Flexible piezoresistive sensor and preparation method thereof

Abstract

The invention provides a flexible piezoresistive sensor and a preparation method thereof, and relates to the technical field of flexible pressure sensor preparation. The preparation method of the flexible piezoresistive sensor comprises the following steps: S1, preparing an AgNWs dispersion liquid and a PVDF packaging solution, and pretreating a PVDF substrate; s2, the surface of the pretreated PVDF substrate is coated with conductive silver paste, AgNWs dispersion liquid is dropwise added after solidification, and an AgNWs / PVDF composite film is prepared; s3, a PVDF packaging solution is dropwise added to the surface of the AgNWs / PVDF composite film, after curing, hot water soaking and vacuum drying are conducted, and the flexible piezoresistive sensor is obtained. According to the flexible piezoresistive sensor, the porosity is as high as 78%, the aperture is 410 nm, the flexible piezoresistive sensor has excellent air and moisture permeability, the stability and durability of a device are enhanced, and the flexible piezoresistive sensor is suitable for high-performance intelligent wearable equipment.

Description

Technical Field

[0001] The present invention relates to the technical field of flexible pressure sensor preparation, and particularly relates to a flexible piezoresistive sensor and a preparation method thereof. Background Art

[0002] As a special type of flexible pressure sensor, the flexible piezoresistive sensor exhibits significant application potential and unique advantages in the fields of medical health monitoring (such as pulse waveform analysis and respiratory pattern recognition) and intelligent wearable devices due to its excellent mechanical flexibility, high sensitivity, wide pressure detection range, and simple preparation process. However, to achieve the long-term physiological monitoring application of flexible piezoresistive sensors on the human skin surface, two key technical bottlenecks still need to be overcome: wearing comfort and device stability.

[0003] Currently, most researchers are committed to solving the problems of wearing comfort and device stability by optimizing the flexible substrate and the internal microstructure of the device. Regarding the problem of wearing comfort, researchers usually use electrospinning technology to prepare porous substrates to improve breathability and moisture permeability. Although this method improves the comfort of the wearer, the resulting sensors often lack sufficient stability. On the contrary, to enhance the stability and durability of the sensor, researchers have tried to use polymer films as encapsulation materials to encapsulate the sensor, thereby improving the mechanical properties and durability of the sensor. However, the dense polymer film materials also sacrifice breathability and moisture permeability, which may lead to adverse consequences such as skin inflammation or allergy after long-term wearing.

[0004] In view of the fact that existing solutions often can only meet the requirements of wearing comfort or device stability singly, therefore, there is an urgent need to develop a flexible piezoresistive sensor that can ensure both breathability and moisture permeability and provide sufficient stability to meet the needs of practical applications. Summary of the Invention

[0005] In view of this, the present invention proposes a flexible piezoresistive sensor and a preparation method thereof, aiming to simultaneously achieve excellent wearing comfort and device stability to meet the needs of long-term intelligent wearing.

[0006] In the first aspect, the present invention provides a preparation method of a flexible piezoresistive sensor, comprising the following steps:

[0007] S1. Prepare an AgNWs dispersion liquid, a PVDF encapsulation solution, and pretreat the PVDF substrate;

[0008] S2. Coat conductive silver paste on the surface of the pretreated PVDF substrate, and after curing, drop the AgNWs dispersion liquid to obtain an AgNWs / PVDF composite film;

[0009] S3. Drop PVDF encapsulation solution on the surface of the AgNWs / PVDF composite film. After curing, soak it in hot water and then dry it under vacuum to obtain a flexible piezoresistive sensor.

[0010] In one or some possible embodiments, in step S1, the AgNWs dispersion is prepared by dispersing AgNWs in absolute ethanol; the concentration of the AgNWs dispersion is 1.3 mg / L to 8.24 mg / L.

[0011] Furthermore, the mass-volume ratio of the AgNWs to the absolute ethanol is (2.02 - 8.24) mg : (1 - 1.5) L.

[0012] In one or some possible embodiments, in step S1, the PVDF encapsulation solution is prepared by dissolving PVDF powder and PVP powder (polyvinylpyrrolidone) in an organic solvent; the concentration of the PVDF encapsulation solution is 9.6 - 10.6 wt%.

[0013] Furthermore, the organic solvent is selected from one of DMF (dimethylformamide), DMSO (dimethyl sulfoxide) or NMP (N-methyl-2-pyrrolidone); the mass ratio of the PVDF powder, the PVP powder and the organic solvent is (4.8 - 5.3) : (0.24 - 0.26) : (44.55 - 45).

[0014] In one or some possible embodiments, in step S1, the diameter of the PVDF substrate can be selected according to actual needs. In the present invention, based on the principle of being easy to carry and wear, the diameter of the substrate is preferably 40 - 50 mm.

[0015] In one or some possible embodiments, in step S2, the amount of the conductive silver paste varies with the diameter of the PVDF substrate.

[0016] In one or some possible embodiments, in step S2, the areal density of the AgNWs on the AgNWs / PVDF composite film is 0.0272 - 0.2261 mg / cm 2 .

[0017] In one or some possible embodiments, in step S2, the areal density of the AgNWs on the AgNWs / PVDF composite film is 0.0452 - 0.1804 mg / cm 2 .

[0018] Further, the dosage of the AgNWs dispersion is 2-10 mL. Specifically, the dosage of the AgNWs dispersion in the present invention can be 2 mL, 4 mL, 6 mL, 8 mL or 10 mL, and any value between two adjacent numbers. The present invention preferably uses the AgNWs dispersion in an amount of 2 mL, 4 mL, 6 mL, 8 mL or 10 mL. At this dosage, the corresponding areal densities are 0.0272 mg / cm 2 , 0.0452 mg / cm 2 , 0.0904 mg / cm 2 , 0.1804 mg / cm 2 , 0.2261 mg / cm 2 .

