# Electroactive Liquid Crystal Elastomers as Soft Actuators

Yakui Deng, Min-hui Li

► To cite this version:

Yakui Deng, Min-hui Li. Electroactive Liquid Crystal Elastomers as Soft Actuators. Advanced Functional Materials, 2025, pp.e18797. 10.1002/adfm.202518797. hal-05371210

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# Electroactive Liquid Crystal Elastomers as Soft Actuators

Yakui Deng and Min-Hui Li*

Liquid crystal elastomers (LCEs) combine the anisotropic properties of liquid crystals (LC) and elastic properties of elastomers. LCEs present outstanding thermotropic properties including large and reversible skeletal-muscle-like strain, tunable processability, and high programmability. LCEs are extensively studied and increasingly considered as one of the competitive smart soft materials. Most LCEs use heat or light as external stimulus, where the temperature variation or light illumination disrupts the LC order and further the anisotropy of LC polymer chain conformation to eventually get material actuation. However, besides heat and light, electrical energy is the most convenient and the most in demand stimulus in actuators. This review discusses all kinds of electroactive LCEs (eLCEs) reported in literature with the emphasis on their chemical structures, their alignment methods to get monodomain sample, the design of conducting materials and electrodes, their actuation mechanism and performance, which will be helpful for the future development of eLCEs. This review discusses eLCEs actuated by electromechanical, electrochemical, and electrothermal mechanisms. Among these, only electrothermal eLCEs involve intrinsic thermo-responsive expansion and contraction, which are induced by Joule heating through electronic and ionic conductive media incorporated into the LCEs. By contrast, electromechanical and electrochemical eLCEs operate as athermal electroactive systems.

## 1. Introduction

At a time when artificial systems are rapidly spreading throughout society, research on soft actuators with remarkable reversible shape-memory properties under external stimuli has become increasingly attractive. The stimuli-responsive smart soft materials including liquid crystal elastomers (LCEs) have been developed

in the fields of artificial muscles, soft robotics and sensors.[1-3] In 1975, Pierre-Gilles de Gennes[4] already predicted that LCEs could present unusual properties that classical rubbers had not, because LCEs combine the anisotropic features of liquid crystals (LC) and elastic properties of elastomers. Since then, and particularly over the past decade, LCEs have been extensively studied and are now widely regarded as one of the most competitive smart soft materials.[5,6]

In LCEs, the thermotropic LC phases appear within a certain temperature range between the isotropic (I) liquid at higher temperatures and crystals (Cr) or glass state (g) at lower temperatures. According to the molecular arrangement, there are different LC phases, including nematic (N), smectic A (SmA) and smectic C (SmC) as the simplest ones (Figure 1a). If LC molecules are connected to form polymer chains, liquid crystal polymers (LCPs) are obtained (Figure 1b). Depending on the way of connection, LCPs can be classified into end-on side-chain LCP, side-on side-chain LCP and main-chain LCP. In general, the LC order is coupled with the polymer chain conformation with different

coupling strength. In main-chain LCPs, the coupling strength reaches its maximum, whereas it progressively diminishes in side-on side-chain LCPs and is weakest in end-on side-chain LCPs.[7] For example, in main-chain nematic LCP, the chain adopts an elongated conformation with possible hairpin defects in the nematic phase;[8-10] and it becomes spherical in the isotropic phase. Consequently, at N-I transition ($T_{NI}$), there is a size contraction of the LCP chain along the nematic director. Like single crystal versus crystal powder samples, LCPs can also have monodomain samples versus polydomain samples according to their state of alignments.[11] Monodomain LCPs exhibit a macroscopically uniform alignment, whereas polydomain LCPs consist of microdomains with locally aligned LC phases, whose directors are randomly oriented throughout the sample. If the crosslinking reaction is performed in monodomain LCP sample, the monodomain LCE is obtained in which the LCP orientation is permanently locked (Figure 1c). In the monodomain LCE, the change of chain conformation across the LC-I phase transition, that is, the chain size contraction along the alignment direction can be translated to the macroscopic shape change of the whole material (sample size contraction); this shape change is totally reversible upon heating and cooling (Figure 1d).[11-13] This

Y. Deng, M.-H. Li
Institut de Recherche de Chimie Paris
UMR8247

CNRS
Chimie ParisTech

Université Paris Sciences & Lettres
11 rue Pierre et Marie Curie, Paris 75005, France
E-mail: min-hui.li@chimieparistech.psl.eu

ID The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.202518797

© 2025 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/adfm.202518797
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Schematics of (a) liquid crystal (LC) molecules, b) liquid crystal polymers (LCP), c) liquid crystal elastomer (LCE) and (d) thermal actuation in the monodomain nematic LCE: the change of polymer conformation across the LC-I phase transition can be translated to the macroscopic shape change of LCE sample and this shape change is totally reversible upon heating and cooling.</figcaption></figure>

is the “artificial muscle” principle of a lot of LCE-based smart soft materials.

