Flexible strain sensor based on dual-resistivity sensitive layer structure and its fabrication method

Abstract

This invention relates to a flexible strain sensor based on a dual-resistivity sensitive layer structure and its fabrication method, belonging to the field of stress sensing technology. The sensor is an elastic fiber with a core-shell structure, consisting of a Gr / TPU shell and an ILs / TPUF core. The invention involves impregnating TPUF with ILs at high temperature to absorb them, followed by coating the surface with a Gr / TPU layer to achieve the flexible strain sensor fabrication. This invention combines a brittle carbon material as the conductive medium in the shell layer with a core layer containing directionally moving ions as the conductive medium. Utilizing the resistance difference between the shell and core layers, a flexible strain sensor with a segmented, multi-sensing mechanism is constructed, featuring a dual-resistivity sensitive layer structure. The sensor exhibits excellent mechanical properties, a strain range of 0–250%, a linearity greater than 0.995, and a sensitivity of up to 34399.9. This flexible strain sensor has significant application value in teleoperation systems, particularly in applications such as dexterous robotic arms.

Description

Technical Field

[0001] This invention belongs to the field of stress sensing technology, specifically relating to a flexible strain sensor based on a dual-resistance sensitive layer structure and its fabrication method. Background Technology

[0002] In the wave of technological innovation and interdisciplinary integration driven by the combination of sensing technology and artificial intelligence, flexible sensors have become an important research direction in the field of sensing technology. The excellent flexibility of flexible sensors allows them to conform to various curved surfaces, improving their adaptability to complex environments and enabling them to accurately acquire effective information in different testing environments. Flexible pressure sensors, flexible strain sensors, flexible temperature sensors, and flexible humidity sensors can sense external stimuli from different dimensions. Among them, flexible strain sensors, when subjected to external tensile stress, change their shape and structure, resulting in a change in the output electrical signal, thus realizing the sensing of external stress stimuli.

[0003] Flexible strain sensors applied to dexterous manipulators have significant value in robot teleoperation. The system captures the human hand's posture and synchronously reproduces it on the dexterous hand, enabling remote manipulation. Therefore, accurately sensing the human hand's posture is a prerequisite for precise dexterous hand operation, especially in specialized applications such as the assembly of precision industrial components and remote medical surgery, where even minute changes in hand posture are crucial to the operational outcome. Joint flexion and coordinated movement produce different hand posture changes, and the strain caused by human joint movement can reach 50% to 150%. This necessitates strain sensors used to monitor joint flexion angles to possess both high sensitivity and a wide strain detection range, thereby achieving accurate capture of human hand movements.

[0004] Currently, research on flexible strain sensors mainly focuses on composite systems of polymer matrices and conductive materials. Introducing microcracks into brittle conductive layers is an effective way to improve the sensitivity of flexible strain sensors. However, the high sensitivity of strain sensors based on microcrack propagation mechanisms is often limited to a narrow strain range, making them insensitive to large strains. Flexible strain sensors using anions and cations as conductive media in ionic liquids, due to the good stability and conductivity of ionic liquids, have a significantly increased strain range but relatively low sensitivity, resulting in a large strain detection range but insensitivity to small strain changes. In summary, it is difficult to simultaneously achieve a wide strain range and high sensitivity in strain sensors through a single material design, posing a significant challenge to flexible strain sensors used in remote operation applications of robotic arms. Therefore, a new design strategy for flexible strain sensors urgently needs to be developed to improve the high sensitivity of sensors over a wide strain range. Summary of the Invention

[0005] To address the existing problems, the purpose of this invention is to construct a multi-sensor mechanism segmented resistive strain sensor through a dual-resistive sensitive layer structural design. This sensor combines a shell layer with brittle carbon material as the conductive medium and a core layer with directionally moving ions as the conductive medium, thus providing a flexible strain sensor with a core-shell structure and its fabrication method. This improves the sensor's sensitivity over a wide strain detection range, solving the problem of existing flexible strain sensors struggling to achieve both a wide strain range and high sensitivity in robotic teleoperation applications.

