Wide-range flexible humidity sensing material, preparation method and application thereof

Abstract

The application discloses a wide-range flexible humidity sensing material, a preparation method and application. The material is composed of a flexible polymer matrix and at least two hygroscopic salts dispersed therein, and the total mass percentage of the salts is 5-40%, preferably, LiCl and CaCl2 / MgCl2 are compounded at a ratio of 1:5-5:1, the salt distribution form is uniform dispersion, concentration gradient or core-shell structure, and the matrix is PVA hydrogel, PAAm hydrogel, gelatin or PDMS. The application also protects the preparation method, a flexible humidity sensor containing the material and application. The material realizes wide-range monitoring of 15-95% RH by means of the synergistic effect of multiple hygroscopic salts and spatial distribution design, combined with the fixed protection of the polymer matrix, and has high sensitivity, fast response, excellent linearity and flexibility, good stability, solves the technical pain points of a single hygroscopic salt sensor, and can be applied to intelligent wear, respiratory monitoring equipment and electronic skin.

Description

Technical Field

[0001] This invention discloses a flexible humidity sensing material for high-sensitivity, wide-range humidity detection and its preparation method, belonging to the field of flexible electronic materials and sensor technology. Background Technology

[0002] Flexible humidity sensors, as devices capable of sensing changes in external humidity, can measure the physical changes caused by these humidity variations and convert them into electrical signals or other easily detectable signals. Their detection of external humidity relies on sensitive materials, such as polymers, metal oxides, and hygroscopic salts.

[0003] Hygroscopic salts such as LiCl have attracted much attention in humidity sensors, but single LiCl hygroscopic salts have their limitations. LiCl-based flexible humidity sensors suffer from insufficient sensitivity and slow response in specific humidity ranges, or poor stability in complex environments. Fuzhou University successfully prepared a highly sensitive, highly elastic, and environmentally stable organic gel humidity sensor by utilizing the dual-network gel structure of polyvinyl alcohol and polyacrylamide, combined with the synergistic effect of lithium chloride and MXene (Advanced Functional Materials, 2024, 34(38): 2402853). However, there is still a lack of flexible sensing materials that can effectively utilize the synergistic effect of multiple hygroscopic salts to achieve a wide range and high linearity humidity response. Summary of the Invention

[0004] To address the above-mentioned shortcomings, this invention provides a wide-range flexible humidity sensing material and its preparation method.

[0005] This invention is achieved through the following technical solution:

[0006] A wide-range flexible humidity sensing material is characterized in that it comprises a flexible polymer matrix and at least two different hygroscopic salts dispersed therein, wherein the hygroscopic salts are distributed in a uniform dispersion, a concentration gradient distribution, or a core-shell structure distribution, and the total mass of the hygroscopic salts accounts for 5% to 40% of the total mass of the sensing material, wherein the total mass of the sensing material specifically refers to the mass of the composite material composed of the flexible polymer matrix and all the hygroscopic salts.

[0007] Preferably, the at least two different hygroscopic salts are lithium chloride (LiCl) and calcium chloride (CaCl2), wherein the mass ratio of LiCl to CaCl2 is 1:5 to 5:1.

[0008] Preferably, the at least two different hygroscopic salts are lithium chloride (LiCl) and magnesium chloride (MgCl2), wherein the mass ratio of LiCl to MgCl2 is 1:5 to 5:1.

[0009] Preferably, the hygroscopic salt exhibits a concentration gradient distribution within the flexible polymer matrix, wherein the concentration gradient distribution is a continuous transitional distribution from a LiCl-rich region to a CaCl2-rich or MgCl2-rich region.

[0010] Preferably, the hygroscopic salt and the polymer matrix together constitute core-shell structured microspheres, and the hygroscopic salts of the core layer and the shell layer are the two hygroscopic salts described in claim 2 or 3, wherein one hygroscopic salt is LiCl and is enriched in the shell layer, and the other is CaCl2 or MgCl2 and is enriched in the core layer.

