Preparation method and application of a double-gradient micro-pleated graphene-based flexible strain sensor

Abstract

The application belongs to the technical field of flexible electronics, and particularly relates to a preparation method and application of a double-gradient micro-creased graphene-based flexible strain sensor. The preparation method comprises the following steps: preparing a graphene oxide GO solution, obtaining two GO films with different thicknesses through vacuum filtration, reducing the films using hydriodic acid to obtain two reduced graphene oxide rGO films with different thicknesses; sequentially transferring the two rGO films with different thicknesses to a pre-stretched acrylic film substrate film using a dry method, and sequentially performing different degrees of shrinkage, when the rGO film and the substrate film are synchronously shrunk, the rGO films with different thicknesses will form different crease structures under different degrees of shrinkage, thereby forming a double-gradient structure with a thickness gradient and a pre-strain gradient. The structure has a large strain range, the early-stage sensitivity can be improved through the mutual cooperation of the double-gradient sensing areas, and the double-gradient structure simultaneously improves the sensitivity and the strain range in the full-range range while the non-coordinated change in the strain process.

Description

Technical Field

[0001] This invention belongs to the field of flexible electronics technology, specifically relating to a method for fabricating and applying a dual-gradient micro-wrinkled graphene-based flexible strain sensor. Background Technology

[0002] Flexible strain sensors are a novel type of sensor. Compared to traditional sensors, they offer greater flexibility, better deformation capabilities, stronger sensitivity and accuracy, and a wider range of applications, showing promising prospects in fields such as health monitoring, medical surgery, human-computer interaction, electronic skin, and smart industry. However, current research on flexible strain sensors faces the challenge of simultaneously improving high sensitivity and a large strain range in the initial stage, hindering the development of strain sensors to higher performance levels. When external stress is applied, high sensitivity under low initial strain is typically achieved by disrupting the conductive layer structure, such as by promoting the propagation of microcracks during strain, thus reducing the strain range. Conversely, a large strain range is achieved by fabricating structures that delay crack initiation, such as micro-folded structures, thereby increasing the strain range by delaying microcrack formation. In summary, high sensitivity under low initial strain and a large strain range are contradictory. Therefore, fabricating flexible strain sensors that combine high sensitivity under low initial strain and a large strain range remains a significant challenge. Summary of the Invention

[0003] The purpose of this invention is to reconcile the contradiction between high sensitivity under small initial strain and large strain range in flexible strain sensors, and to fabricate an rGO flexible strain sensor with a dual-gradient micro-fold structure. This invention features simple material preparation, low manufacturing cost, stable signal transmission, and wide applicability. Its unique structural design simultaneously improves sensitivity under small initial strain and increases the strain range.

[0004] This invention provides a method for fabricating and applying a dual-gradient micro-wrinkled graphene-based flexible strain sensor. The technical solution adopted is as follows:

[0005] A method for fabricating a dual-gradient micro-wrinkled graphene-based flexible strain sensor includes the following steps:

[0006] 1) Graphene oxide (GO) solution was prepared using the improved Hummers method. The GO solution was then diluted with distilled water, stirred, and ultrasonically dispersed to obtain two GO solutions of different concentrations.

[0007] 2) Vacuum filtration was performed on two GO solutions of different concentrations. The thickness of the membrane was controlled by controlling the volume of the GO solution being filtered, so as to obtain a thinner GO membrane and a thicker GO membrane respectively.

[0008] 3) Hydroiodic acid was used to reduce the thinner GO film and the thicker GO film to obtain the thinner rGO film and the thicker rGO film.

[0009] 4) Using acrylic film VHB 4910 as the base film, the base film is fixed on a biaxial pre-stretching platform and pre-stretched in both the horizontal and vertical directions. Then, using a dry transfer method, the thinner rGO film is first transferred to an acrylic film VHB 4910 base film, and then the thicker rGO film is transferred to the same acrylic film VHB 4910 base film. After the transfer, the two rGO films of different thicknesses should be tightly bonded without any gaps. The thinner rGO film is then retracted horizontally, and the thicker rGO film is also retracted horizontally to its initial position. Then, the two rGO films of different thicknesses are simultaneously retracted vertically to their initial positions. When the two rGO films of different thicknesses are retracted to different degrees along with the base film, the two rGO films of different thicknesses will form different wrinkled structures, thus forming a thickness-pre-strain dual-gradient rGO micro-wrinkled film.

