Rehydrative response reversible optical switching hydrogel and preparation method and use thereof

Abstract

The present application belongs to the technical field of intelligent hydrogel and optical response material, and particularly relates to a rehydration response reversible optical switching hydrogel, which is obtained by heating and dissolving a pre-gel water solution and cooling to form a physical gel, wherein the pre-gel water solution comprises, by total mass, PVP 3-12 wt%, PEG 0.5-6 wt%, carrageenan 1-3 wt%, sodium chloride 0.1-1.5 wt%, and the rest is water; the preparation method of the rehydration response reversible optical switching hydrogel comprises the following steps: weighing appropriate amounts of PVP, PEG, carrageenan and sodium chloride, and adding them into water, heating to dissolve them to obtain a uniform solution; cooling the solution to form a physical gel; soaking the physical gel in a gallic acid aqueous solution; taking out the physical gel and drying to obtain a dry transparent material; placing the obtained dry transparent material in water to rehydrate, forming a milky white opaque state; the hydrogel in the present application is dry-wet reversible, response programmable and can be recycled multiple times, and the preparation process is simple.

Description

Technical Field

[0001] This invention belongs to the field of smart hydrogels and optically responsive materials technology, specifically relating to a rehydrated responsive reversible optically switching hydrogel, its preparation method, and its applications. Background Technology

[0002] Hydrogels are widely used in sensing, indication, and anti-counterfeiting fields due to their flexibility and designable network structures. In recent years, with the development of smart materials and functional soft matter, "optically responsive hydrogels" that can visually respond to external stimuli (such as temperature, pH, light, electric field, ionic strength, and moisture) have attracted attention. Among them, optical switching achieved through changes in transparency / turbidity, color depth, or scattering intensity is one of the important technical routes for information display and encryption, anti-counterfeiting labels, and disposable indicators. In existing technologies, optically responsive hydrogels mostly rely on phase transitions, phase separation, or microstructure reconstruction, such as temperature-responsive systems based on lower critical solution temperature (LCST) polymers and switching systems based on pH or ionic strength-controlled microphase separation. However, these systems often require sophisticated chemical synthesis or copolymerization modification, have complex preparation processes, are sensitive to environmental conditions, and some response processes are irreversible or have poor cycle stability, making it difficult to achieve rapid, obvious, and reversible optical switching effects under simple, scalable preparation conditions. On the other hand, research on water-responsive hydrogels mainly focuses on volume swelling and changes in swelling mechanical properties, while there is relatively little research on the regulation of optical scattering induced by dehydration / rehydration processes.

[0003] Physically cross-linked hydrogels constructed from water-soluble polymers such as polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), and carrageenan, along with sodium chloride, offer advantages such as readily available raw materials, mild gelation conditions, and good biocompatibility. They already have a certain application foundation in the food, medical dressing, and soft materials fields. Gallic acid (GA), as a natural polyphenol compound, possesses good water solubility and multi-site hydrogen bonding capabilities. However, current technologies have not fully integrated these material systems with water-induced reversible optical switching. There is a lack of water-responsive hydrogel products with simple structures, mild preparation processes, and the ability to achieve rapid and reversible switching between transparent and opaque (milky white) during drying and rehydration. These hydrogels are intended for constructing intuitive, high-contrast, and reusable optical indication and information display platforms. Therefore, it is necessary to provide a method for preparing a rehydration-responsive reversible optical switching hydrogel and its product, to achieve rapid whitening, significant optical contrast, and good cycling stability while ensuring system safety and process simplicity. Summary of the Invention

[0004] In view of the problems existing in the prior art, the purpose of this invention is to provide a hydrogel and its preparation method that are simple to process, reversible in drying and wetting, programmable in response, reusable multiple times, and suitable for humidity or seepage indication.

