Preparation method of pH quick response hydrogel driver

Abstract

The invention belongs to the field of nanocellulose-based hydrogel drivers, and particularly relates to a preparation method of a pH quick response type hydrogel driver. The preparation method comprises the following steps: sequentially adding a nanocellulose dispersion liquid, a monomer, a chemical cross-linking agent and a photoinitiator into deionized water, and magnetically stirring and dissolving under a dark condition to obtain a clear hydrogel precursor solution; injecting the precursor solution into a specific mold (an interlayer prepared from two glass sheets (5cm * 5cm) and two 500 [mu] m spacing bars), and shearing the precursor at a rate of 5cm / s for dozens of times by using the edge of one glass sheet; and then covering a glass slide on the suspension. Putting the mold under ultraviolet light to polymerize, irradiating for 2 minutes by using the ultraviolet light, stretching the hydrogel by using a clamp, and finally polymerizing for 3 minutes under the ultraviolet light. And finally, the hydrogel driver with high mechanical property and quick response is obtained. The pH response hydrogel driver has the advantages of high mechanical property, quick response and the like, and is wide in application. The method is low in cost, simple to operate, green and environment-friendly.

Description

Technical Field

[0001] This invention belongs to the field of intelligent responsive hydrogels and hydrogel actuators, and relates to a method for preparing a pH-responsive hydrogel actuator. 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] Stimulus-responsive hydrogels are a class of intelligent polymer materials capable of sensing external stimuli (such as temperature, light, electromagnetic fields, pH, ionic strength, redox reactions, etc.) and undergoing reversible swelling / contraction. When these hydrogels are used in actuators, the hydrogels generate internal stress due to local differential swelling in an aqueous environment, thus exhibiting macroscopic deformations such as bending, torsion, and folding.

[0004] Existing stimulus-responsive hydrogel actuators typically have the following shortcomings: First, the response speed is limited by the diffusion and ion exchange processes of water molecules in the network, which can easily lead to slow or asymmetrical responses; second, high water content results in insufficient mechanical strength of the network, making it prone to deformation and breakage, and it is also prone to fatigue decay during repeated stimulation cycles, affecting long-term stable use.

[0005] Therefore, it is of great significance to develop a simple, green, low-cost method for preparing pH-responsive hydrogel actuators that allows for orientation control. Summary of the Invention

[0006] To address the problems of slow response, unstable actuation, and insufficient mechanical properties in existing pH-responsive hydrogel actuators, this invention provides a method for preparing a pH-fast responsive hydrogel actuator. This method induces the orientation of nanocellulose by applying unidirectional shear flow to a precursor solution, followed by directional stretching after partial solidification to further enhance the orientation, and then secondary photocuring to lock the structure, thereby obtaining a hydrogel actuator that combines high strength and rapid actuation response.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: A first aspect of the present invention provides a method for preparing a pH-responsive hydrogel actuator, comprising: A precursor solution was obtained by uniformly mixing an aqueous dispersion of nanocellulose, acrylic monomer, crosslinking agent and photoinitiator in a solvent. The precursor solution is injected into the sandwich mold, and then the precursor is reciprocated by the edge of the glass slide along the same direction to make the nanocellulose oriented under the action of shear flow; after shearing, a glass slide is covered on it to form a stable sandwich and inhibit oxygen polymerization. The sandwich mold is placed under an ultraviolet light source for the first photocuring to form a gel. The gel is then stretched in a directional manner along the shear direction, followed by a second photocuring. After curing, the mold is washed to remove unreacted substances, and the product is obtained.

[0008] This invention prepares a pH-responsive nanocellulose composite hydrogel actuator that combines high mechanical properties with rapid responsiveness by unidirectional shear-induced orientation and directional stretching curing.

[0009] In a second aspect, the present invention provides a pH-responsive hydrogel actuator prepared by the method described above.

[0010] Traditional hydrogels are similar to a sponge, capable of absorbing and shrinking water, but this scaling is omnidirectional, making it difficult to generate "movement" in a specific direction. The hydrogel actuator prepared in this invention is like a muscle, capable of generating thrust, tension, or torque through directional deformation, thereby driving the movement of external objects or itself.

[0011] A third aspect of the present invention provides the application of the above-described pH-responsive hydrogel actuator in the fields of flexible electronics, soft robotics, biomedicine, and environmental adaptation devices.

