A bilayer hydrogel, its preparation method and application

Abstract

This invention discloses a bilayer hydrogel, its preparation method, and its applications. The method includes: adding a first ionic monomer, a second ionic monomer, a crosslinking agent, and an initiator to deionized water and stirring to obtain a reaction solution; pouring the reaction solution into a mold and allowing it to stand until it separates into two layers, both of which are clear; sealing the mold and then subjecting it to ultraviolet light-initiated polymerization to obtain a hydrogel with a bilayer structure. The bilayer hydrogel provided by this invention utilizes the spontaneous phase separation behavior of the system and is prepared in one step, resulting in a simple preparation method; the upper and lower layers of the hydrogel are tightly bonded, making interfacial failure less likely; the bilayer hydrogel possesses asymmetric properties on both sides, and its layer ratio, mechanical properties, and adhesiveness can be adjusted as needed, showing potential applications in asymmetric adhesion and soft-driven processes.

Description

Technical Field

[0001] This invention belongs to the field of hydrogel technology, specifically relating to a bilayer hydrogel, its preparation method, and its application. Background Technology

[0002] Hydrogels are a research hotspot in the field of soft matter materials, leading to the development of various functional and type-specific hydrogels. Among them, bilayer hydrogels have attracted widespread attention due to their unique properties. Unlike traditional homogeneous hydrogels, bilayer hydrogels possess asymmetric properties on both sides, including wettability, adhesion, and crosslinking density. These asymmetric properties can be achieved through component adjustment or structural design, enabling the hydrogel to possess different functions on each side. Due to their unique asymmetric structure and the integration of multiple properties, bilayer hydrogels can achieve functions that homogeneous hydrogels cannot, demonstrating significant advantages in fields such as tissue engineering, flexible sensing, intelligent actuation, and photothermal evaporation.

[0003] Despite extensive research on bilayer hydrogels, several challenges remain. For instance, the construction of bilayer structures is often complex, requiring multi-step polymerization or chemical modification of specific functional groups (Porous Janus materials with unique asymmetries and functionality. Linlin Yan, Xiaobin Yang, Yanqiu Zhang, Yadong Wu, Zhongjun Cheng, Seth B. Darling, Lu Shao. Materials Today, 2021, 51, 626-647). Large-scale, controllable preparation is difficult, and interfacial separation can occur, significantly limiting their applications. One-step hydrogel fabrication is a good approach to address the cumbersome nature of hydrogel preparation. Existing literature reports the preparation of bilayer hydrogels based on monomers with low critical solution temperatures (LCTs) and polymers exhibiting sol-gel transitions. By heating, the LCT monomers transition from a hydrophilic to a hydrophobic state, inducing macroscopic phase separation (Multiscalebilayer hydrogels enabled by macrophase separation. Dong Zhang, Yijing Tang, Kaihang Zhang, Yaoting Xue, Si Yu Zheng, Baoyi Wu, Jie Zheng. Matter, 2023, 6, 1484-1502). While the preparation method is simple, this one-step approach requires both raw material components to possess temperature responsiveness and thermally reversible gelation properties, limiting the range of selectable raw materials. Currently, most reported one-step preparation strategies also rely on specific raw material systems, and their universality needs further improvement.

[0004] Therefore, proposing a simple and widely applicable bilayer hydrogel design strategy is of great significance for the development of functional hydrogels. Summary of the Invention

[0005] In view of the shortcomings of the existing technology, the purpose of this invention is to provide a bilayer hydrogel, its preparation method and application.

[0006] This invention utilizes the spontaneous phase separation effect to prepare a bilayer hydrogel in one step. Specifically, it takes advantage of the difference in ionization ability between the first and second ionic monomers in water, using the first ionic monomer to suppress the ionization degree of the second ionic monomer in water, thereby reducing the solubility of the second ionic monomer and achieving spontaneous phase separation. At the same time, it utilizes the polymerizability of the monomers, allowing the two electrolyte monomers to complete the polymerization reaction while undergoing phase separation, thus achieving the effect of forming a bilayer structure.

[0007] The objective of this invention is achieved through the following technical solutions.

[0008] A method for preparing a bilayer hydrogel includes the following steps:

[0009] (1) Add the first ionic monomer, the second ionic monomer, and the crosslinking agent to deionized water and stir until the solid is completely dissolved to obtain a mixture;

[0010] (2) Inert gas is introduced into the mixture to remove dissolved oxygen, then photoinitiator is added and stirred until the solid is completely dissolved to obtain a reaction solution. The reaction solution is then sonicated to remove bubbles.

[0011] (3) Pour the reaction solution into the mold and let it stand until the reaction solution separates into two layers and both liquids are clear. Then, initiate polymerization under ultraviolet light to obtain a bilayer hydrogel.

