Flexible Pressure Sensor Based on Liquid Metal Electrode and Its Preparation Method

Abstract

The present invention discloses a flexible pressure sensor based on a liquid metal electrode and a preparation method thereof, relating to the technical field of flexible sensing. The flexible pressure sensor includes a liquid metal electrode layer and an ionic dielectric layer. The liquid metal electrode layer includes an upper electrode layer and a lower electrode layer. The upper electrode layer includes an upper electrode encapsulation layer, an upper electrode adhesion layer, and an upper liquid metal layer. The lower electrode layer includes a lower liquid metal layer, a lower electrode adhesion layer, and a lower electrode encapsulation layer. The ionic dielectric layer is disposed between the upper liquid metal layer and the lower liquid metal layer. The present invention uses liquid metal with both high conductivity and ductility as a sensing electrode, and comprehensively utilizes the wettability difference between the liquid metal electrode and the sensor substrate and the intermediate dielectric layer, and utilizes the characteristic that the substrate and the intermediate dielectric layer have a stiffness gradient to ensure the stability of the liquid metal electrode structure during the pressure cycling process, and avoid the interference of the interfacial double-layer capacitance by mechanical deformations such as stretching, bending, and folding.

Description

Technical Field

[0001] The present invention relates to the field of flexible sensing technology, and in particular to a flexible pressure sensor based on liquid metal electrodes and a preparation method thereof. Background Art

[0002] Flexible pressure sensors can convert pressure signals into corresponding electrical signals. They offer advantages such as flexibility and simple fabrication processes, and have been widely used in fields such as electronic skin, human-computer interaction, and smart wearable devices. Typically, in human-computer interaction or intelligent robotics, flexible pressure sensors can be conformally integrated or attached to human skin or the surface of a robotic arm to sense external pressure signals or their own deformation. In addition to requiring fast response speed, high sensitivity, and excellent durability, flexible pressure sensors also need to possess excellent ductility, stability, and robustness to conformally attach to any substrate (e.g., surfaces with varying degrees of softness or hardness or irregular curves). Furthermore, good stability and robustness can also resist the effects of deformation interference (such as stretching, bending, and folding) on pressure sensing performance.

[0003] Flexible pressure sensors typically consist of a flexible substrate, sensitive materials, and sensing electrodes. In recent years, significant research progress has been made in material selection, microstructure design, and pressure sensing methods to improve the performance of flexible pressure sensors. However, many deficiencies remain. A Chinese invention patent, publication number CN119666203A, discloses an ionized flexible capacitive pressure sensor with adjustable linear region. This sensor utilizes a conductive polyurethane sponge elastomer to regulate the linearity of the interfacial double layer between the electrode and the sensitive material. While this significantly improves the sensitivity and linearity of pressure sensing, the use of conductive tape as the sensing electrode (surface-modified with copper and nickel) results in low overall stretchability, making it difficult to shield the sensor from the effects of mechanical deformation on sensing performance.

[0004] Liquid metals can have both good electrical conductivity and ductility, and gallium-based liquid metals also have advantages such as low volatility and good biocompatibility, making them an excellent material choice for flexible sensors. However, due to the large surface tension and low viscosity of liquid metals, their process control is relatively difficult. The Chinese invention patent with the publication number CN110823423A discloses a liquid metal electrode pressure sensor, which constructs a sensing electrode by simply pouring liquid metal into a microchannel, but only improves the stretchability of the electrode and does not consider the influence of mechanical deformation on the pressure sensing signal. The Chinese invention patent with the publication number CN119413322A obtains an embedded liquid metal microstructure electrode by printing liquid metal on the surface of a microcone structure, which not only greatly improves the sensitivity of pressure sensing, but also can avoid the delamination and slip phenomena between multi-layer stacked structures caused by too low interfacial adhesion strength, ensuring that the sensor can also work stably under deformations such as stretching and torsion. However, this invention does not consider the problems of sensing signal stability and robustness. For example, under cyclic pressure loads, liquid metal will inevitably adhere to the dielectric sensitive material.

[0005] In summary, existing flexible pressure sensors generally have disadvantages such as low stretchability, poor stability and robustness. Therefore, it is necessary to propose a flexible pressure sensor using liquid metal as an electrode to achieve pressure sensing performance with high pressure sensitivity, high stability and good stretchability, and solve the problems of serious interference by mechanical deformation, insufficient sensitivity and poor stability of flexible pressure sensors in the prior art. Summary of the Invention

[0006] The purpose of the present invention is to solve at least one of the technical problems existing in the prior art, and provide a flexible pressure sensor based on a liquid metal electrode and a preparation method thereof.

[0007] In view of the problems existing in the current flexible pressure sensing technology, such as the pressure-sensitive signal being easily interfered by mechanical deformation, low detection sensitivity, and poor stability, the present invention uses liquid metal with both high conductivity and ductility as the sensing electrode to ensure the stability of the sensing performance during mechanical deformation processes such as stretching, bending, and folding. The present invention utilizes the wetting property difference between the liquid metal and the substrate (the upper electrode adhesion layer and the lower electrode adhesion layer) and the intermediate dielectric layer (that is, the property that the liquid metal adheres to the substrate but does not adhere to the intermediate dielectric layer) to ensure the stability of the liquid metal electrode structure during the pressure cycling process. On the other hand, the present invention utilizes the property that the substrate and the intermediate dielectric layer have a stiffness gradient (the elastic modulus of the substrate is much smaller than that of the intermediate dielectric layer). Under the action of mechanical deformations such as stretching, bending, and folding, the substrate undergoes large deformations while the intermediate dielectric layer does not deform, ensuring that the contact area between the liquid metal electrode and the intermediate dielectric layer does not change during stretching, and avoiding the interference of the interfacial double-layer capacitance by mechanical deformations such as stretching, bending, and folding. Finally, the intermediate dielectric layer of the present invention uses an ionic composite material with rich microstructures on the surface, greatly improving the sensitivity of pressure sensing.