[0012] Beneficial effects of the present invention (1) By using the process route of “unidirectional shear-induced orientation + stepwise curing + directional stretching and locking”, a stable anisotropic microstructure is constructed inside the hydrogel, which significantly improves the orientation degree of nanocellulose in the network, thereby improving the mechanical strength and fatigue resistance of the hydrogel and accelerating the driving response under pH stimulation.

[0013] (2) Nanocellulose has high crystallinity, high specific strength and good biocompatibility. Introducing it into hydrogel network can improve mechanical properties. At the same time, if nanocellulose is oriented and assembled before gelation and the orientation is further locked after gelation, anisotropic microstructures can be constructed inside the hydrogel, thereby inducing differential swelling and improving driving performance.

[0014] (3) The process is simple, the conditions are mild and environmentally friendly, and the nanocellulose used is widely available and low in cost, making it easy to scale up and industrialize. Attached Figure Description

[0015] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. Exemplary embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0016] Figure 1 SEM image (right) and polarized light microscope image (left) of CP hydrogel before shearing and stretching. Figure 2 SEM image (right) and polarized light microscope image (left) of the sheared and unstretched CPS hydrogel; Figure 3 CPS that has been sheared and stretched to twice its original size 2. SEM image (right) and polarized light microscope image (left) of the hydrogel; Figure 4 The stress-strain curve of a pH-responsive hydrogel; Figure 5 To assess the swelling properties of pH-responsive hydrogels at different pH levels; Figure 6 To illustrate the driving behavior of pH-responsive hydrogels at different pH levels, (a) the hydrogel adheres to the wood block, (b) the hydrogel is lifted, (c) the hydrogel gradually grips the wood block, and (d) the hydrogel unfolds again. Detailed Implementation

[0017] 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.

[0018] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of skill in the art. The reagents and raw materials used in this invention are readily available through conventional means, and unless otherwise specified, they are used in accordance with conventional methods in the art or product instructions. Similarly, unless otherwise specified, the test methods of this invention are performed in accordance with conventional methods in the art or industry-standard methods or practices. Furthermore, any methods and materials similar to or equivalent to those described herein may be applied to the methods of this invention. The preferred embodiments and materials described herein are for illustrative purposes only.

[0019] This invention provides a method for preparing a pH-responsive hydrogel actuator, comprising: A precursor solution was obtained by uniformly mixing an aqueous dispersion of nanocellulose, acrylic monomer, crosslinking agent and photoinitiator in a solvent. The precursor solution is injected into the sandwich mold, and then the precursor is reciprocated by the edge of the glass slide along the same direction to make the nanocellulose oriented under the action of shear flow; after shearing, a glass slide is covered on it to form a stable sandwich and inhibit oxygen polymerization. The sandwich mold is placed under an ultraviolet light source for the first photocuring to form a gel. The gel is then stretched in a directional manner along the shear direction, followed by a second photocuring. After curing, the mold is washed to remove unreacted substances, and the product is obtained.

[0020] The amount of nanocellulose aqueous dispersion can affect the performance of hydrogel. Therefore, the present invention studies the concentration of nanocellulose aqueous dispersion. Preferably, the mass fraction of the nanocellulose aqueous dispersion is 0.5 to 3.0 wt%, more preferably 1.5 wt%, to obtain better mechanical and driving properties.

[0021] The type of monomer affects the performance of hydrogels. Therefore, this invention studies the types of hydrogel monomers, preferably acrylic acid, to prepare a pH-responsive hydrogel actuator.

[0022] This invention does not impose any particular limitation on the type of crosslinking agent; preferably, the crosslinking agent is N,N'. Methylenebisacrylamide is used to achieve better driving performance.

[0023] The amount of crosslinking agent can affect the crosslinking effect of hydrogel. Therefore, the present invention has studied the amount of crosslinking agent. Preferably, the amount of crosslinking agent is 0.1 to 2.0 wt% of the monomer mass to obtain a better crosslinking effect.

[0024] The present invention does not impose any special limitation on the type of initiator. Preferably, the photoinitiator is benzoin dimethyl ether to obtain a better polymerization effect. The amount of initiator affects the rate and extent of free radical polymerization. Therefore, this invention studies the amount of photoinitiator. Preferably, the amount of photoinitiator is 0.1 to 5.0 wt% of the monomer mass to obtain better polymerization effect.