[0006] The flexible strain sensor based on a dual-resistance sensitive layer structure described in this invention is an elastic fiber with a core-shell structure. The shell layer is a single layer of graphene (Gr) / thermoplastic polyurethane elastomer (TPU), and the core layer is 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt (ILs) / thermoplastic polyurethane elastic fiber (TPUF). In this invention, the TPUF absorbs ILs through an impregnation method in a high-temperature environment, and then a layer of Gr / TPU is wrapped on its surface to realize the fabrication of the flexible strain sensor.

[0007] The sensor's shell and core layers exhibit a dual-resistivity sensitive layer structure, with the shell layer in a low-resistivity state and the core layer in a high-resistivity state. Current preferentially flows through the low-resistivity region. When the device is subjected to tensile stress along the fiber length direction, within a small strain range, current mainly propagates through the low-resistivity region, resulting in a low initial resistance of the sensor. As the shell cracks propagate, the sensor resistance gradually increases. With increasing strain range, the shell cracks separate until they completely break, allowing some current to flow through the high-resistivity region of the core layer. This achieves synergy and integration of the two resistivity sensitive layers. The resistance difference between the high-resistivity and low-resistivity regions plays a crucial role in the sensitivity regulation of the sensor. When the tensile stress changes, the electrical properties of the flexible strain sensor change. By measuring the resistance values ​​at both ends of the fiber, a functional relationship between the sensor resistance and the tensile strain rate is established, allowing for the calculation of the sensor's resistive response and sensitivity.

[0008] The resistance response (R) of the strain sensor response R is defined as: response =ΔR / R0=(R-R0) / R0, where R is the resistance value at both ends of the positive and negative electrodes when the sensor generates different tensile strain rates, R0 is the resistance value at both ends of the positive and negative electrodes when the sensor is initially unstretched, and ΔR is the difference between R and R0.

[0009] The tensile strain rate (ε) of a strain sensor is defined as: ε = (ΔL / L0) * 100%, where ΔL is the length of the sensor being stretched and L0 is the initial length of the sensor.

[0010] The sensitivity (GF) of a strain sensor is defined as: GF = ΔR response / (ε1-ε2)ΔR responseThis is the difference between the resistance response generated by the sensor at a tensile strain rate of ε1 and the resistance response generated at a tensile strain rate of ε2.

[0011] The method for fabricating a flexible strain sensor based on a dual-resistance sensitive layer structure according to the present invention comprises the following steps:

[0012] (1) Place TPUF in 10~30mL of deionized water and ultrasonically clean for 5~30min, then dry at 50~70℃ to obtain TPUF with surface impurities removed; TPUF is thermoplastic polyurethane elastic fiber.

[0013] (2) The TPUF obtained in step (1) is immersed in 5~20 mL of ILs with a mass fraction of 95~98% and treated at 50~100℃ for 0.2~6.0 h. After that, it is taken out and the excess ILs on the surface of TPUF are wiped off with filter paper to obtain ILs / TPUF; ILs is 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt;

[0014] (3) Add 0.2~0.3g of TPU to 2~5mL of N,N-dimethylformamide (DMF), heat and stir until the TPU is completely dissolved to obtain a TPU solution; TPU is a thermoplastic polyurethane elastomer;

[0015] (4) Add 0.01~0.40g of Gr to the TPU solution obtained in step (3) and stir until Gr is evenly dispersed to obtain Gr / TPU mixed solution; Gr is monolayer graphene;

[0016] (5) Immerse the ILs / TPUF obtained in step (2) in the Gr / TPU mixed solution obtained in step (4) for 1~5s, take it out, dry it at 50~70℃, and pre-stretch it 20~200 times under a tensile strain rate of 100~300% to obtain a Gr / ILs / TPUF dual resistance sensitive layer, with the core layer being ILs / TPUF and the shell layer being Gr / TPU;

[0017] (6) Place two metal leads at both ends of the outer surface of the Gr / TPU shell of the Gr / ILs / TPUF dual resistance sensitive layer to achieve connection with the external test circuit, thereby obtaining the flexible strain sensor based on the dual resistance sensitive layer structure.