[0011] Preferably, the flexible polymer matrix is ​​polyvinyl alcohol (PVA) hydrogel, polyacrylamide (PAAm) hydrogel, gelatin, or polydimethylsiloxane (PDMS).

[0012] This invention also provides a method for preparing the above-mentioned wide-range flexible humidity sensing material, comprising the following steps:

[0013] (1) Dissolve the at least two hygroscopic salts in deionized water to form a mixed salt solution;

[0014] (2) The polymer monomer or prepolymer is mixed with the mixed salt solution to form a uniform precursor solution. The precursor solution is structured according to the distribution pattern of the hygroscopic salt. If it is a concentration gradient distribution, it is poured layer by layer. If it is a core-shell structure, it is prepared by microfluidic technology. If it is a uniform dispersion, it is directly mixed.

[0015] (3) After the precursor solution is vacuumed and defoamed, it is poured onto the interdigitated electrode substrate that has been cleaned with anhydrous ethanol and dried with nitrogen. It is then solidified by physical crosslinking or chemical crosslinking and dried at room temperature after solidification. The physical crosslinking is either freeze-thaw cycle crosslinking or room temperature air drying crosslinking, and the chemical crosslinking is either thermally initiated polymerization crosslinking or photoinitiated polymerization crosslinking.

[0016] The present invention further protects a flexible humidity sensor comprising the above-mentioned sensing material, the sensor comprising a flexible substrate, electrodes disposed on the flexible substrate, and the above-mentioned wide-range flexible humidity sensing material covering the electrodes.

[0017] This flexible humidity sensor can be applied to the fabrication of smart wearable devices for monitoring human activity, respiratory monitoring devices for monitoring respiratory rate and pattern, and flexible electronic skin for sensing environmental humidity.

[0018] Compared with the prior art, the advantages of the present invention are as follows:

[0019] Hygroscopic salts are responsible for capturing water molecules in the environment and causing changes in ionic conductivity. LiCl and CaCl2 are themselves highly hygroscopic, and their presence is the fundamental reason for generating humidity response signals. The flexible polymer matrix is ​​responsible for fixing and dispersing the hygroscopic salts, preventing their deliquescence and loss. It provides mechanical flexibility and stretchability, allowing the material to conform to skin or curved surfaces. It forms ion transport channels. Adsorbed water molecules dissociate from the salts in the polymer network, forming freely moving ions, thereby changing the material's conductivity. It protects sensitive components and improves the sensor's environmental stability and durability. By cleverly combining specific types and proportions of hygroscopic salts with flexible polymers, a novel composite material with a wide measurement range, high sensitivity, and high flexibility is created, producing a synergistic effect of "1+1>2".

[0020] The shell and core layers are enriched with different types of hygroscopic salts. The shell is enriched with LiCl, while the core layer is enriched with CaCl2 or MgCl2. The greatest advantage of LiCl is its extremely strong hygroscopic capacity and ionic conductivity changes even in low relative humidity environments; it begins to form hydrates at ~11% RH. CaCl2 or MgCl2 are far less sensitive than LiCl in low humidity conditions, but they have a huge hygroscopic capacity in high humidity environments, especially CaCl2, which can form various hydrates until it completely deliquesces.

[0021] In low-humidity environments, water molecules first contact the LiCl shell. LiCl responds rapidly, generating a measurable electrical signal. This ensures high sensitivity of the sensor in the low-humidity range. In high-humidity environments, as humidity increases, water molecules penetrate the LiCl shell before contacting the CaCl2 or MgCl2 core layer. At this point, the core material begins to exert its strong hygroscopic properties under high humidity, generating a stronger signal and thus extending the sensor's measurement range upwards. This design achieves a synergistic effect, with LiCl handling the low-humidity range and CaCl2 or MgCl2 handling the high-humidity range, thereby broadening the measurement range.

