The drawing method is the earliest invented technique for preparing liquid crystal elastomer fibers. Due to the tendency of liquid crystal elements to align along the direction of tensile force under the influence of tensile and shear forces, the drawing method has become the most common and simplest approach for fabricating liquid crystal elastomer fibers. As shown in Figure 3a, by dipping a pointed tool such as tweezers into the liquid crystal elastomer reaction solution or melt and then pulling it upward, followed by cross-linking under light or heat, single-domain-oriented liquid crystal elastomer fibers can be obtained^[43]. By controlling the viscosity of the reaction solution and the stability of the drawing process, the surface structure and dimensional uniformity of the fibers can be improved. Yang et al.^[54] prepared side-chain liquid crystal polymers with a polysiloxane backbone via thiol-ene click chemistry. By controlling the molar ratio and leaving excess thiol groups, they added a diene-containing cross-linker, drew the melt into fibers, and completed secondary cross-linking to obtain single-domain liquid crystal elastomer fibers (Figure 3b). Cheng et al.^[55] copolymerized acrylate-containing azobenzene molecules with hydroxyl-terminated acrylate liquid crystal monomers to form a polymer, drew the polymer melt into fibers, and then immersed the fibers in a solution containing diisocyanate cross-linkers to complete secondary cross-linking, thus achieving liquid crystal elastomer fibers containing azobenzene groups. These fibers exhibited bending deformation away from the light source under UV light stimulation. In addition to the melt-drawing method, Dong et al.^[56] reported a solution-drawing method, where continuous liquid crystal elastomer fibers were prepared vertically by stretching from the viscous fluid of a prepolymer using a metal puller. The drawing method is simple to operate; however, the viscosity of the liquid crystal elastomer reaction solution and the stability of the drawing process can affect the surface structure of the fibers, resulting in poor uniformity in fiber cross-sectional shape and size. Therefore, careful adjustment is required to obtain defect-free continuous fibers. Moreover, the dimensions of liquid crystal elastomer fibers prepared by the drawing method are usually limited, making large-scale production difficult.

The template method involves depositing the liquid crystal elastomer precursor into the pores or onto the surface of a template, followed by removal of the template to obtain a material with the morphology and dimensions specified by the template. The properties of the template determine the size and shape of the liquid crystal elastomer fibers; common templates include polytetrafluoroethylene, silicone, and glass^[53]. Since the template only imparts the shape and dimensions to the material, mechanical orientation techniques are required to align the liquid crystal units, followed by secondary crosslinking to produce liquid crystal elastomer fibers. Yu et al.^[57]injected a liquid crystal elastomer mixture into a glass capillary for primary crosslinking, then removed the multi-domain liquid crystal elastomer fibers after etching the glass template with hydrofluoric acid. Subsequently, the fibers were stretched and oriented, and subjected to secondary crosslinking via UV irradiation to yield single-domain liquid crystal elastomer fibers. Finally, the original liquid crystal elastomer fibers were immersed in a CNT/CH₂Cl₂dispersion solution (0.5 mg/mL) and sonicated to obtain CNT-containing liquid crystal elastomer fibers with a driving strain of up to 45%. Wang et al.^[30]crosslinked a liquid crystal oligomer injected into a polytetrafluoroethylene tube, then stretched and twisted the fibers to fabricate a twisted liquid crystal elastomer fiber capable of providing reversible actuation with good repeatability, achieving rotational deformation of the fiber (Figure 4a). Geng et al.^[58]injected a non-chiral liquid crystal oligomer and a chiral dopant into a low-density polyethylene tube, synthesizing a cholesteric liquid crystal elastomer fiber with continuous, repeatable, and predictable mechanochromic properties (Figure 4b). The template method is simple to operate, but the removal of the template can easily damage the liquid crystal elastomer fibers, affecting their structural integrity and mechanical performance. During fabrication, precise control over template design and removal, reaction conditions, and other factors is necessary to obtain liquid crystal elastomer fibers with superior performance.