[0018] Furthermore, in step (1), the length of the TPUF is 2~5cm and the diameter is 0.5~1.5mm; in step (2), the diameter of the ILs / TPUF is 0.54~2.0mm and the resistivity is 100~5100Ω·m; in step (5), the diameter of the Gr / ILs / TPUF dual resistive sensitive layer is 0.66~2.82mm and the resistivity is 0.05~0.25Ω·m;

[0019] Furthermore, in step (3), the TPU is heated and stirred for 0.5 to 2.0 h under a water bath at 50 to 80°C until it is completely dissolved; in step (4), the TPU is stirred for 5 to 20 h under a water bath at 20 to 30°C until it is uniformly dispersed.

[0020] The flexible strain sensor based on a dual-resistance sensitive layer structure described in this invention is prepared by the above method.

[0021] The advantages of this invention are:

[0022] 1. The sensor designed in this invention has a simple manufacturing process, controllable preparation conditions and processes, and low cost, making it suitable for large-scale production;

[0023] 2. This invention combines a brittle carbon material as the shell layer of the conductive medium and a directionally moving ion as the core layer of the conductive medium to realize a resistive flexible strain sensor dominated by a segmented multi-sensing mechanism.

[0024] 3. The flexible strain sensor with a dual-resistance sensitive layer structure designed in this invention can achieve sensitivity adjustment within a wide strain detection range by regulating the resistance difference between the conductive shell layer and the core layer.

[0025] 4. The flexible strain sensor designed in this invention has good mechanical properties, small nonlinear error, low detection limit, and good stability and repeatability. Attached Figure Description

[0026] Figure 1 These are schematic diagrams of the flexible strain sensors prepared in Embodiments 1-5 of the present invention;

[0027] Figure 2 These are the resistance response curves of the flexible strain sensors prepared in Examples 1-5 of this invention as a function of tensile strain rate;

[0028] Figure 3 This is a bar chart comparing the sensitivity and linearity of the flexible strain sensors prepared in Examples 1-5 of this invention;

[0029] Figure 4 These are the stress-tensile strain rate curves of Gr / ILs / TPUF, TPUF, and ILs / TPUF prepared in Example 5 of this invention.

[0030] Figure 5 This is a bar chart comparing the tensile strength and elongation at break of Gr / ILs / TPUF, TPUF, and ILs / TPUF prepared in Example 5 of this invention;

[0031] Figure 6This is a bar chart comparing the Young's modulus and toughness of Gr / ILs / TPUF, TPUF and ILs / TPUF prepared in Example 5 of this invention;

[0032] Figure 7 These are the resistance response and recovery test curves of the flexible strain sensor prepared in Embodiment 5 of the present invention in the range of 40% to 250% tensile strain rate;

[0033] Figure 8 The right figure shows the response time and recovery time test curves of the flexible strain sensor prepared in Embodiment 5 of the present invention at 90% tensile strain rate. The right figure is an enlarged view of a local area of ​​the left figure.

[0034] Figure 9 These are the resistance response and recovery test curves of the flexible strain sensor prepared in Embodiment 5 of the present invention, which represent the lowest detection limit.

[0035] Figure 10 The image shows the resistance response curve over time of the flexible strain sensor prepared in Embodiment 5 of the present invention when it is stretched to 100% strain 2100 times, as well as a partial magnified view of some of the curves. Detailed Implementation

[0036] Example 1:

[0037] (1) A thermoplastic polyurethane elastic fiber with a length of 3.50 cm and a diameter of 0.80 mm was placed in 20 mL of deionized water and ultrasonically cleaned for 10 min. It was then dried at 60 °C to obtain TPUF with surface impurities removed.

[0038] (2) The TPUF obtained in step (1) was immersed in 10 mL of ILs with a mass fraction of 97%, treated at 90°C for 4 h, and then removed. Excess ILs on the surface were wiped off with filter paper to obtain ILs / TPUF with a diameter of about 1.06 mm and a resistivity of about 177.71 Ω·m.