Most LCE-based materials use heat[13–22] or light[23–37] as external stimulus, where the temperature variation or light illumination disrupts the LC order and further the anisotropy of LCP chain conformation. However, besides heat and light, electrical energy is the most convenient and the most in demand stimulus in the world of actuators. Indeed, nature does use electrical impulses between nerves and muscles for actuation with extraordinary efficacy, and electrical stimulation is more widely utilized as driving forces in industrial devices. From the earliest stages of research on LCEs, significant efforts have been directed toward the development of electroactive LCEs (eLCEs) capable of responding to electrical stimuli.[52,53,61,70] More recently, eLCEs have emerged as a focal point of interest within the materials science and engineering communities (see recent reviews.[38–40])

eLCEs can be divided into three categories according to their actuation mechanism, that is, eLCEs activated by electromechanical, electrochemical and electrothermal effects. In this re-

view, all these three categories of eLCEs will be discussed in detail with updated analyses on systems reported in the literature, and concluding remarks will be given at the end. Among these eLCEs, only electrothermal eLCEs involve intrinsic thermo-responsive expansion and contraction, which are induced by Joule heating through electronic and ionic conductive media incorporated into the LCEs. By contrast, electromechanical and electrochemical eLCEs operate as athermal electroactive systems (Figure 2). To distinguish this work from existing reviews that primarily address either electrothermal systems or the performance and applications of materials,[38–40] the present review emphasizes electromechanical and electrochemical eLCEs. For all systems, the chemical structures, the alignment methods used to obtain monodomain samples, and their actuation mechanisms and performance are highlighted. Rather than providing an exhaustive catalogue of reported systems and applications, we aim to present the fundamental principles underlying each type of system, with the intent of facilitating future developments in eLCEs.
<figure><img src="image_4.png" /><figcaption>Figure 2. Actuation mechanisms of eLCEs by (a) electromechanical, b) electrochemical and (c) electrothermal effects.</figcaption></figure>

## 2. eLCEs by Electromechanical Effects

The reversible shape changes of LCEs in certain cases can be directly achieved by applying electric field, where the electrical energy is directly converted into mechanical energy. These electromechanical effects are produced according to several distinct properties of LCEs, that is, the elastomeric feature of LCEs, the electrostriction effect of nematic LCEs, and the electroclinic effect of chiral smectic A phase in smectic LCEs.

To facilitate the comparison of the actuation performance of different systems, the actuation strain is defined in this review as the change in length ΔL over the initial length L₀, ΔL/L₀, with ΔL = L₀ – L for contraction and ΔL = L – L₀ for extension to keep a positive value.

### 2.1. eLCEs by Maxwell Stress

The dielectric elastomer actuators (DEAs) represent the first man-made analogue to natural muscle in performance from the mechanical engineering perspective.[41] They are simply an elastomer sandwiched between two compliant electrodes. Since their discovery in the early 1990s, they have been developed as exciting soft actuators and transducers. Maxwell stress is generated in the dielectric elastomers like silicone and acrylic elastomers by the electrostatic attraction between two compliant electrodes when high voltage is applied (typically 10–100 V µm⁻¹). Due to Maxwell stress, the actuator is compressed in the vertical thickness direction and expands in the plane direction to produce 10–200% strain (Figure 3a).[42–44] The actuation strain of the dielec-

tric elastomer is dependent on the material’s dielectric constant (permittivity), stiffness, and thickness.[43]

$$s_z = p/Y = \epsilon_0 \epsilon E^2 / Y = \epsilon_0 \epsilon (V/z)^2 / Y \quad (1)$$

with s_z the actuation strain along the thickness (compression), p is the effective pressure, Y is the elastic modulus of the elastomer, ε₀ is the free-space dielectric constant, ε is the relative dielectric constant, E is the electrical field, V is the applied voltage and z is the thickness.

