Preparation method and application of printed flexible humidity sensor

Abstract

The present invention provides a method for preparing a printed flexible humidity sensor, comprising the following steps: first, preparing hexagonal tungsten oxide nanowires; second, preparing the hexagonal tungsten oxide nanowires with a binder and a solvent into a printing ink; finally, using silver paste to print comb-shaped interdigitated electrodes on a flexible substrate, and then covering the interdigitated electrodes with printing ink by screen printing to form a humidity sensitive layer, and drying the interdigitated electrodes to obtain a printed flexible humidity sensor. The printed flexible humidity sensor prepared by the present invention has excellent humidity sensing capability and a relatively fast response recovery speed, as well as good flexibility and repeatability. The printed flexible humidity sensor can be adhered to the inside of a package to monitor the opening state of the package, or can be directly adhered to the surface of human skin or set in a wearable device to monitor the respiratory rate of the human body. The preparation method provided by the present invention has the advantages of low cost and simple process, and is suitable for mass production and commercial application.

Description

Technical Field

[0001] The present invention belongs to the technical field of nanomaterials and functional devices, and relates to a method for preparing a fully printed flexible humidity sensor based on hexagonal tungsten oxide (h-WO3) nanowires, as well as the application of the humidity sensor in respiratory rate monitoring and package opening status monitoring. Background Art

[0002] Humidity, as a crucial environmental parameter, impacts every aspect of human life. Therefore, humidity sensors for monitoring humidity have become a widely used sensor type and have garnered increasing attention in recent years. Currently, most commercial humidity sensors are rigid and bulky, making them unsuitable for wearable applications or for expanding into other scenarios. In contrast, the flexibility and thin film nature of flexible humidity sensors facilitates adhesion to various surfaces, broadening their potential for application. These include, but are not limited to, wearable devices attached to the skin for human respiration or skin humidity monitoring, humidity-triggered contactless switches, environmental humidity monitoring, soil moisture condition monitoring, and package opening status monitoring. Given the current frequent outbreaks of influenza and the accelerated aging of society, there is a pressing need for human respiration monitoring and drug package opening status monitoring as auxiliary tools for disease diagnosis or medication compliance monitoring in the elderly.

[0003] Resistive flexible humidity sensors are currently the most suitable type of flexible humidity sensor for commercial applications due to their simple manufacturing, ease of signal acquisition, and low cost. Their structure primarily consists of a humidity-sensitive layer, an interdigitated electrode layer, and a flexible substrate layer. The humidity-sensitive layer is the most crucial component of a flexible humidity sensor. The choice of sensitive material and the preparation method for the sensing layer not only affect sensor performance but also affect actual production costs and application effectiveness. Among the many methods for preparing flexible humidity sensors, drop coating and spin coating are currently the most common. However, these methods suffer from high production costs and complex processes, making them unsuitable for industrial production applications.

[0004] Based on this, a method for preparing a high-performance resistive flexible humidity sensor that is easy to mass-produce, low-cost, simple in process, and capable of patterning is provided. This is an excellent means to meet the needs of flexible humidity sensors adapting to diverse actual production applications, and is also a technical problem that R&D personnel urgently need to solve. Summary of the Invention

[0005] One of the objectives of the present invention is to provide a method for preparing a printed flexible humidity sensor that has a fast response recovery speed, good flexibility and repeat stability and is suitable for large-scale batch production.

[0006] A second object of the present invention is to provide an application of a printed flexible humidity sensor in monitoring the opening state of a package.

[0007] A third object of the present invention is to provide a printed flexible humidity sensor for use in monitoring human respiratory rate.

