Flexible humidity sensor based on color inkjet printing

Abstract

This invention discloses a flexible humidity sensor based on color inkjet printing. A humidity-sensitive material and a pair of parallel electrodes are disposed on a flexible film. The humidity-sensitive material is located between the pair of electrodes to form a humidity-sensitive resistor. First, a pair of electrodes is prepared on the flexible film. Then, the humidity-sensitive material is inkjet printed on the flexible film with the prepared pair of electrodes. A color inkjet printer is used to print the humidity-sensitive material on the flexible film. Multiple ink cartridges of the color inkjet printer are filled with solutions of different humidity-sensitive materials.

Description

Technical Field

[0001] This invention relates to a flexible humidity sensor based on color inkjet printing. Background Technology

[0002] Conventional humidity sensors are divided into two types: resistive and capacitive. The basic form of both is a moisture-sensitive film formed by coating a substrate with a moisture-sensitive material. When water vapor in the air is adsorbed onto the moisture-sensitive material, the impedance and dielectric constant of the element change significantly, thus creating a humidity-sensitive element.

[0003] Humidity-sensitive resistors are made based on the principle that the resistance of a humidity-sensitive material changes when it absorbs moisture from the air. A key feature of humidity-sensitive resistors is a film made of a moisture-sensitive material coated on a substrate. When water vapor in the air is adsorbed onto this film, the resistivity and resistance of the element change, allowing for the measurement of humidity.

[0004] An important characteristic parameter of humidity sensors is their humidity range. Humidity range refers to the maximum range of ambient humidity that a humidity sensor can accurately measure.

[0005] Because different humidity sensors use different materials and operate on different principles, their characteristics are not universally applicable across the entire relative humidity range of 0–100%RH. They typically have a narrow suitable measurement range; beyond this range, the humidity measurement error increases rapidly, and the reliability of the measurement results decreases drastically. In practice, it is difficult to manufacture humidity sensors with a wide effective humidity sensing range.

[0006] For more detailed background technology and inventive principles, please refer to our prior invention patent application "Wide Measurement Range Resistive Humidity Sensor", application number: CN202111158676X, which will not be elaborated here; the contents of the prior invention patent application are regarded as part of the contents of this application by reference, and we reserve the right to incorporate part or all of its contents into this application.

[0007] In order to solve the above-mentioned technical problems, the following invention is proposed. Summary of the Invention

[0008] The purpose of this invention is to provide a flexible humidity sensor based on color inkjet printing. The resulting humidity sensor is a resistive humidity sensor with a wide measurement range. This flexible humidity sensor maintains accuracy while achieving a wide humidity measurement range, and possesses advantages such as being lightweight, flexible, and easily bendable, allowing for large-area fabrication. Furthermore, it reduces manufacturing costs and the complexity of the humidity sensor. The specific technical solution to achieve the purpose of this invention is as follows:

[0009] A flexible humidity sensor based on color inkjet printing includes a humidity-sensitive material, a flexible thin film, and a pair of electrodes. The humidity-sensitive material is fabricated on the flexible thin film using color inkjet printing technology; the pair of electrodes are also fabricated on the flexible thin film. The humidity-sensitive material has water-absorbing properties, absorbing moisture from the ambient air, causing its resistivity to change with the moisture content.

[0010] To achieve a wide range of humidity measurement, the core technical means adopted in this invention are as follows:

[0011] By using color inkjet printing technology, the composition of the humidity-sensitive material gradually changes along the length of the electrode, resulting in a non-uniform distribution. This causes the resistivity-humidity characteristic curve of the humidity-sensitive material to gradually change along the length of the electrode.

[0012] Alternatively, color inkjet printing technology can be used to make the specific surface area of ​​the humidity-sensitive material gradually change along the length of the electrode, resulting in a non-uniform distribution. This causes the resistivity-humidity characteristic curve of the humidity-sensitive material to gradually change along the length of the electrode.