[0039] (3) Add 0.25g of TPU to 3mL of N,N-dimethylformamide (DMF), heat and stir in a water bath at 60℃ for 1h until the TPU is completely dissolved to obtain a TPU solution;

[0040] (4) Add 0.35g of Gr to the TPU solution obtained in step (3) and stir at 25°C for 12h until Gr is uniformly dispersed to obtain Gr / TPU mixed solution;

[0041] (5) The ILs / TPUF obtained in step (2) is immersed in the Gr / TPU mixed solution obtained in step (4) for 2 seconds and then taken out, dried in an environment of 60°C, and pre-stretched 50 times under a tensile strain rate of 200% to obtain a Gr / ILs / TPUF double resistance sensitive layer with a diameter of about 1.15 mm and a resistivity of about 0.22 Ω·m. The core layer is ILs / TPUF and the shell layer is Gr / TPU.

[0042] (6) Take the middle part of the Gr / ILs / TPUF dual resistance sensitive layer obtained in step (5), cut it to a length of 2.50cm, and place two metal leads at both ends of the outer surface of the Gr / TPU shell of the Gr / ILs / TPUF dual resistance sensitive layer to obtain a flexible strain sensor based on the dual resistance sensitive layer structure. The sensor is named P1.

[0043] Example 2:

[0044] (1) A thermoplastic polyurethane elastic fiber with a length of 3.50 cm and a diameter of 0.80 mm was placed in 20 mL of deionized water and ultrasonically cleaned for 10 min. It was then dried at 60 °C to obtain TPUF with surface impurities removed.

[0045] (2) The TPUF obtained in step (1) was immersed in 10 mL of ILs with a mass fraction of 97%, treated at 90°C for 2 h, and then removed. Excess ILs on the surface were wiped off with filter paper to obtain ILs / TPUF with a diameter of about 1.02 mm and a resistivity of about 223.62 Ω·m.

[0046] (3) Add 0.25g of TPU to 3mL of N,N-dimethylformamide (DMF), heat and stir in a water bath at 60℃ for 1h until the TPU is completely dissolved to obtain a TPU solution;

[0047] (4) Add 0.35g of Gr to the TPU solution obtained in step (3) and stir at 25°C for 12h until Gr is uniformly dispersed to obtain Gr / TPU mixed solution;

[0048] (5) The ILs / TPUF obtained in step (2) is immersed in the Gr / TPU mixed solution obtained in step (4) for 2 seconds and then taken out, dried in an environment of 60°C, and pre-stretched 50 times under a tensile strain rate of 200% to obtain a Gr / ILs / TPUF dual resistance sensitive layer with a diameter of about 1.11 mm and a resistivity of about 0.17 Ω·m. The core layer is ILs / TPUF and the shell layer is Gr / TPU.

[0049] (6) Take the middle part of the Gr / ILs / TPUF dual resistance sensitive layer obtained in step (5), cut it to a length of 2.50cm, and place two metal leads at both ends of the outer surface of the Gr / TPU shell of the Gr / ILs / TPUF dual resistance sensitive layer to obtain a flexible strain sensor based on the dual resistance sensitive layer structure. The sensor is named P2.

[0050] Example 3:

[0051] (1) A thermoplastic polyurethane elastic fiber with a length of 3.50 cm and a diameter of 0.80 mm was placed in 20 mL of deionized water and ultrasonically cleaned for 10 min. It was then dried at 60 °C to obtain TPUF with surface impurities removed.

[0052] (2) The TPUF obtained in step (1) was immersed in 10 mL of ILs with a mass fraction of 97%, treated at 90°C for 1 h, and then removed. Excess ILs on the surface were wiped off with filter paper to obtain ILs / TPUF with a diameter of about 0.96 mm and a resistivity of about 397.80 Ω·m.