[0008] The present invention achieves one of the objectives by adopting a technical solution: providing a method for preparing a printed flexible humidity sensor, comprising the following steps:

[0009] S1. Sodium tungstate dihydrate and potassium sulfate are dissolved in water at a molar ratio of 2:1, and the pH is adjusted to 1.7-2.0 with an acidic solution to obtain a mixed solution; the mixed solution is heated at 180° C. for 12-24 hours, and cooled to obtain a precipitate; the precipitate is washed, dried, and ground to obtain hexagonal tungsten oxide nanowires;

[0010] S2. Add ethyl cellulose to n-propanol and mix well to obtain an additive, and mix the hexagonal tungsten oxide nanowires, the additive, and a certain amount of n-propanol to obtain a printing ink; in the printing ink, the mass ratio of the hexagonal tungsten oxide nanowires, ethyl cellulose, and n-propanol is 9:1:20;

[0011] S3. Use silver paste to print comb-shaped interdigitated electrodes on the surface of polyethylene terephthalate, and then use the screen printing method to cover the comb-shaped interdigitated electrodes with the printing ink to form a humidity sensitive layer. After drying, a printed flexible humidity sensor is obtained.

[0012] In step S1 of the present invention, sodium tungstate dihydrate and potassium sulfate are selected as raw materials. By controlling the molar ratio of the two to 2:1 and the heating temperature to 180°C, the morphology of tungsten oxide is controlled, and the product is rendered into a hexagonal phase. Research has found that, compared with other crystalline tungsten oxide nanomaterials, hexagonal tungsten oxide nanowires can form a loose, porous network structure, which facilitates the adsorption and desorption of water molecules and is easier to formulate into ink for screen printing, making them a high-performance humidity-sensitive material. Furthermore, the humidity sensor prepared from this material exhibits a fast response recovery rate, good flexibility and stability, and low hysteresis.

[0013] Furthermore, in step S1, adjusting the pH to 1.7-2.0 is also necessary to ensure the production of hexagonal tungsten oxide nanowires. A pH that is too low will result in the formation of hexagonal tungsten oxide nanorods. Preferably, adjusting the pH to 1.7 results in the best humidity sensing performance for the sensor.

[0014] In step S2 of the present invention, ethyl cellulose is used as a binder. It is water-insoluble and has low hygroscopicity, and will not affect the performance of the humidity sensor due to excessive swelling due to moisture absorption. n-Propanol is used as a solvent for the humidity-sensitive ink. Its boiling point is about 97.4°C, which is more difficult to volatilize than other organic solvents (ethanol, terpineol, etc.), which is beneficial to ensuring the stability of the ink during use and storage. It can also be removed without high-temperature evaporation, simplifying the preparation process.

[0015] Furthermore, the printing ink requires a controlled mass ratio of hexagonal tungsten oxide nanowires, ethyl cellulose, and n-propanol of 9:1:20. This ratio is crucial for ensuring the direct printing of the prepared humidity-sensitive ink. Extensive research has shown that increasing the amount of tungsten oxide raw material causes the ink to dry out, while an excessive amount of n-propanol solution thins the ink, and an excessive amount of ethyl cellulose additives makes the ink more viscous. These changes in conditions can affect the subsequent screen printing process, making it difficult to evenly coat the surface of the comb-shaped interdigitated electrodes, thus severely impacting the performance of the humidity sensor.

[0016] In step S3 of the present invention, the use of polyethylene terephthalate as the sensor substrate material makes the prepared humidity sensor flexible, allowing it to be attached to a variety of surfaces and also meeting the processing requirements of directly printing the sensor on packaging. Furthermore, the present invention utilizes screen printing technology to produce humidity sensors with different patterns by varying the shape of the interdigitated electrodes and the humidity-sensitive layer. This allows for customized sensor patterning based on product requirements, and facilitates low-cost, industrialized, and large-scale production of humidity sensors.

[0017] Furthermore, in step S1, the acidic solution is selected from hydrochloric acid with a concentration of 2 to 3 mol / L. Preferably, the concentration of the hydrochloric acid is 3 mol / L.