LCEs are dielectric elastomers with high dielectric constant; more interestingly, aligned LCEs are mechanically and dielectrically anisotropic by nature. Therefore, unlike traditional DEAs, which expand equally in all planar directions, DEAs based on aligned LCEs (DLCEAs) expand preferentially in the direction perpendicular to the nematic director (Figure 3b).[44] Davidson et al.[44] first reported this kind of DLCEAs that exhibited anisotropic electric-driven actuation performance with a linear strain of 5% at 37.5 V µm⁻¹. Later, White et al.[45] reported similar DLCEAs but with enhanced electroactive directional actuation through improved material chemistry to prepare LCEs. Their DLCEAs exhibited a linear strain of 17.5% with a rapid strain rate of 18% s⁻¹ under electric field of 117 V µm⁻¹. Both groups used the surface anchoring method to align LC oligomers or monomers to prepare monodomain LCEs.

Since patterning the alignment of LCEs is a straightforward route to realize 3D deformations in response to stimuli,[18] DLCEAs were developed with programmed shape actuation, such as deformations from flat film to film with positive or negative
<figure><img src="image_5.png" /><figcaption>**Figure 3.** a) Schematic of a traditional isotropic DEA in off and on states of voltages.[44] Reproduced from ref. [44]. Copyright (2019), with permission from American Association for the Advancement of Science. b) Schematic of a uniaxial aligned DLCEA in off and on states of voltages. (n indicating the nematic alignment direction).[44] Reproduced from ref. [44]. Copyright (2019), with permission from American Association for the Advancement of Science. c) Locally positive and negative Gaussian curvatures are realized on the voxelated surface of DLCEA. Programmed shape actuation of DLCEA with wavy pattern deformation by patterning the director configuration into an azimuthal-radial defect.[44] Reproduced from ref. [44]. Copyright (2019), with permission from American Association for the Advancement of Science. d) An DLCEA film whose director profile has a +1 azimuthal defect is soft in the radial direction and stiff in the circumferential direction. After voltage is applied, it deforms from flat into a cone reaching a height of more than 8 mm.[45] Reproduced from ref. [45]. Copyright (2021), with permission from Wiley-VCH.</figcaption></figure>

Gaussian curvature (Figure 3c)[44] and transformation from flat sheet to a cone (Figure 3d).[45] For example, in Figure 3d, the spatial nematic director in the LCE varies azimuthally by 360° around a point; the LCE is “soft” in the radial orientation (perpendicular to the director), while it is nearly 15 times stiffer in the circumferential direction (along the director). When subjected to Maxwell stress, this DLCEA expands radially but not circumferentially; consequently, the 2D-3D shape transformation is achieved to obtain a cone. These actuators illustrate the promise to generate topographical surfaces through electroactive settings.

The widespread applications of dielectric elastomers (often in the form of thin film) are restricted by the high voltage requirement (1–10 kV, typically 10–100 V µm⁻¹) that increases the system complexity and has safety issue. Therefore, efforts have been made to reduce the actuation voltage. For example, Cai et al.[46] has taken advantage of the property of soft elasticity of LCEs. They used the method of two-step polymerizations with the mechanical stretching for alignment to prepare LCE sample.[47] Owing to the director rotation under uniaxial stress of loosely crosslinked polydomain LCE, a small mechanical work can generate a large deformation in LCE with a stress plateau (so called soft elasticity). In their work,[46] the mechanical stretching is equibiaxial in the plan of LCE film. By adjusting the stretching ratio, the onset of the strain-stiffening can be tailored without significantly increasing the modulus. The force required to maintain this pre-stretch is lower than conventional elastomers, and the second step crosslinking was used to permanently fix the pre-stretch. Note that the pre-stretching has already been used in traditional dielectric elastomers to realize anisotropic deformations, while rigid frames are necessary to introduce and sustain the mechanical bias.[48,49] Freestanding DL-

CEA film (≈20 µm) thus fabricated by Cai et al.[46] can generate large actuation strains (≈88%) at only 400 V (20 V µm⁻¹), an electric field much lower than those used for comparable deformation by Davidson et al.[44] and White et al.[45] (37.5 V µm⁻¹ for strain of 5%, 117 V µm⁻¹ for strain of 17.5%, respectively). This work is interesting for the development of low voltage DEAs potentially applicable in wearable devices and in lightweight untethered soft robotics.