[0012] Further, in step (1), the first ionic monomer is any one of N,N,N-trimethyl-3-(2-methylallylamino)-1-chloropropylamine, [2-(methacryloyloxy)ethyl]trimethylammonium chloride, acryloyloxyethyltrimethylammonium chloride, and (3-acrylamidopropyl)trimethylammonium chloride.

[0013] The ratio of the amount of the first ionic monomer added to the volume of deionized water is 2 mol / L to 5 mol / L.

[0014] Further, in step (1), the second ionic monomer is any one of dimethylaminoethyl methacrylate, dimethylaminoethyl acrylate, 2-N-morpholinoethyl methacrylate, N-[(3-(dimethylamino)propyl]methacrylamide, N-[(3-dimethylamino)propyl]acrylamide, and N-[2-(dimethylamino)ethyl]methacrylamide.

[0015] The ratio of the amount of the second ionic monomer added to the volume of deionized water is 2 mol / L to 6 mol / L.

[0016] Further, the crosslinking agent in step (1) is N,N'-methylenebispropionamide.

[0017] The amount of crosslinking agent added is 0.1% to 0.5% of the total amount of the first ionic monomer and the second ionic monomer.

[0018] Furthermore, the inert gas mentioned in step (2) is either nitrogen or argon.

[0019] Further, the photoinitiator in step (2) is either 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylphenylacetone or α-ketoglutaric acid.

[0020] The amount of photoinitiator added is 0.1% to 1% of the total amount of the first and second ionic monomers.

[0021] Furthermore, the ultrasonic treatment in step (2) has a power of 50W to 150W and an ultrasonic time of 5min to 10min.

[0022] Further, the mold mentioned in step (3) can be any one of the following: a transparent test tube, a polytetrafluoroethylene groove mold with a transparent cover, or a mold assembled from two glass plates with a gasket sandwiched between them and a long and narrow transparent sealing bag.

[0023] When using a mold assembled from two glass plates with a gasket between them and a long, narrow, transparent sealing bag, first pour the reaction solution into the sealing bag and let it stand so that the reaction solution separates into two layers, both of which are clear. Then place the transparent sealing bag between the two glass plates with a gasket between them, and slowly bring the two glass plates closer together until they are clamped and sealed.

[0024] Furthermore, in step (3), the wavelength of the ultraviolet light is 320nm~365nm, the power is 80W~120W, the irradiation time is 3h~12h, and the reaction temperature is 20℃~50℃.

[0025] This invention provides a bilayer hydrogel prepared by the above-described preparation method.

[0026] The preparation method provided by this invention utilizes the spontaneous phase separation behavior of the system to synthesize a hydrogel with a bilayer structure in one step. The upper and lower layers of the gel are tightly bonded together with no gaps between them, making it less prone to interfacial failure.

[0027] The present invention also provides the application of the bilayer hydrogel in the preparation of asymmetric adhesion or soft actuation devices.

[0028] Compared with the prior art, the present invention has the following advantages and technical effects:

[0029] (1) The method for preparing bilayer hydrogels provided by the present invention utilizes the spontaneous phase separation behavior of the system to obtain a layered gel through one-step polymerization. Specifically, it utilizes the difference in ionization ability between the first ionic monomer and the second ionic monomer in water, using the first ionic monomer to inhibit the ionization degree of the second ionic monomer in water, reducing the solubility of the second ionic monomer, and generating an in-situ phase separation phenomenon to achieve layering; at the same time, it utilizes the polymerizability of the monomers, so that the two electrolyte monomers can complete the polymerization reaction while separating phases, thereby achieving the effect of forming a bilayer structure. This method does not require complex multi-step polymerization or surface modification, greatly simplifying the preparation steps of bilayer hydrogels.

[0030] (2) The bilayer hydrogel provided by the present invention has controllable asymmetric properties on both sides. The thickness, mechanical strength and adhesion of the two layers of hydrogel can be easily adjusted by changing the concentration of the two ionic monomers in water. In addition, since some of the optional second ionic monomers, such as dimethylaminoethyl methacrylate and 2-N-morpholinoethyl methacrylate, are thermosensitive, the layering of the system can also be controlled by changing the polymerization temperature.

[0031] (3) The bilayer hydrogel provided by the present invention exhibits asymmetric mechanical properties on both sides, making it suitable for use as a soft actuator. In addition, the bilayer hydrogel with obvious asymmetric viscosity can also be applied to in vivo tissue patches, wearable devices with human-machine interfaces, and other fields, with broad application prospects.

[0032] (4) The preparation method of the bilayer hydrogel provided by this invention has a certain degree of universality. There are many types of first and second ionic monomers that can be selected. Among them, the first ionic monomer is an ionic monomer that can be completely ionized in water, which is a quaternary ammonium salt cationic monomer; the second ionic monomer is an ionic monomer with a high degree of ionization in water, which is mostly a monomer containing a tertiary amine group. These ionic monomers are easy to obtain, which reduces the preparation cost. By combining different types of first and second ionic monomers, bilayer hydrogels with different properties and functions can be prepared. Attached Figure Description

[0033] Figure 1 Scanning electron microscope and elemental energy dispersive spectroscopy images of the bilayer hydrogel prepared in Example 1.

