WO2024036928A1 - Printable transparent stress sensor and preparation method therefor - Google Patents

Abstract

A printable transparent stress sensor, comprising a conductive layer and an elastic substrate, wherein the conductive layer is a transparent conductive layer (1) composed of conductive nanomaterials, the elastic substrate is a light-transmitting elastic layer with two or more layers and a modulus gradient structure, and the transparent conductive layer (1) is embedded in the elastic substrate and is connected to the highest-modulus layer.

Description

Printable transparent stress sensor and preparation method thereof

This application claims priority to the Chinese patent application filed with the China Patent Office on August 19, 2022, with the application number 202211000536.4 and the invention title "A printable transparent stress sensor and its preparation method", the entire content of which is incorporated by reference. in this application.

Technical field

The invention belongs to the technical field of flexible transparent stress sensors and relates to the technical fields of flexible transparent conductive materials and printed electronic products. In particular, it relates to a mechanical sensor with high light transmittance, high sensitivity and large working range and a preparation method thereof. Mainly used for screen fingerprint unlocking of flexible touch screens and human movement information monitoring. It has great application prospects in fields such as human-computer interaction, electronic skin, and bionic robots.

Background technique

With the promotion of flexible electronic devices, flexible transparent stress sensors have attracted widespread attention. For example, in the field of flexible touch screens, the screen unlocking function of Huawei's flexible screen mobile phone Mate Xs is achieved by using a flexible transparent stress sensor behind the screen backplane. In the field of flexible and transparent wearable devices, "invisibility" and "senseless" properties can increase the wearer's comfort and aesthetics and are important features of future flexible sensors. Therefore, the development of flexible transparent stress sensors is of great significance.

However, so far, researchers' exploration of flexible transparent stress sensors is still not ideal in terms of high sensitivity, large stretching range, and high transparency. The working range is too small to meet the requirements of practical applications, the sensitivity is too low to detect small deformations, and low transparency will significantly reduce the transmittance of the device.

Chinese patent publication number CN 111678623A discloses an ultra-long life self-healing stress sensor based on printable nanocomposite materials and its preparation method. It combines one-dimensional metal nanowires, two-dimensional inorganic nanosheets, polymer materials containing host-guest interactions, and corresponding high-boiling point solvents to prepare nanocomposite colloidal inks with rheological properties through screen printing. The method prepares a stress sensor with in-situ self-healing ability and long cycle service life. During the working process of the sensor, the host-guest polymer materials contained in the sensor can repair internal defects in real time and in situ, greatly extending the service life of the material. At the same time, it has the characteristics of working strain range >50%, sensitivity gauge factor >100, strong self-healing ability, and strong anti-interference ability against sweat. It has great application prospects in fields such as smart wearable devices.

Chinese patent publication number CN211376213U has announced a touch sensor based on a transparent conductive film. The touch sensor can only be bent, which limits the application of the device in wearable electronics.

Chinese patent publication number CN111895902A discloses a transparent and flexible strain sensor based on carbon nanofiber film. The transparent flexible strain sensor uses polyurethane as a base and an ultra-thin carbon nanofiber film as a conductor. The working range of this transparent flexible strain sensor is greater than 70%, and the sensitivity can reach up to 846.7, but the light transmittance is only 50%, which limits the application of this sensor on transparent devices.

Non-patent document 1 (ACS applied materials & interfaces, 2019, 11: 40232-40242) introduces a transparent film prepared by spraying silver nanowires on a polyethylene terephthalate/polysilyl methylsiloxane substrate. Stress sensor, the maximum sensitivity of the transparent stress sensor is 250; when the sheet resistance is 38.5Ω·sq ^-1 , the light transmittance is 77.4%. However, the working range of this transparent stress sensor is only 13%, which limits the application of this device in the wearable field.

Non-patent document 2 (ACS applied materials&interfaces, 2017, 9:26279-26285) introduces a composite strain sensor prepared from solution-processed carbon nanotubes and polydimethylsiloxane. The strain sensor has 92% transmittance and 50% operating range. However, the maximum sensitivity of this strain sensor is only 2.6, which limits the application of this device in the wearable field.

However, stress sensors with high sensitivity, large working range and high light transmittance have not yet been reported.