[0008] The technical solution of the present invention is as follows:

[0009] In a first aspect of the present invention, a flexible pressure sensor based on a liquid metal electrode is provided, including:

[0010] A liquid metal electrode layer, which includes an upper electrode layer and a lower electrode layer arranged below the upper electrode layer; the upper electrode layer sequentially includes an upper electrode encapsulation layer, an upper electrode adhesion layer, and an upper liquid metal layer from top to bottom; the lower electrode layer sequentially includes a lower liquid metal layer, a lower electrode adhesion layer, and a lower electrode encapsulation layer from top to bottom;

[0011] An ionic dielectric layer, which is arranged between the upper liquid metal layer and the lower liquid metal layer, and the ionic dielectric layer includes a dielectric base layer and surface microstructures arranged on the dielectric base layer;

[0012] The contact angle of the upper liquid metal layer with respect to the upper electrode adhesion layer is respectively smaller than the contact angle of the upper liquid metal layer with respect to the ionic dielectric layer and the contact angle of the upper liquid metal layer with respect to the upper electrode encapsulation layer;

[0013] The contact angle of the lower liquid metal layer with respect to the lower electrode adhesion layer is respectively smaller than the contact angle of the lower liquid metal layer with respect to the ionic dielectric layer and the contact angle of the lower liquid metal layer with respect to the lower electrode encapsulation layer;

[0014] The elastic modulus of the ionic dielectric layer is respectively greater than the elastic modulus of the upper electrode encapsulation layer, the upper electrode adhesion layer, the lower electrode encapsulation layer, and the lower electrode adhesion layer.

[0015] The present invention utilizes the wettability differences between the liquid metal electrodes and the sensor substrate (upper electrode adhesion layer and lower electrode adhesion layer) and the intermediate dielectric layer, and utilizes the characteristics that the substrate (upper electrode adhesion layer and lower electrode adhesion layer) and the intermediate dielectric layer have a stiffness gradient to ensure the stability of the liquid metal electrode structure during the pressure cycling process, and avoid the interference of the interfacial double-layer capacitance by mechanical deformations such as stretching, bending, and folding.

[0016] Optionally, the contact angles of the upper liquid metal layer with respect to the upper electrode adhesion layer and the lower liquid metal layer with respect to the lower electrode adhesion layer are both less than 90°; that is, the upper electrode adhesion layer and the lower electrode adhesion layer have adhesiveness to the liquid metal, and the liquid metal can be firmly adhered to the upper electrode adhesion layer and the lower electrode adhesion layer.

[0017] The contact angles of the upper liquid metal layer with respect to the upper electrode encapsulation layer and the lower liquid metal layer with respect to the lower electrode encapsulation layer are both greater than 120°; the upper liquid metal layer and the lower liquid metal layer have non-adhesiveness to the upper electrode encapsulation layer and the lower electrode encapsulation layer, ensuring that the liquid metal does not leak under pressure (the liquid metal is firmly adhered to the upper electrode adhesion layer and the lower electrode adhesion layer).

[0018] The contact angles of the upper liquid metal layer and the lower liquid metal layer with respect to the ion dielectric layer are both greater than 120°; the ion dielectric layer has non-adhesiveness to the liquid metal, ensuring that the liquid metal does not adhere to the ion dielectric layer under cyclic pressure (the liquid metal is firmly adhered to the upper electrode adhesion layer and the lower electrode adhesion layer under cyclic pressure load, thus ensuring structural stability).

[0019] Optionally, the upper electrode adhesion layer, the lower electrode adhesion layer, the upper electrode encapsulation layer, and the lower electrode encapsulation layer have small elastic moduli and are of the same order of magnitude, and can be materials of the same nature, such as all being silicone polymers, to ensure that no interfacial separation occurs between the two layers of materials; however, it is not limited to materials of the same nature, and other materials with the same order of magnitude and no interfacial separation are within the protection scope of the present invention. Preferably, the elastic modulus of the ion dielectric layer is 10 times or more of the elastic moduli of the upper electrode encapsulation layer, the upper electrode adhesion layer, the lower electrode encapsulation layer, and the lower electrode adhesion layer; to ensure that under tensile action, the liquid metal electrode layer deforms preferentially, and the ion dielectric layer hardly deforms, so as to ensure that the contact surface between the sensing electrode and the dielectric layer does not change under mechanical deformations such as stretching, bending, and folding, and shield the influence of mechanical deformations on the pressure sensing performance.

[0020] Optionally, the contact angles of the upper liquid metal layer with respect to the upper electrode adhesion layer and the lower liquid metal layer with respect to the lower electrode adhesion layer are both 10° - 40°;

[0021] The contact angles of the upper liquid metal layer with respect to the upper electrode encapsulation layer and the lower liquid metal layer with respect to the lower electrode encapsulation layer are both 140° - 180°;

[0022] The contact angles of both the upper liquid metal layer and the lower liquid metal layer with the ion dielectric layer are 140° to 180°;

[0023] The elastic moduli of both the upper electrode encapsulation layer and the lower electrode encapsulation layer are 0.5 MPa to 2 MPa;

[0024] The elastic moduli of both the upper electrode adhesion layer and the lower electrode adhesion layer are 0.5 MPa to 2 MPa;

[0025] The elastic modulus of the ion dielectric layer is 50 MPa to 100 MPa.

[0026] Optionally, both the upper electrode encapsulation layer and the lower electrode encapsulation layer are flexible stretch films, and the raw materials for preparing the flexible stretch films are at least one of TPU, Ecoflex silicone polymer, and PDMS; preferably, both the upper electrode encapsulation layer and the lower electrode encapsulation layer are TPU nanofiber films.

[0027] Optionally, both the upper electrode adhesion layer and the lower electrode adhesion layer are flexible composite films, and the raw materials for preparing the flexible composite films are TPU doped with graphene oxide or Ecoflex silicone polymer doped with graphene oxide; preferably, both the upper electrode adhesion layer and the lower electrode adhesion layer are GO / TPU composite nanofiber films.

[0028] Optionally, the raw materials for preparing the upper liquid metal layer and the lower liquid metal layer are selected as liquid metal alloys, specifically alloy materials that can have both high conductivity and ductility at room temperature; preferably, gallium-based liquid metal alloy materials (gallium, indium, tin alloy) are selected; more preferably, the mass ratio of gallium, indium, and tin is 0.6 - 0.7:0.2 - 0.3:0.08 - 0.12; specifically, gallium:indium:tin = 0.685:0.215:0.1.