[0025] The structure of the sandwich mold affects the structure of the hydrogel. Therefore, this invention studies the structure of the sandwich mold. Preferably, the sandwich mold is a rectangular cavity with a sandwich thickness of 200–1000 μm or 500 μm. More preferably, the sandwich mold consists of two glass sheets and a spacer strip, forming a sandwich space with a thickness of 200–1000 μm, preferably 500 μm; the glass sheet size is preferably 5 cm × 5 cm to ensure that the obtained hydrogel has better driving performance.

[0026] The specific steps of the shearing in this invention are as follows: a thin sheet is repeatedly moved on the upper part of the sandwich mold. Preferably, the shearing rate is 1 to 10 cm / s and the number of shearing times is 10 to 100. The shearing direction is kept consistent to obtain a stable orientation so that the hydrogel has better driving performance.

[0027] The conditions for photocuring affect the structure and properties of hydrogels. Therefore, this invention studies the conditions for photocuring. Preferably, the wavelength of the first photocuring is 320–450 nm; the power of the light source is 50–300 W; the distance between the light source and the mold is 5–30 cm; and the time is 1–5 min. Preferably, the wavelength of the second photocuring is 320–450 nm; the power of the light source is 50–300 W; the distance between the light source and the mold is 5–30 cm; and the time is 1–5 min, in order to obtain better mechanical and driving properties.

[0028] The stretching ratio affects the mechanical and actuation properties of hydrogels. Therefore, this invention studies the directional stretching ratio. Preferably, the directional stretching ratio is 1.2 to 2.0 times to obtain better mechanical and actuation properties.

[0029] Furthermore, the obtained hydrogels were placed in solutions with pH values ​​of 2, 7, and 14 for swelling and driving behavior tests.

[0030] More specifically, including: (1) Preparation of hydrogel precursor solution: Add nanocellulose aqueous dispersion, acrylic monomer, crosslinking agent and photoinitiator to deionized water in proportion, and stir magnetically under light-protected conditions until a homogeneous and clear hydrogel precursor solution is obtained.

[0031] (2) Unidirectional shear-induced orientation: The precursor solution is injected into the sandwich mold (avoiding air bubbles during the injection process), and then the precursor is reciprocated by the edge of the glass sheet along the same direction, so that the nanocellulose is oriented under the action of shear flow; after shearing, a glass slide is covered on it to form a stable sandwich and inhibit oxygen polymerization.

[0032] (3) Stepwise photocuring and directional stretching to lock the structure: The sandwich mold is placed under a UV light source for the first photocuring to form a gel with a certain strength; the gel is removed and directionally stretched along the shear direction (stretching ratio of 1 to 2 times the original length), and then photocured a second time under a UV light source to lock the orientation structure; after curing, it is treated with deionized water or Milli-450 water. Q. Rinse thoroughly with water to remove unreacted substances.

[0033] (4) Performance characterization and response test: The obtained hydrogel was subjected to tensile mechanical test (to obtain stress-strain curve), and the hydrogel was placed in solutions of different pH to test swelling behavior and driving deformation.

[0034] The present invention will be further described in detail below with reference to specific embodiments. It should be noted that the specific embodiments are explanations of the present invention and not limitations thereof.

[0035] In the following embodiments, unsheared and unstretched hydrogels are denoted as CP; sheared but unstretched hydrogels are denoted as CPS; and sheared and stretched X times hydrogels are denoted as CPS. X, where X is the stretch ratio.

[0036] The glass slide measures 5 cm × 5 cm.

[0037] Example 1 (CP: Uncut / Unstretched) Preparation of hydrogel precursor solution: Take 12 g of nanocellulose dispersion (concentration 1.5 wt%), 6 g of acrylic acid, and 0.05 g of N,N' Methylenebisacrylamide and 0.5 g of benzoin dimethyl ether were added to an appropriate amount of deionized water and stirred magnetically in the dark until a homogeneous and clear hydrogel precursor solution was obtained.

