A highly stable flexible piezoresistive sensing unit and its preparation method

Abstract

The present invention relates to a highly stable flexible piezoresistive sensing unit and a preparation method thereof. The flexible piezoresistive sensing unit sequentially includes, from top to bottom: a first flexible thin film encapsulation layer, a first substrate layer, a conductive sensing material layer, a second substrate layer, and a second flexible thin film encapsulation layer. The conductive sensing material layer is made by doping conductive fillers in a flexible matrix. The preparation method of the present invention is simple, the steps are easy to operate, the prepared piezoresistive sensing unit has strong stability, good sensitivity and linearity. Among them, the first substrate layer and the first thin film encapsulation layer, and the second substrate layer and the second thin film encapsulation layer are formed by an integral molding method and then the conductive conduction material is encapsulated, which improves the stability of the conductive sensing material and extends the service life of the sensing unit.

Description

Technical Field

[0001] The present invention belongs to the technical field of flexible sensing material preparation, and particularly relates to a high-stability flexible piezoresistive sensing unit and a preparation method thereof. Background Art

[0002] Due to characteristics such as low cost and high sensitivity, piezoresistive sensors have received increasing attention in fields such as electronic skin, wearable medical detection, and motion monitoring. Traditional piezoresistive sensors are limited by packaging means and piezoresistive sensing materials. The sensitivity, linearity, repeatability, and stability of the materials all limit the development of piezoresistive sensors. At the same time, the shapes of many types of piezoresistive sensors are relatively regular and flat, with high process difficulty, which reduces production efficiency.

[0003] The flexible piezoresistive sensor prepared by using a composite conductive material has good plasticity and ductility. However, this flexible piezoresistive sensor is limited by the process and preparation method, and its stability is poor. After being used for a period of time, the conductive ability of the flexible piezoresistive sensor changes, resulting in a decrease in its sensing performance. Summary of the Invention

[0004] The present invention aims to solve the problems that the flexible piezoresistive sensor prepared by using a composite conductive material at present has poor stability and the conductive ability decreases after being used for a certain period of time, leading to a decrease in sensing performance, and provides a high-stability flexible piezoresistive sensing unit and a preparation method thereof.

[0005] The present invention adopts the following technical solution: A high-stability flexible piezoresistive sensing unit, which sequentially includes from top to bottom: a first flexible film encapsulation layer, a first substrate layer, a conductive sensing material layer, a second substrate layer, and a second flexible film encapsulation layer. The conductive sensing material layer is made by doping conductive fillers in a flexible matrix. The conductive fillers are a mixture of carbon black and multi-walled carbon nanotubes. The mass ratio of carbon black to multi-walled carbon nanotubes is 4:1 - 5:1. The first flexible film encapsulation layer and the first substrate layer are integrally formed. The second substrate layer and the second flexible film encapsulation layer are integrally formed. The areas of the first flexible film encapsulation layer and the second flexible film encapsulation layer are both larger than the area of the conductive sensing material layer.

[0006] Further, the materials of the first flexible film encapsulation layer and the first substrate layer are different, and the materials of the second flexible film encapsulation layer and the second substrate layer are different.

[0007] Further, the materials of the first flexible film encapsulation layer and the second flexible film encapsulation layer are polydimethylsiloxane or rubber.

[0008] Further, the materials of the first substrate layer and the second substrate layer are any one of polyimide, polyethylene, or polyester resin.

[0009] Further, the thickness of the first flexible thin film encapsulation layer and the second flexible thin film encapsulation layer is less than or equal to 0.2 mm.

[0010] Further, the flexible substrate is made of polydimethylsiloxane or rubber.

