Fatigue-resistant flexible strain sensor based on multi-scale stress dispersion mechanism

Abstract

The present invention discloses an anti-fatigue flexible strain sensor based on a multi-scale stress dispersion mechanism. The structure of the sensor is as follows: a low-conductivity inner core, a high-conductivity layer, and an encapsulation layer are sequentially arranged from the inside out; the low-conductivity inner core comprises a flexible substrate with a cylindrical structure; the surface of the flexible substrate is uniformly distributed with grooves along the circumference, the grooves penetrate along the axial direction of the flexible substrate, and the cross-section is rectangular; each groove of the flexible substrate is filled with a low-conductivity material; the high-conductivity layer is cylindrical and covers the surface of the low-conductivity inner core; the surface of the high-conductivity layer is uniformly distributed with a number of annular protrusions along the axial direction; the encapsulation layer is coated on the surface of the high-conductivity layer, so that the flexible strain sensor has an overall cylindrical strip structure. The flexible strain sensor prepared by the present invention has the characteristics of anti-fatigue, wide sensing range, high sensitivity, and stability, and can be used in the field of wearable human bodies and robot skin, with good practicality.

Description

Technical Field

[0001] The present invention belongs to the field of sensors, and in particular relates to an anti-fatigue flexible strain sensor based on a multi-scale stress dispersion mechanism and a preparation method thereof. Background Art

[0002] Flexible strain sensors, with their advantages of lightweight flexibility, portability, stretchability, and low cost, have become a research focus in wearable devices, robotic skin, and other fields. Flexible strain sensors based on crack structures have attracted considerable attention due to their simple fabrication, high sensitivity, and rapid response. However, the narrow sensing range, disordered crack structure, and substrate deformation and fracture due to fatigue, which lead to changes in the crack structure and failure, significantly limit the practical application and service life of such sensors.

[0003] In recent years, people have made a lot of research on flexible strain sensors with fatigue resistance and stability:

[0004] The academic paper "An anti-fatigue design strategy for 3D ribbon-shaped flexible electronics (Cheng X, Zhang F, Bo R, et al. An anti-fatigue design strategy for 3D ribbon-shaped flexible electronics. Adv Mater. 2021; 33(37): 2102684.) proposed converting metal-dominated failure into the desired polymer-dominated failure, by adding an ultra-thin polymer layer (such as polyimide, PI) on top of the metal layer (such as copper, copper), significantly extending the fatigue life of 3D ribbon-shaped flexible electronic devices. The demonstrated 3D resistive sensor can be used in 10 5 Long-term vibration measurements during 10 loading cycles.

[0005] Academic paper "Tough and anti-fatigue double network gelatin / polyacrylamide / DMSO / Na2SO4ionic conductive organohydrogel for flexible strainsensor" organohydrogel for flexible strainsensor,European Polymer Journal,Volume 168,2022,111099,ISSN 0014-3057) proposed a hybrid dual-network organic hydrogel with physical and chemical crosslinking. Through sol-gel transition and salting-out effects, gelatin was formed into a physical crosslinking network, and a chemical crosslinking network was constructed with polyacrylamide (PAM) chains. The resulting gelatin / PAM / DMSO / Na2SO4 organic hydrogel exhibited high fatigue resistance and no significant plastic deformation over 1000 stretching cycles at a fixed strain of 300%. The hydrogel was then assembled into a multifunctional sensor with high sensitivity, accuracy, and stability. However, the hydrogel easily loses water and dries out during use, resulting in a short sensor life.

[0006] The academic paper "A wave-inspired ultrastretchable strain sensor with predictable cracks" (Qiang Zou, Jia Zheng, Qi Su, WanLi Wang, Wei Gao, Zhuomin Ma, A wave-inspired ultrastretchable strain sensor with predictable cracks, Sensors and Actuators A: Physical, Volume 300, 2019, 111658, ISSN 0924-4247) proposes a flexible strain sensor based on a wavy GO / PDMS structure. This wavy microstructure better concentrates stress in the grooves, reducing stress from the sides to the center of the grooves and preventing the formation of disordered cracks. However, the sensor has low sensitivity (GF ≈ 80) at strains between 0 and 30%, and because the stress-sensitive element uses a single graphene material, this material is prone to aggregation.

[0007] Patent CN115993086B proposes a method for mechanically modifying flexible sensors based on PEDOT:PSS. By adding polyvinyl alcohol, formaldehyde, and glycerol to PEDOT:PSS, the acetal reaction between the polyvinyl alcohol and formaldehyde produces a polyvinyl alcohol cross-linked network. The numerous hydroxyl groups of glycerol are hydrogen-bonded with the polyvinyl alcohol polymer chains, reinforcing the polyvinyl alcohol cross-linked network. The cross-linked network and PEDOT:PSS form an interpenetrating network, significantly enhancing the mechanical properties of the hybrid network. When used in wearable devices, good air permeability is required, but PEDOT:PSS is hygroscopic and has poor wettability. It absorbs moisture from human sweat and air, causing it to decompose and fail.

[0008] Patent CN115678132B proposes a method for preparing anti-fatigue rubber. The invention introduces multi-dimensional nanofillers into the rubber base material, namely spherical carbon black, tubular nanofiller carbon nanotubes, and flaky nanofiller graphene oxide. These fillers form a strong filler network structure in the rubber matrix, which limits the mobility of the rubber molecular chains. At the same time, it causes the crack tip morphology to continuously evolve during expansion, resulting in new morphologies such as passivation, deflection, and branching, thereby reducing the crack propagation rate.

[0009] Patent CN110823085B proposes a flexible strain sensor with a regular crack structure and its fabrication method. This approach, based on either printing followed by cutting or direct printing, addresses the poor consistency, reliability, and repeatability of existing flexible strain sensors. However, this sensor does not address the problem of crack propagation during repeated use, and the location and number of cracks cannot be controlled.