Jin et al.[50] also reported low e-field (8 V µm⁻¹) driven DLCEAs, but with more complex design, which incorporate the bending dielectric actuation through the space-charge mechanism and can perform multimodal shape-changing under electrical and thermal actuations. Meanwhile, the dynamic covalent bonds (ester bonds with transesterification) activable by heating are also introduced into the system together with the force-directed solvent evaporation[51] for the LCE alignment and shape reconfiguration.

## 2.2. eLCEs by Electrostriction Effect

Electrostriction is a property of all dielectrics without exception.[52] Electrostriction causes these materials to change their shape under the application of an electric field. However, in non-liquid-crystal materials, significant strain due to electrostriction (> 0.1%) occurs only in certain engineered ferroelectric ceramics and ferroelectric polymers.[53–55] For these ferroelectrics, the electrostriction is also called reverse piezoelectricity. The liquid crystal rod-like mesogens present strong dielectric anisotropy and thus have specific electrostriction effect. Here we discuss eLCEs with nematic phases that show significant electroactive strains.
<figure><img src="image_6.png" /><figcaption>**Figure 4.** a) Molecular dipoles (indicated by the arrow) in the classical nematic phase (N) for molecules like n-pentyl cyanobiphenyl (5CB) with longitudinal permanent dipole along **n**.[59] Reproduced from ref.[59] Copyright (2020), with permission from National Academy of Sciences. b) Chemical structure of 5CB, and that of p-azoxyanisole (PAA) with a permanent dipole perpendicular to **n**. and c) Molecular dipoles in the ferroelectric nematic phase (N<sub>F</sub>).[59] Reproduced from ref.[59] Copyright (2020), with permission from National Academy of Sciences. d) Chemical structures and phase sequences of RM734 and DIO with N<sub>F</sub> phase.</figcaption></figure>

### 2.2.1. Classical Nematic Phase and Ferroelectric Nematic Phase

Rod-like nematic molecules align parallel to each other, but they can move freely around showing no long-range positional order. These molecules often have chemically distinct ends (but not always), while in the classical nematic phase (N) molecular interactions do not distinguish between these ends and the fluid is nonpolar (Figure 4a). The apolar character of N is featured by the headless vector **n** (called the director) with **n** = −**n**, which indicates the average orientation of molecules.

The strong dielectric anisotropy (Δε) is one of the major physical properties of liquid crystals. Δε = ε<sub>‖</sub> − ε<sub>⊥</sub>, where ε<sub>‖</sub> and ε<sub>⊥</sub> are the static dielectric constants (relative permittivity) along (‖) and normal (⊥) to the director, respectively. The dielectric anisotropy of N molecules arises from two contributions: (a) the polarizability anisotropy Δα, associated with the dipole moment induced by the applied electric field, and (b) the possible permanent electric dipole moment μ:[56–58]

$$ \Delta\epsilon \propto \left[ \Delta a + Cte \frac{\mu^2}{T} (3\cos^2\beta - 1) \right] \quad (2) $$

where T is the absolute temperature, β is the angle between the permanent dipole moment and the principle molecular axe (the director **n**), and Cte is a constant of local field.

Molecules in N phase respond collectively to the application of an external electric field, which creates a reorientation of the

director.[58] This is the working principle of displays composed of nematic low molecular weight liquid crystals (LMWLCs). The sign of Δε governs the correlation between the directions of the electric field and of molecular reorientation. As shown in Figure 4b, 5CB has a positive Δε = 13.1, while PAA has a negative Δε = −0.2. Most of the rod-like N molecules have positive Δε. The director of molecules with positive or negative Δε tends to align parallel or perpendicular to the field direction, respectively. The dielectric torque exerted on the N molecules is:

$$ \tau_D = \epsilon_0(\epsilon_{\parallel} - \epsilon_{\perp})E^2\sin\theta\cos\theta \quad (3) $$

where E is electric field strength, ε<sub>0</sub> is the vacuum permittivity, and θ is the angle between the electric field and the director **n**.

So far, in the classical nematic phase, permanent molecular dipoles have primarily contributed to the overall dielectric permittivity. Interestingly, as early as 1916, Born proposed a far more pronounced effect: a ferroelectric fluid in which all dipoles point in the same direction (Figure 4c).[59] Excitingly, this long-standing prediction has recently been realized with the demonstration of a ferroelectric nematic phase (N<sub>F</sub>) in rod-like molecules possessing large dipole moments (≈10 D) (Figure 4d).