[0029] Optionally, the ion dielectric layer can be selected as a flexible composite material, preferably a composite fiber film containing ions, typically polytetrafluoroethylene doped with 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquid. The ion dielectric layer contains abundant ions, which can form nanoscale capacitors with the electrons in the sensing electrode, thereby forming abundant double electric layers between the sensing electrode and the ion dielectric layer. Under the action of pressure, the area of the double electric layer will change, so as to be pressure-sensitive and greatly improve the sensitivity.

[0030] Optionally, the preparation methods of the upper electrode encapsulation layer and the lower electrode encapsulation layer include the following steps:

[0031] Fully dissolve the TPU particles in dimethylacetamide and stir to prepare a TPU precursor solution with a concentration of 30 wt.% - 50 wt.%. Load the TPU precursor solution into a syringe and deposit it through the electrospinning process to obtain a TPU nanofiber membrane, which serves as the upper electrode encapsulation layer and the lower electrode encapsulation layer.

[0032] Among them, the applied voltage for electrospinning is 10 kV - 14 kV, the feeding rate is 250 μL / h - 350 μL / h, the spinning distance is 100 mm - 140 mm, the spinning temperature is 20°C - 30°C, and the relative humidity is 30% - 50%.

[0033] Optionally, the preparation method of the upper electrode adhesion layer and the lower electrode adhesion layer includes the following steps:

[0034] Add GO powder to the DMAC solvent, fully dissolve it, then add TPU particles and stir to form a GO / TPU composite precursor solution. Load the GO / TPU composite precursor solution into a syringe and deposit it through the electrospinning process to obtain a composite GO / TPU nanofiber membrane, which serves as the upper electrode adhesion layer and the lower electrode adhesion layer.

[0035] Among them, the mass ratio of GO powder, DMAC solvent, and TPU particles is 0.01:1.5 - 2.5:0.8 - 1.

[0036] The applied voltage for electrospinning is 8 kV - 12 kV, the feeding rate is 180 μL / h - 220 μL / h, the spinning distance is 80 mm - 120 mm, the spinning temperature is 20°C - 30°C, and the relative humidity is 30% - 50%.

[0037] Optionally, the preparation method of the upper liquid metal layer and the lower liquid metal layer includes the following steps:

[0038] Mix gallium, indium, and tin to obtain a liquid metal alloy. Print the liquid metal alloy onto the GO / TPU composite nanofiber membrane through screen printing to obtain the upper liquid metal layer and the lower liquid metal layer.

[0039] Optionally, the preparation method of the ion dielectric layer includes the following steps:

[0040] Dissolve PVDF - HFP particles in the DMAC solvent and mix evenly, then add the [EMIM][TFSI] ionic liquid and stir to obtain a PVDF - HFP / [EMIM][TFSI] composite precursor solution. Load the PVDF - HFP / [EMIM][TFSI] composite precursor solution into a syringe and deposit it through the electrospinning process to obtain a PVDF - HFP / [EMIM][TFSI] composite ion nanofiber membrane, which serves as the ion dielectric layer.

[0041] Among them, the mass ratio of PVDF-HFP particles, DMAC solvent and [EMIM][TFSI] ionic liquid is 0.5-1.5:8-12:0.5-1.5;

[0042] The electrospinning applied voltage is 6 kV-8 kV, the liquid supply rate is 90 μL / h-110 μL / h, the spinning distance is 70 mm-90 mm, the spinning temperature is 20°C-30°C, and the relative humidity is 30%-50%.

[0043] The second aspect of the present invention provides a preparation method of a flexible pressure sensor based on a liquid metal electrode, including the following steps:

[0044] Prepare an ion-containing composite fiber membrane by electrospinning process to obtain an ion dielectric layer;

[0045] Deposit a TPU nanofiber membrane by electrospinning process to obtain a lower electrode encapsulation layer; deposit a GO / TPU composite nanofiber membrane on the lower electrode encapsulation layer by electrospinning process to obtain a lower electrode adhesion layer; then prepare a liquid metal pattern on the lower electrode adhesion layer by screen printing process to obtain a lower liquid metal layer;

[0046] Place the ion dielectric layer on the lower liquid metal layer;

[0047] Deposit a GO / TPU composite nanofiber membrane by electrospinning process, prepare a liquid metal pattern on the GO / TPU composite nanofiber membrane by screen printing process to obtain an upper liquid metal layer; continue to deposit a GO / TPU composite nanofiber membrane by electrospinning process to obtain an upper electrode adhesion layer; finally deposit a TPU nanofiber membrane by electrospinning process to obtain an upper electrode encapsulation layer.

[0048] The present invention has at least one of the following beneficial effects:

[0049] On the one hand, the present invention utilizes the adhesion difference between the liquid metal and the electrode adhesion layer and the ion dielectric layer to ensure the stability of the liquid metal electrode under pressure; on the other hand, the present invention utilizes the stiffness gradient characteristics of the liquid metal electrode layer and the ion dielectric layer to shield the influence of mechanical deformations such as stretching, bending and folding on the pressure sensing performance. The present invention uses liquid metal as the sensing electrode and flexible materials as the substrate and dielectric layer to ensure the overall ductility and the stability of the electrode; the present invention utilizes the double-layer characteristics of the liquid metal electrode layer and the ion dielectric layer to greatly improve the pressure sensing sensitivity. Through the synergistic effect of the above liquid metal electrodes, the sensitivity, stretchability, stability and robustness of the flexible pressure sensing are greatly improved. The flexible pressure sensor proposed by the present invention has broad application potential in the fields of intelligent soft robots and the like. Description of the Drawings

[0050] Figure 1 Schematic diagram of the structure of the flexible pressure sensor based on liquid metal electrodes used in Example 1.

[0051] Figure 2 This is a schematic diagram of the structure of the flexible pressure sensor based on liquid metal electrodes that undergoes internal deformation when a small external pressure load is applied in Example 1.

[0052] Figure 3 This is a schematic diagram of the structure of the flexible pressure sensor based on liquid metal electrodes that undergoes internal deformation when a large external pressure load is applied in Example 1.

[0053] Figure 4 These are the results of the uniaxial stretching experiment of the pure TPU nanofiber membrane in Example 1.

[0054] Figure 5 These are the results of the uniaxial tensile test of the GO / TPU composite nanofiber membrane in Example 1.