[0034] Figure 2 Tensile stress-strain diagrams of the upper and lower gel layers of the bilayer hydrogel prepared in Example 1.

[0035] Figure 3 Scanning electron microscope and elemental energy dispersive spectroscopy images of the homogeneous hydrogel prepared for Comparative Example 1. Detailed Implementation

[0036] The following embodiments further illustrate the specific implementation of the present invention, but the implementation and protection of the present invention are not limited thereto. It should be noted that any processes not specifically described below are those that can be implemented or understood by those skilled in the art based on existing technology. Reagents or instruments whose manufacturers are not specified are considered to be conventional products that can be purchased commercially.

[0037] The present invention will be further described below with reference to the embodiments. The tensile strength, elongation at break, and adhesive strength of the hydrogels obtained in the embodiments were determined using the methods disclosed in J. Mater. Chem. C, 2023, 11, 2316-2327. These embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0038] The stirring described in the following examples was carried out at room temperature.

[0039] Example 1

[0040] At room temperature, 3.5 mol of N,N,N-trimethyl-3-(2-methylallylamino)-1-chloropropylamine and 4 mol of dimethylaminoethyl methacrylate were added to 1 L of deionized water and stirred for 10 min. Then, 0.023 mol of N,N'-methylenebispropionamide was added and stirred until the solid was completely dissolved. Nitrogen gas was then introduced to remove oxygen from the mixture. Finally, 0.015 mol of 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylphenylacetone was added and stirred until the solid was completely dissolved to obtain the reaction solution. The reaction solution was then sonicated for 5 min to remove air bubbles. The reaction solution was poured into a long and narrow transparent sealed bag and allowed to stand for 0.5 hours to allow the reaction solution to separate into two layers, both of which were clear. Then, the transparent sealed bag was placed between two glass plates with a gasket between them. The two glass plates were slowly brought closer together until they were clamped and sealed. Finally, the mold was placed under an 80W 320nm UV lamp for 3 hours to initiate polymerization. The reaction temperature was 20℃, resulting in a bilayer hydrogel.

[0041] Figure 1 Scanning electron microscopy and elemental energy dispersive spectroscopy images of the bilayer hydrogel prepared in Example 1. Figure 1 It can be seen that the hydrogel has a bilayer structure, with the upper and lower layers tightly bonded together without any gaps between them, indicating that the two layers of the bilayer hydrogel constructed using the spontaneous phase separation effect are tightly bonded. Furthermore, there is a significant difference in chlorine content between the two layers, with the upper layer containing very little chlorine and the lower layer containing a large amount. This suggests that the upper gel is mainly formed by the polymerization of dimethylaminoethyl methacrylate monomer, while the lower gel is mainly formed by the polymerization of N,N,N-trimethyl-3-(2-methylallylamino)-1-chloropropylamine monomer.

[0042] Figure 2 Tensile stress-strain diagrams of the upper and lower gel layers of the bilayer hydrogel prepared in Example 1. Figure 1 It can be seen that the upper layer of the bilayer hydrogel prepared in Example 1 has a tensile strength of 1200 kPa and an elongation at break of 300%; the lower layer has a tensile strength of 80 kPa and an elongation at break of 450%, indicating that the bilayer hydrogel has asymmetric mechanical properties and is suitable for use as a soft actuator.

[0043] The bilayer hydrogel prepared in Example 1 has a volume ratio of approximately 1:5 between its upper and lower layers. The upper layer is hard and non-sticky, with a tensile strength of 1200 kPa, an elongation at break of 300%, and an adhesive strength of 5 kPa. The lower layer is soft and sticky, with a tensile strength of 80 kPa, an elongation at break of 450%, and an adhesive strength of 40 kPa. The bilayer hydrogel exhibits significant asymmetric viscosity and has potential applications in in vivo tissue patches, wearable devices with human-machine interfaces, and other fields.

[0044] Example 2

[0045] At room temperature, 5 mol of N,N,N-trimethyl-3-(2-methylallylamino)-1-chloropropylamine and 2 mol of dimethylaminoethyl methacrylate were added to 1 L of deionized water and stirred for 10 min. Then, 0.007 mol of N,N'-methylenebispropionamide was added and stirred until the solid was completely dissolved. Nitrogen gas was then introduced to remove oxygen from the mixture. Finally, 0.007 mol of 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylphenylacetone was added and stirred until the solid was completely dissolved, yielding a reaction solution. The solution was then sonicated for 5 min to remove air bubbles. The reaction solution was poured into a long, narrow, transparent, sealed bag and allowed to stand for 0.5 h to allow the solution to separate into two clear layers. The transparent, sealed bag was then placed between two glass plates with a gasket between them. The two glass plates were slowly brought closer together until they were clamped and sealed. Finally, the mold was placed under an 80 W, 320 nm UV lamp for 3 h to initiate polymerization at a reaction temperature of 20 °C, resulting in a bilayer hydrogel.

