Preparation and application of super-stable flexible strain sensor based on W-shaped vein structure

Abstract

The invention discloses preparation and application of an ultra-stable flexible strain sensor based on a W-shaped vein structure. The preparation method of the sensor comprises the following steps: (1) preparing a W-shaped vein flexible PDMS film by a template method; (2) uniformly covering a Cr / Ag / Cr three-layer metal material on the surface of the flexible PDMS thin film of the W-shaped vein in a magnetron sputtering manner; and (3) stretching and releasing along the W-shaped veins to obtain the flexible strain sensor with the metal coating cracks. The sensor provided by the invention has good stretchability, super stability and excellent durability, and has wide application prospects in multiple aspects of physiological monitoring of pulse beating, swallowing, facial expression, eyebrow wrinkling and the like of a human body.

Description

Technical Field

[0001] The present invention relates to the field of flexible sensing, and in particular to the preparation and application of an ultra-stable flexible strain sensor based on a W-shaped leaf vein structure. Background Art

[0002] As an important technology in the field of intelligent sensing, flexible strain sensors have achieved increasingly rapid development in recent years under the background of cross-integration of multiple disciplines such as materials science, electronic information engineering, and biomedicine. With its unique flexibility, ductility, stability, good repeatability, and durability, this type of flexible sensor has shown broad application prospects in the fields of human physiological monitoring, motion analysis, Braille recognition, human-computer interaction, and smart wearables.

[0003] The development of flexible strain sensors can be traced back to the early stages of sensing technology, but truly flexible sensors only began to emerge after breakthroughs in materials science and nanotechnology. In the early days, due to material limitations, sensors were mostly rigid structures that were difficult to adapt to complex and changing environments. With the emergence of new materials such as polymer materials, conductive polymers, and nanomaterials, flexible strain sensors began to attract people's attention and gradually became a new beginning. Through continuous exploration and optimization, researchers have gradually improved the stability and durability of sensors, promoting their development and application in multiple fields.

[0004] In order to improve the performance of flexible strain sensors, researchers have carried out a lot of work on flexible strain sensing, designing from the aspects of the material's own properties, properties and structure. In terms of its own properties, materials (such as gold, silver, graphene, etc.) will be oxidized in the air, causing their performance to change significantly. In terms of its own properties, various materials (such as organic polymer PDMS, carbon nanomaterials, two-dimensional materials, composite materials, etc.) show excellent mechanical and electrical properties when subjected to external forces, so they can be used to make flexible strain sensors. In terms of structure, researchers will combine flexible substrates with some microstructures existing in nature to prepare deformable microstructures, such as the papilla structure on the surface of lotus leaves, the V-shaped groove structure on the legs of scorpions, the stripe structure on the surface of fingerprints, the serpentine structure, the spider web structure, etc. The design of these structures can withstand the applied mechanical strain to a certain extent, thereby significantly improving the good stretchability and stability of the sensor. For example, some people use a network-like V-shaped groove structure to deposit a silver film, which shows excellent performance such as fast response recovery time, wide working range, and durability. In addition, some people use the fingerprint surface stripe structure and the lotus root filament structure for dual bionics, mix multi-walled carbon nanotubes and graphene nanosheets and cast them on the template, and use silicone to encapsulate them to obtain flexible strain sensing with good stability, repeatability, and fast response recovery time. In addition, by embedding a mechanical material frame, the contact area of ​​the sensor can be increased when it is stretched, thereby further improving the stability of the sensor.

[0005] Recently, someone has used the template method to design a periodic gradient array, and the deposited silver film has an obvious crack extension mechanism under tensile strain, thus showing super stability and a wide working range; or using the high sensitivity of the metal film and the wide working range of the carbon black material to design a sensor with a layered synergistic structure, it shows excellent stability and a wide linear working range. However, some sensors, due to their different structures and materials, not only have complex preparation processes, but also often have poor stability and limited response range when strain is applied, making it difficult to truly apply them to traditional industries.