[0019] More preferably, the dosage of the conductive silver paste is inversely proportional to the areal density. That is, in the present invention, when the areal density is 0.0272 mg / cm 2 -0.2261 mg / cm 2 , the dosage of the conductive silver paste is 10-2 mg. That is, in the present invention, when the areal density is 0.0272 mg / cm 2 , the dosage of the conductive silver paste is 10 mg; when the areal density is 0.0452 mg / cm 2 , the dosage of the conductive silver paste is 8 mg; when the areal density is 0.2261 mg / cm 2 , the dosage of the conductive silver paste is 2 mg, and so on.

[0020] In one or some possible embodiments, in step S3, the dosage of the PVDF encapsulation solution is proportional to the size of the PVDF substrate. That is, in the present invention, when the size of the PVDF substrate is 30 mm, the dosage of the PVDF encapsulation solution is 3 mL; when the size of the PVDF substrate is 40 mm, the dosage of the PVDF encapsulation solution is 4 mL; when the size of the PVDF substrate is 50 mm, the dosage of the PVDF encapsulation solution is 5 mL, and so on.

[0021] In one or some possible embodiments, in step S3, the soaking temperature is 50-65 °C, and the soaking time is 10-20 min.

[0022] By adopting the above technical solution, if the soaking time is too long, it will affect the stability and service life of the device; if the time is too short, the pore size is not large enough to ensure good air permeability and moisture permeability, thereby affecting the comfort during wearing and the performance during long-term wearing.

[0023] In one or some possible embodiments, in step S3, the drying temperature is 50-65 °C, and the drying time is 12-14 h.

[0024] In a second aspect, the present invention relates to a flexible piezoresistive sensor prepared by the above preparation method. The sensor simultaneously achieves the wearing comfort and stability of the device by optimizing the encapsulation process and the areal density of AgNWs (silver nanowires). Specifically, the present invention uses an organic solution of PVDF-PVP as the encapsulation layer material. After the solution is cured on the surface of the electrode layer, it is then treated with water immersion to form a porous structure. During this process, PVP and a small amount of organic solvents in the cured solution will dissolve in water, thereby generating the required porous encapsulation layer. This encapsulation process, combined with the PVDF substrate prepared by electrospinning, together constitutes a permeable structure that is conducive to air and moisture permeability.

[0025] The flexible piezoresistive sensor and its preparation method provided by the present invention have the following beneficial effects compared with the prior art:

[0026] (1) In the preparation method of the flexible piezoresistive sensor of the present invention, the size of the pore diameter and the porosity are precisely controlled by optimizing the soaking time and the areal density of AgNWs, achieving a porosity of up to 78% and a pore diameter of 410 nm.

[0027] (2) In the preparation method of the flexible piezoresistive sensor of the present invention, by optimizing the encapsulation process, not only the influence of long-term use of the sensor on the conductive material is reduced, but also the conductive material is firmly encapsulated inside the sensor to prevent it from falling off, thereby providing higher stability than the existing technologies with air and moisture permeability, and increasing the cycle life by 6 times (MIYAMOTO A, LEE S, COORAY N F, et al. Inflammation-free, gas-permeable, lightweight, stretchable on-skin electronics with nanomeshes[J]. Nat Nanotechnol, 2017, 12(9): 907-13.). In addition, this preparation method also enables the encapsulation layer of the sensor to form a porous structure, which endows the sensor with excellent air permeability and moisture permeability while ensuring the stability of the device, enabling it not to cause skin inflammation when worn continuously for 12 hours, effectively solving the problem of the lack of air permeability and moisture permeability in the stable device structure in the prior art, and successfully preparing a high-performance and comfortable wearable device.

[0028] (3) The flexible piezoresistive sensor prepared by the present invention ensures that it will not cause skin inflammation after 12 hours of long-term wear. At the same time, due to the firm encapsulation layer, the stability of the device is enhanced, enabling it to have a durability of up to 3000 cycles. Moreover, in the actual wear test, the sensor shows an obvious resistance change rate, indicating that while maintaining high stability, it can effectively respond to external pressure changes, reflecting its potential application value in the field of smart wearable devices. Description of the Drawings

[0029] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0030] Figure 1 XRD test diagrams of the flexible piezoresistive sensors prepared in Embodiments 1 - 6 of the present invention;

[0031] Figure 2 Scanning electron microscope diagrams of the flexible piezoresistive sensors prepared in Embodiments 2 - 3 and Comparative Examples 13 - 15 of the present invention;

[0032] Figure 3 Scanning electron microscope diagrams of the flexible piezoresistive sensors prepared in Embodiments 1, 2, 4 and Comparative Examples 3, 4, 18 of the present invention;

[0033] Figure 4 Variation diagrams of the wearable time of the flexible piezoresistive sensors prepared by the present invention at different AgNWs areal densities;

[0034] Figure 5 Variation diagrams of the wearable time of the flexible piezoresistive sensors prepared by the present invention at different soaking times;

[0035] Figure 6 Variation diagrams of the resistance with time when the flexible piezoresistive sensors prepared by the present invention are subjected to wear tests;

[0036] Figure 7 Variation diagrams of the resistance change rate with time when the flexible piezoresistive sensors prepared by the present invention are subjected to wear tests;

[0037] Figure 8 Variation diagrams of the resistance change rate with the cycle time when the flexible piezoresistive sensor prepared in Embodiment 5 of the present invention is subjected to cycle tests. Detailed Embodiments

[0038] Next, in combination with the embodiments of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0039] The present invention will be further described below in combination with specific embodiments. The protection scope of the present invention is not limited by the following embodiments. Unless otherwise specified for the main materials involved in the embodiments, the remaining materials not shown are all conventional commercially available products.

