AU2018102177A4 - Flexible pressure sensor based on hemispheric microstructure and fabrication method therefor - Google Patents

Abstract

The invention is applicable to the technical field of flexible pressure sensors, and discloses a flexible pressure sensor based on a hemispherical microstructure and a fabrication method thereof. The pressure sensor comprises a PDMS flexible 5 substrate, a carbon nanotube film, and a PDMS flexible film layer. The PDMS flexible substrate has convex hemispherical microstructures covered wtih carbon nanotube film. The carbon nanotube film is located between the PDMS flexible substrate and the PDMS flexible film; and the carbon nanotube film is connected with electrodes. The fabrication method is used to fabricate the above-mentioned o flexible pressure sensor. The hemispherical-microstructure-based flexible pressure sensors and the fabrication method thereof provided by the proposed invention improves the measurement range and sensitivity, and reduces the response time of the flexible pressure sensor.

Description

Specification

Hemispherical-microstructure-based Flexible Pressure Sensor and Its Fabrication Method

Technical Field The invention belongs to the technical field of flexible pressure sensors, and particularly relates to hemispherical-microstructure-based flexible pressure sensors and fabrication method thereof.

o Background With the development of society, flexible sensor devices have brought revolutionary changes to many aspects of social life. Because of their advantages in flexibility, they have been gradually applied to robots, wearable electronic devices, human-computer interaction, intelligent skin and other fields. Compared with traditional sensors, flexible sensors have defects in term of performance. Current flexible sensors still have the problems of low sensitivity, small measurement range, large hysteresis and susceptibility to interference from external environmental noise. In recent years, with rapid economic development, people's quality of life has been greatly improved, and the rapid development of wearable flexible sensors has been promoted. It is hoped that the sensor device can be comfortably worn on the body or directly attached to the skin surface to obtain health information such as pulse and blood pressure. In addition, the flexible sensor is also an essential component of the human bionic prosthesis and the intelligent robot to sense the external environment. The research on flexible pressure sensors has become a hotspot this year. The predecessors used a variety of different methods to improve the performance indexes of the flexible pressure sensor, such as sensitivity, range, repeatability, and consistency, etc., which has expanded the application field of the flexible pressure sensor.

In recent years, the process for making flexible pressure sensors mainly includes the following: 1. Make a pyramid groove array mould by photoetching and wet etching, pour PDMS into the mould to make a PDMS substrate with a pyramid structure, prepare a graphene oxide suspension, and then make a graphene film on the PDMS film with a pyramid structure using layer-by-layer self-assembly method. Finally, the PDMS film with graphene is attached to the PET film with ITO coating, and electrodes are drawn out from the film to complete the fabrication of the flexible pressure sensor. This pressure sensor can measure a pressure of 1.5 Pa at minimum, the response o time is only 0.2 ms, and the sensitivity in the pressure range of 0-10Pa is -5.53/kPa. 2. Make a rail wafer mould with a pyramid groove. Mix PDMS and crosslinking agent at a ratio of 5: 1, which is then diluted with hexane and stirred for more than 30 minutes. Coat the diluted 100 pL of the solution on the mould and degas. Treat a 150-micron thick PET film with an ITO conductive layer with ultraviolet light for 20 minutes, and then place the PET film on the PDMS film in a vacuum environment. Apply a pressure of at least 1OOMPa to the film at 70°C for 4 hours, and finally connect wires at both ends of the film to make the sensor. Because the sensor has a microstructure array that is easily deformed, high sensitivity measurement is achieved, and the sensitivity reaches 0.55/kPa in the range of 2 kPa. 3. Mix 10, 30, and 50 mg of single-walled carbon nanotube (SWCNT) powder with 10 mL of deionised water to make SWCNT solutions with different concentrations, followed by mixing 0.1 mL of polystyrene (PSS) solution for 30 minutes of ultrasonic degradation. Pour the PDMS into the mould to make a 500-pm thick PDMS film and treat the PDMS surface with oxygen plasma to obtain a hydrophilic surface. A100-nm thick PU-PEDOT:PSS composite elastomer layer is formed by mixing a solution of polyurethane (60% by weight) and PEDOT:PSS (40% by weight), and deposited on the substrate. After annealing at 150°C for 1 hour, a triethoxysilane solution is used for 30 minutes of self-assembly (SAM) on the surface of the composite membrane to obtain an SWCNT solution coating on the