Contents of the invention

In order to solve the shortcomings of the existing technology, the purpose of the present invention is to provide a printable transparent stress sensor, which embeds a transparent conductive film composed of nano conductive materials in an elastomer base with a modulus gradient to obtain an embedded structure. flexible transparent sensor. The object of the present invention is achieved through the following solutions: providing a printable transparent stress sensor, including a conductive layer and an elastic base, the conductive layer being a transparent conductive layer composed of conductive nanomaterials; the elastic base being two layers or more A light-transmitting elastic layer with a modulus gradient structure. The transparent conductive layer is embedded in the elastic base and connected to the highest modulus layer.

Another object of the present invention is to provide a method for preparing a printable transparent stress sensor according to the above, including:

(1) Conductive nanomaterials are mixed with solvents, ultrasonic oscillated to disperse them evenly, and transparent conductive films are prepared on glass substrates by printing;

(2) Take two or more elastomer materials with large differences in modulus and close elongation at break, and mix them in different proportions to obtain elastomer solutions or precursor liquids with different moduli as elastomers base material;

(3) Spread the elastomer solution or precursor solution obtained in step (2) on the surface of the conductive film in order from high to low modulus, and heat to evaporate the solvent or solidify to form a modulus gradient embedded transparent conductive film. elastic matrix;

(4) Peel off the elastic matrix embedded with the transparent conductive film in step (3) from the glass substrate to obtain a transparent stress sensor.

Another object of the present invention is to provide an application based on the above-mentioned printable transparent stress sensor, which can be used for screen fingerprint unlocking of flexible touch screens, transparent wearable devices, or for detecting human body motion signals, including pulse beats. Deformation signals and large strain signals such as finger or knee bending.

In the present invention, conductive nanomaterials are used to provide a transparent conductive network to ensure higher light transmittance and lower sheet resistance; the relative sliding or deformation of the conductive nanomaterials enables the device to have higher sensitivity; the embedded structure and the mold have The elastomer substrate with quantitative gradient can alleviate the stress concentration of conductive nanomaterials during the stretching process and improve the working range of the sensor. The resulting transparent stress sensor has a large working range (>100%), high sensitivity (>100), and high light transmittance. rate (light transmittance is greater than 80% when the sheet resistance is less than 50Ω·sq ^-1 ), high surface flatness (R _sq <10nm) and excellent tensile stability (more than 1000 times of service life under greater than 30% strain )Features.

Description of the drawings

Figure 1 is a schematic structural diagram of a printable transparent stress sensor;

Figure 2 is the working curve of the printable transparent stress sensor in Example 1;

Figure 3 is the service life of the printable transparent stress sensor in Embodiment 1;

Figure 4 is the light transmittance of the printable transparent stress sensor in Example 1;

Figure 5 is a picture of the prepared sensor detecting pulse;

Figure 6 is a picture of the prepared sensor detecting knee joint bending.

The drawing numbers indicate:

1——Transparent conductive layer;

2—High modulus elastic base layer; 3—Medium modulus elastic base layer; 4—Low modulus elastic base layer.

Detailed ways

As shown in Figure 1, the printable transparent stress sensor includes a conductive layer and an elastic substrate. The conductive layer is a transparent conductive layer 1 composed of conductive nanomaterials; the elastic substrate is three elastic base layers with a modulus gradient structure, which are high-modulus The elastic base layer 2 with a medium modulus, the elastic base layer 3 with a medium modulus and the elastic base layer 4 with a low modulus are provided. The elastic base may be composed of two or more layers of modulus inelastic bodies. The modulus of the base material with a modulus gradient gradually decreases from one side close to the conductive layer to the other side.

The transparent conductive layer 1 is embedded in the elastic base and connected to the highest modulus layer. In elastomers with a modulus gradient structure, as the thickness of the transparent conductive film increases, the transmittance of the transparent stress sensor will decrease accordingly. Among them, as the thickness of the transparent conductive film increases, the light transmittance of the transparent stress sensor will decrease accordingly. When the sheet resistance is less than 50Ω·sq-1, the light transmittance is greater than 80%.

The conductive nanomaterial is prepared into a transparent conductive layer and transferred and embedded in an elastic base layer with a modulus gradient structure. The conductive nanomaterial provides a transparent conductive network to ensure high light transmittance and low sheet resistance. At the same time, the relative sliding of conductive nanomaterials makes the device highly sensitive.

The elastic substrate and embedded structure with a modulus gradient structure can alleviate the stress concentration of conductive nanomaterials during the stretching process and improve the working range of the sensor. The resulting transparent stress sensor has a large working range, high sensitivity, high transmittance, and high Its surface flatness and excellent tensile stability have great application prospects in fields such as flexible touch screens, electronic skins, human-computer interaction, and bionic robots.

