Local regulation and control method for orientation of magnetic particles in magnetic hydrogel and application of local regulation and control method in soft driver

Abstract

The invention discloses a local regulation and control method for orientation of magnetic particles in magnetic hydrogel, which comprises the following steps: carrying out free radical polymerization on AAc and AAm in an aqueous solution system to obtain a P (AAc-co-AAm) solution; the preparation method comprises the following steps: adding Fe-containing magnetic particles into a P (AAc-co-AAm) solution, mixing, and gradually converting at room temperature or under a heating condition to obtain stable metal coordination hydrogel; performing local heating or illumination on the target area of the metal coordination hydrogel, and applying an external magnetic field at the same time, so that new magnetic orientation is formed in the target area; and cooling the target area to room temperature under the condition of keeping the external magnetic field, so that the magnetic particles in the target area are fixed by new magnetic orientation, thereby forming the local orientation reprogrammable magnetic hydrogel. The method is simple and convenient in process, does not need high-temperature annealing, is environment-friendly, and realizes local regulation and control of orientation of magnetic particles in the magnetic hydrogel under mild conditions; the prepared hydrogel simultaneously shows excellent reversible viscoelasticity and magnetic response function, so that the hydrogel can be applied to a soft driver.

Description

Technical Field

[0001] The present invention relates to the field of intelligent soft materials and their applications, and particularly to a method for locally regulating the orientation of magnetic particles in a magnetic hydrogel and its application in a soft actuator. Background Art

[0002] With the rapid development of flexible electronics, wearable devices, and soft robotics technologies, soft materials with remotely controllable or reversible reprogrammable properties have attracted increasing attention in academic research and industrial applications. Magnetic soft materials (especially magnetic hydrogels) have become an important research direction in recent years due to their ability to achieve rapid and reversible deformation and motion under external magnetic field stimulation. For example, by embedding high-performance magnetic particles such as neodymium iron boron (NdFeB) in a polymer matrix, a relatively strong magnetic response can be obtained to a certain extent. However, in most traditional methods, the orientation of magnetic particles often needs to be carried out before molding, and once cured and formed, it is difficult to change their orientation again, which greatly limits the application of magnetic hydrogels in scenarios that require multiple deformation programming or switching between different motion modes.

[0003] On the other hand, the hydrogel matrix is usually a chemically cross-linked or thermoplastic polymer network, which has a strong "pinning" effect on magnetic particles. It is often difficult to change the arrangement direction of magnetic particles under low-temperature or weak magnetic field conditions. Some researchers have used 3D printing to achieve the orderly arrangement of magnetic particles during the molding stage. For example, Zhao et al. (Nature, 2018) achieved a complex-shaped magnetic elastomer structure through the combination of three-dimensional printing and an external magnetic field; however, it is difficult to perform local reorientation reprogramming after curing. Other teams have tried to use high-temperature annealing or local laser heating to re-regulate the arrangement of magnetic domains. For example, Sitti et al. (Sci. Adv., 2020) locally heated magnetic particles above their Curie temperature based on a laser to reset the magnetic orientation, but for strong magnetic materials such as neodymium iron boron (NdFeB) with a high Curie temperature, this method has high difficulty and energy consumption. As Lin et al. (Nat. Commun., 2020) encapsulated magnetic microparticles with a phase-change polymer to reduce the actual operating temperature, but it is mostly limited to the elastomer system and cannot be well applied to the water-rich flexible network of hydrogels. Generally speaking, there is currently a lack of a general strategy that can reversibly reprogram the orientation of magnetic particles after molding and under low-temperature and weak magnetic field conditions, especially in hydrogel materials.

[0004] Inspired by the ability of magnetosome bacteria to easily rearrange magnetic nanoparticles, a possible approach is to make the hydrogel matrix exhibit lower viscoelasticity when needed, allowing the magnetic particles to undergo orientation rearrangement under a weak external magnetic field; after the external stimulus stops, the matrix returns to a higher strength to "lock" the rearranged magnetic orientation. The key to achieving this is that the cross-linking bonds inside the gel network have dynamic reversibility and can respond quickly to stimuli such as temperature and light. For this reason, hydrogels using supramolecular cross-linking methods such as metal coordination become ideal candidates because their cross-linking points can partially dissociate under a certain temperature or energy input, thereby endowing the gel network with adjustable viscoelasticity.