[0011] A preparation method of a highly stable flexible piezoresistive sensing unit includes the following steps:

[0012] (1) Place the first substrate layer and the second substrate layer flat on a heating table respectively. Coat the first flexible thin film encapsulation layer material on the surface of the first substrate, and coat the second flexible thin film encapsulation layer material on the surface of the second substrate layer. Heat and fix them to form, so that the first flexible thin film encapsulation layer is integrally formed with the first substrate layer, and the second flexible thin film encapsulation layer is integrally formed with the second substrate layer;

[0013] (2) According to the shape specifications of the piezoresistive sensing unit, calculate the mass of the flexible substrate of the conductive sensing material layer, weigh each component of the flexible substrate according to the mass ratio, and mix and stir evenly to obtain a flexible substrate solution;

[0014] (3) Dope conductive fillers into the flexible substrate solution and stir well to obtain a conductive sensing material solution, wherein the addition amount of the conductive fillers is 20% of the mass of the flexible substrate. The conductive fillers are carbon black and multi-walled carbon nanotubes, and the mass ratio of the carbon black to the multi-walled carbon nanotubes is 4:1 - 5:1;

[0015] (4) Uniformly spray the conductive sensing material solution prepared in step (3) on the surface of the side of the second substrate layer that is not formed with the second flexible thin film encapsulation layer until all the conductive sensing material solution is sprayed to form a conductive sensing material layer;

[0016] (5) Quickly cover the side of the first substrate layer that is not formed with the first flexible thin film encapsulation layer onto the surface of the second substrate layer, and cure at room temperature under a certain pressure for 20 - 24 h, so that the first flexible thin film encapsulation layer and the second flexible thin film encapsulation layer encapsulate the conductive sensing material.

[0017] Further, when the flexible substrate in step (2) is rubber, the mass ratio of each component is silicone rubber: tetraethyl orthosilicate: dibutyltin dilaurate: dimethyl silicone oil 1:0.05:0.025:0.1.

[0018] Further, in step (1), the heating temperature during heating and fixing is 85 - 95 °C, and the heating time is 8 - 12 min.

[0019] Further, the pressure during curing in step (5) is 800 - 1000 kPa.

[0020] Preparation principle: For the flexible piezoresistive sensing unit prepared in the present invention, it is first necessary to meet the characteristics of bendability and stretchability of the piezoresistive sensing material. Therefore, appropriate flexible matrix and conductive fillers need to be selected. The ductility and bendability of the flexible matrix determine the ductility and bendability of the prepared flexible piezoresistive sensing unit. At the same time, the flexible matrix should also exhibit biocompatibility, chemical stability, and electrical insulation, and these characteristics of the flexible matrix can enable the conductive fillers to effectively play their roles.

[0021] The higher the content of the conductive filler, the lower the resistance, the higher the hardness, and the lower the ductility. Carbon black powder has a large mass and has a greater impact on the ductility of the flexible matrix. Multi-walled carbon nanotubes have a small mass and strong conductivity. The conductive filler is selected as a mixture of carbon black and multi-walled carbon nanotubes, which has less damage to the ductility of the flexible matrix while improving the conductivity.

[0022] The flexible thin film encapsulation layer material has no influence on the conductive sensing material, and at the same time can protect the conductive sensing material from contacting the outside world, improving the stability of the conductive sensing material.

[0023] The advantages of the present invention are as follows:

[0024] (1) The preparation method of the present invention is simple, the steps are easy to operate, the prepared piezoresistive sensing unit has strong stability and good sensitivity, and the linearity of the piezoresistive curve of the flexible piezoresistive sensing unit is still greater than 0.99 within 4 weeks.

[0025] (2) In the present invention, the first substrate layer and the first thin film encapsulation layer, and the second substrate layer and the second thin film encapsulation layer are formed by an integral molding method and then the conductive conduction material is encapsulated, which improves the stability of the internal conductive sensing material and extends the service life of the sensing unit.

[0026] (3) The conductive sensing material layer in the present invention is made of a flexible matrix doped with conductive fillers. The flexible matrix can not only meet the requirements of ductility and bendability but also meet the requirements of conductivity. Description of the drawings

[0027] Figure 1 It is a schematic structural diagram of the flexible piezoresistive sensing unit of the present invention.

[0028] Figure 2 It is a piezoresistive curve graph of the flexible piezoresistive sensing unit in Example 2 of the present invention.

