A flexible strain sensor based on double thread conductive yarn and a preparation method thereof

Abstract

The application relates to a flexible strain sensor based on a double-thread conductive yarn and a preparation method thereof. The flexible strain sensor based on the double-thread conductive yarn comprises a core layer elastic yarn, an intermediate layer conductive material and an outer layer packaging material; the core layer elastic yarn is a double-thread yarn, the intermediate layer conductive material comprises a strain sensitive layer and electrodes fixed at two ends of the strain sensitive layer; the double-thread yarn comprises a supporting core yarn and a double-thread structure, and the double-thread structure is two spiral continuous convex structures formed on the surface of the supporting core yarn; the preparation method comprises the following steps: S1, preparing the double-thread yarn; S2, preparing the double-thread conductive yarn; S3, forming two electrodes at two ends of the double-thread conductive yarn; and S4, preparing the flexible strain sensor based on the double-thread conductive yarn. The preparation method is simple and easy to implement, the prepared flexible strain sensor based on the double-thread conductive yarn has a large strain working range, excellent linearity, low hysteresis and good sensitivity.

Description

Technical Field

[0001] This invention belongs to the field of flexible wearable strain sensor technology, and relates to a flexible strain sensor based on double-threaded conductive yarn and its preparation method. Background Technology

[0002] In recent years, with the increasing demand for real-time detection of human movement and physiological information, miniaturized and intelligent flexible wearable electronic devices have attracted considerable attention. Flexible strain sensors are a crucial component of these devices. However, traditional semiconductor or metal sensors, due to their rigidity and poor stretchability, cannot meet the requirements for detecting large strain movements of the human body. Compared to traditional strain sensors, yarn-based flexible strain sensors exhibit significant advantages in the field of smart wearables due to their superior flexibility, high stretchability, low manufacturing cost, and ease of integration.

[0003] Currently, there are two main methods for fabricating linear strain sensors. One method involves mixing conductive fillers with elastic polymers and preparing conductive fibers through wet spinning or melt spinning. The strain sensing mechanism of the conductive fibers obtained by this method is based on the permeation of the conductive filler within the elastic polymer; the resistance changes with the fiber's geometry and internal conductivity. However, increasing the amount of conductive filler enhances the rigidity of the elastic matrix, thereby reducing the fiber's stretchability and elastic recovery rate, and worsening hysteresis. Furthermore, linearity is difficult to adjust due to its reliance on the mechanical stability of the permeation network. The other method involves loading conductive materials onto the surface of elastic fibers or yarns. The sensing mechanism of the conductive fibers / yarns obtained by this method is mainly based on the mismatch in mechanical properties between the conductive material and the elastic substrate. During stretching, the conductive material generates microcracks, thus altering the resistance. Although coating methods can give strain sensors good mechanical properties and stretchability, the conductive layer itself lacks flexibility, and large longitudinal cracks quickly form under high strain, causing the conductive path to break, thus limiting the effective sensing range of the device.

[0004] Regarding yarn structure design, Reference 1 (Highly stretchable and strain sensitive fibers based on braid-like structure and silver nanowires[J]. Appl Mater Today, 2020, 19(5): 100610.) uses braided yarn as the elastic substrate and silver nanowires as the conductive phase. Through multiple impregnations, a yarn sensor with high sensitivity over a large sensing range is prepared. However, the yarn sensor exhibits significant hysteresis under 30% cyclic stretching strain, with the resistance increasing by 23.6% after one cycle. This is because the yarn microstructure does not recover sufficiently, and microcracks cannot completely merge after release, resulting in high hysteresis and limiting its application in wearable sensors. In addition, Reference 2 (A stretchable and highly sensitive graphene-based fiber for sensing tensile strain, bending, and torsion[J]. Adv Mater, 2015, 27(45): 7365-71.) develops a graphene-based composite yarn with a structure similar to a compression spring. This double-wrap composite yarn exhibits stable performance after multiple stretching cycles and has a wide strain range, but it has low sensitivity and poor linearity, and its relative resistance does not change significantly after strain exceeding 100%.

[0005] In summary, the aforementioned yarn-based flexible strain sensors are prone to high hysteresis and poor repeatability due to inter-fiber slippage. Furthermore, it is difficult to simultaneously achieve good sensitivity and a wide linear strain range by adjusting the sensing performance through wrapping or braiding structures. Therefore, it is necessary to overcome the limitations imposed by inter-fiber slippage on the performance of existing yarn sensors and develop a novel yarn strain sensor with a wide linear strain range, low hysteresis, and good repeatability stability to meet the needs of practical precision sensing applications. Summary of the Invention