[0055] Figure 6 These are the results of the uniaxial tensile test of the PVDF-HFP / [EMIM][TFSI] composite ion fiber membrane in Example 1.

[0056] Figure 7 This is a cross-sectional electron microscope image of the flexible pressure sensor based on liquid metal electrodes in Example 1.

[0057] Figure 8 These are the sensitivity test results of the flexible pressure sensor based on liquid metal electrodes in Example 1.

[0058] Figure 9 The stress distribution simulation results of the flexible pressure sensor based on liquid metal electrodes during the stretching process in Example 1.

[0059] Figure 10 These are the results of the tensile test of the flexible pressure sensor based on liquid metal electrodes in Example 1.

[0060] Figure 11 These are the test results of the application of the flexible pressure sensor based on liquid metal electrodes in Example 1 in a pneumatic soft manipulator.

[0061] Figure 12 This is the result of the uniaxial stretching test of the pure TPU film in Example 2.

[0062] Figure 13 These are the sensitivity test results of the flexible pressure sensor based on liquid metal electrodes in Example 2.

[0063] Figure 14Results of the uniaxial tensile experiment of the PVDF-HFP / [EMIM][TFSI] composite ion fiber membrane in Example 3.

[0064] Figure 15 Results of the sensitivity test of the flexible pressure sensor based on a liquid metal electrode in Example 3.

[0065] Explanation of reference numerals:

[0066] 1. Liquid metal electrode layer; 11. Upper electrode layer; 111. Upper electrode encapsulation layer; 112. Upper electrode adhesion layer; 113. Upper liquid metal layer; 12. Lower electrode layer; 121. Lower electrode encapsulation layer; 122. Lower electrode adhesion layer; 123. Lower liquid metal layer; 2. Ion dielectric layer; 21. Dielectric base layer; 22. Surface microstructure. Detailed implementation manners

[0067] In order to make the technical problems, technical solutions and beneficial effects solved by the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0068] Example 1

[0069] A flexible pressure sensor (ionic capacitive pressure sensor) based on a liquid metal electrode and a preparation method thereof provided in this example are as follows:

[0070] As Figure 1 shown, the structure of the flexible pressure sensor based on a liquid metal electrode in this example includes a liquid metal electrode layer 1 and an ion dielectric layer 2, and the ion dielectric layer 2 is disposed inside the liquid metal electrode layer 1.

[0071] Specifically, the liquid metal electrode layer 1 includes an upper electrode layer 11 and a lower electrode layer 12. The upper electrode layer 11 is disposed above the lower electrode layer 12, and the ion dielectric layer 2 is disposed between the upper electrode layer 11 and the lower electrode layer 12.

[0072] Further, the upper electrode layer 11 includes an upper electrode encapsulation layer 111, an upper electrode adhesion layer 112, and an upper liquid metal layer 113 from top to bottom, and the lower electrode layer 12 includes a lower liquid metal layer 123, a lower electrode adhesion layer 122, and a lower electrode encapsulation layer 121 from top to bottom.

[0073] As Figure 1As shown, the ion dielectric layer 2 is disposed between the upper liquid metal layer 113 and the lower liquid metal layer 123. The ion dielectric layer 2 includes a dielectric base layer 21 and surface microstructures 22 disposed on the dielectric base layer 21; specifically, both the upper surface and the lower surface of the dielectric base layer 21 have a plurality of surface microstructures 22, and the surface microstructures 22 are symmetrically arranged. In this embodiment, since the dielectric base layer 21 is prepared from a nanofiber material, by stacking and depositing the nanofiber material, surface microstructures 22 are formed on the dielectric base layer 21. By providing the surface microstructures 22 on the ion dielectric layer 2, the surface roughness of the ion dielectric layer 2 can be increased, and the sensitivity of the pressure sensor can be greatly improved.

[0074] In this embodiment, both the upper electrode encapsulation layer 111 and the lower electrode encapsulation layer 121 are pure TPU nanofiber membranes, which are prepared by an electrospinning process. The specific preparation method is as follows:

[0075] First, 0.9 g of TPU particles are fully dissolved in 2 g of dimethylacetamide (DMAC), and magnetically stirred at a speed of 200 r / min for 3 hours to obtain a TPU precursor solution with a mass fraction of 40 wt.%. The prepared TPU precursor solution is loaded into a syringe, and a pure TPU nanofiber membrane is deposited by an electrospinning process. The electrospinning applied voltage is 12 kV, the liquid supply rate is 300 μL / h, the spinning distance is 120 mm, the spinning temperature is 25°C, and the relative humidity is 40%. The contact angle of the liquid metal with the pure TPU nanofiber membrane is measured to be 157°; as Figure 4 shown by the uniaxial tensile test results, the elastic modulus of the pure TPU nanofiber membrane is 0.7 MPa.

[0076] In this embodiment, both the upper electrode adhesion layer 112 and the lower electrode adhesion layer 122 are GO / TPU composite nanofiber membranes, which are prepared by an electrospinning process. The specific preparation method is as follows:

[0077] First, 0.01 g of graphene oxide powder (GO powder, with a diameter of 0.5 μm to 5 μm, a thickness of 0.8 nm to 1.2 nm, and a purity of 99%, purchased from Sigma-Aldrich) was added to 2 g of DMAC solvent. After being fully dissolved by ultrasonic treatment, 0.9 g of thermoplastic polyurethane particles (TPU particles, 60A hardness, purchased from Dongguan Jubang Plastic Materials Co., Ltd.) was added, and magnetic stirring was carried out at a speed of 200 r / min for 3 hours to form a GO / TPU composite precursor solution; then the prepared GO / TPU composite precursor solution was filled into a syringe, and a composite GO / TPU nanofiber membrane was deposited through an electrospinning process. The electrospinning applied voltage was 10 kV, the liquid supply rate was 200 μL / h, the spinning distance was 100 mm, the spinning temperature was 25 °C, and the relative humidity was 40%. Because the GO powder is rich in -OH functional groups, it can form an adhesion force with liquid metal through hydrogen bonds. The contact angle of the liquid metal on the GO / TPU composite nanofiber membrane was measured to be 27°. As Figure 5 shown by the uniaxial tensile test results, the elastic modulus of the GO / TPU composite nanofiber membrane was 1.1 MPa.