[0005] Therefore, how to significantly reduce the viscoelasticity of the hydrogel network under relatively mild conditions (such as below 60 °C) so as to achieve local reprogramming of magnetic particles under a weak magnetic field and quickly recover the strength and fix the orientation after cooling or stopping the stimulus has become a technical problem that needs to be solved urgently. Summary of the Invention

[0006] The object of the present invention is to provide a method for locally regulating the orientation of magnetic particles in a magnetic hydrogel and its application in a soft actuator. This method can achieve local regulation of the orientation of magnetic particles in the magnetic hydrogel under mild conditions, and the obtained magnetic hydrogel exhibits excellent reversible viscoelasticity and magnetic response function, so that it can be applied in a soft actuator.

[0007] To achieve the above object of the invention, the present invention provides the following technical solutions:

[0008] A method for locally regulating the orientation of magnetic particles in a magnetic hydrogel, the method comprising the following steps:

[0009] (1) Free radical polymerization of acrylic acid (AAc) and acrylamide (AAm) in an aqueous solution system to obtain an acrylic acid-acrylamide copolymer P(AAc-co-AAm) solution;

[0010] (2) Adding magnetic microparticles containing Fe to the P(AAc-co-AAm) solution, mixing, and gradually transforming at room temperature or under heating conditions to obtain a stable metal coordination hydrogel;

[0011] (3) Locally heating or irradiating the target area of the metal coordination hydrogel and applying an external magnetic field at the same time, and a new magnetic orientation is formed in the target area;

[0012] (4) Cooling the target area to room temperature while maintaining the external magnetic field, so that the magnetic microparticles in the target area are fixed with a new magnetic orientation, thereby forming a magnetic hydrogel with locally reprogrammable orientation.

[0013] The preparation method of the metal coordination hydrogel provided by the present invention is a "one-pot method" in which magnetic particles such as NdFeB are mixed with P(AAc-co-AAm) solution at one time. The Fe3+ ions released by the acidolysis on the particle surface are used to in-situ coordinate and crosslink the polymer network, avoiding complex pretreatment or subsequent solvent replacement, and simplifying the preparation process. Combining laser local heating or similar external stimulation means, the viscoelasticity of a specific area of the hydrogel can be rapidly reduced at low temperature, allowing the magnetic particles to rearrange in a weak magnetic field, and a new magnetic orientation can be "locked" after cooling by reforming the coordination crosslinking, fundamentally improving the deficiencies of the prior art in terms of orientation rewritability and operation mildness.

[0014] The core principle of the present invention is that during the formation of the hydrogel, metal ions (such as Fe3+) coordinate and crosslink with carboxyl and amide groups on the copolymer molecular chain through acidolysis and the released iron ions to form metal coordination bonds with considerable strength and reversibility. The obtained metal coordination hydrogel has excellent mechanical properties and magnetic response ability at room temperature. Under medium temperature (40 - 80 °C) conditions, the above-mentioned coordination bonds are partially dissociated, the viscoelasticity of the hydrogel decreases, thereby greatly weakening the binding force on the NdFeB particles, significantly reducing the "pinning force" of the hydrogel matrix on the magnetic particles, and under the condition of applying a weak magnetic field, the magnetic particles in a local area can be reoriented. After cooling, the metal coordination bonds are reformed, and a new magnetic orientation is obtained and stably fixed in the local area.

[0015] Preferably, in step (1), the molar ratio of acrylic acid AAc to acrylamide AAm in the P(AAc-co-AAm) solution is 1:1 - 1:20, and the total monomer concentration in the P(AAc-co-AAm) solution is 1 wt% - 20 wt%, to ensure that the copolymer has good solution viscosity and subsequent coordination ability.