[0006] Under current circumstances, in order to achieve ultra-stability and excellent durability of flexible sensors, it is necessary to design a suitable and reasonable structure as a flexible substrate, so as to achieve stress transmission and guidance of the substrate to the conductive material in the stress direction, forming a relatively complete conductive network, thereby achieving high stability and excellent durability of the flexible sensor. This patent uses a W-shaped structure as a substrate, and uses template method and magnetron sputtering and other technologies to deposit a silver film on the surface of the substrate, which can produce a flexible strain sensor with a crack array structure, and realize the preparation of ultra-stable and wide working range flexible sensors. Summary of the invention

[0007] In order to overcome the shortcomings of the prior art, the present invention provides a flexible strain sensor with metal plating cracks, improves the performance of flexible strain sensing and applies it to multiple scenarios, overcoming the shortcomings of traditional flexible sensors in ultra-stability and durability.

[0008] To achieve the above object, the present invention adopts the following technical solutions:

[0009] A method for preparing an ultra-stable flexible strain sensor based on a W-shaped leaf vein structure has the characteristics of simplified preparation process, low economic cost and simple operation. The method comprises the following steps:

[0010] (1) Mixing a polydimethylsiloxane prepolymer and a curing agent, and after completely eliminating bubbles, pouring the mixture onto a specific main vein region on the front side of a fern leaf having W-shaped veins, heating to cure the PDMS, and then demolding to obtain a flexible PDMS film with a W-shaped vein structure having a thickness of 1.0 mm to 1.5 mm;

[0011] (2) depositing a first layer of metal chromium on the surface of the W-shaped leaf vein structure flexible PDMS film of step (1) to enhance the bonding force between the substrate and silver, then depositing a second layer of metal silver, and finally depositing a third layer of metal chromium to prevent silver from being oxidized; obtaining a metal coating on the surface of the W-shaped leaf vein structure flexible PDMS film;

[0012] (3) The two ends of the surface of the flexible PDMS film with a W-shaped leaf vein structure in the direction of the main leaf vein are used as electrodes and are connected to external wires. The flexible PDMS film with a W-shaped leaf vein structure is stretched along the direction of the main leaf vein on its surface, and metal plating cracks with a crack width of 5.6μm-18.3μm are generated in the direction perpendicular to the main leaf vein, thereby changing the resistance between the two electrodes and realizing the function of a flexible strain sensor.

[0013] Preferably, in step (2), the thickness of the first metal chromium layer is 2.5nm-5nm; the thickness of the second metal silver layer is 69nm-75nm; the thickness of the third metal chromium layer is 2.5nm-5nm;

[0014] Preferably, in step (3), the stretching is performed along the main vein direction of the surface, and when the tensile strain is 40%, the relative change in resistance reaches a maximum value of 800;

[0015] In step (1), the polydimethylsiloxane prepolymer and the curing agent are mixed in a ratio of 10:1.

[0016] In step (1), the leaf is adhered to a culture dish using double-sided tape, PDMS is poured in, defoamed, and heated to cure, thereby obtaining a flexible PDMS film imitating fern leaves.

[0017] The specifications of the culture dish in step (1) are a bottom diameter of 88.0 mm and a bottom height of 14.49 mm;

[0018] In step (1), 8 g of the polydimethylsiloxane mixture can be poured into a culture dish with a bottom diameter of 88.0 mm and a bottom height of 14.49 mm to obtain a film with a thickness of 1.0 mm to 1.5 mm.

[0019] The leaves in step (1) are cut into a certain size, washed and dried; the culture dish is made of plastic;

[0020] The polydimethylsiloxane prepolymer and curing agent are mixed evenly and stirred for at least 10 minutes, and then left to stand for more than 30 minutes until the bubbles disappear; the curing condition is 80° C. for 4 hours.

[0021] The deposition in step (3) refers to magnetron sputtering, and the device is a JGP560C19 ultra-high vacuum multifunctional magnetron sputtering device;

[0022] In step (3), when the magnetron sputtering is sputtering three metal layers, the chamber pressure is 0.5 Pa; and the sputtering power is 40W.

