Reversible Elastomer–Fluid Transitions for Metamorphosic Robots

Endowing robots with reversible phase transition ability, especially between elastomer and fluid states, can significantly broaden their functionality and applicability. Limited attempts have been made to realize the reversible elastomer–fluid transition. Existing phase transition materials in robotics have over‐hard (≈4 GPa) or over‐soft (≈4 kPa) stiffness in the solid states, which should be further investigated to perform more compliant motions. To address these challenges, a reversible elastomer–fluid transition mechanism enabled by magnetically induced hot melt materials (MIMMs) is presented. The transition principle is explained and material characterizations are conducted. MIMMs‐based metamorphosic robots endow self‐metamorphosing abilities, such as self‐healing, spatial reshaping, self‐division/assembly, and additive manufacturability. When interacting with external environments, MIMMs‐based robots can perform further multifunctional abilities, such as collaborations for structure repairs, swimming by symbiosis with external objects, flowing through a narrow terrain by transiting to fluid, and working with elastomeric structures for stiffness‐variable fluid soft actuators. The proposed elastomer–fluid transitions open a new path for robots to generate more flexible and metamorphosic motions, thereby addressing the cross‐phase transformation challenges that soft robots face.


Introduction
It is challenging to endow robots, including soft robots, with reversible elastomer-fluid transition ability to freely transform robots between a solid elastomeric body and a puddle of fluid.Solid, retaining its shape when not confined, denotes the stable state to interact with objects, while fluid, easily yielding to external interactions, indicates the adaptability to environments. [1,2]nce owning the ability that can rapidly transform between these DOI: 10.1002/adfm.202311981two extreme material states, robots can significantly broaden their application scenarios.
Noteworthy attempts were made to realize the reversible solid-liquid transition ability.Liquid metal transformation and ferrous fluid-based fluidic transformation are the two main methods. [3]Taking advantage of the low melting point, liquid metal, a transitional ferrofluid made by galliumiron mixing and Hydrochloric acid solution, can realize the solid-liquid transition in a predefined temperature range. [4]lthough the transition was realized, the metal in the solid state presents significant stiffness (i.e., Young's modulus of ≈3.6 GPa), which is difficult to deform to conduct compliant motions under this solid state.Regarding the ferrous fluid-based fluidic transformations, [5] the magnetorheological fluid, made of iron particles and water starch, was applied to quickly transform between Newtonian fluid (under a low magnetic field) and Bingham plastic fluid (under a strong magnetic field).As the other extreme against the liquid metal, the magnetorheological fluid in the Bingham plastic fluid state, the so-called solid state, has under-enhanced stiffness (i.e., a shear stress of ≈4 kPa).Moreover, as an aqueous solution-based material, it can be dissolved in water environments which needs to be further addressed. [6]Robots made by both methods can only perform planar motions (i.e., sliding motions under magnetic field guidance), which, however, are difficult to "stand up" operating spatial motions due to the nature of these materials.Current methods reveal the fact that the solid-fluid transition should be further investigated into an advanced elastomer-fluid stage that can practically endow soft robots with unique selfmetamorphosing functions and interaction functions. [7,8]or the transition ability from the soft robotics perspective, it is desired to perform elasticity in the solid state (usually called elastomer state to distinguish between rigid bodies), fluidity in the fluid state, and rapidly reversible metamorphosing ability between these two states (Figure 1a).Specifically, the elasticity is the fundamental requirement of soft robots that can compliantly interact with external objects and operate delicate deformations, such as elongation, bending, and twisting. [9]Fluidity denotes the advanced ability of soft robots to further adapt to external environments.Under the same stimuli for elastomer deformations, taking the magnetic actuation as an example, soft robots in the fluid state perform division, merging, growth, and directional flow motion, respectively. [10]Rapidly reversible metamorphosing ability represents the switch between elastomer and fluid deformations, deconstruction of established structures (elastomer to fluid), and reconstruction (fluid to elastomer).These abilities for soft robots systematically form an advanced transition scheme, defined as the reversible elastomer-fluid transition for metamorphosing robots.
Here, we realize the reversible elastomer-fluid transition by developing a library of magnetically induced hot melt materials (MIMMs), which are magnetically actuatable elastomers with sufficient elasticity in the ambient temperature (i.e., Young's modulus range of 8-50 MPa).MIMMs are also magnetically actuatable fluids with controllable fluidity in the heated temperature (i.e., more than 60 °C).The reversible elastomer-fluid transition of MIMMs can be finished in several seconds by applying induction heating.MIMMs contain thermoplastic powders, magnet-responsive particles (MRPs), and other modifiers (e.g., wax or graphene).We also provide a pathway to prepare a library of MIMMs, which mix the commercially obtained hot melt adhesive (HMA) with neodymium iron boron (NdFeB) magnet powder or ferrous-ferric oxide (Fe 3 O 4 ) powder.
The elasticity, fluidity, and metamorphosing abilities are investigated by conducting material studies and endowing robots with multifunctional self-metamorphosing and interaction abilities.Regarding self-metamorphosing ability, the MIMMs-based robots can realize self-healing, spatial reshaping, self-division, self-assembly, and additive manufacturability.MIMMs are desirable printing materials that can be printed objects, flow through a narrow terrain into robots with a printing pen or a 4D-printing system.Regarding interaction ability, the MIMMs-based robots can collaborate to repair a broken structure, swim underwater like a symbiosis manta ray by transforming from a soft worm to a puddle of fluid, and adjust the stiffness distributions of a fluidic soft actuator.The actuator can conduct field exploration, in situ drug delivery, and intubation navigation tasks.With the reversible elastomer-fluid transition ability, MIMMs-catheters can perform self-division and self-reconfigurations for various purposes, such as path locking, in vivo catheter carriers, and obstacle removal.
As a new type of phase transition material, MIMMs fill the gap in performing elastic behaviors.Enabling phase transition materials with recoverable deformations (e.g., stretching, twisting, and bending) can significantly enrich their functionality in the soft robotics area.This work presents various demonstrations to show MIMMs' advances over existing phase transition materials in application scenarios that require reversible elastomer-fluid transitions.

