Pressure-resistant hydrogel with continuous gradient structure and preparation method of pressure-resistant hydrogel

Abstract

The invention relates to the technical field of hydrogel, in particular to pressure-resistant hydrogel with a continuous gradient structure and a preparation method of the pressure-resistant hydrogel. Bamboo is used as a natural gradient pressure-bearing material, and the longitudinal section direction of the bamboo shows the continuous change characteristics from compactness to looseness and from high modulus to low modulus from bottom to top, so that the bamboo has excellent compressive strength, energy dissipation capacity and fatigue resistance while the overall light weight is kept. On the basis, a thin film with the thickness gradually increased is packaged on the outer side of the pregel in the limited direction, due to the fact that the thicknesses of packaging layers are different, and moisture migration and heat transfer conditions of all areas are different, the pressure-resistant hydrogel of a continuous gradient structure is obtained, the porosity of the pressure-resistant hydrogel is gradually increased in the direction where the packaging thickness is gradually increased, and the crystallinity of the pressure-resistant hydrogel is gradually reduced. The compression modulus is gradually reduced. The spatial distribution of the microstructure in the hydrogel is regulated and controlled, so that the hydrogel can realize partitioned bearing and stress dispersion under the action of compression load, and the compressive strength and mechanical fatigue resistance are improved.

Description

Technical Field

[0001] This invention relates to the field of hydrogel technology, specifically to a pressure-resistant hydrogel with a continuous gradient structure and its preparation method. Background Technology

[0002] The information disclosed in this background section is intended only to enhance understanding of the overall background of the invention and is not necessarily to be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

[0003] Hydrogels are three-dimensional cross-linked networks composed of hydrophilic polymer molecular chains dispersed in water through cross-linking. Due to their high water content, hydrogels possess many excellent properties, making them promising for applications in flexible sensing, electronic skin, wound dressings, health monitoring, and many other fields.

[0004] However, traditional hydrogels often employ uniform cross-linked structures or simple multi-network structures, resulting in a relatively uniform distribution of their internal structure and mechanical properties. However, under compressive loads, these hydrogels are prone to problems such as localized buckling, stress concentration, or structural collapse, leading to limited compressive strength and long-term service stability, making it difficult to meet the high reliability and durability requirements of pressure-bearing applications.

[0005] In recent years, to simulate the structural characteristics of natural pressure-bearing structures, some studies have begun to explore the introduction of layered or partitioned structural designs into hydrogels to achieve differences in mechanical properties across different regions. However, such layered or partitioned structures typically exhibit abrupt property distributions, making it difficult to form a gradient structure similar to the continuous variations found in natural materials. Under compressive loads, stress concentration easily occurs at abrupt interfaces, thus affecting the overall structural stability and fatigue resistance.

[0006] Therefore, how to construct a gradient system with continuously varying structure and mechanical properties in hydrogel materials, and achieve a synergistic improvement in high load-bearing capacity, high buffering performance and long-term stability without significantly sacrificing material flexibility, remains an important technical problem that urgently needs to be solved in this field. Summary of the Invention

[0007] To overcome the above problems, the present invention provides a pressure-resistant hydrogel with a continuous gradient structure and a method for preparing the same.

[0008] To achieve the above technical objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a pressure-resistant hydrogel with a continuous gradient structure, wherein the porosity gradually increases, the crystallinity gradually decreases, and the compressive modulus gradually decreases along a defined direction.

[0009] In one or more embodiments, the defined direction is from the top surface of the hydrogel to the bottom surface.

[0010] A second aspect of the present invention provides a method for preparing the pressure-resistant hydrogel with a continuous gradient structure as described in the first aspect, comprising the following steps: (1) The hydrogel precursor solution was subjected to freeze-thaw cycles to obtain a pregel; (2) Encapsulate a film with gradually increasing thickness along a defined direction on the outside of the pregel; (3) The encapsulated pregel is oriented annealed and then hydrated to obtain a pressure-resistant hydrogel with a continuous gradient structure.

