A micro pneumatic artificial muscle based on phase transition hydrogel actuator, its preparation method and application

Abstract

This invention discloses a micro-pneumatic artificial muscle based on a phase transition hydrogel actuator, its preparation method, and its application, belonging to the field of polymer functional materials technology. The method includes the following steps: (1) using octyl acrylate and acrylamide as monomers, dissolving the monomers, β-cyclodextrin, and crosslinking agent in a dimethylformamide dispersion of sea squirt cellulose nanocrystals, followed by the addition of a photoinitiator to obtain a prepolymer; the prepolymer is then photocured and solvent-replaced to obtain a tubular hydrogel; (2) connecting one end of the tubular hydrogel to an air cavity and sealing the other end to obtain a micro-pneumatic artificial muscle based on a phase transition hydrogel actuator. This method has the advantages of convenient processing, mild reaction, and readily available materials. The micro-pneumatic artificial muscle prepared by this method has adjustable mechanical properties independent of water permeability and fast driving speed, and has broad application prospects in miniaturized biomimetic soft robot devices.

Description

Technical Field

[0001] This invention relates to the field of polymer functional materials technology, and in particular to a micro pneumatic artificial muscle based on a phase transition hydrogel actuator, its preparation method, and its application. Background Technology

[0002] Pneumatic artificial muscles, with their advantages of rapid response, high stroke, and high driving force, are considered the most promising soft actuators. However, the driving source of pneumatic artificial muscles heavily relies on bulky equipment such as compressors and pumps, which limits their miniaturization applications. Furthermore, current pneumatic artificial muscle materials do not contain water, and their structure does not achieve the biomimetic function of natural muscles.

[0003] Recent designs for miniature pneumatic artificial muscles revolve around the idea of ​​achieving internal pressure changes through external stimulation, typically involving two approaches: using internal fluids to generate pressure changes and using actuator materials to alter these pressure changes. Internal fluid designs include liquid-gas phase-change fluids, combustion reactions, chemical reactions that produce gas, and swelling / deswelling. Liquid-gas phase-change fluids are the most widely used method because these low-boiling-point liquids can vaporize upon heating, generating high-pressure expansion, and then liquefy upon cooling, achieving reversible actuation. However, liquid vaporization requires significant time and energy. Another approach utilizes smart membrane materials as pneumatic artificial muscles; a typical example is a dielectric elastomer membrane. Applying a voltage across the membrane causes it to expand in the in-plane direction, resulting in a decrease in internal pressure and thus actuation.

[0004] The inventors disclosed a method for preparing shape memory hydrogels and their applications in Chinese Patent Publication No. CN112940288A. The preparation method includes the following steps: Step 1, acrylamide, acrylic acid, and acrylamide adamantane are dissolved in a certain proportion in an aqueous dispersion of cyclodextrin-grafted sea squirt cellulose nanocrystals, incubated at 55 °C for 2 h, cooled in an ice-water bath, and ammonium persulfate initiator is added. The dispersion is then injected into a mold and polymerized at 55 °C for 5 h to obtain a single-crosslinked hydrogel; Step 2, the single-crosslinked hydrogel obtained in Step 1 undergoes a series of continuous orientation operations to obtain a helical hydrogel; Step 3, the helical hydrogel obtained in Step 2 is immersed in a fixative for 24 h to obtain a shape memory hydrogel. This shape memory hydrogel has two chiral helical structures, which can achieve stretching and deformation in opposite directions under cyclic stimulation of ethanol-water solvent. However, the driving process of this hydrogel material as an artificial muscle is achieved in a swollen state, resulting in a low elastic modulus and thus low driving force and working capacity of the artificial muscle. Meanwhile, this shape memory hydrogel mimics the movement patterns of cucumber tendrils in nature through the shape memory effect, including climbing, winding, and spiraling contractions, but the driving speed is very slow, reaching a maximum of 17.9% / s, which limits its application range.