[0016] More preferably, according to different application requirements, the present invention can adjust the monomer molar ratio of AAc to AAm within the range of 1:5 - 1:15 to balance the mechanical properties and reversible viscoelasticity performance of the hydrogel.

[0017] In step (1), the P(AAc-co-AAm) solution can selectively adjust the monomer ratio and polymerization conditions to obtain a molecular structure suitable for subsequent coordination crosslinking.

[0018] Preferably, in step (1), the addition amount of the magnetic microparticles is 0.1% - 40% based on the volume fraction of the P(AAc-co-AAm) solution.

[0019] Preferably, the addition amount of the magnetic particles is 7.5% - 30% based on the volume fraction of the P(AAc-co-AAm) solution. A more preferable addition range is 10% - 30%, which can balance strong magnetic response and the mechanical stability of the gel network.

[0020] In step (2), the magnetic particles are magnetic particles with an oxide layer that can release Fe3+. Preferably, the magnetic particles are NdFeB.

[0021] Preferably, in step (2), the magnetic particles are NdFeB. Under acidic conditions, acidolysis occurs on the surface of NdFeB, releasing Fe3+ ions and in-situ coordinating with carboxyl or amide groups on the copolymer molecular chain to form a reversible metal coordination crosslinking network. The mixture is left to stand at room temperature or under heating (such as ≤60°C), and a stable magnetic hydrogel can gradually form.

[0022] Preferably, in step (3), the local heating method uses laser irradiation. The wavelength of the laser is 400 - 800 nm, the power density is 0.2 - 2.0 W / cm2, and the heating time is 10 - 120 seconds, which can make the local hydrogel reach the target temperature of 40 - 80°C and achieve reversible reprogramming of the local magnetic particles.

[0023] Preferably, in step (3), the metal coordination hydrogel in the target area is heated to 40 - 80°C to reduce its viscoelasticity, and an external magnetic field of 10 - 300 mT is applied simultaneously.

[0024] In step (3), when the target area is heated to 40 - 80°C, at this time, the reversible coordination bonds are partially dissociated, and the viscoelasticity of the hydrogel is significantly reduced; an external magnetic field of 10 - 300 mT is applied during heating to reorient the magnetic particles in the above area towards the external magnetic field direction; then, after removing the heat source and cooling to room temperature, the coordination bonds reform and lock the new magnetic domain orientation; and this process can be cycled multiple times to achieve local or overall magnetic domain erasure and rewriting of different areas of the same hydrogel.

[0025] Preferably, the metal coordination hydrogel in the target area is heated to 40 - 55°C; more preferably, it is 50 - 55°C.

[0026] In step (4), after the magnetic particle reprogramming is completed under local heating and external magnetic field, it is naturally cooled to room temperature or a low-temperature environment (<30°C), and the magnetic orientation in this area is locked; if it is locally heated to 40 - 80°C again and a magnetic field is applied, the magnetic orientation in this area can be erased or rewritten again.

[0027] The present invention also provides a locally orientation-reprogrammable magnetic hydrogel obtained by the above preparation method.

[0028] The magnetic hydrogel provided by the present invention is a water-saturated flexible network structure with nano- or micron-sized NdFeB particles dispersed therein, and can achieve repeated erasing and rewriting of magnetic orientation under the combined action of external laser and weak magnetic field; the magnetic particles inside the magnetic hydrogel provided by the present invention have a reversible and repeatable reprogrammable orientation distribution in each region, and can generate rapid deformation or movement under the action of an external magnetic field; the locally orientation-reprogrammable magnetic hydrogel prepared by the present invention can achieve rewriting and erasing of magnetic domains in any region inside the same hydrogel through multiple "heating - orientation - cooling" cycles.

[0029] Preferably, the method for erasing or rewriting the magnetic orientation of this region again is: for the reprogrammable orientation metal coordination hydrogel, by locally heating to 40 - 80 °C and applying a magnetic field, the magnetic orientation of the target region can be erased or rewritten again.

