Reprogrammable Magnetic Soft Robots Based on Low Melting Alloys

Magnetic soft robots featuring untethered actuation and high mechanical compliance have promising applications ranging from bionics to biomedicine. However, their fixed magnetization profiles pose a challenge for adaptive shape transformation in unpredictable environments and dynamic tasks. Herein, a reprogrammable magnetic soft composite is reported by encapsulating magnetic neodymium–iron–boron microparticles with low melting alloy (LMA) and embedding them into the elastomer. Utilizing the phase transition of the LMA, the magnetic microparticles can be reoriented under an external magnetic field and they can be immobilized through LMA solidification, allowing the robot to obtain a new magnetization profile corresponding to its temporary shape. By changing the LMA composition, the robot with multiple programming temperatures can be fabricated and its local magnetization profiles can be selectively programmed in different temperature ranges. A bioinspired crawler with multimode locomotion, a reconfigurable robotic gripper capable of adaptable grasping, and reconfigurable electronic circuits are also demonstrated. This work may pave the way for the next‐generation magnetic soft robots and reconfigurable devices.

the local magnetization profiles in different temperature ranges.We also demonstrated a bioinspired crawler with multimode locomotion including overturning, rolling, and creeping, an adaptable robotic gripper for distinguished objects, and a series of reconfigurable electronic circuits including a multimode circuit, a human-interactive circuit, and a reconfigurable antenna.

LMA-Enabled Reprogrammable Magnetic Soft Composite
We selected hard-magnetic NdFeB microparticles as the magnetic filler due to their high-performance magnetism.To create the reprogrammable magnetic soft composite, the NdFeB microparticles were mixed into the InSnBi alloy and magnetized by a strong impulse magnetic field (>1 T).Then, the NdFeB@InSnBi slurry was sheared into the elastomer precursor with forming the reprogrammable NdFeB@InSnBi microparticles.It is noted that the magnetized NdFeB microparticles magnetically attract each other to form a porous structure that can prevent the liquid InSnBi alloy from separating from the NdFeB@InSnBi microparticles during shearing. [29,30]Finally, the obtained mixture was cured in a mold (Figure 1a).When heated above the melting point of the InSnBi alloy, the magnetic microparticles inside the liquid InSnBi shell can realign to the external magnetic field.During cooling, the NdFeB microparticles were immobilized with the solidification of the InSnBi alloy (Figure 1b), creating a new stable magnetization profile in the composite.Moreover, the magnetization profile can be reprogrammed by repeating the above heat-assisted reprogramming process, due to the reversible phase transition of the alloy.Therefore, the composite possessed the reconfigurable magnetic shape morphing.For example, a composite strip exhibited opposite deformations  1c-i) and after (Figure 1c-ii) magnetic programming in response to the same external magnetic field generated by a magnet.