[0013] Alternatively, color inkjet printing technology can be used to make the nanoscale size of the humidity-sensitive material gradually change along the length of the electrode, resulting in a non-uniform distribution. This causes the resistivity-humidity characteristic curve of the humidity-sensitive material to gradually change along the length of the electrode.

[0014] Alternatively, color inkjet printing technology can be used to gradually change the microstructure of the humidity-sensitive material along the length of the electrode, making it non-uniform, so that the resistivity-humidity characteristic curve of the humidity-sensitive material gradually changes along the length of the electrode.

[0015] Of course, it is also possible to make two or more of the components, specific surface area, nanoscale size, and microstructure of the humidity-sensitive material gradually change along the length of the electrode, exhibiting non-uniformity, so that the resistivity-humidity characteristic curve of the humidity-sensitive material gradually changes along the length of the electrode.

[0016] Along the length of the electrode, the resistivity-humidity characteristic curve of the humidity-sensitive material shows a gradual change, indicating that:

[0017] If the lead-out end of either electrode in a pair of electrodes defined on a flexible thin film for connecting the measurement circuit is the near end, and the other end along the electrode away from the near end is the far end, then: from the near end to the far end, the resistivity-humidity characteristic curve of the humidity-sensitive material shows a gradual change.

[0018] At the near end, a humidity-sensitive material can be selected to make it sensitive to low humidity air at that location; a humidity-sensitive material at the middle location will be sensitive to medium humidity air; and at the far end, a humidity-sensitive material can be selected to make it sensitive to high humidity air at that location.

[0019] More specifically, in this invention, along the length of the electrode, i.e. from the proximal end to the distal end, the resistivity-humidity characteristic curve of the humidity-sensitive material gradually changes, such that the humidity-sensitive material at different locations sensitively responds to different ambient humidity levels from the proximal end to the distal end.

[0020] As is common knowledge in this field, changes in the composition, specific surface area, nanoscale size, or microstructure of humidity-sensitive nanomaterials will all lead to changes in the resistivity-humidity characteristic curve of the humidity-sensitive material, thus making it sensitive to air with different humidity levels.

[0021] Of course, in some embodiments of the present invention, the electrode can also be arranged along its length, i.e. from the proximal end to the distal end. In other words, the humidity-sensitive material can also be clearly divided into multiple segments with different properties, and different segments have different resistivity-humidity characteristic curves.

[0022] However, in the above situation, it is not that the humidity-sensitive material other than the one at position x cannot respond to changes in ambient humidity. In fact, it will also respond to changes in humidity to a certain extent, but the humidity sensitivity is relatively low or very low. Therefore, overall, it is mainly the resistance at position x that can sensitively change with humidity.

[0023] When the ambient humidity changes significantly, the humidity-sensitive material at location x no longer has the optimal humidity sensitivity relative to the new humidity. Instead, the humidity-sensitive material at location x' has the optimal humidity sensitivity relative to the new humidity. In this situation, the resistance of the humidity-sensitive material at location x will be outside its optimal humidity range, and its resistance will no longer change significantly with humidity. Overall, the resistance at location x' will be the primary point of sensitivity to humidity changes.

[0024] In practice, in order to enable the humidity-sensitive material at different locations to respond sensitively to different ambient humidity levels, the composition of the humidity-sensitive material can be gradually changed along the electrode direction from the near end to the far end.

[0025] The principle of this invention is explained as follows:

[0026] A component made of a humidity-sensitive material exhibits resistance that is highly sensitive to ambient air humidity within its effective humidity-sensing range. When the humidity is below its effective range, the resistance increases rapidly, approaching that of an insulating material; conversely, when the humidity exceeds this range, the resistance becomes very small. In other words, regardless of whether the ambient humidity is below or above the effective humidity-sensing range, the resistance of the humidity-sensitive material is unlikely to reflect changes in ambient humidity. Only within the effective humidity-sensing range can the resistance of the humidity-sensitive material change significantly and sensitively with variations in ambient humidity. Each manufactured sensor typically has a narrow measurement range; therefore, an appropriate range should be selected according to the required measurement range.

