Temperature-sensitive hydrogel for artificial muscle and method for preparing the same

Abstract

The application discloses a temperature-sensitive hydrogel for artificial muscle and a preparation method thereof, and relates to the technical field of functional polymer materials. The temperature-sensitive hydrogel is prepared by the following steps: forming a deep eutectic precursor solution by using anhydrous citric acid and an amide monomer as raw materials, adding a crosslinking agent and an initiator after vacuum drying, and then performing intermittent cooling polymerization reaction and water replacement treatment. The temperature-sensitive hydrogel prepared by the preparation method has a high entanglement polymer network formed by deep eutectic bulk polymerization, can produce large reversible deformation without extreme temperature stimulation, and has far superior deformation output efficiency and power function to traditional water solution polymerization hydrogels.

Description

Technical Field

[0001] This invention relates to the field of functional polymer materials technology, and more specifically, to a thermosensitive hydrogel for artificial muscle and its preparation method. Background Technology

[0002] Polyamide hydrogels, due to their excellent flexibility, deformability, and environmental responsiveness, show promising applications in artificial muscles, flexible sensors, and soft robots. Among them, poly(N-isopropylacrylamide) hydrogels are a typical example of thermosensitive hydrogels. Their molecular chains contain hydrophilic amide groups and hydrophobic isopropyl groups, enabling them to undergo a reversible volume phase transition in response to external temperature changes. Therefore, they have attracted widespread attention in biomimetic actuators and intelligent responsive devices. Meanwhile, polyacrylamide hydrogels, due to their good hydrophilicity and gel-forming properties, are also widely used in flexible functional material systems.

[0003] In existing technologies, hydrogels such as poly(N-isopropylacrylamide) hydrogels and poly(N-isopropylmethacrylamide) hydrogels are typically prepared using aqueous solution polymerization methods, such as redox-initiated polymerization, thermally initiated polymerization, or photo-initiated polymerization. Specifically, monomers, crosslinking agents, and initiators are generally dissolved in water to form a prepolymer solution, which is then used to initiate the polymerization reaction through heating, light irradiation, or a redox system to ultimately obtain the corresponding hydrogel material.

[0004] However, the concentration of monomers such as N-isopropylacrylamide and N-isopropylmethacrylamide in traditional aqueous systems is limited, which is not conducive to the formation of high-concentration polymer chain stacking and sufficient inter-chain physical entanglement during polymerization. As a result, hydrogels prepared in this way usually have problems such as insufficient network entanglement, limited mechanical properties and low work density, which makes it difficult to meet the requirements of material strength, ductility and stability for applications such as artificial muscles, flexible sensors and soft robots.

[0005] Although existing technologies can improve the properties of hydrogels to some extent by introducing nanofillers for physical reinforcement, copolymerizing with rigid or flexible monomers to regulate the network structure, or using special solvent systems such as ionic liquids to replace water as the polymerization medium, they are often accompanied by new problems such as complex preparation processes, poor biocompatibility, or impaired temperature-sensitive responsiveness.

[0006] In recent years, deep eutectic gels based on deep eutectic solvents have shown broad application prospects in flexible wearable devices, soft robots, energy storage and conversion, and biomedicine due to their excellent mechanical properties, high ionic conductivity, good thermal stability, biocompatibility, and flexible functional design capabilities. Currently, although studies have reported the use of citric acid as a hydrogen bond donor to construct deep eutectic systems with hydrogen bond acceptors such as choline chloride and β-cyclodextrin, which can be further used to dissolve monomers and prepare deep eutectic gels, a technical solution for directly constructing polymerizable deep eutectic solutions with amide monomers using citric acid as a hydrogen bond donor and then further preparing thermosensitive hydrogels for artificial muscles is not disclosed in existing technology, representing a technological gap in this field.

[0007] Therefore, how to provide a new method for preparing thermosensitive hydrogels to increase the concentration of amide monomers in the polymerization system, improve the polymerization process, and obtain thermosensitive hydrogels suitable for applications such as artificial muscles remains a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0008] To overcome the problems of limited monomer concentration, insufficient interchain physical entanglement, weak mechanical properties, and limited application range of polyamide hydrogel materials in the prior art, the first objective of this invention is to provide a method for preparing a thermosensitive hydrogel for artificial muscle, the preparation method specifically including the following steps: Step S1: Under heating conditions, anhydrous citric acid is mixed with amide monomers to form a deep eutectic precursor solution; Step S2: After drying the deep eutectic precursor solution obtained in step S1, add crosslinking agent and initiator, mix evenly to obtain the precursor solution. Step S3: The precursor solution obtained in step S2 is subjected to intermittent cooling polymerization to obtain a deep eutectic gel; Step S4: The deep eutectic gel obtained in step S3 is subjected to water replacement treatment to obtain a thermosensitive hydrogel. In step S1, the molar ratio of anhydrous citric acid to amide monomer is 1:(1-9), and the amide monomer is selected from one of N-isopropylacrylamide, N-isopropylmethacrylamide, N-vinylcaprolactam and acrylamide. In step S2, the crosslinking agent is N,N'-methylenebisacrylamide and / or polyethylene glycol diacrylate, and the initiator is azobisisobutyronitrile and / or azobisisoheptanenitrile; In step S3, the intermittent cooling polymerization reaction includes two steps: first, the precursor solution obtained in step S2 is polymerized at 60-75℃ for 4-8 hours; then the resulting system is transferred to 50-60℃ for further polymerization for 10-18 hours.

