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Page 2 of 26 Xiao et al. Soft Sci 2023;3:11 https://dx.doi.org/10.20517/ss.2023.03

areas, owing to the increasing demand for flexibility, reliability, controllability, and intelligence in
next-generation actuation devices [4-12] . Liquid crystal elastomers (LCEs) are a class of attractive soft active
polymers, with excellent actuation capabilities under various types of stimuli, e.g., heat, light [13,14] , and
electric field, among others. Crosslinked by rigid liquid crystal (LC) mesogens and flexible polymer
networks, LCEs show many outstanding mechanical performances, such as rubber elasticity [4,15,16] , shape
[16]
memory effect [17-19] , and high uniaxial/biaxial actuation strains . The alignment of the mesogens in the
LCEs could be orientated along a specific direction by forces and various external fields [21,22] , allowing their
[20]
transformation into the monodomain state. Moreover, the alignment could be reoriented by physical [19,23,24]
or chemical stimuli, causing a phase transition from the monodomain state to the polydomain state .
[4]
[25]
[26]
According to the types of LCs, LCEs could be classified into three categories, including nematic and
[27]
smectic LCEs, with different functions and actuation capabilities. To date, various fabrication and
alignment methods of LCEs have been reported [28-30] , motivating the applications of LCEs in soft robotics ,
[31]
[33]
[32]
[34]
sensors , and biomedical devices , among others . While the fabrication /alignment methods and
thermal/optical actuation capabilities of LCEs have been discussed thoroughly in several recent reviews, an
overview of various design schemes, actuation strategies, and their wide-ranging applications is still lacking.
In this review, we focus on recent advances in the fundamental and applied studies of LCEs, covering the
fabrication methods, design schemes, actuation mechanisms, and diverse applications, as illustrated in
Figure 1. The second section presents a brief introduction to the fabrication and alignment methods of
LCEs, as these aspects have been addressed in detail by several comprehensive reviews [4,28,30,31,35-37] . The third
section summarizes mechanical responses and actuation strategies triggered by various types of stimuli (e.g.,
heat, light, electric/magnetic field, swelling, etc.). The fourth section discusses recent advances toward
practical applications, including soft robotics, temperature/strain sensors, and biomedical devices, among
others. Finally, perspectives on scientific challenges and opportunities for future research are provided.

FABRICATION OF LCES
Currently, two main types of fabrication methods have been used to fabricate LCEs. The most widely
adopted method is the two-stage thiol-acrylate Michael addition and photopolymerization (TAMAP)
reaction, originally proposed by Küpfer and Finkelmann and later on, developed by Yakacki et al.
[55]
[Figure 2A] . In the first stage, a weakly crosslinked LCE is prepared and then stretched under a
[56]
mechanical loading for the alignment of LC mesogens. In the second stage, the weakly crosslinked LCE is
exposed to UV light to fix the temporary alignment. When the fixed alignment is at the monodomain state,
the LCE possesses a two-way shape memory effect, which could be used for reversible actuation. Based on
this method, 3D/4D printing techniques were developed, where the shear stress was usually exploited for
[57]
the alignment [Figure 2B] . The mesogens could be aligned along the printing path during the
high-operating-temperature direct ink writing (HOT-DIW) process of viscous inks. Based on these 3D/4D
printing techniques, the LCE architectures could be programmed on demand to allow reversible 2D-to-3D
deformations when heated from room temperature to the nematic-isotropic transition temperature.
Additionally, the shear stresses have also been used in the fiber electrospinning [29,58] , drawing , or
[54]
microfluidic aligning [59,60] process, aiming to fabricate monodomain LCE fibers. As an example, Roach et al.
drew very long (~1.5 m) fibers through a nozzle onto a rotating mandrel [Figure 2C] . Such fibers offer
[54]
excellent mechanical performances (2 MPa stiffness, 51% actuation strain, and over 100% failure strain). In
Figure 2D, Ohm et al. used a microfluidic setup to inject the LCE solution into a co-flowing stream of
[59]
silicone oil and fabricated highly oriented fibers .

The other mainstream method is the one-pot synthesis method, in which the mesogens in the former
[61]
solution are aligned before the crosslinking reaction and curing process. In Figure 2E, Zeng et al. used a