Characterization of the Composite
The cross-sectional scanning electron microscopy (SEM) image of the composite shows that the NdFeB@InSnBi microparticles with a size of %20-100 μm were uniformly dispersed in the elastomer (Figure 2a).In addition, the energy-dispersive X-ray spectroscopy (EDS) mapping demonstrates that multiple NdFeB microparticles were encapsulated in the InSnBi shell for a single NdFeB@InSnBi microparticle (Figure 2b).These results confirm that we can create the metallic reprogrammable microparticles directly by shearing.This is in contrast to complicated preparation processes for the polymer-based reprogrammable microparticles such as grinding under liquid nitrogen or phase separation in polyvinyl alcohol aqueous solution.In addition, the differential scanning calorimetry (DSC) curves show that the phase change points of the In 51 Sn 16.5 Bi 32.5 alloy, NdFeB@In 51 Sn 16.5 Bi 32.5 slurry, and corresponding composite were all %62 °C (Figure 2c), suggesting the melting point of the InSnBi alloy was not affected by other components.We also evaluated the reorientation of the NdFeB microparticles in the slurry by measuring the surface magnetic flux density after heat-assisted reprogramming.Figure 2d shows the magnetic flux density increased from 1.40 to 2.45 mT when the NdFeB content increased from 5 to 15 wt%.However, the magnetic flux density decreased to 0.63 mT under 35 wt% NdFeB.This is because the higher content of NdFeB increased the viscosity and drag force of the slurry, making it more difficult to reorient the magnetic NdFeB microparticles. [31,32]Therefore, the slurry containing 15 wt% NdFeB and 85 wt% InSnBi was chosen to prepare the reprogrammable NdFeB@InSnBi microparticles.
We further explored the magnetic performance of the composite.The surface magnetic flux density of the programmed composite increased with increasing the content of the NdFeB@InSnBi microparticles and the intensity of the magnetic field, as shown in Figure 2e.The surface magnetic flux density maximum reached %1.3 mT when the composite contained 70 wt% NdFeB@InSnBi microparticles and was programmed by the external magnetic field of 500 mT.The magnetic capacity of the composite also increased with adding the NdFeB@InSnBi microparticles, as shown in Figure 2f.The composite with 70 wt% NdFeB@InSnBi microparticles (%10 wt% NdFeB microparticles) achieved the highest saturated magnetization value of 14 emu g À1 , remnant magnetization of 9 emu g À1 , and coercivity of 7100 Oe, respectively.Meanwhile, it also maintained high stretchability of %270% and low Young's modulus of %0.1 MPa at room temperature (Figure S1, Supporting Information).Therefore, the composite containing 70 wt% NdFeB@InSnBi microparticles was chosen for the subsequent fabrication of the magnetic soft robot.Moreover, we repeatedly reversed the magnetization directions of the composite and the magnetic densities were stable over 50 programming cycles (Figure 2g), indicating its good magnetic reprogrammability.It is noted that the NdFeB microparticles have poor corrosion resistance and may degrade underwater. [33]Fortunately, the InSnBi shell with stable chemical properties can protect the NdFeB microparticles from complex external environments.The magnetic flux densities of the NdFeB@InSnBi slurry (Figure S2, Supporting Information) and the composite (Figure 2h) remained constant for 50 days underwater, suggesting their good magnetic stability.This result also confirms that the metallic reprogrammable microparticles have the potential for compatibility with various substrates including hydrogel, whereas the reprogrammable microparticles based on the phase change polymers such as the water-soluble PEG may be disabled.As proof of concept, we created a magnetic hydrogel by shearing the NdFeB@InSnBi slurry into the polyvinyl alcohol solution before polymerization.We showed that magnetization profiles of the magnetic hydrogel can also be reprogrammed (Figure S3, Supporting Information) because the LMA cannot be dissolved in water.To our best knowledge, this is the first report that hydrogels were endowed with reprogrammable magnetization profiles, which holds promise for creating intelligent hydrogel machines.
Due to the low programming temperature, we can easily heat the composite into a programmable state using the hot plate or heat oven, which is accessible and practical.In addition, the lightinduced heating method can be utilized to achieve remote programming.To be specific, the reprogrammable microparticles can function as photothermal agents, allowing the composite to transform the light energy into thermal energy.For example, we applied infrared light to heat the composite.As the light power increased, both the rate of temperature increase and the equilibrium temperature also increased (Figure S4, Supporting Information).Under the light with a power of 0.63 W, the temperature increased from room temperature (%30 °C) to 70 °C in a short time (%10 s).We further demonstrated utilizing the infrared light to remotely program and activate a composite film, as shown in Figure S5, Supporting Information.These heating methods offer flexible options to deal with the potential various application scenarios.In the following experiments, we heated the composite using the hot plate.