[0018] Furthermore, in step S1, the drying is carried out in a vacuum environment, the drying temperature is 40 to 60°C, and the drying time is 1 to 24 hours. The specific drying time can be adjusted so that the material is dried to a powder state. The drying temperature here should not exceed 60°C, otherwise it may cause changes in the material (such as the content of oxygen defects, etc.), which will adversely affect the humidity sensing performance. Preferably, the drying temperature is 60°C and the drying time is 4 hours.

[0019] Furthermore, in step S2, the mass fraction of ethyl cellulose in the additive is 5% to 10%.

[0020] Furthermore, in step S3, the screen printing uses a screen printing plate with a mesh size of 150 to 300. Preferably, a screen printing plate with a mesh size of 300 is used to obtain higher printing accuracy.

[0021] Preferably, the comb-shaped interdigitated electrodes are dried at 80-110°C for 10-30 minutes before being covered with ink. After printing the humidity-sensitive layer, the drying process is performed at room temperature for 4-48 hours, with the specific drying time adjusted based on actual conditions to ensure complete evaporation of the solvent (n-propanol) in the ink.

[0022] Furthermore, in step S3, the sensor response recovery speed can be improved by adjusting the thickness of the humidity sensitive layer. In the present invention, the thickness of the prepared single-layer printed humidity sensitive layer is 10-20 μm, preferably 15 μm.

[0023] The technical solution adopted by the present invention to achieve the second purpose is: to provide an application of a printed flexible humidity sensor, wherein the printed flexible humidity sensor is prepared according to the preparation method described in one of the purposes of the present invention, and the application includes: adhering the printed flexible humidity sensor into the package to monitor the opening status of the package.

[0024] Furthermore, the printed flexible humidity sensor operates in a relative humidity range of 11% to 95% RH.

[0025] The technical solution adopted by the present invention to achieve the third purpose is: to provide an application of a printed flexible humidity sensor, wherein the printed flexible humidity sensor is prepared according to the preparation method described in one of the purposes of the present invention, and the application includes: adhering the printed flexible humidity sensor to the surface of human skin or setting it in a wearable device for monitoring the human respiratory rate.

[0026] Furthermore, the printed flexible humidity sensor has a response time of 1.5s and can identify different respiratory states of the human body and monitor respiratory frequency.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] (1) The present invention provides a method for preparing a printed flexible humidity sensor, which uses hexagonal tungsten oxide nanowires as a humidity-sensitive material. This material is easy to prepare, low-cost, and has high humidity sensitivity. Its microscopic state and the final screen-printed film-forming structure can form a loose and porous structure, which is conducive to the adsorption and desorption of water molecules, reducing the response and recovery time of the sensor, and is conducive to application scenarios requiring rapid response and recovery. Furthermore, the present invention can prepare humidity sensors with different patterns by changing the shape of the interdigitated electrodes and the humidity-sensitive layer with the help of screen printing technology. The sensor pattern can be customized according to product requirements, and it is convenient to prepare humidity sensors at low cost, industrially, and in large quantities.

[0029] (2) The present invention provides a method for preparing a printed flexible humidity sensor, in which hexagonal tungsten oxide nanowires are screen-printed to prepare the humidity sensor. By optimizing the type and ratio of the binder and solvent, the hexagonal tungsten oxide nanowires are evenly dispersed in the ink, and the ink viscosity is moderate, ensuring smooth screen printing. In addition, the use of ethyl cellulose as a binder has the advantage of low hygroscopicity, and the high boiling point of the solvent n-propanol can enhance the storage stability and application stability of the humidity-sensitive ink. Polyethylene terephthalate, as a substrate, is flexible and can be adhered to a variety of surfaces for use. It can also meet the processing requirements of directly printing the sensor on packaging. The selection of these materials comprehensively improves the application performance of the humidity sensor.

[0030] (3) The printed flexible humidity sensor produced by the present invention is attached to the inside of a package to monitor the package's opening status. It exhibits rapid response speed and stability in a relative humidity range of 11% to 95%. The printed flexible humidity sensor can be attached to the inside of a medicine bottle cap as a humidity monitoring component to monitor the humidity inside the bottle. It responds to changes in the humidity environment inside the bottle caused by opening the bottle, thereby reflecting the package's opening status and number of times it has been opened.

