Method for regulating surface micro-crack density of flexible strain sensor

Abstract

This invention belongs to the field of flexible strain sensor technology, and specifically relates to a method for controlling the microcrack density on the surface of a flexible strain sensor. First, wrinkles are formed on the surface of the flexible strain sensor substrate material. Then, a film material is covered on the surface of the wrinkled flexible strain sensor substrate material. Afterwards, stretching is performed to control the microcrack density on the surface of the flexible strain sensor. This invention provides a method for controlling the microcrack density on the surface of a flexible strain sensor, achieving control over the microcrack density through pre-stretching, the formation of wrinkled structures, and the formation of microcracks.

Description

Technical Field

[0001] This invention belongs to the field of flexible strain sensor technology, and specifically relates to a method for controlling the density of microcracks on the surface of a flexible strain sensor. Background Technology

[0002] With the rapid development of IoT technology, flexible strain sensors have attracted widespread attention and are rapidly developing in applications such as artificial electronic skin, human health monitoring, and human-computer interaction. Generally, strain sensors require high sensitivity to accurately detect different strains. To improve sensitivity, researchers have designed microcrack structures to enhance the sensitivity of strain sensors. For example, microcrack structures are constructed by pre-stretching a conductive filler layer; the size of microcracks is controlled by applying different strains to the conductive layer during pre-stretching; and a uniformly sized pleated structure is combined with a microcrack structure, and microcrack structures of different densities are achieved through pre-stretching to control the sensing performance of the strain sensor. Sun Hao et al. reported a method for depositing carbon nanotubes and AgNWs on electrospun TPU fiber felt using vacuum-assisted filtration, followed by pre-stretching to construct microcrack structures. The propagation of the microcracks achieved high sensitivity (reaching 11 × 10⁻⁶ within the range of 135%–171%). 4 The conductive fillers at both ends of the microcracks and the extended portions achieve a wide response range. Yu Yuyan et al. reported a flexible strain sensor with a PEDOT:PSS layer drop-coated on a PDMS substrate. The microcrack size was controlled by applying different pre-stretch strains on the substrate. Wu Weitong et al. combined a uniformly sized pleated structure with a microcrack structure, achieving microcrack size control through different pre-stretch strains. They fabricated a stretchable flexible strain sensor with stable sensing performance and high sensitivity.

[0003] Currently, a commonly used method for constructing microcracks involves applying pre-strain to a conductive layer, forming microcrack structures of varying sizes on the conductive layer. When subjected to external stimuli, the microcracks break and reconnect, causing a change in the resistance of the strain sensor, thereby improving the sensor's sensitivity. However, strain sensors with microcrack structures are prone to severe damage to the conductive path during crack propagation, leading to failure. This results in a narrower response range for the strain sensor, making it unsuitable for use in high-strain operating scenarios.

[0004] Furthermore, researchers have achieved a balance between sensitivity and response range by adjusting different pre-stretch strains. However, different pre-stretch strains can only control the size of microcracks; larger sizes result in higher sensitivity over a wider strain range, but a narrower response range. Moreover, strain sensors with large microcrack sizes cannot accurately detect minute strains. For large-size and high-density microcrack structures with the same crack area, the high-density microcrack structure can respond rapidly to minute strains when subjected to minor external stimuli. Therefore, controlling microcrack density remains a challenge. Summary of the Invention

[0005] The purpose of this invention is to provide a method for controlling the density of microcracks on the surface of a flexible strain sensor.

[0006] The technical solution adopted in this invention is as follows:

[0007] A method for controlling the microcrack density on the surface of a flexible strain sensor involves first obtaining wrinkles on the surface of a flexible strain sensor substrate material, then covering the wrinkled flexible strain sensor substrate material with a film material, and finally stretching the film to control the microcrack density on the surface of the flexible strain sensor.