Elastomer-Fluid Transition Principle
The reversible elastomer-fluid transition imposes requirements on smart materials: elasticity, fluidity, and metamorphosing ability corresponding to the elastomer state, fluid state, and transition process.Thermoplastics are polymer materials that are deformable elastomers at ambient temperatures and fluids at a certain elevated temperature, [11,12] e.g., hot melt adhesives. [13]To generate heat and respond to remote controls, MRPs are mixed into the thermoplastics.NdFeB and Fe 3 O 4 are two typically applied MRPs, performing paramagnetism and magnetic hysteresis, respectively. [14,15]NdFeB and Fe 3 O 4 have distinct advantages: the former is a magnetic material with magnetic hysteresis, while the latter shows better heating ability under induction magnetic fields. [16]Recently, induction heating has been applied in soft robotics to conduct, for example, liquid-gas transition, [17] thermoactivated shape memory, [18] remote motion controls, [19] etc.However, it remains unexplored for inducing the elastomer-fluid transitions.
The elastomer-fluid transition principle can be described as follows (Figure 1a): at ambient temperature, stable polymer chains of thermoplastic molecules contribute to the elasticity and morphologic stability of MIMMs in the elastomer state.Mixed MRPs provide MIMMs actuation capability.Under the parallel, vertical, and rotation magnet fields, MIMMs elastomers perform the stretch, bending, and twisting motions, respectively.Radiofrequency (RF) induction transfers electric energy to eddy current heating and hysteresis heating, elevating body temperature, and weakening polymer chains. [20,21]Intermolecular forces are significantly eliminated, transforming the elastomer into the fluid.In this state, motions of MRPs will bring their surrounding melted MIMMs particles moving together (that is, fluidity) due to the viscosity and boundary layer effect. [22]Therefore, under the parallel, vertical, and rotational magnet fields, MRPs perform planar, spatial, and rolling motions leading to overall division, spatial growth, and directional flow motions, respectively.Within several seconds of cooling, the weakened chains are reinforced to the strength as before, denoting the fluid-to-elastomer transition process. [23]argeting robot applications, three significant characteristics of MIMMs (i.e., elasticity, fluidity, and metamorphosing ability) should be investigated.Thermoplastic powders, MRPs, and modifiers are selected: ethylene-vinyl acetate (EVA, 8% VA), polycaprolactone (PCL, 150 μm), and commercially obtained hot melt adhesive particles are selected as alternative thermoplastic materials for different flexibilities and fluidities.The powders affect all three characteristics from the molecular force perspective. [24,25]The metamorphosing ability increases ≈50-100% with the increment of MRPs' proportion.However, the elasticity and fluidity will be negatively influenced (≈50% and ≈42%).Modifiers are additives commonly applied to enhance the performance of thermoplastic materials, such as fluidity, elasticity etc. Wax and graphene are selected to improve the fluidity and metamorphosing ability, respectively.However, excessive proportions of wax and graphene will significantly weaken (≈50%) and harden (≈100%) MIMMs, respectively. [26]MIMMs are fabricated by mixing MRPs and modifiers with the melted thermoplastic powders at an elevated temperature (Figure 1b, see further information in Figure S1, Supporting Information).The proportions of the mentioned material elements can be specifically selected for different application purposes (e.g., elasticitydominated, fluidity-dominated, and actuation-dominated applications).For example, the elasticity-dominated application requires robots to perform recoverable deformations such as bending, crawling, etc.The fluidity-dominated application can be found in the robotic functions of self-healing, merging, division, etc.
Different microstructures of MIMMs are captured and compared by an optical microscope (BX60, Olympus Inc.) and a field emission scanning electron microscope (SEM, Q400F field emission SEM, FEI Inc.).Thermoplastics are observed with better absorbent to NdFeB than Fe 3 O 4 due to different aggregation performances (Figure 1c).NdFeB particles are granularly aggregated, which benefits from large coverage areas of thermoplastic areas.However, Fe 3 O 4 particles aggregate in layers (Figure 1d).Graphene flakes and thermoplastic particles are folded and wrapped with each other, forming stick-like structures, which are covered inside MIMMs, enhancing thermal efficiency (43.7%).Wax, as an amorphous solid, is embedded inside or on the surface of MIMMs to enhance fluidity.

Characteristics of MIMMs for Soft Robotics
To investigate different material characteristics for soft robots, a library of 35 types of MIMMs is developed by fabricating with various material element proportions (Figure 2a; Figure S2, Supporting Information), which are thermoplastics (10 g HMA, EVA, and PCL), MRPs (5, 10, and 15 g Fe 3 O 4 or NdFeB particles), wax (0.3, 0.6, and 1.2 g), and graphene (0.1, 0.2, and 0.3 g).Desired elasticity in the elastomer state, rapid metamorphosing ability in the transition state, and fluidity in the fluid state are observed.

