A flexible humidity sensor based on inkjet printing and its preparation process

Abstract

The present invention discloses a flexible humidity sensor based on inkjet printing and its preparation process, firstly, the flexible substrate is oxidized to change its hydrophilicity, then the bottom circuit, i.e., the bottom electrode plate of the capacitor and the inductor coil, is inkjet printed thereon; after drying the bottom circuit, an insulating layer is covered on the bottom circuit based on inkjet printing technology or coating technology; then a water-absorbing layer is prepared on the insulating layer; after the water-absorbing layer is surface-modified, the top circuit, i.e., the top electrode plate of the capacitor and the lead are processed by inkjet printing technology, so that the lead is connected to the bottom circuit, the bottom electrode plate of the capacitor, the top electrode plate of the capacitor and the water-absorbing layer located therebetween together constitute a flat humidity-sensitive capacitor, and the flat humidity-sensitive capacitor and the inductor coil constitute an LC resonant circuit, and the preparation of the flexible humidity sensor is completed. The present invention has the advantages of simple structure, flexible bendability, wearable patch, etc., simple preparation process, low cost, green and environmentally friendly preparation process, and is suitable for large-scale promotion and application.

Description

Technical Field

[0001] The present invention belongs to the technical field of humidity sensors and relates to a flexible humidity sensor preparation technology, in particular to a flexible humidity sensor based on inkjet printing and a preparation process thereof. Background Art

[0002] Humidity sensors play an important role in agriculture, industry, meteorology, military and other fields. They replace traditional dry-bulb hygrometers and hair hygrometers and can measure environmental humidity stably and accurately. Common humidity sensors are mainly divided into two categories: resistive and capacitive. Their basic structure is to use semiconductor preparation technology (photolithography, sputtering, etc.) to coat a layer of humidity-sensitive material on the substrate to form a humidity-sensitive film. When the humidity-sensitive film absorbs water vapor, the impedance, relative dielectric constant and geometric dimensions of the element will change significantly, thus making a humidity-sensitive element.

[0003] Common resistive humidity sensors include metal oxide hygroresistors, silicon hygroresistors, ceramic hygroresistors, etc., which have the advantages of high precision and high sensitivity, but poor linearity and product interchangeability, and most of them are made of rigid materials. Common capacitive humidity sensors are generally made of polymer films (polystyrene, polyimide, etc.). Although their accuracy is generally lower than that of hygroresistors, their good sensitivity, linearity, interchangeability, response speed, and easy miniaturization, integration, and flexibility make humidity-sensitive capacitors the mainstream in the wearable market.

[0004] Inkjet printing is known as digital writing technology. Its working principle is to deposit a large number of tiny ink droplets produced at the nozzle directly on the substrate under digital control. The pattern of inkjet printing is defined by a digital file rather than a manually set template, so it is easy to change, with low setting cost and short time, which is very suitable for short-term printing. With the improvement of technical stability and robustness, inkjet printing has occupied an increasingly important share in the printing market and is now widely used to print variable information in production environments and even for printing customized products.

[0005] In recent years, with the vigorous development of flexible electronic devices, how to achieve flexibility, ensure sensitivity and reliability while reducing manufacturing costs, shortening R&D cycles, and reducing environmental pollution by designing sensor structures and improving material preparation processes has become a research field that has attracted much attention. The introduction of inkjet printing technology is one of the main solutions at present. Summary of the invention

[0006] In view of the shortcomings of existing humidity sensors based on semiconductor processes, such as high cost and the inability to bend the substrate, the present invention proposes a flexible humidity sensor based on inkjet printing and a preparation process thereof, which overcomes many shortcomings of the original products and processes, achieves flexibility and saves industrial and time costs.

[0007] In order to achieve the above object, the technical solution adopted by the present invention includes:

[0008] The present invention provides a flexible humidity sensor based on inkjet printing, the structure of which includes, from bottom to top:

[0009] Flexible substrates, made of biocompatible materials that are bendable but not stretchable;

[0010] A bottom circuit includes a capacitor bottom electrode plate printed on a flexible substrate by inkjet printing and an inductor coil connected to the capacitor bottom electrode plate;

[0011] Insulation layer, a flexible insulating material covering the bottom circuit;

[0012] A water-absorbing layer, which is a flexible polymer film that can absorb water and swell and is disposed on the insulating layer, and is used for sensing moisture, wherein both the insulating layer and the water-absorbing layer expose the lead ends of the inductor coil;

[0013] The top circuit includes a capacitor top electrode plate printed on the water absorbing layer based on inkjet printing, wherein the capacitor top electrode plate is provided with micropores for facilitating moisture absorption and evaporation of the water absorbing layer; the capacitor top electrode plate is connected to the inductor coil on the bottom circuit through a conductive lead;

[0014] The capacitor bottom electrode plate, the capacitor top electrode plate and the water absorbing layer located therebetween together form a flat-plate humidity-sensitive capacitor.