[0011] In one or more embodiments, in step (1), the pregel includes any one or more of polyvinyl alcohol polymers, polyacrylamide polymers, polyacrylic acid polymers or cellulose compounds, preferably polyvinyl alcohol polymers.

[0012] Preferably, when the pregel is a polyvinyl alcohol polymer, the preparation method of the hydrogel precursor solution includes: mixing polyvinyl alcohol and water and then heating to dissolve them.

[0013] More preferably, the heating temperature is 90~120 ℃.

[0014] More preferably, the mass fraction of polyvinyl alcohol in the hydrogel precursor solution is 5-20%. Since the hydrogel content significantly affects the density and compressive strength of the molded hydrogel, increasing the proportion of hydrogel material increases the number of polymer chains in the system, leading to improved physical entanglement or crystallinity between the chains, thus enhancing the hydrogel's load-bearing capacity under compressive loads. However, when the mass fraction of the hydrogel material exceeds a preset upper limit, the material is difficult to fully dissolve or uniformly disperse in water, easily resulting in an inhomogeneous precursor solution, which in turn affects subsequent molding and the stability of mechanical properties.

[0015] Preferably, when the pregel is a polyvinyl alcohol polymer, in step (1), the freezing temperature of the freeze-thaw treatment is -18 to -22 °C, preferably -20 °C; the freezing time is 5 to 7 h; the thawing temperature is 20 to 40 °C; and the number of freeze-thaw cycles is 3 to 6. Through freeze-thaw cycle treatment, the polymer molecular chains in the hydrogel precursor solution undergo physical cross-linking and rearrangement during ice crystal formation and melting, thereby constructing a stable basic network structure. The number of freeze-thaw cycles and the freezing time will affect the degree of formation of the internal network structure of the hydrogel, and thus affect its subsequent structural regulation and compressive strength.

[0016] Preferably, when the pregel is a polyvinyl alcohol polymer, the film thickness is 55-550 μm. By utilizing the encapsulation layer's restriction of moisture migration and heat transfer during annealing, the hydrogel forms different degrees of crystallization, porous structures, or polymer chain arrangements in different regions, thereby macroscopically constructing a continuous gradient distribution similar to a bamboo structure. An encapsulation layer that is too thin or too thick may lead to uneven control effects, thus affecting the stability of the gradient structure formation.

[0017] Preferably, when the pregel is a polyvinyl alcohol polymer, the directional annealing temperature is 25 ℃ to 75 ℃ and the directional annealing time is 12 to 20 h.

[0018] Preferably, when the pregel is a polyvinyl alcohol polymer, the hydration time is 12-24 h.

[0019] In one or more embodiments, in step (2), the material of the film is an organic silicone material, a copolymer material, a polyamide, a polyimide, a polyester, or a metal inorganic material; preferably, it is a polyimide.

[0020] In one or more embodiments, in step (2), the defined direction is from the top surface of the pregel to the bottom surface.

[0021] The beneficial effects of this invention are as follows: (1) Bamboo, as a typical natural gradient pressure-bearing material, exhibits a continuous change in its longitudinal section from dense to porous and from high modulus to low modulus, thus maintaining its overall lightweight nature while possessing excellent compressive strength, energy dissipation capacity, and fatigue resistance. Based on this, this invention encapsulates a film with gradually increasing thickness along a defined direction on the outside of a pre-gelled layer. During annealing, due to the different thicknesses of the encapsulation layer, the moisture migration and heat transfer conditions in each region differ, resulting in a continuously gradient structure of the pressure-resistant hydrogel. Along the direction of gradually increasing encapsulation thickness, the porosity gradually increases, the crystallinity gradually decreases, and the compressive modulus gradually decreases. In this invention, by controlling the spatial distribution of the microstructure inside the hydrogel, it can achieve zoned load bearing and stress dispersion under compressive load, significantly improving the overall compressive strength and mechanical fatigue resistance.