[0078] In this embodiment, both the upper liquid metal layer 113 and the lower liquid metal layer 123 are prepared from liquid metal. The liquid metal is specifically gallium, indium, and tin with a mass ratio of 0.68:0.215:0.1 and a melting point of 11 °C. The upper liquid metal layer 113 and the lower liquid metal layer 123 are prepared by screen printing on the GO / TPU composite nanofiber membrane (the upper electrode adhesion layer 112 and the lower electrode adhesion layer 122) in a patterned manner.

[0079] In this embodiment, the ionic dielectric layer 2 is a polyvinylidene fluoride - hexafluoropropylene (PVDF-HFP) / N-ethyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI], purchased from Sigma-Aldrich) composite ionic nanofiber membrane, which is prepared by an electrospinning process. The specific preparation method is as follows:

[0080] First, PVDF-HFP particles were dissolved in DMAC solvent at a mass ratio of 1:10. After magnetic stirring at a speed of 200 r / min for 8 hours, an ionic liquid [EMIM][TFSI] with the same mass as PVDF-HFP was added, and then magnetic stirring was continued at 200 r / min for 3 hours until complete miscibility to obtain a PVDF-HFP / [EMIM][TFSI] composite precursor solution. Then, the PVDF-HFP / [EMIM][TFSI] composite precursor solution was loaded into a syringe, and a PVDF-HFP / [EMIM][TFSI] composite ionic nanofiber membrane was deposited through an electrospinning process. The applied voltage for electrospinning was 7 kV, the liquid supply rate was 100 μL / h, the spinning distance was 80 mm, the spinning temperature was 25 °C, and the relative humidity was 40%. The contact angle of the liquid metal with the PVDF-HFP / [EMIM][TFSI] composite ionic fiber membrane was 163°. As Figure 6 shown by the uniaxial tensile test results, the elastic modulus of the ionic dielectric layer 2 (PVDF-HFP / [EMIM][TFSI] composite ionic nanofiber membrane) was 77 MPa, which was 100 times that of the pure TPU nanofiber membrane and 70 times that of the GO / TPU composite nanofiber membrane, respectively.

[0081] The preparation method of the flexible pressure sensor (ionic capacitive pressure sensor) based on the liquid metal electrode in this embodiment includes the following steps:

[0082] S1. Preparation of the ionic dielectric layer 2: A PVDF-HFP / [EMIM][TFSI] composite ionic nanofiber membrane was prepared through an electrospinning process to obtain an ionic dielectric layer 2 with a thickness of 20 μm; it was cut into a square of 4 × 4 mm 2 as the ionic dielectric layer 2 for subsequent use.

[0083] S2. Preparation of the lower electrode layer 12: Then, a pure TPU nanofiber membrane was deposited through an electrospinning process for 2 h as the lower electrode encapsulation layer 121; immediately, a GO / TPU composite nanofiber membrane was deposited through an electrospinning process for 1 h on the TPU nanofiber membrane (lower electrode encapsulation layer 121) as the lower electrode adhesion layer 122; then, a square liquid metal pattern of 4 × 4 mm 2 was prepared on the GO / TPU composite nanofiber membrane (lower electrode adhesion layer 122) through a screen printing process as the lower liquid metal layer 123.

[0084] S3. Preparation of the upper electrode layer 11: Place the ionic dielectric layer 2 on the lower liquid metal layer 123, and then deposit a thin GO / TPU composite nanofiber membrane through electrospinning (about 20 minutes of deposition) to facilitate the manipulation of liquid metal (since liquid metal does not adhere to the PVDF-HFP / [EMIM][TFSI] composite ionic nanofiber membrane). Then, prepare a 4 × 4 mm 2 square (overlapping with the ionic dielectric layer 2 in position) of liquid metal pattern as the upper liquid metal layer 113 through screen printing on this thin GO / TPU composite nanofiber membrane; then deposit a GO / TPU composite nanofiber membrane for 1 h through electrospinning as the upper electrode adhesion layer 112; finally, deposit a TPU nanofiber membrane for 2 h through electrospinning as the upper electrode encapsulation layer 111 to complete the preparation of the ionic capacitive pressure sensor.

[0085] This embodiment also provides a pressure sensing method based on a liquid metal electrode. Compared with the traditional ionic flexible capacitive pressure sensor that only relies on the deformation of the dielectric layer for pressure sensing, this embodiment synergistically utilizes the fluidity of the liquid metal electrode and the elastic deformation behavior of the surface microstructure of the ionic dielectric layer 2 to achieve multi-step pressure sensing stages. The specific sensing method is as follows:

[0086] A1. When a small external pressure is applied, due to the low viscosity and good fluidity of the liquid metal, the liquid metal electrode layer 1 will preferentially flow and deform before the microstructure of the ionic dielectric layer 2; as Figure 2 shown, at this time, the upper liquid metal layer 113 and the lower liquid metal layer 123 tend to wrap the surface microstructure 22 on the ionic dielectric layer 2, and the contact area between the two gradually increases under the action of pressure, and the output capacitance signal also gradually increases;

[0087] A2. Since the liquid metal does not adhere to the intermediate ionic dielectric layer 2 but adheres to the upper electrode adhesion layer 112 and the lower electrode adhesion layer 122, when the applied small pressure is unloaded, no liquid metal will remain on the ionic dielectric layer 2. At the same time, the upper liquid metal layer 113 and the lower liquid metal layer 123 will return to their initial morphology under the action of high surface tension, ensuring the stability of the liquid metal electrode structure, and the output capacitance signal also gradually returns to the initial value as the applied pressure decreases;

[0088] A3. When a large external pressure is applied, at this time, the liquid metal has reached the saturated state of flow deformation; as Figure 3 shown, at this time, the surface microstructure 22 on the ionic dielectric layer 2 will deform under the action of pressure, and the contact area between the upper liquid metal layer 113 and the lower liquid metal layer 123 and the intermediate ionic dielectric layer 2 gradually increases under the action of pressure, and the output capacitance signal also gradually increases;

[0089] A4. When the applied large pressure is unloaded, the surface microstructure 22 on the ion dielectric layer 2 returns to its initial state under the action of elastic force, and the upper liquid metal layer 113 and the lower liquid metal layer 123 also return to their initial states under the action of surface tension. The liquid metal adheres to the upper electrode adhesion layer 112 and the lower electrode adhesion layer 122, but does not adhere to the ion dielectric layer 2. This adhesion difference ensures that the ion dielectric layer 2 will not leave liquid metal under pressure load. In addition, the adhesion difference between the liquid metal and the electrode adhesion layer and the encapsulation layer also ensures that there will be no leakage under the action of pressure, guaranteeing the stability of the sensing performance.