[0031] (4) The printed flexible humidity sensor prepared by the present invention can be adhered to the surface of human skin or set in a wearable device (such as a mask) to monitor the human respiratory rate. It has the advantages of fast response speed and good cycle stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a dynamic response curve diagram of the printed flexible humidity sensor prepared in Example 1 of the present invention to different humidity conditions within the range of 11% to 95% RH;

[0033] Figure 2 This is a response diagram of the printed flexible humidity sensor prepared in Example 1 of the present invention when monitoring the opening state of the package;

[0034] Figure 3 This is a response diagram of the printed flexible humidity sensor prepared in Example 1 of the present invention in monitoring the respiratory rate of the human body. DETAILED DESCRIPTION

[0035] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0036] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0037] The present invention will be further described below with reference to specific examples, but they are not intended to limit the present invention.

[0038] Example 1

[0039] A method for preparing a printed flexible humidity sensor comprises the following steps:

[0040] Step 1: Synthesis of hexagonal tungsten oxide nanowires: Dissolve 2.31g of sodium tungstate dihydrate and 0.61g of potassium sulfate in 50mL of deionized water. Add 3M hydrochloric acid solution to adjust the pH of the mixture to 1.7. The mixture is then placed in a 100mL polytetrafluoroethylene-lined reactor and reacted at 180°C for 12 hours. After the mixture cools to room temperature, the precipitate is washed by centrifugation with deionized water and anhydrous ethanol, and then dried in a vacuum at 60°C for 4 hours. The resulting powder is collected, ground, and stored for later use.

[0041] Step 2: Preparation of hexagonal tungsten oxide nanowire humidity-sensitive ink: Dissolve 1g of ethyl cellulose in 19g of n-propanol solution to obtain a 5% by mass ethyl cellulose-n-propanol mixed solution as an ink additive; stir and mix 0.27g of hexagonal tungsten oxide nanowires, 0.6g of the additive solution, and 0.03g of n-propanol, so that the mass ratio of hexagonal tungsten oxide, ethyl cellulose, and n-propanol is 9:1:20, to obtain a humidity-sensitive ink suitable for screen printing.

[0042] Step 3: Preparation of a flexible humidity sensor. The specific method is as follows: use the screen printing method to print a comb-shaped interdigitated electrode pattern on polyethylene terephthalate using a silver paste through a 300-mesh screen printing plate, and dry it in a 90°C oven for 30 minutes; then, print the hexagonal tungsten oxide nanowire humidity-sensitive ink prepared in the above step on the interdigitated electrode pattern using a 300-mesh screen printing plate, and dry it at room temperature for 24 hours to obtain a flexible humidity sensor, wherein the thickness of the humidity-sensitive layer formed by the humidity-sensitive ink is 15 μm.

[0043] Example 2

[0044] A method for preparing a printed flexible humidity sensor comprises the following steps:

[0045] Step 1: Synthesis of hexagonal tungsten oxide nanowires: Dissolve 1.155g of sodium tungstate dihydrate and 0.305g of potassium sulfate in 20mL of deionized water. Add 2M hydrochloric acid solution dropwise to adjust the pH of the mixed solution to 1.7. The mixed solution is then placed in a 50mL polytetrafluoroethylene-lined reactor and reacted at 180°C for 12 hours. After the mixed solution cools to room temperature, the precipitate is washed by centrifugation with deionized water and anhydrous ethanol, and then dried in a vacuum at 60°C for 4 hours. The resulting powder is collected, ground, and stored for later use.