PDMS substrate. After the SAM treatment, drop the SWCNT solution on the surface of the substrate and spin-coat at 1000 rpm for 10 minutes to obtain a 1.2-pm thick film. Then anneal the samples at 100°C for 1 hour. Finally, coat another layer of PU/PEDOT:PSS solution and anneal at 100°C for 1 hour to complete the sensor fabrication. The sensor has high transparency that can reach 72%, and has excellent repeatability, and the sensitivity coefficient reaches 106. 4. Add 44 mg of tetrachloroauric acid trihydrate to 40 mL of hexane, and then add 1.5 mL of oleylamine. After the gold salt is completely dissolved, add 2.1 mL of triisopropylsilane to the above solution. The mixed solution is allowed to stand at room temperature for two days until the colour of the solution changes from yellow to dark red, indicating the formation of gold nanowires. The mixture is centrifuged and washed several times with a mixed solution of ethanol and hexane (volume ratio 2: 1) to remove residual compounds, and finally concentrated into 2 mL of chloroform solution. Soak 8 x 8 mm 2 Kimberly Clark thin paper in a solution of the gold nanowire in chloroform. After the chloroform evaporates, the colour of the thin paper changes from white to dark red. Repeat the coating and drying process approximately ten times until the resistance of the thin paper reaches 2.5MQ/sq. Plate staggered Ti/Au electrodes on the 30 x 27 mm2 PDMS substrate. The distance between adjacent electrodes is usually 0.1 mm, and the distance between the middle electrodes is 0.5 mm. Two 10 x 10 mm2 contact plates are placed at both ends between the two electrodes and connected to the external circuit. Finally, the thin film with AuNWs is sandwiched between the PDMS film with staggered electrodes and the blank PDMS film to form a sandwich-like structure. The sensor obtained by this method can measure very little pressure, and has a response time of 17 ms, and a sensitivity of 1.14/kPa, which can realise the real-time measurement of the human pulse. Although the above-mentioned flexible sensors can achieve the measurement of external pressure, there are still some disadvantages. 1. The flexible pressure sensor based on a graphene microstructure array has low sensitivity in a pressure range that is too high or too low. This leads to a small application range. 2. The flexible sensor based on a microstructured rubber dielectric layer has low sensitivity, and is only suitable for sensing static pressure. 3. The flexible sensor based on the piezoresistive effect will have a response lag under a large amount of stretching, making it difficult to restore the original state. 4. The flexible pressure sensor based on a gold nanowire has low transparency, and the sensor has low sensitivity under a large amount of stretching, and even fails due to electrode breakage.