[0029] Figure 3 It is a linearity graph of the piezoresistive curve of the flexible piezoresistive sensing unit in Example 2 of the present invention.

[0030] Figure 4 It is a piezoresistive curve graph of the piezoresistive sensing unit in the comparative example of the present invention.

[0031] Figure 5 This is the linearity graph of the piezoresistive curve of the piezoresistive sensing unit in the comparative example of the present invention.

[0032] Explanation of reference numerals: The first flexible film encapsulation layer 1, the first substrate layer 2, the conductive sensing material layer 3, the second substrate layer 4, the second flexible film encapsulation layer 5. Specific embodiments

[0033] In order to make the above objects, features, and advantages of the present invention more obvious and understandable, the specific embodiments of the present invention will be further described below with reference to specific drawings.

[0034] Many specific details are set forth in the following description in order to provide a thorough understanding of the present invention, but the present invention may be practiced in other ways different from those described herein. Those skilled in the art can make similar extensions without departing from the spirit of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed below.

[0035] In the present invention, the carbon black powder model is VCX-72, which is purchased from Cabot Corporation, USA;

[0036] The first substrate layer and the second substrate layer in the present invention are purchased from Shenzhen Yingshida Company;

[0037] The PDMS solution in the present invention is purchased from Dow Corning Corporation, USA;

[0038] The multi-walled carbon nanotubes in the present invention are purchased from Suzhou Hengqiu Company.

[0039] Example 1:

[0040] Flexible piezoresistive sensing unit: The specification size is 50*25*1, unit: mm

[0041] A preparation method of a flexible piezoresistive sensing unit with good stability, comprising the following steps:

[0042] (1) Take the first substrate layer and the second substrate layer made of polyimide, and cut them to 60*30 mm 2 , take the PDMS solution and the curing agent and mix them evenly according to a mass ratio of 10:1 to prepare the raw material of the flexible film encapsulation layer. Place the first substrate layer and the second substrate layer flat on the heating table respectively, coat the raw material of the flexible film encapsulation layer on the surface of the first substrate and the surface of the second substrate layer respectively, heat at 95°C on the heating table for 8 minutes, integrally form the first flexible film encapsulation layer on the surface of the first substrate layer, and integrally form the second flexible film encapsulation layer on the surface of the second substrate layer. The thickness of the first flexible film encapsulation layer and the second flexible film encapsulation layer is 0.2 mm;

[0043] (2)Select silicone rubber as the flexible matrix. The proportions of the components in the flexible matrix are as follows: silicone rubber: tetraethyl orthosilicate: dibutyltin dilaurate: dimethyl silicone oil = 1:0.05:0.025:0.1. Carbon black and multi-walled carbon nanotubes are used as conductive fillers. The mass ratio of carbon black to multi-walled carbon nanotubes is 4:1. The mass of the conductive filler is 20% of the mass of the flexible matrix. After mixing the components of the flexible matrix evenly, slowly add the conductive filler and continue stirring to form a conductive sensing material solution;

[0044] (3)Use a spray head to evenly spray the conductive sensing material solution onto the surface of the second substrate layer until the spraying of the conductive sensing material solution is completed. Immediately cover the first substrate layer onto the surface of the second substrate and cure it at room temperature for 24 h under 1000 kPa to prepare a flexible piezoresistive sensing unit.

[0045] Example 2:

[0046] Flexible piezoresistive sensing unit: The specification size is 50*25*1, unit: mm

[0047] (1)Take the first substrate layer and the second substrate layer made of polyethylene material and cut them to 60*30 mm 2 , take the PDMS solution and the curing agent and mix them evenly according to the mass ratio of 10:1 to prepare the raw material of the flexible film encapsulation layer. Place the first substrate layer and the second substrate layer flat on the heating table respectively, coat the raw material of the flexible film encapsulation layer on the surface of the first substrate and the surface of the second substrate layer respectively, heat it on the heating table at 90 °C for 10 min, integrally form the first flexible film encapsulation layer on the surface of the first substrate layer, and integrally form the second flexible film encapsulation layer on the surface of the second substrate layer. The thickness of the first flexible film encapsulation layer and the second flexible film encapsulation layer is 0.1 mm;