[0022] CaCl2 and MgCl2 are more prone to deliquescence and loss, especially MgCl2, which is very susceptible to overhydration and even dissolution under high humidity, leading to permanent degradation of sensor performance. The LiCl shell acts as a protective barrier: placing the easily lost CaCl2 or MgCl2 in the core layer and encapsulating it with a relatively more stable LiCl / polymer matrix effectively physically blocks their direct contact with the external environment, slowing down their loss rate and significantly improving the sensor's environmental stability and lifespan. Although LiCl generally does not react violently with CaCl2 or MgCl2, spatially isolating them completely avoids any potential adverse ion interactions, ensuring that each salt functions with its most intrinsic properties. Attached Figure Description

[0023] Figure 1This is a schematic diagram of the fabrication process for a core-shell structured microsphere sensor. Detailed Implementation

[0024] The present invention will be further described below with reference to specific embodiments.

[0025] Example 1

[0026] Dissolve 1g of PVA powder in 9mL of deionized water, heat at 90℃ and stir until completely dissolved to obtain a 10 wt% PVA solution.

[0027] Weigh 0.3g LiCl and 0.3g CaCl2, with a mass ratio of 1:1, and add them to the above PVA solution. Stir until completely dissolved to obtain a homogeneous precursor solution.

[0028] After the precursor solution is vacuum-defoamed, it is drop-coated onto the interdigitated electrodes that have been cleaned with anhydrous ethanol and dried with nitrogen. The electrodes are then frozen at -20°C for 12 hours and thawed at room temperature for 4 hours. This freeze-thaw cycle is repeated 3 times to complete the physical cross-linking and form a uniformly dispersed composite PVA hydrogel sensitive film, which is the wide-range flexible humidity sensing material.

[0029] The core equipment used for performance testing of this material includes: a constant temperature and humidity chamber (humidity control accuracy ±1% RH, temperature fluctuation ±0.5℃, test temperature 25℃); a precision LCR meter (test frequency 100 Hz); a data acquisition system; and fixtures and leads.

[0030] The composite PVA hydrogel sensing membrane was placed in a constant temperature and humidity chamber and connected to the measuring equipment. Starting from a low humidity (10% RH), the humidity was gradually increased in 10% RH increments until it reached 95% RH. At each humidity level, the humidity was maintained for 20 minutes to allow the sensor resistance reading to stabilize completely, and the stable resistance value was recorded. The resistance values ​​were recorded, and a humidity-resistance relative change rate (ΔR / R0, where R is the stable resistance value at a certain humidity level, and R0 is the initial resistance value at 30% RH) curve was plotted. The curve shows that ΔR / R0 changes significantly and regularly from 15% RH to 95% RH, thus the measurement range is 15%–95% RH, far exceeding that of a single salt system.

[0031] Observing the humidity-resistance curve, it exhibits different sensitivities in the 30-70% RH and 70-95% RH ranges. In the 30-70% RH range, the average sensitivity S1 = [ (ΔR / R0)@70%RH - (ΔR / R0)@30%RH] / (70 - 30) ≈ -0.8% / %RH. In the 70-95% RH range, the average sensitivity S2 = [ (ΔR / R0)@95%RH - (ΔR / R0)@70%RH] / (95 - 70) ≈ -2.5% / %RH (the negative sign indicates that the resistance decreases as humidity increases).

[0032] The humidity was stepped from 30% RH to 90% RH, and the system started recording the resistance change at high speed (sampling interval 0.1s) from the start of the timer (t0). The response time was defined as the time required for the resistance change to reach 90% of the total change from t0, which was measured to be 5s. Similarly, the humidity was stepped back from 90% RH to 30% RH, and the recovery time was defined as the time required for the resistance to recover to 90% of the total recovery from the start of the timer, which was measured to be 8s.

