Fig. 1. （a） Synthetic methods for preparation of crosslinked LCEs^［37］， including （i） acrylate homopolymerization， （ii） hydrosilylation， （iii） aza-michael chain extension， （iv） thiol-michael chain extension， and （v） radical chain transfer reactions； （b） Configuration and classification of LCEs^［34］， including main-chain LCEs and side-chain LCEs； （c） General alignments of mesogens in LCEs， including homogeneous， homeotropic， twist and hybrid alignments； （d） Change of the polymer backbones in LCEs during the order-disorder phase transition under the external stimulus^［32］； （e） Microscopic order disruption induced macroscopic deformations by （i） thermotropic and （ii） phototropic stimuli^［37］.

Fig. 2. Alignment of mesogens by mechanical stress. （a） Stretching the pre-crosslinked polydomain LCEs during polymerization to align it into monodomain LCEs^［41］. （b） Multi-directional stretch on a pentagram cutting LCE film to realize multiplex alignments of mesogens in a biomimic LCE starfish^［44］. （c） Alignment of mesogens under the compressive stress^［47］. （d） In the cholesteric LCE system， the compressive stress generated by anisotropic deswelling induces the layered and helical orientations of mesogens perpendicular and parallel to the evaporation direction， respectively^［51］. （e） Mesogen alignment in LCEs by combining stretch stress and shear stress during the 4D printing process^［52］. The left picture illustrates the schematic 4D printing diagram， and the right one shows the chemical composition of LCEs and the optical images of aligned LCEs observed under a polarizing optical microscope （POM）. （f） Preparation of aligned LCE-droplets^［53］ by the shear force. （g） Fabrication of well-aligned LCE-shells^［54-55］， LCE-fibers^［55］， LCE-based bionic muscles^［61］ by combining the tensile stress and the shear force. （i） Schematic diagram of artificial muscles assembled by hollow LCE-fibers. （ii） Preparation processes of hollow LCE-fibers： the LCE prepolymer is squeezed into the coaxial nozzle by the pressure. Orientation of the laminar outer shell LCE is controlled by adjusting the moving speed of the nozzle， and then combined with the constant flow rate of the coaxial core layer water flow to form （iii） the core-shell structure of water/LCE. When the water is completely evaporated， the pre-polymerized （iv） LCE shell structure is fully polymerized by UV exposure to form （v） hollow LCE-fibers. （h） Tensile stress， compressive stress and shear force work together to achieve direct preparation of preset patterned structures^［56］.

Fig. 3. Mesogen alignment by surface anchoring. （a） Surface rubbing-enabled mesogen alignment in LCEs^［66］. （b） Photoalignment of azobenzene mesogens induced by linearly polarized blue light （Weigert effect）^［68］. When the linearly polarized blue light is incident onto a monodomain azobenzene with the mesogen alignment parallel to the polarization direction， the azobenzene mesogen molecules within the illumination region absorb the photon energy to the excited state and undergo a cis-trans isomerization transition. As a result， the alignment of the molecules changes from an ordered alignment to a disordered alignment. Since the trans-molecules donot absorb energy when their optical axes are perpendicular to the polarization direction of the incident light， after multiple trans-cis-trans isomerization processes， all azobenzene mesogen molecules within the illumination area are finally reorientated perpendicular to the polarization direction of the incident light.（c） Complex photoalignment in LCEs realized by compiling the polarization of the incident light， such as radial alignment of mesogens^［78］. （d） Microchannels-enabled mesogen alignment in LCEs， which are fabricated by micro/nano-processing techniques^［83］. First， a template with microchannel structures is fabricated using the photolithography technique. Second， the templated structures are transferred to the PDMS and then the epoxy resin subsequently through the soft lithography by replica molding. Finally， two pieces of glass substrates with the epoxy resin microchannel structures are assembled to form a cell. The epoxy resin microchannel structures are used as the alignment layer for LCEs.