[0030] The present invention also provides an application of the above magnetic hydrogel in a soft actuator.

[0031] The present invention also provides an application of the above magnetic hydrogel in soft robots, flexible electronics, or wearable medical devices.

[0032] The present invention utilizes the reversible and reprogrammable magnetic response characteristics of the above reprogrammable orientation metal coordination hydrogel (magnetic hydrogel) (utilizing its locally reprogrammable magnetic orientation and the characteristics of being able to perform various reversible deformations or movements under the action of an external magnetic field) to achieve: various controllable deformation or movement functions, such as crawling, peristalsis, tumbling, target grasping, directional transportation, and other complex movement patterns; or used as a remotely magnetically controllable flexible catheter, gripper, medical operation tool, etc.; can also be used to achieve rewritable magnetic pattern display and information encryption.

[0033] Furthermore, the above reprogrammable orientation metal coordination hydrogel is combined with other polymer elastomers (including but not limited to polydimethylsiloxane PDMS, polyurethane, or silica gel, etc.) to form a firmly bonded composite material for soft robots, wearable medical devices, or other devices that require remotely magnetically controlled driving.

[0034] Compared with the prior art, the beneficial effects of the present invention are specifically reflected in:

[0035] 1. The "one-pot method" is adopted to directly mix magnetic particles with the hydrogel precursor solution, and the in-situ coordination cross-linking is realized by the acid-catalyzed release of Fe3+, without the need to add organic solvents or complex equipment, which is easy for large-scale preparation, and has the advantages of simple preparation process, green preparation process, good reversibility, and broad application prospects.

[0036] 2. The viscoelasticity of the hydrogel decreases significantly in the range of 40 - 80 °C, and the cooperation with an external magnetic field of 10 - 300 mT can make the magnetic particles undergo orientation rearrangement, without the need for high-temperature annealing or strong magnetic field.

[0037] 3. Metal coordination bonds can be repeatedly broken and reorganized, allowing for multi-region and multiple changes in magnetic orientation on the same hydrogel, providing diverse motion patterns for soft robots and the like.

[0038] 4. The prepared magnetic hydrogel simultaneously exhibits excellent mechanical properties, tunable viscoelastic properties, and rewritable magnetic domain orientation function. The hydrogel formed at room temperature has high toughness and adhesion, and can be firmly combined with other elastomers or rigid substrates, expanding its application to scenarios such as medical catheters. Description of the Drawings

[0039] Figure 1 It is the preparation flow chart of the magnetic hydrogel in the embodiment and the characterization diagram at the initial stage of preparation.

[0040] Figure 2 It is the magnetic properties and microstructure diagrams of the magnetic hydrogels prepared in Examples 1-4 and the hydrogel prepared in Comparative Example 1.

[0041] Figure 3 It is the viscoelasticity regulation and magnetic particle rearrangement test diagram obtained from the local heating-magnetic field reprogramming experiment in Application Example 1.

[0042] Figure 4 It is the schematic and performance diagrams for realizing various programmable deformations and motion patterns in Application Example 2.

[0043] Figure 5 It is the application diagram of the magnetic hydrogel on the soft robot after bonding with the elastomer in Application Example 3. Detailed Embodiments

[0044] The following further describes the specific implementation process of the "metal coordination hydrogel with locally tunable viscoelasticity for reprogrammable orientation of magnetic particles" proposed in the present invention and its deformation / motion and composite applications in conjunction with the drawings. It should be noted that the examples listed below are only typical ways to illustrate the technical solutions of the present invention and do not constitute a limitation to the protection scope of the present invention. Equivalent substitutions or changes in the types and amounts of raw materials, temperature, and operating conditions, etc., without departing from the spirit and ideas of the present invention, should be regarded as falling within the protection scope of the present invention.

[0045] Example 1

[0046] In this example, acrylic acid (AAc) and acrylamide (AAm) are used as functional monomers, and a high-viscosity copolymer solution is obtained through free radical polymerization, laying a foundation for subsequent metal coordination and magnetic particle dispersion, as shown in Figure 1 .