In addition, the preparation method of the present invention is simple, there are no harmful substances in the whole process, and it can be mass-produced through screen printing.

The sensing mechanism of the transparent stress sensor is: during the stretching process, poor contact, slippage or cracks between conductive nanomaterials cause a significant increase in resistance, which improves the sensitivity of the transparent sensor. The elastic substrate with a modulus gradient can significantly reduce the stress experienced by the conductive material during the stretching process, reduce the resistance change, and significantly increase the working range of the device.

Elastomer base refers to SBS (styrene-butadiene-styrene block copolymer), SEBS (hydrogenated styrene-butadiene block copolymer), SIS (styrene-isoprene-styrene) , polyurethane, polydimethylsiloxane, polyethylene terephthalate, poly4-methylpentene, polyvinyl alcohol, polyamide thermoplastic elastomer, ethylene-vinyl acetate copolymer, polyacrylate , one or more types of polyolefin thermoplastic elastomers.

Nanoconductive materials include one of zero-dimensional metal nanoparticles, one-dimensional metal nanowires, two-dimensional conductive material graphene, and two-dimensional transition metal carbides or nitrides.

Zero-dimensional metal nanoparticles can be any of gold, silver, copper, iron, chromium, nickel, aluminum, tungsten, platinum, gallium, indium, gallium-indium alloy, and gallium-indium-tin alloy metal nanoparticles. The particle size is 1- 1000nm.

The one-dimensional metal nanowire can be one of gold, silver, copper, iron, nickel, platinum, palladium, and aluminum metal nanowires. The diameter of the metal nanowire is 1-300 nm and the length is 2-100 μm.

One of the two-dimensional transition metal carbides or nitrides. The two-dimensional transition metal carbide or nitride refers to a two-dimensional structure similar to graphene, and its general chemical formula is M _n+1 X _n T _z ,n=1,2,3, _where M is _an early _transition metal _element , , V ₂ C, Nb ₂ C, TiNbC, Nb ₄ C ₃ , Ta ₄ C ₃ , (Ti _0.5 Nb _0.5 ) ₂ C or (V _0.5 Cr _0.5 ) ₃ C ₂ .

This application also provides a method for preparing a printable transparent stress sensor, which specifically includes the following examples.

The printable transparent stress sensor uses a printing method to prepare conductive nanomaterials into a transparent conductive layer, and then transfers and embeds them into an elastic substrate with a modulus gradient structure, so that the transparent stress sensor has good performance: the sensitivity is greater than 100; the working range is greater than 100%; under a strain greater than 30%, the service life exceeds 1,000 times; it has good light transmittance and sheet resistance. When the sheet resistance is less than 50Ω·sq ^-1 , the light transmittance is greater than 80%.

The printing method of conductive film refers to: screen printing, inkjet printing, blade coating, stencil printing, gravure printing, letterpress printing, offset printing, spin coating, dip coating, Meyer rod coating, spray coating, slit coating , micro-contact printing, one of direct writing printing.

Example 1

The printable transparent stress sensor includes a conductive layer and an elastic substrate. The conductive layer is a transparent conductive layer 1 composed of conductive nanomaterials; the elastic substrate is a multi-layer elastic layer with a modulus gradient structure, and the transparent conductive layer 1 is embedded in the elastic substrate. And connected to the highest modulus layer. In this embodiment, silver nanowires are used as the conductive nanomaterial, and films composed of polyurethane 6210 and polyurethane 6290 with different mixing ratios are used as the elastic base. They are prepared according to the following steps:

(1) Preparation of transparent conductive layer: Silver nanowires are used as conductive nanomaterials, dissolved in ethanol, and dispersed evenly by ultrasonic oscillation to obtain a silver nanowire/ethanol dispersion with a concentration of 2 mg/mL. Weigh 100 μL silver nanowires/ethanol. The dispersion is spin-coated on the glass sheet using the printing method. The rotation speed is 800 rpm. The spin-coating time is 15 s. Heating at 60°C for 1 min. Repeat the operation 10 times to obtain a transparent conductive layer of silver nanowires on the glass substrate as an embedding structure;

(2) Weigh polyurethane 6210 and polyurethane 6290 according to the mass ratio of 1:0, 10:1, 5:1, 1:1, 1:5, 1:10, 0:1 respectively. Mix the two and stir evenly. Polyurethane materials with 7 different modulus layers were obtained;