[0006] The purpose of this invention is to solve the aforementioned problems in the prior art and provide a flexible strain sensor based on a double-threaded conductive yarn and its preparation method. The double-threaded structure of the flexible strain sensor of this invention is achieved by hot-pressing a low-melting-point hot-melt wire onto the surface of spandex yarn, forming a double-threaded shape and integrating the low-melting-point hot-melt wire and spandex into a single double-threaded yarn. The low-melting-point hot-melt wire is fused to the surface of the spandex core yarn through hot-melt bonding, overcoming the problems of high hysteresis and poor repeatability caused by fiber slippage during strain in existing yarn-based strain sensors. Compared to pure spandex yarn with uniform tensile strain distribution, this double-threaded yarn can induce a redistribution of strain on the spandex surface during stretching. Specifically, during stretching, the double-threaded yarn gradually straightens, the diameter of the coil decreases, while the spandex elongates longitudinally and contracts radially. Due to the different Poisson's ratios of the two components, the double-threaded design restricts the stretchability of the spandex during elongation. This results in the surface area of ​​the spandex between the double threads being compressed and stretched, leading to smaller strain. Consequently, the surface area of ​​the spandex between the double threads experiences smaller strain, while other uniformly smooth areas of the spandex experience larger strain. This strain distribution effect can be controlled by modifying the structural parameters of the double-threaded design. Furthermore, under axial strain, the thread gradually straightens, exhibiting the smallest strain compared to other areas. This rhythmic strain distribution causes the yarn strain sensor to generate multi-scale cracks during stretching, thus maintaining good sensitivity while achieving a wide linear strain range, making it better suited for real-time monitoring of human movement.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] A flexible strain sensor based on double-threaded conductive yarn includes a core elastic yarn, an intermediate conductive material, and an outer encapsulation material.

[0009] The core layer elastic yarn is a double-threaded yarn, and the intermediate conductive material includes a strain-sensitive layer (i.e., a conductive layer) and electrodes fixed at both ends of the strain-sensitive layer.

[0010] The double-threaded yarn includes a support core yarn and a double-threaded structure. The support core yarn is made of elastic fiber, and the double-threaded structure is made of low-melting-point hot melt wire with a melting point below 110°C. The double-threaded structure is a continuous spiral protrusion structure formed on the surface of the support core yarn.

[0011] The strain-sensitive layer is a carbon-based conductive material;

[0012] The outer encapsulation material covers the outer periphery of the double-threaded conductive yarn, but does not change its double-threaded structural characteristics.

[0013] As a preferred technical solution:

[0014] The flexible strain sensor based on double-threaded conductive yarn, as described above, has a linearity of not less than 0.970, a sensitivity of not less than 11.34, and a hysteresis rate of not more than 13.3% within 100% strain.

[0015] The flexible strain sensor based on double-threaded conductive yarn exhibits a linearity of no less than 0.964, a sensitivity of no less than 9.86, and a hysteresis rate of no more than 6.3% after 2000 cycles of stretching at 50% strain, demonstrating good repeatability.

[0016] As described above, a flexible strain sensor based on double-threaded conductive yarn has a lead length of 2–4 mm (lead refers to the axial distance between two corresponding points on the pitch diameter of two adjacent teeth on a helical thread), a pitch ratio (pitch refers to the minimum axial distance between two corresponding points on the pitch diameter of two adjacent teeth) of 0.4, and a cross-sectional diameter of the double-threaded yarn of 0.56–0.80 mm (the diameter here refers to a reference value). Figure 1 (As shown).

[0017] As described above, a flexible strain sensor based on double-threaded conductive yarn has copper wire as the electrode, and the strain-sensitive layer is connected to the electrode via conductive adhesive.

[0018] As described above, a flexible strain sensor based on double-threaded conductive yarn is provided, wherein the elastic fiber is spandex filament, and the melting point of the low-melting-point hot melt wire is 85-110°C; the outer encapsulation material is a two-component liquid silicone rubber material, preferably Ecoflex 00-10-00-50 series.

[0019] The flexible strain sensor based on double-threaded conductive yarn, as described above, wherein the carbon-based conductive material is one or more of carbon nanotubes, carbon black, and graphene.

[0020] This invention also provides a method for fabricating a flexible strain sensor based on double-threaded conductive yarn, comprising the following steps:

[0021] S1: Low-melting-point hot melt wire with a melting point of 85-110℃ is wound onto the surface of elastic fiber using a wrapping machine, and then the low-melting-point hot melt wire is hot-pressed by a flatbed hot press to partially melt and bond the low-melting-point hot melt wire to the surface of elastic fiber, thus obtaining double-threaded yarn; the double-threaded structure of the double-threaded yarn refers to two continuous spiral protrusions formed by the low-melting-point hot melt wire on the surface of the elastic fiber.

[0022] S2: A layer-by-layer assembly method is used to coat carbon-based conductive material onto the surface of double-threaded yarn to obtain double-threaded conductive yarn;

[0023] S3: Copper wires are fixed to both ends of double-threaded conductive yarn with conductive adhesive to form two electrodes;

[0024] S4: The double-threaded conductive yarn with electrodes is encapsulated with an encapsulation material to obtain a flexible strain sensor based on the double-threaded conductive yarn.

[0025] As described above, in the preparation method of a flexible strain sensor based on double-threaded conductive yarn, step S1 specifically involves hot-pressing a low-melting-point hot melt wire wound on the surface of an elastic fiber under conditions of 120–160°C and 1500–2000 Pa to obtain a double-threaded yarn; the coverage of the low-melting-point hot melt wire is 20–40 twists / cm.

[0026] As described above, the method for preparing a flexible strain sensor based on double-threaded conductive yarn includes step S2, which involves: first immersing the double-threaded yarn in a polyvinyl alcohol aqueous solution for 3-5 minutes, then cleaning and drying it, and then ultrasonically immersing it in a conductive filler suspension for 3-5 minutes, followed by cleaning and drying; repeating the above process 5-10 times to obtain the double-threaded conductive yarn.