[0033] The humidity values ​​across the full range (15%-95% RH) were linearly fitted to ΔR / R0 using Origin software. The coefficient of determination R of the fitted curve was then determined. 2 >0.995, indicating excellent linearity.

[0034] Example 2

[0035] Weigh 1.5g of acrylamide (AAm) monomer, 0.015g of N,N'-methylenebisacrylamide (MBA) crosslinking agent, 0.02g of ammonium persulfate (APS) initiator, and 1.0g of LiCl, and dissolve them in 5mL of deionized water to prepare the core layer prepolymer solution. Separately, dissolve 1.5g of AAM monomer, 0.015g of MBA, 0.02g of APS, and 0.5g of MgCl2 in 5mL of deionized water to prepare the shell layer prepolymer solution.

[0036] Using microfluidic technology, a LiCl-containing core droplet is first formed, then surrounded by a MgCl2-containing shell prepolymer. The resulting core-shell droplet is collected and thoroughly mixed with a liquid PDMS prepolymer (containing a curing agent). After vacuum degassing, the mixture is coated onto interdigitated electrodes cleaned with anhydrous ethanol and dried with nitrogen, and then cured at 70°C for 2 hours. During this process, while the PDMS is thermally cured, the APS in the droplet undergoes a polymerization reaction, simultaneously forming PAAm hydrogel microspheres and a PDMS elastomer matrix, resulting in the final sensitive membrane. A schematic diagram of the fabrication process of this core-shell microsphere sensor is shown below. Figure 1 .

[0037] The core-shell structure sensor exhibits a sensitivity (S1) of -0.7% / %RH in the 20-60% RH range and a sensitivity (S2) of -2.8% / %RH in the 60-90% RH range. Its response time is 4 seconds and its recovery time is 7 seconds. After 100 cycles of testing, the signal attenuation rate is significantly lower than that of a sensor with uniformly dispersed components of the same proportion, demonstrating a significant improvement in stability.

[0038] Example 3

[0039] Two PDMS prepolymers were prepared: Part A was incorporating a high concentration of 20 wt% LiCl, and Part B was incorporating a high concentration of 20 wt% CaCl2. A layer-by-layer casting method was used. First, Part A prepolymer (after vacuum degassing) was poured into a mold cleaned with anhydrous ethanol and dried with nitrogen. It was then briefly pre-cured at 70°C for 5 minutes to form a viscous surface. Next, Part B prepolymer (after vacuum degassing) was poured in. Finally, it was fully cured at 70°C for 2 hours. Due to diffusion and interfacial fusion, a concentration gradient of LiCl and CaCl2 was formed at the interface between the two layers.

[0040] The reason for the concentration gradient distribution is that during the period after the B-component prepolymer is poured in and before it is completely cured together with the A-component, both salt ions tend to diffuse spontaneously from the high concentration area to the low concentration area. However, due to the viscosity of the pre-cured layer, the diffusion is not complete, and thus it is "locked" in the gradient state.

[0041] Component A, poured into the mold, is a liquid PDMS prepolymer containing 20 wt% LiCl. At this point, LiCl ions are uniformly distributed in component A. Heating at 70°C for 5 minutes initiates a cross-linking and curing reaction in the PDMS prepolymer, but it does not completely solidify. Component A transforms from a low-viscosity liquid into a high-viscosity, semi-cured gel state. It possesses sufficient mechanical strength to support the subsequently poured component B without immediate mixing, but its molecular network is not fully formed, and there are still significant voids and degrees of freedom within it. At this stage, although the movement of LiCl ions within component A is hindered, they still possess the ability to diffuse.

[0042] A prepolymer (liquid) containing 20 wt% CaCl2 (part B) was poured in. At the interface between layers A and B, a huge concentration difference was instantly formed, which was significant for Li... + Ions: extremely high concentration (20 wt%) in layer A, and extremely low concentration (~0 wt%) in layer B. For Ca... 2+ Ions: extremely high concentration (20 wt%) in layer B, and extremely low concentration (~0 wt%) in layer A.

