Flexible respiration sensor based on resistance-type humidity sensing and preparation method

Abstract

The invention discloses a flexible respiration sensor based on resistance-type humidity sensing and a preparation method, the flexible respiration sensor comprises a flexible support substrate, a vanadium dioxide sensing material layer and a metal interdigital electrode layer which are sequentially arranged from bottom to top, and during preparation, vanadium dioxide nano powder with a large specific surface area is synthesized through a liquid phase reduction method and a hydrothermal method; dissolving the vanadium dioxide nano powder into an organic solvent to prepare a vanadium dioxide sensing material solution, and attaching the vanadium dioxide sensing material solution to a flexible support substrate by a solution coating method; and finally, depositing a metal interdigital electrode on the vanadium dioxide sensing layer in a thermal evaporation manner. According to the invention, effective monitoring of different breathing modes of a human body can be realized, the different breathing modes of the human body have high sensitivity, ultra-fast response speed (less than 0.5 s) and stable tracking and detection, and meanwhile, the system has high stability and durability, and has good application prospects.

Description

Technical Field

[0001] The present invention relates to a flexible respiration sensor based on resistive humidity sensing and a preparation method thereof, belonging to the technical field of human respiration monitoring. Background Art

[0002] In recent years, the incidence of respiratory diseases has been increasing year by year. Respiratory diseases include emphysema, asthma, chronic obstructive pulmonary disease, obstructive sleep apnea, etc., which have seriously affected the lives and health of patients. Therefore, accurate respiration monitoring has important clinical significance for evaluating and analyzing physical health conditions and preventing respiratory diseases. Currently, the devices used for respiration monitoring mainly include spirometers, polysomnographs, and end-tidal carbon dioxide concentration monitors, etc. These methods can ensure accurate recording and evaluation of various parameter indicators in respiratory activities, but they are costly, have poor portability, high usage thresholds, and complex detection processes, requiring high-end equipment and professional detection personnel.

[0003] With the development of wearable electronic technology, flexible electronic sensors have attracted much attention in the field of personal healthcare. It can convert various stimuli into electrical signals, providing a new way for respiratory monitoring. Among them, flexible humidity sensors show great application prospects in fields such as wearable electronic systems and non-contact sensing. Especially in personal healthcare applications, the effective monitoring of the humidity of the human body's surrounding environment has attracted increasing attention. Its working principle is that when the sensitive material is exposed to an environment with higher humidity, its electrical properties (such as resistance) will change, and measuring the change in electrical properties can determine the change in environmental humidity. Water vapor is one of the most important components in human exhaled gas, so it can be used as an important indicator for discriminating respiratory patterns (including respiratory rate and intensity). Currently, the reported humidity sensing materials mainly include graphene oxide, carbon nanotubes / carbon fibers, TiO2, In2O3, ZnO, and SnO2, etc. Although these materials have high humidity sensitivity, there are obvious deficiencies. Due to its excellent hydrophilicity, the conductive composite material of graphene oxide is easily swollen by water vapor and loses its dimensional stability, so it is easy to lose durability in actual use. Moreover, most of the reported humidity sensors have a response time of up to several seconds or even hundreds of seconds, which will limit their ability to capture sudden changes in respiratory signals. In addition, due to the cumulative effect, for rapid respiratory signals, there will be an obvious drift in the overall change of the recorded data. A flexible respiratory sensor based on vanadium dioxide (B-phase) thin film and its preparation method disclosed in the patent document CN 106308802 A overcome the defects of large measurement errors, complex preparation processes, and difficult applications of VO2 nanowire sensors. However, this respiratory sensor uses a polymer-assisted deposition method to prepare a mixed solution containing vanadium complex as a precursor solution, and then forms a film by spin-coating and heat treatment on a SiO2 / Si substrate. There is a risk of polymer residue in its preparation process, which affects the film-forming uniformity, the film thickness is limited, and it is difficult to prepare a micron-thick film. When mass-producing, the solution stability and the consistency of the coating process need to be strictly controlled, and the cost of polymer reagents and manufacturing costs are relatively high. Summary of the Invention

[0004] In view of this, the purpose of the present invention is to provide a flexible respiratory sensor based on chemiresistive humidity sensing of vanadium dioxide (B-phase), which has high sensitivity, ultrafast response, high stability, and low manufacturing cost to solve the problems in the above background technology.

