A method for preparing hydrogel with highly oriented microstructure

Abstract

The present invention discloses a method for preparing a hydrogel with a highly oriented microstructure. The method comprises the following steps: (1) dissolving PVA with a citric acid solution under heating conditions until the PVA is completely dissolved to obtain a PVA solution; (2) cooling the PVA solution obtained in step (1) to room temperature, and then performing freeze-thaw to obtain a PVA-based hydrogel with a highly oriented pore channel structure. The present invention dissolves PVA in a citric acid solution, and then freeze-thaws, without the need for special equipment or harsh preparation and adjustment parts. The method is simple, gentle, and efficient, and a hydrogel with a highly oriented microstructure is prepared. Dispersing conductive nanomaterials in the hydrogel prepared by the present invention can significantly improve the conductivity of the conductive material, and has the advantages of being more sensitive, real-time, and accurate.

Description

Technical Field

[0001] The invention relates to the technical field of hydrogels, and in particular to a method for preparing a hydrogel with a highly oriented microstructure. Background Art

[0002] As an important class of soft materials, hydrogels exhibit rich and colorful characteristics, which mainly come from their inherent microstructure. Therefore, it is of great significance to realize directional channel structures in hydrogels to give them unique properties. At present, in order to achieve this unique structure, researchers have adopted a variety of methods, among which directional freezing is the most common method. During the directional freezing process, monomers are concentrated in the crystal domain and the freezing of the precursor solution is promoted along a unidirectional temperature gradient, in which ice crystals act as templates to promote the formation of 3D ordered cross-linked polymer microstructures. This unidirectional temperature gradient can be achieved in two ways: one is to immerse the container containing the liquid sample in a cold water bath at a preset controlled speed, and the other is to contact the bottom of the container with cold water and freeze from the bottom to the top with a significant temperature gradient. Generally, both methods require special equipment to control the immersion speed of the pre-polymerized sample, or the uniformity of the structure depends on harsh environmental conditions. In addition, due to too many uncontrollable parameters, the achieved network microstructure usually shows poor homogeneity. Therefore, it is crucial to explore a simple, mild and efficient method to prepare hydrogels with highly oriented microstructures. Summary of the invention

[0003] In view of the above prior art, the purpose of the present invention is to provide a method for preparing a hydrogel with a highly oriented microstructure. The present invention dissolves PVA in a citric acid solution and then freezes and thaws, without the need for special equipment or harsh preparation conditions. The method is simple, mild and efficient, and a hydrogel with a highly oriented microstructure is prepared.

[0004] To achieve the above object, the present invention adopts the following technical solution:

[0005] The first aspect of the present invention provides a method for preparing a hydrogel having a highly oriented microstructure, comprising the following steps:

[0006] (1) dissolving PVA with a citric acid solution under heating conditions until the PVA is completely dissolved to obtain a PVA solution;

[0007] (2) Cooling the PVA solution obtained in step (1) to room temperature, and then freezing-thawing to obtain a PVA-based hydrogel with a highly oriented pore channel structure.

[0008] Preferably, in step (1), the heating is water bath heating; the heating temperature is 90-95°C.

[0009] Preferably, in step (1), the citric acid solution is obtained by mixing citric acid and deionized water, and the pH value of the citric acid solution is 2-6.

[0010] Preferably, in step (1), the concentration of PVA in the PVA solution is 4 to 9 wt %.

[0011] Preferably, in step (2), the freezing temperature is -20°C, and the freezing time is 12 to 36 hours; the thawing temperature is room temperature, and the thawing time is 4 to 5 hours.

[0012] The second aspect of the present invention provides a PVA-based hydrogel having a highly oriented pore channel structure obtained by the above preparation method.

[0013] The third aspect of the present invention provides the use of a PVA-based hydrogel having a highly oriented pore channel structure in preparing a conductive gel or improving the conductivity of a conductive material.

[0014] Beneficial effects of the present invention:

[0015] (1) The present invention dissolves PVA in a citric acid solution and then freezes and thaws to prepare a hydrogel with a highly oriented microstructure. The present invention does not require special equipment or harsh preparation conditions, and the method is simple, mild, and efficient to prepare a hydrogel with a highly oriented microstructure.