Elastomer State: Elasticity and Actuation Capability
MIMMs in the elastomer state are expected to perform magnetresponsive deformations and corresponding morphologic recovery.Therefore, basic considerations of MIMMs in elastomer state are the elasticity and actuation capability by magnetic fields, which are investigated by stretch testing and vibrating sample magnetometer (VSM, PPMS, Quantum Design Inc.) testing, respectively.With similar stiffnesses to commonly applied silicon rubbers, Young's modulus of MIMMs ranges from 5 to 50 MPa.Taking HMA-based MIMMs as examples (Figure 2b), Young's modulus increases (50-400%) with the increasing amount of Fe 3 O 4 , NdFeB, and wax (100-300%).With the same added weight, the stiffening effects of Fe 3 O 4 on MIMMs are more significant than that of NdFeB (440% vs 400%).However, opposite phenomena are observed when excessive graphene is involved (i.e., 1.4% is regarded as an excessive proportion) because its limited material compatibility weakens intermolecular adhesions between thermoplastic particles (see detailed stretch results in Figure S3, Supporting Information).Regarding the actuation capability, viz., the ability to be magnetized and respond to magnetic stimuli, the paramagnetism, magnetic hysteresis, and paramagnetism-hysteresis coupled characteristics are observed (Figure 2c-e).Magnetization abilities steadily enhance with the increment of MRPs' proportions, where Fe 3 O 4 and NdFeB are observed with obvious paramagnetism and magnetic hysteresis, respectively (Figure 2c,d).Fe 3 O 4 -based and NdFeB-based MIMMs soft actuators are fabricated and tested, which perform single directional and bidirectional bending, respectively.When mixing Fe 3 O 4 and NdFeB in thermoplastic powders with a certain ratio (Fe 3 O 4 : NdFeB = 1:4, 1:1, and 4:1), the intercept height of the hysteresis curve is positively correlated with the proportion of involved NdFeB.In contrast, the speed of reaching magnetic saturation positively correlates with the proportion of involved Fe 3 O 4 .There are correlations between the robot's weight capacity and its formation.However, adjusting their weight capacity is usually given less priority than their functionalities for specific application scenarios.Our testing results showed that the payload of MIMMs robot is overall larger than 1 time of their body weight.

Transition State: Metamorphosing Ability
Metamorphosing ability is investigated from two perspectives: melt flow rate (MFR) and transition efficiency under RF heating, which indicate the thermal-responsive ability and inductionresponsive ability, respectively.The melt flow index of MIMMs is tested by a melt flow indexer.Compared with HMA, EVAbased and PCL-based MIMMs perform relatively weak melt flow rates (1-20 g min −1 ).The intention of improving metamorphosing ability by involving wax is verified (Figure 2f).Moreover, the thermal-responsive ability of NdFeB-based MIMMs performs ≈100% higher MFR at 150 °C than that of Fe 3 O 4 due to the larger material area of granularly aggregated NdFeB clusters.Regarding the induction-responsive ability (Figure 2g; Movie S1, Supporting Information), the transition index is defined as the time consumption to melt a gram of MIMMs, by an induction heater with an RF of 280 kHz.Overall, a piece of MIMMs with a length × width × height of 10 × 6 × 3 mm can be melted in ≈1 min with 280 Hz (melt time can be improved to several seconds when the frequency is maximally elevated to 700 kHz).The metamorphosing abilities of EVA-based and PCL-based MIMMs are slightly weaker (72-100%) than that of HMA due to the stronger intermolecular forces.NdFeB-based MIMMs perform insensitivity to induction heating; however, it can be overcome by involving Fe 3 O 4 .The metamorphosing ability enhancements by involving wax and graphene are verified.Graphene locally connects aggregated Fe 3 O 4 clusters, improving the generated eddy current heat, which significantly promotes the metamorphosing ability (Figure 2h).Heat accumulation speeds decrease ≈428% with the increasing heating distance of 5 mm between the specimen and the heating coil (see detailed information in Figure S4, Supporting Information).Note that fluid-to-elastomer processes can be finished in 10-30 s ambient cooling.The time-temperature relation during the heating process can be regarded as the linearity whose gradient depends on the material factor (MNP fractions, heating distances, coil shapes, etc., see detailed information in Figure S19, Supporting Information).However, different from the heating process, external environmental factors dominate the cooling process, such as the ambient temperature, airflow speeds, etc.

Fluid State: Fluidity and Actuation Capability
The actuation ability in the fluid state directly determines the mobility of MIMMs-based soft robots.As the dominant factor of actuation, magnet-guided flow displacements are positively related to the proportions of MRPs.Involving wax enhances the flowing performance (20-40%); however, excessive wax proportions reduce it (≈40%) due to the limited boundary-carrying ability of MRPs (Figure 2i).No obvious relationships between graphene and flowing performances are observed due to the lightweight of graphene particles.The fluidity is further investigated by measuring the viscosities of fluidified MIMMs with a Brookfield viscosimeter.Viscosities of MIMMs decrease 33-45% with the elevating temperature of 20 °C, where NdFeB contributes to significant viscosity reductions due to satisfactory material compatibility (Figure 2j,k).Involving 5.6% wax will make the fluidified MIMMs 25% more viscous, slowing the recovery rate into the elastomer state (see further information in Figure S5, Supporting Information).

Self-Metamorphosing Abilities of MIMMs-Based Robots
MIMMs endow soft robots with multifunctional selfmetamorphosing abilities due to the investigated elasticity, metamorphosing ability, and fluidity.

Self-Healing
The fascinating vitality of soft robots was exemplified by their recovery from damage. [27]MIMMs provide new self-healing strategies to soft robots (Figure 3a; Movie S2, Supporting Information).MIMMs can realize wound healing, fracture healing, and planar growth by the circular guide, rotate guide, and rolling guide, respectively (Figure 3a).Under the circular guide, MRPs in the fluid state can be magnetically actuated to perform sliding motions (understood as being dragged or pushed by magnet fields).The central aggregation of MRPs realizes wound healing.Under the rotation guide, MRPs in the fluid state perform rolling motions, presenting more precise directional motions, that is, fracture healing.Planar reshaping is implemented by roll-drag coupled MRPs motions (Figure 3b).Compared with spatial reshaping, planar reshaping can be accurately extended to a planar location, shaping itself to a new appearance.Captured infrared images present an entire wound healing process containing the heating, guiding, and healing stages (Figure 3c).All healing strategies can be finished in 1-2 mins, regarded as rapid and efficient selfhealing methods. [8]The healing effect on the mechanical properties and actuation ability was investigated.After fracture healing, no significant changes were observed except for random slight changes in the behavior of the plastic stage (see detailed information on the reshaping test in Figure S9, Supporting Information).Further, cyclic heating was conducted on the MIMMs sample to investigate the effects on Young's modulus, flowability, and actuation ability (bending performances).Results indicate no significant relationship exists between cyclic heating and MIMMs performances in about 40 times heating (see detailed information on the cyclic heating in Figures S9 and S10, Supporting Information).