[0005] In Chinese invention patent publication CN116426005A, the inventors provide a temperature-responsive hydrogel artificial muscle, its preparation method, and its application. The method includes: dissolving N-isopropylacrylamide, potassium 3-sulfonopropyl methacrylate, acrylamide, a crosslinking agent, an initiator, and a accelerator in a sulfuric acid-hydrolyzed aqueous dispersion of sea squirt cellulose nanocrystals to obtain a mixture; then injecting the mixture into a mold for polymerization to obtain a chemically crosslinked hydrogel; subjecting the chemically crosslinked hydrogel to continuous orientation to obtain a helical hydrogel; and immersing the helical hydrogel in a zirconium salt solution to obtain the hydrogel artificial muscle. This hydrogel artificial muscle can achieve stretching and deformation through temperature stimulation and can be applied to soft-actuated robots. However, its application is limited by the driving speed, reaching only 0.7% / s.

[0006] In addition, the inventor disclosed a hydrated programmable shape memory artificial muscle and its preparation method and application in Chinese patent CN117820555A. The preparation method includes the following steps: (1) In a solution environment, polyvinyl alcohol, sea squirt cellulose nanocrystals and polymer monomers composed of acrylamide, acrylic acid and N,N'-methylenebisacrylamide are mixed and incubated to obtain a polymerization precursor dispersion; (2) An initiator is added to the polymerization precursor dispersion, and a fibrous hydrogel material is formed by polymerization reaction to obtain a shape memory artificial muscle. The raw materials of this method are widely available and the process is simple and fast. The shape memory artificial muscle prepared has ultra-high elongation at break and toughness, and can realize the shape programming and recovery process under high pre-stretch ratio and constant stress. When used in biomimetic structures, it realizes the antagonistic movement of artificial arms. However, this artificial muscle contracts -60% strain within 3 seconds when heated to 120 ℃, and the driving speed is still low, which also limits its practical application.

[0007] In summary, hydrogel materials possess biomimetic advantages due to their abundant water content, but traditional hydrogel actuators achieve shape changes through swelling / deswelling. This process requires the penetration of water molecules within the hydrogel network, thus limiting the rapid actuation of hydrogel actuators. Improving the actuation speed of artificial muscles is a pressing issue that needs to be addressed, as it is crucial for expanding the applications of artificial muscles. Summary of the Invention

[0008] In view of the above-mentioned deficiencies of the prior art, in a first aspect of the present invention, a method for preparing a micro pneumatic artificial muscle based on a phase transition hydrogel actuator, which is convenient and has a mild reaction, is provided, comprising the following steps:

[0009] (1) Using octyl acrylate and acrylamide as monomers, the monomers, β-cyclodextrin and crosslinking agent were dissolved in dimethylformamide dispersion of sea squirt cellulose nanocrystals, and then a photoinitiator was added to obtain a prepolymer solution; the prepolymer solution was photocured and solvent replaced to obtain a hydrogel with a tubular structure.

[0010] (2) Connect one end of the tubular hydrogel to the air cavity and seal the other end to obtain a micro pneumatic artificial muscle driven by a phase transition hydrogel actuator.

[0011] Preferably, in step (1), the proportion of octyl acrylate to the monomer is 6.7 mol.%-33.3 mol.%.

[0012] Preferably, in step (1), the monomer accounts for 20 wt.%-30 wt.% of the prepolymer solution.

[0013] Preferably, in step (1), the molar ratio of β-cyclodextrin to octyl acrylate is 0-1:1.

[0014] Preferably, in step (1), the crosslinking agent accounts for 1 wt.%-3 wt.% of the prepolymer solution.

[0015] Those skilled in the art can select a suitable crosslinking agent based on the type of polymerization raw material. N,N'-methylenebisacrylamide is a particularly suitable choice for the preparation of this invention. N,N'-methylenebisacrylamide can form stable covalent bonds during polymerization, linking individual polymer chains together to form a robust three-dimensional network structure. Its presence ensures effective linkage of each chain segment, enhancing the overall structural stability and mechanical strength.

[0016] More preferably, the crosslinking agent includes at least one of N,N'-methylenebisacrylamide, ethylene glycol dimethacrylate, and trimethylolpropane triacrylate.

[0017] Preferably, in step (1), the concentration of the dimethylformamide dispersion of sea squirt cellulose nanocrystals is 0.1 wt.%-0.3 wt.%.