[0040] In the following examples, the silver nanowires (AgNWs) are prepared by the following steps:

[0041] Preheat 30 mL of EG (ethylene glycol) solution in an oil bath at 65 °C. Under stirring conditions, dissolve 0.24 g of PVP powder in the EG solution; after the above solution is cooled to room temperature, add 0.5 g of AgNO3 and continue to stir until it dissolves. Subsequently, take 7 mL of FeCL3 / EG solution (0.6 mM) and slowly drop it into the completely dissolved AgNO3 / EG solution. After stirring the above mixed solution at room temperature for 3 min, carry out condensation reflux at 130 °C for 5 h until the mixed solution has a metallic luster and presents a silver-gray color. Let it stand and cool to room temperature, and then centrifuge at 7500 r / min for 5 min. Wash the bottom AgNWs with acetone and EtOH (anhydrous ethanol) solutions multiple times to remove the unreacted EG, and obtain silver nanowires (AgNWs).

[0042] Example 1

[0043] This example provides a preparation method of a flexible piezoresistive sensor, including the following steps:

[0044] S1. Take 2.975 mg of AgNWs and add it to 1 L of EtOH, stir for 1 h to obtain an AgNWs dispersion solution for standby;

[0045] Clean a PVDF filter membrane with a size of 50 mm (purchased from Longjin Membrane Industry Co., Ltd.) with EtOH for standby;

[0046] Dissolve 5 g of PVDF powder and 0.25 g of PVP powder in 44.75 g of N,N-dimethylformamide (DMF) to obtain a PVDF encapsulation solution for standby.

[0047] S2. Coat 6 mg of conductive silver paste on the surface of the PVDF filter membrane, let it stand for 6 h, after curing, drop 6 mL of the AgNWs dispersion solution on its surface and cure to obtain an AgNWs / PVDF composite membrane.

[0048] S3. Drop 5 mL of the PVDF encapsulation solution onto the surface of the AgNWs / PVDF composite membrane. After curing, soak the whole in hot water at 60 °C for 10 min, and then vacuum dry it at 60 °C for 12 h to obtain the flexible piezoresistive sensor.

[0049] Example 2

[0050] This example provides a method for preparing a flexible piezoresistive sensor, which includes the following steps:

[0051] S1. Take 2.975 mg of AgNWs and add them to 1 L of EtOH, stir for 1 h to obtain a dispersed AgNWs solution for standby.

[0052] Clean a PVDF filter membrane with a size of 50 mm (purchased from Longjin Membrane Industry Co., Ltd.) using EtOH for standby.

[0053] Dissolve 5 g of PVDF powder and 0.25 g of PVP powder in 44.75 g of N,N-dimethylformamide (DMF) to obtain a PVDF encapsulation solution for standby.

[0054] S2. Coat 6 mg of conductive silver paste on the surface of the PVDF filter membrane, let it stand for 6 h, and after curing, drop 6 mL of the dispersed AgNWs solution on its surface and cure it to obtain the AgNWs / PVDF composite membrane.

[0055] S3. Drop 5 mL of the PVDF encapsulation solution onto the surface of the AgNWs / PVDF composite membrane. After curing, soak the whole in hot water at 60 °C for 15 min, and then vacuum dry it at 60 °C for 12 h to obtain the flexible piezoresistive sensor.

[0056] Example 3

[0057] This example provides a method for preparing a flexible piezoresistive sensor, which includes the following steps:

[0058] S1. Take 4.453 mg of AgNWs and add them to 1 L of EtOH, stir for 1 h to obtain a dispersed AgNWs solution for standby.

[0059] Clean a PVDF filter membrane with a size of 50 mm (purchased from Longjin Membrane Industry Co., Ltd.) using EtOH for standby.

[0060] Dissolve 5 g of PVDF powder and 0.25 g of PVP powder in 44.75 g of N,N-dimethylformamide (DMF) to obtain a PVDF encapsulation solution for standby.

[0061] S2. Coat 4 mg of conductive silver paste on the surface of the PVDF membrane, let it stand for 6 h. After curing, drop 8 mL of AgNWs dispersion liquid on its surface, and then cure it to obtain the AgNWs / PVDF composite membrane.

[0062] S3. Drop 5 mL of PVDF encapsulation solution on the surface of the AgNWs / PVDF composite membrane. After curing, soak the whole in hot water at 60 °C for 15 min, and then vacuum dry it at 60 °C for 12 h to obtain the flexible piezoresistive sensor.

[0063] Example 4

[0064] This example provides a preparation method of a flexible piezoresistive sensor, including the following steps:

[0065] S1. Take 2.231 mg of AgNWs and add it to 1 L of EtOH, stir for 1 h to obtain the AgNWs dispersion liquid for standby;

[0066] Clean the PVDF membrane with a size of 50 mm (purchased from Longjin Membrane Industry Company) using EtOH for standby;

[0067] Dissolve 5 g of PVDF powder and 0.25 g of PVP powder in 44.75 g of N,N-dimethylformamide (DMF) to obtain the PVDF encapsulation solution for standby.

[0068] S2. Coat 8 mg of conductive silver paste on the surface of the PVDF membrane, let it stand for 6 h. After curing, drop 4 mL of AgNWs dispersion liquid on its surface, and then cure it to obtain the AgNWs / PVDF composite membrane.

[0069] S3. Drop 5 mL of PVDF encapsulation solution on the surface of the AgNWs / PVDF composite membrane. After curing, soak the whole in hot water at 60 °C for 20 min, and then vacuum dry it at 60 °C for 12 h to obtain the flexible piezoresistive sensor.

[0070] Example 5

[0071] This example provides a preparation method of a flexible piezoresistive sensor, including the following steps:

[0072] S1. Take 2.975 mg of AgNWs and add it to 1 L of EtOH, stir for 1 h to obtain the AgNWs dispersion liquid for standby;

[0073] Clean the PVDF membrane with a size of 50 mm (purchased from Longjin Membrane Industry Company) using EtOH for standby;

[0074] Dissolve 5 g of PVDF powder and 0.25 g of PVP powder in 44.75 g of N,N-dimethylformamide (DMF) to prepare a PVDF encapsulation solution for standby.