[0009] Compared with existing technologies, this invention uses anhydrous citric acid and amide monomers to construct a deep eutectic system to replace the traditional aqueous polymerization medium. This increases the effective content of amide monomers in the polymerization system, alleviates the problem of limited monomer concentration in traditional aqueous systems, and provides a more uniform and stable precursor environment for subsequent intermittent cooling polymerization reactions. Furthermore, this invention selects a single amide monomer from N-isopropylacrylamide, N-isopropylmethacrylamide, N-vinylcaprolactam, or acrylamide as the core raw material. All four types of monomers can form stable polymerizable deep eutectic precursors with anhydrous citric acid via hydrogen bonding. Each monomer possesses differentiated temperature-sensitive response characteristics, and the resulting temperature-sensitive hydrogels all exhibit reversible temperature phase transition capabilities, significantly expanding the applicability of this deep eutectic preparation process. Flexible driven hydrogels adapted to different response temperature ranges can be prepared as needed.

[0010] Based on this, the present invention limits the molar ratio of anhydrous citric acid to amide monomers to 1:(1-9), which is beneficial to take into account the gelatinability, uniformity and post-polymerization material properties of the system, thereby giving the resulting thermosensitive hydrogel good comprehensive properties.

[0011] Furthermore, this invention introduces N,N'-methylenebisacrylamide and / or polyethylene glycol diacrylate as crosslinking agents into the deep eutectic system, and combines them with azobisisobutyronitrile and / or azobisisoheptanenitrile as initiators. This allows the amide monomers to polymerize under anhydrous conditions, which is beneficial for improving the degree of interchain interaction and network entanglement in the polymer, thereby improving the tensile strength, elastic modulus, and elongation of the resulting thermosensitive hydrogel, and enhancing the controllability and application adaptability of the material properties. Therefore, this invention can effectively improve the problems of weak mechanical properties and limited application range of existing polyamide hydrogel materials, thus broadening their applications in fields such as artificial muscles, flexible sensors, and soft robots.

[0012] Furthermore, this invention employs a two-step polymerization process with intermittent cooling, consisting of high-temperature prepolymerization at 60-75℃ and low-temperature postpolymerization at 50-60℃. The high-temperature stage rapidly initiates the crosslinking reaction, enabling the system to quickly solidify and form a basic gel network. The low-temperature stage slows down the polymerization rate, allowing unreacted monomers to fully polymerize while releasing the internal stress generated during polymerization. This promotes the full relaxation and entanglement of the polymer chains, resulting in a thermosensitive hydrogel with a denser internal network and fewer defects. This effectively improves the tensile elongation, cyclic thermal stability, and fatigue resistance of the thermosensitive hydrogel.

[0013] In one possible implementation, the heating conditions in step S1 are as follows: heating temperature is 30-50℃, and heating time is 1-5h.

[0014] Compared with the prior art, the present invention, by limiting the heating and mixing parameters in step S1 to 30-50℃ and 1-5h, enables anhydrous citric acid to fully contact and complete hydrogen bonding complexation with the amide monomer, effectively eliminating the problems of uneven local mixing of raw materials and system stratification. This results in a deep eutectic precursor solution with uniform composition and suitable fluidity, avoiding local crosslinking imbalance, gel cracking, or performance differences caused by uneven precursors in subsequent polymerization. Thus, it provides a reliable foundation for the formation of a uniform, complete, and stable gel network in subsequent polymerization.

[0015] In one possible implementation, in step S2, the molar percentage of anhydrous citric acid in the precursor solution is 10%-50%, the molar percentage of amide monomers in the precursor solution is 20%-90%, and the sum of the molar percentages of anhydrous citric acid and amide monomers is less than 100%.

[0016] Compared with existing technologies, this formulation range ensures that anhydrous citric acid can fully exert its hydrogen bonding association effect, constructing a uniform and stable anhydrous polymerization system, and completely breaking through the upper limit of monomer concentration in traditional aqueous polymerization. At the same time, it can flexibly control the crosslinking density, molecular chain entanglement degree and pore structure of the gel's three-dimensional network, realizing gradient adjustable tensile strength, elongation at break, temperature-sensitive response rate and actuation work density of the thermosensitive hydrogel. It can adapt to the driving conditions of artificial muscles and soft robots with different loads and different deformation amplitudes, effectively solving the technical pain points of traditional hydrogels with single performance and difficulty in balancing mechanical and driving performance.