[0027] For example, some humidity sensors have a suitable humidity measurement range (effective sensing range) of 10-30%RH to meet accuracy requirements, with an error of ±2%RH. Outside this range, the resistance of the humidity-sensitive material is either very high or very low, making it difficult to accurately reflect the ambient humidity. Other humidity sensors have a suitable humidity measurement range (effective sensing range) of 30-55%RH. Outside this range, the measurement value becomes unreliable. Still others have an effective sensing range of 70-90%RH.

[0028] In this invention, in order to achieve a gradual change in the resistivity-humidity characteristic curve of the humidity-sensitive material along the length of the electrode, color inkjet printing technology is used to make the composition and / or specific surface area and / or nanoscale size and / or microstructure of the humidity-sensitive material at different locations gradually change, so that the humidity-sensitive material at different locations along the length of the electrode responds sensitively to different environmental humidity.

[0029] In all embodiments of the present invention, the flexible humidity sensor has the following characteristic: along the length direction of the electrode, the resistivity-humidity characteristic curve of the humidity-sensitive material gradually changes, specifically:

[0030] In a pair of electrodes on a flexible thin film, the lead-out terminal of either electrode used to connect to the measurement circuit is defined as the proximal end, and the other end of the electrode away from the proximal end is defined as the distal end. The distance from any point on the electrode to the proximal end along the electrode direction is defined as... Then point Surface resistivity-humidity function of humidity-sensitive materials The partial derivative of surface resistivity with humidity Let x represent the humidity sensitivity of the surface resistivity at point x as a function of humidity. This represents the function curve showing the relationship between the humidity sensitivity of a humidity-sensitive material and the humidity h. The peak value of the function curve represents the optimal humidity sensitivity of the humidity-sensitive material at point x. Therefore: from the near end to the far end, the humidity-sensitive material... The peak value of the function curve varies with Gradually moving; in the above formula R Indicates the surface resistivity of humidity-sensitive materials. Indicates humidity.

[0031] For example, in some embodiments, the composition of the humidity-sensitive material is gradually varied along the length of the electrode, so that the composition of the humidity-sensitive material at different locations is different, resulting in a gradual change in the resistivity-humidity characteristic curve of the humidity-sensitive material.

[0032] As is well known, humidity-sensitive materials with different compositions, different micro-nano structures, and different specific surface areas will exhibit different optimal humidity sensing ranges. The specific sensing range can be determined by rationally selecting the humidity-sensitive materials at the near and far ends based on actual conditions.

[0033] Because different humidity-sensitive materials possess different humidity-sensitive characteristics, meaning each has its own resistivity-humidity characteristic curve, when the composition of the humidity-sensitive material changes at different locations, the resistivity-humidity characteristic curve of the humidity-sensitive material will gradually change along the length of the electrode. This gradual change in the resistivity-humidity characteristic curve allows the humidity-sensitive resistor between a pair of electrodes to sensitively respond to a wide range of humidity changes.

[0034] In this invention, a pair of electrodes, from proximal to distal, have humidity-sensitive materials at different locations that respond sensitively to varying ambient humidity levels. Therefore, when the humidity sensor is located in a low-humidity environment, not all humidity-sensitive materials at any point between the electrodes will respond sensitively to changes in humidity, resulting in changes in resistance. Instead, the humidity-sensitive material at a specific location x has optimal sensitivity relative to that humidity level, allowing the resistance at location x to change sensitively with humidity. When the humidity-sensitive materials between the electrodes of the flexible humidity sensor are configured in a pre-selected manner, the resistivity-humidity characteristic curve of the humidity-sensitive material gradually changes along the length of the electrodes, from proximal to distal, ensuring that the humidity-sensitive materials at different locations respond sensitively to varying ambient humidity levels from proximal to distal.

[0035] By employing the above-described configuration, a flexible humidity sensor can respond to a wide range of humidity variations, thereby achieving a broad humidity measurement range. As can be seen from the core concept of this invention, it provides a resistive flexible humidity sensor with a wide measurement range. While maintaining accuracy, it achieves a broad humidity measurement range, and color inkjet printing technology can further reduce the manufacturing cost and complexity of the flexible humidity sensor, making production easier.