[0010] In summary, although some progress has been made in the research of stability and consistency of flexible strain sensors, the following shortcomings still exist:

[0011] 1. Under alternating loads, the supporting material of a flexible strain sensor undergoes localized deformation in high-stress areas. This repeated deformation can weaken the bonds between atoms or molecules, causing them to shift and rearrange, resulting in permanent fatigue damage to the sensor. This fatigue damage can cause irreversible plastic deformation in the supporting material, altering the conductive layer adhering to it. Consequently, sensor characteristics change over time and with stress, leading to poor sensor stability.

[0012] 2. During the repeated loading and unloading process, fatigue damage of flexible strain sensors continues to develop at the microscopic level, forming macroscopic fine cracks. The sensor may also crack when it hits sharp objects during use. Due to stress concentration at the crack tip, it rapidly expands during each loading cycle. When the crack rapidly expands to a sufficient length and reaches a critical size, the sensor will physically break or electrically fail, thus affecting the use of the sensor.

[0013] 3. To improve the sensitivity of flexible strain sensors, a crack-like surface microstructure is introduced into the sensor's stress-sensitive element. However, flexible strain sensors based on a single-layer crack structure cannot achieve both high sensitivity and a wide operating range. When the stress-sensitive element forms a transverse channel crack, it exhibits ultra-high sensitivity, but the conductive path is rapidly severed under small deformations, and the stretchability is typically less than 5%. When the stress-sensitive element forms a network of cracks with randomly distributed directions and short lengths, the stretchability is typically greater than 40%, but the sensitivity is lower due to the increased number of conductive paths and reduced crack propagation efficiency under deformation.

[0014] 4. Flexible strain sensors based on crack structures often use carbon-based conductive materials mixed into polymers to create stress-sensitive elements. However, the conductive filler is unevenly dispersed within the polymer, resulting in poor sensor stability. Another approach uses conductive polymers to create stress-sensitive elements, but these polymers have poor stretchability and low intrinsic conductivity, making them inadequate for sensor applications.

[0015] 5. When the strain sensor based on the crack structure is cyclically stretched, the stress slowly decreases from both sides to the middle. Since the uniform stress in a large area may cause the generation and connection of random cracks at the same time, the position, number and distribution of the cracks generated in the stress sensitive element are random, and the stability and consistency of the sensor are poor.

[0016] 6. The stress-sensitive element of crack-strain sensors is typically a planar membrane structure. The cracks induced by this structure are confined to a two-dimensional (2D) surface, limiting further improvements in sensor sensitivity. Secondly, due to differences in mechanical and surface properties, debonding or delamination can easily occur between the elastomer and the conductive layer with a flat membrane structure, potentially reducing the mechanical robustness of the sensor. Summary of the Invention

[0017] In order to solve the above technical problems, the present invention provides an anti-fatigue flexible strain sensor based on a multi-scale stress dispersion mechanism and a preparation method. On the one hand, the fatigue threshold is amplified by the synergistic effect of long polymers, clustered particles and strong polymer-particle adhesion in the polymer network formed by silica nanoparticles functionalized with polyethyl acrylate (PEA) and 3-(trimethoxysilyl)propyl methacrylate (TPM). The polymer network is electrically modified to prepare a film with good anti-fatigue performance and controllable electrical properties. On the other hand, the modulus of the polymer is controlled by regulating the content of TPM-functionalized silica nanoparticles in PEA, and polymer fibers with high elastic modulus are embedded in an inner core matrix with low elastic modulus. The fibers and the inner core matrix are adhered together by covalent interconnection, further amplifying the fatigue threshold. The anti-fatigue flexible strain sensor prepared by the present invention can be widely used in the fields of wearable devices and electronic skin.

[0018] In order to solve the above problems, the present invention adopts the following technical solutions:

[0019] The present invention first discloses an anti-fatigue flexible strain sensor based on a multi-scale stress dispersion mechanism. The strain sensor comprises a low-conductivity inner core, a high-conductivity layer and a packaging layer which are sequentially arranged from the inside to the outside.

[0020] The low-conductivity inner core includes a flexible substrate with a cylindrical structure; the surface of the flexible substrate is uniformly distributed with grooves along the circumferential direction, the grooves penetrate the flexible substrate along the axial direction and have a rectangular cross-section; each groove of the flexible substrate is filled with a low-conductivity material, and the low-conductivity material matches the shape of the groove; the low-conductivity inner core is composed of a flexible substrate equipped with a low-conductivity material.

[0021] The high conductive layer is cylindrical and covers the surface of the low conductive inner core; the surface of the high conductive layer is uniformly distributed with a number of annular protrusions along the axial direction;

[0022] The packaging layer is coated on the surface of the high conductive layer, so that the flexible strain sensor has an overall cylindrical strip structure.

[0023] Furthermore, the flexible substrate and the encapsulation layer are prepared from silicon dioxide nanoparticles functionalized with polyethyl acrylate and 3-(trimethoxysilyl)propyl methacrylate.

[0024] Furthermore, the diameter of the flexible substrate is 500 μm to 1 mm, and the stretchable length is not less than 200%; the number of grooves uniformly distributed along the circumference of the flexible substrate is 5 to 10.

[0025] Furthermore, the low-conductivity material is made of polyethyl acrylate, 3-(trimethoxysilyl)propyl methacrylate-functionalized silica nanoparticles, and polypyrrole. The low-conductivity material has an electrical conductivity of no greater than 10 S / cm and a stretchable length of no less than 200%. The electrical conductivity of the low-conductivity material can be adjusted as needed by adjusting the amount of polypyrrole within a specified range. The elastic modulus of the low-conductivity material can be adjusted as needed by adjusting the amount of TPM-functionalized silica nanoparticles within a specified range, and the elastic modulus of the low-conductivity material is higher than the elastic modulus of the flexible substrate.