[0027] As described above, in the preparation method of a flexible strain sensor based on double-threaded conductive yarn, the preparation process of polyvinyl alcohol aqueous solution is as follows: polyvinyl alcohol powder is added to deionized water and heated to 80°C and stirred for 60-90 minutes to obtain a polyvinyl alcohol aqueous solution with a mass concentration of 0.5-1%.

[0028] The method for preparing a flexible strain sensor based on double-threaded conductive yarn as described above is characterized in that the preparation process of the conductive filler suspension is as follows: a dispersant and a carbon-based conductive material are added to deionized water at a mass ratio of 1:2 to obtain a mixture, and the mixture is ultrasonically dispersed at 0-25°C for 0.5-1h under an ultrasonic power of 60W to obtain a conductive filler suspension with a mass concentration of 0.8-1%.

[0029] The dispersant is polyvinylpyrrolidone, sodium dodecyl sulfate, or sodium dodecylbenzenesulfonate.

[0030] The method for preparing a flexible strain sensor based on double-threaded conductive yarn as described above, wherein step S3 specifically involves: first, wrapping copper wire around both ends of the double-threaded conductive yarn 5 to 10 times, then uniformly coating it with conductive adhesive, and curing it at 60°C for 20 to 40 minutes.

[0031] In the above-described method for preparing a flexible strain sensor based on a double-threaded conductive yarn, step S4 involves the following steps: Component A and Component B of a two-component liquid silicone rubber material are uniformly mixed at a mass ratio of 1:1. Then, the mixture is diluted with an organic solvent (the organic solvent must be able to dissolve this two-component liquid silicone rubber material without causing a chemical reaction; it can be hexane, dichloromethane, tetrahydrofuran, etc.) at a ratio of 1g:5ml, and stirred at 25°C for 10 minutes to obtain the encapsulation material.

[0032] As described above, the method for preparing a flexible strain sensor based on double-threaded conductive yarn includes step S4, which involves immersing the double-threaded conductive yarn with electrodes in an encapsulation material for 5–20 seconds, then removing it and placing it in a vacuum oven to cure it at a temperature of 60–90°C for 60–120 minutes to obtain the flexible strain sensor based on double-threaded conductive yarn.

[0033] The principle of this invention is as follows:

[0034] This invention provides a flexible strain sensor based on bifilar threaded conductive yarn. On one hand, the bifilar thread achieves a non-uniform strain distribution effect. The strain concentration area induces longer and wider microcracks in the conductive layer, resulting in more dramatic resistance changes and exhibiting excellent sensitivity. Conversely, the strain separation area induces small and dense microcracks in the conductive layer, ensuring the resistance path remains intact and broadening the strain range of the strain sensor. On the other hand, a thermo-pressing bonding method integrates the low-melting-point hot-melt wire and elastic fiber into a single unit. During stretching, relative slippage between the heterogeneous components of the bifilar threaded yarn is prevented, avoiding stick-slip interactions that could hinder the release or unstable recovery of the composite yarn, thus reducing the yarn's mechanical hysteresis. A layer-by-layer assembly method for coating a carbon-based conductive layer not only ensures a strong and stable interface between the carbon-based conductive layer and the bifilar threaded yarn but also allows for precise control of the carbon-based conductive layer thickness, providing a core foundation for achieving a flexible strain sensor with high linearity, low hysteresis, and high stability. Furthermore, by matching a suitable elastic encapsulation layer to the outer periphery of the bifilar threaded conductive yarn, a hydrophobic flexible strain sensor was successfully fabricated. Without affecting the sensing performance of the double-threaded conductive yarn, the sensitivity of the sensor is further improved, demonstrating great potential in practical applications.

[0035] Beneficial effects:

[0036] (1) The flexible strain sensor based on double-threaded conductive yarn of the present invention has a large strain working range, excellent linearity, low hysteresis, and at the same time ensures that the sensor has good sensitivity.

[0037] (2) The flexible strain sensor based on double-threaded conductive yarn of the present invention has good durability and hydrophobicity;

[0038] (3) The present invention provides a method for preparing a flexible strain sensor based on double-threaded conductive yarn, which is simple to operate, low in cost, and suitable for continuous and large-scale production. Attached Figure Description

[0039] Figure 1 This is a schematic diagram of the structure of the double-threaded yarn of the present invention;

[0040] Figure 2 This is a flowchart illustrating the fabrication process of the flexible strain sensor based on double-threaded conductive yarn according to the present invention.

[0041] Figure 3 This is a schematic diagram of the preparation process of the double-threaded yarn of the present invention;

[0042] Figure 4 The present invention relates to the relative resistance change-strain curve and its linear fitting of the flexible strain sensor based on double-threaded conductive yarn within 100% strain, with a strain rate of 5% / s.

[0043] Figure 5 This is the relative resistance change-strain curve of the flexible strain sensor based on double-threaded conductive yarn of the present invention during the 5th stretching / releasing of 100% strain, with a strain rate of 5% / s.

[0044] Figure 6 It is a multi-cycle test of a flexible strain sensor based on a double-threaded conductive yarn when stretched to 20%, 40%, and 60% strain in 10 stretch-release cycles, with a stretching frequency of 0.05 Hz;

[0045] Figure 7 This is a graph showing the relative resistance change of the flexible strain sensor based on double-threaded conductive yarn of the present invention after 2000 cycles of stretching-releasing under 50% strain.

[0046] Figure 8 This is a schematic diagram of the strain distribution of double-threaded yarn when strain is applied.