[0036] The inventors have now explained the working principle, technical solution, and technical effects of this invention. Detailed Implementation

[0037] To facilitate understanding of the present invention, the technical solution of the present invention will be specifically described below with reference to examples.

[0038] A flexible humidity sensor based on color inkjet printing includes a humidity-sensitive material, a flexible film, and a pair of electrodes; both the electrodes and the humidity-sensitive material are fabricated on the flexible film. The humidity-sensitive material has water-absorbing properties, absorbing moisture from the ambient air, causing its resistivity to change with the moisture content. A color inkjet printer is used to print the humidity-sensitive material on the flexible film, with multiple ink cartridges of the printer containing solutions of different humidity-sensitive materials.

[0039] Before inkjet printing, a distribution image of humidity-sensitive materials is pre-designed, with different colors representing different humidity-sensitive materials; different colored ink cartridges are filled with solutions of different humidity-sensitive materials.

[0040] The flexible film is made of any one of the following materials: polyvinyl alcohol (PVA), polyester (PET), polyimide (PI), polyethylene naphthalate (PEN), and polydimethylsiloxane (PDMS).

[0041] First, a pair of electrodes are prepared on a flexible film, and then a humidity-sensitive material is printed on the flexible film with the pair of electrodes by inkjet printing. The method is characterized by using a color inkjet printer to print the humidity-sensitive material on the flexible film, wherein multiple ink cartridges of the color inkjet printer are filled with solutions of different humidity-sensitive materials, specifically referring to the different properties of the humidity-sensitive materials in different ink cartridges, such as composition, specific surface area, nanoscale size, and microstructure.

[0042] In this invention, multiple ink cartridges of a color inkjet printer are filled with solutions of different humidity-sensitive materials. A pre-designed color gradient image is printed, with different colors representing different humidity-sensitive materials. The specific color representing which humidity-sensitive material can be specified in advance as needed.

[0043] A pair of electrodes can be prepared using vacuum evaporation, magnetron sputtering, chemical electroplating, or other methods, which are not limited here.

[0044] Electrode materials can be metals, conductive organic polymers, or other conductive pastes.

[0045] Preferably, color inkjet printing uses a continuous ink supply system.

[0046] The characteristics of the color inkjet printing process are:

[0047] Along the length of the electrode, the humidity-sensitive materials from different ink cartridges are sequentially inkjet printed onto a flexible film;

[0048] Alternatively, along the length of the electrode, humidity-sensitive materials from different cartridges are printed on a flexible film at gradually varying concentrations, causing the resistivity-humidity characteristic curve of the humidity-sensitive material to show a continuous change. That is, the properties of the humidity-sensitive material change continuously, rather than being divided into several segments.

[0049] Along the length of the electrode, different humidity-sensitive materials from different ink cartridges are sequentially inkjet printed onto a flexible film, resulting in the humidity-sensitive material being divided into multiple segments with different properties, each segment having a different resistivity-humidity characteristic curve.

[0050] The humidity-sensitive material is an organic composite material, an inorganic composite material, or an organic / inorganic composite material. The resistivity of the humidity-sensitive material changes with humidity.

[0051] In practical implementation, a pair of electrodes is first fabricated on a flexible thin film, and then a humidity-sensitive material is printed using color inkjet printing on the flexible thin film with the electrodes. In the completed flexible humidity sensor, the pair of electrodes is positioned between the humidity-sensitive material and the flexible thin film, with the surface of the humidity-sensitive material completely exposed to the environment and not covered by the electrodes. Compared to the method of fabricating the humidity-sensitive material on the flexible thin film first and then fabricating the pair of electrodes, this embodiment of the invention results in a humidity sensor with higher sensitivity.

[0052] In this invention, a pair of electrodes consists of two “parallel” electrodes, which can be of various types, such as two parallel straight electrodes, comb-shaped electrodes, interdigitated electrodes, double-helix electrodes, loop-shaped electrodes, or serpentine electrodes.