[0010] 5) Copper electrodes were assembled on both sides of the dual-gradient rGO micro-wrinkled film using conductive silver paste. After curing, the dual-gradient rGO micro-wrinkled film with assembled electrodes was obtained.

[0011] 6) The assembled electrodes of the dual-gradient rGO micro-wrinkled film were encapsulated using a mixed solution of polydimethylsiloxane prepolymer and curing agent to obtain a dual-gradient micro-wrinkled graphene-based flexible strain sensor.

[0012] Furthermore, in the above preparation method, step 1), the specific method for preparing graphene oxide (GO) solution using the improved Hummers method includes the following steps: 20-25 mL of... 98% concentrated sulfuric acid is placed in an ice bath at -1 to 1°C. Then, 0.8 to 1.5 g of natural graphite is added to the concentrated sulfuric acid in the ice bath and stirred thoroughly. Over 20 to 30 minutes, 5 to 6 g of potassium permanganate is added in 7 to 8 small portions while stirring. After stirring for 3 to 4 hours, the ice bath is removed, and the solution is placed in a constant temperature water bath at 40 to 45°C for the first heating and stirring for 40 to 50 minutes at a stirring rate of 900 to 1000 rpm. This process is accompanied by bubbling and exothermic reaction. The constant temperature water bath is then heated again to 75 to 85°C for a second heating and stirring at a stirring rate of 300 to 500 rpm. While continuously stirring, 5 to 10 mL of distilled water is added in 4 portions, followed by the remaining distilled water in 4 portions, for a total of 70 to 90 mL of distilled water. After stirring, another 60 to 70 mL of distilled water is added for dilution. Finally, 360 to 370 g of potassium permanganate is added. When the solution of 5% hydrogen peroxide turns golden yellow, stop heating and stirring, immediately centrifuge and discard the supernatant, and wash repeatedly with distilled water to bring the pH value to 5-6.

[0013] Furthermore, in the above preparation method, in step 1), the stirring time is 25-35 min and the ultrasonic time is 15-20 min.

[0014] Furthermore, in the above preparation method, in step 1), the concentrations of the two different concentrations of GO solutions are 0.1 mg / mL and 0.32 mg / mL, respectively.

[0015] Furthermore, in the above preparation method, the specific operation steps of step 2) are as follows: two GO solutions of different concentrations are vacuum filtered using a vacuum filtration device to obtain GO films of different thicknesses. The filtration volume of the 0.1 mg / mL GO solution is 1-2 mL, and a thinner GO film is obtained after filtration; the filtration volume of the 0.32 mg / mL GO solution is 4-5 mL, and a thicker GO film is obtained after filtration.

[0016] Furthermore, in the above preparation method, in step 3), the reduction temperature is 95-100℃ and the reduction time is 1-1.5h.

[0017] Furthermore, in the above preparation method, in step 4), the horizontal pre-stretching parameter of the base film is 300%, and the vertical pre-stretching parameter is 200%; the horizontal shrinkage parameter of the thinner rGO film is 285% to 290%; the horizontal shrinkage parameter of the thicker rGO film is 10% to 15%; and the vertical shrinkage parameter of the two different thicknesses of rGO film is 200%.

[0018] Furthermore, in the above preparation method, step 5), the curing conditions are: curing at 60°C for 30 minutes.

[0019] Furthermore, in the above preparation method, the specific operation steps of step 6) are as follows: the mixed solution of polydimethylsiloxane (PDMS) prepolymer and curing agent prepared at a mass ratio of 10:1 is dropped onto the dual-gradient rGO micro-wrinkled film with assembled electrodes using a dropper, and then cured at 60°C for 4 hours to obtain a dual-gradient micro-wrinkled graphene-based flexible strain sensor.

[0020] Applications of the dual-gradient micro-folded graphene-based flexible strain sensor prepared by any of the above methods in health monitoring, medical surgery, human interaction, electronic skin, and smart industry.