(3) Weigh 1g of polyurethane with different modulus obtained in step (2), and react with 0.5g of (5-ethyl-1,3-dioxane-5-yl)methyl acrylate and 0.15g of photoinitiator agent, mix the three and stir evenly, centrifuge to remove air bubbles, and spin-coat on the surface of the transparent conductive layer of silver nanowires obtained in step (1) in order of modulus from high to low, the rotation speed is 900 rpm, the spin-coating time is 60 s, and UV After curing for 3 minutes, a thin film is obtained;

(4) Peel off the film from the glass substrate in step (3) to obtain a transparent stress sensor.

Figure 2 is the operating curve of the transparent stress sensor of this embodiment, showing the operating range and sensitivity of the inventive device.

Figure 3 shows the service life of the transparent stress sensor of this embodiment. In the stretch-release cycle with a strain of 40%, the resistance change remains stable after 1,000 and 4,000 cycles.

Figure 4 shows the light transmittance of the transparent stress sensor according to the embodiment. As the thickness of the transparent conductive film increases, its light transmittance and sheet resistance will decrease; therefore, the greater the thickness of the transparent conductive film, the greater the sheet resistance and light transmittance. Small; on the contrary, the sheet resistance and light transmittance are larger; in Figure 4, the sheet resistance increases, correspondingly the thickness of the transparent conductive film decreases and the light transmittance increases.

Example 2

A printable transparent stress sensor is similar to the steps of Example 1, except that the nano conductive material is two-dimensional transition metal nitride (MXene), and is prepared according to the following steps:

(1) Preparation of transparent conductive layer: Weigh 100 μL of MXene/water dispersion with a concentration of 10 mg/mL, print on the glass sheet with an inkjet printer, heat at 60°C for 1 min, and repeat the operation 5 times to obtain the MXene transparent conductive layer on the glass substrate ;

(2) Weigh the mass ratios respectively: 1:0, 20:1, 15:1, 10:1, 5:1, 1:1, 1:5, 1:10, 1:15, 1:20, 0 : 1 polyurethane 6210 and polyurethane 6290, mix the two and stir evenly to obtain 11 types of polyurethane with different modulus;

(3) Weigh 4g of polyurethane obtained in step (2), 2g of (5-ethyl-1,3-dioxan-5-yl)methyl acrylate and 0.6g of photoinitiator respectively, and mix the three Stir evenly, vacuum to remove bubbles, use a screen printing screen on the surface of the Mxene transparent conductive layer obtained in step (1) in order of modulus from high to low, and UV cure for 5 minutes to obtain an embedded transparent conductive layer with a mold. Elastic film with quantitative gradient;

(4) Peel off the glass substrate to obtain a transparent stress sensor.

Example 3

A printable transparent stress sensor is similar to the steps of Example 1, except that the nano conductive material is copper nanowires, and the elastic base material is polydimethylsiloxane PDMS part A and PDMS part B. It is prepared according to the following steps:

(1) Preparation of transparent conductive layer: Weigh 100 μL of copper nanowire/water dispersion with a concentration of 5 mg/mL, scrape it on the glass sheet with a Meyer rod, heat it at 60°C for 1 min, and repeat the operation 5 times to obtain copper nanowires on the glass substrate. Line transparent conductive layer.

(2) Weigh polydimethylsiloxane PDMS part A and PDMS part B with mass ratios of 10:1, 5:1, and 1:1 respectively. Mix the two and stir evenly to obtain three types with different moduli. PDMS;

(3) Weigh 10g of the PDMS obtained in step (2), vacuum to remove bubbles, and use a Meyer rod to scrape the surface of the transparent conductive layer of copper nanowires obtained in step (1) in order of modulus from high to low, 60°C Heat and solidify for 2 hours to obtain an elastic film embedded with a transparent conductive layer and having a modulus gradient;

(4) Peel off the film from the glass substrate to obtain a transparent stress sensor.

