Engineering Magnetic Soft and Reconfigurable Robots

Abstract

Magnetic control has gained popularity recently due to its ability to enhance soft robots with reconfigurability and untethered maneuverability, among other capabilities. Several advancements in the fabrication and application of reconfigurable magnetic soft robots have been reported. This review summarizes novel fabrication techniques for designing magnetic soft robots, including chemical and physical methods. Mechanisms of reconfigurability and deformation properties are discussed in detail. The maneuverability of magnetic soft robots is then briefly discussed. Finally, the present challenges and possible future work in designing reconfigurable magnetic soft robots for biomedical applications are identified.

Introduction

Soft robots provide remarkable environmental compliance, multiple degrees of freedom, rapid and cost-effective fabrication, and multiple actuation methods. Magnetic actuation has received extensive attention for its ability to provide long-range and untethered manipulation. Magnetic soft robots normally consist of carriers that contain magnetic parts. The carriers are fabricated from compliant materials that usually exhibit good shape deformation properties. Magnetic parts, like magnetic particles and permanent magnets, form part of the actuation and functional factors.

Magnetic soft robots range from the nanometer to centimeter scale. Before 2018, researchers normally combined conventional soft robots or soft matters with magnetic parts through modular assembly or physically combining methods which restricted the scale in millimeters or larger.^1–4 With progressions in material science, smaller scale soft materials have seen application in soft robots and have drawn significant interest from the biomedical community.^5–12 Untethered magnetic control combined with micro- to nano-scale sized robots has been shown to have the potential to be used for drug delivery and targeted therapy in vivo.^13,14

More recently, the reconfigurability (deformation recovery under the interaction of magnetic fields, magnetic parts, and material structures of compliant carriers) of magnetic soft robots has been widely studied. Liquid-like and solid-like reconfigurabilities have been reported in the following areas: Liquid-like: Magnetic manipulated split and merge of liquid metal¹⁵ ferrofluidic based magnetic soft robots¹⁶; or array transformation of swarming magnetic particles^17,18 Solid-like: Break and self-heal or elongate and recovery of polymer based magnetic soft robots.^19,20 The ability to reconfigure has been shown to overcome the locomotion challenges associated with narrow channel environments,^21,22 which is promising for interventional applications. Reconfigurability has also been shown to be beneficial in other complex engineering tasks like mobile charging²³ and circuit repair.²⁴

However, an environmental change, like change in temperature or magnetic field, during fabrication and/or manipulation can affect the reconfigurability properties.^25–27 Hence, in-depth research on how the fabrication method and reconfigurable mechanism affect the properties and maneuverability of magnetic soft robots is required to guide the design of robots for engineering applications.

In the last 5 years, numerous review articles summarized advanced reconfigurable magnetic soft robot fabrication techniques, such as additive manufacturing, direct ink writing, and deposition.^10,28,29 Specific carriers of reconfigurable magnetic robots were also reviewed, including elastomers, liquid metals, gels, and swarming particles.^9,11,30–33 Most of the magnetic responsive sources in these robots were magnetic nanoparticles that could be embedded into the carriers compatibly. Recent works mainly focused on combining magnetic parts with multifunctional carriers possessing excellent reconfigurability characteristics like shape memory, self-assembly, and multifunctional deformation.^34–37

This review discusses recent advancements in facile fabrication strategies for reconfigurable magnetic soft robots and specific reconfigurable mechanisms for representative carriers. Analysis of the reconfigurable properties and maneuverability of magnetic soft robots aims to explore the influence of fabrication methods and reconfigurable mechanisms on magnetic manipulation. The overview of this review is shown in Figure 1.

FIG. 1.

Overview of this review.

Fabrication Methods

The conventional magnetic robot is made of a permanent magnet or magnetic material with a fixed shape. The magnetic part is the driving factor to the robot's locomotion, while the carrier provides flexible and reconfigurable structures. With the rapid development of materials science, material modification and preparation methods for carriers have become more sophisticated. Researchers have applied physical and chemical preparation techniques with structural design to create reconfigurable magnetic soft robot carriers. This section discusses carrier fabrication methods and summarizes them in Table 1. Chemical techniques are covered first which is followed by physical techniques.

Table 1.