[0022] (2) By introducing a spatially programmable control step in the hydrogel preparation process, the compressibility of the hydrogel no longer depends on a single material formulation, but can be precisely designed through process parameters, which significantly improves the freedom of control over the mechanical properties of the hydrogel.

[0023] (3) This invention does not depend on a specific chemical composition and is applicable to a variety of physically cross-linked or chemically cross-linked hydrogel systems, and has good versatility and scalability.

[0024] (4) The preparation method adopted in this invention has simple process steps and strong controllability. By adjusting the process parameters, the gradient structure and compressive strength of the hydrogel can be precisely controlled, which is suitable for material design with different pressure requirements. Attached Figure Description

[0025] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0026] Figure 1 A schematic diagram of the preparation process for a pressure-resistant hydrogel with a continuous gradient structure; Figure 2 Optical photograph of a pressure-resistant hydrogel with a continuous gradient structure; Figure 3 This is a schematic diagram of the shape under compressive load conditions, where a represents the shape before compressive load and b represents the shape during compressive load. Figure 4 The compressive modulus of hydrogels in different regions of the continuously gradient structure obtained in Examples 1 to 4 is calculated. Figure 5 The images shown are scanning electron microscope (SEM) images of different regions of the pressure-resistant hydrogel with a continuous gradient structure obtained in Example 4. In the images, a is a longitudinal cross-sectional image and b is a cross-sectional image. In the images, 100 μm, 300 μm and 500 μm refer to the thickness of the encapsulation layer. Figure 6 The figure shows the crystallinity of different regions of the pressure-resistant hydrogel with a continuous gradient structure obtained in Example 4. In the figure, a is the differential scanning calorimetry (DSC) measurement result of the hydrogel, and b is the crystallinity result of the hydrogel. 100 μm, 300 μm and 500 μm in the figure refer to the thickness of the encapsulation layer. Figure 7 The mechanical properties of different regions of the compressive hydrogel with a continuous gradient structure obtained in Example 4 are shown; where a is the compressive stress-strain curve and b is the compressive modulus diagram; 100 μm, 300 μm and 500 μm in the figure refer to the thickness of the encapsulation layer. Figure 8 The figures show the mechanical stability characteristics of different regions and the whole of the continuously gradient structured hydrogel obtained in Example 4 under cyclic compression loading conditions; 100 μm, 300 μm and 500 μm in the figure refer to the thickness of the encapsulation layer. Figure 9 The compressive modulus of the continuous gradient structure of the hydrogel obtained with different directional annealing times; Figure 10 The compressive modulus of the continuous gradient structure of the hydrogel obtained at different directional annealing temperatures. Detailed Implementation

[0027] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0028] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of exemplary embodiments according to the invention. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0029] To enable those skilled in the art to better understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below with reference to specific embodiments.

[0030] Figure 1 This is a schematic diagram of the preparation process of a pressure-resistant hydrogel with a continuous gradient structure, according to... Figure 1 The method shown synthesizes a pressure-resistant hydrogel with a continuous gradient structure.

[0031] Example 1 (1) Dissolve polyvinyl alcohol particles in deionized water and stir at 90 °C for 4 h to obtain a polyvinyl alcohol hydrogel precursor solution with a mass fraction of 15%.

[0032] The polyvinyl alcohol hydrogel precursor solution was injected into a cylindrical mold (3 cm in diameter and 4 cm in height) and subjected to freeze-thaw treatment. The solution was frozen at -20 °C for 6 h and then thawed at room temperature for 6 h to complete one freeze-thaw cycle. The above freeze-thaw process was repeated 3 times to obtain a pregel with a preliminary physical cross-linking structure.

[0033] (2) On the outside of the pregel, from top to bottom, polyimide encapsulation layers with thicknesses of 100 μm, 300 μm and 500 μm are wound sequentially to form different barrier conditions.