[0075] S2. Coat 6 mg of conductive silver paste on the surface of the PVDF membrane, let it stand for 6 h. After curing, drop 6 mL of AgNWs dispersion liquid on its surface and cure it to obtain an AgNWs / PVDF composite membrane.

[0076] S3. Drop 5 mL of the PVDF encapsulation solution on the surface of the AgNWs / PVDF composite membrane. After curing, soak the whole in hot water at 60 °C for 20 min, and then vacuum dry it at 60 °C for 12 h to obtain a flexible piezoresistive sensor.

[0077] Example 6

[0078] This example provides a preparation method of a flexible piezoresistive sensor, including the following steps:

[0079] S1. Take 4.453 mg of AgNWs and add them to 1 L of EtOH, stir for 1 h to prepare an AgNWs dispersion liquid for standby;

[0080] Clean a PVDF membrane (purchased from Longjin Membrane Industry Company) with a size of 50 mm using EtOH for standby;

[0081] Dissolve 5 g of PVDF powder and 0.25 g of PVP powder in 44.75 g of N,N-dimethylformamide (DMF) to prepare a PVDF encapsulation solution for standby.

[0082] S2. Coat 4 mg of conductive silver paste on the surface of the PVDF membrane, let it stand for 6 h. After curing, drop 8 mL of AgNWs dispersion liquid on its surface and cure it to obtain an AgNWs / PVDF composite membrane.

[0083] S3. Drop 5 mL of the PVDF encapsulation solution on the surface of the AgNWs / PVDF composite membrane. After curing, soak the whole in hot water at 60 °C for 10 min, and then vacuum dry it at 60 °C for 12 h to obtain a flexible piezoresistive sensor.

[0084] Example 7

[0085] This example provides a preparation method of a flexible piezoresistive sensor, including the following steps:

[0086] S1. Take 2.02 mg of AgNWs and add them to 1 L of EtOH, stir for 1 h to prepare an AgNWs dispersion liquid for standby;

[0087] Wash the PVDF membrane with a size of 40 mm (purchased from Longjin Membrane Industry Co., Ltd.) using EtOH and set aside for later use;

[0088] Dissolve 4.8 g of PVDF powder and 0.24 g of PVP powder in 44.55 g of N,N-dimethylformamide (DMF) to prepare a PVDF encapsulation solution and set aside for later use.

[0089] S2. Coat 8 mg of conductive silver paste on the surface of the PVDF membrane, let it stand for 6 h, after curing, drop 2.82 mL of AgNWs dispersion liquid on its surface, and cure to obtain an AgNWs / PVDF composite membrane.

[0090] S3. Drop 4 mL of the PVDF encapsulation solution on the surface of the AgNWs / PVDF composite membrane, after curing, soak the whole in hot water at 50 °C for 20 min, and then vacuum dry at 50 °C for 14 h to obtain a flexible piezoresistive sensor.

[0091] Example 8

[0092] This example provides a preparation method of a flexible piezoresistive sensor, including the following steps:

[0093] S1. Take 8.24 mg of AgNWs and add it to 1.5 L of EtOH, stir for 1 h to prepare an AgNWs dispersion liquid and set aside for later use;

[0094] Wash the PVDF membrane with a size of 40 mm (purchased from Longjin Membrane Industry Co., Ltd.) using EtOH and set aside for later use;

[0095] Dissolve 5.3 g of PVDF powder and 0.26 g of PVP powder in 45 g of N,N-dimethylformamide (DMF) to prepare a PVDF encapsulation solution and set aside for later use.

[0096] S2. Coat 6 mg of conductive silver paste on the surface of the PVDF membrane, let it stand for 6 h, after curing, drop 4.15 mL of AgNWs dispersion liquid on its surface, and cure to obtain an AgNWs / PVDF composite membrane.

[0097] S3. Drop 4 mL of the PVDF encapsulation solution on the surface of the AgNWs / PVDF composite membrane, after curing, soak the whole in hot water at 65 °C for 15 min, and then vacuum dry at 65 °C for 12 h to obtain a flexible piezoresistive sensor.

[0098] Comparative Example 1

[0099] The preparation method of the flexible piezoresistive sensor provided by this comparative example includes the following steps:

[0100] S1. Take 2.231 mg of AgNWs and add it to 1 L of EtOH, stir for 1 h to prepare an AgNWs dispersion for later use.

[0101] Clean a PVDF membrane with a size of 50 mm (purchased from Longjin Membrane Industry Co., Ltd.) using EtOH for later use.

[0102] Dissolve 5 g of PVDF powder and 0.25 g of PVP powder in 44.75 g of N,N-dimethylformamide (DMF) to prepare a PVDF encapsulation solution for later use.

[0103] S2. Coat 8 mg of conductive silver paste on the surface of the PVDF membrane, let it stand for 6 h. After curing, drop 4 mL of the AgNWs dispersion on its surface and cure to obtain an AgNWs / PVDF composite membrane.

[0104] S3. Drop 5 mL of the PVDF encapsulation solution on the surface of the AgNWs / PVDF composite membrane. After curing, soak the whole in hot water at 60 °C for 4 min, and then vacuum dry at 60 °C to obtain a flexible piezoresistive sensor.

[0105] Comparative Example 2

[0106] The preparation method of the flexible piezoresistive sensor provided in this comparative example includes the following steps:

[0107] S1. Take 2.975 mg of AgNWs and add it to 1 L of EtOH, stir for 1 h to prepare an AgNWs dispersion for later use.

[0108] Clean a PVDF membrane with a size of 50 mm (purchased from Longjin Membrane Industry Co., Ltd.) using EtOH for later use.