[0026] Furthermore, the highly conductive layer is prepared from polypyrrole, graphene oxide, and phytic acid. The electrical conductivity of the highly conductive layer is not less than 4000 S / cm, and the elongation at break is not greater than 2%. Polypyrrole is one of the most important conductive polymers, with advantages such as ease of synthesis, good stability, low price, and high electrical conductivity. However, polypyrrole often aggregates severely due to intermolecular interactions and suffers from poor dispersibility and workability. One method to overcome these shortcomings is to deposit polypyrrole (PPy) on a carbon-based material. Because graphene oxide has properties such as high specific surface area, chemical stability, high π conjugation, and hydrophilicity, it provides an excellent electrode platform for the adsorption of other molecules. PPy, which has π bonds on the pyrrole ring, can be adsorbed on the graphene oxide surface through π-π stacking interactions, hydrogen bonds, and van der Waals interactions. Nanocomposites based on polypyrrole and graphene oxide exhibit synergistic properties, enhancing the conductivity and dispersibility of the conductive polymer. The electrical conductivity of the highly conductive layer can be adjusted as needed by adjusting the amounts of pyrrole, graphene oxide, and phytic acid within a specified range.

[0027] Furthermore, the width of each annular protrusion structure on the surface of the highly conductive layer is 400-800 μm, and the protrusion height is 100-400 μm. An annular crack extending circumferentially along the highly conductive layer is formed at the junction of each annular protrusion structure and the surface of the highly conductive layer by pre-stretching.

[0028] The electrical conductivity of the low-conductivity material is no higher than 10 S / cm, and the electrical conductivity of the high-conductivity layer is no lower than 4000 S / cm. The sensitivity of the sensor is defined as the ratio of the change in sensor resistance after a unit of tensile strain to the initial resistance. The different conductivities of the inner and outer conductive layers result in different sensing performance for the entire sensor. The high conductivity of the outer layer ensures that the initial resistance of the sensor is sufficiently low, while the low conductivity of the inner core prevents electrical failure under high tension, resulting in a wide sensing range. Sensors prepared according to the above-mentioned conductivity ratios for the inner and outer layers have a sensitivity of over 2000.

[0029] The elongation at break of the low-conductivity material should be no less than 200%, and the elongation at break of the high-conductivity layer should be no more than 2%. If the elongations at break of the inner and outer layers differ, the inner core will have superior mechanical properties to the outer layer. Therefore, under tension, the deformation of the highly conductive outer layer of the sensor will reach its elongation at break first, causing cracks.

[0030] Fatigue fracture is brittle at a macroscopic level, as the specimen undergoes elastic deformation. However, at a microscopic level, once a crack initiates, a zone of plastic deformation occurs at the crack front during the initial stages of propagation. The fatigue threshold is often used to describe a material's resistance to fatigue damage. A higher fatigue threshold means that fatigue damage only begins to occur at a higher number of cycles or at a greater stress amplitude.

[0031] The low-conductivity inner core is cylindrical, and the high-conductivity layer is coated on the outside of the cylinder. Preferably, the diameter of the cylinder is 500μm to 1mm. The low-conductivity inner core is formed by embedding hard elastomer fibers (i.e., low-conductivity materials) into a soft elastomer matrix. Preferably, grooves for assembling low-conductivity materials are provided on the surface of the flexible matrix at 36° to 72° intervals along the circumference, and the width accounts for 2% to 5% of the circumference of the bottom surface of the cylinder. In the generalized Lake-Thomas model, cracks extend by breaking hard fibers, and the threshold is equal to the elastic energy stored in a layer of fiber per unit area. At the front end of the crack in the composite material, the soft matrix is ​​sheared violently, causing the stress to be dispersed in the hard lattice. The large shearing action of the soft matrix will relieve the crack front, and the stress of the hard fiber will be concentrated. The crack will be blocked by the hard fiber first. Once the crack cuts the fiber, a large amount of elastic energy stored in the fiber will be released, greatly improving the fatigue threshold of the low-conductivity layer.

[0032] Core composites utilize a multiscale stress dispersion mechanism to enhance the fatigue threshold of materials. First, through the covalent interconnection between particles and polymer, the strong adhesion between polymer and particles generated by covalent bonds allows high stresses to be transferred from the polymer to the particles. This allows stress to be dispersed across the many interstitial spaces within the particle cluster. When a single interstitial space ruptures, the energy stored in multiple interstitials is dissipated. Second, the presence of long polymer chains in the composite material facilitates stress dispersion. When a crack strikes a particle cluster, the rigid particles disperse the stress across multiple interstitials within the cluster. The rupture of a single interstitial space releases the energy stored in multiple interstitials, leading to stress dispersion at both the polymer and particle scales. Finally, the aggregation of particles further enhances stress dispersion. Even without particle percolation, clustered particles can disperse stress across multiple thin polymer layers between clustered particles. This multiscale stress dispersion enhances the fatigue threshold of the composite material.

[0033] The method for preparing the fatigue-resistant flexible strain sensor based on the multi-scale stress dispersion mechanism of the present invention comprises the following steps:

[0034] Step 1: Prepare flexible substrate and encapsulation layer slurry

[0035] 3-(trimethoxysilyl)propyl methacrylate functionalized silica nanoparticles and monomer ethyl acrylate were mixed uniformly in N,N-dimethylformamide, tricyclo[5.2.1.02,6]decanedioic acid diacrylate as a crosslinking agent and 2-hydroxy-2-methylpropiophenone as a photoinitiator were added, and stirred at room temperature to obtain a flexible matrix and encapsulation layer slurry; wherein the ratio of the mass of the nanoparticles to the volume of ethyl acrylate was 0.4-0.6 g:1 mL, and the molar ratio of the crosslinking agent to the monomer was 10 -4 ~10 -3 :1, the molar ratio of the photoinitiator to the crosslinking agent is 0.3-0.4:1;