[0017] In one possible implementation, in step S2, the amount of crosslinking agent is 0.5%-2% of the molar amount of the amide monomer; the amount of initiator is 0.2%-2% of the molar amount of the amide monomer.

[0018] Compared with existing technologies, this invention limits the dosage of crosslinking agent and initiator. The effect is that: too low a dosage of crosslinking agent results in insufficient crosslinking network and poor strength; too high a dosage leads to overly dense crosslinking points, increasing gel brittleness and decreasing deformation capacity. Too low a dosage of initiator leads to insufficient monomer conversion, making it difficult to form a complete network; too high a dosage easily causes localized burst polymerization, resulting in poor network uniformity. This dosage range can form a uniform crosslinking network, enabling the thermosensitive hydrogel to possess both good mechanical properties and reversible thermal actuation capability, meeting the needs of artificial muscle actuation.

[0019] In one possible implementation, the drying parameters in step S2 are as follows: temperature 40-45℃, vacuum degree 0.05-0.07MPa, and time 0.5-1h.

[0020] Compared with the prior art, the present invention, by limiting the temperature, vacuum degree and processing time of vacuum drying in step S2, can fully remove trace amounts of free water from the deep eutectic precursor, completely avoid the risk of residual water damaging the anhydrous bulk polymerization system, prevent water from causing premature decomposition of azo initiators, incomplete crosslinking reaction, and pore defects inside the gel, ensure that the subsequent polymerization reaction proceeds smoothly and fully, and improve the uniformity and mechanical integrity of the overall structure of the final deep eutectic gel.

[0021] In one possible implementation, the water replacement treatment in step S4 lasts for 16-24 hours. This invention limits the water replacement soaking time to 16-24 hours, which allows for the complete dissolution of residual anhydrous citric acid solvent within the deep eutectic gel, precisely controlling the balance ratio of hydrophilic and hydrophobic groups within the gel matrix, imparting a suitable minimum critical dissolution temperature to the material, and ensuring that the gel can undergo significant and reversible volume shrinkage and swelling behavior when the temperature changes, thus stably maintaining the continuous actuation deformation and work output capability required for artificial muscles.

[0022] A second objective of this invention is to provide a thermosensitive hydrogel for artificial muscle, prepared using the above-described method.

[0023] The thermosensitive hydrogel prepared by the method of this invention relies on a highly entangled polymer network formed by deep eutectic bulk polymerization. It can generate large-scale reversible deformation without extreme temperature stimulation. Its deformation output efficiency and work capacity are far superior to those of traditional aqueous solution polymerized hydrogels. It combines mechanical flexibility and temperature response characteristics, and can fully meet the core application requirements of flexible drive material deformation and power output for devices such as artificial muscles, soft robots, and flexible temperature sensors. Attached Figure Description

[0024] Figure 1 The stress-strain performance test results of the thermosensitive hydrogels prepared in Examples 2, 4 and 5 and the poly(N-isopropylacrylamide) hydrogel prepared in Comparative Example 1 are shown in the figure. Figure 2 is a schematic diagram of the thermo-actuated properties test of the thermosensitive hydrogel prepared in Example 4. Detailed Implementation

[0025] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention are described in detail below. It should be noted that the following embodiments are only used to illustrate the implementation methods and typical parameters of the present invention, and are not intended to limit the parameter range described in the present invention. Reasonable variations derived therefrom are still within the protection scope of the claims of the present invention.

[0026] It should be noted that the endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0027] Unless otherwise defined, all terms, symbols, and other scientific terms used herein are intended to have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. In some instances, terms having a conventionally understood meaning are defined herein for clarification or ease of reference, and such definitions should not be construed as indicating a significant difference from conventional understanding in the art. The technical methods described or referenced herein are generally well understood by those skilled in the art and employed through conventional methods. Unless otherwise stated, the use of commercially available kits, reagents, and instruments shall be performed according to the manufacturer's instructions and parameters.

[0028] It is worth mentioning that in this invention, a universal tensile testing machine is used for stress-strain testing, and the tensile rate is set to 50 mm / min.

[0029] The work density test is as follows: the hydrogel is immersed in water and fixed at the upper end, with a weight suspended at the lower end. The temperature is raised to above the minimum critical melting temperature. The work done by the artificial muscle is calculated using the formula: W = (GF) × L, where G is the gravity of the heavy load, F is the buoyancy of the heavy load, L is the distance moved, and the heavy load is the sum of the mass of the weight and the clamp. The calculated work is divided by the initial volume of the hydrogel to obtain the work density.