Overview of Magnetic Soft Robot Fabrication Session

References	Year	Material	Methods	Shape	Size	Reconfigurability	Promising application
Sun et al.³⁸	2022	PVA, NdFeB Borax	Crosslink reaction SiO₂ coating	Non-Newtonian fluid slime	Sphere diameters: 1.5–60 mm	Elongate Curl Shrink Swallow Self-heal	Biomedical and wearable devices
Jin et al.³⁹	2019	Fe₃O₄ PDA Au seeds	In situ polymerization	Microswarm	Diameters: 660–675 nm	Ant bridges for electrical connection	Electronic field and bioinspired microrobots
Li et al.⁴⁰	2021	V-MXene Ti-MXene Nanosheets NdFeB Ecoflex DCM	Hydrofluoric acid etching Thermal shrinkage Deposition	Electrode film	Thickness: 170 μm	Stretch	Self-powered soft machines
Zhang et al.⁴¹	2021	NdFeB PI2555, SE1211, ST3866, RM257, DR1A	Spin coated Rubbed Melted UV curing	Liquid crystal film	Thickness: 25–40 μm	In situ deformation	Intelligent wireless soft machines, robots, and devices.
Du et al.⁴²	2020	NdFeB PNIPAM hydrogel	UV-induced polymerization for twice	Leptocephali hydrogel film	Thickness: 174 μm	Swing Roll Crawl Squeeze Helical propulsion	Biomedical and environmental smart robot
Alapan et al.⁴³	2020	CrO₂ PDMS Cross link agent	Mixing Laser heating	Programmable polymer film	Thickness: 25–200 μm	2D-3D shape transformations	Medical robots, wearable health monitoring devices, and bioinspired microrobots
Zhao et al.⁴⁴	2022	Gallium NdFeB PDMS	Mixing Moulding Magnetization	Programmable solid like liquid-metal film	Thickness: 1 mm	Shape fixable Shape deformation	Intelligent soft robots with 3D structures

2D, two-dimensional; 3D, three-dimensional; PDA, polydopamine.

Chemical techniques

A crosslink reaction was utilized to fabricate a sticky non-Newtonian fluid-based magnetic slime that shows no dependence on the substrate environment by Sun et al.³⁸ The crosslink reaction between functional groups forms a polymer structure (Fig. 2a), endowing the macroscopic slime with a porous structure and the ability to coat magnetic particles. To ensure the biocompatibility of any soft robots using this slime as the carrier, SiO₂ is used to cover the toxic magnetic particles in the slime. Besides, the intermolecular hydrogen bonds formed by the crosslink reaction allow the slime to detach and self-heal, enhancing the slime's environmental compatibility. The magnetic slime's excellent conductivity and flexibility make it suitable for biological switches and medical wearables.

FIG. 2.

Fabrication methods for the carriers of magnetic soft robots. (a) Schematic of crosslink reaction for fabrication of the magnetic slime and photograph of the magnetic slime.³⁸ (b) Schematic of in situ coating progress and photograph of the microswarm.³⁹ (c) Schematic of the fabrication process of stretchable Zn-ion hybrid battery magnetic soft robot.⁴⁰ (d) Schematic of magnetic LCE with circular director field and photograph of the LCE film sample.⁴¹ (e) Schematic of twice UV-induced polymerization progress and locomotion of the leptocephali magnetic robot.⁴² (f) Schematic of the laser heating mechanism of the magnetic soft elastomer and the reprogramming process.⁴³ (g) Schematic of maneuverable stiffness liquid metal-based robot fabrication.⁴⁴ LCE, liquid crystal elastomer.

An in situ polymerization method to fabricate fluidic magnetic nanoparticles that can be applied to electrical connections was also presented by Jin et al.³⁹ Gold seeds coat the prepolymerization magnetic particles through electrostatic immobilization and electroless plating due to the significant electrical potential difference between gold and polydopamine (PDA) in an acidic environment. The cooperation of electrostatic attraction and reductive agent protection contributes to the continuous feature of the gold surface (Fig. 2b), which ensures the conductivity and flexibility of the microswarm. In this study, the microswarm hinges on the printed circuit substrate to achieve its electric connection capability. Whether the connection can be maintained in extreme environments remains further explored.

An electrode fabrication was applied to magnetic robot conception resulting in a magnetically controllable power bank robot by Li et al.⁴⁰ The electrode materials are integrated on hydrofluoric acid etched nanosheets with excellent electrical conductivity and rich surface redox chemistry, providing excellent battery reaction conditions for electronic transfer. This design saves space and achieves the miniaturization of the power bank. Through thermal shrinkage the electrode is pre-deformed. Oxygen plasma induced hydrogen bonds strengthen the adhesion on thermal sensitive, shrinkable, substrate. Liquid elastomer is deposited on the shrunk electrode and cured for elasticity, which ensure the stretchable deformation of the electrode (Fig. 2c). Compared to assistive robots, this concept of self-functional magnetic robots combining locomotion and operation will significantly broaden the design ideas for magnetic robots and enhance the application prospects of magnetic robots.

Physical techniques

Moreover, a magnet-embedded liquid crystal elastomer (LCE) membrane was designed by Zhang et al.⁴¹ It combines environmental adaptability from the membrane's mechanical properties and untethered controllability from the magnetic properties of the magnets. Interestingly, a specific circular friction orientation is applied in the liquid crystal fabrication to prove the composite properties of the LCE composite membrane, resulting in a circular arrangement of the liquid crystal molecules that allow the magnets to distribute homogeneously (Fig. 2d). Impressive LCE structure integrity after inserting magnets is demonstrated. A splay molecular director can be formed by slashing laser cutting the sample with a uniform molecular director. Induced helices deformation by enhancing temperature gives materials preprogrammed reconfigurable properties. Materials prepared by this method have excellent application potential in remote magnetic control and soft robotics fields. However, the material's thickness is limited due to the preparation technology, and the performance of its force output and three-dimensional (3D) reconfiguration needs to be further studied.