Spatial Reshaping
In addition to planar growth, a soft robot can rarely spatially expand and reshape its body.The spatial-reshaping of our tailsurvive and regeneration processes of a gecko-shaped MIMMs robot was realized (Figure 3d).The gecko-shaped robot can stay or move (called sliding) on a wall, where the magnetic field is distributed.After passive or active tail-survive (see active tail-survive in the next subsection), the rear region can be locally RF heated and extended out by spatial magnetic guides.The heat-grow-cool process took ≈1 min to complete, where the spatial reshaping took ≈10 s.

Self-Assembly and Self-Division
The reversible elastomer-fluid transition endows robots with self-assembly and self-division abilities, which broaden their application scenarios.The self-assembly was realized by bonding MIMMs after being locally RF heated, while the self-division can be regarded as a reverse fracture healing process (e.g., the active tail-survive).
The assembly-division function can be found in living creatures, which is applied to collaborate, survive, symbiosis, etc.For example, after being cut into two pieces, the original head-end of an earthworm can melt its muscles at the cut region and regenerate new cell clusters for healing. [28]Inspired by the melt-heal phenomenon, we demonstrate an "open sesame" application to illustrate the assembly-division process (Figure 3f).A worm-bot's way home was blocked by a door, which can be opened by pressing any one of the switch-1 and 2. The worm-bot, assembled by two sub-worm bots, first pressed the switch-1 to open the door.Being locally RF heated, the worm-bot was self-divided into two subworm bots.Bot-1 stayed still, kept pressing the switch, and bot-2 crawled to switch-2.When bot-2 pressed switch-2, bot-1 started crawling forward.The self-assembly and self-division abilities of the MIMMs-based soft robot finished the incompletable task for a single robot.

Additive Manufacturability
4D printing has been becoming a hot topic as there is a growing need for fabricating integrated functional materials. [29,30]Here, we provide an additive manufacturing demonstration to show that MIMMs have the potential to contribute to 4D printing development.The investigated fluidity and metamorphosing ability endow MIMMs to be additively fabricated into soft robots.MIMMs were installed into a print pen and printed into letters, which are born actuatable (Figure 3g; Movie S3, Supporting Information).The thinner parts of the letters were applied as joints for grasping and crawling motions.MIMMs can also be molded, viz., the magnet field was applied to squeeze out the air bubbles mixed in the fluidified MIMMs, which is more efficient than the commonly applied air-pressure squeezing method (Figure 3g-iv).Further, MIMMs are overviewed to be 4D-printed potentially.The controlled magnet can guide the nozzle-squeezed MIMMs to operate further real-time shape adjustments (Figure 3h).We should note that the demonstration is in a preliminary form whose motivation is to showcase MIMMs' potential in 4D printing.To implement additive manufacturability to the practical application level, applied techniques in existing literature are instructive and helpful. [29,30]

Functions by Interacting with External Objects
MIMMs-based robots can perform not only smart selfmetamorphosing abilities, but also can gain new functions by interacting with external objects, such as collaborating to repair a fractured structure, symbiosis with a wooden stick to be a manta ray-shaped soft robot swimming underwater, flowing through narrow terrains, and inputted in a soft tube to be stiffness variable and shape memorable soft actuators.

Collaboration and Structure Repairment
Collaborations of soft robots further broadened their application scenarios, which consider more soft robotic functions, controllability, and practical utilization.Due to the simple fabrication and control strategy, MIMMs-based soft robots can realize lively collaboration tasks.Workerbot is a MIMMs-based origami/kirigami coupled soft robot (Figure 4a), which can operate the motion mode, carry mode, and output mode that is applied to generate rapid motions, object transportations, and high payloads, respectively.Therefore, the variable-friction capability was realized by changing contact areas between the robot and the ground.Mode switches can be realized by the rolling guide actuation.Considering the adjustable viscosity of MIMMs in the fluid state, the Workerbot can be applied to stick fractured structures for repair (Figure 4b; Movie S4, Supporting Information).A fractured structure was stacked onto another, which should be moved down for repairs.Workerbot-2 arrived at the carrying site in motion mode and switched to carry mode, using the overlapping point as the fulcrum to move one end of the structure (Figure 4b-i,ii).After moving down the upper structure, Workerbot-2 found it could not move the structure by itself without any fulcrum.Therefore, Workerbot-1 and 2 fit together, adjusting the relative poses of the two fractured structures (Figure 4b-iii,iv).Then they should work separately: in output mode, Workerbot-1 held one end of the fracture structure to avoid slides between structures and ground, while Workerbot-2 moved to the repair site (Figure 4b-v).Responding to the RF heating, Workerbot-2 transformed into fluid, which flowed into the seam between the two fractured structures for repairs (Figure 4b-vi).In addition to the structure repair, the worker bot can connect broken electrical wires and perform the insulation function due to the non-conductive property (Movie S11, Supporting Information).The insulation function has not been found in the existing phase transition materials (e.g., liquid metal and magnetorheological fluid).