[0030] Infrared thermometers are used to monitor the temperature of thermally driven components.

[0031] The following description is based on specific embodiments.

[0032] Example 1: This embodiment provides a thermosensitive hydrogel for artificial muscle. The amide monomer in this embodiment is N-isopropylacrylamide, which is prepared by the following method: Step S1: Add anhydrous citric acid and N-isopropylacrylamide in a molar ratio of 1:1 to a container and heat at 40°C for 2 hours to form a deep eutectic precursor solution. Step S2: The deep eutectic precursor solution obtained in step S1 is dried at 42°C and a vacuum of 0.06 MPa for 0.75 h. Then, N,N'-methylenebisacrylamide and azobisisobutyronitrile are added. The amount of N,N'-methylenebisacrylamide is 0.5% of the molar amount of the amide monomers, and the amount of azobisisobutyronitrile is 0.2% of the molar amount of the amide monomers. The mixture is thoroughly shaken and ultrasonically mixed to obtain a precursor solution, such that the molar percentage of anhydrous citric acid and the molar percentage of the amide monomers in the precursor solution are both 45%. Step S3: Inject the precursor solution into a 1 mm thick silicone mold, place it in a 75°C water bath for 6 h, and then transfer it to a 55°C water bath for 14 h to continue polymerization to obtain a deep eutectic gel. Step S4: The deep eutectic gel obtained in step S3 is subjected to water replacement treatment for 20 hours to obtain a thermosensitive hydrogel.

[0033] Example 2: This example provides a thermosensitive hydrogel for artificial muscle. The only difference from Example 1 is that the molar ratio of anhydrous citric acid to amide monomers is 1:2. Everything else is the same as in Example 1 and will not be repeated here.

[0034] Example 3: This example provides a thermosensitive hydrogel for artificial muscle. The only difference from Example 1 is that the molar ratio of anhydrous citric acid to amide monomers is 1:2, the crosslinking agent is polyethylene glycol diacrylate and the amount used is 1% of the molar amount of amide monomers, and the initiator is azobisisobutyronitrile and the amount used is 0.8% of the molar amount of amide monomers. The rest is the same as in Example 1, and will not be repeated here.

[0035] Example 4: This example provides a thermosensitive hydrogel for artificial muscle. The only difference from Example 1 is that the molar ratio of anhydrous citric acid to amide monomers is 1:3 in this example. Everything else is the same as in Example 1 and will not be repeated here.

[0036] Example 5: This example provides a thermosensitive hydrogel for artificial muscle. The only difference from Example 1 is that the molar ratio of anhydrous citric acid to amide monomers is 1:4 in this example. Everything else is the same as in Example 1, and will not be repeated here.

[0037] Example 6: This example provides a thermosensitive hydrogel for artificial muscle. The only difference from Example 1 is that the molar ratio of anhydrous citric acid to amide monomers is 1:4, the crosslinking agent is N,N'-methylenebisacrylamide and polyethylene glycol diacrylate in a mass ratio of 1:1, and the initiator is azobisisobutyronitrile and azobisisoheptanenitrile in a mass ratio of 1:1. The amounts of crosslinking agent and initiator, as well as other parameters, are the same as in Example 1 and will not be repeated here.

[0038] Example 7: This example provides a thermosensitive hydrogel for artificial muscle. The only difference from Example 1 is that the molar ratio of anhydrous citric acid to amide monomers is 1:5, the molar percentage of anhydrous citric acid in the precursor solution is 16%, and the molar percentage of amide monomers in the precursor solution is 80%. Everything else is the same as in Example 1 and will not be repeated here.

[0039] Example 8: This example provides a thermosensitive hydrogel for artificial muscle. The only difference from Example 1 is that the molar ratio of anhydrous citric acid to amide monomers is 1:6, the molar percentage of anhydrous citric acid in the precursor solution is 13.8%, and the molar percentage of amide monomers in the precursor solution is 83%. In step S4, the water replacement treatment time is 24 hours. The rest is the same as in Example 1 and will not be repeated here.

[0040] Example 9: This example provides a thermosensitive hydrogel for artificial muscle. The only difference from Example 1 is that the molar ratio of anhydrous citric acid to amide monomers is 1:7, the amount of crosslinking agent is 2% of the molar amount of amide monomers, the amount of initiator is 1.5% of the molar amount of amide monomers, and the water replacement treatment time in step S4 is 16 hours. The rest is the same as in Example 1 and will not be repeated here.

[0041] Example 10: This example provides a thermosensitive hydrogel for artificial muscle. The only difference from Example 1 is that the molar ratio of anhydrous citric acid to amide monomers is 1:8, the amount of crosslinking agent is 1.5% of the molar amount of amide monomers, the amount of initiator is 2% of the molar amount of amide monomers, and the heating parameters in step S1 are as follows: heating temperature is 30°C, heating time is 5h, and other parameters are the same as in Example 1, which will not be repeated here.