[0023] The present invention also provides a W-shaped leaf vein structure flexible strain sensor prepared by the above-mentioned preparation method, comprising a W-shaped leaf vein structure flexible PDMS film, on which a surface having a W-shaped leaf vein structure is deposited with three metal layers: a first layer of metal chromium, a second layer of metal silver, and a third layer of metal chromium; the thickness of the W-shaped leaf vein structure flexible PDMS film is 1.0mm-1.5mm; the thickness of the first metal chromium layer is 2.5nm-5nm; the thickness of the second metal silver layer is 69nm-75nm; the thickness of the third metal chromium layer is 2.5nm-5nm; the two ends of the main leaf vein direction of the surface of the flexible PDMS film with the W-shaped leaf vein structure are used as electrodes, which are respectively connected to external wires; the W-shaped leaf vein structure is obtained by demolding with the area of ​​the specific main leaf vein on the front side of the fern leaf as a template.

[0024] The present invention also provides an application of a W-shaped leaf vein structure flexible strain sensor, which is used to prepare any of the following human physiological monitoring equipment: pulse beating, swallowing, facial expression, frowning, and joint bending.

[0025] Preferably, when the tensile strain of the strain sensor is 40%, the relative change in resistance reaches a maximum value of 800; the response time of the strain sensor during stretching is 83ms, and the recovery time is 93ms.

[0026] Preferably, under the condition of 5% plane tensile strain, the strain sensor is cyclically stretched for more than 10,000 times along the main vein direction, and the resistance of the strain sensor after stretching recovery does not change significantly compared with before cyclic stretching; under the condition of 5% inward bending strain, the strain sensor is cyclically bent for more than 10,000 times, and the resistance of the strain sensor after bending recovery does not change significantly compared with before cyclic bending. Compared with other flexible sensors, the present invention has the following advantages:

[0027] 1. The W-shaped leaf vein structure can significantly change the stress on the sensor surface, so that the stress is concentrated at the guide structure, thereby effectively preventing cracks generated during the stretching process. The sensor guide structure of the present invention is a leaf vein morphology that grows crosswise, and the height of the main leaf vein is 80μm.

[0028] 2. The Poisson effect usually occurs in flexible substrates, that is, when stress is applied transversely, obvious displacement will occur in the longitudinal direction and the surface morphology will change significantly. However, the W-shaped leaf vein structure will produce a series of guiding effects on the applied stress on the substrate, thereby forming metal plating cracks with a width of 5.6μm-18.3μm on the sensor surface, obtaining a flexible sensor with ultra-stability and excellent durability.

[0029] 3. The sensor made of the W-shaped leaf vein structure was subjected to nearly 20,000 cycles under 5% strain, and the resistance remained at around 0.5Ω, indicating that it maintained good durability during repeated cycles; the response and recovery times of the prepared sensor were 83ms and 92ms respectively, and it can be used for human activity monitoring; it was bent inward for 10,000 times under 5% strain conditions, and its resistance remained at 4Ω, indicating that it has super stability and excellent durability. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 Schematic diagram of the vacuum internal structure of the magnetron sputtering apparatus used in the present invention: substrate 1, target material 2.

[0031] Figure 2 The fern leaf of Example 1 has a W-shaped vein structure.

[0032] Figure 3 This is a real picture of the fern leaf with W-shaped vein structure in Example 1.

[0033] Figure 4 This is a real picture of the W-shaped main vein of Example 1 after the mold is reversed

[0034] Figure 5 This is a process flow chart of the flexible strain sensor with a W-shaped leaf vein structure crack array prepared in Example 1.

[0035] Figure 6This is a real picture of the W-shaped main vein structure of Example 1 after being plated with silver film.

[0036] Figure 7 The actual image of the flexible strain sensor with W-shaped main vein-free silver-plated mold in Example 2

[0037] Figure 8 This is a physical picture of the flexible strain sensor with a W-shaped main vein structure crack array prepared in Example 1.

[0038] Fig. 9 This is a scanning electron microscope image of the silver-plated film with a W-shaped main vein structure in Example 1.

[0039] Fig.10 This is a cross-sectional line diagram of the W-shaped main vein structure of Example 1.