[0021] The beneficial effects of this invention are:

[0022] 1. The flexible strain sensor prepared by this invention utilizes the gradual unfolding of dual-gradient folds formed by rGO films of different thicknesses during the strain process. The fold structures formed by the two rGO films of different thicknesses will undergo non-coordinated changes during the unfolding process. Due to the small horizontal pre-strain during the preparation of the thick rGO film, the folds formed will exhibit microcracks under small strain, resulting in high early-stage sensitivity of the strain sensor. On the other hand, the folds formed by the thin rGO film have a much larger horizontal pre-strain during preparation than the thick rGO film, providing a large strain range. The cooperation between these two fold structures can achieve both high sensitivity under small early-stage strain and a large strain range.

[0023] 2. The strain sensor prepared by this invention uses only rGO as the sensing layer material. This material is simple to prepare, low in cost, easy to use, green and environmentally friendly, and can be easily mass-produced.

[0024] 3. This invention uses a simple substrate pre-strain shrinkage method to prepare a dual-gradient micro-folded structure. The experimental instruments used in the preparation process are simple and the process is stable.

[0025] 4. The flexible strain sensor prepared by this invention has stable signal transmission, a wide range of applications, and is green, environmentally friendly, and safe.

[0026] 5. The flexible strain sensor prepared by this invention has high initial sensitivity, and even small strains will cause stable signal changes, making it suitable for various small strain detections. Attached Figure Description

[0027] Figure 1 XRD patterns of natural graphite and GO and rGO composites prepared using natural graphite.

[0028] Figure 2 The diagram shows a comparison of the sensitivity and strain range of the three flexible strain sensors prepared in Example 1, where ① is a flexible strain sensor of rGO with a dual-gradient micro-fold structure; ② is a flexible strain sensor of rGO with a single pre-strain gradient micro-fold structure; and ③ is a flexible strain sensor of rGO with a single thickness gradient micro-fold structure.

[0029] Figure 3 The image shows a SEM image of the rGO wrinkled film with a thickness-pre-strain dual gradient prepared in Example 1. Detailed Implementation

[0030] Example 1

[0031] (I) The fabrication method of the rGO flexible strain sensor with a dual-gradient micro-fold structure is as follows:

[0032] 1) GO solution was prepared using a modified Hummers method. 20–25 mL of 98% concentrated sulfuric acid (H₂SO₄) was placed in an ice bath at -1–1°C. 0.8–1.5 g of natural graphite was then added to the concentrated sulfuric acid in the ice bath, and the mixture was stirred thoroughly. Over 20–30 minutes, 5–6 g of potassium permanganate (KMnO₄) was added in 7–8 small portions, stirring continuously for 3–4 hours. The ice bath was then removed, and the solution was placed in a constant-temperature water bath at 40–45°C for initial heating and stirring for 40–50 minutes at a stirring rate of 900–10⁻⁶. The process involves bubbling and exothermic reactions. The constant temperature water bath is then heated again to 75-85°C for a second heating and stirring at a rate of 300-500 r / min. 70-90 mL of distilled water is added slowly and in small amounts (first 5-10 mL in 4 portions, then the remaining water in 4 portions). After stirring, another 60-70 mL of distilled water is added for dilution. After dilution, 360-370 g of 5% hydrogen peroxide (H2O2) is added. The solution is observed to turn golden yellow. Heating and stirring are then stopped, the solution is immediately centrifuged, and the supernatant is discarded. The solution is washed several times with distilled water to bring the pH to 5-6.

[0033] 2) Dilute the GO solution obtained in step 1) with distilled water to 0.1 mg / mL and 0.32 mg / mL respectively. Then stir for 25-35 min and sonicate for 15-20 min to obtain GO solutions with a concentration of 0.1 mg / mL and GO solutions with a concentration of 0.32 mg / mL.

[0034] 3) Vacuum filtration was performed on the two GO solutions of different concentrations obtained in step 2) to obtain GO films of different thicknesses. The filtration volume of the 0.1 mg / mL GO solution was 1-2 mL, and a thinner GO film was obtained after filtration. The filtration volume of the 0.32 mg / mL GO solution was 4-5 mL, and a thicker GO film was obtained after filtration.

[0035] 4) The thinner GO film and the thicker GO film obtained in step 3) are reduced with hydroiodic acid (HI) to obtain a thinner rGO film and a thicker rGO film. The reduction temperature is 95-100℃ and the reduction time is 1-1.5h.

