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Xiao et al. Soft Sci 2023;3:11 https://dx.doi.org/10.20517/ss.2023.03 Page 15 of 26

Owing to the relatively large actuation stress, high actuation strain, and versatile actuation modes, LCEs can
be used as grippers at different length scales [44,88,143] . Figure 8F presents a LCE-based tubular actuator with
[45]
multiple actuation modes powered by electricity . The gripper, capable of grasping and releasing 50 g vials,
is composed of LCE actuators, a fixing plate, and a microcontroller. The gripper can be fabricated in
micro-scale size via various methods (e.g., soft lithography, 3D printing, embossing, et. at.) [141,144,145] .
3
Figure 8G shows a photothermally actuated microgripper (20 × 200 × 20 µm ) fabricated by soft
lithography . This microgripper recognizes objects by the response to certain spectra from the resonant
[141]
laser lines of the target material. The microgripper only grabs the recognized material.
Specially prepared LCEs can change their shape and colors spontaneously, which can be used to develop the
LCE-based camouflage robot [102,142,146-149] . Figure 8H shows an example based on the LCE that incorporates
[142]
tetraphenylethene and spiropyran moieties. The resulting robot resembles a caterpillar and can walk (by
shape-morphing) and camouflage (by discoloration) in response to light with different wavelengths.

Temperature/Strain sensors
The nematic LCEs could change transmittance following the phase transition induced by mechanical or
thermal loadings, which have been exploited to develop temperature or strain sensors. LCEs are opaque at
the nematic polydomain state and become transparent when heated into the isotropic phase. Based on this
optical behavior, a temperature sensor with integrated a light intensity sensor, light-emitting diodes (LEDs),
and a buzzer indicator was developed, as shown in Figure 9A . When the environment temperature
[150]
changes, the light intensity across the LCE film changes and is measured by the light intensity sensor. Based
on the magnitude of the temperature, the LEDs display different colors (red, yellow, and green). When the
temperature reaches a dangerous level, the LEDs turn red and the buzzer indicator sounds to inform the
users. Based on the transmittance changes of LCEs caused by thermal and mechanical loadings, a strain
sensor consisting of a LCE cantilever actuator and an infrared radiation (IR) LED [Figure 9B] was
[14]
prepared . The sensor possesses two modes of bending deformations, including thermally induced and
nonthermally induced modes. In the thermal bending mode, the beam becomes more transparent at a
higher temperature, resulting in a stronger optical signal, and this optical signal becomes stronger with the
increase of the bending strain. In the nonthermal bending mode, the LCE beam is almost opaque at the
nematic state, yielding a weaker signal than the case of the thermal bending mode. According to the
measured optical signals and temperature history, the bending deformation can then be determined. In
[151]
Figure 9C, Wei et al. reported a photoresponsive device composed of LCE, graphene-doped
polydimethylsiloxane (PDMS), and polyvinylidene fluoride (PVDF) layers, which can effectively convert
photothermal and mechanical energies into electricity. It is noteworthy that the device could be powered by
near-infrared radiation (NIR). Because of the temperature fluctuation induced by NIR light, the bending
angles are time-varying, which can be measured for temperature sensing. Figure 9D presents LCE-LM
[152]
coaxial fibers consisting of LCE fibers with a hollow structure and LM in the core . Through Joule
heating, the inner LM can generate heat to actuate the outer LCE shell. When the LCE–LM fiber is
uniaxially stretched, the resistance of the inner LM changes, thereby allowing us to determine the extension
of the fibers or the mass of loading from the resistance change of the inner LM.

Unlike the nematic LCEs, cholesteric LCEs usually exhibit color changes under mechanical or thermal
loadings, and this feature can be used for strain/temperature sensing. For example, a strain sensor was
designed based on a stretchable and highly uniform cholesteric LCE film [Figure 9E] . When the film is
[48]
under a bending strain with zero Gaussian curvature, the film turns blue gradually. Differently, when the
film is under a bending strain with nonzero Gaussian curvature, the two parts separated by a gap show
different colors (e.g., red and blue). By monitoring the color changes, the local bending strain could be
determined, and the deformation modes with different curvatures could be distinguished. Additionally, the