Page 79 - Read Online

Page 16 of 25 Lin et al. Soft Sci 2023;3:14 https://dx.doi.org/10.20517/ss.2023.05

©
magnetic field generated by a permanent magnet (bottom). Reproduced with permission from Ref. [160] . Copyright 2018. Springer
Nature; (D) An origami crawler with a four-unit Kresling origami structure and four magnetic plates. Reproduced with permission from
©
Ref. [161] . Copyright 2022. The American Association for the Advancement of Science; (E) A reprogrammable magnetic soft robot. Left
frame: schematic illustration of the heat-assisted 3D magnetic programming method. R.T., room temperature. Right frames:
©
illustrations and images of various 3D shapes. Reproduced with permission from Ref. [164] . Copyright 2020. The American Association
for the Advancement of Science; (F) A biodegradable robot based on soft-magnetic composites. Left frame: Schematic illustration of
the multi-legged array and nanofiber-constructed body. Right frame: mechanical analysis of a single leg. Reproduced with permission
©
from Ref. [170] . Copyright 2022. Elsevier. EL: Eudragit L; ES: Eudragit S; NdFeB: neodymium iron boron.

Another effective means to broaden the motion modalities of soft robots based on hard-magnetic
composites is to prepare multiple hard-magnetic composites with uniform/nonuniform magnetizations and
assemble them with controlled position and direction. Figure 5B shows a method of manufacturing
untethered magnetic soft robots using modular magnetization units embedded into a network of adhesive
[154]
sticker layers . Each unit contains NdFeB particles (average diameter of 38.0, 75.0, or 150.0 μm),
magnetized into either uniform or nonuniform magnetization profiles by the template-assisted method.
Selectively sticking the units onto a double-sided adhesive (i.e., polyetherimide (PEI) tape) forms the soft
robot with complex patterns, with a planar resolution of 40 μm. After cutting out from the tape, the
magnetic torque induced by the actuating magnetic field will deform the robots into various 3D geometries.
In areas without magnetic particles, their shapes remain flat under a magnetic field, thereby providing the
desired spaces for further integration of other functional modules, such as temperature/UV sensors and
radio frequency identifiers.

The above examples mainly exploit 2D patterns with programmed magnetizations to yield multimodal
locomotion. Constructing soft robots in sophisticated 3D geometries can further broaden the motion
modalities [155-159] . Soft robots based on magnetic composites are compatible with 3D printing techniques, as
the composites usually undergo a precured condition that can be printed through a nozzle. Figure 5C
illustrates an advanced 3D printing process that allows magnetization of the composite during the
[160]
construction of the 3D structure . Here, the ink used for printing is a mixture of NdFeB particles, silica
nanoparticles and an uncured elastomer matrix. Controlling the weight ratio of silica nanoparticles enables
adjustment of mechanical properties (e.g., shear thinning, shear yielding) of the ink to meet the
requirements for printing. Applying a magnetic field generated by a permanent magnet or an electromagnet
placed around the dispensing nozzle can reorient the NdFeB particles during the printing process.
Therefore, this strategy can control the position and direction of magnetization during the construction of
various 3D structures. The resulting 3D soft robots exhibit many advanced geometries and motion
modalities, ranging from a thin-walled structure that can elongate in its diagonal direction to a set of auxetic
structures that can shrink in different directions, and to a hexapedal structure that can warp, roll, and hold
objects.

Although soft robots with programmed magnetization exhibit a diverse set of motion modalities, the
relatively low modulus (typically ranging from tens of kPa to dozens of MPa) of the composite materials
limits the ability of the robots to overcome the large environmental resistance introduced by confined
spaces. Designing origami structures to cooperate with distributed magnetic programming represents an
[161]
effective approach to tackling this issue. Figure 5D shows a crawler that exploits a four-unit Kresling
origami structure to generate axial contraction under either torque or compressive force [162,163] . Rational
design allows the structure to cancel out internal twists for efficient straight motion. Specifically, the
structure incorporates four magnetic plates made of a mixture of silicone elastomer and hard-magnetic