[0015] The insulating layer is used to separate the bottom electrode plate of the capacitor and the top electrode plate of the capacitor to prevent the capacitor from short-circuiting and breaking down.

[0016] The flat humidity-sensitive capacitor and the inductor coil together form an LC resonant circuit. When the ambient humidity increases, the water-absorbing layer absorbs water and expands, and its thickness and relative dielectric constant change, causing the flat capacitor composed of the bottom electrode plate and the top electrode plate of the capacitor to change. The resonant frequency change of the LC resonant circuit is detected by an external vector network analyzer, and the humidity response curve of the component is calibrated with reference to the experimental results to achieve the humidity measurement function.

[0017] Preferably, the capacitor bottom electrode plate and the capacitor top electrode plate are parallel to each other, relatively positioned and have the same size, thereby improving the stability and symmetry of the sensor structure.

[0018] Preferably, the water absorbing layer is made of polyvinyl alcohol (PVA) material which has good adhesion and film-forming properties and is biodegradable.

[0019] Preferably, the thickness of the bottom circuit and the top circuit processed by inkjet printing are both 100-300nm, and the optimal thickness is 200nm, so that during the preparation and packaging process, the thickness of the bottom circuit and the top circuit can be ignored, and two adjacent layers of flexible materials can be directly stacked to achieve packaging without the need for other packaging structures such as adhesives.

[0020] Preferably, the thickness of the flexible substrate is 50-100 μm, and the thickness of the insulating layer and the water absorbing layer is 200-500 nm.

[0021] Preferably, the overall thickness of the flexible humidity sensor is 10-100 μm and the area is 10-100 mm 2 .

[0022] Preferably, the biocompatible material is polyimide (PI), but is not limited thereto.

[0023] Preferably, the flexible insulating material is polydimethylsiloxane (PDMS) and / or polytrimethylene carbonate (PTMC), but is not limited thereto.

[0024] Preferably, the insulating layer and the water absorbing layer are prepared by inkjet printing or coating.

[0025] Preferably, the capacitor top electrode plate is a metal film mesh structure, the mesh holes on the metal film have a diameter of micrometer level, and the shape is not limited to square, circular, triangular, etc., and the size and spacing can be changed according to actual needs.

[0026] Preferably, both the bottom layer circuit and the top layer circuit are printed with nano silver ink.

[0027] The present invention also provides a process for preparing the flexible humidity sensor as described in any one of the above items, comprising the following steps:

[0028] S1, prepare flexible substrate;

[0029] S2, oxidizing the flexible substrate to change its hydrophilicity so that inkjet printing can be performed thereon;

[0030] S3, based on inkjet printing technology, the bottom circuit is processed, namely the bottom electrode plate of the capacitor and the inductor coil, and the two are connected;

[0031] S4, drying the bottom circuit so that it has the ability to conduct electricity after solidification;

[0032] S5, an insulating layer is covered on the bottom circuit based on inkjet printing technology or coating technology, and the lead head needs to be exposed;

[0033] S6, preparing a water absorbing layer on the insulating layer based on inkjet printing technology or coating technology, and the lead head needs to be exposed;

[0034] S7, performing oxidation treatment on the water absorbing layer to change its hydrophilicity so that inkjet printing can be performed thereon;

[0035] S8, processing the top circuit, i.e., the top electrode plate and the lead of the capacitor, based on the inkjet printing technology, and printing the lead onto the inductor coil so that the lead is connected to the bottom circuit;

[0036] S9, drying the top layer circuit so that it has the ability to conduct electricity after solidification.

[0037] Preferably, the oxidation treatment in steps S2 and S7 is performed by irradiation with an ultraviolet ozone lamp.

[0038] Preferably, the polymer material solution of the insulating layer in step S5 and the water-absorbing layer in the aforementioned step S6 is easy to clog the nozzle of the inkjet printing device, and a common coating process in semiconductor preparation can also be used instead of inkjet printing.

[0039] Preferably, a flexible protective layer may be covered on the top circuit, and a moisture-sensitive window may be provided on the flexible protective layer at a position corresponding to the capacitor top electrode plate to facilitate moisture absorption and evaporation.

[0040] The present invention also provides a use of the above-mentioned flexible humidity sensor, which is characterized by being used in conjunction with a vector network analyzer in the fields of biological surface sweat detection, robot electronic skin, curved surface humidity detection, etc.