[0034] (3) The pregel after encapsulation is oriented annealed at a temperature of 70 °C for 24 h. Then the hydrogel that has undergone oriented annealing is immersed in deionized water for 24 h to restore it to a stable water-containing state and obtain a pressure-resistant hydrogel with a continuous gradient structure.

[0035] Figure 2 Optical photographs of a pressure-resistant hydrogel with a continuous gradient structure; from Figure 2Structural differences can be observed in different regions of the hydrogel.

[0036] Figure 3 The diagram illustrates the morphology under compressive load conditions, demonstrating that the hydrogel prepared by the method of this invention exhibits good structural integrity and load-bearing capacity under compression.

[0037] Example 2 (1) Dissolve polyvinyl alcohol particles in deionized water and stir at 90 °C for 4 h to obtain a 5% polyvinyl alcohol hydrogel precursor solution.

[0038] The polyvinyl alcohol hydrogel precursor solution was injected into a cylindrical mold (3 cm in diameter and 4 cm in height) and subjected to freeze-thaw treatment. The solution was frozen at -20 °C for 6 h and then thawed at room temperature for 6 h to complete one freeze-thaw cycle. The above freeze-thaw process was repeated 3 times to obtain a pregel with a preliminary physical cross-linking structure.

[0039] (2) On the outside of the pregel, from top to bottom, polyimide encapsulation layers with thicknesses of 100 μm, 300 μm and 500 μm are wound sequentially to form different barrier conditions.

[0040] (3) The pregel after encapsulation is oriented annealed at a temperature of 70 °C for 24 h. Then the hydrogel that has undergone oriented annealing is immersed in deionized water for 24 h to restore it to a stable water-containing state and obtain a pressure-resistant hydrogel with a continuous gradient structure.

[0041] Example 3 (1) Dissolve polyvinyl alcohol particles in deionized water and stir at 90 °C for 4 h to obtain a 10% polyvinyl alcohol hydrogel precursor solution.

[0042] The polyvinyl alcohol hydrogel precursor solution was injected into a cylindrical mold (3 cm in diameter and 4 cm in height) and subjected to freeze-thaw treatment. The solution was frozen at -20 °C for 6 h and then thawed at room temperature for 6 h to complete one freeze-thaw cycle. The above freeze-thaw process was repeated 3 times to obtain a pregel with a preliminary physical cross-linking structure.

[0043] (2) On the outside of the pregel, from top to bottom, polyimide encapsulation layers with thicknesses of 100 μm, 300 μm and 500 μm are wound sequentially to form different barrier conditions.

[0044] (3) The pregel after encapsulation is oriented annealed at a temperature of 70 °C for 24 h. Then the hydrogel that has undergone oriented annealing is immersed in deionized water for 24 h to restore it to a stable water-containing state and obtain a pressure-resistant hydrogel with a continuous gradient structure.

[0045] Example 4 (1) Dissolve polyvinyl alcohol particles in deionized water and stir at 90 °C for 4 h to obtain a polyvinyl alcohol hydrogel precursor solution with a mass fraction of 20%.

[0046] The polyvinyl alcohol hydrogel precursor solution was injected into a cylindrical mold (3 cm in diameter and 4 cm in height) and subjected to freeze-thaw treatment. The solution was frozen at -20 °C for 6 h and then thawed at room temperature for 6 h to complete one freeze-thaw cycle. The above freeze-thaw process was repeated 3 times to obtain a pregel with a preliminary physical cross-linking structure.

[0047] (2) On the outside of the pregel, from top to bottom, polyimide encapsulation layers with thicknesses of 100 μm, 300 μm and 500 μm are wound sequentially to form different barrier conditions.

[0048] (3) The pregel after encapsulation is oriented annealed at a temperature of 70 °C for 24 h. Then the hydrogel that has undergone oriented annealing is immersed in deionized water for 24 h to restore it to a stable water-containing state and obtain a pressure-resistant hydrogel with a continuous gradient structure.