Spinning is a process that uses various textile techniques to transform liquid or solid precursors into fibers with specific structures and properties. Common methods for preparing liquid crystal elastomer fibers include melt spinning, electrospinning, and solution spinning.

Melt spinning involves heating and melting polymers, extruding them through spinnerets, and cooling them in air to solidify into fibers. The extrusion through spinnerets exerts shear forces on the liquid crystal elements, aligning them; further alignment of these elements can be achieved through gravity, mechanical traction, and other methods. The size of the spinnerets and the applied pressure can be adjusted to control fiber dimensions. Hou et al.^[23]inspired by spider silk spinning, developed a new technology combining melt spinning with roller-drawing techniques to continuously and rapidly produce liquid crystal elastomer microfibers under mild processing conditions, achieving a manufacturing speed of up to 8400 m/h and enabling large-scale production of liquid crystal elastomer fibers (Figure 5a,b). Liao et al.^[59]employed an improved melt spinning method, allowing the liquid crystal elastomer prepolymer to stretch, align, and crosslink under gravity, thereby fabricating hollow liquid crystal elastomer fibers. By vacuum filling with liquid metal (LM), they obtained integrated drive-and-sensing LCE-LM coaxial fibers, achieving a reversible contraction rate of 40% and excellent durability (Figure 5c,d). Melt spinning for preparing liquid crystal elastomer fibers offers advantages such as simple equipment and a short process flow, making it suitable for large-scale production.

The principle of electrospinning is that the spinning solution is rapidly ejected under the action of a high-voltage electric field, and the solvent evaporates quickly during the ejection process, ultimately yielding nanofibers. The preparation of liquid crystal elastomer fibers by electrospinning is influenced by factors such as the viscosity and viscoelasticity of the liquid crystal elastomer precursor, electric field properties, and environmental temperature and humidity^[36]. The rheological properties of liquid crystal elastomers affect fiber ejection and formation, thereby influencing the morphology and performance of liquid crystal elastomer fibers. Meanwhile, properties such as the strength of the electric field affect the orientation of liquid crystal elements, thus impacting the reversible deformation capability and mechanical properties of liquid crystal elastomer fibers^[60]. He et al.^[37]used electrospinning technology to prepare liquid crystal elastomer fibers with diameters ranging from 10 to 100 µm. During electrospinning, the liquid crystal elastomer precursor was placed in a syringe, and when a high-voltage electric field of 6 kV was applied, numerous micron-sized fibers were ejected from the syringe and collected on a metal collector. The liquid crystal elastomer fibers obtained by this method were in a multi-domain state, requiring weights to be suspended at the bottom of the multi-domain liquid crystal elastomer fibers for orientation, thereby achieving thermal actuation (Figure 6a). Javadzadeh et al.^[18]prepared carbon nanotube-doped liquid crystal elastomer fibers using electrospinning technology. They twisted and stretched the CNT/LCE composite fibers to obtain CNT/LCE yarns, and finally oriented the CNT/LCE yarns through stretching and cured them with ultraviolet light, synthesizing a CNT/LCE yarn with rapid photothermal responsiveness and achieving a reversible contraction rate of 70% (Figure 6b). Melt Electrospinning Writing (MEW) technology combines melt electrospinning with 3D printing technology, enabling the preparation of microfibers and nanofibers. Feng et al.^[61]prepared liquid crystal elastomer fiber actuators ranging from micrometer scale to over centimeter scale using MEW technology. By controlling printing parameters, the fiber diameter could be variably adjusted between 4.5 and 70 μm, while achieving a work density of up to 160 J/kg (Figure 6c). Electrospinning offers advantages such as low cost and controllable particle size for preparing liquid crystal elastomer fibers, and can produce ultrafine fibers; however, it faces challenges in preparing oriented fibers, necessitating techniques such as traction stretching to achieve single-domain liquid crystal elastomer fibers.