[0047] Figure 9 It is a longitudinal cross-sectional SEM image of a flexible strain sensor based on double-threaded conductive yarn at different locations with different strains applied.

[0048] Figure 10 This is a schematic diagram of a water droplet falling on a flexible strain sensor based on double-threaded conductive yarn to test the contact angle.

[0049] Figure 11 The sensor is attached to the elbow, and the relative resistance changes are measured when the elbow is bent 10 times each at 45°, 90° and 135°.

[0050] Among them, 1-spandex filament, 2-feed roller, 3-hollow spindle, 4-low melting point hot melt wire, 5-winding shaft, 6-gear, 7-conveyor belt, 8-motor, 9-winding roller, 10-yarn guide, 11-flat hot press, 12-double thread yarn. Detailed Implementation

[0051] The present invention will be further described below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0052] The substances used in this invention are as follows:

[0053] (1) Polyvinyl alcohol: It is derived from Shanghai Aladdin Biochemical Technology Co., Ltd., with the brand name PVA-1788 and a weight-average molecular weight of 44050.

[0054] (2) Polyvinylpyrrolidone: sourced from Sinopharm Chemical Reagent Co., Ltd., brand name is PVP-K30;

[0055] (3) Carbon nanotubes: sourced from Suzhou Carbon Abundant Graphene Technology Co., Ltd. (MWCNTs, purity >95wt%, inner diameter 3-5nm, outer diameter 8-15nm, length 3-12μm);

[0056] (4) Carbon black: sourced from Suzhou Carbon Graphene Technology Co., Ltd., nano carbon powder (purity >99.9wt%, particle size 20nm);

[0057] (5) Graphene: Sourced from Suzhou Carbon-Feng Graphene Technology Co., Ltd., chemically produced few-layer graphene (purity >95wt%, thickness 3-8nm, sheet diameter 10-50μm, number of layers 1-5, specific surface area 100-500m²). 2 / g);

[0058] (6) Conductive adhesive: It is double bond DB2011 copper powder conductive adhesive, which comes from Wuhan Double Bond Kem Sealing Materials Co., Ltd., and its brand name is DB2011;

[0059] (7) Two-component liquid silicone rubber material: sourced from Smooth-on, USA, brand name Ecoflex00-50;

[0060] (8) n-Hexane: derived from Changshu Yangyuan Chemical Co., Ltd., CH3(CH2)4CH3, analytical grade;

[0061] (9) Dichloromethane: derived from Shanghai Aladdin Biochemical Technology Co., Ltd., CH2Cl2, analytical grade;

[0062] (10) Tetrahydrofuran: derived from Shanghai Aladdin Biochemical Technology Co., Ltd., C4H8O, analytical grade.

[0063] The testing method used in this invention is as follows:

[0064] The sensor electrodes were fixed at both ends on a universal tensile testing machine and stretched at a speed of 90 mm / min. When the strain reached 100%, the sensor was returned to its initial position at the same speed. This process was repeated for 5 cycles. The hysteresis rate was calculated by taking the relative resistance change-strain curve of the 5th cycle. The hysteresis rate of the sensor is Hs = Δ(Ru-Rd)max / Rmax × 100%. Δ(Ru-Rd) is the maximum difference in resistance between the stretching and recovery processes under the same strain in a certain cycle. Rmax is the maximum resistance change during the stretching and recovery process in that cycle. Ru and Rd are the resistances corresponding to the stretching and recovery processes, respectively.

[0065] The linearity of the sensor is δ = ΔYmax / Y × 100%, where ΔYmax is the maximum deviation between the sensor's test curve and the fitted straight line, and Y is the percentage of full-scale output. Within the 100% strain range, the least squares method is used to fit the straight line. The sensitivity is GF = (R - R0) / R0ε, where R is the instantaneous resistance, R0 is the initial resistance, and ε is the strain.

[0066] Example 1

[0067] A method for fabricating a flexible strain sensor based on double-threaded conductive yarn, such as... Figure 2 As shown, it includes the following steps:

[0068] S1: Prepare double-threaded yarn;

[0069] like Figure 3As shown, the core spandex filament 1 (1120D) is unwound from the feed roller 2 at a speed of 9 r / min and passes parallel to the hollow spindle 3. Using the core spandex filament 1 as the axis, the winding shaft 5 drives the low-melting-point hot melt wire 4 (150D, melting point 85℃) to be uniformly wound around the surface of the spandex filament 1 at a speed of 750 r / min. The winding shaft 5 is fitted onto the hollow spindle 3, and its rotation is controlled by a motor 8 via gear 6 and conveyor belt 7. The coverage of the low-melting-point hot melt wire is 35%. The twist rate is 10000 / cm. The speed difference between the winding roller 9 (winding speed is 10r / min) and the feed roller 2 controls the draft ratio of the core yarn spandex filament 1. After the low melting point hot melt yarn 4 is wound on the spandex filament 1, it is fed into the flat hot press 11 through the yarn guide 10 for hot pressing (hot pressing temperature 140℃, hot pressing pressure 1600Pa, hot pressing time is 20s). Two spiral continuous convex structures are formed on the surface of the spandex filament 1 by the low melting point hot melt yarn 4, namely double thread yarn 12.

[0070] like Figure 1 As shown, in the obtained double-threaded yarn, the length of one lead is 2.5 mm, the pitch to lead ratio is 0.4, and the diameter of the cross-section of the double-threaded yarn is 0.72 mm.