Unlike simple physical mixing fabrication methods, the leptocephali asymmetric structural design alleviated the conjugation between the driving and reconfigurable parts (Fig. 2e), which significantly utilizes the advantages of composite parts. With this, a magnetic hydrogel-based millirobot fabricated by two simple photo-initiated steps was introduced by Du et al.⁴² According to the soft and transparent nature of the hydrogel carrier, the adjustment of the material composition of the embedded particles imparts bonus properties to magnetic soft robots, like masking and temperature-sensitive color change, which increases the application space of magnetic soft robots.

With the assistance of laser heating, magnetic elastomers can be selectively activated in specific areas due to the unique properties of ferromagnetic materials. A laser heating method to create a paramagnetic magnetic modifying condition for ferromagnetic elastomers was then introduced by Alapan et al.⁴³ Magnetic domains can be eliminated or formed by controlling the temperature to be greater or less than the Curie temperature so that the magnetic field can controllably program the denaturation ability of the material for precise programming (Fig. 2f). This method facilitates the fabrication of magnetic elastomers with complex structures and shapes, and it can be used to create asymmetric structures to realize the desired motion and deformation of the robot. Besides, these programming processes are reversible to enhance fabrication sustainability.

In contrast, a liquid metal-based magnetic robot with maneuverable stiffness was presented by Zhao et al.⁴⁴ The magnetic part of the robot is covered with an elastomer shell (Fig. 2g) that enables the liquid metal base to be durable with multiple geometry structures rather than only droplets for an extended application environment. Phase transition of the liquid metal enhances the robot stiffness controllability in the compressive strength test. Noticeably, Young's modulus of the elastomer in this study is four to five orders of magnitude smaller than the magnetic material it encapsulates, which causes low tensile strength, and also indicates that the balance of stretchability and environmental adaptivity is a future challenge.

Carrier modification techniques contribute to the reconfigurability of magnetic soft robot significantly. Combining magnetic particles with carriers shows various composite compatibilities. How to ensure that the compliance and other properties of the magnetic particles and the carrier will be unaffected when combined during the fabrication of magnetic soft robots remains to be deeply studied.

Reconfigurable Mechanisms

The reconfigurable mechanisms of magnetic soft robots vary due to the diversity of carriers and the properties of different magnetic particles. It has flexibility and diversity in deformation. These mechanisms, mainly liquid-like and solid-like reconfigurability mechanisms, are affected by ambient conditions such as temperature, pH, and magnetic field strength. Due to the particularity of the preparation method, the operating environment of some reconfigurable magnetic soft robots will also be limited. Representative reconfigurable mechanisms are introduced in this section and summarized in Table 2.

Table 2.

Overview of Magnetic Soft Robot Reconfigurable Mechanism Session

References	Year	Material	Shape	Mechanism of reconfigurability	Environment restriction	Reconfigurability	Features
Chen et al.⁴⁵	2021	Calcium alginate Gelatine Fe₃O₄	Tilting doll droplet	Gravity, pH sensitive	Withstand stomach acid, release in intestinal fluid	Release and sustain	PH-sensitive triple layer structure
Chen et al.⁴⁶	2021	Fe₃O₄ PDA PCL TPU	Elastomer film	Thermoplastic properties	N/A	Light and magnetic responsive deformation Self-healing	Light responsive self-heal ability
Zhang et al.⁴⁷	2022	Iron Galinstan	Droplet	Magnetic force and surface tension interaction, size effect	Superhydrophobic substrate	Passive deformability Split and merge	Phase transition-based shape locking/unlocking
Kuang et al.⁴⁸	2021	NdFeB Furan Maleimide	Programmable elastomer film	Thermally reversible chemical reaction Reprogrammable magnetization	N/A	3D structure assembly Modular Assembly	Covalent adaptive network inside the material
Song et al.⁴⁹	2020	NdFeB Oligomeric PEG, Silicone	Programmable elastomer film	Solid-to-liquid phase transition	N/A	Reprogrammable transformation	Fast reprogrammable shaping
Deng et al.⁵⁰	2020	NdFeB PCL Silicone	Programmable elastomer film	Thermoplastic properties Reprogrammable magnetization	N/A	3D structure assembly	Encoded magnetic anisotropies for 2D-3D deformation
Fan et al.⁵¹	2020	Ferrofluids EMG 901 MFR-DP1	Droplet	External magnetic potential energy distribution	Water-based fluidic environment	Split Merge Deformation	Can be controlled by programming spatiotemporal magnetic fields

PCL, polycaprolactone.