[0048] (2)Select silicone rubber as the flexible matrix. The proportions of the components in the flexible matrix are as follows: silicone rubber: tetraethyl orthosilicate: dibutyltin dilaurate: dimethyl silicone oil = 1:0.05:0.025:0.1. Carbon black and multi-walled carbon nanotubes are used as conductive fillers. The mass ratio of carbon black to multi-walled carbon nanotubes is 5:1. The mass of the conductive filler is 20% of the mass of the flexible matrix. After mixing the components of the flexible matrix evenly, slowly add the conductive filler and continue stirring to form a conductive sensing material solution;

[0049] (3)Use a spray head to evenly spray the conductive sensing material solution onto the surface of the second substrate layer until the spraying of the conductive sensing material solution is completed. Immediately cover the first substrate layer onto the surface of the second substrate and apply a pressure of 1000 kPa on the surface of the first flexible film encapsulation layer, and cure it at room temperature for 24 h to prepare a flexible piezoresistive sensing unit.

[0050] Example 3:

[0051] Flexible piezoresistive sensing unit: The specification size is 50*25*1, unit: mm;

[0052] (1) Take the first and second substrate layers made of polyethylene material, cut them to 60*30 mm2. Take the PDMS solution and the curing agent and mix them evenly according to the mass ratio of 10:1 to prepare the raw material for the flexible film encapsulation layer. Place the first and second substrate layers flat on the heating table respectively, coat the raw material for the flexible film encapsulation layer on the surface of the first substrate layer and the second substrate layer respectively, heat at 85 °C on the heating table for 12 min, integrally form the first flexible film encapsulation layer on the surface of the first substrate layer, and integrally form the second flexible film encapsulation layer on the surface of the second substrate layer. The thickness of the first flexible film encapsulation layer and the second flexible film encapsulation layer is 0.2 mm;

[0053] (2) Select silicone rubber as the flexible matrix. The mass ratio of each component in the flexible matrix is silicone rubber: tetraethyl orthosilicate: dibutyltin dilaurate: dimethyl silicone oil = 1:0.05:0.025:0.1. Weigh each component material and mix and stir. Carbon black and multi-walled carbon nanotubes are conductive fillers. The mass ratio of the carbon black to the multi-walled carbon nanotubes is 5:1. The mass of the conductive filler is 20% of the mass of the flexible matrix. After mixing the components of the flexible matrix evenly, slowly add the conductive filler and continue to stir to form a conductive sensing material solution;

[0054] (3) Use a spray head to evenly spray the conductive sensing material solution onto the surface of the second substrate layer until the spraying of the conductive sensing material solution is completed. Immediately cover the first substrate layer onto the surface of the second substrate, apply a pressure of 1200 kPa on the surface of the first flexible film encapsulation layer, and cure at room temperature for 24 h to prepare a flexible piezoresistive sensing unit.

[0055] Example 4:

[0056] Flexible piezoresistive sensing unit: The specification size is 50*25*1, unit: mm

[0057] (1) Take the first and second substrate layers made of polyester resin material, cut them to 60*30 mm2. Take rubber as the raw material for the flexible film encapsulation layer. Place the first and second substrate layers flat on the heating table respectively, coat the raw material for the flexible film encapsulation layer on the surface of the first substrate layer and the second substrate layer respectively, heat at 90 °C on the heating table for 10 min, integrally form the first flexible film encapsulation layer on the surface of the first substrate layer, and integrally form the second flexible film encapsulation layer on the surface of the second substrate layer. The thickness of the first flexible film encapsulation layer and the second flexible film encapsulation layer is 0.2 mm;

[0058] (2) Select PDMS silicone rubber as the flexible matrix, carbon black and multi-walled carbon nanotubes as the conductive fillers. The mass ratio of the carbon black to the multi-walled carbon nanotubes is 5:1, and the mass of the conductive fillers is 20% of the mass of the flexible matrix. After mixing the components of the flexible matrix evenly, slowly add the conductive fillers and continue stirring to form a conductive sensing material solution;

[0059] (3) Spray the conductive sensing material solution evenly onto the surface of the second substrate layer using a spray head until the spraying of the conductive sensing material solution is completed. Immediately cover the first substrate layer onto the surface of the second substrate, apply a pressure of 800 kPa on the surface of the first flexible film encapsulation layer, and cure at room temperature for 24 h to prepare a flexible piezoresistive sensing unit.