[0049] Figure 4 To obtain the compressive modulus of the hydrogel in different regions of the continuously gradient structured hydrogel obtained in Examples 1 to 4, from... Figure 4 As can be seen, in hydrogels of the same concentration, the compressive modulus gradually decreases with the increase of the encapsulation film thickness, thus forming a compressive hydrogel with a continuous gradient structure along a defined direction. Furthermore, pregels of different concentrations, after undergoing the same encapsulation, directional annealing, and hydration processes, can all produce compressive hydrogels with a continuous gradient.

[0050] Figure 5 The images shown are scanning electron microscope (SEM) images of different regions of the pressure-resistant hydrogel with a continuous gradient structure obtained in Example 4, where a is a longitudinal cross-sectional image and b is a cross-sectional image. Figure 5 As can be seen, the hydrogel in different regions exhibits significantly different pore structures and structural density, and these structures gradually change along the gradient direction from one end to the other, indicating that a continuously varying microstructural gradient can be constructed within the hydrogel. Figure 5As can be seen from Figure a, with the gradual decrease in the thickness of the encapsulation film, the evaporation rate and directionality of water inside the hydrogel increase during directional annealing, and the dehydration-induced stress gradually increases. This causes the polymer chains inside the hydrogel to gradually converge and rearrange their orientation along a specific direction, exhibiting more obvious chain orientation characteristics and a trend towards structural ordering, resulting in a more compact and ordered microstructure in the obtained region. Figure 5 As can be seen from Figure b, as the thickness of the encapsulation film increases, the outward diffusion of moisture is restricted. During the directional annealing process, some moisture is retained inside the hydrogel and becomes locally enriched, thus forming more porous structures during the structural evolution process, resulting in an increase in the number of pores in the cross-section of the hydrogel.

[0051] Figure 6 The image shows the crystallinity of different regions of the pressure-resistant hydrogel with a continuous gradient structure obtained in Example 4. In this image, a represents the differential scanning calorimetry (DSC) measurement result of the hydrogel, and b represents the crystallinity result of the hydrogel. Figure 6 As can be seen, there are significant differences in crystallinity at different locations along the gradient direction of the hydrogel, and the crystallinity exhibits a gradual change along the gradient direction. Combined with the aforementioned microstructure characterization results, it can be found that the crystallinity decreases with increasing encapsulation film thickness. Regions with higher crystallinity correspond to relatively dense structures, while regions with lower crystallinity exhibit a more porous structure. These crystallinity distribution results further illustrate that the continuous gradient structure is reflected not only in the pore structure and morphology but also in the polymer crystal structure.

[0052] Figure 7 The mechanical property characterization results of different regions of the compressive hydrogel with a continuous gradient structure obtained in Example 4 are shown; where a is the compressive stress-strain curve and b is the compressive modulus diagram; from Figure 7 As can be seen, different regions exhibit different mechanical responses under the same compressive loading conditions, and their compressive modulus or load-bearing capacity shows a gradual change trend along the gradient direction. These results correspond to the microstructure characterization and crystallinity gradient results, indicating that the internal microstructure and internal crystallinity gradient of the hydrogel can be effectively converted into a mechanical property gradient, thereby achieving spatial regulation of compressive strength.

[0053] Figure 8This document presents the mechanical stability characterization results of different regions and the entire continuous gradient hydrogel obtained in Example 4 under cyclic compression loading conditions. During the test, the continuous gradient hydrogel sample was subjected to multiple cyclic loadings under the same cyclic compression strain or compression load conditions. The results show that the stress-strain response of the continuous gradient hydrogel remains relatively stable during cyclic compression, without significant mechanical property degradation or structural damage. These results indicate that the continuous gradient structure can effectively disperse cyclic compression loads and alleviate local stress concentration, thereby significantly improving the fatigue resistance and structural stability of the hydrogel under long-term compression conditions, demonstrating its suitability for applications involving long-term load-bearing or repeated stress.