Fig. 4. Mesogen alignment by external field effects. （a） Mesogen radial alignment in a confined area at millimeter scale through three-dimensional magnetic field control to realize the bionic iris actuation^［89］. （i） Schematic diagram and the physical image of the magnetic field device for LCE alignment. There are eight groups of ferromagnetic blocks connecting the inner and outer double-layer ferromagnetic rings， and the eight groups of ferromagnetic blocks are evenly distributed along the circumference. Combined with （ii） the embedded platinum metal loop that can generate heat， （iii） the mesogen alignment， preparation and driving of the bionic iris can be achieved. （b） Micrometer-scale uniformly aligned LCE actuators fabrication by combining three-dimensional electric field control with 4D printing^［100］. （i） The schematic diagram of LCE 4D printing device with electrically controlled alignment. The zooming part illustrates the schematic diagram of electrodes for the electric field generation； （ii） Physical images， POM images， and scanning electron microscope （SEM） images of the LCE micro-actuators in the x-y plane and z-y plane， confirming good alignment.

Fig. 5. Schematic illustration of four basic alignment configurations of LCEs including （a） homogenous， （b） homeotropic， （c） hybrid， and （d） twisted， and their corresponding deformations^［101］.

Fig. 6. （a） Schematic illustration of twist-aligned LCE samples cut along different angles and their corresponding bending deformation modes^［103］； （b） Cholesteric LCEs with the helical axis parallel to the substrate： （i） schematic illustration of the alignment configuration， （ii） the POM image， and （iii） the stimuli-driven deformation^［104］.

Fig. 7. Deformation characteristics for LCEs with in-plane patterned alignment. （a） LCE samples with azimuthal （+1 defect） and radial （-1 defect） alignment and their corresponding deformations^［76］； （b） Schematic illustration of the alignment configurations， POM images and corresponding deformations of LCE samples with alignment defects ranging from -（5/2） to +（5/2）^［77］； （c） POM image and corresponding deformation of the 3×3 arrayed LCE sample with +1 alignment defect^［1］； （d） POM images and alignment configurations of LCE samples with +1 and -1 defect， and the deformations for different arrayed samples （1×1， 2×2， 3×3）^［107］； （e） Box-folding deformation based on topology optimization of alignment： （i） optimized hinge design （left） where the grayscale corresponds to different alignment angle， and its corresponding POM image （right）， （ii） optimized box alignment design （left） and its POM image （right）， （iii） the deformation of the sample under different temperatures^［108］； （f） Schematic diagram and physical image of complex face deformation based on inverse design^［82］； （g） Accordion-like deformation based on the combined design of in-plane patterned alignment and twist alignment along the film thickness^［109］.

Fig. 8. Deformation characteristics of mechanically aligned LCEs. （a） Schematic illustration of molecular alignment of LCE samples with unidirectional stretching and their photos before and after stretching^［40］； （b） Wrinkle deformation of LCE samples based on the radial stretching， the centers of which are circular （top） and square （bottom） polydomain regions， inducing different wrinkle distribution^［43］； （c） Bending deformation based on curling and winding^［111］； （d） Complex face deformation based on embossing^［63］.

Fig. 9. Deformation of bulk LCEs based on external field-induced alignment. （a） Heat-induced deformation of LCE micropillars with unidirectional alignment induced by a magnetic field^［97］； （b） Heat-induced deformation of different 3D structures in centimeter scale with unidirectional alignment induced by a magnetic field， including tube， pyramid， ellipsoid and hemispheroid^［112］； （c） Stimuli-responsive deformations of LCE samples fabricated by DIW based on different print paths： the cone deformation based on the rotating print path around the center， the twist deformation based on double layers of rectangular structures with perpendicular alignment， the mesh shrinkage based on stacked multi-layer porous structures with staggered alignment^［58］； （d） Complex deformations based on the DIW technique including cone deformation， saddle deformation， mesh shrinkage and deformation of a cone array^［55］.

Fig. 10. Typical applications of LCEs. （a） Soft robots based on distributed alignment： schematic illustration of alignment distribution and its stimuli-responsive deformation^［114］； （b） Tunable photonic crystals： （i） microscopic images and （ii） transmission spectra at different temperatures^［116］； （c） Light-driven tunable metasurfaces： （i） SEM image of the nanostructure array and simulated electric field distribution of the unit structure， （ii） transmission spectra of the sample stimulated by light with different intensities^［29］； （d） Tunable cylindrical microlens arrays： schematic illustration of deformation and focal length change for samples with alignments （i） parallel and （ii） perpendicular to the long axis of the microlens^［117］； （e） Reflection spectra of cholesteric LCEs changing with the stress^［118］.