[0036] Step 2: Prepare low conductive material slurry

[0037] 3-(trimethoxysilyl)propyl methacrylate functionalized silica nanoparticles and ethyl acrylate monomer are mixed uniformly in N,N-dimethylformamide, tricyclo[5.2.1.02,6]decanedioic acid diacrylate as a crosslinking agent and 2-hydroxy-2-methylpropiophenone as a photoinitiator are added, stirred at room temperature for reaction, and then polypyrrole is added and ultrasonically mixed to obtain a low conductive material slurry; wherein: the ratio of the mass of the nanoparticles to the volume of ethyl acrylate is 0.7-0.9 g:1 mL; the molar ratio of the crosslinking agent to the monomer is 10 -4 ~10 -3:1, the molar ratio of the photoinitiator to the crosslinking agent is 0.3-0.4:1; the mass ratio of the nanoparticles to the polypyrrole is 1600-1700:1;

[0038] Step 3: Prepare high conductive layer slurry

[0039] Graphene oxide nanosheets are dispersed in water, pyrrole is added and mixed evenly, and then FeCl3-6H2O is added. A stirring polymerization reaction is carried out at 0-4°C. The resulting product is filtered, washed, and vacuum-dried to obtain a polypyrrole / graphene oxide nanocomposite material. The amount ratio of graphene oxide nanosheets, FeCl3-6H2O, and pyrrole is 15-20 mg: 0.3-0.4 g: 30 μL.

[0040] Ultrasonic dispersion of the polypyrrole / graphene oxide nanocomposite in a phytic acid solution to obtain a highly conductive layer slurry; wherein the amount ratio of the polypyrrole / graphene oxide nanocomposite to the phytic acid is 100-110 g:1 mol;

[0041] Step 4: Prepare the sensor by using a casting process combined with mold forming

[0042] The flexible substrate and encapsulation layer slurry is poured into a substrate mold and UV-cured to form a flexible substrate with grooves uniformly distributed along the circumference of the surface; a low-conductive material slurry is then injected into the grooves of the flexible substrate and UV-cured to form a low-conductive inner core; the low-conductive inner core is repeatedly stretched 2000-3000 times at a strain of 15-30% to stabilize the inner core microstructure;

[0043] The low-conductivity inner core is straightened and suspended and positioned at the central axis of the high-conductivity mold. The high-conductivity layer slurry is cast into the high-conductivity layer mold to wrap the inner core, and then air bubbles are removed by vacuum, infrared curing and drying are performed. The device is removed and repeatedly stretched 10 to 50 times at a strain of 15 to 30% to form regular annular cracks extending circumferentially along the high-conductivity layer at the junction of each annular protrusion structure and the surface of the high-conductivity layer. Two wires are fixed at both ends of the axial direction of the high-conductivity layer.

[0044] The prepared device is straightened and suspended in the center of a cylindrical packaging layer mold, and then the flexible substrate and packaging layer slurry are poured into the packaging layer mold and UV-cured to obtain a fatigue-resistant flexible strain sensor based on a multi-scale stress dispersion mechanism.

[0045] Compared with the prior art, the beneficial effects of the present invention are embodied in:

[0046] 1. The flexible strain sensor proposed in this invention has a high fatigue threshold and good stability. Its inner core has an extremely high fatigue threshold of more than 1000Jm -2. The inner core uses ethyl acrylate as a monomer to synthesize a polymer. Since the polymer chain is very long, the number of entanglements far exceeds the number of crosslinks. The dense entanglements enable the tension to be transmitted to many other chains along the length of the polymer chain. The sparse crosslinks prevent the polymer chains from separating, making the polymer have high toughness and fatigue resistance. By adding TPM-functionalized nano-silica particles to polyethyl acrylate, the polymer chains and particles are connected to each other through strong bonds. When a crack hits a cluster of particles, the rupture of a single particle-particle gap will dissipate the energy stored in many particle-particle gaps in the cluster, further increasing the fatigue threshold of the polymer. At the same time, it limits the movement of the polymer chain and inhibits the permanent plastic deformation of the polymer.

[0047] 2. The flexible strain sensor proposed in this invention features an inner core structure with two alternating moduli. By embedding hard elastomer fibers within a soft elastomer core matrix, the sensor significantly slows crack propagation. When small cracks form in the core due to cyclic loading and unloading, the high shear action of the soft matrix disperses the stress at the crack tip. Further crack propagation requires breaking the hard fibers, which then break through the elastic energy stored in the hard fibers. This hinders crack propagation, while simultaneously achieving high toughness and low hysteresis.

[0048] 3. The flexible strain sensor proposed in the present invention uses a low-conductivity fiber embedded in a matrix and a high-conductivity outer layer bonded together to form a strain sensitive element of the flexible strain sensor, so that the flexible strain sensor has high sensitivity and a wide working strain range.

[0049] 4. The flexible strain sensor proposed in this invention features a highly conductive layer made of polypyrrole, graphene oxide, and phytic acid. This hybrid conductive material exhibits superior performance by integrating the advantages of its components. Due to strong π-π interactions and van der Waals forces, pyrrole can adsorb on the surface of graphene oxide, where it grows and bonds with it, making carrier mobility easier, thereby increasing conductivity. This also effectively improves the uniformity of graphene oxide and pyrrole, suppressing the high intersheet contact resistance of graphene and pyrrole aggregation. The addition of phytic acid increases the number of additional carriers, and the anions in phytic acid can migrate through the polypyrrole, forming ionic conductive pathways that promote ion conduction in the polypyrrole and further enhance conductivity.

[0050] 5. The flexible strain sensor proposed in the present invention adopts a stress concentration strategy in the high conductive layer, so that the stress of the sensor is concentrated under pre-stretching strain, avoiding the random generation of cracks in the sensor, thereby obtaining high consistency in the manufacturing process and high stability during use.

[0051] 6. The flexible strain sensor proposed in the present invention has various sensitivity adjustment methods. The sensitivity can be flexibly adjusted by changing the number, width and depth of the substrate grooves, and the sensitivity of the sensor can also be flexibly adjusted by changing the conductivity of the low-conductive material of the inner core and the high-conductive outer layer.