[0046] The bilayer hydrogel prepared in Example 2 has a volume ratio of approximately 1:10 between its upper and lower layers. The upper layer is hard and non-sticky, with a tensile strength of 1000 kPa, an elongation at break of 400%, and an adhesive strength of 5 kPa. The lower layer is soft and sticky, with a tensile strength of 100 kPa, an elongation at break of 500%, and an adhesive strength of 60 kPa.

[0047] Example 3

[0048] At room temperature, 2 mol of N,N,N-trimethyl-3-(2-methylallylamino)-1-chloropropylamine and 6 mol of dimethylaminoethyl methacrylate were added to 1 L of deionized water and stirred for 10 min. Then, 0.040 mol of N,N'-methylenebispropionamide was added and stirred until the solid was completely dissolved. Nitrogen gas was then introduced to remove oxygen from the mixture. Finally, 0.024 mol of 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylphenylacetone was added and stirred until the solid was completely dissolved, yielding a reaction solution. The solution was then sonicated for 5 min to remove air bubbles. The reaction solution was poured into a long, narrow, transparent sealed bag and allowed to stand for 0.5 h to allow the solution to separate into two clear layers. The transparent sealed bag was then placed between two glass plates with a gasket between them. The two glass plates were slowly brought closer together until they were clamped and sealed. Finally, the mold was placed under an 80 W, 320 nm UV lamp for 4 h to initiate polymerization at a reaction temperature of 20 °C, resulting in a bilayer hydrogel.

[0049] The bilayer hydrogel prepared in Example 3 has a volume ratio of approximately 1:2 between its upper and lower layers. The upper layer is hard and non-sticky, with a tensile strength of 1400 kPa, an elongation at break of 200%, and an adhesive strength of 5 kPa. The lower layer is soft and weakly sticky, with a tensile strength of 60 kPa, an elongation at break of 300%, and an adhesive strength of 10 kPa.

[0050] Example 4

[0051] At room temperature, 5 mol of N,N,N-trimethyl-3-(2-methylallylamino)-1-chloropropylamine and 2 mol of dimethylaminoethyl acrylate were added to 1 L of deionized water and stirred for 10 min. Then, 0.007 mol of N,N'-methylenebispropionamide was added and stirred until the solid was completely dissolved. Nitrogen gas was then introduced to remove oxygen from the mixture. Finally, 0.035 mol of 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylphenylacetone was added and stirred until the solid was completely dissolved, yielding a reaction solution. The solution was then sonicated for 5 min to remove air bubbles. The reaction solution was poured into a long, narrow, transparent sealed bag and allowed to stand for 0.5 h to allow the solution to separate into two clear layers. The transparent sealed bag was then placed between two glass plates with a gasket between them. The two glass plates were slowly brought closer together until they were clamped and sealed. Finally, the mold was placed under an 80 W, 320 nm UV lamp for 4 h to initiate polymerization at a reaction temperature of 20 °C, resulting in a bilayer hydrogel.

[0052] The bilayer hydrogel prepared in Example 4 has a volume ratio of approximately 1:10 between its upper and lower layers. The upper layer is hard and non-sticky, with a tensile strength of 800 kPa, an elongation at break of 400%, and an adhesive strength of 5 kPa. The lower layer is soft and sticky, with a tensile strength of 100 kPa, an elongation at break of 500%, and an adhesive strength of 60 kPa.

[0053] Example 5

[0054] At room temperature, 2 mol of N,N,N-trimethyl-3-(2-methylallylamino)-1-chloropropylamine and 6 mol of dimethylaminoethyl acrylate were added to 1 L of deionized water and stirred for 10 min. Then, 0.040 mol of N,N'-methylenebispropionamide was added and stirred until the solid was completely dissolved. Nitrogen gas was then introduced to remove oxygen from the mixture. Finally, 0.056 mol of 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylphenylacetone was added and stirred until the solid was completely dissolved, yielding a reaction solution. The solution was then sonicated for 5 min to remove air bubbles. The reaction solution was poured into a long, narrow, transparent sealed bag and allowed to stand for 1 h to allow the solution to separate into two clear layers. The transparent sealed bag was then placed between two glass plates with a gasket between them. The two glass plates were slowly brought closer together until they were clamped and sealed. Finally, the mold was placed under an 80 W, 320 nm UV lamp for 5 h to initiate polymerization at a reaction temperature of 20 °C, resulting in a bilayer hydrogel.