[0042] Example 11: This example provides a thermosensitive hydrogel for artificial muscle. The only difference from Example 1 is that the molar ratio of anhydrous citric acid to amide monomers is 1:9 in this example, and the heating parameters in step S1 are as follows: heating temperature is 50°C, heating time is 1 hour, and the rest are the same as in Example 1, which will not be repeated here.

[0043] Example 12: This example provides a thermosensitive hydrogel for artificial muscle. The only difference from Example 1 is that the amide monomer in this example is acrylamide, and the molar ratio of anhydrous citric acid to the amide monomer is 1:2. In step S3, the polymerization is first carried out at 70°C for 8 hours, and then transferred to 60°C for 10 hours. The rest is the same as in Example 1, and will not be repeated here.

[0044] Example 13: This example provides a thermosensitive hydrogel for artificial muscle. The only difference from Example 1 is that the amide monomer in this example is acrylamide, and the molar ratio of anhydrous citric acid to the amide monomer is 1:6. The crosslinking agent is polyethylene glycol diacrylate, and the initiator is azobisisobutyronitrile. The amounts of crosslinking agent and initiator are the same as in Example 1. In addition, in step S3, polymerization is first carried out at 60°C for 4 hours, and then transferred to 50°C for 18 hours. The rest is the same as in Example 1, and will not be repeated here.

[0045] Example 14: This example provides a thermosensitive hydrogel for artificial muscle. The only difference from Example 1 is that the amide monomer in this example is acrylamide, and the molar ratio of anhydrous citric acid to the amide monomer is 1:8. In step S2, the drying parameters are as follows: temperature is 45°C, vacuum degree is 0.05 MPa, time is 0.5 h, the crosslinking agent is N,N'-methylenebisacrylamide and polyethylene glycol diacrylate in a mass ratio of 1:1, and the initiator is azobisisobutyronitrile and azobisisoheptanenitrile in a mass ratio of 1:1. The amount of crosslinking agent and initiator and other parameters are the same as in Example 1, and will not be repeated here.

[0046] Example 15: This example provides a thermosensitive hydrogel for artificial muscle. The only difference from Example 1 is that the amide monomer in this example is acrylamide, and the molar ratio of anhydrous citric acid to the amide monomer is 1:9. In step S2, the drying parameters are as follows: temperature is 40°C, vacuum degree is 0.07MPa, and time is 1h. The other parameters are the same as in Example 1 and will not be repeated here.

[0047] Example 16: This example provides a thermosensitive hydrogel for artificial muscle. The only difference from Example 1 is that the amide monomer in this example is N-isopropylmethacrylamide, and the molar ratio of anhydrous citric acid to the amide monomer is 1:4. The crosslinking agent is N,N'-methylenebisacrylamide, and the initiator is azobisisoheptanenitrile. The amount of crosslinking agent and initiator and other parameters are the same as in Example 1, and will not be repeated here.

[0048] Example 17: This example provides a thermosensitive hydrogel for artificial muscle. The only difference from Example 1 is that the amide monomer in this example is N-vinylcaprolactam, and the molar ratio of anhydrous citric acid to the amide monomer is 1:3. The rest is the same as in Example 1, and will not be repeated here.

[0049] Comparative Example 1: This comparative example provides a poly(N-isopropylacrylamide) hydrogel, which differs from Example 2 only in that it does not contain anhydrous citric acid. Otherwise, it is the same as Example 2 and will not be repeated here.

[0050] Comparative Example 2: This comparative example provides a hydrogel, which differs from Example 1 only in that the specific operation of step S3 in this comparative example is as follows: the precursor solution is injected into a 1 mm thick silicone mold and polymerized in a 65°C water bath for 20 h to obtain a gel; the rest is the same as in Example 1, and will not be repeated here.

[0051] Comparative Example 3: This comparative example provides a hydrogel, which differs from Example 1 only in that step S4 is not performed in this comparative example, and the deep eutectic gel obtained in step S3 is finally obtained.

[0052] Comparative Example 4: This comparative example provides a hydrogel, which differs from Example 11 only in that the molar ratio of anhydrous citric acid to amide monomers in this comparative example is 1:10. Everything else is the same as in Example 11, and will not be repeated here.

[0053] Application Examples

[0054] Figure 2 Using a 50g weight as a load, the thermodynamic reversible properties of the thermosensitive hydrogel prepared in Example 4 of this invention were verified: at a low temperature of 25°C, the hydrogel absorbs water and swells, resulting in a longer overall length; when the temperature is raised to 40°C, the gel undergoes a thermosensitive phase transition, becomes hydrophobic, loses water, and shrinks, shortening the length of the gel segment and causing displacement when the weight is pulled upwards; when the temperature drops back to 25°C, the gel reabsorbs water and swells, restoring its initial length. Figure 2 This demonstrates that the hydrogel can achieve reversible contraction-swelling deformation under heavy loads by relying on temperature changes, possessing the temperature-controllable driving capability required for artificial muscles, and can be applied to the fields of biomimetic actuation and soft robotics.