[0109] Dissolve 5 g of PVDF powder and 0.25 g of PVP powder in 44.75 g of N,N-dimethylformamide (DMF) to prepare a PVDF encapsulation solution for later use.

[0110] S2. Coat 6 mg of conductive silver paste on the surface of the PVDF membrane, let it stand for 6 h. After curing, drop 6 mL of the AgNWs dispersion on its surface and cure to obtain an AgNWs / PVDF composite membrane.

[0111] S3. Drop 5 mL of the PVDF encapsulation solution on the surface of the AgNWs / PVDF composite membrane. After curing, soak the whole in hot water at 60 °C for 4 min, and then vacuum dry at 60 °C to obtain a flexible piezoresistive sensor.

[0112] Comparative Example 3

[0113] The preparation method of the flexible piezoresistive sensor provided in this comparative example includes the following steps:

[0114] S1. Add 4.453 mg of AgNWs to 1 L of EtOH and stir for 1 h to prepare an AgNWs dispersion for later use.

[0115] Wash a PVDF membrane with a size of 50 mm (purchased from Longjin Membrane Industry Co., Ltd.) using EtOH for later use.

[0116] Dissolve 5 g of PVDF powder and 0.25 g of PVP powder in 44.75 g of N,N-dimethylformamide (DMF) to prepare a PVDF encapsulation solution for later use.

[0117] S2. Coat 4 mg of conductive silver paste on the surface of the PVDF membrane, let it stand for 6 h, after curing, drop 8 mL of the AgNWs dispersion on its surface and cure to obtain an AgNWs / PVDF composite membrane.

[0118] S3. Drop 5 mL of the PVDF encapsulation solution on the surface of the AgNWs / PVDF composite membrane, after curing, soak the whole in hot water at 60 °C for 4 min, and then vacuum dry at 60 °C to obtain a flexible piezoresistive sensor.

[0119] Comparative Example 4

[0120] The preparation method of the flexible piezoresistive sensor provided in this comparative example includes the following steps:

[0121] S1. Add 2.686 mg of AgNWs to 1 L of EtOH and stir for 1 h to prepare an AgNWs dispersion for later use.

[0122] Wash a PVDF membrane with a size of 50 mm (purchased from Longjin Membrane Industry Co., Ltd.) using EtOH for later use.

[0123] Dissolve 5 g of PVDF powder and 0.25 g of PVP powder in 44.75 g of N,N-dimethylformamide (DMF) to prepare a PVDF encapsulation solution for later use.

[0124] S2. Coat 10 mg of conductive silver paste on the surface of the PVDF membrane, let it stand for 6 h, after curing, drop 2 mL of the AgNWs dispersion on its surface and cure to obtain an AgNWs / PVDF composite membrane.

[0125] S3. Drop 5 mL of the PVDF encapsulation solution on the surface of the AgNWs / PVDF composite membrane, after curing, soak the whole in hot water at 60 °C for 5 min, and then vacuum dry at 60 °C to obtain a flexible piezoresistive sensor.

[0126] Comparative Example 5

[0127] The preparation method of the flexible piezoresistive sensor provided by this comparative example includes the following steps:

[0128] S1. Take 2.231 mg of AgNWs and add it to 1 L of EtOH, stir for 1 h to prepare an AgNWs dispersion solution for later use;

[0129] Wash a PVDF filter membrane with a size of 50 mm (purchased from Longjin Membrane Industry Company) using EtOH for later use;

[0130] Dissolve 5 g of PVDF powder and 0.25 g of PVP powder in 44.75 g of N,N-dimethylformamide (DMF) to prepare a PVDF encapsulation solution for later use.

[0131] S2. Coat 8 mg of conductive silver paste on the surface of the PVDF filter membrane, let it stand for 6 h, after curing, drop 4 mL of the AgNWs dispersion solution on its surface and cure it to obtain an AgNWs / PVDF composite membrane.

[0132] S3. Drop 5 mL of the PVDF encapsulation solution on the surface of the AgNWs / PVDF composite membrane, after curing, soak the whole in hot water at 60 °C for 5 min, and then vacuum dry it at 60 °C to obtain a flexible piezoresistive sensor.

[0133] Comparative Example 6

[0134] The preparation method of the flexible piezoresistive sensor provided by this comparative example includes the following steps:

[0135] S1. Take 2.975 mg of AgNWs and add it to 1 L of EtOH, stir for 1 h to prepare an AgNWs dispersion solution for later use;

[0136] Wash a PVDF filter membrane with a size of 50 mm (purchased from Longjin Membrane Industry Company) using EtOH for later use;

[0137] Dissolve 5 g of PVDF powder and 0.25 g of PVP powder in 44.75 g of N,N-dimethylformamide (DMF) to prepare a PVDF encapsulation solution for later use.

[0138] S2. Coat 6 mg of conductive silver paste on the surface of the PVDF filter membrane, let it stand for 6 h, after curing, drop 6 mL of the AgNWs dispersion solution on its surface and cure it to obtain an AgNWs / PVDF composite membrane.

[0139] S3. Drop 5 mL of the PVDF encapsulation solution on the surface of the AgNWs / PVDF composite membrane, after curing, soak the whole in hot water at 60 °C for 5 min, and then vacuum dry it at 60 °C to obtain a flexible piezoresistive sensor.

[0140] Comparative Example 7

[0141] The preparation method of the flexible piezoresistive sensor provided by this comparative example includes the following steps:

[0142] S1. Take 4.453 mg of AgNWs and add it to 1 L of EtOH, stir for 1 h to obtain an AgNWs dispersion solution for standby;

[0143] Wash a PVDF filter membrane with a size of 50 mm (purchased from Longjin Membrane Industry Company) using EtOH for standby;

[0144] Dissolve 5 g of PVDF powder and 0.25 g of PVP powder in 44.75 g of N,N-dimethylformamide (DMF) to obtain a PVDF encapsulation solution for standby.