[0055] The bilayer hydrogel prepared in Example 5 has a volume ratio of approximately 1:2 between its upper and lower layers. The upper layer is hard and non-sticky, with a tensile strength of 1000 kPa, an elongation at break of 200%, and an adhesive strength of 5 kPa. The lower layer is soft and weakly sticky, with a tensile strength of 60 kPa, an elongation at break of 300%, and an adhesive strength of 10 kPa.

[0056] Example 6

[0057] At room temperature, 5 mol of N,N,N-trimethyl-3-(2-methylallylamino)-1-chloropropylamine and 2 mol of 2-N-morpholinoethyl methacrylate were added to 1 L of deionized water and stirred for 10 min. Then, 0.007 mol of N,N'-methylenebispropionamide was added and stirred until the solid was completely dissolved. Nitrogen gas was then introduced to remove oxygen from the mixture. Finally, 0.063 mol of 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylphenylacetone was added and stirred until the solid was completely dissolved, yielding a reaction solution. The solution was then sonicated for 5 min to remove air bubbles. The reaction solution was poured into a polytetrafluoroethylene grooved mold and allowed to stand for 1 h to allow the reaction solution to separate into two clear layers. A transparent cover was then placed on the mold, and finally, the mold was placed under a 100 W 320 nm UV lamp for 5 h to initiate polymerization at a reaction temperature of 20 °C, resulting in a bilayer hydrogel.

[0058] The bilayer hydrogel prepared in Example 6 has a volume ratio of approximately 1:10 between its upper and lower layers. The upper layer is hard and non-sticky, with a tensile strength of 1100 kPa, an elongation at break of 400%, and an adhesive strength of 5 kPa. The lower layer is soft and sticky, with a tensile strength of 100 kPa, an elongation at break of 500%, and an adhesive strength of 60 kPa.

[0059] Example 7

[0060] At room temperature, 2 mol of N,N,N-trimethyl-3-(2-methylallylamino)-1-chloropropylamine and 6 mol of 2-N-morpholinoethyl methacrylate were added to 1 L of deionized water and stirred for 10 min. Then, 0.040 mol of N,N'-methylenebispropionamide was added and stirred until the solid was completely dissolved. Nitrogen gas was then introduced to remove oxygen from the mixture. Finally, 0.080 mol of 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylphenylacetone was added and stirred until the solid was completely dissolved, yielding a reaction solution. The solution was then sonicated for 5 min to remove air bubbles. The reaction solution was poured into a polytetrafluoroethylene grooved mold and allowed to stand for 1 h to allow the reaction solution to separate into two clear layers. A transparent cover was then placed on the mold, and finally, the mold was placed under a 100 W 320 nm UV lamp for 6 h to initiate polymerization at a reaction temperature of 20 °C, resulting in a bilayer hydrogel.

[0061] The bilayer hydrogel prepared in Example 7 has a volume ratio of approximately 1:2 between its upper and lower layers. The upper layer is hard and non-sticky, with a tensile strength of 1500 kPa, an elongation at break of 200%, and an adhesive strength of 5 kPa. The lower layer is soft and weakly sticky, with a tensile strength of 60 kPa, an elongation at break of 300%, and an adhesive strength of 10 kPa.

[0062] Example 8

[0063] At room temperature, 5 mol of [2-(methacryloyloxy)ethyl]trimethylammonium chloride and 2 mol of N-[(3-(dimethylamino)propyl]methacrylamide] were added to 1 L of deionized water and stirred for 10 min. Then, 0.007 mol of N,N'-methylenebispropionamide was added and stirred until the solid was completely dissolved. Argon gas was then introduced to remove oxygen from the mixture. Finally, 0.007 mol of α-ketoglutaric acid was added and stirred until the solid was completely dissolved to obtain a reaction solution. The reaction solution was then sonicated for 5 min to remove air bubbles. The reaction solution was poured into a polytetrafluoroethylene grooved mold and allowed to stand for 1.5 h to allow the reaction solution to separate into two clear layers. A transparent cover was then placed on the mold, and finally, the mold was placed under a 100 W 365 nm UV lamp for 6 h to initiate polymerization. The reaction temperature was 35 °C, resulting in a bilayer hydrogel.

[0064] The bilayer hydrogel prepared in Example 8 has a volume ratio of approximately 1:30 between its upper and lower layers. The upper layer is hard and non-sticky, with a tensile strength of 900 kPa, an elongation at break of 400%, and an adhesive strength of 5 kPa. The lower layer is soft and sticky, with a tensile strength of 120 kPa, an elongation at break of 500%, and an adhesive strength of 60 kPa.