[0055] The properties of the thermosensitive hydrogels prepared in Examples 1-17 and the hydrogels prepared in Comparative Examples 1-4 were tested, and the test results are shown in Table 1. Table 1: Performance test results of the thermosensitive hydrogels prepared in Examples 1-17 and the hydrogels prepared in Comparative Examples 1-4

[0056] As shown in Table 1, this invention, through multi-dimensional synergistic design involving raw material selection, proportion control, stepwise polymerization, and water replacement post-treatment, enables differentiated and targeted controllable regulation of the three core mechanical properties of hydrogels: tensile strength, elastic modulus, and elongation at break. The independent influence of each parameter on different properties is basically consistent with the preparation principle of this invention. Among them, tensile strength is mainly affected by monomer activity, deep eutectic system ratio, selection of crosslinking agent / initiator combination, and intermittent cooling polymerization process. Highly active acrylamide monomers, a suitable citric acid-monomer hydrogen bond ratio, and a dense crosslinked network constructed by gradient cooling polymerization are beneficial to improving the hydrogel's tensile strength. The tensile strength and elastic modulus of the hydrogel are mainly determined by the crosslinking density, network compactness, residual small molecules, and hydration state of the system. Generally, the higher the crosslinking density and the tighter the network, the greater the elastic modulus, the stronger the material rigidity, and the weaker the flexibility. However, the plasticizing effect of residual small molecules or changes in the hydration state can also weaken the network rigidity and reduce the elastic modulus. Therefore, a comprehensive judgment based on the specific system is required. The elongation at break is mainly regulated by the flexibility of the polymer chains, the degree of molecular chain entanglement, the network pore structure, and the hydration swelling state. Flexible monomer substrates, sufficient molecular chain relaxation entanglement, and a uniform and loose hydration network can improve the material's reversible deformation ability and elongation at break. Based on the preparation process and raw material ratio range defined by this invention, the various mechanical properties of the hydrogel can be effectively controlled in a gradient manner, and a series of temperature-sensitive hydrogel products adapted to different artificial muscles and flexible device driving conditions can be prepared, effectively improving the problems of single mechanical properties, poor adaptability, and limited application range of traditional polyamide hydrogels.

[0057] Comparing the test results of Example 11 and Comparative Example 4, it can be seen that when the molar ratio of anhydrous citric acid to amide monomers exceeds the 1:(1-9) limit of this invention and reaches 1:10, the tensile strength of the thermosensitive hydrogel prepared in Example 11 is 121 kPa and the elongation at break is 336%, while the tensile strength of the hydrogel prepared in Comparative Example 4 is 116 kPa and the elongation at break is 323%. The two core mechanical properties of the hydrogel prepared in Comparative Example 4 are lower than those of the thermosensitive hydrogel prepared in Example 11. This test result supports the rationality of limiting the molar ratio of anhydrous citric acid to amide monomers to 1:(1-9) in this invention. This ratio range is beneficial to ensure sufficient hydrogen bond complexation and uniform and stable composition in the deep eutectic system, reducing network defects caused by ratio imbalance, and enabling the relatively stable construction of a uniform and dense polymer crosslinking network. This provides a foundation for the hydrogel to have good comprehensive mechanical properties from the raw material system level.

[0058] The stress-strain properties of the thermosensitive hydrogels prepared in Examples 2, 4, and 5, as well as the conventional aqueous polymer (N-isopropylacrylamide) hydrogel prepared in Comparative Example 1, were tested, and the results are as follows: Figure 1As shown. Comparative Example 1 uses a traditional pure aqueous polymerization system. Due to the limitation of the aqueous solvent, the effective polymerization concentration of monomers is low, the physical entanglement between polymer chains is insufficient, the network structure is loose, and the final tensile strength is only 38 kPa, resulting in weak mechanical properties. In contrast, this invention uses anhydrous citric acid to construct a deep eutectic polymerization system, effectively replacing the traditional aqueous polymerization medium. This helps to overcome the technical bottleneck of limited monomer concentration in traditional aqueous systems, increase the effective polymerization content of amide monomers, and, with the addition of a preferred combination of crosslinking agent and initiator, strengthen the interaction and physical entanglement between polymer molecular chains. Therefore, the tensile strength and elongation at break of the thermosensitive hydrogels prepared in Examples 2, 4, and 5 are all superior to the traditional aqueous poly(N-isopropylacrylamide) hydrogel prepared in Comparative Example 1. Meanwhile, with the optimized control of the molar ratio of anhydrous citric acid to N-isopropylacrylamide, the hydrogel can achieve a gradient performance change with gradually increasing tensile strength, moderately adjustable elastic modulus, and a significant increase in elongation at break. This confirms that the deep eutectic system design of the present invention can realize the directional and designable control of the mechanical properties of the hydrogel, effectively improving the core defects of traditional polyamide hydrogels, such as loose network and weak mechanical properties.