[0016] (2) The present invention induces ice crystals to grow in a certain direction through spatial extension of citric acid anions, thereby forming cylindrical ice crystals. Then, in the presence of ice crystals, regular and dense PVA-CA microporous channels are formed, thereby realizing a directional pore channel structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 :Microstructure of the hydrogel prepared in Example 1; a. Electron microscopic image of the cross section of the PVA hydrogel at 200 times magnification. b. Electron microscopic image of the longitudinal section of the PVA hydrogel at 200 times magnification. c. Electron microscopic image of the cross section of the PVA hydrogel at 1000 times magnification. d. Electron microscopic image of the longitudinal section of the PVA hydrogel at 1000 times magnification.

[0018] Figure 2 : Electrical sensing response and recovery time of PVA-MXene hydrogel with citric acid as solvent.

[0019] Figure 3: Hydrogels used as sensors for detecting human heart health. (a1) and (a2) show the traditional detection of human heart health using metal as electrodes and hydrogel as electrodes, (a3) ​​and (a4) show the enlarged images of the wrist and ankle when using hydrogel as a sensor. (a5) Data is collected and transmitted to the data platform. PVA gel with water as solvent. (b) Hydrogel with water as solvent, (c) gel with citric acid as solvent as electrode sheets for detecting human heart health test results (ECG).

[0020] Figure 4 : Optical microscope photographs of (a) water and (b) citric acid ice crystals after freezing. DETAILED DESCRIPTION

[0021] It should be noted that the following detailed descriptions are illustrative and are intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present application belongs.

[0022] As described in the background technology section, in the prior art, a directional channel structure is realized in a hydrogel, and its preparation method needs to rely on special equipment, and there are also defects of complex operation and poor product homogeneity. In addition, the patent with application number 201810205930.9 discloses a citric acid cross-linked chitosan hydrogel and a preparation method thereof, wherein chitosan is dissolved in an acetic acid solution, heated and then citric acid is added, followed by freezing and thawing to obtain a preliminary hydrogel model, and then the hydrogel model is subjected to an amidation cross-linking reaction, and finally a porous, macroporous and different-shaped hydrogel is obtained. Although this method adds citric acid to chitosan, it only premixes citric acid and chitosan together, and then performs the next step of amidation cross-linking to form a citric acid cross-linked chitosan hydrogel. And the initial gel obtained after freezing and thawing does not form a highly oriented microstructure. Finally, after cross-linking, only a hydrogel with a porous and macroporous structure is formed, rather than a hydrogel with a directional pore channel structure.

[0023] Based on this, the purpose of the present invention is to provide a method for preparing a hydrogel with a highly oriented microstructure. The present invention uses citric acid as a medium to prepare a hydrogel with a highly oriented microstructure. Studies on the mechanism of network formation have shown that the spatially extended anion skeleton of the ternary weak acid plays an important role in the formation of the oriented microstructure. The spatial extension of the citric acid ion plays a key role in the formation of a regular ice crystal columnar structure through oriented growth and uniform distribution. During the freezing process, phase separation occurs, and ice crystals are formed with water as the liquid molecule. During the thawing process, the ice crystals melt and a micron-sized porous structure is generated in the hydrogel. When H2O is used as a solvent, PVA-H2O leads to an irregular microporous structure due to the inability to form regular ice crystals. On the contrary, when a citric acid solution is used, the spatial extension of the citric acid anions may induce ice crystals to grow in a certain direction, thereby forming cylindrical ice crystals. Then, in the presence of ice crystals, regular and dense PVA-CA microporous channels are formed.

[0024] In order to enable those skilled in the art to more clearly understand the technical solution of the present application, the technical solution of the present application will be described in detail below in conjunction with specific embodiments.

[0025] The test materials used in the examples of the present invention are all conventional test materials in the art and can be purchased through commercial channels.