Solution spinning involves dissolving a polymer in a solvent to form a spinning solution, which is then extruded from spinnerets into a coagulation bath or hot gas to solidify into fibers. Solution spinning is divided into wet spinning and dry spinning. In wet spinning, the prepared spinning solution is extruded through spinnerets into a coagulation bath to form fibers. Martinez et al.^[62]developed a wet spinning technique for preparing multidomain liquid crystal elastomer fibers. After stretching and twisting treatments, these fibers can achieve both conventional actuation (contraction) and more complex actuation (rotation and contraction). By programming the different orientations of liquid crystal elastomer fibers and textile structures, material properties and deformation behaviors can be adjusted (Figure 7a).In dry spinning, the spinning solution is extruded from spinnerets, and the solvent evaporates at sonic speed in hot air to form fibers. Wu et al.^[38]prepared a novel photo/electro/thermo multi-stimuli-responsive liquid crystal elastomer fiber using a simple dry spinning method combined with a two-step crosslinking process. Studies have shown that these fibers not only respond to external stimuli but also exhibit excellent optoelectronic properties (Figure 7b).Liquid crystal elastomer fibers produced by solution spinning have uniform diameters and are easy to control. However, due to the presence of solvents, direct orientation cannot be achieved during extrusion, requiring mechanical stretching and drawing after spinning to achieve orientation. Additionally, solvent evaporation may lead to an uneven internal structure of the fibers.

Printing technology can create complex internal structures and geometries, providing a new approach for the preparation of liquid crystal elastomer fibers. The method of preparing liquid crystal elastomer fibers through printing is influenced by factors such as the rheological properties of liquid crystal elastomers and printing head parameters^[63]. During the printing process, parameters such as the diameter and shape of the printing head, extrusion speed, and printing temperature affect the forming quality of liquid crystal elastomer fibers. It is necessary to rationally adjust these parameters to maintain the structural integrity and functional characteristics of liquid crystal elastomer fibers^[64]. When using three-dimensional (3D) printing to prepare liquid crystal elastomer fibers, shear forces cause liquid crystal elements to align directionally within the fibers. Chen et al.^[65]spun liquid crystal elastomer fibers prepared via 3D printing into a strand, which was then woven with flexible heating filaments into a special knot structure. This structure demonstrated faster and larger actuation compared to simply parallel liquid crystal elastomer fibers, while also exhibiting strong environmental adaptability, achieving stable cyclic actuation at 1 Hz under equivalent water pressure at a depth of 3000 m (Figure 8a). Although 3D printing has limitations in preparing liquid crystal elastomers with complex shapes, four-dimensional (4D) printing can construct intricate micro- and macrostructures, controlling liquid crystal orientation during processing and thus precisely regulating the actuation strain of liquid crystal elastomer fibers. Wang et al.^[22]developed a direct-write 4D printing method based on liquid crystal elastomers (Figure 8b). This printing technique allows for controllable printing structures by adjusting the offset position of fibers within the composite material. The fibers can withstand loads up to 2805 times their own weight and achieve a bending curvature of 0.33 mm^‒1under conditions of 150 ℃. Direct ink writing (DIW) technology excels in material flexibility and the feasibility of multi-material printing. Li et al.^[66]used cholesteric liquid crystal elastomers and dye solutions to prepare a dynamically color-switchable cholesteric liquid crystal elastomer fiber via DIW technology, simulating the skin color and texture switching function of cephalopods, demonstrating excellent color controllability and mechanical responsiveness. Liquid crystal elastomer fibers prepared via 4D printing possess greater actuation energy and more complex actuation deformations, enabling broader applications across various fields.