[0047] (1) Weigh 0.14 M acrylic acid and 1.26 M acrylamide separately, and dissolve them in an appropriate amount of deionized water. The total monomer concentration in the resulting solution is about 10.5 wt% (which can be adjusted as needed), and the total volume of the solution is 20 mL. Add 0.1 wt% - 0.5 wt‰ potassium persulfate (KPS) as an initiator to the solution and stir to dissolve it. Pass high-purity argon gas for 20 - 30 minutes to remove dissolved oxygen, and then transfer the solution to a low temperature (such as 4 °C) or room temperature (25 °C) and let it stand for 24 - 48 hours to initiate free radical polymerization. When the viscosity of the solution increases significantly and becomes transparent or semi-transparent, a high-viscosity P(AAc-co-AAm) copolymer solution, denoted as "precursor solution", is obtained. This precursor solution is a viscous solution of the polymer with a high viscosity, which is beneficial to uniformly disperse neodymium iron boron (NdFeB) magnetic particles in the subsequent steps and prevent sedimentation.

[0048] (2) Weigh NdFeB magnetic particles with a volume fraction of 30 vol% (particle size 5 μm) and add them to the prepared high-viscosity P(AAc-co-AAm) solution.

[0049] (3) Transfer the uniformly mixed solution to a mold with a thickness of 1 mm, and let it stand at room temperature (25 °C) or heat (50 °C) for 0.5 - 1 hour; through the acidolysis of the NdFeB surface and the release of Fe3+, metal coordination cross-linking is formed with the carboxyl groups on the copolymer chain, thus completing the sol-gel transition to obtain a magnetic hydrogel. After demolding, a magnetic hydrogel sample can be obtained.

[0050] (4) Place the magnetic hydrogel sample containing 30 vol% NdFeB at room temperature (25 °C) and record its initial magnetization direction or deformation; irradiate the target area with a 520 nm laser (power density about 0.62 W / cm2) for about 30 seconds to raise the temperature of the target area to 50 - 55 °C; at this time, part of the metal coordination bonds dissociate, and the pinning force of the gel on the magnetic particles decreases; apply an external magnetic field of about 80 mT while irradiating to reorient the magnetic particles in this area; after stopping the irradiation, keep the magnetic field for dozens of seconds and wait for natural cooling to "lock" the new magnetic domain orientation.

[0051] Example 2

[0052] The difference from Example 1 is that in this example, NdFeB magnetic particles with a volume fraction of 22.5 vol% are weighed.

[0053] Example 3

[0054] The difference from Example 1 is that in this example, NdFeB magnetic particles with a volume fraction of 15 vol% are weighed.

[0055] Example 4

[0056] The difference from Example 1 is that in this example, NdFeB magnetic particles with a volume fraction of 7.5 vol% are weighed.

[0057] Comparative Example 1

[0058] The difference from Example 1 is that in this example, no NdFeB magnetic particles are added.

[0059] Testing and Characterization

[0060] In Examples 1-4 and Comparative Example 1, a series of hydrogels with different mechanical properties and magnetic response characteristics were prepared by changing the addition amount of NdFeB magnetic microparticles (0 vol% - 30 vol%). The tensile stress-strain properties of the hydrogels prepared in Examples 1-4 and Comparative Example 1 refer to Figure 1 , where: Figure 1 a) in Figure 1 is a schematic diagram of a high-viscosity copolymer solution obtained after free radical polymerization of acrylic acid (AAc) and acrylamide (AAm); and a schematic diagram of the network structure of a supramolecular hydrogel formed after adding NdFeB magnetic microparticles to the solution and standing for several minutes; Figure 1 b) in

[0061] Figure 1 is the corresponding pH-time change curve, showing that Fe3+ released by surface acidolysis gradually initiates the metal coordination crosslinking process;