Gain New Motion Abilities Through External Interactions
Inspired by the symbiosis activities of living creatures, MIMMsbased soft robots can gain new locomotor abilities by attaching to external objects.It is known that soft robots perform more efficient motions when rigid bones are involved.For example, a magnetized MIMMs wing can perform limited bending; however, when attached to a rigid bone, it can generate manta ray-like flapping motions under magnetic actuation (Figure 4c; Movie S5, Supporting Information).Here, we demonstrate the "symbiosis" process.Under magnetic guides, the magnetized MIMMs wing can position its way, aligning to the center of a small wooden stick (Figure 4d-i).After alignment, RF heating was locally applied at the overlap region between the wing and stick.The reversible elastomer-fluid transition can assemble both parts in ≈10 s.The MIMMs wing, therefore, gained the manta ray-like swimming ability.Then, a rolling magnet field (with a 5 Hz oscillating magnetic field strength of 60-150 mT) was applied to actuate the swimming motions (Figure 4d-ii).The MIMMs manta ray robot can swim underwater with an average speed of ≈1 BL s −1 .Note that, different from the ferrous fluid-based fluidic transformations, the reported MIMMs-based soft robots are not dis-solved in water compared with ferrofluids materials.The symbiosis ability of the MIMMs-based robots significantly enhances the interaction capability with external objects.It should be noted that MIMMs are eco-friendly and recyclable materials that can be reused after the symbiosis process.We should also note that, the swimming motion is contributed simultaneously by the flipping motion and magnetic gradient field pulling motion (please see proof experiment in Figure S14, Supporting Information).Proposing the manta-ray-like soft swimmer is to showcase that the MIMMs can gain new motion abilities through external interactions instead of focusing on designing a high-speed underwater soft robot that is competitive with other magnetic swimmers.

Environmental Adaptability
Soft robots are known for their impressive possibility to adapt to complex environments.Characteristics of the reversible elastomer-fluid transition (i.e., elasticity, metamorphosing ability, and fluidity) endowed soft robots with unique environmental adaptability.A MIMMs-based Wormbot can operate crawling motions navigating its adventure by responding to pulse magnetic stimuli (Figure 4e).However, an extremely narrow terrain and a slope blocked its way forward (Figure 4f-i,ii).To pass the terrains, it transited to the fluid state under RF heating and flowed through the narrow seam under the rolling magnetic field (Figure 4f-iii).After cooling (Figure 4f-iv,v), it can head up due to its maintained magnetization profile (see proof in Figure S13, Supporting Information) and crawl forward, continuing its journey (see Movies S6 and S19, Supporting Information).It should be noted that the environmental adaptability is illustrated by the flowing through/over extreme terrains and reshaping back to the worm shape, simultaneously.Additionally, the MIMMs-based robot was applied to transport an endoscope through a narrow hole, demonstrating the environmental adaptability (i.e., a triangular hole with a height × base of 6 × 6 mm, Movie S12, Supporting Information).

Stiffness-Tuning, Redistribution and Shape Memory for Fluidic Actuators
MIMMs can also play their creative roles from the soft actuator perspective.Endowing variable stiffnesses to soft actuators has long been the focus of soft roboticists, which can perform more controllable and delicate bending motions operating various tasks. [7,31]Here, we developed a MIMMs-based fluidic soft actuator with stiffness-tuning, stiffness-redistribution, and shape memory (Figure 4g).The actuator contains an elastic shell with homogeneous stiffness and MIMMs inside the shell.Involving MIMMs makes the elastic shell actuatable and, simultaneously, stiffness-tunable.The varied stiffness effect is reflected in two levels: one is MIMMs can perform reversible elastomer-fluid transitions freely self-divide and redistribute in the channel contributing to overall stiffness variability (Figure 4h-i,ii; Movie S7, Supporting Information).Bending tests are conducted to compare the performances of soft actuators with different overall stiffness redistributions (Figure 4h-iii).MIMMs in the elastomer state provided stiffened effects to their locations in the actua-  tor, letting the rest hollow parts perform high-curvature bending (Figure 4h-iv).When MIMMs are in a fluid state, the stiffening effect can be eliminated, providing similar stiffness as the hollowed elastic shell (Figure 4h-v).The contributing local stiffness tunability (i.e., regions MIMMs located at are stiffness tunable); the other is MIMMs shape memory phenomenon was observed when MIMMs were cooled down while the soft actuator was bent (see further analysis in Figure S6, Supporting Information).
The actuators were demonstrated to be applied in field exploration and biomedical scenarios.Regarding the field exploration application, the actuator can stick into a closed space to grab an object out (Figure 5a,b; Movie S8, Supporting Information) and, therefore, realize the object locking-setting function (i.e., the object carrying ability). [14]The distributed regions responded to the magnetic field simultaneously to navigate its directions.It should be noted that the grabbing process benefited from the metamorphosing of MIMMs.The RF heating was locally applied at the tip of the actuator, where the MIMMs flowed onto the object.Af-ter cooling down, the actuator and object were stuck together for the grabbing out process.Different MIMMs can be selected to meet various grabbing strengths.The actuator can grab an object weighing 900 g (Figure 5c).For proof-of-concept biomedical applications, the actuator can perform in situ drug deliveries or treatments while operating the endoscopy (Figure 5d-f; Movie S9, Supporting Information).A meltable connection was applied between the actuator and drug, which can be melted to fracture, realizing the drug release task.An ex vivo pig intestine was used to demonstrate the application.Even with complex environmental frictions, the actuator was remotely controlled and approached the target region.The in situ membrane replacement can also be realized for biofilm repairs.Another biomedical application was demonstrated to highlight the stiffness-tunability (Figure 5g,h; Movie S10, Supporting Information).An ex vivo pig trachea was applied to present the intubation navigation application.The actuator was inserted through the oropharynx and magnetically guided from the trachea to different bronchi.The NdFeBbased MIMMs were distributed at the catheter tip for precise tip guidance, and the Fe3O4-based MIMMs were distributed in the middle of the actuator for sufficient feeding forces.Therefore, coupled adjustments of the remotely controlled actuator were realized by controlling both regions simultaneously.
It should be noted that the size of MIMMs-catheters can be reduced for more practical applications.MIMMs-catheters can also be endowed with self-division and reconfiguration abilities, which can realize more complicated and interesting tasks, such as "catheter carrier", "path lock", and "obstacle grab", as shown in Supplementary Movies S15-S18 (Supporting Information).