[0036] 5) Using acrylic film VHB 4910 as the base film, the base film is fixed on a biaxial pre-stretching platform and pre-stretched by 300% horizontally and 200% vertically. Then, using a dry transfer method, the thinner rGO film is first transferred onto an acrylic film VHB 4910 base film, and then the thicker rGO film is transferred onto the same acrylic film VHB 4910 base film. On the 4910 base film, the two rGO films of different thicknesses should be tightly bonded together after transfer, without any gaps. The thinner rGO film is retracted by 285% to 290% in the horizontal direction, and the thicker rGO film is retracted by 10% to 15% in the horizontal direction to the initial position in the horizontal direction (the position before pre-stretching of the base film). Then, the two rGO films of different thicknesses are simultaneously retracted by 200% in the vertical direction to the initial position (the position before pre-stretching of the base film). When the two rGO films of different thicknesses are retracted to different degrees along with the base film, the two rGO films of different thicknesses will form different wrinkled structures, thus forming a thickness-pre-strain dual-gradient rGO micro-wrinkled film.

[0037] 6) Use conductive silver paste to assemble copper electrodes on both sides of the dual-gradient rGO micro-wrinkled film obtained in step 5), and then cure at 60°C for 30 min to obtain the dual-gradient rGO micro-wrinkled film with assembled electrodes.

[0038] 7) The dual-gradient rGO micro-wrinkled film with assembled electrodes obtained in step 6) is encapsulated with a mixed solution of polydimethylsiloxane (PDMS) prepolymer and curing agent. The mass ratio of PDMS prepolymer to curing agent is 10:1. The prepared PDMS prepolymer and curing agent mixed solution is dropped onto the dual-gradient rGO micro-wrinkled film with assembled electrodes using a dropper, and then cured at 60°C for 4 hours to obtain a flexible rGO strain sensor with a dual-gradient micro-wrinkled structure.

[0039] (II) A flexible strain sensor of rGO with a single pre-strained gradient micro-folded structure is fabricated as follows:

[0040] 1) GO solution was prepared using a modified Hummers method. 20–25 mL of 98% concentrated sulfuric acid (H₂SO₄) was placed in an ice bath at -1–1°C. 0.8–1.5 g of natural graphite was then added to the concentrated sulfuric acid in the ice bath, and the mixture was stirred thoroughly. Over 20–30 minutes, 5–6 g of potassium permanganate (KMnO₄) was added in 7–8 small portions, stirring continuously for 3–4 hours. The ice bath was then removed, and the solution was placed in a constant-temperature water bath at 40–45°C for initial heating and stirring for 40–50 minutes at a stirring rate of 900–10⁻⁶. The process involves bubbling and exothermic reactions. The constant temperature water bath is then heated again to 75-85°C for a second heating and stirring at a rate of 300-500 r / min. 70-90 mL of distilled water is added slowly and in small amounts (first 5-10 mL in 4 portions, then the remaining water in 4 portions). After stirring, another 60-70 mL of distilled water is added for dilution. After dilution, 360-370 g of 5% hydrogen peroxide (H2O2) is added. The solution is observed to turn golden yellow. Heating and stirring are then stopped, the solution is immediately centrifuged, and the supernatant is discarded. The solution is washed several times with distilled water to bring the pH to 5-6.

[0041] 2) Dilute the GO solution obtained in step 1) with distilled water to a concentration of 0.1 mg / mL, stir for 25-35 min, and then sonicate for 15-20 min to obtain a GO solution with a concentration of 0.1 mg / mL.

[0042] 3) The GO solution obtained in step 2) is vacuum filtered using a vacuum filtration device to obtain a GO membrane. The GO solution is filtered in a volume of 1-2 mL. After filtration, a GO membrane is obtained.

[0043] 4) The GO film obtained in step 3) is reduced with hydroiodic acid (HI) to obtain rGO film. The reduction temperature is 95-100℃ and the reduction time is 1-1.5h.

[0044] 5) Using acrylic film VHB 4910 as the base film, the base film is fixed on a biaxial pre-stretching platform and pre-stretched by 300% in the horizontal direction and 200% in the vertical direction. Then, two rGO films are transferred to the same acrylic film VHB 4910 base film in two separate dry transfers. After the transfer, the two rGO films should be tightly bonded without gaps. The rGO film transferred first is retracted by 285% to 290% in the horizontal direction, and the rGO film transferred later is retracted by 10% to 15% in the horizontal direction to its initial position. Then, both rGO films are simultaneously retracted by 200% in the vertical direction to their initial positions. When the rGO films transferred one after the other retract to different degrees with the base film, the rGO films will form different wrinkled structures, thus forming a single pre-strain gradient rGO micro-wrinkled film.