o Summary The objective of the proposed invention is to overcome the above-mentioned shortcomings of the prior art, and to provide a hemispherical-microstructure-based flexible pressure sensor and a fabrication method thereof, which improve the measurement range and sensitivity, and reduce the response time of the flexible pressure sensor. The technical solution of the proposed invention is: a flexible pressure sensor based on hemispherical microstructures, comprises a PDMS flexible substrate, a carbon nanotube film, and a PDMS flexible film layer. The PDMS flexible substrate has convex hemispherical microstructures covered wtih carbon nanotube film. The carbon nanotube film is located between the PDMS flexible substrate and the PDMS flexible film; and the carbon nanotube film is connected with electrodes. Optionally, the microstructure is hemispherical. The invention also provides a fabrication method of a flexible pressure sensor, comprising the following steps: fabricate a PDMS flexible substrate with convex hemispherical microstructures; fabricate a carbon nanotube film; cover the convex hemispherical microstructures with the carbon nanotube film; fabricate a PDMS flexible film layer and place the PDMS flexible film layer on top of the carbon nanotube film; connect electrodes to the carbon nanotube film. Optionally, wherein fabricating the PDMS flexible substrate comprises the following steps: produce silicon wafer mould with photoresist hemispherical concave microstructure using photolithography; mix and stir PDMS and agent at a weight ratio of 10: 1 to obtain a mixed solution, and then coat the mixed solution to the silicon wafer mould with a hemispherical concave microstructure; heat the silicon wafer mould with the mixed PDMS; let the silicon wafer mould and the mixed PDMS cool down to room temperature that forms a PDMS flexible substrate layer after it is cooled and solidified, and separate the PDMS flexible substrate layer from the silicon wafer mould to obtain a PDMS flexible substrate with hemispherical microstructures. Optionally, wherein fabricating the carbon nanotube film comprises the following steps: Step 1: mix a hydrogen chloride solution and a hydrogen peroxide solution to obtain a mixed solution, and add carbon nanotube powders to the mixed solution to heat. Step 2: add the carbon nanotube in Step 1 to a dimethylformamide solution and evacuate to leak, and finally obtain a layer of carbon nanotube film attached on a leakage film. Step 3: insert the leakage film obliquely into deionised water, and a layer of carbon nanotube film is separated from the carbon nanotube film obtained in Step 2. Step 4: take out the carbon nanotube film floating on deionised water and dry it with nitrogen flow. Optionally, in Step 3, a layer of carbon nanotube film with a thickness of 50-60 nm is separated from the carbon nanotube film with a thickness of 200-300 pm. Optionally, cover the convex hemispherical microstructures with the carbon nanotube film comprises the following steps: transfer the carbon nanotube film onto the PDMS flexible substrate with a hemispherical microstructure and heat it. Optionally, wherein fabricating the PDMS flexible film layer comprises the following steps: mix and stir PDMS and crosslinking agent at a weight ratio of 10: 1 to obtain a solution; spin-coat the solution on the silicon wafer and heat, then cool it down, and separate the semi-cured PDMS flexible film layer on the silicon wafer. Optionally, the semi-cured PDMS flexible film layer is bonded to the carbon nanotube film and the PDMS flexible substrate and heated. Optionally, after bonding, electrodes are drawn on both sides of the carbon nanotube film of the intermediate layer. The hemispherical-microstructure-based flexible pressure sensor and a fabrication method thereof provided by the proposed invention, of which the flexible pressure sensor adopts a hemispherical internal structure, and the measurement range of the sensor is greatly improved. Combined with carbon nanotubes with high conductivity, it has a higher sensitivity. And by improving the fabrication method, the fabrication of the flexible sensor is easier and more feasible, which reduces the fabrication difficulty and labour cost, improves the manufacturing efficiency, and realises the standardised manufacturing process.

Description of drawings In order to illustrate the technical solutions of embodiments of the proposed invention more clearly, the drawings required to be used in the embodiments will be briefly described below. It is evident that the drawings described below are only some embodiments of the proposed invention. It will be apparent to one of ordinary skill in the art that other drawings may be obtained based on the accompanying drawings without inventive effort. Fig. 1 is a schematic cross-sectional view of a hemispherical-microstructure-based flexible pressure sensor provided by an embodiment of the proposed invention;

Fig. 2 is a schematic cross-sectional view of a silicon wafer mould used in a fabrication method of a flexible pressure sensor provided by an embodiment of the proposed invention; Fig. 3 is a schematic plan view of a silicon wafer mould used in a fabrication method of a flexible pressure sensor provided by an embodiment of the proposed invention; Fig. 4 is a reference flowchart of a fabrication method of a flexible pressure sensor provided by an embodiment of the proposed invention.