[0040] Fig.11 This is a 50x optical microscope image of the W-shaped main vein structure of Example 1.

[0041] Fig.12 This is a 100x optical microscope image of the W-shaped main vein structure of Example 1.

[0042] Fig.13 This is a 200x optical microscope image of the W-shaped main vein structure of Example 1.

[0043] Fig.14 This is a 500x optical microscope image of the W-shaped main vein structure of Example 1.

[0044] Fig.15 This is a 1000x optical microscope image of the W-shaped main vein structure of Example 1.

[0045] Fig.16 The resistance change diagram of the flexible strain sensor with and without main leaf veins prepared in Example 1-2.

[0046] Fig.17 The crack width of the W-shaped main vein structure of Example 1 at 40% strain.

[0047] Fig.18 This is a resistance change diagram of the flexible strain sensor prepared in Example 1 under 40% strain.

[0048] Fig.19 This is a resistance change diagram of the flexible strain sensor prepared in Example 1 under different strains.

[0049] Fig. 20 Response and recovery time of the flexible strain sensor prepared in Example 1.

[0050] Fig.21This is a resistance change diagram of the flexible strain sensor prepared in Example 1 under different continuously changing strains.

[0051] Fig. 22 This is a resistance change diagram of the flexible strain sensor prepared in Example 1 at different frequencies.

[0052] Fig.23 This is a resistance change diagram of the flexible strain sensor prepared in Example 1 during cyclic testing under 5% tensile strain.

[0053] Fig.24 This is a schematic diagram of the flexible strain sensor prepared in Example 1 bending inward.

[0054] Fig.25 This is a resistance change diagram of the flexible strain sensor prepared in Example 1 after 10,000 cycles under 5% inward bending strain.

[0055] Fig.26 This is a resistance change diagram of the flexible strain sensor prepared in Example 1 when testing pulse beating.

[0056] Fig. 27 This is a resistance change diagram of the flexible strain sensor prepared in Example 1 during swallowing test.

[0057] Fig.28 This is a resistance change diagram of the flexible strain sensor prepared in Example 1 when testing knee bending.

[0058] Fig.29 This is a graph showing the resistance change of the flexible strain sensor prepared in Example 1 when the finger is bent at different angles.

[0059] Fig.30 This is a graph showing the resistance change of the flexible strain sensor prepared in Example 1 when the wrist is bent at different angles.

[0060] Fig.31 This is a resistance change diagram of the flexible strain sensor prepared in Example 1 when testing facial expressions.

[0061] Fig.32 This is a resistance change diagram of the flexible strain sensor prepared in Example 1 when testing a frown. DETAILED DESCRIPTION

[0062] The preparation and application of a super-stable flexible strain sensor based on a W-shaped leaf vein structure include the following steps:

[0063] (1) First, cut the fern leaves with W-shaped vein structure into a certain size, rinse with deionized water, and place on a heating platform to dry. Then use double-sided tape to stick the leaves in a culture dish with the front side facing up. Mix the polydimethylsiloxane prepolymer and curing agent in a ratio of 10:1, let it stand for half an hour to completely eliminate bubbles, cast it on the surface of the leaves, and heat it on a heating platform at 80°C for 4 hours. When the PDMS is completely cured, cut the PDMS and peel it off from the culture dish with the W-shaped vein structure to obtain a flexible PDMS film with a W-shaped vein structure with a thickness of 1.0mm-1.5mm;

[0064] (2) Place the W-shaped leaf vein structure flexible PDMS film into a magnetron sputtering instrument, fix the target on the cathode, and place the anode facing the substrate surface. After evacuation, introduce inert gas, and apply voltage between the cathode and the anode. The discharge between the two electrodes causes positive ions to fly toward the target and collide with the atoms on the target surface, thereby depositing a thin film on the substrate surface. On the W-shaped leaf vein structure flexible PDMS film, firstly, a first layer of thin film metal chromium (Cr) is uniformly plated to enhance the bonding force between the substrate and silver (Ag), then a second layer of metal silver (Ag) is plated, and finally a third layer of metal chromium (Cr) is plated to prevent silver (Ag) from being oxidized;

[0065] (3) After the coating is completed, the shape of the film before stretching is determined as the original state, and the flexible PDMS film with a W-shaped vein structure is stretched along the main vein to obtain a flexible strain sensor with a W-shaped vein structure, the surface of which has a guide crack with a crack width of 5.6μm-18.3μm.