Microfluidic technology is a technique that uses microchannels to systematically manipulate fluids. It can regulate the size and structure of fibers by adjusting solution parameters such as the viscosity of the spinning solution, process parameters such as the flow rates of fluids in inner and outer channels, and chip parameters such as the dimensions of the microchannels^[67]. In microfluidic channels, the orientation of liquid crystal elements can be controlled by applying shear stress, electric fields, magnetic fields, or surface anchoring methods. Ohm et al.^[68] used a microfluidic device to inject a photocrosslinkable smectic main-chain polymer solution into a flowing silicone oil phase (Figure 9a). As the solution diffused into the silicone oil, fibers were formed, and subsequent UV irradiation induced secondary cross-linking, ultimately fixing the orientation and structure of the fibers. These fibers exhibited reversible actuation deformation under thermal stimulation. Zhang et al.^[69] developed an electrically responsive liquid crystal elastomer fiber (Figure 9b). Two liquid metals (LM), gallium and mercury, were injected as conductive cores into hollow liquid crystal elastomer fibers. Microfluidic technology was employed to control the distribution of LM within the core, such that the mercury segments, which have higher electrical resistivity, contracted more noticeably than the gallium segments with lower resistivity when electricity was applied, thus achieving precise control and editing of the deformation of the liquid crystal elastomer fibers. Microfluidic technology enables chemical reactions to occur in a controlled manner at the ultra-small scale within fibers, providing a simple, green, and efficient pathway for synthesizing micro- and nanoscale liquid crystal elastomer fibers.

In summary, there are diverse methods for preparing liquid crystal elastomer fibers, ranging from early techniques such as the dip-drawing method and template method to more advanced approaches like spinning, printing, and microfluidic methods. The dip-drawing method is simple to operate and suitable for small-scale laboratory preparation, but the fiber's cross-sectional shape and size uniformity are relatively poor. The template method operates under mild conditions and is ideal for fabricating fibers with complex structures; however, the removal of the template can easily damage the fibers. The spinning method enables the mass production of continuous fibers and is well-suited for large-scale industrial manufacturing, yet it is challenging to control the alignment of liquid crystal molecules. The printing method allows for rapid fabrication of fibers with intricate internal structures and complex geometries, making it ideal for personalized customization; however, it has higher technical complexity and cost. The microfluidic method is suitable for producing fibers with high precision and excellent uniformity, but it involves a complex process and higher costs. Each method has its unique advantages and disadvantages, making it appropriate for different application scenarios. By improving materials and optimizing process parameters, the stability and efficiency of liquid crystal elastomer fiber preparation can be enhanced, providing better solutions for practical applications.

The coordination mechanism of human muscle movement is a complex physiological process, primarily accomplished through the collaborative efforts of the central and peripheral nervous systems. After receiving information from sensory organs, the brain sends signals to muscles, which then contract, stretch, and perform other movements in response. Artificial muscles are a type of soft actuator that can mimic various human movement patterns when stimulated by external factors such as temperature, light, or pressure^[70]. Fiber-based artificial muscles share structural and functional similarities with natural muscles. Studies have shown that liquid crystal elastomer-based artificial muscle fibers exhibit excellent flexibility, high energy conversion efficiency, and power density, making them widely applicable in fields such as soft robotics and rehabilitation medicine^[71]. Hou et al.^[23]used a set of liquid crystal elastomer microfibers (diameter 78 µm, total of 30 fibers) as an artificial masseter muscle. Upon near-infrared irradiation, the liquid crystal elastomer microfibers rapidly contracted, driving mandibular closure and achieving the biological function of biting food (Figure 10a). Additionally, artificial biceps and quadriceps muscles fabricated from microfiber bundles can respectively simulate arm flexion and leg kicking movements. Inspired by the coupled behavior of muscles, bones, and the nervous system in mammals, Liu et al.^[72]proposed a concentric tube/rod-shaped artificial muscle based on liquid crystal elastomers and low-melting-point alloys, capable of stable bending deformations with upper and lower bending angles of approximately 45° and -35°, respectively. The outer liquid crystal elastomer enables reversible contraction and recovery, while the inner alloy locks the deformation, corresponding respectively to the functions of muscles and bones. By adjusting laser energy and irradiation direction, the bending angle and direction of the artificial muscle can be controlled. The deformation state of the artificial muscle can be monitored by testing changes in the resistance of the alloy, providing a simple method for intelligent operation. Furthermore, Roach et al.^[73]employed the DIW method to design and fabricate a liquid crystal elastomer fiber that mimics the driving characteristics of muscle contraction and relaxation (Figure 10b,c). Using 3D printing technology, a robotic arm was created, with liquid crystal elastomer fibers placed between the simulated biceps origin and insertion points. Upon heating, the fibers contract, enabling the robotic arm to achieve a bending angle of 70°. By increasing the number of woven fibers to enhance driving force, this liquid crystal elastomer fiber can lift loads equivalent to 250 times its own weight. With their excellent flexibility and reversible deformation properties, artificial muscle fibers demonstrate great application potential in biomedical devices and other fields.