[0060] Comparative example

[0061] Flexible piezoresistive sensing unit: The specification size is 50*25*1, unit: mm; without a flexible film encapsulation layer.

[0062] (1) Select silicone rubber as the flexible matrix. The proportion of each component in the flexible matrix is: silicone rubber: tetraethyl orthosilicate: dibutyltin dilaurate: dimethyl silicone oil = 1:0.05:0.025:0.1. Carbon black and multi-walled carbon nanotubes are used as the conductive fillers. The mass ratio of the carbon black to the multi-walled carbon nanotubes is 5:1, and the mass of the conductive fillers is 20% of the mass of the flexible matrix. After mixing the components of the flexible matrix evenly, slowly add the conductive fillers and continue stirring to form a conductive sensing material solution;

[0063] (2) Uniformly spray the conductive sensing material solution in step (1) onto the surface of the second substrate to form a conductive sensing material layer. Immediately cover the first substrate and cure at room temperature for 24 h under a certain pressure to prepare a flexible piezoresistive sensing unit.

[0064] Select the flexible piezoresistive sensing unit prepared in Example 2 of the present invention and the piezoresistive sensing unit in the comparative example for piezoresistive performance testing. The results are as Figures 2 - 5 shown. The flexible piezoresistive sensor has strict requirements for stability and the linearity of the piezoresistive curve. Under the same pressure, the change in the resistance value of the flexible piezoresistive sensor will seriously affect the accuracy of the measured pressure.

[0065] Figure 2 This is the result of measuring the piezoresistive curve of the flexible piezoresistive sensing unit sample prepared by the present invention. The abscissa represents the magnitude of the pressure, and the ordinate represents the resistance value.

[0066] It can be seen from Figure 2 that the piezoresistive curve of the flexible piezoresistive sensing unit prepared by the present invention remains stable within 4 weeks. Figure 3It is the linearity curve graph of the piezoresistive curve of the flexible piezoresistive sensing unit. The closer the linearity of the piezoresistive curve is to 1, the more accurate the fitting of the relationship between pressure and resistance value is, which is more conducive to obtaining the magnitude of pressure through the piezoresistive curve and measuring the resistance value, and also indicates that the piezoresistive characteristics are better. Excellent piezoresistive characteristics can better meet the requirements for fabricating a flexible piezoresistive sensor. The flexible piezoresistive sensing unit prepared by the present invention has a linearity greater than 0.99 within 4 weeks, so it can fully meet the performance requirements of the flexible piezoresistive sensor.

[0067] Figure 4 is the piezoresistive sensing unit obtained from the comparative example, which consists of Figure 4 As can be seen, the resistance of the piezoresistive sensing unit without encapsulation by the flexible thin film encapsulation material layer increases as a whole after 2 weeks, and there is an obvious difference in the resistance value under the same pressure compared with the first week, which is not conducive to the long-term use of the flexible piezoresistive sensor. Comparing Figure 5 with the piezoresistive curve linearity, its long-term linearity is unstable, and the linearity drops greatly after 2 weeks. Although the subsequent linearity has a certain recovery, this piezoresistive change cannot meet the performance requirements of the flexible piezoresistive sensor.