[0005] The present invention achieves the above-mentioned objective by adopting the following technical solution: a rehydration-responsive reversible optically switching hydrogel, wherein the hydrogel is obtained by heating and dissolving an aqueous solution before gelation and then cooling it to form a physical gel, wherein the aqueous solution before gelation comprises, by total mass: 3-12 wt% PVP, 0.5-6 wt% PEG, 1-3 wt% carrageenan, 0.1-1.5 wt% sodium chloride, and the remainder being water; the physical gel formed by cooling the aqueous solution before gelation is treated by soaking in gallic acid aqueous solution and then dried to a dry transparent state, and the hydrogel changes from a transparent state to a milky white opaque state after rehydration.

[0006] Preferably, the gallic acid in the gallic acid aqueous solution is 3-5 wt%.

[0007] A method for preparing a rehydrated, reversibly optically switching hydrogel includes the following steps:

[0008] S1 Dissolution: Weigh appropriate amounts of PVP, PEG, carrageenan and sodium chloride, add them to water, heat to dissolve, and obtain a homogeneous solution;

[0009] S2 gelation: The solution obtained in S1 is cooled to form a physical gel;

[0010] S3 soaking: The physical gel obtained in S2 is soaked in gallic acid aqueous solution;

[0011] S4 Drying: Remove the physical gel that was soaked in S3 and dry it to obtain a dry transparent material;

[0012] S5 Rehydration: The dry transparent material obtained in S4 is placed in water for rehydration, forming a milky white opaque state.

[0013] Preferably, in step S1, the mixture is heated to 70-95°C and stirred for 10-60 minutes to dissolve it until the solution is clear and homogeneous.

[0014] Preferably, the operation steps of step S2 are as follows: pour the solution obtained in S1 into a mold, cool it to 10-30℃, and let it stand for 0.5-12 hours to form a physical gel.

[0015] Preferably, the physical gel obtained in step S2 is a sheet, and the thickness L of the sheet is 0.2-6 mm.

[0016] Preferably, in step S3, the temperature for soaking the physical gel in gallic acid aqueous solution is 25-40℃, and the soaking time is 0.1-48h.

[0017] Preferably, in step S4, the drying temperature of the physical gel is 40-60℃, and the drying time is 2-24h.

[0018] Preferably, the hydrogel that has been soaked in gallic acid aqueous solution in step S3 but has not yet been dried is gently rinsed for no more than 30 seconds.

[0019] Use of a rehydration-responsive reversible optically switching hydrogel in humidity or seepage indication.

[0020] The beneficial effects of this invention are:

[0021] This invention achieves reversible optical switching between dry and wet states in hydrogels through gallic acid (GA) impregnation and the synergistic regulation of sodium chloride, polyethylene glycol (PEG), polyvinylpyrrolidone (PVP), and carrageenan, without the need for complex polymerization reactions. This design not only simplifies the preparation process but also avoids the complex chemical reactions and high costs associated with traditional methods while maintaining high performance. By using GA solution impregnation, specific physical forces are introduced into the hydrogel structure, enabling rapid whitening upon rehydration and thus achieving rapid optical response switching between dry and wet states.

[0022] Furthermore, the whitening response after rehydration is extremely fast, typically completing the whitening process in less than 90 seconds, giving the hydrogel an instantaneous response characteristic. This feature provides significant advantages in real-time applications such as humidity change monitoring or water seepage indication. In addition, the whitening response time exhibits a good linear relationship with the hydrogel thickness, thus enabling programmable kinetic control of the hydrogel's optical response. By simply adjusting the gel thickness or other relevant parameters, whitening responses at different time scales can be achieved, meeting the needs of various application scenarios.

[0023] The hydrogel of this invention also possesses the ability to be recycled multiple times, and its optical response remains stable after each cycle, exhibiting high repeatability and reliability. This hydrogel, capable of reversible optical switching between wet and dry states, has broad application prospects, especially in environmental monitoring and sensing technologies requiring rapid, accurate, and controllable responses. Attached Figure Description

[0024] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments of the present invention or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1Curves showing the whitening time of GA after immersion in gels of different thicknesses.