[0053] (3) Add 0.25g of TPU to 3mL of N,N-dimethylformamide (DMF), heat and stir in a water bath at 60℃ for 1h until the TPU is completely dissolved to obtain a TPU solution;

[0054] (4) Add 0.35g of Gr to the TPU solution obtained in step (3) and stir at 25°C for 12h until Gr is uniformly dispersed to obtain Gr / TPU mixed solution;

[0055] (5) The ILs / TPUF obtained in step (2) is immersed in the Gr / TPU mixed solution obtained in step (4) for 2 seconds and then taken out, dried in an environment of 60°C, and pre-stretched 50 times under a tensile strain rate of 200% to obtain a Gr / ILs / TPUF double resistance sensitive layer with a diameter of about 1.05 mm and a resistivity of about 0.11 Ω·m. The core layer is ILs / TPUF and the shell layer is Gr / TPU.

[0056] (6) Take the middle part of the Gr / ILs / TPUF dual resistance sensitive layer obtained in step (5), cut it to a length of 2.50cm, and place two metal leads at both ends of the outer surface of the Gr / TPU shell of the Gr / ILs / TPUF dual resistance sensitive layer to obtain a flexible strain sensor based on the dual resistance sensitive layer structure. The sensor is named P3.

[0057] Example 4:

[0058] (1) A thermoplastic polyurethane elastic fiber with a length of 3.50 cm and a diameter of 0.80 mm was placed in 20 mL of deionized water and ultrasonically cleaned for 10 min. It was then dried at 60 °C to obtain TPUF with surface impurities removed.

[0059] (2) The TPUF obtained in step (1) was immersed in 10 mL of ILs with a mass fraction of 97%, and treated at 90°C for 0.5 h. After that, it was taken out and the excess ILs on the surface were wiped off with filter paper to obtain ILs / TPUF with a diameter of about 0.90 mm and a resistivity of about 715.70 Ω·m.

[0060] (3) Add 0.25g of TPU to 3mL of N,N-dimethylformamide (DMF), heat and stir in a water bath at 60℃ for 1h until the TPU is completely dissolved to obtain a TPU solution;

[0061] (4) Add 0.35g of Gr to the TPU solution obtained in step (3) and stir at 25°C for 12h until Gr is uniformly dispersed to obtain Gr / TPU mixed solution;

[0062] (5) The ILs / TPUF obtained in step (2) is immersed in the Gr / TPU mixed solution obtained in step (4) for 2 seconds and then taken out, dried in an environment of 60°C, and pre-stretched 50 times under a tensile strain rate of 200% to obtain a Gr / ILs / TPUF double resistance sensitive layer with a diameter of about 0.99 mm and a resistivity of about 0.18 Ω·m. The core layer is ILs / TPUF and the shell layer is Gr / TPU.

[0063] (6) Take the middle part of the Gr / ILs / TPUF dual resistance sensitive layer obtained in step (5), cut it to a length of 2.50cm, and place two metal leads at both ends of the outer surface of the Gr / TPU shell of the Gr / ILs / TPUF dual resistance sensitive layer, thereby obtaining a flexible strain sensor based on the dual resistance sensitive layer structure, which is named P4.

[0064] Example 5:

[0065] (1) A thermoplastic polyurethane elastic fiber with a length of 3.50 cm and a diameter of 0.80 mm was placed in 20 mL of deionized water and ultrasonically cleaned for 10 min. It was then dried at 60 °C to obtain TPUF with surface impurities removed.

[0066] (2) The TPUF obtained in step (1) was immersed in 10 mL of ILs with a mass fraction of 97%, and treated at 90°C for 0.25 h. After that, it was taken out and the excess ILs on the surface were wiped off with filter paper to obtain ILs / TPUF with a diameter of about 0.87 mm and a resistivity of about 4994.73 Ω·m.