[0071] S2: Prepare double-threaded conductive yarn;

[0072] Polyvinyl alcohol powder was added to deionized water and heated to 80°C and stirred for 60 minutes to obtain a polyvinyl alcohol aqueous solution with a mass concentration of 0.5%.

[0073] Polyvinylpyrrolidone and carbon nanotubes were added to deionized water at a mass ratio of 1:2 to obtain a mixture, and the mixture was ultrasonically dispersed at 5°C for 1 h under an ultrasonic power of 60 W to obtain a carbon nanotube suspension with a mass concentration of 1%.

[0074] First, soak the double-threaded yarn in a polyvinyl alcohol aqueous solution for 3 minutes, then wash and dry it. Then, ultrasonically soak it in a conductive filler suspension for 3 minutes, then wash and dry it. Repeat the above process 6 times to obtain double-threaded conductive yarn.

[0075] S3: Electrode preparation;

[0076] First, wrap copper wire around both ends of double-threaded conductive yarn 10 times, then evenly coat it with conductive adhesive, and cure it at 60°C for 30 minutes to form two electrodes.

[0077] S4: Fabrication of a flexible strain sensor based on double-threaded conductive yarn;

[0078] The A and B components of the two-component liquid silicone rubber material were uniformly mixed in a mass ratio of 1:1, then diluted with n-hexane at a ratio of 1g:5ml, and stirred at 25°C for 10 minutes to obtain the encapsulation material.

[0079] The double-threaded conductive yarn with electrodes was immersed in the encapsulation material for 10 seconds, then removed and placed in a vacuum oven to be cured at 60°C for 90 minutes to obtain a flexible strain sensor based on the double-threaded conductive yarn.

[0080] The fabricated flexible strain sensor based on double-threaded conductive yarn includes a core elastic yarn, an intermediate conductive material, and an outer encapsulation material. The core elastic yarn is a double-threaded yarn, and the intermediate conductive material includes a strain-sensitive layer (i.e., carbon nanotubes) and electrodes fixed at both ends of the strain-sensitive layer. The strain-sensitive layer and the electrodes are connected by conductive adhesive. The outer encapsulation material covers the outer periphery of the double-threaded conductive yarn but does not change its double-threaded structural characteristics.

[0081] This flexible strain sensor exhibits good resistance response and stable repeatability under different strains, such as Figure 6 As shown; Figure 4 , 5 As shown, the flexible strain sensor based on double-threaded conductive yarn exhibits a linearity of 0.978, a sensitivity of 12.43, and a hysteresis rate of 13.3% within 100% strain. After 2000 repeated cycles at 50% strain, the linearity of this flexible strain sensor is evaluated as 0.97, the sensitivity as 11.36, and the hysteresis rate as 6.3%. Figure 7 As shown, the resistance change with strain is completely reversible and repeatable over 2000 cycles, which means that the flexible strain sensor has reliable mechanical properties and electrical response in long-term use.

[0082] The performance of the flexible strain sensor based on double-threaded conductive yarn prepared in Example 1 was compared with the performance of different existing flexible strain sensors. The results are shown in Table 1:

[0083]

[0084] Table 1

[0085] The references are as follows:

[0086] [1]WANG W T,LU L S,LI Z H,et al.Fingerprint-Inspired Strain Sensorwith Balanced Sensitivity and Strain Range Using Laser-Induced Graphene[J].Acs Applied Materials&Interfaces,2022,14(1):1315-25;

[0087] [2]CAI G,HAO B,LUO L,et al.Highly stretchable sheath-core yarns formultifunctional wearable electronics[J].ACS Appl Mater Interfaces,2020,12(26):29717-27;

[0088] [3]CAO Z R,WANG R R,HE T Y,et al.Interface-controlled conductivefibers for wearable strain sensors and stretchable conducting Wires[J].ACSAppl Mater Interfaces,2018,10(16):14087-96;

[0089] [4]LI X,HUA T,XU B.Electromechanical properties of a yarn strainsensor with graphene-sheath / polyurethane-core[J].Carbon,2017,118:686-98;

[0090] [5]ZHANG S C,SUN Y,XU J T.(3-Mercaptopropyl)triethoxysilane-ModifiedReduced Graphene Oxide-Modified Polyurethane Yarn Enhanced by Epoxy / ThiolReactions for Strain Sensors[J].Acs Applied Materials&Interfaces,2021,13(29):34865-76;

[0091] [6] ZHAO M, LI DW, HUANG JY, et al. A multifunctional and highly stretchable electronic device based on silver nanowire / wrap yarn composite for a wearable strain sensor and heater [J]. Journal of Materials Chemistry C, 2019, 7(43): 13468-76;

[0092] [7]YAN T, ZHOU H, NIU HT, et al. Highly sensitive detection of subtle movement using a flexible strain sensor from helically wrapped carbon yarns[J]. Journal of Materials Chemistry C, 2019, 7(32): 10049-58;

[0093] [8] LI XT, HU HB, HUA T, et al. Wearable strain sensing textile based on one-dimensional stretchable and weavable yarn sensors [J]. Nano Res, 2018, 11(11): 5799-811;

[0094] [9] WU H, LIU Q, CHEN H, et al. Fibrous strain sensor with ultra-sensitivity, wide sensing range, and large linearity for full-range detection of human motion [J]. Nanoscale, 2018, 10(37): 17512-9.