Aqueous affected mechanisms

A pH-sensitive biocompatible tilting doll-shaped magnetic robot for drug delivery was designed by Chen et al. (Fig. 3a).⁴⁵ This capsule robot can pass through the stomach acid to the small intestine to release anticancer drugs. The acid-resistant calcium alginate (CaA) hydrogel shell prevents the drug from being released before the capsule arrives in the intestine, and the gelatin layer ensures that the drug does not get scattered and left in the CaA shell during the release. The gelatin-covered drug can spread evenly in the target site due to the proposed electrodeposition method for tilting doll shape, utilizing gravity to promote the separation of CaA shell and gelatin and increase the accuracy of the release progress. The gravity-based shape mechanism was not further discussed in empirical therapy but showed the flexibility of the magnetic robotic in vivo drug delivery.

FIG. 3.

Reconfigurable mechanisms of magnetic soft robots. (a) Image of the tilting doll-like TCMR and the schematics of releasing progress.⁴⁵ (b) Photographs of reconfiguration progression under light and magnetic control and the mechanism of the reconfigurability.⁴⁶ (c) Images reflecting shape-lock process of liquid metal droplet robot and the schematic of the temperature affected reconfigurable mechanism.⁴⁷ (d) Image of the modular assembly under magnetic attraction and temperature control mechanism.⁴⁸ (e) Images of two ferromagnetic states transformation.⁴⁹ (f) Schematics and images of reprogrammable magnetization.⁵⁰ (g) Schematics of the reconfigurability affected by external magnetic field.⁵¹ TCMR, triple-configurational magnetic robot.

Thermal affected mechanisms

Thermoplastic polymer polycaprolactone (PCL) can be embedded in the elastomer carrier holding the magnetic particles. A polymer microcomposite sensitive to light and magnetic fields and schematizing the configurable mechanism through thermoplastic properties of the elastomer carrier was developed by Chen et al.⁴⁶ The PCL is light sensitive because its melting point is only around 60℃, and exposure to light can raise the temperature of the PCL past the melting point. The elastomer can be manipulated by light and magnetic fields, which can be reconfigured and fixed by removing the light and magnetic fields (Fig. 3b). The reversibility and controllability of the thermoplastic allow the elastomer to repair cracks through exposure to light automatically. This self-healing mechanism is different from the self-healing of magnetic slime,³⁸ as the thermoplastic intercalation polymer relies on light stimulation for healing. However, its efficient self-healing still gives it excellent application prospects in soft robots and intelligent wearables.

The magnetic liquid metal robot was applied to control the droplet. The reconfiguration mechanisms are illustrated by Zhang et al. with the intriguing phase transition of low melt point metal, which can control the shape duration of the magnetic robot.⁴⁷ A further study on the size effect shows that the spilt capability of the robot is dominated by surface tension in small sizes and by magnetic force in large sizes (Fig. 3c). The phase transition mechanism shows high integration of magnetic particles and liquid metal carriers. However, the hydrophilic robot requires a hydrophobic environment, which is not fit for the moist human body environment. However, the highly integrated and reconfigurable magnetic robot has the potential to be applied in drug delivery and micro biopsy.

Since the temperature affects the direction of the thermochemical reaction, the Diels–Alder reaction is applied to control the structure of the magnetic particle-encapsulated network to manipulate the freedom of the internal magnetic particles. A facile method of fabricating a magnetic material with a polymer network was developed by Kuang et al. using the thermally reversible chemical reaction mechanism.⁴⁸ Because of the paramagnetic properties of the ferromagnetic particles, the magnetization enables the particles' magnetic poles to be reversible, making the composite modular assembled and integrated by temperature-responsive carrier network structure transformation (Fig. 3d).

Furthermore, a shape memory alloy-like reprogrammable magnetic soft robot that applies oligomers to encapsulate magnetic particles was presented by Song et al.⁴⁹ The solid–liquid phase transition of the encapsulation material results in two ferromagnetic states of the magnetic particles, causing the robot to exhibit different magnetic responsiveness. The liquid oligomer phase makes the magnetic particles reprogrammable in magnetic fields when heated over the critical temperature (Fig. 3e). The solid state can stabilize particles' ferromagnetic state beneath the critical temperature, which causes memory alloy-like responsiveness to the applied magnetic field. This work shows that the confinement of magnetic particles can improve the shaping efficiency of robots, and the deformation between 2D and 3D structures can be widely used in fields where robot flexibility is required.

Magnetic affected mechanisms

A magnetic responsive soft material that combines PCL and ferrimagnetic particles to achieve a 3D configuration was also developed by Deng et al.⁵⁰ Compared to Chen's work,⁴⁶ this study alternates the ferrimagnetic particles so that the robot will be reprogrammed by the magnetic field when the PCL enters the liquid phase (Fig. 3f). The magnetization process creates magnetic anisotropy in the composite film, which allows for conversion between 2D and 3D configurations by controlling the external magnetic field.