[0067] (3) Add 0.25g of TPU to 3mL of N,N-dimethylformamide (DMF), heat and stir in a water bath at 60℃ for 1h until the TPU is completely dissolved to obtain a TPU solution;

[0068] (4) Add 0.35g of Gr to the TPU solution obtained in step (3) and stir at 25°C for 12h until Gr is uniformly dispersed to obtain Gr / TPU mixed solution;

[0069] (5) The ILs / TPUF obtained in step (2) is immersed in the Gr / TPU mixed solution obtained in step (4) for 2 seconds and then taken out, dried in an environment of 60°C, and pre-stretched 50 times under a tensile strain rate of 200% to obtain a Gr / ILs / TPUF double resistance sensitive layer with a diameter of about 0.97 mm and a resistivity of about 0.10 Ω·m. The core layer is ILs / TPUF and the shell layer is Gr / TPU.

[0070] (6) Take the middle part of the Gr / ILs / TPUF dual resistance sensitive layer obtained in step (5), cut the length to 2.50cm, and place two metal leads at both ends of the Gr / TPU shell of the Gr / ILs / TPUF dual resistance sensitive layer to obtain a flexible strain sensor based on the dual resistance sensitive layer structure. The sensor is named P5.

[0071] Schematic diagrams of the flexible strain sensors prepared in Examples 1-5 are shown below. Figure 1 As shown, the ILs / TPUF core layer 1 is wrapped by the Gr / TPU shell layer 2. Two metal leads 3 and 3' are respectively connected to the outer surface of the Gr / TPU shell layer 2 at both ends of the sensor before being connected to the external test circuit.

[0072] The resistance response curves of the flexible strain sensors prepared in Examples 1-5 as a function of tensile strain rate are shown below. Figure 2 As shown, the strain range of the sensor can reach 0~250%, and as the resistivity of the sensor core increases, the resistance difference between the core and shell increases, and the resistive response of the sensor gradually increases.

[0073] The sensitivity and linearity comparison bar charts of the flexible strain sensors prepared in Examples 1-5 are shown below. Figure 3As shown, linearity is defined as the linear relationship between the resistive response of the sensor and the tensile strain rate. The sensitivities of sensors P1 to P5 are 696.9, 2115.5, 6650.3, 23043.1, and 34399.9, respectively. The sensitivity of the sensors increases with the increase of the core resistivity, reaching a maximum sensitivity of 34399.9. The linearity of sensors P1 to P5 is greater than 0.995, exhibiting a small linear error.

[0074] The stress-tensile strain rate curves of Gr / ILs / TPUF, TPUF, and ILs / TPUF prepared in Example 5 are shown below. Figure 4 As shown, stress is defined as the ratio of the external force required to stretch the sensor to the cross-sectional area. The Gr / ILs / TPUF treated with Gr and ILs exhibits a larger tensile strain rate range and a smaller slope.

[0075] The bar chart comparing the tensile strength and elongation at break of Gr / ILs / TPUF, TPUF, and ILs / TPUF prepared in Example 5 is shown below. Figure 5 As shown, tensile strength is defined as the maximum stress the sensor experiences before breaking under tension, and elongation at break is defined as the ratio of the maximum elongation of the sensor before breaking under tension to its initial length. Compared to TPUF and ILs / TPUF, Gr / ILs / TPUF exhibits lower tensile strength and higher elongation at break, indicating that Gr / ILs / TPUF treated with Gr and ILs is easier to stretch and less prone to breakage.

[0076] The Young's modulus and toughness comparison histograms of Gr / ILs / TPUF, TPUF, and ILs / TPUF prepared in Example 5 are shown below. Figure 6 As shown, Young's modulus is defined as the slope of the curve of sensor stress versus tensile strain rate, and toughness is defined as the area under the curve of sensor stress versus tensile strain rate. Compared with TPUF and ILs / TPUF, Gr / ILs / TPUF exhibits a smaller Young's modulus and stronger toughness, which indicates that Gr / ILs / TPUF treated with Gr and ILs has a greater elastic deformation capacity when subjected to stress.

[0077] The resistance response and recovery test curves of the flexible strain sensor prepared in Example 5 at tensile strain rates of 40%, 70%, 100%, 130%, 160%, 190%, 220%, and 250% are shown below. Figure 7 As shown, when the same tensile strain rate was repeated four times, the sensor's resistance response remained consistent, and the curve baseline remained stable during the test at different tensile strain rates, indicating that the sensor has good repeatability and stability.