[0018] Preferably, in step (1), the proportion of photoinitiator in the prepolymer solution is 1 wt.%-3 wt.%.

[0019] Photocuring offers mild reaction conditions, enabling polymerization to be initiated under ambient light irradiation and forming a robust polymer network. Photocuring allows for precise control of the curing location of materials in complex three-dimensional spaces, making it ideal for manufacturing hydrogels with highly complex internal structures. Those skilled in the art can also select appropriate photoinitiators and curing times based on the type of polymerizing raw material and the production scale. As presented in one or more embodiments of the present invention, I2959, with its superior water solubility, light absorption properties, high photopolymerization efficiency, and good biocompatibility, is a particularly suitable photoinitiator choice.

[0020] More preferably, the photoinitiator includes at least one of 2-hydroxy-1-(4-(2-hydroxyethoxy)phenyl)-2-methyl-1-propanone (I2959), 2-hydroxy-2-methyl-1-phenyl-1-propanone (1173), and lithium phenyl-2,4,6-trimethylbenzoyl phosphite (LAP).

[0021] The tubular hydrogel can be synthesized by placing the reactants in a transparent tubular mold, or by using other processing methods suitable in the art to form the tubular structure.

[0022] In a second aspect of the present invention, a micro pneumatic artificial muscle based on a phase transition hydrogel actuator with fast driving speed and adjustable mechanical properties is provided. Its structure includes a tubular hydrogel actuator and an air cavity, and it is prepared using the preparation method of the first aspect of the present invention.

[0023] Based on the above technical solutions, the design concept and principle of this invention are as follows: using octyl acrylate and acrylamide as monomers, they are dissolved with β-cyclodextrin and a crosslinking agent in a dimethylformamide dispersion of sea squirt cellulose nanocrystals. A hydrogel is then prepared via photocuring, enabling the tubular hydrogel to achieve controlled mechanical properties without relying on external water exchange. This is due to the switching between hydrophobic interactions and host-guest interactions within the hydrogel. At room temperature, the hydrophobic chains on the side groups of the hydrogel polymer chains form strong hydrophobic interactions after solvent replacement, increasing the hydrophobic crosslinks within the hydrogel network and enhancing its mechanical properties. However, when the hydrogel is heated, these hydrophobic groups form host-guest interactions with the free cyclodextrin within the hydrogel network, leading to the opening of hydrophobic crosslinks and a decrease in the hydrogel's mechanical properties, achieving a thermally stimulated phase transition from microphase separation to a homogeneous phase.

[0024] Therefore, micro-pneumatic artificial muscles can achieve a phase transition effect upon heating, transforming from microphase separation caused by hydrophobic association and cross-linking to a homogeneous hydrogel generated by host-guest interactions, leading to a decrease in hydrogel modulus and an increase in toughness. With one end sealed, pressurizing and filling the other end with fluid allows the hydrogel to expand, with radial expansion strain exceeding axial expansion strain. The pressure-volume curve of the hydrogel under pressure during expansion decreases with increasing temperature, indicating a decrease in critical pressure. When the hydrogel is connected to a gas cavity, at low temperatures, the volume change of the gas cavity pressurizes the hydrogel to near the critical pressure, where it is in a stable state of high pressure and small volume. However, when the hydrogel is heated, the phase transition causes a decrease in modulus, and the internal pressure exceeds the critical pressure. This triggers a sudden instability, resulting in rapid volume expansion, ultimately reaching a stable state of low pressure and large volume.

[0025] In a third aspect of the invention, an application of the micro pneumatic artificial muscle based on the phase transition hydrogel actuator of the second aspect of the invention is provided, specifically as an application of the artificial muscle in the fabrication of a robot.

[0026] Preferably, the robot includes a diving robot and a rolling robot.

[0027] More preferably, the inner diameter of the tubular hydrogel is 1-3 mm, the outer diameter is 3-6 mm, and the internal volume of the air cavity is 0.5-5 mL.