o Detailed Description The proposed invention will be further described in detail below in conjunction with the accompanying drawings and embodiments to make the objectives, technical solutions and advantages of the proposed invention clearer. It should be understood that the specific embodiments described herein are only used to explain the proposed invention, and are not intended to limit the proposed invention. It should be noted that when an element is referred to as being "fixed" or "disposed on" another element, it may be directly on another element or there may be a centring element at the same time. When an element is referred to as being "connected to" another element, it may be directly connected to the other element, or there may be a centring element at the same time. It should also be noted that the terms of left, right, top, and bottom in the embodiments of the proposed invention are only relative concepts or refer to the regular use state of the product, and should not be considered as limiting. As shown in Fig. 1, a hemispherical-microstructure-based flexible pressure sensor provided by an embodiment of the proposed invention comprises a PDMS flexible substrate 1, a carbon nanotube film 2, and a PDMS flexible film layer 3. Both the PDMS flexible substrate 1 and the PDMS flexible film layer 3 are made of PDMS. PDMS is the abbreviation of polydimethylsiloxane. It has high transparency and excellent adhesion to silicon wafers, chemical inertness, light transmittance, and biocompatibility, can be easily joint with various materials at room temperature, and has high elasticity due to low Young's modulus. The PDMS flexible substrate 1 has a microstructure 11, and the microstructure 11 has convex hemispherical microstructures, that is, the microstructure 11 may be spherically crowned. Preferably, the microstructure 11 may be hemispherical. A plurality of microstructures 11 are provided, and the plurality of microstructures 11 are integrally formed in a matrix on one side of the PDMS flexible substrate 1. The PDMS flexible substrate 1 has one side of the microstructure 11 covered with the carbon nanotube film 2, and the carbon nanotube film 2 is uniformly and fitly covering the microstructure 11 and the PDMS flexible substrate 1. Moreover, the o carbon nanotube film 2 is located between the PDMS flexible substrate 1 and the PDMS flexible film layer 3, and the carbon nanotube film 2 is connected with electrodes. The working principle of the flexible sensor is the piezoresistive effect. When the external environment applies a load to the flexible sensor, the internal hemispherical microstructure 11 deforms, and the contact area between the hemispherical microstructure 11 and the substrate decreases, making the resistance of the flexible pressure sensor smaller, resulting in an increase in current intensity. After the load is released, due to the elastic properties of PDMS, the hemispherical microstructure 11 returns to the initial state, the flexible pressure sensor can thus realise the pressure measurement by measuring the current, the measurement range and sensitivity of the flexible pressure sensor are improved, and the response time is shortened. An embodiment of the proposed invention also provides a fabrication method of a flexible pressure sensor, which can be used to fabricate the above-mentioned hemispherical-microstructure-based flexible pressure sensor, comprising the following steps: fabricate a PDMS flexible substrate 1 with a microstructure 11 in convex hemispherical microstructures; fabricate a carbon nanotube film 2; cover the carbon nanotube film 2 on the surface of the PDMS flexible substrate 1 with a microstructure 11; fabricate a PDMS flexible film layer 3 and cover the PDMS flexible film layer 3 on the carbon nanotube film 2; connect electrodes to the carbon nanotube film 2. Specifically, wherein fabricate the PDMS flexible substrate 1 comprises the following steps: as shown in Fig. 2 and Fig. 3, a silicon wafer mould 4 with a hemispherical concave microstructure 41 is produced using photolithography. The hemispherical concave microstructure 41 can be used to form the hemispherical microstructure 11; mix and stir PDMS and crosslinking agent at a weight ratio of 10: 1 to obtain a o mixed solution, and then coat the mixed solution to the silicon wafer mould 4 with a hemispherical concave microstructure 41; heat the silicon wafer mould 4 and the mixed solution; cool down the silicon wafer mould 4 and the mixed solution which forms a PDMS flexible substrate 1 after the mixed solution is cooled down and solidified 1, and separate the PDMS flexible substrate 1 from the silicon wafer mould 4 to obtain a PDMS flexible substrate 1 with a hemispherical microstructure 11. Specifically, wherein fabricating the carbon nanotube film 2 comprises the following steps: Step 1: mix a hydrogen chloride solution and a hydrogen peroxide solution to obtain a mixed solution, add carbon nanotube powders to the mixed solution, and heat them. Step 2: add the carbon nanotube in Step 1 to a dimethylformamide solution and evacuate to leak, and finally obtain a layer of the carbon nanotube film 2 attached to a leakage film. Step 3: insert the leakage film obliquely into deionised water, and a layer of carbon nanotube film 2 is separated from the carbon nanotube film 2 obtained in Step 2. Step 4: take out the carbon nanotube film 2 floating on deionised water and dry it with nitrogen flow. Specifically, in Step 3, a layer of carbon nanotube film 2 with a thickness of

50-60 nm is separated from the carbon nanotube film 2 with a thickness of 200-300 pm.