[0059] Further mechanism analysis was conducted on the ultra-high toughness samples of the present invention: The present invention selects multiple types of thermosensitive amide monomers, among which the acrylamide monomers selected in Examples 12 and 13 have structural characteristics of extremely simple side groups, extremely low steric hindrance, and excellent main chain flexibility. Compared with thermosensitive monomers such as N-isopropylacrylamide, N-isopropylmethacrylamide, and N-vinylcaprolactam, which have hydrophobic side groups and large steric hindrance, the molecular chain slippage and relaxation resistance is smaller. This provides the core structural basis for achieving an ultra-high elongation at break of over 0.1% and breaking through the technical bottleneck of traditional thermosensitive hydrogels that are difficult to balance strength and toughness. This also broadens the applicability and performance adjustable range of the process of the present invention. Two sets of ultra-high elongation examples, relying on the differentiated formulation and intermittent cooling two-step polymerization process of this invention, formed high strength and high toughness properties with different focuses: Example 12 adopted a 1:2 low citric acid-acrylamide ratio, combined with an intermittent polymerization regime of 70℃ high-temperature prepolymerization for 8 hours + 60℃ low-temperature postpolymerization for 10 hours, which appropriately weakened the hydrogen bond crosslinking strength of the system, retained sufficient molecular chain slip space, and released the deformation ability of acrylamide molecular chains to a large extent, ultimately obtaining an optimal elongation at break of 1152%, which is suitable for ultra-large deformation biomimetic driving scenarios; Example 13 adopted A 1:6 optimized formulation, combined with a mild polymerization process of 60℃ low-temperature prepolymerization for 4 hours + 50℃ long-term low-temperature postpolymerization for 18 hours, aligns with the core design concept of intermittent cooling polymerization in this invention. The high-temperature stage rapidly forms the basic chemical cross-linked framework, while the low-temperature stage slows down the polymerization rate and fully releases internal polymerization stress, promoting the relaxation and entanglement of polymer chains. This results in a hierarchical composite network structure of "dense chemical cross-linked framework + highly physically entangled flexible molecular chains," achieving an ultra-high tensile strength of 669 kPa and an ultra-high elongation at break of 1075%. In summary, this invention, through the synergistic innovative design of flexible monomer selection, deep eutectic ratio optimization, and intermittent cooling stepwise polymerization, effectively constructs a high-strength, high-toughness, and highly stable hydrogel cross-linked network, effectively solving the problem of traditional hydrogels' difficulty in balancing strength and toughness, and improving the material's flexibility, adaptability, and application range.

[0060] Comparative Example 2 serves as a control sample for the one-step isothermal polymerization process. The only difference is that the core intermittent cooling two-step polymerization process of this invention was replaced with traditional isothermal polymerization. All other raw materials, proportions, drying, and post-treatment processes were identical to those in Example 1. This example was used to independently verify the technical gain effect of the stepwise polymerization process of this invention. Test results show that the hydrogel prepared in Comparative Example 2 has a tensile strength of 75 kPa and an elongation at break of 153%, both of which are lower than the performance of the temperature-sensitive hydrogel prepared in Example 1. In essence, traditional one-step isothermal polymerization has an excessively fast reaction rate, resulting in instantaneous cross-linking and setting of monomers. This leaves insufficient time for molecular chains to relax, stretch, and entangle, leading to concentrated internal polymerization stress and prominent issues such as network defects and uneven porosity. In contrast, the two-step polymerization process of high-temperature prepolymerization and low-temperature stress release, employed in this invention, is a crucial factor in improving the gel network structure and enhancing mechanical properties. It allows unreacted monomers to fully polymerize, effectively reducing network defects and improving cross-linking uniformity. Simultaneously, it fully releases internal polymerization stress, strengthening the molecular chain entanglement effect. From a process perspective, this improves the strength and ductility of the hydrogel to a certain extent, supporting the rationality of the stepwise polymerization process of this invention.