[0065] Example 9

[0066] At room temperature, 2 mol of [2-(methacryloyloxy)ethyl]trimethylammonium chloride and 6 mol of N-[(3-(dimethylamino)propyl]methacrylamide] were added to 1 L of deionized water and stirred for 10 min. Then, 0.040 mol of N,N'-methylenebispropionamide was added and stirred until the solid was completely dissolved. Argon gas was then introduced to remove oxygen from the mixture. Finally, 0.016 mol of α-ketoglutaric acid was added and stirred until the solid was completely dissolved to obtain a reaction solution. The solution was then sonicated for 5 min to remove air bubbles. The reaction solution was poured into a polytetrafluoroethylene grooved mold and allowed to stand for 1.5 h to allow the reaction solution to separate into two clear layers. A transparent cover was then placed on the mold, and finally, the mold was placed under a 100 W 365 nm UV lamp for 7 h to initiate polymerization. The reaction temperature was 35 °C, resulting in a bilayer hydrogel.

[0067] The bilayer hydrogel prepared in Example 9 has a volume ratio of approximately 1:3 between its upper and lower layers. The upper layer is hard and non-sticky, with a tensile strength of 1300 kPa, an elongation at break of 200%, and an adhesive strength of 5 kPa. The lower layer is soft and weakly sticky, with a tensile strength of 80 kPa, an elongation at break of 300%, and an adhesive strength of 10 kPa.

[0068] Example 10

[0069] At room temperature, 5 mol of acryloyloxyethyltrimethylammonium chloride and 2 mol of N-[(3-dimethylamino)propyl]acrylamide were added to 1 L of deionized water and stirred for 10 min. Then, 0.007 mol of N,N'-methylenebispropionamide was added and stirred until the solid was completely dissolved. Argon gas was then introduced to remove oxygen from the mixture. Finally, 0.021 mol of α-ketoglutaric acid was added and stirred until the solid was completely dissolved to obtain a reaction solution. The solution was then sonicated for 5 min to remove air bubbles. The reaction solution was poured into a polytetrafluoroethylene grooved mold and allowed to stand for 1.5 h to allow the reaction solution to separate into two clear layers. A transparent cover was then placed on the mold, and finally, the mold was placed under a 100 W 365 nm UV lamp for 8 h to initiate polymerization. The reaction temperature was 35 °C, resulting in a bilayer hydrogel.

[0070] The bilayer hydrogel prepared in Example 10 has a volume ratio of approximately 1:10 between its upper and lower layers. The upper layer is hard and non-sticky, with a tensile strength of 700 kPa, an elongation at break of 400%, and an adhesive strength of 5 kPa. The lower layer is soft and non-sticky, with a tensile strength of 110 kPa, an elongation at break of 500%, and an adhesive strength of 5 kPa.

[0071] Example 11

[0072] At room temperature, 2 mol of acryloyloxyethyltrimethylammonium chloride and 6 mol of N-[(3-dimethylamino)propyl]acrylamide were added to 1 L of deionized water and stirred for 10 min. Then, 0.040 mol of N,N'-methylenebispropionamide was added and stirred until the solid was completely dissolved. Argon gas was then introduced to remove oxygen from the mixture. Finally, 0.040 mol of α-ketoglutaric acid was added and stirred until the solid was completely dissolved to obtain a reaction solution. The solution was then sonicated for 5 min to remove air bubbles. The reaction solution was poured into a transparent test tube and allowed to stand for 1.5 h to allow the reaction solution to separate into two clear layers. The tube opening was then sealed with sealing film. Finally, the mold was placed under a 120 W 365 nm UV lamp for 9 h to initiate polymerization at a reaction temperature of 35 °C, resulting in a bilayer hydrogel.

[0073] The bilayer hydrogel prepared in Example 11 has a volume ratio of approximately 1:2 between its upper and lower layers. The upper layer is hard and non-sticky, with a tensile strength of 1100 kPa, an elongation at break of 200%, and an adhesive strength of 5 kPa. The lower layer is soft and non-sticky, with a tensile strength of 70 kPa, an elongation at break of 300%, and an adhesive strength of 5 kPa.

[0074] Example 12

[0075] At room temperature, 5 mol of (3-acrylamidopropyl)trimethylammonium chloride and 2 mol of N-[2-(dimethylamino)ethyl]methacrylamide were added to 1 L of deionized water and stirred for 10 min. Then, 0.007 mol of N,N'-methylenebispropionamide was added and stirred until the solid was completely dissolved. Argon gas was then introduced to remove oxygen from the mixture. Finally, 0.049 mol of α-ketoglutaric acid was added and stirred until the solid was completely dissolved to obtain the reaction solution. The reaction solution was then sonicated for 5 min to remove air bubbles. The reaction solution was poured into a transparent test tube and allowed to stand for 1.5 h to allow the reaction solution to separate into two clear layers. The tube opening was then sealed with sealing film. Finally, the mold was placed under a 120 W 365 nm UV lamp for 10 h to initiate polymerization at a reaction temperature of 35 °C, resulting in a bilayer hydrogel.