[0043] According to Fick's law of diffusion, substances spontaneously migrate from areas of high concentration to areas of low concentration to eliminate the concentration difference and eventually achieve a uniform distribution. This concentration difference is the original driving force of diffusion. During the two hours from the pouring of component B to its final complete curing, two competing processes occurred. + Ions attempt to diffuse from layer A (high concentration) across the interface to layer B (low concentration). Ca 2+ Ions will attempt to diffuse from layer B (high concentration) across the interface into layer A (low concentration). If there is no obstruction, the entire material will eventually become a uniformly distributed mixture of LiCl and CaCl2 (each with a concentration of ~10 wt%).

[0044] The final two hours of heating allowed layer A to continue solidifying, while layer B also began to solidify from a liquid state. As the PDMS crosslinking network formed and matured, the viscosity of the system increased dramatically, and the degrees of freedom of molecular and ionic movement decreased sharply. Ultimately, before diffusion was complete and the concentration was uniform, the PDMS network had completely solidified. Thus, the "diffusion process" of the ions was permanently fixed: in the region near the original layer A, Li... + The concentration of ions remains the highest, Ca. 2+ The concentration of ions is low. In the region near the original B layer, Ca... 2+ The concentration of ions remains the highest, Li + The ion concentration is low. At the interface between the two layers, a concentration gradient is formed that transitions continuously from LiCl-rich to CaCl2-rich.

[0045] The advantages of concentration gradient distribution are: (1) Expanding the humidity range: Different types of salt have different sensitivities to different humidity ranges. Through gradient distribution, the sensor can make the optimal response to different humidity ranges at different depths, thereby achieving "seamless" monitoring from low humidity to high humidity and effectively expanding the effective range of the sensor. (2) Improving response linearity: Uniformly distributed salt may respond too drastically (non-linearly) in a certain humidity range and respond slowly in another range. Gradient distribution can "smooth out" such drastic changes, making the entire humidity-resistance curve more linear, which is convenient for signal processing and calibration. (3) Optimizing response / recovery kinetics: The surface salt can respond quickly to the initial changes in ambient humidity, while the inner salt is responsible for the subsequent depth response and stability, which may accelerate the overall response and recovery speed. (4) Enhancing stability: Placing the hygroscopic salt in the inner layer can reduce its chance of direct contact with the outside world, thereby slowing down the loss of salt and the performance degradation of the sensor.

[0046] Using PDMS as the substrate, this sensor boasts a tensile strength of up to 150% and can withstand 1000 bending cycles without significant performance degradation. In high-temperature / 80% RH and high-low temperature cycling (-10℃ to 50℃) tests, performance fluctuations are less than ±5%, demonstrating excellent environmental stability. When integrated into a mask, it can clearly and stably record waveforms under different breathing modes, including calm breathing, deep breathing, and rapid breathing, with a high signal-to-noise ratio.

[0047] Example 4

[0048] Weigh 1.5g of acrylamide (AAm) monomer, 0.015g of N,N'-methylenebisacrylamide (MBA) crosslinking agent, 0.3g of LiCl, and 0.3g of CaCl2 (mass ratio 1:1), and dissolve them in 5mL of deionized water, stirring until completely dissolved. Add 0.02g of ammonium persulfate (APS) as an initiator, and stir to dissolve to form a homogeneous precursor solution. After vacuum degassing the precursor solution, drop-coat it onto interdigitated electrodes that have been cleaned with anhydrous ethanol and dried with nitrogen. Place the electrodes in a 60℃ oven and heat for polymerization for 2 hours to complete chemical crosslinking and curing, forming a uniformly dispersed PAAm hydrogel sensitive film.

[0049] The sensor exhibits a wide measurement range (16%–93% RH) and high sensitivity (S2≈-2.3% / %RH@70–93% RH) similar to Example 1, demonstrating the effectiveness of the chemical crosslinking method and providing a faster curing speed.