[0005] The purpose of the present invention is achieved through the following technical solutions:

[0006] A flexible respiration sensor based on resistive humidity sensing, comprising a flexible support substrate, a vanadium dioxide sensing material layer, and a metal interdigital electrode layer arranged in sequence from bottom to top. The vanadium dioxide sensing material layer is used to monitor water molecules in human respiration, and the metal interdigital electrode layer serves as the source and drain for electrical signal output.

[0007] A preparation method of a flexible respiration sensor based on resistive humidity sensing, comprising the following steps:

[0008] Step S1: Synthesize an active semiconductor humidity sensing material with a large specific surface area, namely vanadium dioxide nano-powder, by the liquid-phase reduction method + hydrothermal method;

[0009] Step S2: Dissolve the obtained vanadium dioxide nano-powder into an organic solvent to prepare a vanadium dioxide sensing material solution;

[0010] Step S3: Attach the vanadium dioxide sensing material solution to the flexible support substrate by the solution coating method;

[0011] Step S4: Deposit a metal interdigital electrode on the vanadium dioxide sensing layer by thermal evaporation.

[0012] Further, in step S1, the liquid-phase reduction method + hydrothermal method specifically includes:

[0013] Add a reducing agent, deionized water to the vanadium source, and synthesize a vanadium dioxide nano-solution according to certain process parameters; after the vanadium dioxide nano-solution reacts in a hydrothermal environment for a certain time, perform repeated centrifugation, filtration, and washing with absolute ethanol, and then naturally dry to obtain vanadium dioxide nano-powder.

[0014] Further, the mass ratio of the reducing agent, vanadium source, and deionized water is 1:0.8 - 1:180 - 200. The reducing agent is oxalic acid or hydrazine hydrate, and the vanadium source is vanadium pentoxide or ammonium metavanadate. Oxalic acid or hydrazine hydrate is used as the reducing agent, and vanadium pentoxide or ammonium metavanadate is used as the vanadium source. Add them to deionized water in sequence and continuously stir magnetically to prepare a uniform yellow-green vanadium dioxide nano-solution.

[0015] Further, the temperature and time ranges for the reaction of the vanadium dioxide nano-solution in the hydrothermal environment are 180 - 230 °C and 24 - 36 h, respectively.

[0016] Further, in step S2, the organic solvent is absolute ethanol. Dissolve the obtained vanadium dioxide nano-powder into absolute ethanol, and then perform ultrasonic treatment for no less than 30 min to prepare a uniform vanadium dioxide suspension.

[0017] Further, in step S3, the coating method of the vanadium dioxide nanomaterial includes solution impregnation, spraying, drop coating or spin coating; the thickness of the coated vanadium dioxide nanomaterial thin film ranges from 1 to 5 μm; the material of the flexible support substrate is selected from polyimide (PI), polydimethylsiloxane (PDMS) or polyethylene terephthalate (PET), and the thickness is ≥ 0.2 mm.

[0018] Further, the specific coating steps of the vanadium dioxide sensing layer material include:

[0019] S3.1. Clean the flexible support substrate with acetone, ethanol and deionized water, and dry it naturally.

[0020] S3.2. Preheat the flexible support substrate and heat the flexible support substrate to 50 - 80 °C.

[0021] S3.3. Coat the vanadium dioxide sensing layer material onto the surface of the heated flexible support substrate to form a continuous, dense and uniform-thickness vanadium dioxide nanobelt network structure, and dry it naturally.