In 2017, Mandle, Cowling, and Goodby[60,61] synthesized a family of highly polar rod-like compounds that exhibited both a classical nematic (N) and a lower-temperature nematic of unknown structure, one of which (RM734) is shown in Figure 4d. At the same time, Kikuchi and coworkers[62] reported a
<figure><img src="image_7.png" /><figcaption>**Figure 5.** a) Deformation of eLCEs activated by mesogen reorientation through dielectric anisotropy of N phase. b) A plane strip of nematic elastomer with embedded aligned carbon nanotubes subjected to an electric field E perpendicular to the initial director/CNT alignment.[68] Reproduced from ref.[68] Copyright (2003), with permission from EPL / EDP Sciences. In the fixed-length (isostrain) conditions, an actuator force of sample contraction was registered along the initial alignment axis. c) Schematics of the correlations between the macroscopic deformation of I-PNE and mesogens realignment from polydomain to monodomain under E-fields.[71] Reproduced from ref.[71] Copyright (2011), with permission from Royal Society of Chemistry. d) Chemical composition of I-PNE.[71] In (a–c) the mesogen represented by an ellipsoid has positive Δε.</figcaption></figure>

different material (DIO, Figure 4d) with a dipole moment of ≈9.4 D that exhibited a ferroelectric-like phase. Finally, in 2020, Chen et al.[63] definitively demonstrated that the lower-temperature nematic in RM734 is the ferroelectric nematic (N<sub>F</sub>) envisioned by Born, formed by macroscopic domains with uniform polarization that can adopt one of two orientations along **n**, **P** or **−P**. The polar ordering makes N<sub>F</sub> incredibly more sensitive to the electric field than N. Fields create a torque:

$$ \tau_p = PE \sin\theta \quad (4) $$

where the measured polarization density is very high, *P* = 40–60 mC m<sup>−2</sup>. The electric field as low as 10<sup>2</sup> V m<sup>−1</sup> (10<sup>−4</sup> V µm<sup>−1</sup>) are sufficient to overcome the orientational elasticity of N<sub>F</sub> to reorient **P** along the electric field direction. Note that the same field 10<sup>2</sup> V m<sup>−1</sup> (10<sup>−4</sup> V µm<sup>−1</sup>) acting on N with Δε = 10 would produce a torque that is orders of magnitude weaker and incapable of causing strong twists.

In principle, the mesogen reorientation of both N and N<sub>F</sub> activated by electric field can be used to develop eLCEs with electroactive deformations (electrostriction).

### 2.2.2. eLCEs with Classical Nematic Phase

In classical nematic LCEs, a very high electric field is required to induce director reorientation and achieve actuation, since a strong field *E* is needed to increase the dielectric torque τ<sub>D</sub> (Equation 3) sufficiently to overcome the resistance proportional to the elastic modulus of the elastomer (Figure 5a).[64] Several strategies have been explored to lower the required field and enhance actuation, either by reducing the elastic modulus (e.g., swelling LCEs

with LMWLCs) or by increasing Δε (e.g., incorporating carbon nanotubes into LCEs to enhance their sensitivity to the electric field, Figure 5b).

In 1986, Zentel first observed the shape change of a LCE swollen with a phenyl-cyclohexane nematic mixture (1132TNC from Merck) under electric fields.[65] In 1994, Kishi et al.[66] reported ≈10% shape changes of swollen polydomain LCEs under a DC electric field 0.3 V µm<sup>−1</sup>. Later, Yusuf et al.[64] investigated in detail electromechanical effects in polydomain and monodomain LCEs swollen with 5CB and observed a maximum of 13% contraction in fields ranging from 0.01 to 1 V µm<sup>−1</sup> when 5CB was in its nematic phase (between 24 and 34.5 °C). Meanwhile, Urayama et al.[67] studied the role of dielectric anisotropy of each mesogen in both LCE and LMWLC solvent. Four combinations of the LCEs and LMWLC solvents with Δε(LCE) > 0 and Δε(LMWLC) > 0, Δε(LCE) > 0 and Δε(LMWLC) < 0, etc. were used to construct swollen LCEs. The deformation of the systems composed of a LCE and a LMWLC solvent with identical signs of dielectric anisotropy is the favourable situation for electromechanical effect with maximum strain observed as large as 20% at *E* < 0.5 V µm<sup>−1</sup>.