[0028] The tubular hydrogel possesses tunable mechanical properties independent of water permeability. When connected to a miniaturized gas chamber, it remains stable under pressure until external thermal stimulation triggers a sudden instability, causing the actuator to expand rapidly. The gas chamber can employ a sealed but variable-volume injection pump structure. In applications, miniature pneumatic artificial muscles can be driven by external thermal stimulation after separating from components such as compressors and pumps. First, the gas chamber is pressurized at room temperature, where the hydrogel actuator is in a stable state of high pressure and small volume. Heating the hydrogel causes its volume to expand rapidly because heating reduces its modulus and increases its toughness. The internal pressure exceeds the actuator's critical pressure, triggering a sudden instability that leads to a stable state of low pressure and large volume, thus completing the actuation. Phase transition hydrogels can achieve switching of mechanical properties using non-water-involved mechanisms. Therefore, combining phase transition hydrogels with pneumatic artificial muscles makes miniaturized biomimetic soft robotic devices possible.

[0029] For submersible robots, the micro-pneumatic artificial muscles, after pre-pressurization, will sink in cold water; however, in hot water, due to the rapid and unstable expansion of the actuators, the robot's buoyancy increases, causing it to sink first and then quickly rise. For rolling robots, after pre-pressurization, the expansion of the hydrogel in a thermal response propels the robot to roll, enabling it to roll a relatively long distance in a short time.

[0030] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0031] This invention provides a method for preparing a micro pneumatic artificial muscle based on a phase transition hydrogel actuator, which has the advantages of convenient process, mild reaction and wide availability of materials.

[0032] This invention provides a micro pneumatic artificial muscle based on a phase transition hydrogel actuator, which has adjustable mechanical properties that do not depend on water permeation and has a fast driving speed.

[0033] This invention provides an application of a micro pneumatic artificial muscle based on a phase transition hydrogel actuator, which has broad application prospects in miniaturized biomimetic soft robot devices. Attached Figure Description

[0034] Figure 1 A physical image of a hydrogel actuator with a tubular structure undergoing rapid expansion;

[0035] Figure 2 The images show a submersible robot diving in cold water and surfacing in hot water.

[0036] Figure 3 This is a photograph of a rolling robot rolling after being subjected to thermal stimulation. Detailed Implementation

[0037] The present invention is further illustrated below by way of embodiments, but the invention is not limited to the scope of the embodiments described herein. Experimental methods in the following embodiments that do not specify specific conditions were performed according to conventional methods and conditions, or as selected according to the product instructions.

[0038] In the following embodiments:

[0039] A transparent pipe mold with an inner diameter of 2 mm and an outer diameter of 5 mm is used.

[0040] Example 1

[0041] A method for fabricating a micro-pneumatic artificial muscle based on a phase transition hydrogel actuator, comprising the following steps:

[0042] (1) Dissolve 0.077 g acrylamide, 82 μL octyl acrylate, 0.448 g β-cyclodextrin, and 0.012 g N,N'-methylenebisacrylamide in a dimethylformamide dispersion of 0.8 g sea squirt cellulose nanocrystals (0.2 wt.%), and then add 0.01 g photoinitiator I2959 to obtain a prepolymer solution. Inject the prepolymer solution into a transparent tube mold, irradiate it under ultraviolet light at room temperature for 2 h, and then soak it in water for solvent replacement for 3 days to obtain a hydrogel with a tubular structure.

[0043] (2) Connect one end of the tubular hydrogel to an initial volume of 6 mL of air cavity and seal the other end to obtain a micro pneumatic artificial muscle driven by a phase transition hydrogel actuator.

[0044] Example 2

[0045] A method for fabricating a micro-pneumatic artificial muscle based on a phase transition hydrogel actuator, comprising the following steps:

[0046] (1) Dissolve 0.077 g acrylamide, 82 μL octyl acrylate, 0.448 g β-cyclodextrin, and 0.012 g N,N'-methylenebisacrylamide in a dimethylformamide dispersion of 0.8 g sea squirt cellulose nanocrystals (0.2 wt.%), and then add 0.01 g photoinitiator I2959 to obtain a prepolymer solution. Inject the prepolymer solution into a transparent tube mold, irradiate it under ultraviolet light at room temperature for 2 h, and then soak it in water for solvent replacement for 3 days to obtain a hydrogel with a tubular structure.

