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Page 12 of 25 Lin et al. Soft Sci 2023;3:14 https://dx.doi.org/10.20517/ss.2023.05

SOFT ROBOTICS BASED ON MAGNETIC NANOMEMBRANES/NANOSTRUCTURES
Another significant application of magnetic nanomaterials is in the field of robotics. Magnetic actuation
offers advantages over other actuation strategies such as light, thermal, and electric, including remote and
wireless operation, which eliminates the need for optical fibers and electrical wires. Additionally, static and
low-frequency magnetic fields do not attenuate through natural tissue and organs, making them suitable for
use in biomedicine.

Magnetic nanomembranes in soft magnetic materials, such as iron, can be deposited and patterned using
conventional microelectronic processes. Robots based on these materials, therefore, can have overall
dimensions down to microscale, with promising applications in minimally invasive surgery and
[133]
micromanipulation. Figure 4A shows an untethered magnetic gripper for single-cell manipulation . The
gripper involves a bilayer of silicon monoxide and silicon dioxide (thickness: 30 nm in total) with internal
stress mismatch to bend planar pattern into 3D geometries, a thin iron layer (thickness: 100 nm) for remote
control under a magnetic field, and a thermally responsive layer made of wax for on-demand activation
[middle frame of Figure 4A]. The gripper can be controlled remotely to navigate through narrow conduits
and to fix tissue sections ex vivo. Compared with previously demonstrated grippers [134-136] , the gripper
demonstrated in this example has a tip-to-tip size of 70 μm when open and 15 μm after folding. The
dimension is comparable to the size of human arterioles (diameter: 50-300 μm), making the gripper suitable
for future in vivo applications of cell capture or excision at the single-cell level.

Magnetostrictive materials can deform under the action of external magnetic fields and, therefore, represent
promising candidates for constructing magnetic-controlled robots [137,138] . Typical magnetostrictive materials
involve Fe-Co-Ni-rich alloys, Terfenol-D, and Galfenol [139-141] . These materials are also compatible with
conventional microelectronic processes and can be deposited through magnetron sputtering coating. The
example shown in Figure 4B proposes a method of directly depositing a magnetostrictive nanomembrane
(Fe Co alloy; thickness: 100 nm) on a flexible microfiber of spider silk thread (diameter: 9 ± 5 µm) in a
50
50
scalable fashion. The resulting magnetostrictive fiber retains excellent properties in mechanical robustness,
[142]
electrical conductivity, as well as magneto-mechanical coupling . For example, the fiber can maintain the
original mechanical characteristics of spider silk to prevent irreversible plasticization, and exhibits
capabilities in lifting loads up to 0.12 mN (maximal stress: ~10 MPa) under a magnetic field of ~0.1 T.
These features allow the magnetic silk threads to serve as a basic component in soft robotics.

A noteworthy point of nanoscale magnetic materials is that their coercivities and remanences show
dependence on their sizes [143,144] . As shown in the coercivity curve in Figure 4C, the particles in cobalt have
almost no remanence and coercivity at sizes smaller than 100 nm (regarded as superparamagnetic). When
the size of cobalt is larger than the critical size (100 nm) but remains within hundreds of nanometers, the
particles show single-domain characteristics, including high remanence and the positive correlation
between coercivity and size. At scales larger than 500 nm, magnetic particles in cobalt exhibit multidomain
features, where the coercivity reduces with the magnet size. Based on the effect that the coercivities of
nanomagnets change with their sizes, researchers have developed a magnetic programming strategy that can
[53]
encode multiple deformation instructions into magnetic robots . The four-panel robot in the left frame of
Figure 4C is programmed by arranging panels with nanomagnets in different sizes (panel I: 520 nm × 60
nm, illustrated with red color; panel II: 398 nm × 80 nm, illustrated with blue color). Due to the positive
correlation between coercivity and size, the robot can be coded through a series of magnetized fields in
different intensities. For example, a magnetic field larger than 140 mT in one direction magnetizes both
panel I and panel II. A subsequent magnetic field between 90 and 140 mT in the opposite direction reverses
the magnetization of panel II. The intensity of the subsequent magnetic field (< 140 mT) is not sufficient to