[0050] Example 5

[0051] Prepare a 10 wt% gelatin solution. Take half of the gelatin solution, add 0.4 g MgCl2, stir to dissolve, then remove bubbles under vacuum, cast into a film, and air dry at room temperature to form the bottom film. Immerse the dried bottom film in a saturated aqueous solution containing 0.8 g LiCl for 30 minutes. LiCl ions diffuse from the film surface to the interior, forming a gradient distribution with high LiCl concentration on the surface and high MgCl2 concentration inside. Remove the film, gently wipe off surface droplets, and attach it to an interdigitated electrode that has been cleaned with anhydrous ethanol and dried with nitrogen gas for use.

[0052] Due to the gradient effect, the sensor exhibits a detectable response in low humidity (20% RH), with a range extending to 20%–90% RH. Its response recovery kinetics have been optimized, reducing the response time to 4 seconds.

[0053] Example 6

[0054] 0.1g LiCl and 0.1g CaCl2, totaling 5% of the PDMS prepolymer mass, were ground into a fine powder in a mortar. The salt powder was added to 2g of PDMS prepolymer (Sylgard 184 A:B = 10:1), mechanically stirred, and ultrasonically treated for 30 minutes to ensure uniform dispersion. After vacuum degassing, the powder was poured onto interdigitated electrodes that had been cleaned with anhydrous ethanol and dried with nitrogen. The electrodes were then cured at 70℃ for 2 hours. The sensor's range is 30%–85% RH. Although its absolute sensitivity is lower than that of high-salt samples, its linearity is excellent (R0). 2 >0.998), excellent mechanical properties, with an elongation exceeding 200%.

[0055] Example 7

[0056] 1.2g LiCl and 1.2g CaCl2, totaling 40% of the PDMS prepolymer mass, were ground into a fine powder in a mortar. The salt powder was added to 2g of PDMS prepolymer (Sylgard 184 A:B=10:1), mechanically stirred, and ultrasonically treated for 30 minutes to ensure uniform dispersion. After vacuum degassing, the powder was poured onto interdigitated electrodes that had been cleaned with anhydrous ethanol and dried with nitrogen. The mixture was then cured at 70℃ for 2 hours. This sensor exhibited high sensitivity (S²≈-3.8% / %RH) in high humidity conditions (>80% RH), but its mechanical strength decreased, with an elongation of approximately 80%.

[0057] Example 8

[0058] Weigh 0.1g LiCl and 0.5g CaCl2 (mass ratio 1:5) and add them to 10g of 10 wt% PVA solution. Stir until completely dissolved to obtain a homogeneous precursor solution. After vacuum degassing, the precursor solution is drop-coated onto interdigitated electrodes that have been cleaned with anhydrous ethanol and dried with nitrogen. The electrodes are then frozen at -20℃ for 12 hours and thawed at room temperature for 4 hours. This freeze-thaw cycle is repeated three times to complete physical cross-linking, forming a uniformly dispersed composite PVA hydrogel sensitive film. Tests show that the sensor exhibits good response in a moderate humidity range, with a range of 25%–88% RH.

[0059] Example 9

[0060] Weigh 0.5g LiCl and 0.1g CaCl2 (mass ratio 5:1) and add them to 10g of 10 wt% PVA solution. Stir until completely dissolved to obtain a homogeneous precursor solution. After vacuum degassing, the precursor solution is drop-coated onto interdigitated electrodes that have been cleaned with anhydrous ethanol and dried with nitrogen. The electrodes are then frozen at -20℃ for 12 hours and thawed at room temperature for 4 hours. This freeze-thaw cycle is repeated three times to complete physical cross-linking, forming a uniformly dispersed composite PVA hydrogel sensitive film. Tests show that the sensor exhibits excellent sensitivity in the high humidity range (S²≈-2.2% / %RH) with a range of 20%–92% RH.