[0052] 7. The flexible strain sensor proposed in the present invention adopts a linear structure to expand the crack into a three-dimensional structure, making the sensor more sensitive and easier to sew with the fabric; at the same time, the outermost layer is encapsulated with a PEA and TPM functionalized silica composite material, which effectively inhibits the shedding of the high-conductivity layer, protects the film and provides hydrophobicity.

[0053] 8. The materials used in the flexible strain sensor proposed in the present invention, such as ethyl acrylate, pyrrole, TPM-functionalized silica, phytic acid, etc., are non-toxic and biocompatible, and can be safely used in the wearable field without causing harm to the human body. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 This is a schematic diagram of the appearance structure of the fatigue-resistant flexible strain sensor based on the multi-scale stress dispersion mechanism of the present invention;

[0055] Figure 2 Schematic diagram of the low-conductivity inner core and high-conductivity layer of the fatigue-resistant flexible strain sensor based on the multi-scale stress dispersion mechanism of the present invention;

[0056] Figure 3 Schematic diagram of the cross-sectional structure of the fatigue-resistant flexible strain sensor based on the multi-scale stress dispersion mechanism of the present invention;

[0057] Figure 4 A schematic diagram of a mold used to manufacture a flexible substrate of a flexible strain sensor according to the present invention;

[0058] Figure 5 A schematic diagram of a mold used to manufacture the high conductive layer of the flexible strain sensor of the present invention;

[0059] Figure 6 This is a schematic diagram of the principle of the flexible strain sensor of the present invention in which the fatigue threshold is amplified by the synergistic effect between the long polymer single chain and the clustered particles of the low-conductive inner core matrix material;

[0060] Figure 7 This is a schematic diagram showing the principle of preventing crack propagation in the flexible strain sensor of the present invention by embedding hard fibers into the soft matrix of the low-conductivity inner core;

[0061] Figure 8 This is a schematic diagram of the principle of the present invention for preventing the coagulation of polypyrrole / graphene composite conductive materials;

[0062] Figure 9Schematic diagram of tensile stress distribution of the bionic structure of the high-conductivity layer of the anti-fatigue flexible strain sensor based on multi-scale stress of the present invention;

[0063] Figure 10 Schematic diagram of the sensing mechanism and local conductive path of the fatigue-resistant flexible strain sensor based on the multi-scale stress dispersion mechanism of the present invention;

[0064] Figure 11 The stress-strain characteristic curve of the fatigue-resistant flexible strain sensor based on the multi-scale stress dispersion mechanism of the present invention;

[0065] Figure 12 The strain-resistance characteristic curve of the fatigue-resistant flexible strain sensor based on the multi-scale stress dispersion mechanism of the present invention;

[0066] Figure 13 Cyclic stretching-resistance characteristic curves of the fatigue-resistant flexible strain sensor based on the multi-scale stress dispersion mechanism under different strains of the present invention;

[0067] Figure 14 This is the step strain-resistance characteristic curve of the fatigue-resistant flexible strain sensor based on the multi-scale stress dispersion mechanism of the present invention;

[0068] Figure 15 This is the electrical stability curve of the fatigue-resistant flexible strain sensor based on the multi-scale stress dispersion mechanism of the present invention. DETAILED DESCRIPTION

[0069] The embodiments of the present invention are described in detail below with reference to the accompanying drawings. The following embodiments are implemented based on the technical solutions of the present invention, and provide detailed implementation methods and specific operating procedures. However, the protection scope of the present invention is not limited to the following embodiments.

[0070] Ethyl acrylate used in the following examples was purchased from Shanghai MacLean Biochemical Technology Co., Ltd. (AR 99%).

[0071] Tricyclo[5.2.1.02,6]decanedioic acid diacrylate used in the following examples was purchased from Shanghai MacLean Biochemical Technology Co., Ltd.

[0072] The 2-hydroxy-2-methylpropiophenone used in the following examples was purchased from Shanghai MacLean Biochemical Technology Co., Ltd.

[0073] The N,N-dimethylformamide used in the following examples was purchased from Shanghai MacLean Biochemical Technology Co., Ltd. (AR 99.8%).

[0074] The xylene used in the following examples was purchased from Shanghai MacLean Biochemical Technology Co., Ltd. (AR 99%).

[0075] The silicon dioxide particles used in the following examples were purchased from Shanghai MacLean Biochemical Technology Co., Ltd. (average particle size 110 nm, purity >99.5%).

[0076] 3-(Trimethoxysilyl)propyl methacrylate used in the following examples was purchased from Sigma-Aldrich (AR 98%).

[0077] The pyrrole used in the following examples was purchased from Shanghai MacLean Biochemical Technology Co., Ltd. (AR 98%).

[0078] The polypyrrole used in the following examples was purchased from Shanghai MacLean Biochemical Technology Co., Ltd.

[0079] The graphene oxide used in the following examples was purchased from Shanghai MacLean Biochemical Technology Co., Ltd. (sheet diameter: 0.5-5 μm, thickness: 0.8-1.2 nm).

[0080] The phytic acid used in the following examples was purchased from Shanghai MacLean Biochemical Technology Co., Ltd. (AR 50%).

[0081] The ferric chloride hexahydrate used in the following examples was purchased from Shanghai MacLean Biochemical Technology Co., Ltd. (purity 99%).

[0082] Example 1

[0083] like Figures 1 to 3 As shown, the fatigue-resistant flexible strain sensor based on the multi-scale stress dispersion mechanism provided in this embodiment has an overall cylindrical strip structure (or a linear structure with a circular cross-section), including a low-conductivity inner core, a high-conductivity layer, and an encapsulation layer sequentially arranged from the inside to the outside;

[0084] The low-conductivity inner core includes a flexible substrate with a cylindrical structure; the surface of the flexible substrate is evenly distributed with grooves along the circumference, the grooves penetrate the flexible substrate along the axial direction, and the cross-section is rectangular; each groove of the flexible substrate is filled with a low-conductivity material, and the low-conductivity material matches the shape of the groove; the low-conductivity inner core is composed of a flexible substrate equipped with a low-conductivity material.