[0076] The bilayer hydrogel prepared in Example 12 has a volume ratio of approximately 1:10 between its upper and lower layers. The upper layer is hard and non-sticky, with a tensile strength of 800 kPa, an elongation at break of 400%, and an adhesive strength of 5 kPa. The lower layer is soft and sticky, with a tensile strength of 90 kPa, an elongation at break of 500%, and an adhesive strength of 60 kPa.

[0077] Example 13

[0078] At room temperature, 2 mol of (3-acrylamidopropyl)trimethylammonium chloride and 6 mol of N-[2-(dimethylamino)ethyl]methacrylamide were added to 1 L of deionized water and stirred for 10 min. Then, 0.040 mol of N,N'-methylenebispropionamide was added and stirred until the solid was completely dissolved. Argon gas was then introduced to remove oxygen from the mixture. Finally, 0.072 mol of α-ketoglutaric acid was added and stirred until the solid was completely dissolved to obtain the reaction solution. The reaction solution was sonicated for 5 min to remove air bubbles. The reaction solution was poured into a transparent test tube and allowed to stand for 2 h to allow the reaction solution to separate into two clear layers. The tube opening was then sealed with sealing film. Finally, the mold was placed under a 120 W 365 nm UV lamp for 11 h to initiate polymerization at a reaction temperature of 35 °C, resulting in a bilayer hydrogel.

[0079] The bilayer hydrogel prepared in Example 13 has a volume ratio of approximately 1:2 between its upper and lower layers. The upper layer is hard and non-sticky, with a tensile strength of 1200 kPa, an elongation at break of 200%, and an adhesive strength of 5 kPa. The lower layer is soft and weakly sticky, with a tensile strength of 50 kPa, an elongation at break of 300%, and an adhesive strength of 10 kPa.

[0080] Example 14

[0081] At room temperature, 5 mol of N,N,N-trimethyl-3-(2-methylallylamino)-1-chloropropylamine and 2 mol of dimethylaminoethyl methacrylate were added to 1 L of deionized water and stirred for 10 min. Then, 0.007 mol of N,N'-methylenebispropionamide was added and stirred until the solid was completely dissolved. Nitrogen gas was then introduced to remove oxygen from the mixture. Finally, 0.007 mol of 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylphenylacetone was added and stirred until the solid was completely dissolved, yielding a reaction solution. The solution was sonicated for 5 min to remove air bubbles. The reaction solution was poured into a long, narrow, transparent sealed bag and allowed to stand for 0.5 h to allow the solution to separate into two clear layers. The transparent sealed bag was then placed between two glass plates with a gasket between them. The two glass plates were slowly brought closer together until they were clamped and sealed. Finally, the mold was placed under an 80 W, 320 nm UV lamp for 3 h to initiate polymerization at a reaction temperature of 50 °C, resulting in a bilayer hydrogel.

[0082] The bilayer hydrogel prepared in Example 14 has a volume ratio of approximately 1:7 between its upper and lower layers. The upper layer is hard and non-sticky, with a tensile strength of 1100 kPa, an elongation at break of 400%, and an adhesive strength of 5 kPa. The lower layer is soft and sticky, with a tensile strength of 90 kPa, an elongation at break of 500%, and an adhesive strength of 60 kPa.

[0083] Example 15

[0084] At room temperature, 5 mol of N,N,N-trimethyl-3-(2-methylallylamino)-1-chloropropylamine and 2 mol of 2-N-morpholinoethyl methacrylate were added to 1 L of deionized water and stirred for 10 min. Then, 0.007 mol of N,N'-methylenebispropionamide was added and stirred until the solid was completely dissolved. Nitrogen gas was then introduced to remove oxygen from the mixture. Finally, 0.063 mol of 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylphenylacetone was added and stirred until the solid was completely dissolved, yielding a reaction solution. The solution was then sonicated for 5 min to remove air bubbles. The reaction solution was poured into a polytetrafluoroethylene grooved mold and allowed to stand for 1 h to allow the reaction solution to separate into two clear layers. A transparent cover was then placed on the mold, and finally, the mold was placed under a 100 W 320 nm UV lamp for 5 h to initiate polymerization at a reaction temperature of 50 °C, resulting in a bilayer hydrogel.

[0085] The bilayer hydrogel prepared in Example 15 has a volume ratio of approximately 1:7 between its upper and lower layers. The upper layer is hard and non-sticky, with a tensile strength of 1200 kPa, an elongation at break of 400%, and an adhesive strength of 5 kPa. The lower layer is soft and sticky, with a tensile strength of 90 kPa, an elongation at break of 500%, and an adhesive strength of 60 kPa.

[0086] A comparison of Examples 2 and 14, and Examples 6 and 15 shows that by simply changing the polymerization temperature, the bilayer hydrogels obtained in Examples 2 and 14, or Examples 4 and 15, have different layering ratios. This is because the second ionic monomers, dimethylaminoethyl methacrylate and 2-N-morpholinoethyl methacrylate, are thermosensitive, and the layering of the system can be controlled by temperature.