[0008] Furthermore, the matrix material is first pre-stretched, then the conductive filler is embedded into the surface of the matrix material while it is in a pre-stretched state, and then the matrix material is released to obtain wrinkles on the surface of the flexible strain sensor matrix material.

[0009] Taking spandex yarn as an example, the spandex yarn is first pre-stretched, and then carboxylated multi-walled carbon nanotubes (MWCNT-COOH) are embedded into the surface of the stretched and swollen spandex fibers. After cleaning and drying, the pre-stretched state is released.

[0010] Preferably, the spandex yarn can be first ultrasonically cleaned in deionized water and ethanol solutions for 10 minutes to remove impurities and oil from the yarn surface, and then dried in a 60°C oven for 1 hour. Pre-stretching is then performed afterward.

[0011] When the base material is spandex yarn, the pre-stretch ratio can be determined to be 0-150%, but not 0; for example, it could be 50%, 100%, or 150%. In practice, there are no strict limitations on the pre-stretch ratio, as long as it is within the tensile range that the base material can withstand. If the base material is changed, the stretch ratio should be determined based on the specific base material.

[0012] Furthermore, the pre-stretched yarn is immersed in the organic solution of MWCNT-COOH and ultrasonically treated for 20-30 minutes at an ultrasonic intensity of 150W-300W.

[0013] The concentration of the organic solution of MWCNT-COOH can be selected from 2 mg / mL to 5 mg / mL, preferably 4 mg / mL.

[0014] The solvent for the organic solution may be selected from tetrahydrofuran, acetone, N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, dimethyl sulfoxide, dioxane, etc.

[0015] Furthermore, after MWCNT-COOH is embedded into the surface of the stretched and swollen spandex fiber, it is rinsed with deionized water, then placed in a fume hood for 20 minutes, and then dried at 60°C for 20 minutes before releasing the sample.

[0016] Furthermore, after covering the surface of the flexible strain sensor substrate material with a film material to form wrinkles, it is stretched again. The stretching is carried out on the basis of pre-stretching at a stretching ratio of 150-300%.

[0017] Because there is a release process after the pre-stretching and conductive material embedding in the matrix material, the stretch ratio after release will be lower than that after pre-stretching. Subsequent stretching will achieve a stretch ratio of 150-300% based on the first pre-stretch.

[0018] Specifically, the released matrix material is immersed in an aqueous solution of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonic acid)PEDOT:PSS for dip coating and drying, and finally stretched.

[0019] Preferably, the concentration of the PEDOT:PSS aqueous solution is 4 mg / mL to 12 mg / mL, and more preferably 4 mg / mL.

[0020] Repeat the process of immersing in PEDOT:PSS solution for coating and drying 3-4 times.

[0021] Preferably, each dip-coating time is 20 minutes, followed by 20 minutes in a 60°C oven.

[0022] Finally, EG was used to treat the spandex yarn to enhance the conductivity of the sample by separating the PEDOT and PSS phases.

[0023] The treatment can be performed by, but is not limited to, immersing the stretched sample in EG organic solvent for 5 minutes and then drying it in an oven at 60°C for 1 hour.

[0024] This invention discloses a method for controlling the microcrack density on the surface of a flexible strain sensor. First, spandex yarn is pre-stretched. Then, carboxylated multi-walled carbon nanotubes (MWCNT-COOH) are embedded into the surface of the stretched and swollen spandex fibers. After cleaning and drying, the fibers are released and then immersed in an aqueous solution of poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonic acid) PEDOT:PSS for coating and drying. Finally, the fibers are stretched again. This invention constructs a wrinkled structure of different densities using one-dimensional carboxylated multi-walled carbon nanotubes (MWCNT-COOH) on a PU matrix with varying degrees of pre-stretching. The wrinkle density increases with increasing pre-stretching strain. Furthermore, this invention forms a microcrack layer by coating with the conductive polymer PEDOT:PSS and then performs another stretching treatment. The use of wrinkled structures of different densities in conjunction with the stretching treatment achieves precise control of the microcrack density. As the applied strain increases, microcracks preferentially form at the crests of the wrinkles, and the higher the wrinkle density, the denser the formed microcracks.