Courty et al.[68] first demonstrated the electromechanical response in an LCE embedded with a low concentration (~ 0.01%) of carbon nanotubes aligned along the nematic director (Figure 5b). The nanotubes create a very large effective dielectric anisotropy of the composite. Their local field-induced torque is transmitted to LCE and produces uniaxial stress of ~ 1 kPa with a strain of 0.1% in response to a constant field of ~ 1 V µm<sup>−1</sup>. Recently, White et al.[69] prepared nanocomposite in which both the orientation of the LCE and single-walled carbon nanotube (SWNT) are locally oriented in discrete voxels via photoalignment surfaces. Topological 3D shape change, that is, protrusion of voxelate cone shape, is obtained by applying a direct current (DC)
<figure><img src="image_8.png" /><figcaption>**Figure 6.** a) An electric field is coupled to the polymer film by the electrodes to induce the formation of protrusions.[72] b) Chemical composition of the liquid crystal mixture to form the LCN by photopolymerization.[72] c) Schematic representation of the LCN polymer. The orientation of the LC molecules perpendicular to the AC field provides the torque that induces oscillatory stresses.[72] d) Schematic representation of disruption of mesogens order and translating to form surface topographies in macroscopy under E-fields.[72] Reproduced from ref.[72] Copyright (2017), with permission from Springer Nature.</figcaption></figure>

field of 1.5 V µm⁻¹. When the field is released, the film quickly returns to its original shape in less than 1 s. However, this shape change occurs only at high temperature (90 °C), but not at room temperature. Then, Guin and Naskar et al.[70] improved the material chemistry and got a SWNT–LCE actionable at ambient conditions and household available voltages (<120 V, 1.4 V µm⁻¹ DC field), which contracts > 4% against a 140 kPa load, comparable to the blocking force of many natural muscular tissues.

Swollen LCEs and LCE nanocomposites present certain drawbacks, such as reduced mechanical properties due to the presence of LMWLC solvents, or increased complexity in the preparation of nanocomposites. In contrast, dry and homogeneous systems are particularly attractive. The first dry LCE with a large strain reaching 35% was reported by Urayama et al. (Figure 5c,d).[71] An isotropic-genesis polydomain nematic LCE (I-PNE) was obtained by crosslinking the mesogens in the isotropic state at high temperature and then cooling down to a temperature in the nematic state (e.g., 90 °C). Under electric field of 25 V µm⁻¹, I-PNE exhibits a large expansion strain of 35% in the plane normal to the field axis, as well as a polydomain-to-monodomain transition (Figure 5c). The authors attribute this electrical actuation to the softness of director rotation in the polydomain structure formed by crosslinking mesogens in the high-temperature isotropic state. Nevertheless, one may ask whether Maxwell stress

also plays a role. Most likely, Maxwell stress and mesogen reorientation through dielectric anisotropy act synergistically. Such a combination of electromechanical effects could be highly promising for the development of eLCEs.

Broer et al.[72] demonstrated surface-oscillating actuation in thin LC polymer networks (LCNs, 2.5 µm) using an alternating electric field applied via interdigitated electrodes (Figure 6). The LCN, similar to an LCE but with higher crosslinking density, was prepared with mesogens homeotropically aligned at the surface, while the alternating current (AC) field was applied perpendicularly (Figures 6a–c). Thus, cyanobiphenyl mesogens with positive Δε experience oscillatory torque under the AC field, which, although insufficient to fully reorient them due to the rigidity of the LCN, generates oscillatory stress and increases free volume (Figure 6d). Resonance is achieved when the driving AC frequencies match the intrinsic frequencies of the LCN, leading to strongly amplified deformation.

### 2.2.3. eLCEs with Ferroelectric Nematic Phase

As discussed in Section 2.2.1, polar ordering makes N<sub>F</sub> phase significantly more sensitive to the electric field than the N phase. Electric fields several orders of magnitude weaker should be
<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /><figcaption>**Figure 7.** a) Chemical structures of two series of N<sub>F</sub> main-chain LCPs with a benzyl (Bn<sub>m</sub>) or nitrobenzene (Nt<sub>m</sub>) end group and b) their phase sequences obtained during cooling.[73] Reproduced from ref.[73] Copyright (2021), with permission from American Chemical Society. c) Chemical structures of N<sub>F</sub> side-chain LCPs with different joint positions in mesogens and d) their possible molecular packing mechanisms. Only SP1 shows a stable N<sub>F</sub> phase with a phase sequence of N<sub>F</sub>-85 °C-N-126 °C-Iso. Side-jointed SP is between middle-jointed MP (equivalent to side-on in Figure 1) and end-jointed EP (equivalent to end-on in Figure 1).[74] Reproduced from ref.[74] Copyright (2021), with permission from American Chemical Society.</figcaption></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