[0145] S2. Coat 4 mg of conductive silver paste on the surface of the PVDF filter membrane, let it stand for 6 h, after curing, drop 8 mL of the AgNWs dispersion solution on its surface, and cure to obtain an AgNWs / PVDF composite membrane.

[0146] S3. Drop 5 mL of the PVDF encapsulation solution on the surface of the AgNWs / PVDF composite membrane, after curing, soak the whole in hot water at 60 °C for 5 min, and then vacuum dry at 60 °C to obtain a flexible piezoresistive sensor.

[0147] Comparative Example 8

[0148] The preparation method of the flexible piezoresistive sensor provided by this comparative example includes the following steps:

[0149] S1. Take 4.465 mg of AgNWs and add it to 1 L of EtOH, stir for 1 h to obtain an AgNWs dispersion solution for standby;

[0150] Wash a PVDF filter membrane with a size of 50 mm (purchased from Longjin Membrane Industry Company) using EtOH for standby;

[0151] Dissolve 5 g of PVDF powder and 0.25 g of PVP powder in 44.75 g of N,N-dimethylformamide (DMF) to obtain a PVDF encapsulation solution for standby.

[0152] S2. Coat 2 mg of conductive silver paste on the surface of the PVDF filter membrane, let it stand for 6 h, after curing, drop 10 mL of the AgNWs dispersion solution on its surface, and cure to obtain an AgNWs / PVDF composite membrane.

[0153] S3. Drop 5 mL of the PVDF encapsulation solution on the surface of the AgNWs / PVDF composite membrane, after curing, soak the whole in hot water at 60 °C for 5 min, and then vacuum dry at 60 °C to obtain a flexible piezoresistive sensor.

[0154] Comparative Example 9

[0155] The difference from Example 1 is as follows: In step S2, 10 mg of conductive silver paste was coated on the surface of the PVDF membrane, left standing for 6 h, and after curing, 2 mL of the AgNWs dispersion was dropped on its surface, and then cured to obtain the AgNWs / PVDF composite membrane. The remaining steps remained unchanged.

[0156] Comparative Example 10

[0157] The difference from Example 1 is as follows: In step S2, 8 mg of conductive silver paste was coated on the surface of the PVDF membrane, left standing for 6 h, and after curing, 4 mL of the AgNWs dispersion was dropped on its surface, and then cured to obtain the AgNWs / PVDF composite membrane. The remaining steps remained unchanged.

[0158] Comparative Example 11

[0159] The difference from Example 1 is as follows: In step S2, 4 mg of conductive silver paste was coated on the surface of the PVDF membrane, left standing for 6 h, and after curing, 8 mL of the AgNWs dispersion was dropped on its surface, and then cured to obtain the AgNWs / PVDF composite membrane. The remaining steps remained unchanged.

[0160] Comparative Example 12

[0161] The difference from Example 1 is as follows: In step S2, 2 mg of conductive silver paste was coated on the surface of the PVDF membrane, left standing for 6 h, and after curing, 10 mL of the AgNWs dispersion was dropped on its surface, and then cured to obtain the AgNWs / PVDF composite membrane. The remaining steps remained unchanged.

[0162] Comparative Example 13

[0163] The difference from Example 2 is as follows: In step S2, 10 mg of conductive silver paste was coated on the surface of the PVDF membrane, left standing for 6 h, and after curing, 2 mL of the AgNWs dispersion was dropped on its surface, and then cured to obtain the AgNWs / PVDF composite membrane. The remaining steps remained unchanged.

[0164] Comparative Example 14

[0165] The difference from Example 2 is as follows: In step S2, 8 mg of conductive silver paste was coated on the surface of the PVDF membrane, left standing for 6 h, and after curing, 4 mL of the AgNWs dispersion was dropped on its surface, and then cured to obtain the AgNWs / PVDF composite membrane. The remaining steps remained unchanged.

[0166] Comparative Example 15

[0167] The difference from Example 2 is as follows: In step S2, 2 mg of conductive silver paste is coated on the surface of the PVDF membrane, left standing for 6 h, and after curing, 10 mL of the AgNWs dispersion is dropped on its surface and then cured to obtain the AgNWs / PVDF composite membrane. The remaining steps remain unchanged.

[0168] Comparative Example 16

[0169] The difference from Example 4 is as follows: In step S2, 10 mg of conductive silver paste is coated on the surface of the PVDF membrane, left standing for 6 h, and after curing, 2 mL of the AgNWs dispersion is dropped on its surface and then cured to obtain the AgNWs / PVDF composite membrane. The remaining steps remain unchanged.

[0170] Comparative Example 17

[0171] The difference from Example 4 is as follows: In step S2, 2 mg of conductive silver paste is coated on the surface of the PVDF membrane, left standing for 6 h, and after curing, 10 mL of the AgNWs dispersion is dropped on its surface and then cured to obtain the AgNWs / PVDF composite membrane. The remaining steps remain unchanged.

[0172] Comparative Example 18

[0173] The preparation method of the flexible piezoresistive sensor provided by this comparative example includes the following steps:

[0174] S1. Take 7.18 mg of AgNWs and add them to 1 L of EtOH, stir for 1 h to obtain the AgNWs dispersion for standby;

[0175] Use EtOH to wash the PVDF membrane with a size of 50 mm (purchased from Longjin Membrane Industry Co., Ltd.) for standby;

[0176] Dissolve 5 g of PVDF powder and 0.25 g of PVP powder in 44.75 g of N,N-dimethylformamide (DMF) to obtain the PVDF encapsulation solution for standby.

[0177] S2. Coat 8 mg of conductive silver paste on the surface of the PVDF membrane, leave it standing for 6 h, and after curing, drop 4 mL of the AgNWs dispersion on its surface and cure it to obtain the AgNWs / PVDF composite membrane.