[0095] To theoretically analyze the impact of the double-threaded structure on the conductive network during tensile testing, a FEA simulation was performed using ABAQUS software, revealing the strain distribution, such as... Figure 8As shown, because the pitch of this double-threaded design is less than half the lead, it divides the exposed spandex surface into two regions with unequal surface areas, defined as region I and region II. Simulation results show that, in the tensile direction, the strain in region I is significantly lower than that in region II, which leads to a rhythmic strain distribution on the spandex surface during stretching. Furthermore, because the hot-melt wire is helically bonded to the spandex core yarn surface, when tensile strain is applied, the hot-melt wire does not experience relative slippage due to friction with the spandex, and the strain generated by the hot-melt wire is relatively small; while the interface between the hot-melt wire and the spandex experiences large strain due to stress concentration, mainly occurring at the inner edge of region II.

[0096] During the stretching process, due to the induced strain redistribution effect, multi-scale cracks are generated in the carbon nanotube conductive layer on the surface of the flexible strain sensor based on double-threaded conductive yarn, such as... Figure 9 As shown in the diagram, the greater the applied external strain, the greater the elongation of the sample, followed by an increase in the number of microcracks and a gradual increase in the gap between the cracks. This results in a sharp and proportional increase in the relative resistance change of the yarn strain sensor with strain. In region I, between the double-threaded strands, the cracks are short in length and have small gaps; the outer encapsulation layer does not crack, only wrinkles, ensuring good conductivity of the sensor under high strain, thus providing a wide strain range. In region II, the carbon nanotube conductive layer on the uniform and smooth fiber structure experiences larger local strain under high strain tension, resulting in longer and wider cracks and a more pronounced resistance response, thus ensuring good sensor sensitivity. Furthermore, since the surface strain of the conductive layer on the threaded strands is minimal, the resulting cracks are smaller and fewer, which helps the sensor maintain a stable conductive path under high strain. Finally, due to the inherent mechanical properties of the elastic spandex core yarn, the flexible strain sensor based on the double-threaded conductive yarn exhibits good resilience, almost recovering its original length. The microcracks decrease proportionally during the release process. Once the strain is fully recovered, the sensing layer shows no significant damage. When the cracks re-engage, only a few tiny cracks remain.

[0097] like Figure 10 As shown, the contact angle test reveals that the contact angle of the sensor surface coated with Ecoflex is greater than 90°, indicating that the sensor surface has good hydrophobicity.

[0098] like Figure 11 As shown, when the sensor is attached to the skin of the elbow, it responds accurately to different bending angles of the elbow. Furthermore, as the bending angle increases, the strain of the elbow skin increases, and the relative resistance of the sensor also increases. This indicates that the sensor has good sensing performance and has good application potential in human motion detection.

[0099] Example 2

[0100] A method for fabricating a flexible strain sensor based on double-threaded conductive yarn, such as... Figure 2 As shown, it includes the following steps:

[0101] S1: Prepare double-threaded yarn;

[0102] like Figure 3 As shown, the core spandex filament (840D) is unwound from the feed roller at a speed of 7 r / min and passes parallel to the hollow spindle. Using the core spandex filament as the axis, the winding shaft drives the low melting point hot melt yarn (150D, melting point of 85℃) to be uniformly wound on the surface of the spandex filament at a speed of 780 r / min. The winding shaft is sleeved on the hollow spindle and is controlled by a motor through gears and a conveyor belt. The coverage of the low melting point hot melt yarn is 40 twists / cm. The draw ratio of the core spandex filament is then controlled by the speed difference between the winding roller (winding speed of 9 r / min) and the feed roller. After the low melting point hot melt yarn is wound on the spandex filament 1, it is fed into a flat hot press through a yarn guide for hot pressing (hot pressing temperature of 120℃, hot pressing pressure of 2000Pa, hot pressing time of 40s). On the surface of the spandex filament, two spiral continuous convex structures are formed by the low melting point hot melt yarn, namely double-threaded ribbed yarn.

[0103] like Figure 1 As shown, in the obtained double-threaded yarn, the length of one lead is 2mm, the pitch to lead ratio is 0.4, and the diameter of the cross-section of the double-threaded yarn is 0.64mm.

[0104] S2: Prepare double-threaded conductive yarn;

[0105] Polyvinyl alcohol powder was added to deionized water and heated to 80°C and stirred for 90 minutes to obtain a polyvinyl alcohol aqueous solution with a mass concentration of 0.7%.

[0106] Sodium dodecyl sulfate and carbon black were added to deionized water at a mass ratio of 1:2 to obtain a mixture. The mixture was then ultrasonically dispersed at 0°C for 0.5 h under an ultrasonic power of 60 W to obtain a carbon black suspension with a mass concentration of 0.9%.

[0107] First, the double-threaded yarn is soaked in a polyvinyl alcohol aqueous solution for 4 minutes, then washed and dried. Then, it is ultrasonically soaked in a conductive filler suspension for 5 minutes, then washed and dried. The above process is repeated 9 times to obtain double-threaded conductive yarn.

[0108] S3: Electrode preparation;

[0109] First, wrap the copper wire around both ends of the double-threaded conductive yarn 5 times, then coat it evenly with conductive adhesive, and cure it at 60°C for 20 minutes to form two electrodes.

[0110] S4: Fabrication of a flexible strain sensor based on double-threaded conductive yarn;

[0111] The A and B components of the two-component liquid silicone rubber material were uniformly mixed in a mass ratio of 1:1, then diluted with dichloromethane at a ratio of 1g:5ml, and stirred at 25°C for 10 minutes to obtain the encapsulation material.