[0022] Further, in step S4, use an electron beam evaporation or thermal evaporation device to deposit a gold interdigital electrode on the vanadium dioxide sensing layer, and the size of the gold interdigital electrode matches the area of the vanadium dioxide sensing layer.

[0023] Further, the metal interdigital electrode layer is a conventional metal layer or a composite metal layer, and the thickness is ≥ 30 nm.

[0024] Compared with the prior art, the beneficial effects of the present invention are:

[0025] (1) The flexible respiration sensor based on resistive humidity sensing prepared by the present invention can effectively monitor different human respiration modes (normal respiration, rapid respiration and deep respiration), has high sensitivity to different human respiration modes, an ultra-fast response speed (< 0.5 s), as well as stable tracking and detection, and at the same time has high stability and durability.

[0026] (2) The present invention uses a liquid phase reduction method + hydrothermal method to prepare an active semiconductor material with a large specific surface area, namely vanadium dioxide (B-phase) nanobelts. Its preparation method has a simple process and low cost, can precisely control the size and morphology of vanadium nanoparticles, and ensure the directional growth of vanadium dioxide in a hydrothermal environment, promote the crystallization of the vanadium dioxide (B-phase) nanostructure, reduce defects, and improve chemical stability and activity. And the prepared flexible respiration sensor based on resistive humidity sensing has a wide relative humidity range detection and high sensitivity (its relative humidity detection range reaches 26% RH - 93% RH, and the sensitivity reaches more than 163.4%), and has good application prospects.

[0027] Other advantages, objects, and features of the present invention will be set forth in part in the description which follows, and in part will be obvious to those having ordinary skill in the art upon examination of the following, or may be learned from practice of the invention. The objects and other advantages of the invention may be realized and attained by the means of the instrumentalities and combinations particularly pointed out hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to make the objects, technical solutions, and advantages of the present invention clearer, the present invention will be described in further detail below with reference to the accompanying drawings, where:

[0029] Figure 1 Schematic diagram of the structure of a flexible respiration sensor based on resistive humidity sensing provided for an embodiment of the present invention;

[0030] Figure 2 Structural diagram of the synthesized vanadium dioxide (B-phase) sample;

[0031] Figure 3 Raman spectrum of the synthesized vanadium dioxide (B-phase) sample;

[0032] Figure 4 XRD pattern of the synthesized vanadium dioxide (B-phase) sample;

[0033] Figure 5 XPS spectrum of the synthesized vanadium dioxide (B-phase) sample;

[0034] Figure 6 Relationship curve of the resistance change of the flexible respiration sensor with time under different relative humidities;

[0035] Figure 7 Relationship curve of the bending curvature of the flexible respiration sensor with the resistance change;

[0036] Figure 8 Relationship curve of the resistance change of the flexible respiration sensor for respiration detection with time;

[0037] Figure 9 Change curve of the response time and recovery time of the flexible respiration sensor for respiration detection within a complete respiration cycle;

[0038] Figure 10 Relationship curve of the resistance change of the flexible respiration sensor for different respiration detections (normal respiration, rapid respiration, and deep respiration) with time;

[0039] Figure 11 Relationship curve of the resistance change of the flexible respiration sensor with time during an 80-s continuous respiration test. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0040] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts belong to the scope of protection of the present invention.

[0041] As Figure 1 shown, the first embodiment of the present invention provides a flexible respiratory sensor based on resistive humidity sensing, which includes a flexible support substrate, a vanadium dioxide sensing material layer, and a metal interdigital electrode layer arranged in sequence from bottom to top. The vanadium dioxide sensing material layer is used to monitor water molecules in human breath, and the metal interdigital electrode layer serves as the source and drain for electrical signal output.