Specifically, covering the carbon nanotube film 2 on the surface of the PDMS flexible substrate 1 with a microstructure 11 comprises the following steps: transfer the carbon nanotube film 2 onto the PDMS flexible substrate 1 with a hemispherical microstructure 11 and heat. Specifically, wherein fabricating the PDMS flexible film layer 3 comprises the following steps: mix and stir PDMS and crosslinking agent at a weight ratio of 10: 1 to obtain a o solution; spin-coat the solution on the silicon wafer and heat, then cool it down, and separate the semi-cured PDMS flexible film layer 3 on the silicon wafer. Specifically, the semi-cured PDMS flexible film layer 3 is bonded to the carbon nanotube film 2 and the PDMS flexible substrate 1, and heated. Specifically, after bonding, electrodes are drawn on both sides of the carbon nanotube film 2 of the intermediate layer. Specific applications can refer to the following process, as shown in Fig. 1: Step 1: produce a silicon wafer mould 4 with a hemispherical concave microstructure 41 using photoetching technology, as shown in Fig. 2 and Fig. 3. Step 2: mix and stir polydimethylsiloxane (PDMS) and crosslinking agent at a weight ratio of 10: 1 for 10 minutes, then coat the solution to the silicon wafer mould 4 with a hemispherical concave microstructure 41, and heat at 85°C for 60 minutes. Step 3: cool down the solution obtained above at room temperature, and after the solution is solidified, separate the film from the silicon wafer mould 4 to obtain a PDMS film (PDMS flexible substrate 1) with a hemispherical microstructure 11. Step 4: mix a hydrogen chloride solution and a hydrogen peroxide solution with a ratio of 3:1, add 5 g of carbon nanotube powder to the mixed solution, and heat at 60°C for 4 hours. Step 5: add the treated carbon nanotube to a dimethylformamide solution and evacuate to leak, and finally obtain a layer of carbon nanotube film attached on a leakage film. Step 6: insert the leakage film obliquely into deionised water at an oblique angle of 45 degrees, and a layer of carbon nanotube film 2 with a thickness of 50-60 is separated from the carbon nanotube film with a thickness of 200-300 pm. Step 7: take out the carbon nanotube film 2 floating on deionised water and dry it with nitrogen flow. Step 8: transfer the carbon nanotube film 2 onto the PDMS film (PDMS flexible substrate 1) with a hemispherical microstructure 11 and heat at 200-220°C for half an hour. Step 9: mix and stir polydimethylsiloxane (PDMS) and crosslinking agent at a weight ratio of 10: 1 for 10 minutes to obtain a solution, spin-coat the solution on the silicon wafer at 900-1100 rpm and heat at 85°C for half an hour. Cool down the solution at room temperature, and separate the semi-cured PDMS film (PDMS flexible film layer 3) on the silicon wafer. Step 10: the semi-cured PDMS flexible film layer 3 is bonded to the carbon nanotube film 2 and the PDMS flexible substrate 1, and heated for 30 minutes for tight-binding (see Fig. 1). Step 11: draw electrodes on both sides of the carbon nanotube film 2 of the intermediate layer to complete the fabrication of the flexible pressure sensor. The hemispherical-microstructure-based flexible pressure sensor and a fabrication method thereof provided by embodiments of the proposed invention, of which the flexible pressure sensor adopts a hemispherical internal structure, and the measurement range of the sensor is greatly improved. Combined with carbon nanotubes with high conductivity, it has higher sensitivity. And by improving the fabrication method, the fabrication of the flexible sensor is easier and more feasible, which reduces the fabrication difficulty and labour cost, improves the manufacturing efficiency, and realises the standardised manufacturing process. The foregoing is only preferred exemplary embodiments of the proposed invention and is not intended to be limiting of the proposed invention. Any modifications, equivalent substitutions, or improvements and the like within the spirit and principles of the proposed invention are intended to be embraced by the protection range of the proposed invention.