[0062] c) in Figure 2 is the stress-strain curve of hydrogels with different NdFeB contents (such as 7.5 vol%, 15 vol%, 22.5 vol%, 30 vol%) under tensile state. Figure 2 It shows that as the NdFeB content increases, both the elastic modulus and the fracture strength of the gel increase significantly. For example, at 30 vol%, the fracture stress can reach about 850 kPa. Figure 2 a) in Figure 2 is a schematic diagram of the hysteresis loop measured at different ratios of NdFeB content; s ) r ), the remanent magnetization (B c ) and the coercivity (H Figure 2Figures c) and d) are SEM comparison diagrams of the distribution of magnetic particles inside the gel before and after magnetization, showing the differences between random distribution and chain arrangement.

[0063] Rewriting of magnetic domains in Application Example 1

[0064] In Examples 1 - 4, at room temperature, the viscoelasticity of the gel was reduced by local laser heating, and the orientation of NdFeB particles was selectively reprogrammed under a weak magnetic field (80 mT).

[0065] Figure 3 It is a test diagram of viscoelasticity regulation and magnetic particle rearrangement obtained from the local heating - magnetic field reprogramming experiment in Example 1, where: Figure 3 a) in it is a graph showing the change of the complex viscosity (η*) of the hydrogel measured at 25 °C and 50 °C respectively with frequency; Figure 3 b) in it is a trend diagram showing the coercivity (H c ) decreasing with the increase of temperature, indicating that a weak magnetic field can orient the particles; Figure 3 c) in it is a comparison diagram of the local magnetic orientation change in the laser - irradiated area and the "locked" state after re - cooling.

[0066] From Figure 3 it can be seen that the gel has a low coercivity (about 53 mT) at 55 °C, so 80 mT is sufficient to rearrange the particles; when it returns to 25 °C, the new magnetic orientation can be stably maintained, and its remanence can be maintained above 80%. As Figure 3 shown in a) - c) in it, the increase in temperature causes a significant decrease in the complex viscosity and the coercivity to drop from 600 mT to about 53 mT; after reprogramming, the hydrogel has good magnetic domain retention at room temperature, and the deformation and magnetic response remain stable after repeating the "heating - orientation - cooling" cycle many times.

[0067] Application Example 2

[0068] Based on the aforementioned rewritability of magnetic domains, this application example demonstrates how to achieve multiple deformation modes (such as U - shaped, S - shaped, spiral - shaped, etc.) and different crawling / rolling motions of the same hydrogel under the same external magnetic field.

[0069] Figure 4 It is a schematic and performance diagram of a magnetic hydrogel prepared in Example 1 to achieve multiple programmable deformations and motion modes, where: Figure 4 a) in it is a comparison photo of a flat - sheet hydrogel showing U - shaped, S - shaped, wavy, and spiral deformations respectively after reprogramming different magnetic domains in turn under the same applied magnetic field (about 80 mT); Figure 4 b) in it is a schematic diagram of the motion trajectories of the hydrogel realizing "earthworm - like" crawling and "coccidia - like" rolling when a rotating magnetic field (2 Hz, 80 mT) is applied;

[0070] From Figure 4 It can be seen that by performing "local heating + magnetic field orientation" on multiple regions of the hydrogel sheet (about 2 mm thick), magnetic domain distributions in several different directions are achieved; when a magnetic field of about 80 mT is applied, the particles in each region couple with the magnetic field, and the whole generates different bends such as U-shaped, S-shaped, wavy, and spiral shapes; after removing the magnetic field, the hydrogel flat sheet can automatically return to its original state. Under the condition of a rotating magnetic field (2 Hz, 80 mT), if staggered magnetic orientations are written to different segments of the hydrogel, a movement mode similar to that of "caterpillars" or "coccidia" will be exhibited. These movements have adjustable directionality and instantaneity, and the maximum movement speed can reach 14.3 mm / s, exceeding the response rate of general hydrogels.

[0071] Application Example 3

[0072] In this application example, the prepared magnetic hydrogel is adhesively bonded to a common elastomer (such as PDMS) at the interface to demonstrate its potential applications in the fields of soft robots, minimally invasive medicine, etc.