[0041] Compared with the prior art, the beneficial effects of the present invention include:

[0042] 1. The flexible humidity sensor based on inkjet printing provided by the present invention has the advantages of simple structure, flexible bendability, wearable patch, etc., and can be processed into various shapes. In particular, the water-absorbing layer adopts a polymer film material that can absorb water and swell, and can achieve a single response to humidity with high sensitivity, high reliability and long life. It can be used in biological sweat detection, robot electronic skin, curved surface humidity detection and other fields;

[0043] 2. The preparation process of the flexible humidity sensor based on inkjet printing provided by the present invention has the advantages of low cost, environmental protection and non-toxicity. No auxiliary materials are required in the preparation process, no special process requiring large equipment is involved, no toxic reagents (such as photoresist) are used, and all consumables are used on the finished product, and no additional waste is generated;

[0044] 3. The preparation process of the humidity sensor based on inkjet printing provided by the present invention adopts fully digital manufacturing. Except for the flexible substrate, all parts are processed by inkjet printing equipment. After laying the base, it can be continuously manufactured in situ and formed in one step. Moreover, the inkjet printed pattern is defined by a digital file and is easy to modify. It has the advantages of being editable and having a short R&D cycle. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 Schematic diagram of the structure of a flexible humidity sensor based on inkjet printing in an embodiment of the present invention.

[0046] Figure 2 It is a schematic diagram of the structural decomposition of a flexible humidity sensor based on inkjet printing in an embodiment of the present invention.

[0047] Figure 3 It is a cross-sectional schematic diagram of a flexible humidity sensor based on inkjet printing in an embodiment of the present invention.

[0048] For the convenience of expression, Figure 1 and Figure 2 In the figure, the thickness of the bottom circuit and the top circuit is relatively magnified many times, but in fact the thickness is very small, and the substrate, insulating layer and water absorbing layer are all flexible, so the substrate, insulating layer and water absorbing layer are actually attached together. Figure 3 shown.

[0049] 1-flexible substrate, 2-bottom circuit, 3-insulating layer, 4-water absorbing layer, 5-top circuit, 21-capacitor bottom electrode plate, 22-inductor coil, 51-capacitor top electrode plate, 52-lead, 53-micropore. DETAILED DESCRIPTION

[0050] The device of the present invention is described in detail below with reference to the accompanying drawings.

[0051] like Figure 1 As shown, the present invention provides a flexible humidity sensor based on inkjet printing, comprising

[0052] A flexible substrate 1, made of a bendable but non-extensible biocompatible material;

[0053] The bottom circuit 2 includes a capacitor bottom electrode plate 21 printed on a flexible substrate by inkjet printing and an inductor coil 22 connected to the capacitor bottom electrode plate;

[0054] Insulation layer 3, a flexible insulating material covering the bottom circuit;

[0055] The water-absorbing layer 4 is a flexible polymer film that can absorb water and swell and is disposed on the insulating layer, and is used for sensing moisture. Both the insulating layer and the water-absorbing layer are exposed to the inductor coil;

[0056] The top circuit 5 includes a capacitor top electrode plate 51 printed on the water-absorbing layer based on inkjet printing, and the capacitor top electrode plate is provided with micropores 53 for facilitating moisture absorption and evaporation of the water-absorbing layer; the capacitor top electrode plate is connected to the inductor coil 23 on the bottom circuit through a conductive lead 52;

[0057] The capacitor bottom electrode plate 21, the capacitor top electrode plate 51 and the water absorbing layer 4 located therebetween together form a flat humidity-sensitive capacitor, and the flat humidity-sensitive capacitor and the inductor coil 22 form an LC resonant circuit. When the ambient humidity increases, the water absorbing layer 4 absorbs water and expands, and its thickness and relative dielectric constant change, so that the flat capacitor formed by the capacitor bottom electrode plate 21 and the capacitor top electrode plate 51 changes. The resonant frequency change of the LC resonant circuit is detected by an external vector network analyzer, and the humidity response curve of the component is calibrated with reference to the experimental results to achieve the humidity measurement function.

[0058] Specifically, this embodiment of the present invention includes the following steps:

[0059] (1) Cleaning and drying the 8 mm*10 mm*74 μm PI-based flexible substrate 1;

[0060] (2) The flexible substrate 1 is oxidized for 2 min using an ultraviolet ozone lamp to change its hydrophilicity so that inkjet printing can be performed on it;

[0061] (3) Printing a bottom circuit 2 with a thickness of 200 nm on a flexible substrate 1 using nanosilver ink, namely, a capacitor bottom electrode plate 21 and an inductor coil 22;

[0062] (4) Drying the bottom circuit 2 at 150°C for 30 minutes to make it conductive after curing;

[0063] (5) Using a coating machine, a 500 nm thick PDMS film, i.e., insulating layer 3, is coated on the bottom circuit 2;

[0064] (6) A layer of PVA solution is coated on the insulating layer 3 by a coating machine, and then the water in the PVA is dried at 45°C to obtain a PVA film with a thickness of 180 nm (about 360 nm after complete water absorption and swelling), i.e., the water absorption layer 4;