[0090] A5. When mechanical deformations such as stretching, bending, and folding are applied, since the elastic modulus of the ion dielectric layer 2 is more than 10 times larger than that of the liquid metal electrode layer 1, this stiffness gradient causes the ion dielectric layer 2 (i.e., the overall ion dielectric layer 2 has a small deformation) to have a small deformation during mechanical deformations such as stretching, bending, and folding. Most of the elastic stress is concentrated on the liquid metal electrode layer with a smaller elastic modulus (the good conductivity of the liquid metal ensures the stability of the deformed electrode). Therefore, the contact area between the upper liquid metal layer 113 and the lower liquid metal layer 123 and the ion dielectric layer 2 changes little, and the output capacitance signal hardly changes, guaranteeing the robustness of the sensing performance (not affected by mechanical deformations such as stretching, bending, and folding).

[0091] As Figure 7 shown is the cross-sectional electron micrograph of the ion-type capacitive pressure sensor prepared in this embodiment, specifically the cross-sectional electron micrograph of the upper liquid metal layer 113, the ion dielectric layer 2, and the lower liquid metal layer 123. It can be seen from the figure that the prepared ion-type capacitive pressure sensor has good stretchability, and the overall structure is a "sandwich" structure, with the ion dielectric layer 2 sandwiched between the upper and lower liquid metal electrodes.

[0092] This embodiment also conducts a simulation analysis on the pressure sensing method of the prepared ion-type capacitive pressure sensor. Among them, when simulating, the ion dielectric layer 2 is replaced by a porous model and has protruding microstructures on the surface. In the first stage of the simulation analysis, when a small pressure is applied, at this time, the liquid metal electrodes (the upper liquid metal layer 113 and the lower liquid metal layer 123) undergo flow deformation and wrap the surface microstructure 22 on the ion dielectric layer 2, and the ion dielectric layer 2 does not deform. In the second stage of the simulation analysis, when a large pressure is applied, at this time, the flow deformation of the liquid metal electrodes reaches saturation, and the ion dielectric layer 2 deforms under the action of pressure.

[0093] As Figure 8The sensitivity curve of the ionic capacitive pressure sensor prepared in this embodiment measured is shown. The curve includes two stages, corresponding to the first stage and the second stage in the simulation analysis respectively. The first stage is the capacitance signal response generated by the flow deformation of the liquid metal (the upper liquid metal layer 113 and the lower liquid metal layer 123), and the sensitivity in this stage is 2 kPa -1 ; the second stage is the capacitance signal response generated by the elastic deformation of the ion dielectric layer 2, and the sensitivity in this stage is 1.2 kPa -1 .

[0094] Both the simulation analysis and the experimental results show that the pressure sensing method proposed by the present invention that synergistically utilizes the fluidity of the liquid metal electrode and the elastic deformation behavior of the surface microstructure of the ion dielectric layer can achieve multi-step pressure sensing capabilities.

[0095] As Figure 9 shown, the simulation analysis results of the deformation behavior of the ionic capacitive pressure sensor prepared in this embodiment during the stretching process are shown. In order to simplify the model and save computing resources, the substrate (the upper electrode adhesion layer 112 and the lower electrode adhesion layer 122) is set as a block structure with a low modulus (1 Mpa) during the simulation, and the middle ion dielectric layer 2 is set as a block structure with a high modulus (70 Mpa), and a 30% tensile deformation is applied to observe the tensile stress distribution of the ionic capacitive pressure sensor prepared in this embodiment. It can be seen from the figure that most of the tensile stress is concentrated on the substrate with a low modulus, and the high modulus ion dielectric layer 2 hardly deforms; at this time, the contact area between the liquid metal electrodes (the upper liquid metal layer 113 and the lower liquid metal layer 123) and the surface microstructure 22 on the ion dielectric layer 2 does not change, so the capacitance output value hardly changes, proving that the tensile deformation has little influence on the ionic capacitive pressure sensor prepared in the embodiment.

[0096] As Figure 10The following shows a tensile test experiment on the ionic capacitive pressure sensor prepared in this embodiment. During the test, the ionic capacitive pressure sensor was pressed twice with a finger in the initial state, and the capacitance output value was recorded at the same time. Then, a 100% tensile deformation was applied to the ionic capacitive pressure sensor, and the ionic capacitive pressure sensor was pressed twice with a finger in the tensile state, and the capacitance output value was recorded at the same time. Finally, the tensile load applied to the ionic capacitive pressure sensor was released, and the ionic capacitive pressure sensor was pressed twice with a finger in the recovery state, and the capacitance output value was recorded at the same time. As can be seen from the figure, in the initial state, after the finger applied pressure, the ionic capacitive pressure sensor prepared in this embodiment increased significantly from 2 pF to 600 pF; after the ionic capacitive pressure sensor was stretched by 100%, the initial capacitance value decreased from 2 pF to 1.6 pF, and after the finger applied pressure, the capacitance of the ionic capacitive pressure sensor prepared in this embodiment increased significantly from 2 pF to 600 pF in the tensile state; after the tensile load was released, the initial value of the ionic capacitive pressure sensor prepared in this embodiment recovered to 2 pF, and after the finger applied pressure, the capacitance of the ionic capacitive pressure sensor prepared in this embodiment increased significantly from 2 pF to 600 pF in the recovery state. The experiment proves that the ionic capacitive pressure sensor prepared in this embodiment is hardly affected by 100% tensile deformation and has good robustness.

[0097] Finally, the application of this embodiment in the field of soft manipulators was explored. The prepared ionic capacitive pressure sensor was adhered to the surface of a pneumatic soft manipulator. When adhering, liquid silicone was coated on the adhesion interface, and the integration of the sensor was completed after curing. Since the ionic capacitive pressure sensor prepared in this embodiment has good flexibility, the adhered sensor did not show delamination and wrinkling even when the manipulator was bent.