[0073] The NdFeB particle-P(AAc-co-AAm) mixture prepared in Example 1 (still in the process of crosslinking) was uniformly coated on the surface of PDMS; after standing for about 1 hour, the hydrogel completed coordination crosslinking and formed a firm interfacial bond with the PDMS surface; the results of the 180° peel strength test showed that the adhesion energy between the magnetic hydrogel and elastomers such as PDMS and Ecoflex could reach 300 - 400 J / m2, or even higher, indicating that it has good adhesion performance on a variety of polymer substrates, as Figure 5 shown in a)-c) of Figure 5 where a) in Figure 5 is a schematic diagram of the adhesion process between the magnetic hydrogel and the elastomer; Figure 5 b) in Figure 5 is the test result of the peel force after the freshly prepared hydrogel is coated on the surface of the PDMS film to form a firm composite;

[0074] According to Figure 5 d) in

[0075] In summary, the magnetic hydrogels prepared by the present invention can rapidly generate various programmable deformations or movement modes (such as bending, wave deformation, rolling or crawling, etc.) under an external magnetic field, and can be firmly bonded to elastomers to form composite devices, which are widely applicable to soft robots, wearable medical devices, information encryption / display, and other occasions requiring remote or dynamic control.

Claims

1. A method for local regulation of magnetic particle orientation in a magnetic hydrogel, characterized in that: The method comprises the following steps: (1) free radical polymerization of acrylic acid AAc and acrylamide AAm in an aqueous solution system to obtain an acrylic acid-acrylamide copolymer P (AAc-co-AAm) solution; (2) After adding Fe-containing magnetic particles to the P(AAc-co-AAm) solution and mixing, the mixture gradually transforms at room temperature or under heating conditions to obtain a stable metal coordination hydrogel; (3) Local heating or illumination of the target area of ​​the metal coordination hydrogel and simultaneous application of an external magnetic field cause the target area to form a new magnetic orientation; (4) The target area is cooled to room temperature while maintaining an external magnetic field, so that the magnetic particles in the target area are fixed in a new magnetic orientation, thereby forming a magnetic hydrogel with locally reprogrammable orientation.

2. The method for local regulation of magnetic particle orientation in magnetic hydrogel according to claim 1, characterized in that: In step (1), the molar ratio of acrylic acid AAc to acrylamide AAm in the P(AAc-co-AAm) solution is 1:1 to 1:20, and the total monomer concentration in the P(AAc-co-AAm) solution is 1 wt% to 20 wt%.

3. The method for local regulation of magnetic particle orientation in magnetic hydrogel according to claim 2, characterized in that: In step (1), the amount of the magnetic particles added is 0.1% to 40% by volume of the P(AAc-co-AAm) solution.

4. The method for local regulation of magnetic particle orientation in magnetic hydrogel according to claim 1, characterized in that: In step (2), the magnetic particles are NdFeB.

5. The method for local regulation of magnetic particle orientation in magnetic hydrogel according to claim 1, characterized in that: In step (3), local heating is performed by laser irradiation, wherein the wavelength of the laser is 400 to 800 nm, the power density is 0.2 to 2.0 W / cm2, and the heating time is 10 to 120 seconds.

6. The method for local regulation of magnetic particle orientation in magnetic hydrogel according to claim 1, characterized in that: In step (3), the metal coordination hydrogel in the target area is heated to 40-80° C., and an external magnetic field of 10-300 mT is applied simultaneously.

7. A locally oriented reprogrammable magnetic hydrogel obtained by the method according to any one of claims 1 to 6.

8. The magnetic hydrogel according to claim 7, characterized in that: By locally raising the temperature to 40-80°C and applying a magnetic field, the magnetic orientation of the target area can be erased or rewritten again.

9. Use of the magnetic hydrogel according to claim 7 in a soft actuator.

10. The use according to claim 9, characterized in that: The magnetic hydrogel is used in soft robots, flexible electronics or wearable medical devices.

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