Rigid robots can withstand large working loads and dominate in practical robotic applications. In contrast to rigid robots, soft robots composed of flexible structures offer advantages in terms of continuity, environmental adaptability, and safety. Inspired by the muscle structure of an elephant's trunk, Hu et al.^[74]combined multiple liquid crystal elastomer fibers with different orientations (axial, radial, circumferential, and helical) to design a three-dimensional tubular flexible actuator, achieving various deformation modes such as elongation, bending, and twisting, as well as complex deformation patterns resulting from combinations of these modes. They also developed a novel flexible pump and a soft robotic tentacle (Figure 11a, b). Drawing on the principle that mammalian hearts enhance ejection efficiency through torsional deformation, the actuator's torsional motion induces a wringing deformation, increasing fluid discharge rate and improving the pumping efficiency of the flexible pump (η = 84%). Furthermore, as a tentacle, this actuator can be light-driven to grasp and rotate bolts for unscrewing. When multiple segments of the tentacle are bent, torque can be applied to unscrew the bolt. Liao et al.^[59]prepared an integrated sensing and actuating LCE-LM coaxial fiber using an improved melt-spinning method and constructed a flexible three-arm Delta robot for object sorting (Figure 11c). The arm length of the Delta robot is precisely controlled by the current applied to three individual LCE-LM coaxial fibers. Through coordinated movements of its three arms, the Delta robot can move its gripper to specific positions in three-dimensional space. Additionally, by monitoring the feedback voltage of each arm, the movement paths and timing of the three arms can be determined. Soft robots based on liquid crystal elastomer fibers exhibit excellent shape memory effects and stimulus-responsive capabilities, enabling controllable shape changes and motion under external stimuli such as temperature and light, demonstrating broad application potential in fields like medical devices and smart furniture.

Through techniques such as twisting, sewing, and weaving, liquid crystal elastomer fibers can be processed into textiles. Liquid crystal elastomer textiles can respond upon detecting changes in human physiological states or environmental signals, offering broad application prospects in smart clothing and wearable devices, as well as sports assistance^[14]. Sun et al.^[75]prepared liquid crystal elastomer fibers using melt spinning and utilized a commercial knitting machine to create liquid crystal elastomer fiber textiles with varying geometries and deformation capabilities (Figure 12a). By quantitatively controlling the position, type, and size of the knitted patterns, the geometric shape and deformation characteristics of this liquid crystal elastomer fiber actuator can be precisely regulated, including parameters such as dimensions, bending location, direction, and angle. Combining knitted patterns with basic deformation modes enables more complex deformations, such as twisting and folding. Since the knitted structure consists of physically entangled continuous fibers, this liquid crystal elastomer fiber actuator can be disassembled back into its original fibers and re-knitted, achieving zero-loss recycling. To address the limitation of the single driving mode of liquid crystal elastomer fiber textiles contracting along the fiber axis, Lee et al.^[13]synthesized a nematic liquid crystal elastomer fiber with excellent mechanical properties using melt spinning technology. Unlike traditional liquid crystal elastomers oriented along the fiber axis, the liquid crystal elements in this fiber are arranged orthogonally relative to the fiber axis (Figure 12b). The different driving modes are determined by the liquid crystal phase during extrusion; nematic and smectic phases extruded separately result in fibers exhibiting thermal contraction (about 40%) and thermal elongation (about 30%), respectively. Integrating fibers with thermal elongation and thermal contraction within a single textile allows for two distinct spatial driving modes. The controllability of the driving direction in liquid crystal elastomer fiber textiles makes innovations in smart clothing and liquid crystal elastomer fiber actuators possible.