[0047] (2) Connect one end of the tubular hydrogel to an initial volume of 1 mL of air cavity and seal the other end to obtain a micro pneumatic artificial muscle driven by a phase transition hydrogel actuator.

[0048] Example 3

[0049] In this embodiment, after injecting 0.9 mL of compressed gas into the air cavity of the micro-pneumatic artificial muscle prepared in Example 1, the hydrogel with the tubular structure was heated to 70 °C. Figure 1 As shown, the hydrogel expands radially by 350% within 10 s, with an expansion rate of 200% / s, and its working capacity reaches 27.2 J / kg.

[0050] The miniature pneumatic artificial muscle fabricated in Example 1 was integrated into a submersible robot, and its effectiveness in practical applications was investigated. Figure 2As shown, when the submersible robot is placed in room temperature water at 20°C, it remains submerged. When the ambient temperature changes to 70°C, under the same conditions, the submersible robot rapidly rises within 3 seconds. This is because it expands rapidly in hot water, increasing its buoyancy and thus allowing it to float.

[0051] Example 4

[0052] After pressurizing the air cavity of the miniature pneumatic artificial muscle prepared in Example 2 by 0.3 mL, it was used to prepare a rolling robot. Figure 3 As shown, when the hydrogel is heated to 70 °C, it expands radially by 210% within 10 s, with an expansion rate of 130% / s. Under thermal response, the expansion of the hydrogel propels the robot to roll, enabling it to roll forward 8.3 cm within 6 s.

[0053] Example 5

[0054] (1) Dissolve 0.126 g acrylamide, 26.5 μL octyl acrylate, 0.144 g β-cyclodextrin, and 0.012 g N,N'-methylenebisacrylamide in a 0.8 g dimethylformamide dispersion of sea squirt cellulose nanocrystals (0.3 wt.%), and then add 0.01 g photoinitiator I2959 to obtain a prepolymer solution; inject the prepolymer solution into a transparent tube mold, irradiate it under ultraviolet light at room temperature for 2 h, and then soak it in water for solvent replacement for 3 days to obtain a hydrogel with a tubular structure;

[0055] (2) Connect one end of the tubular hydrogel to an initial volume of 6 mL of air cavity and seal the other end to obtain a micro pneumatic artificial muscle driven by a phase transition hydrogel actuator.

[0056] Example 6

[0057] (1) Dissolve 0.065 g acrylamide, 96 μL octyl acrylate, 0.522 g β-cyclodextrin, and 0.012 g N,N'-methylenebisacrylamide in a dimethylformamide dispersion of 0.8 g sea squirt cellulose nanocrystals (0.1 wt.%), and then add 0.01 g photoinitiator I2959 to obtain a prepolymer solution. Inject the prepolymer solution into a transparent tube mold, irradiate it under ultraviolet light at room temperature for 2 h, and then soak it in water for solvent replacement for 3 days to obtain a hydrogel with a tubular structure.

[0058] (2) Connect one end of the tubular hydrogel to an initial volume of 6 mL of air cavity and seal the other end to obtain a micro pneumatic artificial muscle driven by a phase transition hydrogel actuator.

[0059] In summary, compared to traditional hydrogel actuators that achieve shape changes through swelling / deswelling, the micro-pneumatic artificial muscle of this invention achieves mechanical property control without relying on external water exchange by switching between hydrophobic interactions and host-guest interactions within the hydrogel. At room temperature, the hydrophobic chains on the side groups of the hydrogel polymer chains form strong hydrophobic interactions after solvent replacement, increasing the hydrophobic crosslinks within the hydrogel network and enhancing its mechanical properties. However, when the hydrogel is heated, these hydrophobic groups form host-guest interactions with free cyclodextrins within the hydrogel network, leading to the opening of hydrophobic crosslinks and a decrease in the hydrogel's mechanical properties, achieving a thermally stimulated phase transition from microphase separation to a homogeneous phase.