Broken Wires Insulation
Benefiting from the electric nonconductivity of the MIMMs (see proof experiments in Figure S15 and Movie S21, Supporting Information), MIMMs-enabled soft robots can operate wire insulation in dangerous scenarios, as shown in Figure 6a and Movie S20 (Supporting Information).To connect and insulate broken wires, which were exposed directly to the water leading to potential dangers such as electroshocks, the MIMMs-enabled robot symbiosis to a broken wire and carried it approaching the other wire in water (shown in Figure 6b).After connecting two wires to each other, the MIMMs-robot was transformed into the fluid state, flowing over the exposed wires for insulation purpose (see Figure 6c).Compared with other phase transition materials, the wire insulation function exhibits unique functions of MIMMs, including electric nonconductivity, water-insolubility, and elasticity.
As a material-based robotic discipline, soft robotics is continuously studied from three perspectives: material study, robotic functions, and application scenarios.The proposed elastomerfluid transition provides a research system that shows close correspondences and relations between material properties (i.e., elasticity, metamorphosing ability, and fluidity), robotic functions (i.e., crawling, swimming, flowing, etc.), and application scenar-ios (i.e., structural health monitoring, field exploration, actuator operations, etc.).Therefore, regarding different application tasks, corresponding robotic functions can be realized by preparing related MIMMs.

Discussion
Here, we have provided a reversible elastomer-fluid transition technique by developing a series type of magnetically induced hot melt materials (MIMMs) for metamorphosic robots.The satisfactory elasticity in the elastomer state (8-50 MPa), rapid metamorphosing ability in the transition state (≈1 min), and desirable fluidity in the fluid state (≈7 BL flowing) contribute simultaneously to the unique characteristics of MIMMs-based soft robots.MIMMs can be fabricated by a straightforward process and easily available materials, as demonstrated.The transition principle is investigated by proportion analysis and microscopic observations.A library of 35 types of MIMMs is fabricated with various proportions to study the unique characteristics of the elastomerfluid transition.MIMMs in the elastomer state are observed with satisfactory strength (8-50 MPa) while magnetic actuation capability remains.MIMMs in the transition state can rapidly operate reversible phase transitions in several seconds to several minutes.Fluidity is measured with low viscosity (58.2 Pa s) while remaining desirable actuation ability.These characteristics directly refer to various self-metamorphosing abilities and interaction abilities of MIMMs-based robots.MIMMs-based robots have creative self-metamorphosing abilities, such as self-healing, spatial reshaping, self-division, self-assembly, and additive manufacturability.By interacting with external objects, the robots can gain new functions like a creature, such as collaboration to repair a fractured structure, self-assemble to swim, overcome complex external environments, and become stiffness-variable and stiffnessredistributed fluidic soft actuators.The multi-functionality of the actuator was demonstrated by field exploration, in situ drug delivery/treatment, and intubation navigation applications.
Regarded as an advanced solid-fluid transition method, the elastomer-fluid transition provides soft robots abilities to perform spatial motions, such as folding, crawling, swimming, spatial reshaping, etc.Therefore, the phase transition-based soft robots can "stand up," performing more flexible and lively motions.Compared with previous transition techniques, MIMMs are born flexible and actuatable elastomers (not metals nor pastes), which are the material substances of soft robots.Moreover, due to the substantial of being a copolymer, MIMMs-based robots can normally work in water, oil, and even acid/alkaline environments without dissolving observed (Figure 4d-iii).Due to the non-conductive property of the copolymer, MIMMs-based robots can connect broken electrical wires and perform the insulation function.
Basic and direct robotic demonstrations have been presented in this work.Moreover, MIMMs-based soft robots can be further applied to design more advanced soft robots.For example, as an important additive of MIMMs, Fe 3 O 4 is also commonly used to operate in vivo ablations.MIMMs-based soft robots are therefore outlooked to conduct targeted ablation tasks by in vivo crawling or swarm collaborations.From the structural health perspective, MIMMs-based soft robots could self-position to the damaged site for repairs and move into crevices that humans cannot reach for repair or detection.
Similar to other soft robots actuated by environmental stimuli, MIMMs-enabled robots also face challenges in overcoming environmental factors (e.g., temperature change, humidity, light, etc.), which may affect performance stability.The extent to which the MIMMs-enabled robots' stability is thermally affected depends on the scale of working temperature ranges (see proof experiments in Figure S8, Supporting Information).Weakening stability is the inevitable result of pursuing performance flexibility.However, the weakened stability can be enhanced by engineering techniques.It should be noted that we proposed a library of MIMMs that contains various base thermoplastic materials and different proportions of additives.Melting points of PCL, EVA, and HMA are 60 °C, 90-120 °C, and 100-230 °C, respectively, which can cover most conditions keeping MIMMs in the elastomer state.
Regarding concerns about MNP aggregation behaviors under the fluid state, we have conducted a series of experiments (Figure S16, Supporting Information).Due to the high viscosity of the thermoplastic substrates and the boundary layer effect under rotational magnetic control strategies, MNPs do not significantly aggregate affecting MIMMs' normal functions.
EVA and PCL are often used in adhesives and may, to some extent, stick to materials with high surface energies, such as metals.Adjusting the bonding strength of adhesives is a long-established topic, and we can use these techniques in reverse.Taking EVAbased hot melt adhesives as an example, reducing the fraction of VA and increasing the wax fraction will help in overcoming the stickiness issue.On the other hand, the stickiness of MIMMs can also be regarded as an advantage in some specific scenarios, such as structure repairment applications.To totally overcome the stickiness issue, further material studies need to be conducted.
MIMMs, as the magnet particles involved composite, can be thermally demagnetized; however, in certain conditions, such as non-flowing transitions and self-divisions, their magnetization profiles will not be randomized under reversible elastomer-fluid transitions (see proof experiments in Figure S13 and heating process in Movie S13, Supporting Information).There are also some engineering approaches to enhance the heat tolerance of the MIMMs' magnetization profiles, such as lowering the melting point by increasing the wax's proportions and enhancing the demagnetization temperature of MIMMs by involving heat-proof magnet particles (e.g., AlNiCo).
Overall, the proposed elastomer-fluid transitions may open a new path for soft robots to operate more flexible and metamorphosic motions, thereby addressing the cross-phase transformation challenges that soft robots face.