[0046] Step 2: Preparation of hexagonal tungsten oxide nanowire humidity-sensitive ink: Dissolve 0.5g of ethyl cellulose in 5.75g of n-propanol solution to obtain an 8% by mass ethyl cellulose-n-propanol mixed solution as an ink additive; stir and mix 0.36g of hexagonal tungsten oxide nanowires, 0.5g of the additive solution, and 0.34g of n-propanol, so that the mass ratio of hexagonal tungsten oxide, ethyl cellulose, and n-propanol is 9:1:20, to obtain a humidity-sensitive ink suitable for screen printing.

[0047] Step 3: Preparation of a flexible humidity sensor. The specific method is as follows: use the screen printing method to print a comb-shaped interdigitated electrode pattern on polyethylene terephthalate using a silver paste through a 200-mesh screen printing plate, and dry it in an oven at 100°C for 15 minutes; then, print the hexagonal tungsten oxide nanowire humidity-sensitive ink prepared in the above step on the interdigitated electrode pattern using a 200-mesh screen printing plate, and dry it at room temperature for 24 hours to obtain a flexible humidity sensor, wherein the thickness of the humidity-sensitive layer formed by the humidity-sensitive ink is 10 μm.

[0048] Example 3

[0049] A method for preparing a printed flexible humidity sensor comprises the following steps:

[0050] Step 1: Synthesis of hexagonal tungsten oxide nanowires: Dissolve 2.31g of sodium tungstate dihydrate and 0.61g of potassium sulfate in 50mL of deionized water. Add 2.5M hydrochloric acid solution dropwise to adjust the pH of the mixed solution to 2.0. The mixed solution is then placed in a 100mL polytetrafluoroethylene-lined reactor and reacted at 180°C for 16 hours. After the mixed solution cools to room temperature, the precipitate is washed by centrifugation with deionized water and anhydrous ethanol, and then dried in a vacuum at 50°C for 6 hours. The resulting powder is collected, ground, and stored for later use.

[0051] Step 2: Preparation of hexagonal tungsten oxide nanowire humidity-sensitive ink: Dissolve 1g of ethyl cellulose in 9g of n-propanol solution to obtain a 10% by mass ethyl cellulose-n-propanol mixed solution as an ink additive; stir and mix 0.27g of hexagonal tungsten oxide nanowires, 0.3g of the additive solution, and 0.33g of n-propanol, so that the mass ratio of hexagonal tungsten oxide, ethyl cellulose, and n-propanol is 9:1:20, to obtain a humidity-sensitive ink suitable for screen printing.

[0052] Step 3: Preparation of a flexible humidity sensor. The specific method is as follows: use the screen printing method to print a comb-shaped interdigitated electrode pattern on polyethylene terephthalate using a silver paste through a 150-mesh screen printing plate, and dry it in an oven at 110°C for 12 minutes; then, print the hexagonal tungsten oxide nanowire humidity-sensitive ink prepared in the above step on the interdigitated electrode pattern through a 150-mesh screen printing plate, and dry it at room temperature for 36 hours to obtain a flexible humidity sensor, wherein the thickness of the humidity-sensitive layer formed by the humidity-sensitive ink is 20 μm.

[0053] Application Example 1

[0054] The printed flexible humidity sensor prepared in Example 1 is used to monitor the opening status of a package. The specific method of use is as follows:

[0055] The printed flexible humidity sensor is attached to the inside of the package, such as the inside of the bottle cap of the medicine bottle packaging, to monitor the changes in humidity conditions inside the medicine bottle. When the medicine bottle is opened, the humidity environment inside the bottle changes, and the sensor will generate a corresponding response to reflect the current opening status of the medicine bottle (opening state and number of times). The sensor response is as follows: Figure 2 shown.

[0056] Depend on Figure 1 It can be seen that the humidity sensor prepared by the present invention has a fast response speed, high sensitivity and stability to different humidity conditions within the relative humidity range of 11 to 95%. Figure 2 It can be seen that the humidity sensor made by the present invention can be used as a medicine bottle humidity monitoring component attached to the inside of the bottle cap to monitor the humidity inside the medicine bottle, and respond to the changes in the humidity environment inside the bottle caused by opening the medicine bottle, thereby reflecting the opening status and number of times the package has been opened.