[0112] The double-threaded conductive yarn with electrodes was immersed in the encapsulation material for 5 seconds, then removed and placed in a vacuum oven to be cured at 60°C for 60 minutes to obtain a flexible strain sensor based on the double-threaded conductive yarn.

[0113] The fabricated flexible strain sensor based on double-threaded conductive yarn includes a core elastic yarn, an intermediate conductive material, and an outer encapsulation material. The core elastic yarn is a double-threaded yarn, and the intermediate conductive material includes a strain-sensitive layer (i.e., carbon black) and electrodes fixed at both ends of the strain-sensitive layer. The strain-sensitive layer and the electrodes are connected by conductive adhesive. The outer encapsulation material covers the outer periphery of the double-threaded conductive yarn but does not change its double-threaded structural characteristics.

[0114] The flexible strain sensor based on double-threaded conductive yarn exhibits a linearity of 0.989, a sensitivity of 13.52, and a hysteresis rate of 9.4% within 100% strain. At 50% strain, after 2000 repeated cycles, the linearity is evaluated as 0.975, the sensitivity as 12.26, and the hysteresis rate as 5.8%. This flexible strain sensor demonstrates good resistance response and stable repeatability under different strain conditions.

[0115] Example 3

[0116] A method for fabricating a flexible strain sensor based on double-threaded conductive yarn, such as... Figure 2 As shown, it includes the following steps:

[0117] S1: Prepare double-threaded yarn;

[0118] like Figure 3As shown, the core spandex filament (1680D) is unwound from the feed roller at a speed of 10 r / min and passes parallel to the hollow spindle. Using the core spandex filament as an axis, a winding shaft drives a low-melting-point hot melt wire (150D, melting point 110℃) to be uniformly wound around the surface of the spandex filament at a speed of 680 r / min. The winding shaft is fitted onto the hollow spindle, and its rotation is controlled by a motor via gears and a conveyor belt. The coverage of the low-melting-point hot melt wire is 2. The core yarn spandex filament has a twist of 0 / cm. The draw ratio is then controlled by the speed difference between the winding roller (winding speed of 11 r / min) and the feed roller. After the low melting point hot melt yarn is wound on the spandex filament, it is fed into a flat hot press through a yarn guide for hot pressing (hot pressing temperature 160℃, hot pressing pressure 1500Pa, hot pressing time 30s). Two spiral continuous raised structures are formed on the surface of the spandex filament by the low melting point hot melt yarn, which is a double-threaded ribbed yarn.

[0119] like Figure 1 As shown, in the obtained double-threaded yarn, the length of one lead is 4mm, the pitch to lead ratio is 0.4, and the diameter of the cross-section of the double-threaded yarn is 0.76mm.

[0120] S2: Prepare double-threaded conductive yarn;

[0121] Polyvinyl alcohol powder was added to deionized water and heated to 80°C and stirred for 75 minutes to obtain a polyvinyl alcohol aqueous solution with a mass concentration of 0.75%.

[0122] Sodium dodecylbenzenesulfonate and graphene were added to deionized water at a mass ratio of 1:2 to obtain a mixture. The mixture was then ultrasonically dispersed at 25°C for 0.75 h under an ultrasonic power of 60 W to obtain a graphene suspension with a mass concentration of 0.8%.

[0123] First, soak the double-threaded yarn in a polyvinyl alcohol aqueous solution for 5 minutes, then wash and dry it. Then, ultrasonically soak it in a conductive filler suspension for 4 minutes, then wash and dry it. Repeat the above process 5 times to obtain double-threaded conductive yarn.

[0124] S3: Electrode preparation;

[0125] First, wrap copper wire around both ends of double-threaded conductive yarn 10 times, then evenly coat it with conductive adhesive, and cure it at 60°C for 40 minutes to form two electrodes.

[0126] S4: Fabrication of a flexible strain sensor based on double-threaded conductive yarn;

[0127] The A and B components of the two-component liquid silicone rubber material were uniformly mixed in a mass ratio of 1:1, then diluted with tetrahydrofuran at a ratio of 1g:5ml, and stirred at 25°C for 10 minutes to obtain the encapsulation material.

[0128] The double-threaded conductive yarn with electrodes was immersed in the encapsulation material for 20 seconds, then removed and placed in a vacuum oven to be cured at 90°C for 120 minutes to obtain a flexible strain sensor based on the double-threaded conductive yarn.

[0129] The fabricated flexible strain sensor based on double-threaded conductive yarn includes a core elastic yarn, an intermediate conductive material, and an outer encapsulation material. The core elastic yarn is a double-threaded yarn, and the intermediate conductive material includes a strain-sensitive layer (i.e., graphene) and electrodes fixed at both ends of the strain-sensitive layer. The strain-sensitive layer and the electrodes are connected by conductive adhesive. The outer encapsulation material covers the outer periphery of the double-threaded conductive yarn but does not change its double-threaded structural characteristics.

[0130] The flexible strain sensor based on double-threaded conductive yarn exhibits a linearity of 0.97, a sensitivity of 11.34, and a hysteresis of 12.7% within 100% strain. At 50% strain, after 2000 repeated cycles, the linearity is evaluated as 0.964, the sensitivity as 9.86, and the hysteresis as 6.2%. This flexible strain sensor demonstrates good resistance response and stable repeatability under different strain conditions.