sufficient to induce reorientation of N<sub>F</sub> mesogens compared with those required for N. Due to the very recent discovery of the N<sub>F</sub> phase,[63] eLCEs possessing N<sub>F</sub> has not yet been reported in the literature. However, we believe this is only a matter of time. Polymer materials involving N<sub>F</sub>, such as polymer-dispersed N<sub>F</sub>, N<sub>F</sub> LCPs and N<sub>F</sub> LCEs, are expected to attract great interest. In the following, we analyze a few recently published examples of NF LCPs.[73,74]

Huang and coworkers designed and synthesized a series of main-chain N<sub>F</sub> LCPs and side-chain N<sub>F</sub> LCPs based on *p*-oxybenzoate-based mesogens structurally analogous to RM734.[73,74] For the main-chain N<sub>F</sub> LCPs, the well-defined polymer and oligomer chains were terminated with either benzyl (Bn<sub>m</sub>) or nitrobenzene (Nt<sub>m</sub>) group (Figure 7a). Interestingly, the conventional nonpolar N phase was absent in these systems, while the N<sub>F</sub> phase was stabilized and persisted over a wide temperature range (Figure 7b). Moreover, in the Nt<sub>m</sub> series (m = 1–4), crystallization was suppressed down to room temperature.

In the side-chain LCPs, the N<sub>F</sub> mesogens were covalently attached to polyacrylate backbones at various linking positions, giving rise to a series of side-chain N<sub>F</sub> LCPs (Figure 7c). Distinct packing mechanisms in these side-chain architectures were found to strongly influence the formation of the N<sub>F</sub> phase (Figure 7d). Among them, only one of the side-jointed LCPs (SP1) exhibited a clear N–N<sub>F</sub> phase transition. In contrast, other side-jointed LCPs (SP2, SP3), the middle-jointed and end-jointed analogues (MP and EP) promoted either poorly ordered

dipoles or antiparallel dipole packing, resulting in weak or negligible polarity and the absence of the N<sub>F</sub> phase. The side-jointed configuration (SP) represents an intermediate geometry between the middle-jointed (MP, equivalent to side-on in Figure 1) and end-jointed (EP, equivalent to end-on in Figure 1) architectures.

We propose that both the main-chain N<sub>F</sub> LCPs (Bn<sub>m</sub> and Nt<sub>m</sub>) and side-chain N<sub>F</sub> LCP (SP1) are promising candidates for the development of N<sub>F</sub>-based LCEs exhibiting pronounced electromechanical responses. A key question arises as to whether the polymer chain conformation undergoes a transformation, such as from oblate to prolate, during the reorientation of the polarization *P* of N<sub>F</sub> mesogen under an applied electric field. If such coupling between LC order and polymer chain conformation exists, the resulting material deformation could be substantial, analogous to that observed in thermo-activated nematic LCEs.

## 2.3. eLCEs by Electroclinic Effect of Chiral Smectic A Phase

The electroclinic effect was first demonstrated by Garoff and Meyer,[75] which exists in a smectic A phase composed of chiral molecules. In the chiral smectic A (SmA*) molecule, there is a permanent traverse electric dipole (Figure 8a), in contrast to the longitudinal dipole in non-chiral nematogen like 5CB. When an electric field is applied to the SmA* phase in the layer plane
<figure><img src="image_13.png" /><figcaption>**Figure 8.** a) Shape change of a LCE with a chiral smectic A (SmA*) phase under an applied electric field (E-field).[78,79] The mesogen with a transverse dipole is depicted as an orange ellipsoid. b) Chemical structures of monomers and crosslinker for the preparation of LCE, the final LC showing the phase transition sequence: Tg 28 °C SmA* 128 °C Iso.[79] Reproduced from ref.[79] Copyright (2016), with permission from American Chemical Society.</figcaption></figure>

(indicated by E in Figure 8a), the transverse dipole of the molecules directly couples to the electric field and tilts the molecules in a plane perpendicular to the applied field direction (Figure 8a). This phenomenon is called the electroclinic effect. In summary, the LC molecules tilt within the plane perpendicular to the applied electric field, resulting in an induced ferroelectric smectic C* (SmC*) state.