Experimental Section
Preparations of MIMMs: MIMMs were prepared in a constanttemperature heating furnace (NDJ-1, DECCA Inc.), which can remain in a temperature range of 30-240 °C.The thermoplastic powders were first melted in the steel tube of the furnace, and the rest additive powders were added several times separately.After 5-10 min mixing with a stirring rod, the mixture was poured out for cooling.A convenient fabrication method using a heat gun to melt several sticks of commercially obtained hot melt adhesives, which can be put in a beaker was presented.Add additive powders several times when HMAs are melted and mix them by continuous stirring.After cooling, the mixtures can be applied as a basic type of MIMMs.The 35 types of MIMMs consisted of 23 HMA-based cases, 6 EVA-based cases, and 6 PCL-based cases.HMA was set as the control group, which, therefore, has more cases than others (see detailed case table in Figure S2, Supporting Information).
Mechanical Tests: The mechanical tests were conducted following instructions of ISO/DIS37-1990.The no.2-typed (2#) specimens were prepared, whose test region was dimensioned with length × width × thickness of 20 × 4 × 3 mm.The presented Young's modulus was fitted by the linear elastic model.
VSM Tests: VSM test samples (cylinders) were dimensioned with a diameter and height of 2 mm.Every sample was separately installed in a transparent tube with a ≈2 mm channel, which was further installed in the VSM test module.The applied magnet field ranged from 0-7500 Oe.Note that the 10 g Fe 3 O 4 mixed HMA sample and 10 g NdFeB mixed HMA samples were tested using a magnetic field range of −24 000-24 000 Oe for full magnetization loops.
MFR Tests: An MFR indexer (MFI, DECCA Inc.) was applied to conduct tests.Samples applied for MFR tests were preprocessed into particles that can be easily input into the indexer.Different load levels were applied due to the various MFR performances between cases.The applied temperature range was 100-160 °C.An interval of 10 °C was applied for every case.A duration of 30 s was set and the extruded materials were cut six times.The six extruded MIMMs were cooled down and measured to calculate the average extruded weight.Collected results were unified into the extruded weight by per kilogram load in 10 min.
RF Heating Tests: An induction heater (HHCG-6, Honghe Inc.) was applied for RF heating tests.Frequencies of the heater can be adjusted from 280-700 kHz.To distinguish metamorphosing ability between cases, the lowest frequency (i.e., 280 kHz) was selected.Namely, the metamorphosing ability can be further improved when a higher frequency is selected.A qualitative test showed that the test sample melted immediately once it approached the heating region.
The RF heating response tests were conducted by two setups, which were applied for Figure 2g,h, respectively.In Figure 2g, RF heating test samples were cuboid and dimensioned with length × width × thickness of 10 × 10 × 3 mm.They were installed in the center of the heating coil with a fixed position.In Figure 2h, to study the distance effect, spherical samples with a diameter of 10 mm were applied.The distance origin was set up at the upper surface center of the heating coil.
Viscosity Tests: A Brookfield viscosimeter (NDJ-1, DECCA Inc.) was applied to conduct tests.The viscosities of samples were tested under heating environments.The 29# spindle was selected to operate the tests.Rotation speed ranged from 5 to 9 rpm with 1 rpm intervals.Every sample was tested twice for repeatability, and 2 min testing was set for every case.The pre-heating under 200 °C was applied to eliminate air bubbles in the melted samples for repeatability.
Actuation Tests in the Flow State: Test samples were dimensioned with length × width × thickness of 10 × 10 × 3 mm.Samples were flatly put on a stable platform, and heated by 500 °C hot air for 30 s.The rotating magnetic field (150 mT with 60 rpm) was activated when the heating process ended.Flow displacements were recorded.
Fabrication of Demonstrated Soft Robots: MIMMs are eco-friendly materials that can be melted and reshaped for reuse.Demonstrated robots in this work were fabricated by various methods.
The thin films presented in Figure 2d,e were dimensioned with length × width × thickness of 20 × 5 × 1 mm.MIMMs were heated into fluid and laid flat in grooves with corresponding dimensions.After ≈1 min cooling, the thin films were taken out of the grooves by tweezers.
The gecko-shaped robot was molded.A gecko mold was 3D printed and inverted into a silicon rubber mold (Ecoflex 00-30, Smooth-on Inc.).Excessive MIMMs in the elastomer state were stuffed into the mold and heated into fluid by a heat gun.The MIMMs were evenly distributed in the mold and a magnet was applied to attract the fluid under the mold to squeeze out air bubbles.After cooling down, the gecko-shaped robot was easily removed from the mold.
The Wormbot was first fabricated by a similar method as the thin film.Post-shaping was conducted by applying external pressures to shape predefined curves.The Wormbot presented in Figure 3f was made from two fabricated worm bots symmetrically.Local heating at the center of the two bots was applied to stick them together.
The Workerbot was cut from the flatly laid thin film.The cut Workerbot was symmetrically folded along the center line by external pressures to shape predefined curves.Magnetizations were conducted by a cuboid NdFeB magnet (150 mT).The magnetic wing (presented in Figure 4b) was cut from the flatly laid thin film and magnetized following the strategy presented in Figure 4b.
Finite Element Analysis of Magnetic Guides: Finite element analysis was carried out to simulate the magnetic guides (i.e., rolling guide and circulate guide).Regarding the rolling guide, a cuboid magnet was modeled with a dimension of 30 × 15 × 30 mm.N35 (Sintered NdFeB) was assigned to the magnet.The magnet was set in a box which is dimensioned 80 × 40 × 46 mm.According to the B-H constitutive relation, the remanent magnetic flux density model was selected to govern the simulation, which can be written as and where μ rec , ‖B r ‖, e denote the recovery permeability, modulo of residual magnetic flux intensity, and the direction of residual magnetic flux that was set along the height direction of the magnet, respectively.In the simulation, the magnet moved along the x-axis with a speed of 40 mm min −1 , while rotating around the y-axis with a rotation speed of 120 degrees min −1 .Regarding the circulate guide, a cylindrical magnet was modeled with dimensions of 20 mm diameter and 20 mm height.The magnet was set in a test box, which is dimensioned 100 × 100 × 28 mm.Other numerical setups were the same as that of the rolling guide.