[0045] 6) Use conductive silver paste to assemble copper electrodes on both sides of the single pre-strain gradient rGO micro-wrinkled film obtained in step 5), and then cure at 60°C for 30 min to obtain the single pre-strain gradient rGO micro-wrinkled film with assembled electrodes.

[0046] 7) The rGO micro-wrinkled film with assembled electrodes obtained in step 6) is encapsulated with a mixture of polydimethylsiloxane (PDMS) prepolymer and curing agent. The mass ratio of PDMS prepolymer to curing agent is 10:1. The prepared PDMS prepolymer and curing agent mixture is dropped onto the rGO micro-wrinkled film with assembled electrodes using a dropper. Then, it is cured at 60°C for 4 hours to obtain a flexible rGO strain sensor with a single pre-strain gradient micro-wrinkled structure.

[0047] (III) A flexible strain sensor based on rGO with a single thickness gradient micro-fold structure is fabricated as follows:

[0048] 1) GO solution was prepared using a modified Hummers method. 20–25 mL of 98% concentrated sulfuric acid (H₂SO₄) was placed in an ice bath at -1–1°C. 0.8–1.5 g of natural graphite was then added to the concentrated sulfuric acid in the ice bath, and the mixture was stirred thoroughly. Over 20–30 minutes, 5–6 g of potassium permanganate (KMnO₄) was added in 7–8 small portions, stirring continuously for 3–4 hours. The ice bath was then removed, and the solution was placed in a constant-temperature water bath at 40–45°C for initial heating and stirring for 40–50 minutes at a stirring rate of 900–10⁻⁶. The process involves bubbling and exothermic reactions. The constant temperature water bath is then heated again to 75-85°C for a second heating and stirring at a rate of 300-500 r / min. 70-90 mL of distilled water is added slowly and in small amounts (first 5-10 mL in 4 portions, then the remaining water in 4 portions). After stirring, another 60-70 mL of distilled water is added for dilution. After dilution, 360-370 g of 5% hydrogen peroxide (H2O2) is added. The solution is observed to turn golden yellow. Heating and stirring are then stopped, the solution is immediately centrifuged, and the supernatant is discarded. The solution is washed several times with distilled water to bring the pH to 5-6.

[0049] 2) Dilute the GO solution obtained in step 1) with distilled water to 0.1 mg / mL and 0.32 mg / mL respectively. Then stir for 25-35 min and sonicate for 15-20 min to obtain GO solutions with a concentration of 0.1 mg / mL and GO solutions with a concentration of 0.32 mg / mL.

[0050] 3) Vacuum filtration was performed on the two GO solutions of different concentrations obtained in step 2) to obtain GO films of different thicknesses. The filtration volume of the 0.1 mg / mL GO solution was 1-2 mL, and a thinner GO film was obtained after filtration. The filtration volume of the 0.32 mg / mL GO solution was 4-5 mL, and a thicker GO film was obtained after filtration.

[0051] 4) The thinner GO film and the thicker GO film obtained in step 3) are reduced with hydroiodic acid (HI) to obtain a thinner rGO film and a thicker rGO film. The reduction temperature is 95-100℃ and the reduction time is 1-1.5h.

[0052] 5) Using acrylic film VHB 4910 as the base film, the base film is fixed on a biaxial pre-stretching platform and pre-stretched by 300% in the horizontal direction and 200% in the vertical direction. Then, two rGO films of different thicknesses are transferred to the same acrylic film VHB 4910 base film in two stages using a dry transfer method. After the transfer, the two rGO films of different thicknesses should be tightly bonded together without gaps. At the same time, the two rGO films of different thicknesses are retracted by 300% in the horizontal direction to their initial position, and then the two rGO films of different thicknesses are retracted by 200% in the vertical direction to their initial position. When the two rGO films of different thicknesses are transferred one after the other and retracted to the same extent as the base film, the two rGO films of different thicknesses will form the same wrinkled structure, thus forming a single thickness gradient rGO micro-wrinkled film.