Single Temperature Programming
For the conventional method of template-assisted magnetization, the deformed robot was magnetized to saturation under a strong uniform magnetic field (>1 T), resulting in a continuous magnetization profile in the robot. [1,34]Inspired by this, we can create the desired magnetization profile in the robot by combining the shape programming with heat-assisted reprogramming.For example, we fabricated a strip-like robot with a single programming temperature of 62 °C (Figure 3a and S6, Supporting Information).This robot was deformed into a temporary bent (Figure 3b) and twisted (Figure 3c) shape successively.The deformed robot was heated above its programming temperature and a constant upward magnetic field was applied to reorient the NdFeB microparticles.After cooling, the magnetic microparticles were immobilized by the solidified InSnBi alloy, creating a new magnetization profile corresponding to the temporary shape in the robot (Figure 3b,c).The programmed trip-like robot regenerated the bent (Figure 3d) and twisted (Figure 3e) shape upon the upward magnetic field.Due to the elastic resilience, the robot can rapidly recover to its original shape once removing the external magnetic field (Movie S1, Supporting Information).
Bioinspired soft crawling robots with high mechanical compliance demonstrate effective interactions with unstructured environments and humans, making them promising for various applications such as infrastructure inspection, environmental exploration, and biomedicine. [35]However, existing soft crawlers with a specific structure typically perform fixed locomotion mode, hindering their flexibility and functionality. [36]Herein, we designed a magnetic soft crawling robot with multimodal locomotion by connecting three reprogrammable body segments with two elastomer joints, as shown in Figure 3f.Inspired by the art of origami, this planar crawling robot was folded along the joints into different configurations during the heat-assisted reprogramming process.Three distinguished magnetization profiles were created in the robot successively (Figure 3g-i), and the robot achieved three corresponding locomotion modes including creeping, overturning, and rolling upon the dynamic magnetic field (Figure S7 and Movie S2, Supporting Information).Specifically, by applying the upward and downward magnetic field alternatively using a rotating magnet (Figure S7b, Supporting Information), the robot with a single magnetization direction (Figure 3g) performed a continuous overturning motion of %8 mm/cycle (Figure 3j), which can be used for climbing over obstacles.Alternatively, we programmed the middle segment and the two end segments with opposite magnetization directions (Figure 3h).The counterclockwise rotating magnet actuated the robot to curl a tubelike shape and achieve a rolling motion like a pill bug (speed of %10 mm/cycle), as shown in Figure 3k and S7c, Supporting Information.The transformation of a tubelike shape can be used to carry objects during movement.Furthermore, as the three body segments were programmed with different magnetization directions (Figure 3i), the robot exhibited a creeping motion like an inchworm (Figure 3l and S7d, Supporting Information).During a creeping motion cycle, the magnetic repulsion caused the middle and front segments to arch, and the back segment was pulled rightward.Subsequently, when the magnetic field faded or reversed, the robot spread to push the middle and front segments rightward due to the anchorage of the back segment.
The asymmetrical arching and spreading motions alternated, resulting in a subtle creeping movement (<1 mm/cycle).

Multitemperature Programming
Selective programming of local magnetization profiles is attractive to create the intricate magnetization profile but typically relies on a focused laser beam for local heating. [21,23,28]This required a high-cost laser heating system, and the penetration depth of the laser heating is limited. [12]Herein, we adjusted the programming temperature by changing the composition of the InSnBi alloy.Thus, we can fabricate the robot with multiple programming temperatures and selectively program its local magnetization profiles in different temperature ranges.
For instance, a cross-like robot composed of a lateral arm and a vertical arm was fabricated by combining two composites (In 55 Sn 25 Bi 20 , melting point: 47 °C and In 51 Sn 16.5 Bi 32.5 , melting point: 62 °C), as shown in Figure 4a and S8, Supporting Information.The magnetization profiles of the two arms were both in a programmable state when this cross-like robot was heated above the higher programming temperature (62 °C).During the cooling process, the magnetization profile in the vertical arm was fixed once the temperature dropped below 62 °C, whereas the lateral arm remained programmable until the temperature dropped below 47 °C.A constant external magnetic field encoded the two arms with identical magnetization directions (Figure 4b).However, when we reversed the magnetic field during cooling, the two arms were programmed with opposite magnetization directions (Figure 4c).As shown in Movie S3, Supporting Information, the arms with identical magnetization directions both bent upward under the upward magnetic field (Figure 4d), while the vertical and lateral arms with the opposite magnetization directions bent upward and downward, respectively (Figure 4e).Photographs showing the comb-like and octopus-like robot performing as a 2D-gripper (l) and 3D-gripper (m), respectively.Purple and black arrows represent the external magnetic field direction and the magnetization direction in the robots, respectively.The programming magnetic field is %300 mT and the actuation magnetic field is %200 mT.Scale bars: 5 mm for (d,e,l,m) and 3 mm for (g,h,j,k).
Furthermore, a more complex comb-like robot composed of six parallel arranged arms connected by a spine was demonstrated, as shown in Figure 4f.We encoded the comb-like robot with two magnetization profiles successively, similar to the crosslike robot.The six arms with identical magnetization directions performed uniform bending deformation upon the magnetic actuation (Figure 4g).However, when the adjacent arms were programmed with opposite magnetization directions, this planar comb-like robot can perform as a 2D gripper for ultrathin objects (Figure 4h and Movie S4, Supporting Information).Specifically, the adjacent arms bent in opposite directions to accommodate the paper object (%100 μm in thickness) under the rightward magnetic field, as shown in Figure 4l-i.Then, the arms clamped and carried the paper under the reversed external magnetic field (Figure 4l-ii).At the aimed location, the external magnetic field was reversed again, allowing the comb-like robot to release the paper (Figure 4l-iii).It is noted that the clamping force can provide sufficient static friction to manipulate the paper without causing any deformation.
Moreover, we reconfigured this planer comb-like into an octopus-like robot with a 3D structure by looping its spine (Figure 4i), leading to more complex shape morphing (Figure 4j,k).For example, the adjacent arms of the octopus-like robot bent outward and inward in response to the downward magnetic field, generating a flower-like structure (Figure 4k).The arms with the other magnetization profile bent outward or inward synchronously upon magnetic actuation (Figure 4j).In this case, the octopus-like robot can function as a 3D gripper for the manipulation of small objects such as a spitball (Figure 4m and Movie S5, Supporting Information).To be specific, the six arms bent outward under the downward magnetic field, opening to accommodate the spitball (Figure 4m-i).The arms bent inward to hold ) Circuit switching between "1-1" and "0-0" states upon magnetic actuation (mode 1).c) Circuit switching between "1-0" and "0-1" states upon magnetic actuation (mode 2).d) Schematic diagram of the human-interactive circuit.e,f ) Ga-based switch (programming temperature: 29 °C) being triggered by a glass rod (e) or a human finger (f ).g) Schematic diagram of the reconfigurable antenna.h,i) Schematic diagram of three deformation modes of the antenna (h) and corresponding S11 parameters (i).Purple, black, and green arrows represent the external magnetic field direction, the magnetization direction in the robots, and the applied force direction, respectively.Scale bars: 5 mm for (b,c,e,f,h).
the spitball for transport by applying the upward magnetic field (Figure 4m-ii), and then the spitball was released by reversing the magnetic field (Figure 4m-iii).Overall, the comb/octopuslike robot can perform as a soft gripper capable of adaptable grasping for distinguished objects by taking advantage of its magnetic reprogrammability and morphological reconfigurability.