[0078] The response time and recovery time test curves of the flexible strain sensor prepared in Example 5 at 90% tensile strain rate are shown below. Figure 8 As shown, the sensor's response time and recovery time are both approximately 260ms. This fast response and recovery time is beneficial for the sensor to respond accurately and quickly under external stress.

[0079] The resistance response and recovery curves of the flexible strain sensor prepared in Example 5, representing the lowest detection limit, are shown below. Figure 9 As shown, the lower detection limit is defined as the smallest tensile strain rate that the sensor can stably detect. This sensor can accurately identify tensile strain rates as low as 0.023%, demonstrating its sensitive response to minute tensile strain rates.

[0080] The resistance response curve of the flexible strain sensor prepared in Example 5 under repeated stretching is shown in the figure, along with a magnified view of a portion of the curve. Figure 10 As shown, under the same stress, after 2100 cycles of the same tensile strain rate, the sensor's resistance response and baseline tend to stabilize. Comparing the curves from two test intervals of approximately 275~293s and 5653~5672s, the sensor exhibits good stability and durability during repeated tests lasting up to 6000s.

Claims

1. A method for fabricating a flexible strain sensor based on a dual-resistance sensitive layer structure, characterized in that: The steps are as follows: (1) Place TPUF in 10~30mL of deionized water and ultrasonically clean for 5~30min, then dry at 50~70℃ to obtain TPUF with surface impurities removed; (2) Immerse the TPUF obtained in step (1) in 5~20 mL of ILs with a mass fraction of 95~98%, treat it at 50~100℃ for 0.2~6.0 h, take it out, and wipe off the excess ILs on the surface of TPUF with filter paper to obtain ILs / TPUF; (3) Add 0.2~0.3g of TPU to 2~5mL of N,N-dimethylformamide, heat and stir until the TPU is completely dissolved to obtain a TPU solution; (4) Add 0.01~0.40g of Gr to the TPU solution obtained in step (3) and stir until Gr is evenly dispersed to obtain Gr / TPU mixed solution; (5) Immerse the ILs / TPUF obtained in step (2) in the Gr / TPU mixed solution obtained in step (4) for 1~5s, take it out, dry it at 50~70℃, and pre-stretch it 20~200 times under a tensile strain rate of 100~300% to obtain a Gr / ILs / TPUF dual resistance sensitive layer, with the core layer being ILs / TPUF and the shell layer being Gr / TPU; (6) Place two metal leads at both ends of the outer surface of the Gr / TPU shell of the Gr / ILs / TPUF dual resistance sensitive layer to obtain the flexible strain sensor based on the dual resistance sensitive layer structure; wherein, TPUF is thermoplastic polyurethane elastic fiber, ILs is 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt, TPU is thermoplastic polyurethane elastomer, and Gr is monolayer graphene.

2. The method for fabricating a flexible strain sensor based on a dual-resistance sensitive layer structure as described in claim 1, characterized in that: In step (1), the length of TPUF is 2~5cm and the diameter is 0.5~1.5mm; in step (2), the diameter of ILs / TPUF is 0.54~2.0mm and the resistivity is 100~5100Ω·m; in step (5), the diameter of Gr / ILs / TPUF dual resistive sensitive layer is 0.66~2.82mm and the resistivity is 0.05~0.25Ω·m.

3. The method for fabricating a flexible strain sensor based on a dual-resistance sensitive layer structure as described in claim 1, characterized in that: In step (3), the TPU is heated and stirred for 0.5 to 2.0 h under a water bath at 50 to 80°C until it is completely dissolved; in step (4), the TPU is stirred for 5 to 20 h under a water bath at 20 to 30°C until it is uniformly dispersed.

4. A flexible strain sensor based on a dual-resistance sensitive layer structure, characterized in that: It is prepared by the preparation method described in any one of claims 1, 2 or 3.

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