Besides, a programmable method for a ferrofluid-based magnetic droplet robot capable of in situ reconfigurations was also introduced by Fan et al.⁵¹ They pointed out that magnetic potential energy distribution plays the leading role in magnetic robot configurability, and an external magnetic field can manipulate this distribution. Simulation results reveal the energy potential field reconfiguration mechanism. The magnetic forces concentration results in the lowest magnetic potential energy area having in situ reconfigurability, so that the splitting and merging occur (Fig. 3g). Various deformations can be achieved by controlling the magnitude and direction of the external magnetic field.

Reconfigurable Properties

The fabrication method of the reconfigurable magnetic soft robot will directly affect its macro shape and structural composition, affecting its reconfigurable properties, such as response time, deformation speed, or degree. Representative reconfigurable properties are introduced in this section and summarized in Table 3.

Table 3.

Overview of Magnetic Soft Robot Reconfigurable Property Session

References	Year	Material	Shape	Size	Reconfigurability	Promising application	Features
Joyee and Pan⁵²	2020	EMG 1200 Nonaqueous photocurable resin Spot E elastic	Inchworm elastomer	Length: 40 mm Width: 5 mm	Bending deformation	Medical robot	Bidirectional locomotion
Hilby et al.⁵³	2022	N52 NdFeB disc magnets LDPE	Yoshimura origami structure	Diameter: 6 mm	Elongation Bending Buckling	Space exploration, soft robot endoscope	800% elongation and enhanced deformation
Cao et al.⁵⁴	2020	NdFeB Galinstan	Liquid-metal film Blocky liquid-metal	Thickness: From microns to millimeters	Deformation, Assembly	Flexible electronics and soft robotics	Mechanical demagnetization
Liu et al.⁵⁵	2019	Carbonyl-iron DiAPLEX MM5520 IROGRANPS 455–203	Elastomer film	Thickness: ∼100 μm	Shape memory deformation	In vivo and aerospace reconfigurable soft robots	Shape memory
Li et al.⁵⁶	2021	NdFeB RM257 EDDET PETMP Dragon Skin	Elastomer film	Thickness: 60–400 μm	Reversible shape-morphing	Shape changing robots	Thermal responsive spiral reconfigurability
Ha et al.⁵⁷	2021	NdFeB DiAPLEX GMR sensors	Origami film	Thickness: 25–300 μm	Self-Guided Assembly	Soft robots	Self-guide assembly
Li et al.⁵⁸	2020	Iron Galinstan	Liquid-metal droplet	Diameter: 2.6–7.8 mm	Split and merge	Targeted drug delivery and therapy	pH and magnetic particle proportion influenced deformation

GMR, giant magnetoresistance.

Solid-like reconfigurabilities

An untethered, fully 3D printed elastomer-based magnetic soft robot for in vivo drug delivery was designed by Joyee and Pan.⁵² In the preparation process, the distribution of magnetic particles in the resin is controlled by an external magnetic field, and the magnetic particles are successfully concentrated at both ends of the robot (Fig. 4a). The structure separating the magnetic and carrier parts shows good composite properties. This structure ensures the elasticity and flexibility of the robot's torso, and the magnetic particles at both ends bend the robot up to 146° under a magnetic field.

FIG. 4.

Reconfigurable properties of magnetic soft robot. (a) Images showing the untethered, fully 3D printed magnetic soft robot's bending performance.⁵² (b) Image of the enhanced deformation performance of soft origamic robots through joint magnets.⁵³ (c) Mechanical demagnetization process.⁵⁴ (d) Images of faster shape memory and responsiveness than the initial cycle in the cycle test.⁵⁵ (e) Reconfiguring the magnetic composite and modulus enhancement with increased magnetic particle fraction.⁵⁶ (f) Schematic of reconfigurability performance corresponding to different magnetic particle concentrations.⁵⁷ (g) Images of splitting magnetic liquid metal droplets with different particle concentrations and splitting processes.⁵⁸

Furthermore, a magnet combined reconfigurable Yoshimura structure was also developed by Hilby et al.⁵³ Applying magnets as a joint in a conventional elastic structure can be used to upgrade soft robots' reconfigurability with conventional power sources (Fig. 4b). This combination demonstrates improved bending deformation capabilities compared to single pneumatic soft robots; the influence of this modification method on the external magnetic field can be further investigated.

