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Lin et al. Soft Sci 2023;3:14 https://dx.doi.org/10.20517/ss.2023.05 Page 17 of 25

particles (i.e., NdFeB), each of which locates between the two units. Adjusting the volume fraction of the
magnetic particles can tune the magnetization density, thereby affecting the direction and magnitude of the
torque. Such design strategies enable the programming of the torque distribution on the Kresling crawler
under magnetic fields. As a result, the magnetic field can regulate the stiffness of the crawler along its axis
due to the programmable torque on each Kresling unit. In the case that all the units contract
simultaneously, the actuation force during crawling is sufficient to overcome the large environmental
resistance. Using magnetic actuation, the crawler has the capability to steer its trajectory through rotation,
suggesting promises for many medical applications such as navigation in narrow spaces inside the human
body.

Many soft robots based on hard-magnetic composites leverage programmable magnetization position,
direction and intensity to achieve various locomotion, but their magnetization profiles remain constant and
cannot be reprogrammed to adapt to different applications. At elevated temperatures (i.e., above Curie
temperature), magnetic particles can demagnetize. Therefore, simultaneously controlling the distributions
of temperature and magnetic field over the hard-magnetic composites can enable reprogrammable shape
[164]
transformation of magnetic soft robots . Figure 5E shows a soft robot based on a mixture of chromium
dioxide (CrO ) microparticles (average diameter: 10 μm) and a PDMS elastomer. A laser can heat a specific
2
region of the composite to 118 ℃ (above Curie temperature) to demagnetize the magnetic particles.
Applying an external magnetic field during cooling can reprogram the magnetic domains. As shown in the
right frames of Figure 5E, multiple steps of laser heating and magnetization yield discrete magnetization in
various 3D directions across the bodies and extremities of the robots. Such magnetization profiles allow the
robot to exhibit sophisticated 3D deformations under a constant vertical magnetic field (intensity: 60 mT).
The thermal-assisted strategy enables magnetic reprogramming at the microscale, with spatial resolutions
up to ~38 μm, showing great potential in microrobots for minimally invasive medical applications. Besides
[165]
laser heating, magnetothermal effect also enables reprogrammable shape conversion of soft robots .
Compared with hard-magnetic particles, the coercivity and the remanence of soft-magnetic materials, such
as iron and nickel- or silicon-based alloys of iron, are relatively low, leading to small magnetic hysteresis
and instability under interference. However, the high magnetic susceptibility and saturation magnetization
of soft-magnetic materials make them highly sensitive to magnetic fields and easy to be magnetized, thereby
creating many opportunities for robotic applications [166-169] . Figure 5F shows a biodegradable soft magnetic
millirobot (Fibot) where the body exploits a core-shell structure in drug-coated nanofiber (core: Eudragit L
(EL) 100 and drug2; shell: Eudragit S [ES] 100) and the legs are based on magnet-drug composites EL 100-
55 containing iron particles and drug1) . The soft-magnetic property of iron particles renders a positive
[170]
correlation between the magnetization of the legs and the strength of the external magnetic field, and
enables stable and controllable actuation of Fibot. The right frame of Figure 5F illustrates the mechanical
analysis of a single leg. The angle difference θ between the easy magnetization axis of the leg i and the
direction of the applied magnetic field causes a magnetic torque T which keeps acting on the leg until the
Mi
difference disappears. Placing a permanent magnet nearby and moving it in a predesigned trajectory allows
[171]
the Fibot to achieve various locomotion , such as flap-wave or inverted-pendulum motion, realizing
better movement in the complex environment inside the human body. Due to the high magnetic
susceptibility of soft-magnetic materials, Fibot enables the embedded iron particles to be well-arranged in
an orderly manner along the leg during the fabrication. The resulting benefits include improved efficiency
and quality in device manufacturing, and rapid and precise response in vivo, even under a small magnetic
field.