[0053] 6) Use conductive silver paste to assemble copper electrodes on both sides of the single-thickness gradient rGO micro-wrinkled film obtained in step 5), and then cure at 60°C for 30 min to obtain the single-thickness gradient rGO micro-wrinkled film with assembled electrodes.

[0054] 7) The rGO micro-wrinkled film with assembled electrodes obtained in step 6) is encapsulated with a mixed solution of polydimethylsiloxane (PDMS) prepolymer and curing agent. The mass ratio of PDMS prepolymer to curing agent is 10:1. The prepared mixed solution of PDMS prepolymer and curing agent is dropped onto the rGO micro-wrinkled film with assembled electrodes using a dropper. Then, it is cured at 60°C for 4 hours to obtain a flexible rGO strain sensor with a single-thickness gradient micro-wrinkled structure.

[0055] (iv) Characterization

[0056] Figure 1 XRD patterns of natural graphite, GO, and rGO materials. Figure 1 The results show that the present invention successfully prepared natural graphite into GO and reduced GO into rGO. Figure 1 The crystal structures of GO and rGO were analyzed using X-ray diffraction. The XRD patterns showed that the highest diffraction peak of natural graphite was at 26.5°, while that of GO was at 11.45°. This indicates that natural graphite was successfully oxidized to GO, carrying a large number of oxygen-containing functional groups. During the reduction of GO to rGO, the highest diffraction peak of the GO film changed from 11.45° to 24.45°, indicating that the oxygen-containing functional groups on the GO surface were reduced, successfully preparing rGO.

[0057] Figure 2This diagram illustrates a comparison of the sensitivity and strain range of the rGO flexible strain sensor with a dual-gradient micro-wrinkled structure (①), a single-pre-strain gradient (②), and a single-thickness gradient (③) fabricated in Example 1. A digital multimeter was used to record the real-time changes in the film's resistance under different strains. ① The strain range of the rGO flexible strain sensor with a dual-gradient micro-pleated structure is 300%. Within the strain range of 0–60%, GF = 211.79; within the strain range of 60%–220%, GF = 2332.26; and within the strain range of 220%–300%, GF = 12326.34. ② The strain range of the rGO flexible strain sensor with a single pre-strain gradient is 300%. Within the strain range of 0–150%, GF = 18.18; within the strain range of 150%–230%, GF = 36.82; and within the strain range of 230%–300%, GF = 632.52. ③ The strain range of the rGO flexible strain sensor with a single thickness gradient is 300%. Within the strain range of 0–150%, GF = 0.41; within the strain range of 150%–250%, GF = 11.08; and within the strain range of 250%–300%, GF = 574.22. The above demonstrates that combining two single-gradient flexible strain sensor structures into a dual-gradient structure not only maintains a wide strain range but also significantly improves the sensor's sensitivity across the entire range.

[0058] Figure 3 This is a SEM image of the thickness-pre-strain dual-gradient rGO wrinkled film prepared in Example 1. The surface microstructure of the film was characterized using scanning electron microscopy. The dual-gradient structure of the film is clearly visible. Figure 3 The image on the left shows a relatively thick rGO film that has undergone a 10-15% horizontal shrinkage and a 200% vertical shrinkage. Figure 3 The right side shows a thinner rGO film that has been retracted by 285-290% in the horizontal direction and 200% in the vertical direction.

Claims

1. A method for fabricating a dual-gradient micro-wrinkled graphene-based flexible strain sensor, characterized in that, Includes the following steps: 1) Graphene oxide (GO) solution was prepared using the improved Hummers method. The GO solution was then diluted with distilled water, stirred, and ultrasonically dispersed to obtain two GO solutions of different concentrations. 2) Vacuum filtration was performed on two GO solutions of different concentrations. The thickness of the membrane was controlled by controlling the volume of the GO solution being filtered, so as to obtain a thinner GO membrane and a thicker GO membrane respectively. 3) Thinner and thicker GO films were reduced using hydroiodic acid to obtain thinner and thicker rGO films. 4) Using acrylic film VHB 4910 as the base film, the base film is fixed on a biaxial pre-stretching platform and pre-stretched in both the horizontal and vertical directions. Then, using a dry transfer method, the thinner rGO film is first transferred to an acrylic film VHB 4910 base film, and then the thicker rGO film is transferred to the same acrylic film VHB 4910 base film. After the transfer, the two rGO films of different thicknesses should be tightly bonded without any gaps. The thinner rGO film is then retracted horizontally, and the thicker rGO film is also retracted horizontally to its initial position. Then, the two rGO films of different thicknesses are simultaneously retracted vertically to their initial positions. When the two rGO films of different thicknesses are retracted to different degrees along with the base film, the two rGO films of different thicknesses will form different wrinkled structures, thus forming a thickness-pre-strain dual-gradient rGO micro-wrinkled film. 5) Copper electrodes were assembled on both sides of the dual-gradient rGO micro-wrinkled film using conductive silver paste. After curing, the dual-gradient rGO micro-wrinkled film with assembled electrodes was obtained. 6) The assembled electrodes of the dual-gradient rGO micro-wrinkled film were encapsulated using a mixed solution of polydimethylsiloxane prepolymer and curing agent to obtain a dual-gradient micro-wrinkled graphene-based flexible strain sensor.