[0085] The high-conductivity layer is cylindrical and covers the surface of the low-conductivity inner core; the surface of the high-conductivity layer is evenly distributed with a number of annular protrusion structures along the axial direction;

[0086] The packaging layer is coated on the surface of the high conductive layer.

[0087] The method for preparing the fatigue-resistant flexible strain sensor of this embodiment includes the following steps:

[0088] Step 1: Prepare flexible substrate and encapsulation layer slurry

[0089] Preparation of TPM-functionalized silica nanoparticles: 30 g of dried silica nanoparticles were suspended in 600 mL of xylene. 180 mL of the xylene solution and 50 mL of 3-(trimethoxysilyl)propyl methacrylate solution were added over 25 minutes, and the suspension was refluxed at 140°C under nitrogen for 8 hours. After cooling to room temperature, the suspension was filtered, washed with toluene, and dried under vacuum at 100°C for 6 hours. The resulting TPM-functionalized silica nanoparticles were stored in a P2O5 desiccator.

[0090] In a fume hood, weigh 6.2 g of TPM-functionalized silica nanoparticles and pour them into a conical tube. Then, add 10.8 mL of ethyl acrylate (density: 0.918 g / mL) and 30 mL of N,N-dimethylformamide solution to the conical tube. Stir the mixture at 3000 rpm for 1 minute in a vortex mixer and sonicate at room temperature for 3 minutes. Then, add 0.024 mL of tricyclo[5.2.1.02,6]decanedioic acid diacrylate (density: 1.1 g / mL) and 0.0053 g of 2-hydroxy-2-methylpropiophenone. Stir again at 3000 rpm for 1 minute to obtain a flexible substrate and encapsulation layer slurry.

[0091] Step 2: Prepare low conductive material slurry

[0092] In a fume hood: 8.3 g of TPM-functionalized silica nanoparticles were weighed and poured into a conical tube. Then, 10.8 mL of ethyl acrylate (density 0.918 g / mL) and 30 mL of N,N-dimethylformamide solution were added to the conical tube. The mixture was stirred at 3000 rpm in a vortex mixer for 1 minute and sonicated at room temperature for 3 minutes. Then, 0.024 mL of tricyclo[5.2.1.02,6]decanedioic acid diacrylate (density 1.1 g / mL) and 0.0053 g of 2-hydroxy-2-methylpropiophenone were added, and the mixture was stirred again at 3000 rpm for 1 minute. Subsequently, 5 mg of polypyrrole was added and sonicated at room temperature for 10 minutes to obtain a low-conductivity material slurry.

[0093] Step 3: Prepare high conductive layer slurry

[0094] 20 mg of graphene oxide nanosheets were dispersed in 40 mL of water to obtain a yellow-brown dispersion with a concentration of 0.5 mg / mL. 30 μL of pyrrole was added to the graphene oxide solution and magnetically stirred for more than 1 hour. Subsequently, 20 mL of an aqueous solution containing 0.32 g of FeCl3-6H2O was added and stirred at 0-4 ° C for 4 hours. The obtained product was filtered, washed with ethanol and deionized water in sequence, and then vacuum dried at 45 ° C for 12 hours to obtain a polypyrrole / graphene oxide nanocomposite material.

[0095] 10 mg of polypyrrole / graphene oxide nanocomposite material was dispersed into 10 mL of 0.01 mol / L phytic acid aqueous solution and ultrasonically treated for 2 h to form a stable polypyrrole / graphene oxide nanocomposite material suspension to obtain a highly conductive layer slurry.

[0096] Step 4: Prepare the sensor by using a casting process combined with mold forming

[0097] Use silicone release agent to spray on the base glass mold (its structure is as follows Figure 4 The flexible substrate and encapsulation layer slurry were poured into the glass mold and the glass mold was dried for 5 minutes before use. -2 The sample was cured by irradiating it with an ultraviolet lamp for 12 hours, and then taken out of the substrate mold and placed in a fume hood for 1 day to evaporate the unreacted monomer and DMF, thereby forming a flexible substrate with grooves uniformly distributed on the surface along the circumference.

[0098] The low-conductivity material slurry was injected into the grooves of the flexible substrate and again irradiated with an intensity of 1.5 mW cm -2 The sample was cured by irradiation with a UV lamp for 12 hours and placed in a fume hood for one day to obtain a low-conductivity inner core. The inner core was then repeatedly stretched 2000 times at 30% strain to stabilize the inner core microstructure.

[0099] The low-conductivity inner core is straightened and suspended and positioned on the high-conductivity layer mold (its structure is as follows Figure 5 The high conductive layer slurry was cast into the high conductive layer mold to wrap the linear core. The mold was then placed in a vacuum pump with a vacuum degree of 200 Pa and allowed to stand for 30 minutes to remove bubbles. The high conductive layer slurry was cast into the high conductive layer mold to wrap the linear core. The high conductive layer slurry was then placed in a vacuum pump with a vacuum degree of 200 Pa and allowed to stand for 30 minutes to remove bubbles. The high conductive layer slurry was then cast into the high conductive layer mold to wrap the linear core. The high conductive layer slurry was ... -2 The device was then irradiated with infrared light for 6 hours and then placed in a 60°C oven for 4 hours. The device was then removed and repeatedly stretched 20 times at 20% strain to create regular annular cracks extending along the circumference of the highly conductive layer at the junction of each annular protrusion and the surface of the highly conductive layer. Two wires were fixed at both ends of the highly conductive layer's axial direction.

[0100] The silicone release agent was sprayed on the cylindrical encapsulation layer glass mold again. The prepared device was straightened and suspended in the center of the encapsulation layer mold. Then the flexible substrate and encapsulation layer slurry were poured into the encapsulation layer mold. The device was heated with a strength of 1.5 mW cm -2 . The sample was irradiated with an ultraviolet lamp for 12 hours to solidify it, and the sample was taken out of the mold and placed in a fume hood for 1 day to obtain a fatigue-resistant flexible strain sensor based on a multi-scale stress dispersion mechanism.

