive flexible film, the mass content of graphene is 20-60%.

3. The bionic high linear fiber-shaped flexible strain sensor according to claim 1 or 2, characterized in that: The material of the viscoelastic polymer includes SEBS, PDMS, SEPS or silicone; The thickness of the annular conductive flexible film is 0.1-0.3 mm.

4. The bionic high linear fiber-shaped flexible strain sensor according to claim 1, characterized in that: The material of the metal wire is one or more of gold, silver and copper, and the material of the packaging metal is silver or tin.

5. The bionic high linear fiber-shaped flexible strain sensor according to claim 1, characterized in that: The length of the bionic high linear fiber-shaped flexible strain sensor is 50 to 2500 mm; The inner diameter of the outer silicone tube is 1.0-2.5 mm, and the thickness is 0.9-1.1 mm.

6. The method for preparing the bionic high linear fiber-shaped flexible strain sensor according to any one of claims 1 to 5, characterized in that: The following steps are involved: mixing a viscoelastic polymer, graphene and an organic solvent to obtain a polymer solution; Injecting the polymer solution into the interior of the silicone tube to perform a first curing to volatilize the organic solvent, thereby obtaining a silicone tube with an inner wall bonded to a ring-shaped conductive flexible film; Two metal wires with ends covered with encapsulated metal paste are respectively inserted into the two ends of the silicone tube whose inner wall is attached to the annular conductive flexible film, so that the metal paste is in contact with the annular conductive flexible film, and a second curing is performed to obtain a bionic high-linear fibrous flexible strain sensor.

7. The preparation method according to claim 6, characterized in that: The temperature of the first curing is 24-26° C., and the time is 1-2 hours.

8. The preparation method according to claim 6, characterized in that: The temperature of the second curing is 90-120° C., and the time is 20-30 minutes.

9. The method for connecting the biomimetic high linear fiber-shaped flexible strain sensor according to any one of claims 1 to 5, characterized in that: The following steps are involved: A plurality of bionic high-linear fiber-shaped flexible strain sensors are connected in series through metal wires, and packaging devices are provided at the connection points of the metal wires.

10. Application of the bionic high linear fibrous flexible strain sensor described in any one of claims 1 to 5 or the bionic high linear fibrous flexible strain sensor prepared by the preparation method described in any one of claims 6 to 8 in the field of human motion monitoring, building structure health monitoring or greenhouse environment monitoring.

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