Example 4

A printable transparent stress sensor, the steps are similar to those in Example 1, except that the nano conductive material is two-dimensional transition metal nitride (MXene), and the elastic base material is polydimethylsiloxane PDMS part A and PDMS part B. Press Prepare by following steps:

(1) Preparation of transparent conductive layer: Weigh 100mL of MXene/water dispersion with a concentration of 8 mg/mL, soak the glass piece in it for 10 minutes, heat and dry at 80°C, repeat the operation 7 times, and obtain the Mxene transparent conductive layer on the glass substrate, as embedded structure;

(2) Weigh 10:1, 8:1, 5:1, 3:1, 1:1 PDMS part A and PDMS part B respectively, mix them and stir evenly to obtain five types of PDMS with different moduli. ;

(3) Weigh 5g of the PDMS obtained in step (2), and screen-print on the surface of the MXene transparent conductive layer obtained in step (1) in order of modulus from high to low, and heat it at 80°C for 1 hour to obtain a package. An elastic film with a transparent conductive layer embedded in it and a modulus gradient;

(4) Peel off the film from the glass substrate to obtain a transparent stress sensor.

Example 5

A printable transparent stress sensor is similar to the steps of Embodiment 1, except that graphene is selected as the nano conductive material, and is prepared according to the following steps:

(1) Preparation of the transparent conductive layer: Use dimethylformamide (DMF) as the solvent to prepare a graphene/DMF dispersion with a concentration of 6 mg/mL. Weigh 10 mL of the graphene/DMF dispersion and screen-print it on the glass sheet, 40 Heat and dry at ℃, repeat the operation three times to obtain a graphene transparent conductive layer on the glass substrate;

(2) Weigh the mass ratios respectively as 1:0, 7:1, 6:1, 5:1, 4:1, 3:1, 2:1, 1:1, 1:2, 1:3, 1 :4, 1:5, 1:6, 1:7, 0:1 polyurethane 6210 and polyurethane 6290, mix the two and stir evenly to obtain 15 kinds of polyurethane with different modulus;

(3) Weigh 2g of polyurethane obtained in step (2), 1g of (5-ethyl-1,3-dioxan-5-yl)methyl acrylate and 0.3g of photoinitiator, mix the three and stir evenly , use a slit coater to coat the surface of the graphene transparent conductive layer obtained in step (1) in order of modulus from high to low, and cure with UV for 10 minutes to obtain an elastic film with embedded transparent conductive layer and modulus gradient;

(4) Peel off the film from the glass substrate to obtain a transparent stress sensor.

This application also provides the application of printable transparent stress sensors, which can be used for on-screen fingerprint unlocking of flexible touch screens, transparent wearable devices, or for detecting human body motion signals, including pulse beat-like micro-deformation signals and finger or knee bending. Large strain signal.

As shown in Figure 5, the present invention can be used as a pulse beat sensor and has high detection sensitivity for the tiny deformation signals of the pulse beat.

As shown in Figure 6, the present invention also has excellent effects in detecting knee joint bending movements.

The above embodiments are only to further illustrate the present invention and should not be limited to the contents shown in the embodiments. Each specific substance in the product components disclosed in the technical solution of the present invention can be implemented by the present invention, and the same technical effects can be obtained as in the examples. Here, the examples will not be cited one by one for explanation. Therefore, any equivalents or modifications made without departing from the spirit disclosed in the present invention fall within the scope of protection of the present invention.

Claims (15)

1. A printable transparent stress sensor, including a conductive layer and an elastic base, wherein the conductive layer is a transparent conductive layer composed of conductive nanomaterials; the elastic base is two or more light-transmissive layers with a modulus gradient structure The elastic layer, the transparent conductive layer is embedded in the elastic base and connected to the highest modulus layer.
2. The printable transparent stress sensor according to claim 1, wherein as the thickness of the transparent conductive film increases, the light transmittance of the transparent stress sensor decreases accordingly. When the sheet resistance is less than 50Ω·sq ^-1 , the transmittance The light rate is greater than 80%.
3. The printable transparent stress sensor according to claim 1, wherein

   The elastomer base refers to: SBS, SEBS, SIS, polyurethane, polydimethylsiloxane, polyethylene terephthalate, poly4-methylpentene, polyvinyl alcohol, polyamide thermoplastic elastomer One or more of the following: ethylene-vinyl acetate copolymer, polyacrylate, and polyolefin thermoplastic elastomer.
4. The printable transparent stress sensor according to claim 1 or 3, wherein the light-transmitting elastic layer having a modulus gradient has a modulus that gradually decreases from one side close to the transparent conductive layer to the other side.
5. The printable transparent stress sensor according to claim 1, wherein