[0042] A flexible respiratory sensor based on resistive humidity sensing and its preparation method are as follows:

[0043] Step S1: Synthesize an active semiconductor humidity sensing material with a large specific surface area, namely vanadium dioxide (B-phase) nanostructure, by the liquid-phase reduction method + hydrothermal method. Specifically, weigh ~0.16 g of oxalic acid as a reducing agent and ~0.14 g of vanadium pentoxide as a vanadium source, and add them to 30 mL of deionized water in sequence, and continuously stir magnetically until a uniform yellow-green solution is formed; then transfer the solution to the quartz inner liner of the reaction kettle, raise the temperature to 180 - 230 °C, and keep it for 24 - 36 h; after the reaction kettle naturally cools to room temperature, centrifuge the above solution at a speed of 6000 - 8000 rpm, filter, and wash with absolute ethanol, and repeat this process three times; finally, dry naturally to obtain blue-black vanadium dioxide (B-phase) powder.

[0044] In step S1, vanadium nanoparticles are reduced by the liquid-phase reduction method, which can precisely control the size and morphology of vanadium nanoparticles. During the reduction process, magnetic stirring can promote the uniform mixing of reactants, accelerate mass transfer, avoid local high concentration or precipitation aggregation, and the particles have good dispersibility; then the hydrothermal method is used to use the vanadium nanoparticles synthesized by the liquid-phase method as "seeds" to guide the directional growth of vanadium dioxide in a hydrothermal environment, promote the crystallization of vanadium dioxide (B-phase) nanostructure, reduce defects, improve chemical stability and activity, and control the morphology of vanadium dioxide (B-phase) nanostructure to increase the specific surface area. Specifically, the morphology of the vanadium dioxide (B-phase) nanostructure includes granular, one-dimensional belt / rod / fiber-like, and two-dimensional sheet-like.

[0045] The morphology and structure characterization of the synthesized vanadium dioxide (B-phase) sample are as Figures 2 - 5As shown in the figure, the synthesized vanadium dioxide (phase B) sample presents a nanoribbon structure with an average length and width of about 2.5 μm and 68 nm, respectively; the peaks at 138, 278, 403, 680, and 986 cm -1 The peak positions can be attributed to the characteristic Raman peaks of vanadium dioxide (phase B) (520 cm -1 is the Raman peak of silicon, used for peak position calibration); the diffraction peaks (110), (002), (-401), (003) and (020) in the XRD spectrum are consistent with the reported characteristic peaks of vanadium dioxide (B phase); the V 2p peak with a binding energy of 516.36 eV in the XPS spectrum is consistent with the reported characteristic peaks of vanadium dioxide (B phase). 3 / 2 The peak was assigned to V 4+ , indicating that the synthesized material is high-purity vanadium dioxide (phase B).

[0046] Step S2: Prepare a vanadium dioxide (phase B) sensing material solution. Weigh an appropriate amount of the vanadium dioxide (phase B) powder obtained in step 1, place it in a centrifuge tube, add an appropriate amount of anhydrous ethanol to redissolve it, and then perform ultrasonic treatment (time ~30min) to form a uniform vanadium dioxide suspension. The concentration of this solution can be controlled by the amount of solute vanadium dioxide (phase B) powder and solvent anhydrous ethanol, and the concentration is ~5mg / mL. -1 .

[0047] Step S3: Use a flexible polyimide (PI) material as the device substrate. Before applying the vanadium dioxide (Phase B) sensing layer material, clean the PI substrate with acetone, ethanol, and deionized water and allow it to air dry. Subsequently, use a micropipette to draw an appropriate amount (40-80 μL) of the vanadium dioxide (Phase B) solution from Step 2 and apply it to the surface of the PI substrate heated to 50-80°C. The preheated substrate better absorbs the sensing nanomaterial, forming a continuous, dense, and uniformly thick vanadium dioxide (Phase B) nanoribbon network structure, which is then air-dried.

[0048] In addition, the flexible substrate material may also be polydimethylsiloxane (PDMS), polyethylene terephthalate (PET), etc., with a thickness of ≥0.2 mm.

[0049] Step S4: Depositing metal electrodes: Using electron beam evaporation or thermal evaporation equipment, combined with a metal interdigitated electrode template whose size matches the area of the sensing layer, deposit 50 nm thick gold interdigitated electrodes on the vanadium dioxide (B phase) sensing layer.