Magnetization can cause remnant magnetization in a liquid metal-based ferromagnetic material, which can be exploited to give the material reconfigurable properties.⁵⁴ Cao's work proposed a mechanical demagnetization method through the reorder of the magnetic particles, without an external magnetic field (Fig. 4c), and this method enhances reusability and eliminates demagnetization power consumption.⁵⁴

Compared to the liquid metal-based droplet magnetic robot, the solid-like liquid metal-based ferromagnetic material has the advantages of easier controllability and less magnetic dependence to be applied in low magnetic field environments. Moreover, thermoplastic magnetic elastomers' properties are studied by Liu et al. for testing different structures and comparing them to the simulation.⁵⁵ They pointed out that these light and magnetic responsive materials show faster shape memory and responsiveness than the initial cycle in the cycle test because of the residual temperature inside the material (Fig. 4d), and practical illumination can cause uneven heating. Asymmetric magnetic particle distribution is also found in these polymers' base magnetic structures. A solvent casting chained premagnetization method in a uniform magnetic field has the asymmetric load situation in continuous linear geometry structure, while in the radial structure it cannot come into effect.

A bulking fabrication method is developed by Li et al. to thicken a film of LCEs and enhances the elasticity by embedding magnetic particles.⁵⁶ The magnetic composite shows enhanced fracture stress and high reconfigurability under a specific particle concentration range. The fourfold increase in thickness compared to conventional LCEs allows the material to be programmed into various structures and realize 2D to 3D reconfiguration (Fig. 4e). Besides, the encapsulated magnetic particles ensure dynamic maneuverability and mechanical strengthening. Ha et al. indicated that the magnetic particle concentration influences reconfiguration properties, so magnetic materials with different concentrations of magnetic particles should be used according to different application scenarios.⁵⁷ This study shows that high magnetic particle concentration causes particle agglomeration deposition in the material, which restricts reconfigurability (Fig. 4f). With the application of giant magnetoresistance sensors in this study, the origami-structured magnetic robot in this work can self-detect the magnetization state and arrange the folding sequence.

Liquid-like reconfigurabilities

Moreover, a magnetic manipulation platform for a liquid-like magnetic liquid metal robot was also designed by Li et al.⁵⁸ A galvanic cell in an acidic environment is applied to embed nano iron particles into hydrophilic liquid metal. The locomotion and deformation of the magnetic robot are studied in this work. The magnetic field-induced reconfigurability is also considered the factor affecting the mass fraction of magnetic particles. Interestingly, the separation speed of the magnetic robot is not positively correlated with the mass fraction of magnetic particles due to the specific alkaline platform environment. After the mass fraction of magnetic particles exceeds a specific value, the separation speed of the magnetic robot gradually becomes slower because the galvanic reaction consumes the metals (Fig. 4g). This phenomenon shows that this liquid metal-based magnetic robot is highly dependent on the environment. The performance of such robots needs further testing for harsh application environments.

The properties of a reconfigurable magnetic soft robot reflect the composite compatibility between the carrier and the magnetic particles during the preparation process, and they can be used as indicators for optimizing the preparation method. Experiments on reconfigurable properties will help to find the optimal concentrations of magnetic particles and tune the composite method to increase the robustness or enhance the flexibility of magnetic soft robots.

Maneuverability

The shape and size of a reconfigurable magnetic soft robot determine its motion under an external magnetic field. By controlling the size and angle of the applied magnetic field, the reconfigurability of the magnetic soft robot will be reflected, but its maneuverability will vary due to environmental factors and structure. Representative deformation and locomotion maneuverabilities are introduced in this section and summarized in Table 4.

Table 4.

Overview of Magnetic Soft Robot Maneuverability Session

References	Year	Material	Shape	Size	Magnetic steering	Moving speed	Promising application
Ju et al.⁵⁹	2021	NdFeB Silicone rubber	Inchworm film Six-arm film	Body length: 20 mm	Cylindrical permanent magnet Helmholtz coil	Locomotion maximum speed of the inchworm: 22 mm s⁻¹	Grippers, bionic robots, and wearable flexible actuators
Manamanchaiyaporn et al.⁶⁰	2020	Magnetic part Silicone rubber	Head-tail morphology elastic-soft film	Thickness: 0.1 mm Length: 10 mm	Electromagnetic actuation system	Swimming: 1.5–4.5 mm s⁻¹	Minimally invasive medicine device
Xie et al.⁶¹	2019	Hematite colloidal particles	Peanut-shaped particle	Long axis: 3 μm Short axis: 2 μm	Alternating magnetic field generated by five identical coils	Chains swarm: 6.5 μm s⁻¹	Imaging and targeted drug delivery
Wang et al.⁶²	2022	Fe₃O₄	Sphere particle	Diameter: 300 nm	Solenoid coil system	In six times diluted mouse blood: 170 μm s⁻¹	Targeted tumor therapy
Wang et al.⁶³	2022	Carbonyl iron water suspension	Droplet	Diameter: 3 μm	Three-axis Helmholtz coil	Angular velocity: 0–10 rad s⁻¹	Targeted drug delivery
Wang et al.⁶⁴	2019	Carbonyl iron water suspension	Droplet	Diameter: 3 μm	Three-axis Helmholtz coil	Angular velocity: 0–10 rad s⁻¹	Targeted drug delivery
Zhao et al.⁶⁵	2022	[Fe(CN)₆]⁴⁻ Fe₃O₄	Sphere particle	Diameter: 406.7 nm	Three-axis Helmholtz coil	Climbing degree from 15° to 60°: 309–529 μm s⁻¹	Clinical treatment of numerous ROS-related diseases

ROS, reactive oxygen species.