[0101] Specifically, in this embodiment, the flexible substrate has a diameter of 1 mm and a stretchable length of 230%. There are eight grooves evenly distributed along the circumference of the flexible substrate, and the groove width accounts for 4% of the circumference of the cylindrical base of the substrate. The low-conductivity material has an electrical conductivity of 8.3 S / cm and a stretchable length of 210%. The high-conductivity layer has an electrical conductivity of 5263 S / cm and an elongation at break of 2%. Each annular protrusion on the surface of the high-conductivity layer has a width of 600 μm and a height of 300 μm.

[0102] Figure 6 Schematic diagram of the high fatigue threshold achieved by a polymer of TPM-functionalized silica particles and poly(ethyl acrylate). Entanglement refers to the physical interweaving and interlacing of polymer chains or molecules within a chain. These interlacings are not covalent bonds, but rather physical interweaving of the chains. Crosslinking refers to the covalent bonds formed within a polymer chain, binding different polymer chains or segments together. When a crack strikes a polymer chain, the number of interchain entanglements in the polymer used far exceeds the number of crosslinks. The dense entanglements transfer tension within the polymer chain to many other chains along its length. The sparse crosslinks prevent the polymer chains from unraveling, and the low friction between the polymer chains distributes stress throughout the polymer chain. When a crack strikes a particle cluster, the strong polymer-particle adhesion transfers high stress from the polymer to the particles. Because the particles are rigid, the stress is distributed across the many interparticle gaps within the cluster. This multi-scale stress distribution significantly improves the fatigue threshold.

[0103] like Figure 7 As shown in the figure, when the inner core produces small cracks due to long-term use, the stress at the crack tip is dispersed under the large shear action of the soft matrix. As the crack tip expands, the stress of the hard fiber is concentrated, and the crack is blocked by the hard fiber, which greatly alleviates the expansion of the small crack.

[0104] Figure 8 Schematic diagram of a conductive composite material made of polypyrrole and graphene oxide. PPy, with its pi-bonded pyrrole rings, can adsorb onto the graphene oxide surface through pi-pi stacking interactions, hydrogen bonding, and van der Waals interactions. The strong pi-pi interactions facilitate carrier mobility in polypyrrole, thereby enhancing conductivity. The high surface area of ​​graphene oxide allows for ample adsorption of pyrrole monomers, inhibiting their aggregation.

[0105] Figure 9 This is a schematic diagram of the controllable regularly distributed cracks in the highly conductive layer of the sensor under tension. It can be seen from the figure that: no cracks are generated in the highly conductive layer in the initial state; when a longitudinal tensile force is applied to the sensor, the stress on the highly conductive layer will be concentrated at both ends of the link, causing cracks to be concentrated at these stress concentration points.

[0106] Figure 10Schematic diagram of the sensing mechanism and local conductive path of the strain sensor prepared in this embodiment. It can be seen that the main transmission path of electrons before pre-stretching is the high-conductivity layer of the sensor. After the sensor is pre-stretched, cracks are generated in the stress concentration area of ​​the high-conductivity layer. At this time, the transmission path of electrons is from the low-conductivity fiber to the high-conductivity outer layer, and so on. It is equivalent to connecting several low-resistance inner cores and high-resistance outer layers in series. Therefore, when the sensor is subjected to strain, the crack gap increases, resulting in an increase in the sensor resistance, which effectively improves the sensing performance of the sensor. In the figure, R represents the resistance of the high-conductivity film layer when it is not stretched, and R' refers to the resistance of the low-conductivity inner core when it is not stretched. R is much smaller than R'. R1, R2, and R3 respectively represent the resistance of each section separated by the cracks in the high-conductivity film layer after the cracks are generated. R1' and R2' respectively refer to the resistance of the current transmitted through the low-conductivity inner core at the crack after the cracks are generated. R1' and R2' are much larger than R1, R2, and R3.

[0107] Figure 11 The stress-strain test result curve of the strain sensor prepared in this embodiment shows that the elongation at break of the sensor can reach 120%, which means that the sensor has good stretchability and can fully meet the needs of the wearable field of the human body.

[0108] Figure 12 The strain resistance change characteristic curve of the strain sensor prepared in this embodiment shows that: when the strain is in the range of 0-20%, the sensor sensitivity coefficient GF is about 675.06; when the strain range is 20-40%, the sensitivity coefficient GF is about 314.2.

[0109] Figure 13 The resistance change characteristic curve of the strain sensor prepared in this embodiment during the cyclic loading and release process shows that the resistance of the sensor does not shift significantly when it is repeatedly loaded and released with different strains, indicating the good stability of the sensor under different strains.

[0110] Figure 14 The strain sensor prepared in this embodiment was subjected to a step test. The results showed that the sensor had very small creep effect under a tensile strain of 0% to 40% with a strain step of 5%.

[0111] Figure 15 The strain sensor prepared in this embodiment was subjected to 3000 cycles of loading / unloading (strain range 0% to 30%) to test the electrical stability and repeatability of the sensor. It can be seen that after 3000 cycles of testing, the sensor has stable electrical characteristics, thus proving that the sensor has good consistency.

[0112] The above description is merely an exemplary embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention shall be included in the scope of protection of the present invention.