[0178] S3. Drop 5 mL of the PVDF encapsulation solution on the surface of the AgNWs / PVDF composite membrane, and after curing, soak the whole in hot water at 60 °C for 25 min, and then vacuum dry at 60 °C to obtain the flexible piezoresistive sensor.

[0179] Comparative Example 19

[0180] The difference from Comparative Example 18 is as follows: S2. 6 mg of conductive silver paste was coated on the surface of the PVDF membrane, left standing for 6 h, and after curing, 6 mL of AgNWs dispersion was dropped on its surface and cured to obtain an AgNWs / PVDF composite membrane. The remaining steps remained unchanged.

[0181] Comparative Example 20

[0182] The difference from Comparative Example 18 is as follows: S2. 4 mg of conductive silver paste was coated on the surface of the PVDF membrane, left standing for 6 h, and after curing, 8 mL of AgNWs dispersion was dropped on its surface and cured to obtain an AgNWs / PVDF composite membrane. The remaining steps remained unchanged.

[0183] The inventors performed XRD tests on the flexible piezoresistive sensors prepared in Examples 1 to 6, and the results are as Figure 1 shown.

[0184] It can be Figure 1 seen that silver nanowires were successfully spread on the PVDF substrate.

[0185] The inventors performed scanning electron microscope tests on the flexible piezoresistive sensors prepared in Examples 2 to 3 and Comparative Examples 13 to 15, and the results are as Figure 2 shown.

[0186] It can be Figure 2 seen that either too high or too low areal density will affect the conductive network and pore size inside the device, thereby having a certain impact on the air permeability and moisture permeability and the change of electrical signal of the sensor.

[0187] The inventors performed scanning electron microscope tests on the flexible piezoresistive sensors prepared in Comparative Examples 3 to 4, Examples 1, 2, 4 and Comparative Example 18, and the results are as Figure 3 shown.

[0188] It can be Figure 3 seen that with the increase of the soaking time, the pore size inside the device becomes larger. Although a larger pore size results in better air permeability and better force conduction, the stability of the device will gradually decrease.

[0189] Taking the above Examples 1 to 6 and Comparative Examples 1 to 20 as examples, the prepared flexible piezoresistive sensors were subjected to a wearing comfort test. The test method was as follows: Considering that the swallowing and eating habits of most people usually concentrate within 12 hours a day, in the present invention, whether there is an inflammatory reaction on the skin surface after wearing for 12 hours was used as the criterion for evaluating whether it is suitable for long-term wearing. The specific judgment criterion was: if the wearing time reached or exceeded 12 hours and no skin inflammation was caused, it was judged as "√"; if the wearing time was less than 12 hours or skin inflammation occurred, it was judged as "×". The test results are shown in Table 1.

[0190] Table 1 Explanation table of wearable time

[0191]

[0192]

[0193] The inventor further analyzed the flexible piezoresistive sensors prepared in the above-mentioned examples and comparative examples as follows, and the results are as Figure 4 、 5 shown.

[0194] Combined with Table 1 and Figure 4 it can be seen that when the soaking time in step S3 is fixed, as the surface density of AgNWs increases, the wearable time gradually decreases. This shows that: when the influence of the soaking time on the pore size is fixed, the increase in the number of silver nanowires will gradually cover the porous structure of the sensor, resulting in poor air permeability and moisture permeability of the device, and thus affecting the wearable time.

[0195] Combined with Table 1 and Figure 5 it can be seen that when the surface density of AgNWs in step S2 is fixed, as the soaking time increases, the wearable time also gradually extends. This shows that: in terms of air permeability and moisture permeability, when the influence of the AgNWs surface density on the pore size is fixed, the longer the soaking time, the larger the pore size and the porosity, and thus the better the air permeability and moisture permeability.

[0196] In order to balance the influence of the AgNWs surface density and the soaking time on the wearable device, taking Examples 1-6, Comparative Examples 4-5, Comparative Examples 9-10, Comparative Examples 13-14, Comparative Example 16 and Comparative Examples 18-20 as examples, a test for monitoring physiological information during wearing was carried out.

[0197] The electrical signal acquisition device (DMM7510 digital multimeter) was connected to the interdigital electrodes of the prepared flexible piezoresistive sensor with wires, and the flexible piezoresistive sensor was pasted on the Adam's apple of the volunteer for physiological information monitoring. Combining the signal acquisition software (Kick Start 2), the signal of the resistance changing with time during swallowing was recorded to obtain the resistance change rate (△R / R0). Whether to perform a cyclic stability test next was judged by the magnitude of the obtained resistance change rate. If the resistance change rate did not exceed 1% or the signal deviated, the cyclic test was not considered.

[0198] The inventor first selected samples with AgNWs surface densities of 0.0272 mg / cm 2 and 0.0452 mg / cm 2 and soaking times ranging from 5 min to 20 min, including Comparative Example 4, Comparative Example 9, Comparative Example 13, Comparative Example 16, Comparative Example 5, Comparative Example 10, Comparative Example 14 and Example 4 for testing. The test results are as Figure 6 shown.

[0199] It can be seen from Figure 6 that: In Comparative Examples 4 to 16, since the content of silver nanowires was too low, no obvious change in the electrical signal occurred during the compression process. For Comparative Example 5 to Example 4, as the soaking time became longer, a weak electrical signal appeared at the soaking time of 20 minutes, indicating that a low areal density required a long soaking time to create large pores to improve force conduction, thereby generating an electrical signal.

[0200] The inventor further conducted the above tests on Examples 1 to 3, Examples 5 to 6, and Comparative Examples 18 to 20, and obtained the test results as Figure 7 shown.

[0201] Combined with Examples 1, 2, 5 and Figure 7 it can be seen that: When the areal density remains unchanged, as the soaking time increases, the resistance change rate becomes higher. This shows that under an excellent internal conductive network, the longer the soaking time, the larger the pore size, which is more conducive to force conduction, thereby increasing the change in the electrical signal.