Deformation maneuverability

To configure elastomer-based magnetic robots, pulsed, high magnetic field focusing methods were used by Ju et al.⁵⁹ Magnetic soft robots can be shaped into a few figures by various moulds and can be transformed into different shapes under the control of magnetic fields of different directions and strengths. This method of magnetically inducing deformation reduces the complexity of robot design. A six-arm soft robot is magnetized to demonstrate the maneuverability of circle deformation (Fig. 5a). Besides, the one-step magnetization properties and the synchronous magnetic orientation also shorten the time-consuming preparation process. However, the energy consumed by frequent magnetization and demagnetization is not discussed.

FIG. 5.

Maneuverability of magnetic soft robots. (a) Schematic of a six-arm magnetic robot undertaking versatile tasks.⁵⁹ (b) Image of triangular head-tail magnetic robot locomotion under magnetic field.⁶⁰ (c) Images of different microswarm shapes under different magnetic fields.⁶¹ (d) Schematic and images of microswarm robots crossing a gorge-like obstacle.⁶² (e) Images of needle guided aggregation of droplet magnetic robots.⁶³ (f) Images showing droplet magnetic robots' cargo transportation process.⁶⁴ (g) Ultrasonic image of microswarm manipulation.⁶⁵

In addition, a magnetic soft robot with a triangular head-tail morphology was also developed by Manamanchaiyaporn et al., which can be maneuvered by changing the magnetic field.⁶⁰ Due to the triangle's asymmetry, the magnetization of the magnetic soft robot exhibits an oscillating distribution from the head to the tail during the magnetization process, which enables the robot to actuate in a wave-like deformation pattern in an oscillating field (Fig. 5b). Compared with a rectangular magnetic soft robot, the difference in magnetization enables the triangular robot to have a more significant motion tendency and higher maneuverability in actual manipulation.

Furthermore, to realize the reconfiguration of magnetic particles microswarm, a facile method of using an alternating magnetic field was also used by Xie et al.⁶¹ Under the alternating magnetic field, the magnetic particle cluster can be transformed into various swarm arrays (Fig. 5c). The programmable external magnetic field shows the potential to maneuver simple magnetic particles to undertake specific tasks under complex environments. Only single magnetic particles are tested in this study, so the maneuverability of complex magnetic composites remains to be researched.

Locomotion maneuverability

In addition, a magnetic particle swarm was used in tumor therapy method by Wang et al.⁶² In an external magnetic field with adjustable angle and frequency, the swarm successfully crosses a gorge-like obstacle with the help of the motion inertia of the vortex swarm structure (Fig. 5d). In vitro sixfold diluted blood locomotion test shows vortex swarms' impressive obstacle crossing capability and magnetic navigation capability, indicating that the magnetic swarm would exhibit robust locomotion in the vascular system. Besides, the photothermal effect of the magnetic nanoparticles can be harnessed to generate relatively intense heat to kill tumor cells.

A magnetic needle-guided magnetic droplet robot control method that enhances the precision of control was presented by Wang et al.⁶³ The nonlinear coil system has a high error in the control of droplets. However, with the magnetic needle-guided method, droplets can be precisely controlled to traverse planned paths to improve the working efficiency of magnetic droplets (Fig. 5e). In addition, the guidance method increases the responsiveness of the magnetic robot when encountering obstacles. Unlike magnetic particle clusters, the control of magnetic droplets is complicated because of the hydrodynamic interactions between droplets. As a result, a dynamic self-assembly control strategy to facilitate the maneuverability of magnetic droplets was introduced by Wang et al.⁶⁴ By controlling the magnetic field's angle and frequency, aggregated magnetic droplets can be successfully separated (Fig. 5f).

An ultrasonic imaging-guided steering method was applied by Zhao et al. to assist in manipulating the reactive oxygen species magnetic robot (ROSrobots) in osteoarthritis therapy.⁶⁵ Real-time visualization from ultrasonic imaging guides the locomotion trajectory of the ROSrobots. However, the hyperechoic structure's highlight property in ultrasonic images can blur the ROSrobots' images, which hinders the feedback for instantaneous magnetic field adjustment. Raising the concentration of ROSrobots can address the interference (Fig. 5g).

During the remote manipulation of a magnetic robot, the angle and magnitude of the applied magnetic field provide power support for the reconfigurability of the robot. Reconfigurable magnetic robots of different sizes and shapes exhibit structure-guided reconfigurable maneuverability both in vivo and in vitro.