Claims

1. A preparation method of a highly stable flexible piezoresistive sensing unit for preparing a highly stable flexible piezoresistive sensing unit, characterized in that: It includes the following steps: (1) Place the first substrate layer and the second substrate layer flat on the heating table respectively. Coat the first flexible thin film encapsulation layer material on the surface of the first substrate, and coat the second flexible thin film encapsulation layer material on the surface of the second substrate layer. Heat and fix them to form, so that the first flexible thin film encapsulation layer is integrally formed with the first substrate layer, and the second flexible thin film encapsulation layer is integrally formed with the second substrate layer; (2) According to the shape specifications of the piezoresistive sensing unit, calculate the mass of the flexible matrix of the conductive sensing material layer, weigh each component of the flexible matrix according to the mass ratio, and mix and stir evenly to obtain the flexible matrix solution; (3) Dope conductive fillers into the flexible matrix solution and stir well to obtain the conductive sensing material solution, wherein the addition amount of the conductive fillers is 20% of the mass of the flexible matrix. The conductive fillers are carbon black and multi-walled carbon nanotubes, and the mass ratio of the carbon black to the multi-walled carbon nanotubes is 4:1 - 5:1; (4) Uniformly spray the conductive sensing material solution prepared in step (3) on the surface of the side of the second substrate layer that is not formed with the second flexible thin film encapsulation layer until all the conductive sensing material solution is sprayed to form a conductive sensing material layer; (5) Quickly cover the side of the first substrate layer that is not formed with the first flexible thin film encapsulation layer onto the surface of the second substrate layer, and cure at room temperature under a certain pressure for 20 - 24 h, so that the first flexible thin film encapsulation layer and the second flexible thin film encapsulation layer encapsulate the conductive sensing material; Among them, when the flexible matrix in step (2) is rubber, the mass ratio of each component is silicone rubber: tetraethyl orthosilicate: dibutyltin dilaurate: dimethyl silicone oil 1:0.05:0.025:0.1; The high-stability piezoresistive sensing unit includes, from top to bottom in sequence: a first flexible thin film encapsulation layer (1), a first substrate layer (2), a conductive sensing material layer (3), a second substrate layer (4), and a second flexible thin film encapsulation layer (5). The conductive sensing material layer is made by doping conductive fillers in a flexible matrix. The conductive fillers are a mixture of carbon black and multi-walled carbon nanotubes, and the mass ratio of the carbon black to the multi-walled carbon nanotubes is 4:1 - 5:

1. The first flexible thin film encapsulation layer (1) is integrally formed with the first substrate layer (2), the second substrate layer (4) and the second flexible thin film encapsulation layer (5) are integrally formed, and the areas of the first flexible thin film encapsulation layer (1) and the second flexible thin film encapsulation layer (5) are both larger than the area of the conductive sensing material layer.

2. The preparation method of the highly stable flexible piezoresistive sensing unit according to claim 1, characterized in that: In step (1), the heating temperature during heating and fixing to form is 85 - 95 °C, and the heating time is 8 - 12 min.

3. The preparation method of the highly stable flexible piezoresistive sensing unit according to claim 1, characterized in that: In step (5), the pressure during curing is 800 - 1000 kPa.

4. The preparation method of the highly stable flexible piezoresistive sensing unit according to claim 1, characterized in that: The materials of the first flexible thin film encapsulation layer and the first substrate layer are different, and the materials of the second flexible thin film encapsulation layer and the second substrate layer are different.

5. The preparation method of the highly stable flexible piezoresistive sensing unit according to claim 4, characterized in that: The materials of the first flexible thin film encapsulation layer and the second flexible thin film encapsulation layer are polydimethylsiloxane or rubber.

6. The preparation method of the highly stable flexible piezoresistive sensing unit according to claim 5, characterized in that: The materials of the first substrate layer and the second substrate layer are any one of polyimide, polyethylene or polyester resin.

7. The preparation method of the highly stable flexible piezoresistive sensing unit according to claim 1, characterized in that: The thicknesses of the first flexible thin film encapsulation layer and the second flexible thin film encapsulation layer are less than or equal to 0.2 mm.

8. The preparation method of the highly stable flexible piezoresistive sensing unit according to claim 1, characterized in that: The flexible substrate is made of polydimethylsiloxane or rubber material.

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