[0065] (7) The water-absorbing layer 4 is oxidized by an ultraviolet ozone lamp for 10 min to change its hydrophilicity so that inkjet printing can be performed on it;

[0066] (8) Printing a top circuit 5 with a thickness of 200 nm on the water-absorbing layer 4 using nanosilver ink, i.e., a capacitor top electrode plate 51 and a lead 52 having a mesh structure, and connecting the lead 52 and the inductor 22 to form an LC resonant circuit, and then drying the bottom circuit 2 at 150°C for 30 min to make it conductive after curing;

[0067] (9) The obtained flexible humidity sensor is connected to a vector network analyzer. The humidity is changed through a temperature and humidity test chamber. The resonant frequency change of the LC resonant circuit of the sensor at 10%~90%RH is measured. The humidity response curve of the component is calibrated to realize the humidity measurement function.

[0068] The flexible humidity sensor of this embodiment is soft and skin-friendly, easy to wear and apply, has a simple structure, low cost, is environmentally friendly and non-toxic, and can achieve a single response to humidity with high sensitivity, high reliability and long life. Its preparation process integrates inkjet printing technology, and has the advantages of digital manufacturing editability, short R&D cycle, and one-time molding. It is suitable for large-scale industrial production and can be applied to biological sweat detection, robot electronic skin, arc surface depth detection and other fields.

[0069] It should be pointed out that the above is only a preferred embodiment of the present invention, the purpose is to enable those skilled in the art to understand the content of the present invention and implement it accordingly, and it is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A flexible humidity sensor based on inkjet printing, characterized in that: include Flexible substrates, made of biocompatible materials that are bendable but not stretchable; A bottom circuit includes a capacitor bottom electrode plate printed on a flexible substrate by inkjet printing and an inductor coil connected to the capacitor bottom electrode plate; Insulation layer, a flexible insulating material covering the bottom circuit; A water-absorbing layer, which is a flexible polymer film capable of absorbing water and swelling, and is disposed on the insulating layer and is used for sensing moisture, wherein both the insulating layer and the water-absorbing layer are exposed to the inductor coil; The top circuit includes a capacitor top electrode plate processed on the water-absorbing layer based on inkjet printing, and the capacitor top electrode plate is provided with micropores for facilitating moisture absorption and evaporation of the water-absorbing layer; The top electrode plate of the capacitor is connected to the inductor coil on the bottom circuit through a conductive lead; The capacitor bottom electrode plate, the capacitor top electrode plate and the water absorbing layer therebetween together form a flat humidity-sensitive capacitor, and the flat humidity-sensitive capacitor and the inductor coil form an LC resonant circuit.

2. The flexible humidity sensor according to claim 1, characterized in that: The capacitor bottom electrode plate and the capacitor top electrode plate are parallel to each other, opposite in position and have the same size.

3. The flexible humidity sensor according to claim 1, characterized in that: The thickness of the bottom and top circuits based on inkjet printing is 100-300nm.

4. The flexible humidity sensor according to claim 1, characterized in that: The thickness of the flexible substrate is 50-100 μm, and the thickness of the insulating layer and the water absorbing layer is 200-500 nm.

5. The flexible humidity sensor according to claim 1, characterized in that: The overall thickness of the flexible humidity sensor is 10-100 μm and the area is 10-100 mm 2 .

6. The flexible humidity sensor according to any one of claims 1 to 5, characterized in that: The biocompatible material is polyimide.

7. The flexible humidity sensor according to any one of claims 1 to 5, characterized in that: The flexible insulating material is polydimethylsiloxane and / or polytrimethylene carbonate.

8. The flexible humidity sensor according to any one of claims 1 to 5, characterized in that: The insulating layer and the water absorbing layer are prepared by printing or coating.

9. A process for preparing the flexible humidity sensor according to any one of claims 1 to 8, characterized in that: The steps include: S1, prepare flexible substrate; S2, oxidizing the flexible substrate to change its hydrophilicity so that inkjet printing can be performed thereon; S3, based on inkjet printing technology to process the bottom circuit, namely the capacitor bottom electrode plate and inductor coil; S4, drying the bottom circuit so that it has the ability to conduct electricity after solidification; S5, covering the bottom circuit with an insulating layer based on inkjet printing technology or coating technology; S6, preparing a water absorbing layer on the insulating layer based on inkjet printing technology or coating technology; S7, performing oxidation treatment on the water absorbing layer to change its hydrophilicity so that inkjet printing can be performed thereon; S8, processing the top circuit, namely the top electrode plate and lead of the capacitor based on inkjet printing technology, so that the lead is connected to the bottom circuit.

10. The preparation process according to claim 9, characterized in that: The oxidation treatment in steps S2 and S7 is performed by irradiation with an ultraviolet ozone lamp.

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