Challenges and Future Works

Composite properties

Most reconfigurable magnetic robots reported in the past 5 years are composed of magnetic parts and carrier parts, but enhancing the performance of these composite robots is still a research challenge. The influence of the carrier coating on the magnetic induction of the magnetic particles and the limitation of motion will be reflected in the reconfigurable properties of magnetic robots. Surprisingly, some research groups have achieved the composite compatibility of the carrier and magnetic particles through fabrication methods involving material modification, where the motion properties of magnetic particles and the deformation properties of carriers can achieve instantaneously. However, some magnetic robots show performance degradation, such as in tensile strength, which will affect the reconfigurability of magnetic robots.

In addition, the type of composite carriers can also impact the composite properties. Polymer-based magnetic soft robots may range in size from the millimeter to the micrometer scale, and they can prevent leakage of magnetic particles, but the composite deformation is likely to be impacted by the magnetic particle distribution. Droplet-based magnetic soft robots highly depend on the substrate environment but exhibit impressive high-speed maneuverability. Besides, magnetic soft robot microswarms show more nonlinear reconfigurability at the nanometer scale, which will be more suitable for in vivo medical applications.

Biocompatibility

The untethered magnetic manipulation and reconfigurability of magnetic robots make them promising for biomedical applications. Since medical equipment hazards can be lethal, patient and medical staff safety is one of the most crucial factors to consider while constructing a medical device. Fabrication techniques suitable for biocompatible magnetic materials and carriers must be comprehensively researched. Developing carriers that can prevent soft robots from injuring patients and leaking magnetic particles is a current challenge. For polymer-based magnetic soft robots, the size and shape of the robot must be conducive to in vivo manipulation. Substrate-free droplet-based magnetic soft robots, like the slime robot, may be suitable for diverse working environments. To ensure the biocompatibility of magnetic soft robot microswarms, measures must be taken to mitigate toxicity and retrieve particles.

Maneuverability

The fabrication method affects the manipulation performance of magnetic reconfigurable robots, and a robot's shape, size, and operating environment can affect its maneuverability. For particular application purposes, these factors should be considered in the design of magnetic robots to eliminate or downsize adverse effects on maneuverability. Preprogrammed polymer-based magnetic soft robots show high potential in specified motivation design, but the interactions between the carriers and magnetic particles still require in-depth research. Droplet-based magnetic robots have excellent velocity and shape morphing performance, but restrictions on their operation environment limit their applicability, so modification methods for the droplet surface should be further researched. Future works on magnetic soft robot microswarms should focus on integrated locomotion and internal interaction.

Conclusion

Reconfigurable magnetic soft robots are capable of untethered manipulation and reconfigurability in locomotion and deformation. These properties could potentially lend magnetic soft robots to interventional applications in the medical field. The fabrication method and composite materials affect the reconfigurability of magnetic soft robots in many aspects.

In this review, novel material modification methods are reported with breakthroughs in fabrication techniques that allow for smaller scale and reconfigurable carriers to be made for magnetic soft robots. This decrease in scale makes magnetic soft robots more suitable for in vivo task. Polymers, liquid metals, and microswarms prove to impress reconfigurabilities in deformation, self-healing, and aggregating with the change of ambient condition. These environmentally sensitive reconfigurabilities of magnetic soft robots provide the possibility to match specific clinical case conditions. The remote locomotion maneuverability under magnetic field shows the automation assistant control potential for surgery and diagnosis.

Study on robot biocompatibility and retrieval, quantified magnetic control, robot locomotion, and deformation learning will be beneficial for reconfigurable magnetic soft robot applications in the biomedical field. The biocompatibility of the carrier in the in vivo environment needs to be thoroughly studied. Further modification of the surface and internal network of carriers will enhance the robustness of magnetic soft robots while maneuvering in and around vessels and organs. Smart self-healing reconfigurable carriers are promising for handling disintegration of the magnetic soft robots, either from physical friction or chemical/thermal dissolution.

The magnetic control for current reconfigurable magnetic soft robots is rarely concluded through mathematical methods. Theoretical models for controlling magnetic soft robots are necessary for reliable robot design and magnetic field guiding. Finite element analysis of reconfigurable magnetic robots is still relatively new. The irregular shapes and various material compositions of the carriers add to the complexity, and slow progress, of the field. However, simulation analysis is key to understanding the locomotion and deformation of reconfigurable magnetic soft robots in vivo. Combined with other advanced biomedical technologies, such as ultrasound imaging and magnetic needle control, reconfigurable magnetic soft robots can achieve major clinical breakthroughs.

Footnotes

Author Disclosure Statement

No competing financial interests exist.

Funding Information

This study was supported, in part, by the China Scholarship Council, Academy of Medical Sciences Professorship, Royal Society Wolfson Fellowship, Cancer Research UK (EDDPMA-Nov21\100026), and National Institutes of Health (NIH) Bench-to-Bedside Award. This study was also supported by the NIH Center for Interventional Oncology: Grant ZID# BC011242 & CL040015, and the Intramural Research Program of the National Institutes of Health.

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