[0061] Comparative Example 3 is a control sample with the water replacement process omitted. It fully executes the deep eutectic polymerization steps S1-S3 of this invention, except for the water replacement treatment step S4, which is omitted. This is used to independently verify the important role of the water replacement process in the microstructure, mechanical properties, and temperature-sensitive driving properties of the hydrogel. Test data show that, compared to Example 1, the tensile strength of the hydrogel prepared in Comparative Example 3 significantly increased from 80 kPa to 461 kPa, but the elastic modulus decreased from 31.6 kPa to 21.5 kPa, and the elongation at break decreased from 160% to 139%, indicating a significant deterioration in material ductility. The mechanism of this differential performance change is basically consistent with the design principle of the water replacement process of this invention: the deep eutectic gel without water replacement retains a large number of anhydrous citric acid molecules. The residual citric acid can act as temporary physical crosslinking points to fill network pores and compress gel gaps, macroscopically manifesting as an instantaneous increase in tensile strength; however, the residual small molecules restrict the movement space of the polymer chains, limiting the stretching, slippage, and relaxation of the molecular chains, directly leading to a decrease in the material's deformation capacity and ductility. Meanwhile, residual citric acid disrupts the hydrophilic-hydrophobic group balance of the gel, interferes with its temperature-sensitive phase transition properties, and affects its reversible swelling-contraction ability, making it difficult to achieve the thermo-actuated function of artificial muscles. This invention, through a 16-24h water replacement treatment, can fully remove residual anhydrous citric acid from the gel, reduce ineffective physical cross-linking points, regulate the hydrophilic-hydrophobic balance and network pore structure of the gel, and release space for polymer chain movement. Although the tensile strength decreases significantly, it improves the material's ductility, endowing the gel with better flexibility, ductility, and reversible temperature-sensitive response, achieving a comprehensive balance between mechanical properties and thermal actuation performance. This is a crucial step in ensuring the hydrogel's suitability for long-term actuation applications in artificial muscles.

[0062] Combining the mechanical properties and thermally driven test results, it can be seen that the thermosensitive hydrogel prepared by this invention, relying on a self-designed deep eutectic anhydrous polymerization system, ratio control, intermittent cooling stepwise polymerization, and quantitative water replacement post-treatment, possesses an optimized gradient crosslinking network and a highly entangled polymer chain structure. Simultaneously, thermally driven performance tests were conducted on the samples of each embodiment and comparative example. The thermosensitive hydrogel prepared by this invention has a thermally driven temperature of 30-50℃, an actuation strain of 30%-60%, and a work density of 6-15 kJ / m³. 3 The comparative sample could not simultaneously meet the above driving indicators, proving that the thermosensitive hydrogel obtained by the preparation process of this invention has both excellent mechanical properties and the thermal actuation output capability required by artificial muscles.

[0063] While the disclosure is as stated above, its scope of protection is not limited thereto. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of this disclosure, and all such changes and modifications will fall within the protection scope of this invention.

Claims

1. A method for preparing a thermosensitive hydrogel for artificial muscle, characterized in that, The preparation method specifically includes the following steps: Step S1: Under heating conditions, anhydrous citric acid is mixed with amide monomers to form a deep eutectic precursor solution; Step S2: After drying the deep eutectic precursor solution obtained in step S1, add crosslinking agent and initiator, mix evenly to obtain the precursor solution. Step S3: The precursor solution obtained in step S2 is subjected to intermittent cooling polymerization to obtain a deep eutectic gel; Step S4: The deep eutectic gel obtained in step S3 is subjected to water replacement treatment to obtain a thermosensitive hydrogel. In step S1, the molar ratio of anhydrous citric acid to amide monomer is 1:(1-9), and the amide monomer is selected from one of N-isopropylacrylamide, N-isopropylmethacrylamide, N-vinylcaprolactam and acrylamide. In step S2, the crosslinking agent is N,N'-methylenebisacrylamide and / or polyethylene glycol diacrylate, and the initiator is azobisisobutyronitrile and / or azobisisoheptanenitrile; In step S3, the intermittent cooling polymerization reaction includes two steps: first, the precursor solution obtained in step S2 is polymerized at 60-75℃ for 4-8 hours; then the resulting system is transferred to 50-60℃ for further polymerization for 10-18 hours.

2. The preparation method according to claim 1, characterized in that, In step S1, the heating conditions are as follows: heating temperature is 30-50℃, and heating time is 1-5h.

3. The preparation method according to claim 1, characterized in that, In step S2, the molar percentage of anhydrous citric acid in the precursor solution is 10%-50%, the molar percentage of amide monomers in the precursor solution is 20%-90%, and the sum of the molar percentages of anhydrous citric acid and amide monomers is less than 100%.

4. The preparation method according to claim 1, characterized in that, In step S2, the amount of crosslinking agent is 0.5%-2% of the molar amount of amide monomer; the amount of initiator is 0.2%-2% of the molar amount of amide monomer.

5. The preparation method according to claim 1, characterized in that, In step S2, the drying parameters are as follows: temperature 40-45℃, vacuum degree 0.05-0.07MPa, and time 0.5-1h.

6. The preparation method according to claim 1, characterized in that, In step S4, the water replacement treatment time is 16-24 hours.

7. A thermosensitive hydrogel for use in artificial muscle, characterized in that, It is prepared by the preparation method described in any one of claims 1-6.

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