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

reverse panel I due to the higher coercivity associated with the larger size. Such programming processes
allow the robot to possess distinct magnetization directions at different positions, thus generating complex
deformations under the trigger of the magnetic field. Additional design strategies to achieve more
complicated deformations include constructing nanomagnet arrays with more than two aspect ratios and
directions on multiple panels and conceiving sophisticated planar layouts that resemble recognizable
objects. As an example, the bottom right frame of Figure 4C shows an origami microscale ‘bird’ consisting
of nanomagnet arrays with eight different configurations. The ‘bird’ can exhibit morphological changes that
resemble flapping under a magnetic field with a fixed direction and varied intensity (from 1 to 10 mT), and
hovering under a rotating magnetic field with a fixed intensity of 1.5 mT.

In conclusion, magnetic nanomembranes afford remote controllability for many soft structures, thereby
supporting applications in biopsy sampling, targeted drug delivery, artificial muscles, and others. Magnetic
materials with other nanoscale structures, such as nanorods in cobalt, exhibit size-dependent coercivity that
can be used for programmable magnetization. These advantages suggest promise for the development of
soft robots with multifunctionality and multimodality.

SOFT ROBOTICS BASED ON MAGNETIC COMPOSITES
Compared with magnetic nanomembranes, magnetic composites—mixtures of magnetic particles/NWs and
polymer matrices—exhibit lower modulus and, sometimes, intrinsic stretchability to enable more complex
motion modalities. Unlike magnetic nanomembranes that exploit conventional lithography and deposition
processes, manufacturing of magnetic composites usually adopts 3D printing, soft lithography, and laser/
mechanical cutting [132,145-149] . The overall dimensions of soft robots based on magnetic composites are usually
in millimeter or centimeter scale, larger than those constructed with lithographic techniques.

One advantage of magnetic composites is that their mechanical and magnetic properties can be tailored for
different applications. The polymer matrix determines the mechanical properties, while the magnetic
particles/NWs play a major role in magnetic properties and motion modalities. Typically, magnetic particles
can be classified into three categories: hard-magnetic (large hysteresis, high coercivity and remanence), soft-
magnetic (small hysteresis, low coercivity and remanence) and superparamagnetic (no hysteresis, zero
remanence) , depending on the magnetic hysteresis loop.
[150]
Hard-magnetic particles retain constant magnetism once magnetized, because of their large magnetic
hysteresis characterized by high coercivity and high remanence. The magnetic moments in both isotropic
(e.g., NdFeB particles) and anisotropic (e.g., platelet-shaped barium hexaferrite particles) hard-magnetic
[58]
particles can act as distributed and stable actuation sources to induce bending and rotation of the robots .
Furthermore, the remanent magnetization within the composite matrix can be programmed in a
nonuniform format through template-assisted magnetization, maskless lithography, modular assembly, and
other approaches to enable complex deformations [121,151,152] . Figure 5A demonstrates a magneto-elastic
multimodal millimeter-scale robot composed of a silicone elastomer (i.e., Ecoflex 00-10) doped with hard-
magnetic NdFeB particles (average diameter: 5 μm) , with a single-wavelength harmonic magnetization
[153]
profile along its body. The magnetization process starts with wrapping a rectangular composite on a
cylinder. Applying a strong magnetic field of 1.65 T along the radial direction of the cylinder magnetizes the
ring-shaped composite. Unwrapping the composite yields a flat membrane (length: 3.7 mm, width: 1.5 mm,
thickness: 0.185 mm) with harmonic magnetization. Such a configuration enables different modes of
locomotion when combined with spatiotemporal control of the actuating magnetic fields. For example, a
periodic magnetic field is able to sequentially adapt the robot’s tilting angle and curvature, making it walk in
a desired direction; while a rotating magnetic field can produce a longitudinal traveling wave to propel the
robot along the direction of the wave, allowing it to crawl across obstacles.