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Xiao et al. Soft Sci 2023;3:11 https://dx.doi.org/10.20517/ss.2023.03 Page 7 of 26

[55]
Historically, Finkelmann et al. demonstrated, for the first time, the thermal actuation of LCEs, by
preparing a uniaxially aligned monodomain LCE through mechanical alignment. More versatile
self-adaptive deformations can be achieved by altering the strategic regions of alignment and their
directions. Figure 4A shows that the flat LCE film, when integrated with a non-actuation material, can
realize reversible thermal deformations into various 3D shapes, such as tubes, helices, and waves . More
[79]
precisely-controlled 3D shape morphings can also be achieved [18,23] . Figure 4B shows a disulfide LCE film
fabricated by the embossing method. Here, the film is compressed between male and female molds under
100 °C. The disulfide bonds can be cleaved into thiyl radicals at high temperatures, thus temporarily
removing the shape change. When the temperature cools down, the disulfide bonds are formed again, thus
emerging the embossed pattern. Moreover, the 2D-to-3D shape morphing actuation can be archived by the
origami-inspired designs (or the designs inspired by topological defects) [19,80] . A miura-origami-based LCE
[81]
film is aligned by the azobenzene-based photoalignment method, enabling 100 distinct director
orientations in an area of 100 µm by 100 µm. The film can achieve self-folding and unfolding upon heating
and cooling. Figure 4C presents a curing-based shape morphing method through 4D-printing techniques,
[19]
enabling more degrees of design freedom in the fabrication . The patterning precision for the embossing
and the azobenzene-based photoalignment methods is ~0.1 mm. Smaller characteristic sizes of LCE
structures can be realized through the template-induced alignment technique [62,82,83] . Using this method, LCE
films with 1 μm microchannel patterns (or topological defects) can be fabricated. The film has excellent
[84]
mechanical performances, and the actuation strain can reach ~50% . Moreover, the film can lift over 700
times its own weight .
[85]
The thermally actuated LCEs were widely used in practical applications because of the simple control and
the easy synthesis. However, the nematic-isotropic transition temperature of the LCE with thermal
actuation is typically from 70-80 °C , thus limiting its applications under room temperature (20-22 °C) or
[86]
human-body temperature (~37 °C). Recently, research showed that LCEs could be heated by the
surrounding body temperature or the human-body temperature [52,87] , thus broadening its application in the
bio-devices or the drug delivery system. However, the thermally actuated LCEs usually rely on thermal
[88]
conduction and convection for heating, but the response time (e.g., 10-100 s) is a bit long [88-91] . He Q et al.
fabricate the water channel inside the LCE film in Figure 4D, and the LCE film is actuated by the heat
transferring under the water heating/cooling process. The actuation time for the thermal LCE actuator is
~25 s, and the corresponding cooling time is ~200 s.

Optical actuation
By the incorporation of nano-phase materials into LCEs, the LCE-based composites can absorb light almost
over the wide spectrum range and convert the absorbed light into heat to achieve fast and precise
remotely-controlled actuation. The optical actuation methods can be classified into the photothermal
method and the photochemical method. The photothermal LCEs transform the absorbed light into heat,
which results in macroscopic actuation. The photochemical LCEs rely on the isomerization method, which
is a molecular scale motion upon light exposure in a certain range of wavelength.

The photothermal actuation can be realized by incorporating nano-phase materials [e.g., carbon nanotubes
(CNTs), graphenes, metal nanoparticles, dyes, and conjugated polymers (CPs)]. Figure 5A shows a LCE/
CNT fiber (with a modulus of 40 MPa along the long axis) that offers excellent actuation performances .
[92]
Its photothermal response time is within 10 s. The LCE/CNT fiber can absorb light almost over a wide
spectrum range [93,94] . The LCE can incorporate the RMGO (reduced chemically modified graphene oxide) to
convert light to thermal. According to reported experimental results, an actuation temperature of ~ 122 °C
and a contraction of 30% can be achieved. The metal nanoparticles (e.g., nano-Au or nano-Ag) can be
incorporated into LCEs to enable excellent optical actuation performances and versatile actuation