[0061] Comparative Example 1

[0062] Dissolve 1 g of PVA powder in 9 mL of deionized water and heat at 90 °C with stirring until completely dissolved to obtain a 10 wt% PVA solution. Weigh 0.6 g of LiCl (consistent with the total mass of the two salts in Example 1) and add it to the above PVA solution. Stir until completely dissolved. After vacuum degassing the precursor solution, drop it onto an interdigitated electrode that has been cleaned with anhydrous ethanol and dried with nitrogen. Perform physical cross-linking according to the freeze-thaw cycle of Example 1 to form a single LiCl PVA hydrogel sensitive membrane.

[0063] Under the same testing conditions, the sensor exhibits extremely weak response and near-zero sensitivity in low humidity (<30% RH). In high humidity (>80% RH), although the sensitivity is higher, LiCl is highly hygroscopic, leading to damage to the sensitive film structure, irreversible drift of the resistance signal, and poor device stability. Its effective range is limited to 40%–90% RH, and its performance degrades rapidly during cyclic testing.

[0064] Comparative Example 2

[0065] Prepare a 10 wt% PVA solution (as above). Weigh 0.6 g of CaCl2, keeping the total mass consistent with the dual salts in Example 1, and add it to the PVA solution. Stir until completely dissolved to obtain a homogeneous precursor solution. After vacuum degassing the precursor solution, drop-coat it onto interdigitated electrodes that have been cleaned with anhydrous ethanol and dried with nitrogen. Perform physical cross-linking according to the freeze-thaw cycle of Example 1 to form a single CaCl2 PVA hydrogel sensitive film.

[0066] Under the same testing conditions, the sensor exhibited good response and stability in the medium humidity range (40–70% RH). However, in the high humidity range (>85% RH), its sensitivity was significantly lower than that of the dual-salt sensor in Example 1 (S2≈-1.1% / %RH), limiting its upper range and making it unable to effectively monitor high humidity environments above 90%.

[0067] Comparative Example 3

[0068] After physically mixing 0.3g LiCl and 0.3g CaCl2, the mixture was directly applied to an interdigitated electrode coated with adhesive, which had been cleaned with anhydrous ethanol and dried with nitrogen. The sensor initially exhibited high sensitivity, but after 10 minutes of testing in an 85% RH environment, the salt powder rapidly deliquesced and was lost, causing irreversible drift and failure of the resistance signal. This demonstrates that the polymer matrix is ​​crucial for immobilizing the salt and maintaining the sensor's stability.

[0069] Comparative Example 4

[0070] Referring to Example 1, but changing the salt ratio to LiCl:CaCl2 = 1:10, 10g of 10 wt% PVA solution was added and stirred until completely dissolved to obtain a homogeneous precursor solution. After vacuum degassing the precursor solution, it was drop-coated onto interdigitated electrodes that had been cleaned with anhydrous ethanol and dried with nitrogen. Physical cross-linking was completed using the freeze-thaw cycle of Example 1 to form a composite PVA hydrogel sensitive film. Due to the excessively high proportion of CaCl2, the sensor response characteristics approached those of a single CaCl2 sensor. In high humidity areas (>85%RH), the sensitivity was significantly lower than that of Example 1 (S2≈-1.2% / %RH), losing the advantage of dual-salt synergy, thus proving the necessity of the specific ratio range in claim 2.

[0071] The above embodiments and comparative examples demonstrate that the flexible humidity sensor and its sensing material described in this invention, with their excellent flexibility, high sensitivity, wide measurement range, and environmental stability, are highly suitable for application in various smart scenarios. For example, integrating them into smartwatch straps or clothing as smart wearable devices can monitor human activity. Attaching them to the inside of masks or using them in respirators as respiratory monitoring devices can monitor respiratory rate and pattern in real time, providing early warning of respiratory diseases. Combining them with flexible circuits to fabricate a large-area array as electronic skin can provide humidity sensing capabilities for robots, enhancing their ability to interact with the environment.