Claims

1. Fatigue-resistant flexible strain sensor based on multi-scale stress dispersion mechanism, characterized by: It includes a low-conductivity inner core, a high-conductivity layer and a packaging layer which are sequentially arranged from the inside to the outside; The low-conductivity inner core comprises a flexible substrate with a cylindrical structure; the surface of the flexible substrate is uniformly distributed with grooves along the circumference, the grooves penetrate the flexible substrate along the axial direction and have a rectangular cross-section; each groove of the flexible substrate is filled with a low-conductivity material, and the low-conductivity material conforms to the shape of the groove; the low-conductivity inner core is formed by the flexible substrate equipped with the low-conductivity material; The high conductive layer is cylindrical and covers the surface of the low conductive inner core; the surface of the high conductive layer is uniformly distributed with a number of annular protrusions along the axial direction; The packaging layer is coated on the surface of the high conductive layer, so that the flexible strain sensor has an overall cylindrical strip structure.

2. The flexible strain sensor according to claim 1, wherein: The flexible substrate and the encapsulation layer are prepared from silica nanoparticles functionalized with polyethyl acrylate and 3-(trimethoxysilyl)propyl methacrylate.

3. The flexible strain sensor according to claim 1 or 2, characterized in that: The diameter of the flexible substrate is 500 μm to 1 mm, and the stretchable length is not less than 200%. The number of grooves uniformly distributed along the circumference of the flexible substrate is 5 to 10.

4. The flexible strain sensor according to claim 1, wherein: The low-conductivity material is made of polyethyl acrylate, 3-(trimethoxysilyl)propyl methacrylate-functionalized silica nanoparticles, and polypyrrole.

5. The flexible strain sensor according to claim 1 or 4, characterized in that: The low-conductive material has an electrical conductivity of no higher than 10 S / cm and a stretchable length of no lower than 200%.

6. The flexible strain sensor according to claim 1, characterized in that: The highly conductive layer is prepared from polypyrrole, graphene oxide and phytic acid.

7. The flexible strain sensor according to claim 1 or 6, characterized in that: The electrical conductivity of the highly conductive layer is not less than 4000 S / cm, and the elongation at break is not greater than 2%.

8. The flexible strain sensor according to claim 1, characterized in that: The width of each annular protrusion structure on the surface of the highly conductive layer is 400-800 μm, and the protrusion height is 100-400 μm. An annular crack extending along the circumference of the highly conductive layer is formed at the junction of each annular protrusion structure and the surface of the highly conductive layer by pre-stretching.

9. A method for preparing a fatigue-resistant flexible strain sensor based on a multi-scale stress dispersion mechanism according to any one of claims 1 to 8, characterized in that: The steps include: Step 1: Prepare flexible substrate and encapsulation layer slurry 3-(trimethoxysilyl)propyl methacrylate functionalized silica nanoparticles and monomer ethyl acrylate were mixed uniformly in N,N-dimethylformamide, tricyclo[5.2.1.02,6]decanedioic acid diacrylate as a crosslinking agent and 2-hydroxy-2-methylpropiophenone as a photoinitiator were added, and stirred at room temperature to obtain a flexible matrix and encapsulation layer slurry; wherein the ratio of the mass of the nanoparticles to the volume of ethyl acrylate was 0.4-0.6 g:1 mL, and the molar ratio of the crosslinking agent to the monomer was 10 -4 ~10 -3 :1, the molar ratio of the photoinitiator to the crosslinking agent is 0.3-0.4:1; Step 2: Prepare low conductive material slurry 3-(trimethoxysilyl)propyl methacrylate functionalized silica nanoparticles and ethyl acrylate monomer are mixed uniformly in N,N-dimethylformamide, tricyclo[5.2.1.02,6]decanedioic acid diacrylate as a crosslinking agent and 2-hydroxy-2-methylpropiophenone as a photoinitiator are added, stirred at room temperature for reaction, and then polypyrrole is added and ultrasonically mixed to obtain a low conductive material slurry; wherein: the ratio of the mass of the nanoparticles to the volume of ethyl acrylate is 0.7-0.9 g:1 mL; the molar ratio of the crosslinking agent to the monomer is 10 -4 ~10 -3 :1, the molar ratio of the photoinitiator to the crosslinking agent is 0.3-0.4:1; the mass ratio of the nanoparticles to the polypyrrole is 1600-1700:1; Step 3: Prepare high conductive layer slurry Graphene oxide nanosheets are dispersed in water, pyrrole is added and mixed evenly, and then FeCl3-6H2O is added. A stirring polymerization reaction is carried out at 0-4°C. The resulting product is filtered, washed, and vacuum-dried to obtain a polypyrrole / graphene oxide nanocomposite material. The amount ratio of graphene oxide nanosheets, FeCl3-6H2O, and pyrrole is 15-20 mg: 0.3-0.4 g: 30 μL. Ultrasonic dispersion of the polypyrrole / graphene oxide nanocomposite in a phytic acid solution to obtain a highly conductive layer slurry; wherein the amount ratio of the polypyrrole / graphene oxide nanocomposite to the phytic acid is 100-110 g:1 mol; Step 4: Prepare the sensor by using a casting process combined with mold forming The flexible substrate and encapsulation layer slurry is poured into a substrate mold and UV-cured to form a flexible substrate with grooves uniformly distributed along the circumference of the surface; a low-conductive material slurry is then injected into the grooves of the flexible substrate and UV-cured to form a low-conductive inner core; the low-conductive inner core is repeatedly stretched 2000-3000 times at a strain of 15-30% to stabilize the inner core microstructure; The low-conductivity inner core is straightened and suspended and positioned at the central axis of the high-conductivity mold. The high-conductivity layer slurry is cast into the high-conductivity layer mold to wrap the inner core, and then air bubbles are removed by vacuum, infrared curing and drying are performed. The device is removed and repeatedly stretched 10 to 50 times at a strain of 15 to 30% to form regular annular cracks extending circumferentially along the high-conductivity layer at the junction of each annular protrusion structure and the surface of the high-conductivity layer. Two wires are fixed at both ends of the axial direction of the high-conductivity layer. The prepared device is straightened and suspended in the center of a cylindrical packaging layer mold, and then the flexible substrate and packaging layer slurry are poured into the packaging layer mold and UV-cured to obtain a fatigue-resistant flexible strain sensor based on a multi-scale stress dispersion mechanism.

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