[0026] Figure 2 Comparison chart after 10 cycles in Example 1. Detailed Implementation

[0027] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0028] It should be noted that, in specific embodiments of the present invention, terms such as "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, the use of phrases such as "comprising one" to define an element does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element. Those skilled in the art will understand the specific meaning of the above terms in the present invention through the specific circumstances.

[0029] Further analysis: The raw materials used in the experiments of the embodiments and comparative examples of the present invention are as follows, but are not limited to the following raw materials. The present invention only uses the following raw materials as specific examples to further illustrate the effect of the rehydration-responsive reversible optical switching hydrogel described in the present invention.

[0030] Example 1

[0031] 4g carrageenan (KA), 9.75g polyvinylpyrrolidone (PVP), 0.75g sodium chloride, and 3g polyethylene glycol (PEG) were added to 232.5g of water and heated to 90℃ to obtain a homogeneous and transparent mixed solution. The mixed solution was transferred to a mold and cooled to room temperature to obtain a hydrogel with a thickness of 3mm. The hydrogel prepared in the previous step was placed in a gallic acid solution at 30℃ and soaked for 30min. After soaking, it was removed to obtain a rehydration-responsive reversible optical switching hydrogel. The mass fraction of gallic acid in the gallic acid aqueous solution was 3%.

[0032] Example 2

[0033] Example 2 is basically the same as Example 1, except that 4g of carrageenan (KA), 9.75g of polyvinylpyrrolidone (PVP), 0.75g of sodium chloride and 5.25g of polyethylene glycol (PEG) are added to 230.25g of water in the mixed solution.

[0034] Example 3

[0035] 4g of carrageenan (KA), 9.75g of polyvinylpyrrolidone (PVP), 0.75g of sodium chloride, and 3g of polyethylene glycol (PEG) were added to 232.5g of water and heated to 90℃ to obtain a homogeneous and transparent mixed solution. The mixed solution was transferred to a mold and cooled to room temperature to obtain a hydrogel with a thickness of 3mm. The hydrogel prepared in the previous step was placed in a gallic acid solution and immersed in a water bath at 40℃ for 10min. After immersion, it was removed to obtain a rehydration-responsive reversible optical switching hydrogel. The mass fraction of gallic acid in the gallic acid aqueous solution was 3%.

[0036] Example 4

[0037] Example 4 is basically the same as Example 3, except that the mass fraction of gallic acid in the gallic acid aqueous solution is 4%.

[0038] Example 5

[0039] Example 5 is basically the same as Example 3, except that the mass fraction of gallic acid in the gallic acid aqueous solution is 5%.

[0040] Example 6

[0041] 4g of carrageenan (KA), 9.75g of polyvinylpyrrolidone (PVP), 0.75g of sodium chloride, and 3g of polyethylene glycol (PEG) were added to 232.5g of water and heated to 90℃ to obtain a homogeneous and transparent mixed solution. The mixed solution was transferred to a mold and cooled to room temperature to obtain a hydrogel with a thickness of 0.9mm. The hydrogel prepared in the previous step was placed in a gallic acid solution and immersed in a water bath at 40℃ for 10min. After immersion, it was removed to obtain a rehydration-responsive reversible optical switching hydrogel. The mass fraction of gallic acid in the gallic acid aqueous solution was 3%.

[0042] Example 7

[0043] The content of Example 7 is basically the same as that of Example 6, except that the thickness of the hydrogel is 2 mm.

[0044] Example 8

[0045] The content of Example 8 is basically the same as that of Example 6, except that the thickness of the hydrogel is 4 mm.

[0046] Example 9

[0047] The content of Example 9 is basically the same as that of Example 6, except that the thickness of the hydrogel is 6 mm.

[0048] Comparative Example 1

[0049] The content of Comparative Example 1 is basically the same as that of Example 1, except that the mixed solution contains only 4g of carrageenan and 246g of water.

[0050] Comparative Example 2

[0051] The content of Comparative Example 2 is basically the same as that of Example 1, except that the mixed solution contains only 4g of carrageenan, 3g of polyethylene glycol and 243g of water.