Reconfigurable Electronic Circuits
In addition to its applications in soft robots, this reprogrammable composite offers the potential for the integration with electronic circuits to create reconfigurable electronics.To illustrate this, we designed a multimode circuit composed of a parallel circuit with two light-emitting diodes (LEDs) and a reconfigurable magnetic switch, as shown in Figure 5a.The magnetic switch was composed of two composite membranes (In 51 Sn 16.5 Bi 32.5 , melting point: 62 °C) jointed with Ecoflex film.Copper tapes adhered to the composite membranes for electrical conduction.The switch can be programmed with two magnetization profiles, resulting in two modes for the switch deformation and the circuit operation.In mode 1, the two membranes with a single magnetization direction exhibited the same deformation under the external downward or upward magnetic field, resulting in the turning off (0-0 state, Figure 5b-i) or on (1-1 state, Figure 5b-ii) the two LEDs.In mode 2, when the membranes were encoded with opposite magnetization directions, only one LED was turned on under the external downward (1-0 state, Figure 5c-i) or upward magnetic field (0-1 state, Figure 5c-ii).This circuit can be viewed as a two-bit memory with four distinct states, which can also be used in mechanical computation. [37]e further reduced the programming temperature of the composite by using pure gallium (Ga, melting point: 29.8 °C) instead of the InSnBi alloy.This composite can be programmed by body heat, offering a unique opportunity for humaninteractive electronics.For example, we designed a body heattriggered switch using the Ga-based composite and connected it in series with an LED circuit (Figure 5d).When subjected to a downward magnetic field, the switch opened the circuit (Figure 5e-i and f-i).Although we can use external objects (e.g., a glass rod) with room temperature to press the switch and turn on the LED (Figure 5e-ii), the switch was turned off upon magnetic actuation after removing the glass rod (Figure 5e-iii).However, when we pressed the switch with a finger, heat from the finger allowed the composite in the programmable state, and the magnetization direction was reversed by the external magnetic field (Figure 5f-ii).Thus, even after removing the finger, the circuit remained closed (Figure 5f-iii).Thus, this reconfigurable switch with body heat trigger may promise the development of intelligent systems for human-machine interaction.
Furthermore, we created a reconfigurable dipole antenna by patterning the liquid metal of eutectic gallium-indium (EGaIn, melting point: 15.7 °C) on a composite membrane (programming temperature: 62 °C) and encapsulating it with Ecoflex 00-30, as shown in Figure 5g and S9, Supporting Information.We can regulate the antenna resonant frequency by reconfiguring membrane shape under magnetic actuation (Figure 5h).The reflection coefficients (S11) corresponding to different antenna shapes were measured, as shown in Figure 5i.The original 2D antenna (Figure 5h-i) showed an 11.1 dB return loss at 1.22 GHz.By programming the composite with two magnetization profiles, the 2D antenna could transform into two folding configurations upon magnetic actuation.When a single fold was introduced, the resonant frequency of the antenna shifted from the original 1.22 to 2.4 GHz (mode 1, Figure 5h-ii).Additionally, the original resonant frequency of 1.22 GHz disappeared in the antenna with dual folds (mode 2, Figure 5h-iii).