[0066] Preferably, the inert gas is argon, the vacuum degree of magnetron sputtering is 0.5 Pa, and the sputtering power is 40W.

[0067] Preferably, the substrate is a flexible substrate PDMS (Dow Corning 184).

[0068] Preferably, the target material is a chromium target material and a silver target material, wherein the first layer of metal chromium film serves to enhance the bonding force between the substrate and the silver, and the second layer of metal chromium film prevents the silver from being oxidized.

[0069] Preferably, the thickness of the chromium film is 2.5nm-5nm, and the thickness of the silver film is 69nm-75nm.

[0070] Preferably, the height of the W-shaped main vein is selected to be 80 μm.

[0071] The present invention will be further described below in conjunction with specific embodiments and drawings.

[0072] In the following descriptions, unless otherwise specified, all instruments, equipment and raw materials can be purchased from commercial channels. Unless otherwise specified, all methods are conventional.

[0073] like Figure 1 As shown, it is a schematic diagram of the structure in the vacuum chamber of the magnetron sputtering instrument used in the present invention, the target material is located at the cathode, the substrate is fixed at the anode directly above the target surface, and an inert gas is introduced after vacuuming, and finally the coating is performed. The first layer of thin film metal chromium (Cr) has a thickness of 2.5nm-5nm to enhance the bonding force between the substrate and silver, and then the second layer of metal thin film silver (Ag) is plated with a thickness of 69nm-75nm, and finally the third layer of metal thin film chromium (Cr) is plated with a thickness of 2.5nm-5nm to prevent silver from being oxidized.

[0074] Example 1 Preparation method of flexible strain sensor with W-shaped leaf vein structure

[0075] (1) Preparation of flexible substrate

[0076] First, Sylgard 184 (polydimethylsiloxane: PDMS) from Dow Corning of the United States was selected. The prepolymer and the curing agent were mixed in a ratio of 10:1. After stirring evenly, the mixture was allowed to stand for half an hour to completely eliminate bubbles. 8 g of the mixture was poured into a culture dish with a bottom diameter of 88.0 mm and a bottom height of 14.49 mm. The mixture was heated at 80°C for 4 hours to be completely cured. The thickness of the PDMS film was about 1.0 mm-1.5 mm.

[0077] (2) Preparation method of silver film with W-shaped leaf vein structure

[0078] First, cut the fern leaves with W-shaped vein structure into a certain size, rinse with deionized water, and place on a heating platform to dry. Then use double-sided tape to stick the leaves in a culture dish with the front side facing up. The configured PDMS is cast on the surface of the leaves, heated on a heating platform at 80°C for 4 hours. When the PDMS is completely cured, cut the PDMS and peel it off from the culture dish with the W-shaped vein structure to obtain a W-shaped vein structure flexible PDMS film with a thickness of 1.0mm-1.5mm, such as Figure 4 shown.

[0079] Then, the flexible PDMS film with the W-shaped leaf vein structure is placed in a magnetron sputtering apparatus, the target is fixed on the cathode, the anode is facing the substrate surface, and after evacuation, an inert gas is introduced. The vacuum degree of the magnetron sputtering is 0.5 Pa, and the sputtering power is 40 W. On the flexible PDMS film with the W-shaped leaf vein structure, a first layer of metal thin film chromium (Cr) with a thickness of 2.5nm-5nm is firstly uniformly plated to enhance the bonding force between the substrate and silver, and then a second layer of metal thin film silver (Ag) with a thickness of 69nm-75nm is plated, and finally a third layer of metal thin film chromium (Cr) with a thickness of 2.5nm-5nm is plated to prevent the metal silver from being oxidized, such as Figure 6 shown.