[0057] Application Example 2

[0058] The printed flexible humidity sensor prepared in Example 1 is used to monitor the respiratory rate of a human body. The specific method of use is as follows:

[0059] The printed flexible humidity sensor can be attached to the lower part of the philtrum of the human head, or to the inner side of the mask near the nostril to monitor the human respiratory rate. The humidity sensor can identify different breathing states of the human body, such as normal breathing, fast and slow breathing, and apnea. The sensor response is as follows: Figure 3 shown.

[0060] Depend on Figure 3 It can be seen that the printed flexible humidity sensor prepared by the present invention can effectively identify different respiratory states, such as normal breathing, rapid breathing, slow breathing and apnea, and has a short response time (1.5s). It can accurately monitor each breath and different respiratory rates and has good cyclic stability. It can be adhered to the surface of human skin or the inside of a mask to monitor the respiratory rate of the human body under different respiratory states.

[0061] In summary, the printed flexible humidity sensor provided by the present invention combines hexagonal oxide nanowires with a screen printing process. Compared to currently available conventional commercial humidity sensors, the present invention produces a flexible resistive sensor. The flexible substrate enables adhesion to a variety of surfaces, and the circuit structure for outputting the resistance signal is simple, making it easy to monitor. This humidity sensor has advantages such as fast response speed, good cycle stability, and high measurement accuracy. It performs well in monitoring package opening status and human respiratory rate. Furthermore, the printed flexible humidity sensor has a simple preparation process, is easy to mass-produce, and has broad prospects for promotion and application.

[0062] The above are only preferred embodiments of the present invention and do not limit the implementation mode and protection scope of the present invention. For those skilled in the art, it should be aware that all solutions obtained by equivalent substitutions and obvious changes made using the contents of the present invention specification should be included in the protection scope of the present invention.

Claims

1. A method for preparing a printed flexible humidity sensor, characterized in that: The following steps are involved: S1. Sodium tungstate dihydrate and potassium sulfate are dissolved in water at a molar ratio of 2:1, and the pH is adjusted to 1.7-2.0 with an acidic solution to obtain a mixed solution; the mixed solution is heated at 180° C. for 12-24 hours, and cooled to obtain a precipitate; the precipitate is washed, dried at 40-60° C. under vacuum for 1-24 hours, and then ground to obtain hexagonal tungsten oxide nanowires; S2. Ethyl cellulose is added to n-propanol and mixed to obtain an additive having a mass fraction of 5% to 10% of ethyl cellulose; and hexagonal tungsten oxide nanowires, the additive, and n-propanol are mixed in a mass ratio of 9:1:20 to obtain a printing ink. S3. Use silver paste to print comb-shaped interdigitated electrodes on the surface of polyethylene terephthalate, use a 150-300 mesh screen printing plate to print the printing ink on the comb-shaped interdigitated electrodes to form a 10-20 μm thick humidity sensitive layer, and dry it to obtain a printed flexible humidity sensor.

2. The preparation method according to claim 1, wherein In step S1, the acidic solution is selected from hydrochloric acid with a concentration of 2-3 mol / L.

3. Application of a printed flexible humidity sensor, wherein the printed flexible humidity sensor is prepared by the preparation method according to claim 1 or 2, characterized in that: The printed flexible humidity sensor is adhered to the inside of the package to monitor the opening status of the package.

4. The use according to claim 3, characterized in that The printed flexible humidity sensor operates in a relative humidity range of 11% to 95% RH.

5. Application of a printed flexible humidity sensor, wherein the printed flexible humidity sensor is prepared by the preparation method according to claim 1 or 2, characterized in that: The printed flexible humidity sensor is adhered to the surface of human skin or arranged in a wearable device to monitor the respiratory rate of the human body for non-diagnostic and therapeutic purposes.

6. The use according to claim 5, characterized in that The response time of the printed flexible humidity sensor is 1.5 s.

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