2. The preparation method according to claim 1, characterized in that, In step 1), the specific method for preparing graphene oxide (GO) solution using the improved Hummers method includes the following steps: 20-25 mL of 98% concentrated sulfuric acid is placed in an ice bath at -1 to 1°C. Then, 0.8-1.5 g of natural graphite is added to the concentrated sulfuric acid in the ice bath, and the mixture is stirred until homogeneous. Over 20-30 minutes, 5-6 g of potassium permanganate is added in 7-8 small portions while stirring. After stirring for 3-4 hours, the ice bath is removed, and the solution is placed in a constant-temperature water bath at 40-45°C for initial heating and stirring for 40-50 minutes at a stirring rate of 900-1000 r / min. This process is accompanied by bubbling and exothermic reactions. The constant-temperature water bath is then heated again to 75-85°C for a second heating and stirring at a stirring rate of 300-500 r / min. While continuously stirring, 5-10 mL of distilled water is added in 4 portions, followed by the remaining distilled water in 4 portions, for a total of 70-90 mL of distilled water. After stirring, another 60-70 mL of distilled water is added. Dilute with mL of distilled water, then add 360~370g of 5% hydrogen peroxide. Observe that the solution turns golden yellow. Stop heating and stirring, centrifuge immediately and discard the supernatant. Wash with distilled water several times to bring the pH value to 5~6.

3. The preparation method according to claim 1, characterized in that, In step 1), the stirring time is 25-35 min and the ultrasonic time is 15-20 min.

4. The preparation method according to claim 1, characterized in that, In step 1), the concentrations of the two different GO solutions are 0.1 mg / mL and 0.32 mg / mL, respectively.

5. The preparation method according to claim 4, characterized in that, The specific operation steps of step 2) are as follows: two GO solutions of different concentrations are vacuum filtered using a vacuum filtration device to obtain GO films of different thicknesses. The filtration volume of the 0.1 mg / mL GO solution is 1~2 mL, and a thinner GO film is obtained after filtration; the filtration volume of the 0.32 mg / mL GO solution is 4~5 mL, and a thicker GO film is obtained after filtration.

6. The preparation method according to claim 1, characterized in that, In step 3), the reduction temperature is 95~100℃ and the reduction time is 1~1.5 h.

7. The preparation method according to claim 1, characterized in that, In step 4), the horizontal pre-stretch parameter of the base film is 300%, and the vertical pre-stretch parameter is 200%; the horizontal shrinkage parameter of the thinner rGO film is 285%~290%; the horizontal shrinkage parameter of the thicker rGO film is 10%~15%; and the vertical shrinkage parameter of the two different thicknesses of rGO film is 200%.

8. The preparation method according to claim 1, characterized in that, In step 5), the curing conditions are: curing at 60°C for 30 min.

9. The preparation method according to claim 1, characterized in that, The specific operation steps of step 6) are as follows: the mixed solution of polydimethylsiloxane prepolymer and curing agent prepared at a mass ratio of 10:1 is dropped onto the dual-gradient rGO micro-wrinkled film with assembled electrodes using a dropper, and then cured at 60°C for 4 h to obtain a dual-gradient micro-wrinkled graphene-based flexible strain sensor.

10. The application of the dual-gradient micro-wrinkled graphene-based flexible strain sensor prepared by the preparation method according to any one of claims 1-9 in health monitoring, human interaction, electronic skin, and smart industry.

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