[0054] Example 5 Compared with Example 4, the directional annealing time in this embodiment was adjusted to 12 h, 14 h, 16 h, 18 h and 20 h, respectively, to obtain anti-compression hydrogels with continuous gradient structures.

[0055] The compressive modulus of the continuous gradient structure hydrogels obtained by different directional annealing times is as follows: Figure 9 As shown, from Figure 9 As can be seen, the hydrogel samples exhibit different mechanical responses under compressive loads with varying directional annealing times, and their compressive modulus increases with increasing annealing time. These results indicate that, based on the formation of a continuous gradient structure, the mechanical properties of the hydrogel can be further adjusted within a certain range by controlling the directional annealing time.

[0056] Example 6 Compared with Example 4, this embodiment adjusts the directional annealing temperature to 30 ℃, 40 ℃, 50 ℃ and 60 ℃ respectively to obtain a pressure-resistant hydrogel with a continuous gradient structure.

[0057] The compressive modulus of the continuous gradient structure hydrogels obtained at different directional annealing temperatures is as follows: Figure 10 As shown, from Figure 10 As can be seen, the degree of formation of polyvinyl alcohol crystalline domains within the hydrogel varies under different directional annealing temperatures, leading to changes in the hydrogel's crystallinity. With varying crystallinity, the hydrogel exhibits different mechanical responses under compressive loads. These results indicate that, based on the formation of a continuous gradient structure, the crystalline structure and mechanical properties of the hydrogel can be further adjusted within a certain range by controlling the directional annealing temperature.

[0058] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A pressure-resistant hydrogel with a continuous gradient structure, characterized in that, Along the defined direction, the porosity gradually increases, the crystallinity gradually decreases, and the compressive modulus gradually decreases.

2. The method for preparing the pressure-resistant hydrogel with a continuous gradient structure according to claim 1, characterized in that, Includes the following steps: (1) The hydrogel precursor solution was subjected to freeze-thaw cycles to obtain a pregel; (2) Encapsulate a film with gradually increasing thickness along a defined direction on the outside of the pregel; (3) The encapsulated pregel is oriented annealed and then hydrated to obtain a pressure-resistant hydrogel with a continuous gradient structure.

3. The preparation method according to claim 2, characterized in that, In step (1), the pregel includes any one or more of polyvinyl alcohol polymers, polyacrylamide polymers, polyacrylic acid polymers or cellulose compounds, preferably polyvinyl alcohol polymers.

4. The preparation method according to claim 3, characterized in that, When the pregel is a polyvinyl alcohol polymer, the preparation method of the hydrogel precursor solution includes: mixing polyvinyl alcohol and water and then heating to dissolve.

5. The preparation method according to claim 4, characterized in that, The heating temperature is 90~120℃; Alternatively, the mass fraction of polyvinyl alcohol in the hydrogel precursor solution is 5-20%.

6. The preparation method according to claim 3, characterized in that, When the pregel is a polyvinyl alcohol polymer, in step (1), the freezing temperature of the freeze-thaw treatment is -18 to -22 ℃, preferably -20 ℃; the freezing time is 5 to 7 h; the thawing temperature is 20 to 40 ℃; and the number of freeze-thaw cycles is 3 to 6.

7. The preparation method according to claim 3, characterized in that, When the pregel is a polyvinyl alcohol polymer, the film thickness is 55~550 μm.

8. The preparation method according to claim 3, characterized in that, When the pregel is a polyvinyl alcohol polymer, the directional annealing temperature is 25 ℃~75 ℃ and the directional annealing time is 12~20 h; Alternatively, when the pregel is a polyvinyl alcohol polymer, the hydration time is 12-24 h.

9. The preparation method according to claim 2, characterized in that, In step (2), the film is made of silicone, copolymer, polyamide, polyimide, polyester and inorganic metal materials; preferably polyimide.

10. The preparation method according to claim 2, characterized in that, In step (2), the direction is defined as from the top surface of the pregel to the bottom surface.

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