Claims

1. A flexible strain sensor based on double-threaded conductive yarn, characterized in that: It includes a core layer of elastic yarn, a middle layer of conductive material, and an outer layer of encapsulation material; The core layer elastic yarn is a double-threaded yarn, and the intermediate conductive material includes a strain-sensitive layer and electrodes fixed at both ends of the strain-sensitive layer. The double-threaded yarn includes a supporting core yarn and a double-threaded structure. The supporting core yarn is made of elastic fiber, and the double-threaded structure is made of low-melting-point hot-melt wire with a melting point below 110℃. The low-melting-point hot-melt wire is partially melted and bonded to the surface of the elastic fiber. The double-threaded structure consists of two continuous spiral protrusions formed on the surface of the supporting core yarn, with the two spiral continuous protrusions winding in the same direction. The length of one lead in the double-threaded yarn is 2~4mm, and the pitch to lead ratio is 0.

4. The strain-sensitive layer is a carbon-based conductive material; The electrode is a copper wire, and the strain-sensitive layer is connected to the electrode by conductive adhesive. The elastic fiber is spandex filament, and the melting point of the low-melting-point hot melt wire is 85~110℃; the outer encapsulation material is a two-component liquid silicone rubber material.

2. A flexible strain sensor based on double-threaded conductive yarn according to claim 1, characterized in that, The flexible strain sensor based on double-threaded conductive yarn has a linearity of not less than 0.970, a sensitivity of not less than 11.34, and a hysteresis rate of not more than 13.3% within 100% strain. The flexible strain sensor based on double-threaded conductive yarn has a linearity of no less than 0.964, a sensitivity of no less than 9.86, and a hysteresis rate of no more than 6.3% after 2000 cycles of stretching at 50% strain.

3. A flexible strain sensor based on double-threaded conductive yarn according to claim 1, characterized in that, The carbon-based conductive material is one or more of carbon nanotubes, carbon black, and graphene.

4. A method for preparing a flexible strain sensor based on double-threaded conductive yarn as described in any one of claims 1 to 3, characterized in that... Includes the following steps: S1: Low melting point hot melt wire with a melting point of 85~110℃ is wound onto the surface of elastic fiber by a wrapping machine, and then the low melting point hot melt wire is hot-pressed by a flat hot press to obtain double-threaded yarn; the double-threaded structure of the double-threaded yarn refers to two spiral continuous protrusions formed by the low melting point hot melt wire on the surface of the elastic fiber. S2: A layer-by-layer assembly method is used to coat carbon-based conductive material onto the surface of double-threaded yarn to obtain double-threaded conductive yarn; S3: Copper wires are fixed to both ends of double-threaded conductive yarn with conductive adhesive to form two electrodes; S4: The double-threaded conductive yarn with electrodes is encapsulated with an encapsulation material to obtain a flexible strain sensor based on the double-threaded conductive yarn.

5. The method according to claim 4, characterized in that, Specifically, step S1 involves hot-pressing a low-melting-point hot melt wire wound on the surface of an elastic fiber under conditions of 120~160℃ and 1500~2000Pa to obtain a double-threaded yarn. The coverage of low-melting-point hot melt wire is 20~40 twists / cm.

6. The method according to claim 4, characterized in that, Step S2 specifically involves: first immersing the double-threaded yarn in a polyvinyl alcohol aqueous solution for 3-5 minutes, then cleaning and drying it, and then ultrasonically immersing it in a conductive filler suspension for 3-5 minutes, followed by cleaning and drying; repeating the above process 5-10 times to obtain the double-threaded conductive yarn.

7. The method according to claim 6, characterized in that, The preparation process of polyvinyl alcohol aqueous solution is as follows: polyvinyl alcohol powder is added to deionized water and heated to 80℃ and stirred for 60~90 min to obtain a polyvinyl alcohol aqueous solution with a mass concentration of 0.5~1%.

8. The method according to claim 6, characterized in that, The preparation process of the conductive filler suspension is as follows: a dispersant and a carbon-based conductive material are added to deionized water at a mass ratio of 1:2 to obtain a mixture, and the mixture is ultrasonically dispersed at 0~25℃ for 0.5~1h under an ultrasonic power of 60W to obtain a conductive filler suspension with a mass concentration of 0.8~1%; The dispersant is polyvinylpyrrolidone, sodium dodecyl sulfate, or sodium dodecylbenzenesulfonate.

9. The method according to claim 4, characterized in that, Step S3 specifically involves: first, wrapping copper wire around both ends of the double-threaded conductive yarn 5 to 10 times, then uniformly coating it with conductive adhesive, and curing it at 60°C for 20 to 40 minutes.

10. The method according to claim 4, characterized in that, In step S4, the encapsulation material is prepared by uniformly mixing component A and component B of the two-component liquid silicone rubber material at a mass ratio of 1:1, then diluting it with an organic solvent at a ratio of 1g:5ml, and stirring at 25°C for 10 minutes to obtain the encapsulation material.

11. The method according to claim 10, characterized in that, Step S4 specifically involves immersing the double-threaded conductive yarn with electrodes in the encapsulation material for 5-20 seconds, then removing it and placing it in a vacuum oven to cure it at a temperature of 60-90°C for 60-120 minutes, thereby obtaining a flexible strain sensor based on the double-threaded conductive yarn.

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