[0025] Compared with the prior art, the present invention has the following advantages:

[0026] This invention discloses a method for controlling the microcrack density on the surface of a flexible strain sensor. By pre-stretching, forming a pleated structure, and stretching again, the formation density of microcracks is controlled by using pleated structures of different densities in synergistic stretching. In other words, the microcrack density on the surface of the flexible strain sensor can be directly controlled by using different pleat densities, which effectively overcomes the problems of existing methods that can only control the size of microcracks and cannot achieve a good balance between sensitivity and response range. Attached Figure Description

[0027] Figure 1 The effect of different fold densities on microcrack density of the product obtained in Example 1 is shown in the top and bottom columns, which are the obtained fold structures and their corresponding microcrack structures, respectively; from left to right, the microcrack density increases with the increase of fold density.

[0028] Figure 2 In Example 1, different pre-stretching ratios were used, but a re-stretching ratio of 150% was used to obtain the corresponding wrinkle density and microcrack density.

[0029] Figure 3 To further study the sensing performance of the PEDOT:PSS / MWCNT-COOH@PU strain sensor obtained by using different pre-stretch ratios but with a re-stretch ratio of 150%, Figure a shows... Figure 3 Enlarged view of the portion of the medium pre-stretch ratio in the range of 0-150%. Detailed Implementation

[0030] The technical solution of the present invention is illustrated below with specific embodiments, but the scope of protection of the present invention is not limited thereto:

[0031] The sources of materials in the following examples are described below, but this does not constitute a limitation on the sources of materials: Spandex yarn (1680D) was sourced from Haining Kaiwei Textile Co., Ltd.; Carboxylated multi-walled carbon nanotubes (MWCNT-COOH) (diameter = 20-30 nm, average length = 10-30 μm) were purchased from Chengdu Organic Chemistry Co., Ltd., Chinese Academy of Sciences; Poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonic acid) (PEDOT:PSS) was purchased from Beijing Innocare Technology Co., Ltd.; Ethylene glycol (EG) was purchased from Sigma-Aldrich Shanghai Trading Co., Ltd.

[0032] Example 1

[0033] A method for controlling the density of microcracks on the surface of a flexible strain sensor, comprising the following steps:

[0034] 1) Add 600 mg MWCNT-COOH to 150 mL THF and sonicate for 1 h to prepare a MWCNT-COOH / THF solution with a concentration of 4 mg / mL for later use. Dilute the PEDOT:PSS solution with deionized (DI) water to a concentration of 4 mg / mL for later use.

[0035] 2) The spandex yarn was ultrasonically soaked in deionized water and ethanol solution for 10 minutes in turn to remove impurities and oil stains from the yarn surface, and then dried in a 60℃ oven for 1 hour.

[0036] 3) After pre-stretching the cleaned spandex yarn to different strains (0%, 50%, 100%, 150%), immerse it in a MWCNT-COOH / THF solution and sonicate for 25 minutes to allow MWCNT-COOH to embed into the surface of the stretched and swollen spandex fiber. Rinse with deionized water, place in a fume hood for 20 minutes, and dry in a 60℃ oven for 20 minutes. Remove the clamp that holds the pre-stretched sample and release the sample, forming a wrinkled structure of different densities on the surface of the spandex fiber.

[0037] 4) Spandex yarns with different pleated structures were immersed in PEDOT:PSS solution and subjected to four dip-coating-drying cycles, each dip-coating time being 20 min, and then placed in a 60℃ oven for 20 min. Afterwards, samples with different pleated densities were stretched again. To ensure consistency, the tensile strain was fixed at 150% of the effective strain to construct microcrack structures, thereby achieving the control of microcrack density by pleated structures of different densities.

