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Xiao et al. Soft Sci 2023;3:11 https://dx.doi.org/10.20517/ss.2023.03 Page 5 of 26

As the most widely adopted synthesis method, the two-stage synthesis method shows advantages in the
fabrication of monodomain LCEs with feature sizes [45,63] . The alignment of the LCEs is usually implemented
using mechanical stretching and other relevant techniques (e.g., 4D printing and electrospinning), whose
[47]
pattern precisions are usually above 20 μm . Therefore, the complex alignment pattern of the LC mesogens
in a small local region (~10 μm) remains a challenge for the two-stage synthesis method. The one-pot
synthesis method is usually adopted to fabricate thin LCE films (e.g., 10-50 μm in thickness) with complex
alignment patterns [64,65] . However, the LCEs with large sizes (> 1 cm) are difficult to fabricate using the
one-pot synthesis method. Except for the LCEs with regular covalent bonds that are fabricated by the
one-pot or two-stage synthesis method, the LCEs with dynamic covalent bonds were initially reported by
[66]
Pei Z et al. , based on reversible and dynamic chemistries. Different from the LCEs with regular covalent
bonds, the LCEs with dynamic covalent bonds can be reshaped or reprogrammed by mechanical
deformations above the topology-freezing transition temperature (i.e., an activation barrier temperature for
[67]
the fast breaking and reforming of the ester bonds) . Despite the reprogramming capability, the LCEs with
dynamic covalent bonds are difficult to be designed with complex patterns in a small local region (~10 μm)
because the dynamic exchange reaction for the reprogrammable LCEs needs mechanical stretching, whose
precision for the alignment is above 100 μm.

DESIGN AND ACTUATION MECHANISMS OF LCES
Mechanical responses
LCEs are composed of LC mesogens (i.e., rod-like molecular segments) connected to a flexible polymer
network. Figure 3A shows that the mesogenic groups can be characterized by the type of ordering, such as
the ordered (smectic or nematic) phase and the disordered (isotropic) phase . The smectic LC means the
[32]
molecules are positionally ordered along one direction. The nematic LCEs means that the LCEs molecules
are aligned in the same direction, and the nematic LCEs are usually classified further into the fully-enforced
alignment (monodomain LCEs) and the macroscopically-unaligned alignment (polydomain LCEs) . The
[38]
representative micro-structures of LCEs are illustrated in Figure 3B. Meanwhile, LCEs can be divided into
the main-chain LCEs (with the mesogen embedded into the backbone of a polymer chain) and the
side-chain LCEs (with the mesogen attached to the main chain as side groups), depending on the mesogenic
[32]
group incorporated methods [Figure 3A] . The alignment of LC mesogens is quantitatively described by
the average orientation of mesogens, characterized by the director n. This order degree of the alignment is
defined by the order parameter: S. It is noteworthy that the LC alignment and the crosslinking type have a
strong impact on the mechanical responses of LCEs.

Generally, the polydomain nematic LCEs can be divided into the isotropic-genesis polydomain nematic
LCEs (I-PNLCEs) and nematic-genesis polydomain nematic LCEs (N-PNLCEs) according to the LC phase
during crosslinking process [39,68,69] . They both show a relatively linear relationship between applied stress and
strain at small strains and a stress plateau at intermediate stress [Figure 3C], which is denoted as soft
elasticity (or semisoft elasticity) . The N-PNLCEs possess high modulus and stress plateau than the
[70]
I-PNLCEs. During the loading process, the director is reoriented to align with the loading direction. When
the domains are fully aligned to the loading direction, stiffer stress-strain responses are observed at high
strains after the regime of soft elasticity. The monodomain LCE shows an evident anisotropic effect .
[16]
Specifically, the monodomain LCE offers a rather linear stress-strain response when the loading direction is
close to the initial orientation of mesogens. Under the orthogonal-dominated loadings, the stress-strain
response shows an apparent soft elasticity and a strain-hardening feature. The anisotropic effect can be well
captured by the theoretical modeling considering the director reorientation , as evidenced by the
[16]
agreements with results of the finite element analysis (FEA) in Figure 3D.