[0050] In step S4, the metal interdigital electrode layer is a conventional metal layer, such as gold; or a composite metal layer such as gold / nickel, gold / chromium, gold / titanium, etc., with a thickness of ≥30 nm.

[0051] Based on the above steps S1-S4, the structure of the prepared vanadium dioxide (phase B) flexible respiratory sensor is as followsFigure 1 As shown, its bottom layer is a flexible substrate such as PI, the middle layer is a vanadium dioxide (B-phase) nanobelt sensing layer, and the top layer is a metal interdigital electrode connected to an external circuit.

[0052] Humidity response test of the sensor. The sensor was successively placed in relative humidity environments of 26%RH, 37%RH, 52%RH, 73%RH, 84%RH, and 93%RH (the reference relative humidity was 10%RH), and the response performance of the vanadium dioxide (B-phase) sensor at different relative humidities was obtained, as Figure 6 shown. The sensitivity of the humidity sensor is defined as:

[0053] S = (R - R0 / R0) × 100%,

[0054] where R0 is the resistance value of the vanadium dioxide (B-phase) sensing layer at a reference relative humidity of 10%RH, and R is the resistance value at the tested relative humidity (26%, 37%, 52%, 73%, 84%, 93%). The test results show that the resistance of the device increases significantly with the increase of relative humidity, indicating that the sensor has a wide relative humidity detection range (26% - 93%RH) and a high sensitivity of 163.4%.

[0055] The mechanical robustness of the sensor was evaluated using a bending test platform. As Figure 7 shown, when the bending curvature increased from 0° to 20°, the resistance fluctuation of the sensor was always lower than 12%, much lower than the response sensitivity of 163.4% at high humidity. This is due to the stress buffer layer design between the flexible PI substrate and the gold electrode, ensuring the integrity of the vanadium dioxide (B-phase) sensing layer structure in actual applications, indicating that the sensor has good mechanical robustness.

[0056] Respiratory sensing performance test of the vanadium dioxide (B-phase) sensor. The respiratory sensor was attached below the volunteer's nasal cavity, and the real-time change of the sensor resistance with respiration was observed at room temperature. As Figure 8 shown, when the volunteer exhaled (Out), the resistance of the vanadium dioxide (B-phase) sensitive layer increased rapidly; while when inhaling (In), the resistance quickly returned to the initial resistance.

[0057] As Figures 9 - 10 shown, in order to accurately calculate the response time and recovery time of the sensor to respiration, a complete respiratory cycle was selected. The response time and recovery time of the sensor to respiration are respectively defined as the time required for the signal to change between the specified lower limit (10%) and upper limit (90%). The calculated result shows that the response time t response = 0.32s, and the recovery time t recovery= 0.41 s, indicating the ultrafast response of the respiration sensor to the breathing process. The response time and recovery time of the prepared VO₂ (B-phase) nanobelt network respiration sensor are both within 0.5 s, which is sufficient to detect human respiration at different breathing frequencies (normal breathing, rapid breathing, and deep breathing).

[0058] As Figure 11 shown, during the 80-s respiration test, the change amplitude of the sensor resistance with the breathing cycle is stable and the signal drift is very small, indicating that the prepared sensor has good reliability as a long-term respiration monitoring device.

[0059] In summary, the present invention uses a hydrothermal method to prepare an active semiconductor material with a large specific surface area, VO₂ (B-phase) nanobelts, and develops a VO₂ (B-phase) flexible respiration sensor based on chemiresistive humidity sensing. With a wide relative humidity detection range (26% - 93% RH) and high sensitivity (163.4%), it can achieve high sensitivity, ultrafast response speed (<0.5 s), and stable tracking and detection of different human breathing patterns (normal breathing, rapid breathing, and deep breathing).