[0098] As Figure 11 shown, the manipulator was driven to grasp an empty water bottle. After the grasping was stable, water was poured into the bottle. Finally, the grasping air pressure was reduced, and the capacitance output value of the ionic capacitive pressure sensor adhered to the surface of the manipulator was recorded in real time. As can be seen from the figure, when the driving air pressure of the manipulator increased, the capacitance output value of the ionic capacitive pressure sensor prepared in Example 1 increased; when water was poured into the bottle, the capacitance output value of the ionic capacitive pressure sensor showed jitter, which was caused by the oscillation of the water poured into the bottle; when the driving air pressure of the manipulator decreased, the capacitance output value of the ionic capacitive pressure sensor prepared in Example 1 also decreased. The experimental results prove that the ionic capacitive pressure sensor prepared in this embodiment has great application potential in the field of intelligent soft robots.

[0099] Example 2

[0100] This embodiment provides a flexible pressure sensor (ionic capacitive pressure sensor) based on liquid metal electrodes and a preparation method thereof, which differs from Example 1 in that the preparation processes of the upper electrode packaging layer 111 and the lower electrode packaging layer 121 are different.

[0101] In this embodiment, the upper electrode packaging layer 111 and the lower electrode packaging layer 121 are both pure TPU films, which are prepared by a mold inversion process. The specific preparation method is as follows:

[0102] The TPU precursor solution prepared in Example 1 was placed in the corresponding mold and cured to obtain a pure TPU film. The contact angle of the liquid metal on the pure TPU film was measured to be 153°; Figure 12 The results of the uniaxial tensile test show that the elastic modulus of the pure TPU film is 2 MPa, which is 35 times smaller than that of the ionic dielectric layer 2.

[0103] The preparation method of the flexible pressure sensor (ionic capacitive pressure sensor) proposed in this embodiment is the same as that in Example 1.

[0104] like Figure 13 The figure shows the measured sensitivity curve of the ionic capacitive pressure sensor prepared in this example. The curve consists of two parts, corresponding to the first and second stages of the pressure sensing method in Example 1. The first stage is the capacitance signal response generated by the flow deformation of the liquid metal, and the sensitivity of this stage is 1.5 kPa. -1 The second stage is the capacitance signal response generated by the elastic deformation of the surface microstructure 22 on the ionic dielectric layer 2. The sensitivity of this stage is 0.82 kPa. -1 .

[0105] Example 3

[0106] This embodiment provides a flexible pressure sensor based on liquid metal electrodes, a preparation method thereof, and applications thereof.

[0107] The difference from Example 1 is that the surface microstructure 22 on the ionic dielectric layer 2 is an inverted pyramid structure, and the surface microstructure 22 is obtained by mold casting, as follows:

[0108] In this embodiment, the ionic dielectric layer 2 is a PVDF-HFP / [EMIM][TFSI] composite ionic membrane with a surface microstructure of an inverted pyramid structure, which is prepared by a mold casting process. The specific method includes: pouring the PVDF-HFP / [EMIM][TFSI] composite precursor solution prepared in Example 1 into a corresponding mold, and after curing, a PVDF-HFP / [EMIM][TFSI] composite ionic membrane with a microstructure on the surface can be obtained. The preparation process of the flexible pressure sensor proposed in this embodiment is the same as that in Example 1, except that the surface microstructure 22 on the ionic dielectric layer 2 is different. As Figure 14 shown by the uniaxial tensile test results, the elastic modulus of the PVDF-HFP / [EMIM][TFSI] composite ionic nanofiber membrane is 92 MPa, and the elastic modulus is 90 times larger than that of the substrate (the upper electrode adhesion layer 112 and the lower electrode adhesion layer 122).

[0109] As Figure 15 shown is the sensitivity curve of the ionic capacitive pressure sensor prepared in this embodiment. The curve includes two stages, corresponding to the first stage and the second stage in the pressure sensing method in Example 1 respectively. The first stage is the capacitance signal response generated by the flow deformation of the liquid metal, and the sensitivity in this stage is 1.3 kPa -1 ; the second stage is the capacitance signal response generated by the elastic deformation of the surface microstructure 22 on the ionic dielectric layer 2, and the sensitivity in this stage is 0.66 kPa -1 . It can be seen that the performance such as the sensitivity and stability of the ionic capacitive pressure sensor proposed in the present invention can be regulated by the geometric configurations of the liquid metal, the flexible substrate (the upper electrode adhesion layer 112 and the lower electrode adhesion layer 122), and the surface microstructure 22 on the ionic dielectric layer 2.

[0110] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, makes equivalent substitutions or changes, and all should be covered within the protection scope of the present invention.

Claims

1. A flexible pressure sensor based on a liquid metal electrode, characterized in that, Comprising: A liquid metal electrode layer, which includes an upper electrode layer and a lower electrode layer disposed below the upper electrode layer; The upper electrode layer sequentially includes an upper electrode encapsulation layer, an upper electrode adhesion layer, and an upper liquid metal layer from top to bottom; the lower electrode layer sequentially includes a lower liquid metal layer, a lower electrode adhesion layer, and a lower electrode encapsulation layer from top to bottom; An ion dielectric layer, which is disposed between the upper liquid metal layer and the lower liquid metal layer. The ion dielectric layer includes a dielectric base layer, and both the upper surface and the lower surface of the dielectric base layer have a plurality of surface microstructures, and the surface microstructures are symmetrically arranged; The contact angle of the upper liquid metal layer with the upper electrode adhesion layer is respectively smaller than the contact angle of the upper liquid metal layer with the ion dielectric layer and the contact angle of the upper liquid metal layer with the upper electrode encapsulation layer; The contact angle of the lower liquid metal layer with the lower electrode adhesion layer is respectively smaller than the contact angle of the lower liquid metal layer with the ion dielectric layer and the contact angle of the lower liquid metal layer with the lower electrode encapsulation layer; The elastic modulus of the ion dielectric layer is respectively greater than the elastic modulus of the upper electrode encapsulation layer, the upper electrode adhesion layer, the lower electrode encapsulation layer, and the lower electrode adhesion layer; Both the upper electrode encapsulation layer and the lower electrode encapsulation layer are flexible stretchable films, both the upper electrode adhesion layer and the lower electrode adhesion layer are flexible composite films, and the ion dielectric layer is a composite fiber film containing ions.