[0038] Curing and crosslinking: The precursor solution was injected into a sandwich mold (the mold consists of two parallel glass slides, separated by 0.40 mm spacers at the front, back, and left sides, forming a rectangular cavity). It was then irradiated under a UV light source (365 nm, 50 W, 20 cm distance) for 5 min to complete curing. After curing, it was processed using Milli-Einstein crosslinking technology. Q. Wash with water to remove any remaining unreacted substances.

[0039] Structural characterization: As shown in Figure 1, SEM observation shows that the hydrogel has a closed-pore structure with a pore size of about 10 μm; polarized light microscopy shows that the fiber orientation is disordered and the whole structure does not have anisotropy.

[0040] Mechanical property testing: Stress-strain curves were obtained using a tensile testing machine (Figure 4). The maximum fracture strain was approximately 440%, and the maximum stress was approximately 0.2 MPa.

[0041] pH Response: The swelling properties of the hydrogel were tested by placing it in solutions with pH values ​​of 2, 7, and 14 (Figure 5). At pH 2, the swelling rate was approximately 500% after 20 minutes; at pH 7, the swelling rate was approximately 3800% after 20 minutes; and at pH 14, the swelling rate was approximately 6000% after 20 minutes. The hydrogel showed virtually no bending in solutions of different pH values.

[0042] Example 2 (CPS: Shear / Unstretched) Preparation of hydrogel precursor solution: Same as in Example 1.

[0043] Precursor solution shearing: The precursor solution is injected into a sandwich mold consisting of two glass slides (5 cm × 5 cm) and three 500 μm spacers. Then, the glass slides are sheared 50 times along the same direction at a rate of 5 cm / s (using manual shearing of thin sheets at a shearing rate of 10 cm / s, with the shearing direction kept consistent). After shearing, a glass slide is placed over the solution.

[0044] Curing and crosslinking: The mold was placed under a UV light source (365 nm, 50 W, 20 cm distance) for 5 min to complete curing; after curing, it was treated with Milli-Crystal Crosslinking. Q. Wash with water.

[0045] Structural characterization: As shown in Figure 2, SEM observation shows that the nanofibers in the upper part of the hydrogel exhibit a certain degree of orientation; polarized light microscopy also shows that the fibers as a whole tend to be horizontally oriented along the shear direction, exhibiting anisotropy.

[0046] Mechanical property test: As shown in Figure 4, the maximum fracture strain is about 4100% and the maximum stress is about 0.6 MPa.

[0047] pH response: The swelling properties and driving behavior of the hydrogel were tested by placing it in solutions with pH = 2, 7, and 14 respectively. Figure 5 (Figure 6). When pH=2, the swelling rate is about 500% after 15 min; when pH=7, the swelling rate is about 3000%; when pH=14, the swelling rate is about 6000% after 15 min. Due to the anisotropy of the upper fibers, the hydrogel bends in solutions of different pH values, but the bending speed is relatively slow.

[0048] Example 3 (CPS) 1: Shear / stretch by one time) Preparation of hydrogel precursor solution: Same as in Example 1.

[0049] Shearing precursor solution: Same as in Example 2.

[0050] Stepwise curing and stretching: The sandwich mold was pre-cured for 2 min under a UV light source (320 nm, 50 W, 20 cm distance) to form a workable gel; then the gel was removed and stretched oriented along the shear direction to twice its original length (i.e., stretched from 3 cm to 6 cm); it was then placed under a UV light source for another 3 min to complete the secondary curing. After curing, it was processed using Millimeter... Q. Wash with water to remove any remaining unreacted substances.

[0051] Example 4 (CPS) 2: Shear / stretch twice) Preparation of hydrogel precursor solution: Same as in Example 1.

[0052] Shearing precursor solution: Same as in Example 2.

[0053] Stepwise curing and stretching: The sandwich mold was pre-cured for 2 min under a UV light source (365 nm, 50 W, 20 cm distance) to form a workable gel; then the gel was removed and stretched oriented along the shear direction to twice its original length (i.e., the stretching length was twice the original length, from 3 cm to 9 cm); it was then placed under a UV light source for another 3 min to complete the secondary curing. After curing, it was processed using Milli-450 micrometers. Q. Wash with water to remove any remaining unreacted substances.

[0054] Structural characterization: As shown in Figure 3, SEM observation shows that the nanofibers in the upper part of the hydrogel are more neatly arranged; polarized light microscopy shows obvious anisotropy.

