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Page 10 of 26 Xiao et al. Soft Sci 2023;3:11 https://dx.doi.org/10.20517/ss.2023.03

irradiation by a 366-nm laser beam. The mechanical stress generated by photo-irradiation reached about
220 kPa, similar to the contraction stress of human muscles (~300 kPa).

The optical actuated LCEs can easily achieve the remote control. However, the thickness for the optically
actuated LCEs is typically in the range of 10-100 μm [41,113-115] . When the optically actuated LCEs are too thick,
the light will be difficult to fully penetrate the opaque LCE, leading to undesired inhomogeneous
deformations along the thickness direction. The photothermal LCEs undergo severe temperature change
(> 30 °C) upon actuation ; thus, the photothermal LCEs are not suitable for biomedical applications. The
[106]
photochemical LCEs are actuated with a negligible temperature change. However, the photosensitive
molecules for preparing the photochemical LCEs are limited because they only include azobenzene (azo)
and its derivatives. Furthermore, the photochemical LCEs respond under selected wavelength ranges
[116]
(320-380 nm and 400-450 nm) .
Electric actuation
[118]
[117]
The LCEs integrated with the conductive wires or conductive polymer layers can be actuated by
resistive heating. Usually, the conductive wires can realize local actuation of LCEs at selective regions.
Figure 6A shows free-standing ultrathin (~2.6 µm thick) serpentine-shaped wires attached to the LCE
film . The generated Joule heat (~0.237 W) from the wires (under a voltage of 8.2 V) induces 24%
[42]
[45]
shrinkage along the longitudinal direction. He Q et al. demonstrated the integration of LCE with
conductive wire to fabricate an actuator capable of multi-directional bending under a voltage of 1.5V. In
comparison to actuators made of dielectric elastomers , the low-voltage actuation of LCE/wires represents
[119]
an advantage. The conductive layer can also be made from organic materials, such as
[118]
poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) . The deformation of the bilayer film reaches
-1
20% actuation strain and 18% s strain rate.
Ferroelectric LCEs can be actuated by electricity due to the electroclinic effect [21,120] , in which the smectic
layer tilts under an electric field. Figure 6B shows the schematic diagram illustrating the electroclinic
effect . Ferroelectric LCEs offer a fast actuation, with a response time on the order of 10ms. Ferroelectric
[121]
LCEs are typically much more rigid than most nematic LCEs. Ferroelectric LCEs act as a strong resistance
to the dielectric realigning force, with high electric field strength (e.g., 1.5 MV·m ) and low actuation
-1 [21]
strain (4%).

[125]
Composites that integrate LCEs with liquid metals (LMs) through embedded channels [122-124] and vapor
can also achieve electrically-induced actuation. Figure 6C shows LCE-based coaxial fibers with LMs in the
[123]
center . The fiber has a large actuation strain (nearly 50%) upon the Joule heating. Figure 6D shows the
LM vapor trapped in the LCE network . Under the actuation through Joule heating, LCEs/LM can be
[125]
actuated at rates faster than 2 Hz, and cycled to 50% reversible strain by 15,000 times at 0.007 Hz (and 2.5%
reversible strain by > 100,000 times at 1 Hz) while retaining > 90% of its original actuation strain.

Based on the customized circuit pattern, the LCEs with electric actuation could be easily controlled to
achieve complex shape transformation through strategic local deformations. The speed of the electric
actuation is faster than that of the thermal conduction and convection, with the response time as low as
[126]
~0.01 s . Compared with the optically actuated LCEs, the local deformation of the electrically actuated
LCEs could be controlled more precisely by adjusting the voltage. However, excessive electric heating is very
[125]
common to induce material failure, i.e., the delamination between the LCEs and wires/polymer layers . It
is difficult to fabricate a miniatured actuator by the integrated LCEs with the LMs because the size of the
integrated LCEs is around a millimeter scale.