Claims (10)

Patent Claims

1. A flexible pressure sensor based on a hemispherical microstructure, which comprises a PDMS flexible substrate, a carbon nanotube film, and a PDMS flexible film layer. The PDMS flexible substrate has convex hemispherical microstructures covered wtih carbon nanotube film. The carbon nanotube film is located between the PDMS flexible substrate and the PDMS flexible film; and the carbon nanotube film is connected with electrodes.

2. The hemispherical-microstructure-based flexible pressure sensor according to Claim 1, wherein the microstructure is hemispherical.

3. A fabrication method of a flexible pressure sensor that comprises the following steps: fabricate a PDMS flexible substrate with convex hemispherical microstructures; fabricate a carbon nanotube film; cover the convex hemispherical microstructures with the carbon nanotube film; fabricate a PDMS flexible film layer and place the PDMS flexible film layer on top of the carbon nanotube film; connect electrodes to the carbon nanotube film.

4. The fabrication method of a flexible pressure sensor according to Claim 3, wherein fabricating the PDMS flexible substrate comsists of the following steps: produce silicon wafer mould with photoresist hemispherical concave microstructure using photolithography; mix and stir PDMS and agent at a weight ratio of 10: 1 to obtain a mixed solution, and then coat the mixed solution to the silicon wafer mould with a hemispherical concave microstructure; heat the silicon wafer mould with the mixed PDMS; let the silicon wafer mould and the mixed PDMS cool down to room temperature that forms a PDMS flexible substrate layer after it is cooled and solidified, and separate the PDMS flexible substrate layer from the silicon wafer mould to obtain a PDMS flexible substrate with hemispherical microstructures.

5. The fabrication method of a flexible pressure sensor according to Claim 3 or

4, wherein fabricating the carbon nanotube film comprises the following steps: Step 1: mix a hydrogen chloride solution and a hydrogen peroxide solution to obtain a mixed solution, and add carbon nanotube powders to the mixed solution to heat; Step 2: add the carbon nanotube in Step 1 to a dimethyformamide solution and o evacuating to leak, and finally obtain a layer of carbon nanotube film attached on a leakage film. Step 3: insert the leakage film obliquely into deionised water, and a layer of carbon nanotube film is separated from the carbon nanotube film obtained in Step 2. Step 4: take out the carbon nanotube film floating on deionised water and drying it with nitrogen flow.

6. The fabrication method of a flexible pressure sensor according to Claim 5, wherein in Step 3, a layer of carbon nanotube film with a thickness of 50-60 nm is separated from the carbon nanotube film with a thickness of 200-300 pm.

7. The fabrication method of a flexible pressure sensor according to Claim 3, wherein covering the carbon nanotube film on the surface of the PDMS flexible substrate layer with a microstructure comprises the following steps: transfer the carbon nanotube film onto the PDMS flexible substrate layer with a hemispherical microstructure and heat it.

8. The fabrication method of a flexible pressure sensor according to Claim 3, wherein fabricating the PDMS flexible film layer comprises the following steps: mixand stir PDMS with crosslinking agent at a weight ratio of 10: 1 to obtain a solution; spincoat the solution on the silicon wafer, heat and then cool down, and separate the semi-cured PDMS flexible film layer on the silicon wafer.

9. The fabrication method of a flexible pressure sensor according to Claim 8, wherein the semi-cured PDMS flexible film layer is bonded to the carbon nanotube film and the PDMS flexible substrate, and then heated.

10. The fabrication method of a flexible pressure sensor according to Claim 9, wherein after bonding, electrodes are drawn on both sides of the carbon nanotube film of the intermediate layer.