[0087] Comparative Example 1

[0088] At room temperature, 1 mol of N,N,N-trimethyl-3-(2-methylallylamino)-1-chloropropylamine and 1 mol of dimethylaminoethyl methacrylate were added to 1 L of deionized water and stirred for 10 min. Then, 0.002 mol of N,N'-methylenebispropionamide was added and stirred until the solid was completely dissolved. Argon gas was then introduced to remove oxygen from the mixture. Finally, 0.020 mol of α-ketoglutaric acid was added and stirred until the solid was completely dissolved, yielding a reaction solution. The solution was sonicated for 5 min to remove air bubbles. The reaction solution was poured into a transparent test tube and allowed to stand for 2 h to allow the solution to separate into two clear layers. The tube was then sealed with sealing film. Finally, the mold was placed under a 120 W 365 nm UV lamp for 12 h to initiate polymerization at a reaction temperature of 50 °C, yielding a hydrogel.

[0089] The hydrogel prepared in Comparative Example 1 was a homogeneous system and did not exhibit stratification. Figure 3 Scanning electron microscope and elemental energy dispersive spectroscopy images of the homogeneous hydrogel prepared in Comparative Example 1 were obtained from... Figure 3 It can be seen that the carbon, oxygen, and chlorine elements are evenly distributed in the hydrogel, and there is no difference in element distribution, indicating that a layered hydrogel cannot be prepared at this formulation concentration.

[0090] As can be seen from Comparative Example 1, when the amount of the first ionic monomer N,N,N-trimethyl-3-(2-methylallylamino)-1-chloropropylamine and the second ionic monomer dimethylaminoethyl methacrylate added is small, that is, when the concentration of the first ionic monomer or the second ionic monomer in water does not reach the concentration range described in this invention, the system will not undergo phase separation, and a hydrogel with a bilayer structure cannot be prepared.

Claims

1. A method for preparing a bilayer hydrogel, characterized in that, Includes the following steps: (1) Add a first ionic monomer, a second ionic monomer, and a crosslinking agent to deionized water, and stir until the solid is completely dissolved to obtain a mixture; the first ionic monomer is any one of N,N,N-trimethyl-3-(2-methylallylamino)-1-chloropropylamine, [2-(methacryloyloxy)ethyl]trimethylammonium chloride, acryloyloxyethyltrimethylammonium chloride, and (3-acrylamidopropyl)trimethylammonium chloride; the ratio of the amount of the first ionic monomer added to the volume of deionized water is 2 mol / L to 5 mol / L; the second ionic monomer is any one of dimethylaminoethyl methacrylate, dimethylaminoethyl acrylate, 2-N-morpholinoethyl methacrylate, N-[(3-(dimethylamino)propyl]methacrylamide, N-[(3-dimethylamino)propyl]acrylamide, and N-[2-(dimethylamino)ethyl]methacrylamide; the ratio of the amount of the second ionic monomer added to the volume of deionized water is 2 mol / L to 6 mol / L; (2) Inert gas is introduced into the mixture, then photoinitiator is added and stirred until the solid is completely dissolved to obtain a reaction solution. The reaction solution is then sonicated. (3) Pour the reaction solution into the mold and let it stand until the reaction solution separates into two layers and both liquids are clear. Then, initiate polymerization under ultraviolet light to obtain a bilayer hydrogel.

2. The method for preparing a bilayer hydrogel according to claim 1, characterized in that, The crosslinking agent in step (1) is N,N'-methylenebispropionamide; the amount of crosslinking agent added is 0.1% to 0.5% of the total amount of the first ionic monomer and the second ionic monomer.

3. The method for preparing a bilayer hydrogel according to claim 1, characterized in that, The photoinitiator in step (2) is either 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylphenylacetone or α-ketoglutaric acid; the amount of photoinitiator added is 0.1% to 1% of the total amount of the first ionic monomer and the second ionic monomer.

4. The method for preparing a bilayer hydrogel according to claim 1, characterized in that, The inert gas mentioned in step (2) is either nitrogen or argon.

5. The method for preparing a bilayer hydrogel according to claim 1, characterized in that, The mold mentioned in step (3) can be any one of the following: a transparent test tube, a polytetrafluoroethylene groove mold with a transparent cover, or a mold assembled from two glass plates with a gasket sandwiched between them and a long and narrow transparent sealing bag.

6. The method for preparing a bilayer hydrogel according to claim 1, characterized in that, The wavelength of the ultraviolet light in step (3) is 320nm~365nm, the power is 80W~120W, the irradiation time is 3h~12h, and the reaction temperature is 20℃~50℃.

7. A bilayer hydrogel prepared by the preparation method according to any one of claims 1-6, characterized in that, It has asymmetric properties.

8. The use of the bilayer hydrogel of claim 7 in the preparation of asymmetric adhesion or soft actuation devices.

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