[0052] Comparative Example 3

[0053] The contents of Comparative Example 3 are basically the same as those of Example 1, except that the mixed solution contains only 4g of carrageenan, 9.75g of polyvinylpyrrolidone (PVP) and 236.25g of water.

[0054] Comparative Example 4

[0055] The content of Comparative Example 4 is basically the same as that of Example 1, except that the mixed solution contains only 4g of carrageenan, 9.75g of polyvinylpyrrolidone (PVP), 3g of polyethylene glycol and 233.25g of water.

[0056] Comparative Example 5

[0057] The content of Comparative Example 5 is basically the same as that of Example 1, except that the mixed solution contains only 4g of carrageenan, 9.75g of polyvinylpyrrolidone (PVP), 0.75g of sodium chloride and 235.5g of water.

[0058] Comparative Example 6

[0059] The content of Comparative Example 6 is basically the same as that of Example 3, except that the mass fraction of gallic acid in the gallic acid aqueous solution is 0.5%.

[0060] Comparative Example 7

[0061] The content of Comparative Example 7 is basically the same as that of Example 3, except that the mass fraction of gallic acid in the gallic acid aqueous solution is 1%.

[0062] Comparative Example 8

[0063] The content of Comparative Example 8 is basically the same as that of Example 3, except that the mass fraction of gallic acid in the gallic acid aqueous solution is 2%.

[0064] Comparative Example 9

[0065] The content of Comparative Example 9 is basically the same as that of Example 6, except that the thickness of the hydrogel is 8 mm.

[0066] Comparative Example 10

[0067] The content of Comparative Example 10 is basically the same as that of Example 6, except that the thickness of the hydrogel is 10 mm.

[0068] Table 1. Comparison of the appearance of hydrogels prepared with different component ratios and changes before and after rehydration.

[0069]

[0070] Table 2. Component ratios and changes after GA soaking in Examples 1-9 and Comparative Examples 1-10

[0071]

[0072] Table 3. Whitening time of Examples 3-9

[0073]

[0074] analyze:

[0075] Comparing Comparative Examples 1 and 2 (without PVP) with the series of samples containing PVP, it is evident that PVP is a key component affecting gel discoloration and maintaining gel structural integrity. During GA immersion treatment, the gels in Comparative Examples 1 and 2 showed no color change, but their structures completely dissolved, failing to maintain morphological stability. In contrast, the samples with added PVP, after the same GA immersion treatment, exhibited a whitening effect, and their structures remained stable without dissolution or collapse. This phenomenon indicates that PVP can synergistically interact with other components in the system to enhance the gel network's tolerance to the GA immersion environment, effectively inhibiting gel structure dissolution and providing crucial support for maintaining gel morphology.

[0076] The comparative experiment between Comparative Example 5 (without PEG) and Example 1 (containing 1.2% PEG) showed that after two drying and rehydration operations, the gel structure of Comparative Example 5 underwent significant dissolution, exhibiting poor morphological and performance stability, and could not meet the application requirements for repeated processing. In contrast, after multiple drying-rehydration cycles, the macroscopic morphology and microstructure of the gel in Example 1 showed almost no significant changes, demonstrating excellent stability against repeated processing. This phenomenon indicates that PEG can reduce the damage to the gel network during the drying-rehydration process by adjusting the osmotic pressure and intermolecular forces within the gel, thereby improving its structural stability during repeated use.

[0077] A comparison of Comparative Examples 3 and 4 (without NaCl) with Examples 1 and 5 (containing NaCl) shows that the gels formed by Comparative Examples 3 and 4 without NaCl are soft, have low mechanical strength, and exhibit strong interfacial adhesion to the substrate, making the peeling process difficult and prone to causing gel structure damage. In contrast, the gels formed by Examples 1 and 5 with added NaCl show significantly improved mechanical strength, a harder texture, and effectively reduced interfacial adhesion to the substrate, making peeling easier and maintaining the integrity of the gel structure. This difference stems from NaCl's ability to regulate the cross-linking density of the gel network. It can promote the cross-linking and aggregation of gel molecular chains through ionic bonding, optimizing gel mechanical properties, while also altering the gel surface energy, reducing interfacial bonding forces, and improving peeling characteristics.