[0060] After integrating miniature pneumatic artificial muscles into the robot, the air chamber is first pressurized at room temperature, at which point the hydrogel actuator is in a stable state of high pressure and small volume. The submersible robot can dive in cold water, but in hot water, the hydrogel's modulus decreases and its toughness increases after heating. The internal air pressure exceeds the actuator's critical pressure, triggering a sudden instability that causes the hydrogel to reach a stable state of low pressure and large volume. Its volume expands rapidly, reaching a rate of 200% / s, increasing the robot's buoyancy and propelling it upwards. The rolling robot, relying on the expansion of the hydrogel, destabilizes its center of gravity, thus achieving rolling, advancing 8.3 cm in 6 seconds.

[0061] The preferred embodiments of the present invention have been described in detail above. It should be understood that those skilled in the art can make numerous modifications and variations based on the concept of the present invention without creative effort. Therefore, all technical solutions that can be obtained by those skilled in the art based on the concept of the present invention through logical analysis, reasoning, or limited experimentation on the basis of existing technology should be within the scope of protection defined by the claims.

Claims

1. A method for preparing a micro-pneumatic artificial muscle based on a phase transition hydrogel actuator, characterized in that, Includes the following steps: (1) Using octyl acrylate and acrylamide as monomers, the monomers, β-cyclodextrin and crosslinking agent were dissolved in dimethylformamide dispersion of sea squirt cellulose nanocrystals, and then a photoinitiator was added to obtain a prepolymer solution; the prepolymer solution was photocured and solvent replaced to obtain a hydrogel with a tubular structure. (2) Connect one end of the tubular hydrogel to the air cavity and seal the other end to obtain a micro pneumatic artificial muscle driven by a phase transition hydrogel actuator.

2. The method for preparing a micro pneumatic artificial muscle based on a phase transition hydrogel actuator according to claim 1, characterized in that: In step (1), the proportion of octyl acrylate to the monomer is 6.7 mol.%-33.3 mol.%.

3. The method for preparing a micro pneumatic artificial muscle based on a phase transition hydrogel actuator according to claim 1, characterized in that: In step (1), the monomer accounts for 20 wt.%-30 wt.% of the prepolymer solution.

4. The method for preparing a micro pneumatic artificial muscle based on a phase transition hydrogel actuator according to claim 1, characterized in that: In step (1), the molar ratio of β-cyclodextrin to octyl acrylate is 0-1:

1.

5. The method for preparing a micro pneumatic artificial muscle based on a phase transition hydrogel actuator according to claim 1, characterized in that: In step (1), the crosslinking agent accounts for 1 wt.%-3 wt.% of the prepolymer liquid; the crosslinking agent includes at least one of N,N'-methylenebisacrylamide, ethylene glycol dimethacrylate, and trimethylolpropane triacrylate.

6. The method for preparing a micro pneumatic artificial muscle based on a phase transition hydrogel actuator according to claim 1, characterized in that: In step (1), the concentration of the dimethylformamide dispersion of sea squirt cellulose nanocrystals is 0.1 wt.%-0.3 wt.%.

7. The method for preparing a micro pneumatic artificial muscle based on a phase transition hydrogel actuator according to claim 1, characterized in that: In step (1), the proportion of photoinitiator in the prepolymer liquid is 1 wt.%-3 wt.%; the photoinitiator includes at least one of 2-hydroxy-1-(4-(2-hydroxyethoxy)phenyl)-2-methyl-1-propanone, 2-hydroxy-2-methyl-1-phenyl-1-propanone, and phenyl-2,4,6-trimethylbenzoyl lithium phosphite.

8. A miniature pneumatic artificial muscle based on a phase transition hydrogel actuator, characterized in that: It is prepared by any one of the preparation methods described in claims 1-7.

9. An application of the micro pneumatic artificial muscle based on a phase transition hydrogel actuator as described in claim 8, characterized in that: As an application of artificial muscles in the fabrication of robots.

10. The application of the micro pneumatic artificial muscle based on a phase transition hydrogel actuator according to claim 9, characterized in that: The robots include diving robots and rolling robots; the inner diameter of the tubular hydrogel is 1-3 mm and the outer diameter is 3-6 mm; the internal volume of the air cavity is 0.5-5 mL.

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