[0053] Preferably, the surface area of ​​the electrode is 1 / 3 to 2 / 3 of the surface area of ​​the humidity-sensitive material.

[0054] Preferably, the flexible humidity sensor of the present invention further includes a resistance measurement module for monitoring the change in resistance of the humidity-sensitive material between a pair of electrodes, thereby obtaining the ambient humidity value.

[0055] In some embodiments, the humidity-sensitive material is an organic material; in some embodiments, the humidity-sensitive material is an inorganic material; and in some embodiments, the humidity-sensitive material is an organic / inorganic composite material.

[0056] In some embodiments, along the length of the electrode, i.e., from the proximal end to the distal end, the humidity-sensitive material is clearly divided into multiple segments with different properties, each segment having a different resistivity-humidity characteristic curve. Along the length of the electrode, humidity-sensitive materials from different ink cartridges are sequentially inkjet printed onto a flexible film, resulting in the humidity-sensitive material being divided into multiple segments with different properties, each segment having a different resistivity-humidity characteristic curve.

[0057] In some embodiments, the humidity-sensitive material is selected as a nanoparticle material. Along the length of the electrode, the specific surface area of ​​the humidity-sensitive material nanoparticle material is set to gradually change, so that the specific surface area of ​​the humidity-sensitive material at different locations is different, resulting in a gradual change in the resistivity-humidity characteristic curve of the humidity-sensitive material.

[0058] For example, solutions of TiO2 nanorods with diameters of 100 nm, 200 nm, and 400 nm are injected into different ink cartridges of a color inkjet printer, respectively. Three segments of humidity-sensitive material are then sequentially printed along the length of the electrode using inkjet printing and dried. The resulting humidity-sensitive materials are: TiO2 nanorods with diameters of 100 nm, 200 nm, and 400 nm. Clearly, the change in diameter of the TiO2 nanorods at the microscale means a gradual change in specific surface area, which leads to different resistivity-humidity characteristic curves. This allows the humidity sensor composed of three different TiO2 nanorod segments to respond to a wider range of humidity changes than a sensor using any single-diameter TiO2 nanorod.

[0059] In this invention, the humidity-sensitive material can also be an organic composite material, an inorganic composite material, or an organic / inorganic composite material. These humidity-sensitive materials are all composite materials, and the resistivity of the composite material changes with humidity.

[0060] In this invention, the composite material refers to a composite of humidity-sensitive nanomaterials and non-humidity-sensitive nanomaterials. Alternatively, the composite material refers to a composite of highly humidity-sensitive nanomaterials and low humidity-sensitive nanomaterials.

[0061] In some implementations, the humidity sensor can also be a humidity sensor with a wide measurement range due to changes in the composition of the humidity-sensitive material. For example, a solution of WO3 nanomaterials and a solution of TiO2 nanowires are injected into two ink cartridges of a color inkjet printer, respectively. An image with a color gradient from near to far end is pre-designed on a computer. The color inkjet printer then prints WO3 nanomaterials with gradually increasing concentration and TiO2 nanomaterials with gradually decreasing concentration along the length of the electrodes. This results in a humidity sensor with a wide measurement range due to changes in the composition of the humidity-sensitive material.