Conclusion
In summary, we created a reprogrammable magnetic soft composite in a facile manner by shearing the magnetized NdFeb@InSnBi slurry directly into the elastomer precursor.We can reorient the magnetic microparticles and reprogram the magnetization profile of the composite at a low temperature utilizing the phase change of the InSnBi alloy.The programming temperature of the composite can be adjusted by changing the composition of the InSnBi alloy.In addition, we created the desired magnetization profiles by shape programming or selectively programming the local magnetization profiles.Utilizing the reprogrammable magnetization profile, a crawler with multimode locomotion including overturning, rolling, and creeping, a reconfigurable robotic gripper capable of adaptable grasping, and reconfigurable electronic circuits were demonstrated.
In addition to InSnBi alloy, other metals with lower melting points such as gallium-based metals can also be selected as the phase change materials to decrease the energy consumption and enable reprogramming the composite by body heat.Moreover, this magnetic reprogrammable mechanism has the potential to be applied to a wide range of functional materials such as shape memory polymers, liquid crystal elastomers, and hydrogels for reconfigurable intelligent material systems.We expect that the reprogrammable magnetic soft composites and the programming strategies in this work could inspire diverse applications in magnetic soft microrobots, reconfigurable electronics, and biomedical engineering.
Preparation of Composites: The NdFeB microparticles were mixed into the liquid InSnBi alloy by mechanical stirring at 70 °C and then magnetized using a strong impulse magnetic field (>1 T) at room temperature.The obtained NdFeB@InSnB slurry was sheared into part B of Ecoflex 00-30 at 70 °C to form NdFeB@InSnBi microparticles, followed by mixing with the equal part A of Ecoflex 00-30 at room temperature.This precursor of the composite was cured in a mold at room temperature for 4 h.The Ga-based reprogrammable composite was prepared at room temperature.
Preparation of the Reprogrammable Magnetic Hydrogel: 15 wt% PVA aqueous solution was heated at 90 °C for 2 h with mechanical stirring, followed by degassing at 80 °C for 2 h.70 wt% NdFeB@InSnBi slurry was sheared into the PVA solution at 70 °C.The as-prepared solution was then transferred into a mold.The reprogrammable PVA hydrogel was obtained by cooling down to À20 °C for 12 h and thawing at room temperature for 2 h.
Fabrication of the Reconfigurable LM Antenna: The LM antenna was patterned on a reprogrammable composite (40 mm Â 12 mm Â 0.4 mm) using utilizing an LM magnetic printing method that we developed in our previous work. [31]Specifically, a thin tape on the composite was fabricated into a shadow mask by laser engraving.The LM mixed with 5 wt% Ni microparticles was dropped on the mask followed by spreading it out using a magnet.Then, the excess LM was removed using a syringe, and the LM antenna was obtained after peeling off the tape mask.Finally, a subminiature coaxial cable connector was connected to the antenna and the antenna was encapsulated by Ecoflex 00-30 (0.4 mm in thickness).
Characterization: SEM image and EDS maps were obtained using a microscope system (UltraPlus Zeiss).Magnetic hysteresis loops were acquired at 25 °C using a Vibrating Specimen Magnetometer (Lakeshore 7407).Magnetic flux densities were measured with a commercial digital Tesla meter (TD8620, Tunkia).The surface temperature was measured using an infrared camera (FLIR E5).A differential scanning calorimeter (American TA DSC 250) was adopted to determine the thermal properties.S11 parameters of the antenna were measured using a vector network analyzer (Keysight E5063A ENA).