[0202] Combined with Examples 2 to 3, Examples 5 to 6 and Figure 7 it can be seen that: When the soaking time remains unchanged, the resistance change rate gradually decreases. This indicates that the internal conductive network with an areal density of 0.1804 mg / cm 2 is too dense, resulting in fewer changes in the contact points inside after compression compared to that with an areal density of 0.0904 mg / cm 2 .

[0203] Combined with Comparative Examples 18 to 20 and Figure 7 it can be seen that: Within the range of a reasonable AgNWs areal density, too long a soaking time will cause the pore size to be too large, resulting in an unstable structure and signal drift.

[0204] The resistance change rates during the wearing tests of Examples 1 to 6 are recorded and integrated in Table 2 below.

[0205] Table 2 Wearing test results

[0206] Resistance change rate / % Example 1 3.5％ Example 2 5％ Example 3 4％ Example 4 1％ Example 5 6％ Example 6 4.5％

[0207] Taking the flexible piezoresistive sensors prepared in Examples 1 to 6 as an example, the inventor conducted a cyclic stability test. The test method was: After connecting the prepared sensor and the electrical signal acquisition device, place them in a pressure test device (INSTRON5967 universal testing machine), and perform a dynamic cyclic test. Continuously load-unload cycle tests were carried out on it at 10 kPa. The test results are shown in Table 3 and Figure 8 shown.

[0208] Table 3 Cyclic test results

[0209] Resistance change rate / % Cycle time Example 1 5％ 3000s Example 2 10％ 2000s Example 3 11％ 2000s Example 4 1％ 1000s Example 5 9.5％ 1000s Example 6 10.5％ 1000s

[0210] Combined with Tables 1 to 3 and Figure 8 it can be seen that:

[0211] (1) From the perspective of the resistance change rate of Example 1 and Example 2, when the areal density is 0.0904 mg / cm 2 , to achieve a higher electrical signal, the soaking time needs to be lengthened.

[0212] (2) From the resistance change rate of Example 2 and Example 3, it can be seen that under the same soaking time, the resistance change rate is gradually increasing.

[0213] (3) From Example 1 - 2 and Example 4 - 6, it can be seen that as the soaking time lengthens, the cycling stability decreases.

[0214] (4) In Example 4 under long soaking time, due to the lower areal density, the resistance change rate is smaller.

[0215] (5) From Example 4 - 6, it can be seen that when the areal density gradually increases, the resistance change rate also increases. When it exceeds 0.0904 mg / cm 2 , it starts to show a downward trend.

[0216] Thus, it can be explained that: in the case of realizing long - term wearability, both the areal density and the soaking time affect the resistance change rate. Among them, when the areal density is a fixed value, a long soaking time can increase the resistance change rate, but too long will lead to a decrease in the cycling life; when the soaking time is a fixed value, an increase in the areal density can increase the resistance change rate, but continuous increase in the areal density will not cause the resistance change rate to continuously rise, but will instead increase the production cost.

[0217] Therefore, in order to further balance the relationship between the areal density and the soaking time and prepare a flexible piezoresistive sensor with both wearing comfort and device stability, the inventor further optimizes the areal density of AgNWs to 0.0452 - 0.1804 mg / cm 2 , and the soaking time is 10 - 20 min.

[0218] The above - mentioned is only the preferred embodiment of the present invention and is not intended to limit the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A preparation method of a flexible piezoresistive sensor, characterized in that, It includes the following steps: S1. Prepare an AgNWs dispersion, a PVDF encapsulation solution and pre-treat a PVDF substrate; S2. Coat a conductive silver paste on the surface of the pre-treated PVDF substrate, and after curing, dropwise add the AgNWs dispersion to obtain an AgNWs / PVDF composite film; S3. Dropwise add the PVDF encapsulation solution on the surface of the AgNWs / PVDF composite film, after curing, soak in hot water, and dry in vacuum to obtain a flexible piezoresistive sensor.

2. The preparation method of the flexible piezoresistive sensor according to claim 1, characterized in that In step S1, the AgNWs dispersion is prepared by dispersing AgNWs in absolute ethanol; the concentration of the AgNWs dispersion is 1.3 mg / L to 8.24 mg / L.

3. The preparation method of the flexible piezoresistive sensor according to claim 2, wherein, The mass-volume ratio of the AgNWs to the absolute ethanol is (2.02 to 8.24) mg:(1 to 1.5) L.

4. The preparation method of the flexible piezoresistive sensor according to claim 1, characterized in that, In step S1, the PVDF encapsulation solution is prepared by dissolving PVDF powder and PVP powder in an organic solvent; the concentration of the PVDF encapsulation solution is 9.6 to 10.6 wt%.

5. The preparation method of the flexible piezoresistive sensor according to claim 4, characterized in that, The organic solvent is selected from one of DMF, DMSO or NMP; The mass ratio of the PVDF powder, the PVP powder and the organic solvent is (4.8 to 5.3):(0.24 to 0.26):(44.55 to 45).

6. The preparation method of the flexible piezoresistive sensor according to claim 1, characterized in that, In step S2, the areal density of the AgNWs on the AgNWs / PVDF composite membrane is 0.0272 - 0.2261 mg / cm 2 .

7. The preparation method of the flexible piezoresistive sensor according to claim 6, wherein, In step S2, the areal density of the AgNWs on the AgNWs / PVDF composite membrane is 0.0452 - 0.1804 mg / cm 2 .

8. The preparation method of the flexible piezoresistive sensor according to claim 6, wherein, In step S3, the soaking temperature is 50 to 65 °C and the soaking time is 10 to 20 min.

9. The preparation method of the flexible piezoresistive sensor according to claim 8, characterized in that, In step S3, the drying temperature is 50 to 65 °C and the drying time is 12 to 14 h.

10. A flexible piezoresistive sensor, characterized in that, Prepared by the preparation method according to any one of claims 1 to 9.

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