[0072] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art can make other variations or modifications based on the above description. It is impossible to exhaustively list all embodiments here. All obvious variations or modifications derived from the technical solutions of the present invention are still within the protection scope of the present invention.

Claims

1. A wide-range flexible humidity sensing material, characterized in that, The sensor comprises a flexible polymer matrix and at least two different hygroscopic salts dispersed therein, wherein the hygroscopic salts are distributed in a uniform dispersion, a concentration gradient distribution, or a core-shell structure distribution, and the total mass of the hygroscopic salts accounts for 5% to 40% of the total mass of the sensing material. The total mass of the sensing material specifically refers to the mass of the composite material composed of the flexible polymer matrix and all the hygroscopic salts.

2. The wide-range flexible humidity sensing material as described in claim 1, characterized in that, The at least two different hygroscopic salts are lithium chloride (LiCl) and calcium chloride (CaCl2), wherein the mass ratio of LiCl to CaCl2 is 1:5 to 5:

1.

3. The wide-range flexible humidity sensing material as described in claim 1, characterized in that, The at least two different hygroscopic salts are lithium chloride (LiCl) and magnesium chloride (MgCl2), wherein the mass ratio of LiCl to MgCl2 is 1:5 to 5:

1.

4. The wide-range flexible humidity sensing material according to any one of claims 1 to 3, characterized in that, The hygroscopic salt exhibits a concentration gradient distribution within the flexible polymer matrix, wherein the concentration gradient distribution is a continuous transitional distribution from a LiCl-rich region to a CaCl2-rich or MgCl2-rich region.

5. The wide-range flexible humidity sensing material according to any one of claims 1 to 3, characterized in that, The hygroscopic salt and the polymer matrix together constitute core-shell structured microspheres. The hygroscopic salts in the core and shell layers are the two hygroscopic salts described in claim 2 or 3, and one hygroscopic salt is LiCl and is enriched in the shell layer, while the other is CaCl2 or MgCl2 and is enriched in the core layer.

6. The wide-range flexible humidity sensing material as described in claim 1, characterized in that, The flexible polymer matrix is ​​polyvinyl alcohol (PVA) hydrogel, polyacrylamide (PAAm) hydrogel, gelatin, or polydimethylsiloxane (PDMS).

7. A method for preparing a wide-range flexible humidity sensing material as described in any one of claims 1 to 6, characterized in that, Includes the following steps: (1) Dissolve the at least two hygroscopic salts in deionized water to form a mixed salt solution; (2) The polymer monomer or prepolymer is mixed with the mixed salt solution to form a uniform precursor solution. The precursor solution is structured according to the distribution pattern of the hygroscopic salt. If it is a concentration gradient distribution, it is poured layer by layer. If it is a core-shell structure, it is prepared by microfluidic technology. If it is a uniform dispersion, it is directly mixed. (3) After the precursor solution is vacuumed and defoamed, it is poured onto the interdigitated electrode substrate that has been cleaned with anhydrous ethanol and dried with nitrogen. It is then solidified by physical crosslinking or chemical crosslinking and dried at room temperature after solidification. The physical crosslinking is either freeze-thaw cycle crosslinking or room temperature air drying crosslinking, and the chemical crosslinking is either thermally initiated polymerization crosslinking or photoinitiated polymerization crosslinking.

8. A flexible humidity sensor, characterized in that, It includes a flexible substrate, electrodes disposed on the flexible substrate, and a wide-range flexible humidity sensing material as described in any one of claims 1 to 6 covering the electrodes.

9. The application of the flexible humidity sensor as described in claim 8, characterized in that, Applications include the development of smart wearable devices for monitoring human activity status, respiratory monitoring devices for monitoring respiratory rate and pattern, and flexible electronic skin for sensing environmental humidity.

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