[0055] Mechanical property test: As shown in Figure 4, the maximum fracture strain is about 350% and the maximum stress is about 0.8 MPa.

[0056] pH response: The swelling properties and driving behavior of the hydrogel were tested by placing it in solutions with pH = 2, 7, and 14 respectively. Figure 5 (Figure 6). At pH = 2, the swelling rate was approximately 500% after 5 minutes; at pH = 7, the swelling rate was approximately 2000% after 5 minutes; and at pH = 14, the swelling rate was approximately 6000% after 5 minutes. Due to the significantly enhanced fiber orientation, the hydrogel bent relatively quickly in solutions of different pH values. Figure 6 shows an example of a robotic gripper composed of two sheets of hydrogel. Two sheet-like hydrogels (CPS) were then used. 2. The material is fixed to the rod in a cross shape and placed in an alkaline solution (NaOH) with a pH of 14. It is then placed in contact with a wooden block (10 cm × 10 cm × 1 cm). After immersion for 1 second, the hydrogel can be seen adhering the wooden block. Figure 6 As shown in (a); then, lift the hydrogel, and you can see that the wood block is still bonded to the hydrogel, as shown in (a). Figure 6 As shown in (b); upon continued observation, it can be seen that the hydrogel gradually grips the wooden block, as shown in (b). Figure 6 As shown in (c); finally, the hydrogel was placed in an acidic solution (HCl) with pH = 2, and the hydrogel re-spread out, gradually returning to its initial state, as shown. Figure 6 As shown in (d).

[0057] 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 method for preparing a pH-responsive hydrogel actuator, characterized in that, include: A precursor solution was obtained by uniformly mixing an aqueous dispersion of nanocellulose, acrylic monomer, crosslinking agent and photoinitiator in a solvent. The precursor solution is injected into a sandwich mold, and then the precursor is reciprocated by the edge of a glass sheet along the same direction, so that the nanocellulose is oriented under the action of shear flow. After shearing, a glass slide is placed on top to form a stable interlayer and inhibit oxygen polymerization. The sandwich mold is placed under an ultraviolet light source for the first photocuring to form a gel. The gel is then stretched in a directional manner along the shear direction, followed by a second photocuring. After curing, the mold is washed to remove unreacted substances, and the product is obtained.

2. The method for preparing the pH-responsive hydrogel actuator as described in claim 1, characterized in that, The mass fraction of the nanocellulose aqueous dispersion is 0.5–3.0 wt% or 1.5 wt%.

3. The method for preparing the pH-responsive hydrogel actuator as described in claim 1, characterized in that, The monomer is acrylic acid; Alternatively, the crosslinking agent is N,N' Methylenebisacrylamide; Alternatively, the amount of crosslinking agent used is 0.1–2.0 wt% of the monomer mass; Alternatively, the photoinitiator may be benzoin dimethyl ether; Alternatively, the amount of photoinitiator used is 0.1 to 5.0 wt% of the monomer mass.

4. The method for preparing the pH-responsive hydrogel actuator as described in claim 1, characterized in that, The sandwich mold is a rectangular cavity with a sandwich thickness of 200–1000 μm or 500 μm.

5. The method for preparing the pH-responsive hydrogel actuator as described in claim 1, characterized in that, The shear rate is 1 to 10 cm / s, and the number of shearing cycles is 10 to 100.

6. The method for preparing the pH-responsive hydrogel actuator as described in claim 1, characterized in that, The wavelength of the first photocuring is 320–450 nm; the power of the light source is 50–300 W; the distance between the light source and the mold is 5–30 cm; and the time is 1–5 min.

7. The method for preparing the pH-responsive hydrogel actuator as described in claim 1, characterized in that, The wavelength of the second photocuring is 320–450 nm; the power of the light source is 50–300 W; the distance between the light source and the mold is 5–30 cm; and the time is 1–5 min.

8. The method for preparing the pH-responsive hydrogel actuator as described in claim 1, characterized in that, The directional stretching ratio is 1.2 to 2.0 times.

9. A pH-responsive hydrogel actuator prepared by the method of any one of claims 1-8.

10. The application of the pH-responsive hydrogel actuator of claim 9 in the fields of flexible electronics, soft robotics, biomedicine, and environmental adaptation devices.

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