[0062] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A flexible humidity sensor based on color inkjet printing, a humidity sensitive material and a pair of parallel electrodes are provided on a flexible film, the humidity sensitive material is located between the pair of electrodes to form a humidity sensitive resistor, characterized in that: The humidity sensitive material is printed on the flexible film by using a color inkjet printer, different humidity sensitive material solutions are injected into multiple ink cartridges of the color inkjet printer, and a pre-designed color image is used to make the physical or chemical properties of the humidity sensitive material gradually change along the length direction of the electrode, so that the humidity sensitive material is non-uniformly distributed, and the resistivity-humidity characteristic curve of the humidity sensitive material gradually changes along the length direction of the electrode, and the humidity sensitive material at different positions has different optimal humidity sensing sensitivities; the obtained flexible humidity sensor is a wide-measurement-range resistive humidity sensor; The physical or chemical properties of the humidity sensitive material gradually change along the length direction of the electrode, and the humidity sensitive material is non-uniformly distributed, and the composition, specific surface area, nanoscale size, and microstructure of the humidity sensitive material at different positions along the length direction of the electrode change, and the humidity sensitive material is non-uniformly distributed; The humidity sensitive material is printed on the flexible film by using a color inkjet printer, different humidity sensitive material solutions are injected into multiple ink cartridges of the color inkjet printer, and the composition, specific surface area, nanoscale size, and microstructure of the humidity sensitive material in different ink cartridges are different; the color inkjet printing adopts a continuous ink supply system.

2. The flexible humidity sensor of claim 1, wherein: The material of the flexible film is any one of polyvinyl alcohol (PVA), polyester (PET), polyimide (PI), polyethylene naphthalate glycol (PEN), and polydimethylsiloxane (PDMS).

3. The flexible humidity sensor of claim 2, wherein: Along the length direction of the electrode, the resistivity-humidity characteristic curve of the humidity sensitive material gradually changes, which means that: The lead-out end of any one of the pair of electrodes defined on the flexible film for connecting the measuring circuit is the proximal end, the other end of the electrode away from the proximal end is the distal end, and the length of any point on the electrode away from the proximal end along the direction of the electrode is The surface resistivity of the humidity-sensitive material at point is a function of humidity The partial derivative of the surface resistivity with respect to humidity represents the humidity sensitivity of the surface resistivity at point x with respect to humidity, and then represents the functional curve between the humidity sensitivity of the humidity-sensitive material and humidity h, The peak of the functional curve represents the optimal humidity sensitivity of the humidity-sensitive material at point x, and then: from the proximal end to the distal end, the The peak of the functional curve gradually moves with ; in the above formula R represents the surface resistivity of the humidity-sensitive material, represents the humidity; The humidity sensitive material is non-uniformly distributed from the proximal end to the distal end, such that the peak of the function curve of the humidity sensitive material moves gradually. moves gradually.

4. The flexible humidity sensor of claim 1, wherein: The pair of electrodes are double-helix electrodes, parallel straight-line electrodes, comb-shaped electrodes, interdigital electrodes, snake-shaped electrodes, or hairpin-shaped electrodes.

5. A method of manufacturing the flexible humidity sensor according to any one of claims 1 to 4, wherein a pair of electrodes is first prepared on a flexible film, and then a humidity sensitive material is inkjet printed on the flexible film prepared with the pair of electrodes, characterized in that: The humidity sensitive material is printed on the flexible film by using a color inkjet printer, different humidity sensitive material solutions are injected into multiple ink cartridges of the color inkjet printer, and the composition, specific surface area, nanoscale size, and microstructure of the humidity sensitive material in different ink cartridges are different; The inkjet printing step is characterized in that: Along the length direction of the electrode, the humidity sensitive material in different ink cartridges is sequentially inkjet-printed on the flexible film; Alternatively, along the length direction of the electrode, the humidity sensitive material in different ink cartridges is printed on the flexible film at gradually changing concentrations.

6. The method of claim 5, wherein: Along the length direction of the electrode, the humidity sensitive material in different ink cartridges is sequentially inkjet-printed on the flexible film, so that the humidity sensitive material is divided into multiple segments with different properties, and different segments have different resistivity-humidity characteristic curves.

7. The method of claim 5, wherein: Along the length direction of the electrode, the humidity sensitive material in different ink cartridges is printed on the flexible film at gradually changing concentrations, so that the resistivity-humidity characteristic curve of the humidity sensitive material continuously changes, that is, the properties of the humidity sensitive material continuously change.

8. The method of claim 5, wherein: Before inkjet printing, a distribution image of the humidity sensitive material is pre-designed, different colors represent different humidity sensitive materials, and different colors of ink cartridges are injected with different humidity sensitive material solutions.