[0078] The rehydration whitening characteristic time of hydrogels treated with gallic acid aqueous solution is linearly correlated with the square of the hydrogel thickness L, and decreases as the thickness L decreases.

[0079] The hydrogel can undergo no less than 10 drying-rehydration cycles while maintaining reversible whitening switching, and the time to achieve 50% whitening through rehydration is ≤90 seconds.

[0080] In summary, this invention demonstrates optimal performance in terms of structural stability, recyclability, and mechanical exfoliation properties through the synergistic effect of PVP, PEG, and NaCl.

[0081] The above description, in conjunction with specific embodiments, provides a further detailed explanation of the present invention. It should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, various simple deductions or substitutions can be made without departing from the concept of the present invention, and all such deductions or substitutions should be considered within the scope of protection of the present invention.

Claims

1. A reversible optically switching hydrogel with a rehydration response, characterized in that: The hydrogel is obtained by heating and dissolving an aqueous solution before gelation and then cooling it to form a physical gel. The aqueous solution before gelation includes, by total mass: 3-12 wt% PVP, 0.5-6 wt% PEG, 1-3 wt% carrageenan, 0.1-1.5 wt% sodium chloride, and the remainder is water. The physical gel formed by cooling the aqueous solution before gelation is treated with gallic acid aqueous solution and then dried to a dry transparent state. After rehydration, the hydrogel changes from a transparent state to a milky white opaque state.

2. The rehydration-responsive reversible optically switching hydrogel according to claim 1, characterized in that: The gallic acid aqueous solution contains 3-5 wt% gallic acid.

3. A method for preparing a rehydration-responsive reversible optically switching hydrogel according to any one of claims 1-2, characterized in that, Includes the following steps: S1 Dissolution: Weigh appropriate amounts of PVP, PEG, carrageenan and sodium chloride, add them to water, heat to dissolve, and obtain a homogeneous solution; S2 gelation: The solution obtained in S1 is cooled to form a physical gel; S3 soaking: The physical gel obtained in S2 is soaked in gallic acid aqueous solution; S4 Drying: Remove the soaked physical gel from S3 and dry it to obtain a dry transparent material; S5 Rehydration: The dry transparent material obtained in S4 is placed in water for rehydration, forming a milky white opaque state.

4. The method for preparing the rehydration-responsive reversible optically switching hydrogel according to claim 3, characterized in that, In step S1, the mixture is heated to 70-95°C and stirred for 10-60 minutes to dissolve it until the solution is clear and homogeneous.

5. The method for preparing the rehydration-responsive reversible optically switching hydrogel according to claim 3, characterized in that, The steps of step S2 are as follows: pour the solution obtained in S1 into a mold, cool it to 10-30℃, and let it stand for 0.5-12 hours to form a physical gel.

6. The method for preparing the rehydration-responsive reversible optically switching hydrogel according to claim 5, characterized in that, The physical gel obtained in step S2 is a sheet, and the thickness L of the sheet is 0.2-6 mm.

7. The method for preparing the rehydration-responsive reversible optically switching hydrogel according to claim 6, characterized in that, In step S3, the physical gel is soaked in gallic acid aqueous solution at a temperature of 25-40℃ for 0.1-48 hours.

8. The method for preparing the rehydration-responsive reversible optically switching hydrogel according to claim 6, characterized in that, In step S4, the drying temperature of the physical gel is 40-60℃, and the drying time is 2-24h.

9. The method for preparing the rehydration-responsive reversible optically switching hydrogel according to claim 3, characterized in that: The hydrogel that has been soaked in gallic acid aqueous solution in step S3 but has not yet been dried is gently rinsed for no more than 30 seconds.

10. Use of the rehydration-responsive reversible optically switching hydrogel according to any one of claims 1-2 in humidity or water seepage indication.

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