Figure 1 .
Figure 1.Reversible elastomer-fluid transition principle and material characterization of MIMMs.a) Illustration of the reversible elastomer-fluid transition principle.MIMMs in elastomer and fluid states perform different motions under the same actuation strategy.b) The fabrication process of MIMMs contains the pre-heating and mixing stages (i-iii).Thin-film-shaped MIMMs can be transformed into fluid state by radio frequency (RF) heating.c) Optical microscope-captured images of MIMMs are post-colored to distinguish between different thermoplastic materials.d) Scanning electron microscope (SEM) captured images of applied additives, including MRPs clusters (Fe 3 O 4 and NdFeB), graphene, and wax.The SEM images of the elastomer-state MIMMs were captured at the ambient temperature.

Figure 2 .
Figure 2. Characteristics of MIMMs for soft robots.a) Materials setup presenting a library of 35 types of MIMMs with various proportions.Characteristics in elastomer state: b) Elasticity investigated by tensile tests, showing Young's modulus of MIMMs ranging from 8-50 MPa.Note that the min, mid, and max refer to the material proportion ranges marked on the label.c-e) Magnetic actuators under VSM tests showing paramagnetism, magnetic hysteresis, and paramagnetism-hysteresis coupled characteristics, respectively.Characteristics in transition state: f) Meltability investigated by MFI test, showing additive effects and characteristic differences between alternative thermoplastic materials.g) Transition index under 280 kHz RF heating, which denotes the time consumption to transform per gram elastomer into the fluid.Abbreviations at the horizontal axis denote the initials of additives.h) Relations between time, temperature, and distances between specimen and induction heater.Graphene significantly enhanced the metamorphosing ability with respect to elevated temperature and corresponding time consumption.Characteristics in fluid state: i) Actuation capability investigated by flow motion test, performing displacements ranging from 2.5-7 body lengths (BLs) in 10 s. j) Fluidity, also known as viscosity, is measured by a Brookfield viscometer, which shows additive influences on fluidified MIMMs' viscosities.k) Density plots of relations between the rotation speed, temperature, and detected shear in different additive conditions.

Figure 3 .
Figure 3. Self-metamorphosing abilities of MIMMs-based soft robots.a) Self-healing, which contains cut-wound healing, fracture healing, and spatial reshaping.Under b) Magnetic guidance, the MIMMs in the fluid state can heal themselves by their fluidity.c) Infrared images show the thermal distribution during the healing process.d) The gecko-shaped soft robot can be endowed with spatial reshaping ability.The robot mimics the tail survival and tail-regenerating processes.Its fractured tail can regenerate rapidly (i.e., ≈10 s).e) "Open sesame" demonstrates self-assembly and self-division ability.f) MIMMs can be applied as an additive manufacturing material.(i)-(iii) present the manually printed characters, which are born actuatable, and (iv) actively model fabricated characters.g) An outlook for 4D printing by applying MIMMs.

Figure 4 .
Figure 4. Functions by interacting with external objects.Collaboration and structure repairment: a) Foldable workerbots perform the motion, carry, and output modes, which are applied to operate rapid motions, carry objects with satisfactory movability, and output the highest payloads, respectively.b) Workerbots can collaborate to repair a damaged structure.Gain new motion abilities: c) MIMMs-based magnetic wings can self-assemble to a bone structure to be a manta ray-inspired soft (continued) robot, which can flap wings under magnetic fields.d) The manta ray-inspired soft robot can swim underwater at high speeds, illustrating that the MIMMs-based robot can gain new motion functions by incorporating external objects.Environmental adaptability: e) Working principle of the MIMMs-based Wormbot, which can adapt to complex environmental conditions by reversible elastomer-fluid transitions.f) A demonstration of environmental adaptability.To go through a narrow terrain and over the slope, the Wormbot can transit into a fluid state to flow through and over the terrains.The solidified worm can reshape back to the worm shape by adapting to the external terrains.Stiffness adjustable and redistributable fluidic actuators: g) Working principle of MIMMs-based fluidic soft actuator, which can be of various stiffness tuning and redistribution by fluidity and elasticity of the MIMMs.h) The fluidic soft actuator performs various bending motions under different stiffness distribution conditions.Moreover, reversible elastomer-fluid transitions endowed the actuator with shape memory ability.

Figure 5 .
Figure 5. Biomedical applications of MIMMs-based actuator.Grab objects in a closed space: a) Illustration of the experimental setup.The closed space contains complex terrains inside.To reach the target object, the MIMMs-based actuator should go through every door set at the walls inside.b)The reaching and grabbing processes.i) The actuator can be magnetically navigated to reach the target object.Local RF heating was applied to melt the MIMMs at the tip, which flowed out and stuck to the object.ii) After cooling down in a few seconds, the actuator grabbed the object out.c) The actuator can grab an object weighing 900 g.In situ drug delivery/treatment: d) Illustration of the in-situ drug delivery/treatment.e) The actuator can carry an endoscopic camera (with a light source, CMOS camera-OV6946, OmniVision, CA, USA) to operate the endoscopy while delivering the drug.f) The actuator carried and released the drug to the target region.Intubation navigation: g) Illustration of the intubation navigation experimental setup.h) The actuator was applied to operate the intubation navigation in the trachea.Different MIMMs were applied to realize coupled adjustment.

Figure 6 .
Figure 6.Wire insulation in a dangerous scenario.a) A MIMMs-robot can operate wire connection and insulation tasks in a dangerous scenario, such as electroshock.b) A MIMMs-robot can symbiosis to wire and approach another wire in water.c) The MIMMs-robot can connect the wires and be transformed into a fluid state to insulate the exposed wires.