[0060] The above are only the preferred embodiments of the present invention, and do not impose any form of confidentiality restrictions on the present invention. Any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical content of the present invention and the technical essence of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A flexible respiration sensor based on resistive humidity sensing, characterized in that, It includes a flexible support substrate, a vanadium dioxide sensing material layer, and a metal interdigital electrode layer arranged in sequence from bottom to top. The vanadium dioxide sensing material layer is used to monitor water molecules in human breath, and the metal interdigital electrode layer serves as the source and drain for electrical signal output.

2. A preparation method of a flexible respiration sensor based on resistive humidity sensing, characterized in that, It includes the following steps: Step S1: Synthesize an active semiconductor humidity sensing material with a large specific surface area, namely vanadium dioxide nanopowder, by the liquid-phase reduction method + hydrothermal method; Step S2: Dissolve the obtained vanadium dioxide nanopowder into an organic solvent to prepare a vanadium dioxide sensing material solution; Step S3: Attach the vanadium dioxide sensing material solution to the flexible support substrate by the solution coating method; Step S4: Deposit metal interdigital electrodes on the vanadium dioxide sensing layer by thermal evaporation.

3. The preparation method according to claim 2, characterized in that, In step S1, the liquid-phase reduction method + hydrothermal method specifically includes: Adding a reducing agent and deionized water to a vanadium source, synthesizing a vanadium dioxide nano-solution according to certain process parameters; after the vanadium dioxide nano-solution reacts in a hydrothermal environment for a certain time, perform repeated centrifugation, filtration, and washing with absolute ethanol, and naturally dry to obtain vanadium dioxide nanopowder.

4. The preparation method according to claim 3, characterized in that, The mass ratio of the reducing agent, vanadium source, and deionized water is 1:0.8 - 1:180 - 200. The reducing agent is oxalic acid or hydrazine hydrate, and the vanadium source is vanadium pentoxide or ammonium metavanadate. Using oxalic acid or hydrazine hydrate as the reducing agent and vanadium pentoxide or ammonium metavanadate as the vanadium source, add them to deionized water in sequence and continuously stir magnetically to prepare a uniform yellow-green vanadium dioxide nano-solution.

5. The preparation method according to claim 3, characterized in that, The temperature and time ranges for the reaction of the vanadium dioxide nano-solution in the hydrothermal environment are 180 - 230 °C and 24 - 36 h, respectively.

6. The preparation method according to claim 2, wherein, In step S2, the organic solvent is absolute ethanol. Dissolve the obtained vanadium dioxide nanopowder into absolute ethanol, and then perform ultrasonic treatment for a treatment time of not less than 30 min to prepare a uniform vanadium dioxide suspension.

7. The preparation method according to claim 2, characterized in that, In step S3, the coating method of the vanadium dioxide nano-material includes solution impregnation, spraying, drop coating, or spin coating; the thickness of the coated vanadium dioxide nano-material thin film is 1 - 5 μm; the material of the flexible support substrate is selected from polyimide (PI), polydimethylsiloxane (PDMS), or polyethylene terephthalate (PET), and the thickness is ≥0.2 mm.

8. The preparation method according to claim 7, characterized in that, The specific coating steps of the vanadium dioxide sensing layer material include: S3.1: Clean the flexible support substrate with acetone, ethanol, and deionized water, and naturally dry; S3.2: Preheat the flexible support substrate and heat the flexible support substrate to 50 - 80 °C; S3.3: Coat the vanadium dioxide sensing layer material onto the surface of the heated flexible support substrate to form a continuous, dense, and uniform-thickness vanadium dioxide nanobelt network structure, and naturally dry.

9. The preparation method according to claim 2, wherein, In step S4, use an electron beam evaporation or thermal evaporation device to deposit gold interdigital electrodes on the vanadium dioxide sensing layer, and the size of the gold interdigital electrodes matches the area of the vanadium dioxide sensing layer.

10. The preparation method according to claim 9, characterized in that, The metal interdigital electrode layer uses a conventional metal layer or a composite metal layer, and the thickness is ≥30 nm.

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