Figure 1 .
Figure 1.Preparation and working principle of the reprogrammable composite.a) Schematic diagram showing the fabrication and components of the composite.b) Heat-assisted reprogramming procedure based on the solid-liquid transition of the InSnBi alloy.c) Photographs showing a strip responding to the same external magnetic field before (i) and after (ii) magnetic programming.Scale bar: 5 mm.

Figure 2 .
Figure 2. Characterization of the reprogrammable composite.a) SEM image showing the cross section of the composite.Scale bar: 100 μm.b) Related element mapping of the elements including Si, Nd, Fe, In, Sn, and Bi.c) DSC curves of the In 51 Sn 16.5 Bi 32.5 alloy, NdFeB@In 51 Sn 16.5 Bi 32.5 slurry (15 wt% NdFeB and 85 wt% InSnBi), and composite (70 wt% NdFeB@In 51 Sn 16.5 Bi 32.5 ).d) Surface magnetic field density of the NdFeB@In 51 Sn 16.5 Bi 32.5 slurry (0.3 mm in thickness) after heat-assisted reprogramming (70 °C, 150 mT).e) Effect of the external magnetic field and the content of the NdFeB@InSnBi microparticles on the magnetic programming performance of the composite (0.4 mm in thickness).f ) Magnetization hysteresis loops of the composites (6 mm in diameter and 3 mm in thickness) with different contents of NdFeB@InSnBi microparticles at 25 °C.g) Surface magnetic density of the composite (0.4 mm in thickness) during 50 reprogramming cycles (70 °C, 300 mT).h) Surface magnetic density of the underwater composite for 50 days.

Figure 3 .
Figure 3. Single temperature programming.a) Strip-like robot (20 mm Â 5 mm Â 0.4 mm) with a programming temperature of 62 °C.b,c) Programming the (b) bent and (c) twisted robot.d,e) Schematic diagrams and photographs showing the bent (d) and twisted (e) shape of the robot upon the upward magnetic field.f ) Crawling robot with three body segments (8 mm Â 8 mm Â 0.4 mm) and two joints (2 mm Â 8 mm Â 0.4mm).g-i)Programming the robot with three magnetization profiles.j-l) Photographs of the overturning (j), rolling (k), and creeping (l) motion.Purple and black arrows represent the external magnetic field direction and the magnetization direction in the robots, respectively.The programming magnetic field is %300 mT and the actuation magnetic field is %200 mT.Scale bars: 5 mm for (d,e) and 8 mm for (j-l).

Figure 4 .
Figure 4. Multitemperature programming.a) Cross-like soft magnetic robot consisted of a lateral arm and a vertical arm (20 mm Â 2.5 mm Â 0.4 mm).b,c) Programming the robot using a constant (b) or changing (c) magnetic field during cooling.d,e) Magnetic actuation of the comb-like robot with the identical (d) or opposite (e) magnetization directions.f-k) Structure and magnetic actuation of the comb-like (f-h) and octopus-like (i-k) robots.(l,m)Photographs showing the comb-like and octopus-like robot performing as a 2D-gripper (l) and 3D-gripper (m), respectively.Purple and black arrows represent the external magnetic field direction and the magnetization direction in the robots, respectively.The programming magnetic field is %300 mT and the actuation magnetic field is %200 mT.Scale bars: 5 mm for (d,e,l,m) and 3 mm for (g,h,j,k).

Figure 5 .
Figure 5. Reconfigurable electronic circuits.a) Schematic diagram of the multimode circuit.b) Circuit switching between "1-1" and "0-0" states upon magnetic actuation (mode 1).c) Circuit switching between "1-0" and "0-1" states upon magnetic actuation (mode 2).d) Schematic diagram of the human-interactive circuit.e,f ) Ga-based switch (programming temperature: 29 °C) being triggered by a glass rod (e) or a human finger (f ).g) Schematic diagram of the reconfigurable antenna.h,i) Schematic diagram of three deformation modes of the antenna (h) and corresponding S11 parameters (i).Purple, black, and green arrows represent the external magnetic field direction, the magnetization direction in the robots, and the applied force direction, respectively.Scale bars: 5 mm for (b,c,e,f,h).