   The nano-conductive material refers to: zero-dimensional metal nanoparticles, one of gold, silver, copper, iron, chromium, nickel, aluminum, tungsten, platinum, gallium, indium, gallium-indium alloy, and gallium-indium-tin alloy metal nanoparticles. species, particle size 1-1000nm.
6. The printable transparent stress sensor according to claim 1, wherein

   The nano conductive material refers to: one-dimensional metal nanowires, one of gold, silver, copper, iron, nickel, platinum, palladium and aluminum metal nanowires, the diameter of the metal nanowires is 1-300nm, Length is 2-100μm.
7. The printable transparent stress sensor according to claim 1, wherein

   The nano-conductive material refers to one of two-dimensional conductive materials graphene, two-dimensional transition metal carbide or nitride, and the two-dimensional transition metal carbide or nitride refers to a two-dimensional graphene-like conductive material. dimensional structure, its general chemical formula is M _n+1 X _n T _z , n=1,2,3, where M is an early transition metal element, Reactive functional groups, including Ti ₂ C, Ti ₃ C ₂ , Ti ₃ CN, V ₂ C, Nb ₂ C, TiNbC, Nb ₄ C ₃ , Ta ₄ C ₃ , (Ti _0.5 Nb _0.5 ) ₂ C or (V _0.5 Cr _0.5 ) ₃ C ₂ in one.
8. A method for preparing a printable transparent stress sensor according to any one of claims 1 to 7, comprising:

   (1) The conductive nanomaterials are mixed with solvents, ultrasonic oscillated to disperse them evenly, and a transparent conductive layer is prepared on the glass substrate by printing;

   (2) Take two or more elastomer materials with large differences in modulus and close elongation at break, and mix them in different proportions to obtain elastomer solutions or precursor liquids with different moduli as elastic bases Material;

   (3) Spread the elastomer solution or precursor solution obtained in step (2) on the surface of the conductive layer in order from high to low modulus, and heat to evaporate the solvent or solidify to form a modulus gradient embedded transparent conductive layer. elastic matrix;

   (4) Tear off the elastic matrix embedded with the transparent conductive layer in step (3) from the glass substrate to obtain a transparent stress sensor.
9. The method for preparing a printable transparent stress sensor according to claim 8, wherein in step (1), the printing method of the conductive film refers to: screen printing, inkjet printing, scraping, stencil printing, gravure printing , letterpress printing, offset printing, spin coating, dip coating, Meyer rod coating, spray coating, slit coating, micro contact printing, one of direct writing printing.
10. The method for preparing a printable transparent stress sensor according to claim 8, wherein it is prepared according to the following steps:

    (1) Preparation of transparent conductive layer: Silver nanowires are used as conductive nanomaterials, dissolved in ethanol, and dispersed evenly by ultrasonic oscillation to obtain a silver nanowire/ethanol dispersion with a concentration of 2 mg/mL. Weigh 100 μL silver nanowires/ethanol. The dispersion is spin-coated on the glass sheet using the printing method. The rotation speed is 800 rpm. The spin-coating time is 15 s. Heating at 60°C for 1 min. Repeat the operation 10 times to obtain a transparent conductor of silver nanowires on the glass substrate. Electrical layer, as an embedded structure;

    (2) Weigh polyurethane 6210 and polyurethane 6290 according to the mass ratio of 1:0, 10:1, 5:1, 1:1, 1:5, 1:10, 0:1 respectively. Mix the two and stir evenly. Polyurethane materials with 7 different modulus layers were obtained;

    (3) Weigh 1g of the polyurethane with different modulus obtained in step (2), mix it with 0.5g of methyl acrylate and 0.15g of photoinitiator respectively, mix the three evenly, and remove bubbles by centrifugation. Spin coating on the surface of the silver nanowire transparent conductive layer obtained in step (1) in sequence, the rotation speed is 900 rpm, the spin coating time is 60 s, and UV curing is performed for 3 minutes to obtain a thin film;

    (4) Peel off the film from the glass substrate in step (3) to obtain a transparent stress sensor.
11. The method for preparing a printable transparent stress sensor according to claim 8, wherein it is prepared according to the following steps:

    (1) Preparation of transparent conductive layer: Weigh 100 μL of MXene/water dispersion with a concentration of 10 mg/mL, print on a glass sheet with an inkjet printer, heat at 60°C for 1 min, and repeat the operation 5 times to obtain a transparent conductive layer of MXene on the glass substrate;