[0080] (3) Preparation method of flexible sensor with W-shaped leaf vein structure

[0081] First, a conductive silver paste was applied to both ends of the PDMS with a W-shaped leaf vein structure on which a silver film was deposited, and heated at 150°C for 4 hours until it was completely cured. Then, two copper wires were glued to the silver electrodes at both ends of the sensor to obtain a flexible strain sensor with a W-shaped leaf vein structure, such as Figure 8 shown.

[0082] Example 2 Preparation method of flexible strain sensor with W-shaped leaf vein structure

[0083] The difference between Example 2 and Example 1 is the presence or absence of main veins. Figure 7 The main veins are removed and replaced with PET or silicon strips.

[0084] Example 3 Preparation of a flexible strain sensor with a W-shaped leaf vein structure

[0085] like Fig.16 As shown, the flexible strain sensor of the above-mentioned Example 1 and Example 2 tests the changing relationship between strain and resistance.

[0086] like Fig.18 As shown, the flexible strain sensor prepared in the above example 1 has a relative resistance change of 800Ω at a maximum value under a strain of 40%, and has a good response time and high sensitivity.

[0087] like Fig.19 As shown, the flexible strain sensor prepared in the above example 1 is cycled under different strain conditions, and its resistance does not change much under different strains, indicating good stability.

[0088] like Fig. 20 As shown, the response and recovery time of the flexible strain sensor prepared in the above example 1, wherein the response time is 83ms, and the recovery time is 92ms.

[0089] like Fig.21 As shown, when the flexible strain sensor prepared in the above example 1 increases the strain by 1% each time, the resistance change under each strain presents a stable state, and the resistance change under each strain presents an obvious step state.

[0090] like Fig. 22 As shown, the flexible strain sensor prepared in the above example 1 shows good stability when testing the relationship between different frequencies and resistance under 5% strain.

[0091] like Fig.23As shown, the flexible strain sensor prepared in the above example 1 maintained a resistance change of about 0.5Ω after 20,000 cycles under 5% strain, indicating good durability during repeated cycles.

[0092] like Fig.25 As shown, the flexible strain sensor prepared in the above example 1 maintained a resistance change of about 4Ω after 10,000 cycles under 5% inward bending strain, indicating good durability during repeated cycles.

[0093] The flexible strain sensor with a W-shaped leaf vein structure, due to its unique geometric structure, will concentrate stress in the structural area, and can produce larger deformation under smaller strain conditions, thereby improving the sensitivity of the sensor. This structure can produce more obvious deformation when subjected to external force, making the output signal of the sensor more obvious. When the flexible strain sensor with a W-shaped leaf vein structure is stretched or compressed, due to the structural characteristics of its shape, it can better disperse and transfer stress, thereby improving the mechanical properties of the sensor. The W-shaped structure can provide a more uniform stress distribution and can withstand greater stress without damage, allowing it to maintain good stability during multiple cycles, increasing the durability and reliability of the sensor.

[0094] The above tests show that the flexible strain sensor with W-shaped leaf vein structure mentioned in this article has high sensitivity, ultra-stability and good durability. In practical applications, a series of tests such as human physiological monitoring such as wrist bending and facial expression changes are performed to observe the response of its electrical signals. Figure 26-32 shown.

[0095] (1) Pulse, swallowing, knee bending

[0096] The flexible strain sensor is attached to the physiological test site, specifically the pulse of the wrist, the Adam's apple and the knee joint. The test response results are as follows: Figure 26-28 As shown in the figure, the test results show that the resistance will change significantly, indicating that the sensor has high sensitivity.

[0097] (2) Bend your fingers and wrists

[0098] The prepared flexible strain sensor was pasted on the finger and tested at different bending degrees. The test results are as follows: Fig.29 The prepared flexible strain sensor was pasted on the wrist and tested at different bending degrees. The test results are shown in Fig.30 shown.

[0099] (3) Facial expressions, frowns

[0100] The prepared flexible strain sensor was pasted on the cheek and brow bone, and its resistance change was tested during smiling or frowning. The test results are as follows: Figure 31-32 shown.