[0026] Example 1

[0027] In a water bath at 93°C, PVA was dissolved in a citric acid solution with a pH of 3, and magnetic stirring was used until the PVA was completely dissolved to obtain a PVA solution with a concentration of 4.75wt%. The obtained PVA solution was first cooled to room temperature, then transferred to a refrigerator with a preset temperature of -20°C and kept cooled for 24 hours, and then thawed at room temperature for 4.5 hours to obtain a PVA-based hydrogel with a highly oriented pore channel structure.

[0028] Example 2

[0029] In a water bath at 90°C, PVA was dissolved in a citric acid solution with a pH of 2, and magnetic stirring was used until the PVA was completely dissolved to obtain a PVA solution with a concentration of 4 wt%. The obtained PVA solution was first cooled to room temperature, then transferred to a refrigerator with a preset temperature of -20°C and kept cooled for 12 hours, and then thawed at room temperature for 4 hours to obtain a PVA-based hydrogel with a highly oriented pore channel structure.

[0030] Example 3

[0031] In a water bath at 95°C, PVA was dissolved in a citric acid solution with a pH of 6, and magnetic stirring was used until the PVA was completely dissolved to obtain a PVA solution with a concentration of 9 wt%. The obtained PVA solution was first cooled to room temperature, then transferred to a refrigerator with a preset temperature of -20°C and kept cooled for 36 hours, and then thawed at room temperature for 5 hours to obtain a PVA-based hydrogel with a highly oriented pore channel structure.

[0032] Comparative Example

[0033] The difference from Example 1 is that the citric acid solution with a pH of 2 is replaced by deionized water of equal mass.

[0034] After freezing, the optical microscope photos of the hydrogels prepared in the comparative example and Example 1 are as follows: Figure 4 As shown by Figure 4 It can be seen that in Example 1, the spatial extension of the citric acid anions induces ice crystals to grow in a certain direction, thereby forming cylindrical ice crystals. Then, in the presence of ice crystals, regular and dense PVA-CA microporous channels are formed, achieving a directional pore channel structure.

[0035] Application Examples

[0036] Conductive nanomaterials (MXene) were added to the hydrogels prepared in Example 1 and Comparative Example, respectively.

[0037] Experimental method: First, 0.015 g of MXene was dispersed in 1 ml of deionized water under ultrasound for 2 minutes to obtain a MXene dispersion. With a mass ratio of MXene to PVA of 3:1, the MXene dispersion was added to a PVA solution with citric acid as solvent (Example 1) and a PVA solution with water as solvent (Comparative Example), and the MXene was evenly dispersed by magnetic stirring. After the solution was cooled to room temperature, it was transferred to a refrigerator, frozen at -20°C for 24 hours, and then thawed at room temperature for 4 hours. The resulting hydrogels were labeled PVA-CS-MX hydrogel and PVA-water-MX hydrogel.

[0038] Electrocardiogram monitoring.

[0039] The hydrogel samples were cut into discs of appropriate size (1 cm in diameter) and used as electrodes for ECG measurement. The hydrogels were connected to the chest, wrists and ankles and connected to the ECG monitoring equipment (iE 12A, Shenzhen BiocareBio-Medical Equipment Co., Ltd.) via wires. The ECG activity signals were sensed by the hydrogel electrodes and ultimately transmitted to the ECG monitoring equipment. The analysis criteria for the ECG spectrum include characteristics, frequency, repeatability, etc.

[0040] Depend on Figure 3 b and Figure 3 c It can be seen that the sensor made of PVA-CS-MX hydrogel has the advantages of being more sensitive, real-time and accurate when monitoring human body movement compared with the hydrogel using only water as solvent.

[0041] The above description is only the preferred embodiment of the present application and is not intended to limit the present application. For those skilled in the art, the present application may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. The use of citric acid in improving the high orientation of PVA-based hydrogels, characterized in that: In a 93°C water bath, PVA was dissolved with a citric acid solution with a pH of 3, and magnetic stirring was used until the PVA was completely dissolved to obtain a PVA solution with a concentration of 4.75wt%; the obtained PVA solution was first cooled to room temperature, then transferred to a refrigerator with a preset temperature of -20°C and kept cooled for 24 hours, and then thawed at room temperature for 4.5 hours to obtain a PVA-based hydrogel with a highly oriented pore channel structure.

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