    (2) Weigh the mass ratios respectively: 1:0, 20:1, 15:1, 10:1, 5:1, 1:1, 1:5, 1:10, 1:15, 1:20, 0 : 1 polyurethane 6210 and polyurethane 6290, mix the two and stir evenly to obtain 11 types of polyurethane with different modulus;

    (3) Weigh 4g of the polyurethane obtained in step (2), 2g of methyl acrylate and 0.6g of photoinitiator respectively, mix the three and stir evenly, vacuum to remove air bubbles, and proceed in step (() in order of modulus from high to low. 1) The surface of the obtained Mxene transparent conductive film is screen printed with a screen printing screen, and UV cured for 5 minutes to obtain an elastic film with a transparent conductive layer embedded and a modulus gradient;

    (4) Peel off the glass substrate to obtain a transparent stress sensor.
12. The method for preparing a printable transparent stress sensor according to claim 8, wherein it is prepared according to the following steps:

    (1) Preparation of transparent conductive layer: Weigh 100 μL of copper nanowire/water dispersion with a concentration of 5 mg/mL, scrape it on the glass sheet with a Meyer rod, heat it at 60°C for 1 min, and repeat the operation 5 times to obtain copper nanowires on the glass substrate. Line transparent conductive layer;

    (2) Weigh polydimethylsiloxane PDMS part A and PDMS part B with mass ratios of 10:1, 5:1, and 1:1 respectively. Mix the two and stir evenly to obtain three types with different moduli. PDMS;

    (3) Weigh 10g of the PDMS obtained in step (2), vacuum to remove bubbles, and use a Meyer rod to scrape the surface of the copper nanowire transparent conductive film obtained in step (1) in order of modulus from high to low, 60°C Heat and solidify for 2 hours to obtain an elastic film embedded with a transparent conductive layer and having a modulus gradient;

    (4) Peel off the film from the glass substrate to obtain a transparent stress sensor.
13. The method for preparing a printable transparent stress sensor according to claim 8, wherein it is prepared according to the following steps:

    (1) Preparation of transparent conductive layer: Weigh 100 mL of MXene/water dispersion with a concentration of 8 mg/mL, soak the glass piece in it for 10 min, heat and dry at 80°C, repeat the operation 7 times, and obtain the Mxene transparent conductive layer on the glass substrate as a package buried structure;

    (2) Weigh 10:1, 8:1, 5:1, 3:1, 1:1 PDMS part A and PDMS part B respectively, mix them and stir evenly to obtain five types of PDMS with different moduli. ;

    (3) Weigh 5g of the PDMS obtained in step (2), and screen-print on the surface of the MXene transparent conductive layer obtained in step (1) in order of modulus from high to low, and heat it at 80°C for 1 hour to obtain a package. An elastic film with a transparent conductive layer embedded in it and a modulus gradient;

    (4) Peel off the film from the glass substrate to obtain a transparent stress sensor.
14. The method for preparing a printable transparent stress sensor according to claim 8, wherein it is prepared according to the following steps:

    (1) Preparation of transparent conductive layer: Use dimethylformamide as solvent to prepare graphene/DMF dispersion with a concentration of 6 mg/mL. Weigh 10 mL of graphene/DMF dispersion and screen-print it on a glass sheet. Heat and bake at 40°C. Dry and repeat the operation three times to obtain a graphene transparent conductive layer on the glass substrate;

    (2) Weigh the mass ratios respectively as 1:0, 7:1, 6:1, 5:1, 4:1, 3:1, 2:1, 1:1, 1:2, 1:3, 1 :4, 1:5, 1:6, 1:7, 0:1 polyurethane 6210 and polyurethane 6290, mix the two and stir evenly to obtain 15 kinds of polyurethane with different modulus;

    (3) Weigh 2g of the polyurethane obtained in step (2), 1g of methyl acrylate and 0.3g of photoinitiator respectively, mix the three and stir evenly, and add the transparent conductive graphene obtained in step (1) in order of modulus from high to low. The surface of the layer is coated with a slit coater and UV cured for 10 minutes to obtain an elastic film with a transparent conductive layer embedded and a modulus gradient;

    (4) Peel off the film from the glass substrate to obtain a transparent stress sensor.
15. An application of the printable transparent stress sensor according to any one of claims 1 to 7, wherein it is used for screen fingerprint unlocking of flexible touch screens, transparent wearable devices, or for detecting human body motion signals, including pulse beats Small deformation-like signals and large strain signals like finger or knee bending.