[0101] The above is a relatively good implementation case of the present invention, and does not limit the present invention in any form. Under the premise of not exceeding the technical solution of the claims, certain improvements can be made, and these improvements are also regarded as the protection scope of the present invention.

Claims

1. A method for preparing an ultra-stable flexible strain sensor based on a W-shaped leaf vein structure, characterized in that: The following steps are involved: (1) Mixing a polydimethylsiloxane prepolymer and a curing agent, and after eliminating bubbles, pouring the mixture onto a specific main vein region on the front side of a fern leaf having W-shaped veins, heating to cure the PDMS, and then demolding to obtain a flexible PDMS film with a W-shaped vein structure having a thickness of 1.0 mm to 1.5 mm; (2) depositing a first layer of metal chromium on the surface of the W-shaped leaf vein structure flexible PDMS film of step (1) to enhance the bonding force between the substrate and silver, then depositing a second layer of metal silver, and finally depositing a third layer of metal chromium to prevent silver from being oxidized, thereby obtaining a metal coating on the surface of the W-shaped leaf vein structure flexible PDMS film; (3) The two ends of the surface of the flexible PDMS film with a W-shaped leaf vein structure in the direction of the main leaf vein are used as electrodes and are connected to external wires. The flexible PDMS film with a W-shaped leaf vein structure is stretched along the direction of the main leaf vein on its surface, and metal plating cracks with a crack width of 5.6μm-18.3μm are generated in the direction perpendicular to the main leaf vein, thereby changing the resistance between the two electrodes and realizing the function of a flexible strain sensor.

2. The preparation method according to claim 1, characterized in that: In step (2), the thickness of the first metal chromium layer is 2.5nm-5nm; the thickness of the second metal silver layer is 69nm-75nm; and the thickness of the third metal chromium layer is 2.5nm-5nm.

3. The preparation method according to claim 2, characterized in that: In step (3), the leaf is stretched along the main vein direction of its surface. When the stretching strain is 40%, the relative change in resistance reaches a maximum value of 800.

4. The preparation method according to claim 2, characterized in that: The flexible strain sensor has a response time of 83 ms and a recovery time of 93 ms when stretched.

5. A W-shaped leaf vein structure flexible strain sensor obtained by the preparation method according to any one of claims 1 to 4, characterized in that: The invention comprises: a flexible PDMS film with a W-shaped leaf vein structure, on which a surface with the W-shaped leaf vein structure is deposited with three metal layers: a first layer of metal chromium, a second layer of metal silver, and a third layer of metal chromium; the thickness of the flexible PDMS film with the W-shaped leaf vein structure is 1.0 mm to 1.5 mm; the thickness of the first layer of metal chromium is 2.5 nm to 5 nm; the thickness of the second layer of metal silver is 69 nm to 75 nm; the thickness of the third layer of metal chromium is 2.5 nm to 5 nm; two ends of the surface of the flexible PDMS film with the W-shaped leaf vein structure in the direction of the main leaf vein serve as electrodes, which are respectively externally connected to wires; the W-shaped leaf vein structure is obtained by demoulding using the specific main leaf vein area on the front side of a fern leaf as a template.

6. The application of a W-shaped leaf vein structure flexible strain sensor according to claim 5, characterized in that: It is used to prepare any of the following human physiological monitoring equipment: pulse beating, swallowing, facial expression, frowning.

7. The application of a W-shaped leaf vein structure flexible strain sensor according to claim 6, characterized in that: When the tensile strain of the strain sensor is 40%, the relative change in resistance reaches a maximum value of 800; the response time of the strain sensor during stretching is 83ms, and the recovery time is 93ms.

8. The application of a W-shaped leaf vein structure flexible strain sensor according to claim 6, characterized in that: Under the condition of 5% plane tensile strain, the strain sensor is cyclically stretched for more than 10,000 times along the main vein direction, and the resistance of the strain sensor after stretching recovery does not change significantly compared with that before cyclic stretching; When subjected to cyclic bending for more than 10,000 times under the condition of 5% inward bending strain, the resistance of the strain sensor after bending recovery did not change significantly compared with that before the cyclic bending.

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