[0038] 5) Treatment of spandex yarn with EG: The pre-stretched sample was immersed in EG organic solvent for 5 min and then dried in an oven at 60℃ for 1 h. The conductivity of the sample was enhanced by separating the PEDO:PSS phase.

[0039] Conductive silver paste was uniformly coated on both ends of the prepared PEDOT:PSS / MWCNT-COOH@PU sample. After the conductive silver paste was slightly dry, copper-nickel conductive tape with a length of 50 mm and a width of 5 mm was attached to both ends of the sample as electrodes to obtain the PEDOT:PSS / MWCNT-COOH@PU strain sensor.

[0040] from Figure 1 , Figure 2 , Figure 3 As can be seen, with the gradual increase of tensile strain, different initial pre-stretching can cause the MWCNT-COOH layer to form a wrinkled structure of different densities, and the wrinkle density increases with the increase of the initial pre-stretching strain. After coating with PEDOT:PSS and then stretching again, microcrack structures of different densities can be formed based on the wrinkled structures of different densities. In the initial stage of stretching, the wrinkled structure gradually expands, generating a small number of microcracks. As the tensile strain increases, when the applied strain exceeds the initial tensile strain, microcracks will continue to form at the crests of the wrinkles. The greater the wrinkle density, the denser the formed microcracks. Based on the microcrack structures of different densities, it is possible to achieve… Figure 3 The performance trend is as follows: the greater the microcrack density, the more obvious the response of the sensor to external forces during tensile testing, resulting in high sensitivity; the stronger the resistance to external forces, the wider the response range while maintaining high sensitivity.

[0041] Within the scope of protection of the claims of this invention, the concentration of the solution is varied. The concentration of the solution will affect the amount of conductive filler load. However, the experimental results show that the control trend of microcrack density by different fold densities is not affected, and the response trend in tensile applications is also not affected.

Claims

1. A method for controlling the density of microcracks on the surface of a flexible strain sensor, characterized in that, First, wrinkles are obtained on the surface of the flexible strain sensor substrate material. Then, a film material is covered on the surface of the wrinkled flexible strain sensor substrate material. After stretching, the density of microcracks on the surface of the flexible strain sensor is controlled. Specifically, the substrate material is first pre-stretched, then conductive filler is embedded into the surface of the substrate material while maintaining the pre-stretched state, and then the substrate material is released to obtain wrinkles on the surface of the flexible strain sensor substrate material. The matrix material is made of spandex yarn. The spandex yarn is first pre-stretched, and then carboxylated multi-walled carbon nanotubes (MWCNT-COOH) are embedded into the surface of the stretched and swollen spandex fiber. After cleaning and drying, the pre-stretched state is released. The released matrix material is immersed in an aqueous solution of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonic acid)PEDOT:PSS for dip coating and drying, and finally stretched.

2. The method for controlling the microcrack density on the surface of a flexible strain sensor as described in claim 1, characterized in that, The stretching is performed based on pre-stretching with a stretching ratio of 150-300%.

3. The method for controlling the microcrack density on the surface of a flexible strain sensor as described in claim 1, characterized in that, PEDOT: The concentration of PSS in aqueous solution is 4 mg / mL to 12 mg / mL.

4. The method for controlling the surface microcrack density of a flexible strain sensor as described in claim 3, characterized in that, Repeat the process of immersing in PEDOT:PSS solution for coating and drying 3-4 times.

5. The method for controlling the surface microcrack density of a flexible strain sensor as described in claim 1, characterized in that, The pre-stretching ratio is 0-150%, but not 0.

6. The method for controlling the surface microcrack density of a flexible strain sensor as described in claim 1, characterized in that, The pre-stretched yarn is immersed in the organic solution of MWCNT-COOH and ultrasonically treated for 20-30 minutes at an intensity of 150W-300W.

7. The method for controlling the surface microcrack density of a flexible strain sensor as described in claim 6, characterized in that, The concentration of the organic solution of MWCNT-COOH is 2 mg / mL to 5 mg / mL.

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