2. The flexible pressure sensor according to claim 1, wherein The contact angle of the upper liquid metal layer with the upper electrode adhesion layer and the contact angle of the lower liquid metal layer with the lower electrode adhesion layer are both less than 90°; The contact angle of the upper liquid metal layer with the upper electrode encapsulation layer and the contact angle of the lower liquid metal layer with the lower electrode encapsulation layer are both greater than 120°; The contact angle of the upper liquid metal layer and the lower liquid metal layer with the ion dielectric layer are both greater than 120°; The elastic modulus of the ion dielectric layer is 10 times or more of the elastic modulus of the upper electrode encapsulation layer, the upper electrode adhesion layer, the lower electrode encapsulation layer, and the lower electrode adhesion layer respectively.

3. The flexible pressure sensor according to claim 1, wherein The contact angle of the upper liquid metal layer with the upper electrode adhesion layer and the contact angle of the lower liquid metal layer with the lower electrode adhesion layer are both 10° - 40°; The contact angle of the upper liquid metal layer with the upper electrode encapsulation layer and the contact angle of the lower liquid metal layer with the lower electrode encapsulation layer are both 140° - 180°; The contact angle of the upper liquid metal layer and the lower liquid metal layer with the ion dielectric layer are both 140° - 180°; The elastic modulus of both the upper electrode encapsulation layer and the lower electrode encapsulation layer is 0.5 MPa - 2 MPa; The elastic modulus of both the upper electrode adhesion layer and the lower electrode adhesion layer is 0.5 MPa - 2 MPa; The elastic modulus of the ion dielectric layer is 50 MPa - 100 MPa.

4. The flexible pressure sensor according to claim 1, wherein The raw material for preparing the flexible stretchable film is at least one of TPU, Ecoflex silicone polymer, and PDMS; The raw material for preparing the flexible composite film is TPU doped with graphene oxide or Ecoflex silicone polymer doped with graphene oxide; The raw materials for preparing the upper liquid metal layer and the lower liquid metal layer are both gallium, indium and tin. Among them, the mass ratio of gallium, indium and tin is 0.6~0.7:0.2~0.3:0.08~0.

12.

5. The flexible pressure sensor according to claim 4, characterized in that, The preparation methods of the upper electrode encapsulation layer and the lower electrode encapsulation layer include the following steps: Fully dissolve TPU particles in dimethylacetamide, stir to obtain a 30 wt.%~50 wt.% TPU precursor solution; load the TPU precursor solution into a syringe and deposit it by electrospinning process to obtain a TPU nanofiber membrane, which is the upper electrode encapsulation layer and the lower electrode encapsulation layer; Among them, the electrospinning applied voltage is 10 kV~14 kV, the feeding rate is 250 μL / h~350 μL / h, the spinning distance is 100 mm~140 mm, the spinning temperature is 20℃~30℃, and the relative humidity is 30 %~50 %.

6. The flexible pressure sensor according to claim 4, wherein, The preparation methods of the upper electrode adhesion layer and the lower electrode adhesion layer include the following steps: Add GO powder into DMAC solvent, after fully dissolving, add TPU particles and stir to form a GO / TPU composite precursor solution; load the GO / TPU composite precursor solution into a syringe and deposit it by electrospinning process to obtain a composite GO / TPU nanofiber membrane, which is the upper electrode adhesion layer and the lower electrode adhesion layer; Among them, the mass ratio of GO powder, DMAC solvent and TPU particles is 0.01:1.5~2.5:0.8~1; The electrospinning applied voltage is 8 kV~12 kV, the feeding rate is 180 μL / h~220 μL / h, the spinning distance is 80 mm~120 mm, the spinning temperature is 20℃~30℃, and the relative humidity is 30 %~50 %.

7. The flexible pressure sensor according to claim 6, wherein The preparation methods of the upper liquid metal layer and the lower liquid metal layer include the following steps: Mix gallium, indium and tin to obtain a liquid metal alloy; print the liquid metal alloy on the GO / TPU composite nanofiber membrane by screen printing to obtain the upper liquid metal layer and the lower liquid metal layer.

8. The flexible pressure sensor according to claim 4, wherein The preparation method of the ionic dielectric layer includes the following steps: Dissolve PVDF-HFP particles in DMAC solvent and mix evenly, add [EMIM][TFSI] ionic liquid and stir to obtain a PVDF-HFP / [EMIM][TFSI] composite precursor solution; load the PVDF-HFP / [EMIM][TFSI] composite precursor solution into a syringe and deposit it by electrospinning process to obtain a PVDF-HFP / [EMIM][TFSI] composite ionic nanofiber membrane, which is the ionic dielectric layer; Among them, the mass ratio of PVDF-HFP particles, DMAC solvent and [EMIM][TFSI] ionic liquid is 0.5~1.5:8~12:0.5~1.5; The electrospinning applied voltage is 6 kV~8 kV, the feeding rate is 90 μL / h~110 μL / h, the spinning distance is 70 mm~90 mm, the spinning temperature is 20℃~30℃, and the relative humidity is 30 %~50 %.

9. A method for preparing a flexible pressure sensor according to any one of claims 1 to 8, characterized in that, Include the following steps: Prepare an ion-containing composite fiber membrane through an electrospinning process to obtain an ion dielectric layer; Deposit a TPU nanofiber membrane through an electrospinning process to obtain a lower electrode encapsulation layer; deposit a GO / TPU composite nanofiber membrane on the lower electrode encapsulation layer through an electrospinning process to obtain a lower electrode adhesion layer; then prepare a liquid metal pattern on the lower electrode adhesion layer through a screen printing process to obtain a lower liquid metal layer; Place the ion dielectric layer on the lower liquid metal layer; Deposit a GO / TPU composite nanofiber membrane through an electrospinning process, and prepare a liquid metal pattern on the GO / TPU composite nanofiber membrane through a screen printing process to obtain an upper liquid metal layer; Continue to deposit a GO / TPU composite nanofiber membrane through an electrospinning process to obtain an upper electrode adhesion layer; finally, deposit a TPU nanofiber membrane through an electrospinning process to obtain an upper electrode encapsulation layer.

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