In a LCE film with chiral smectic A phase, where the smectic layer is oriented perpendicular to the film surface (Figure 8a), the electric field applied along the direction of the smectic layer plane and perpendicular to the film surface, the LCE film undergoes significant shear deformation (a distortion from a rectangle to a parallelogram) as shown in Figure 8a. Consequently, a reduction in the thickness of the smectic layer and shearing in the direction of the layer plane are observed. Kremer et al.[76] first described this kind of chiral smectic LCE made of side-chain polysiloxane in free-standing film of tiny size (1-mm-width and 75-nm-thick) and reported 4% contraction strain of the smectic layer under an electric field of 1.5 V µm⁻¹. However, Stannarius et al.[77] observed a layer contraction of only 1% under a 3 V µm⁻¹ electric field in the LCE film with the same chemical structure and in similar conditions (sample size: 1.4 mm × 0.76 mm × 240 nm or 324 nm). Naciri et al.[78] reported a macroscopic LCE film sample made of a side-chain polyacrylate with a large temperature range of chiral smectic A phase (Figure 8b) and observed 1.0% shrinking strain of the smectic layer along the length direction for a 60-mm-thick film under an electric field of 16.7 V µm⁻¹ (Figure 8a,b). Recently, the same chiral smectic A LCE with typical dimensions of 2 cm × 0.5 cm × 50 µm was studied again by Selinger et al.[79] Interestingly, instead of the expected length contraction and in-plane shear deformation, a rapid and reversible twisting out of the plane was observed with a helical sense depending on the sign of the field (±16.7 V µm⁻¹). This 3D actuation can be attributed to the asymmetry between the front and back sides of the film caused by preparation conditions. Consequently, the shearing deformations by electroclinic effect in both sides are anisotropic, which result in curvature out of the plane. In literature, eLCE by electroclinic effect is often directly attributed to the SmC* phase; however, this is not the case. Only LCEs exhibiting the SmA* phase display electroclinic behavior and are capable of layer shearing. Different eLCEs driven by electromechanical effects are summarized in Table 1.

### 3. eLCEs by Electrochemical Effects

Electrically conducting polymers (ECPs) are key components of ionic electroactive polymers (EAPs) used as artificial muscles.[80–84] Their actuation relies on ion and solvent transport that induces volume changes. ECPs are conjugated organic semiconductors with alternating single and double bonds, where electron removal creates delocalized positive charges along the backbone. To maintain charge neutrality, anions are incorporated into the polymer. Applying a negative potential reverses this process, returning electrons and neutralizing the backbone.

Volume change primarily arises from ion and solvent transport: the polymer expands when ions enter and contracts when they leave. Ion insertion can occur in either the oxidized (Equation 5) or reduced (Equation 6) states, where P⁺ and P⁰ denote oxidized and neutral polymers, respectively. P⁺(A⁻) represents anion (counter-ion) incorporation, while P⁰(AC) represents cation (co-ion) insertion. To maintain charge neutrality, electronic charge variations are balanced by ionic charge exchange with the electrolyte, leading to volume change (Figure 9a). Polymers with small, mobile anions expand upon oxidation (Equation 5), whereas those with large, immobile anions expand upon reduction as charge compensation occurs via cation insertion (Equation 6).

$$P^{+}(A^{-})+e^{-} \leftrightarrow P^{0}+A^{-} \quad (5)$$

$$P^{+}(A^{-})+C^{+}+e^{-} \leftrightarrow P^{0}(AC) \quad (6)$$

A common ionic electroactive device (i-EAD) is typically a tri-layer system consisting of an ionic conducting membrane sandwiched by two ECPs as electrodes. Poly(3,4-ethylenedioxythiophene)/polystyrene sulfonate (PEDOT:PSS) are the most used ECP.[85,86] In the presence of ions, PEDOT:PSS presents reversible volume variations upon electrical stimulation that corresponds to the process of Equation 6 because of the non-mobile macro-anions PSS⁻. When PEDOT:PSS are electrochemically reduced or oxidized in the presence of an electrolyte, the insertion or expulsion of cations within the PEDOT:PSS to insure the overall electroneutrality leads to the swelling or the deswelling of the electrode layers. With a tri-layer configuration, these volume variations result in a large bending movement as shown in Figure 9b. Polyelectrolytes including ionic liquids
<table><caption>**Table 1.** Summary of eLCEs driven by electromechanical effects.</caption><thead><tr><th>eLCEs actuators</th><th>Electrical field [V µm<sup>-1</sup>]</th><th>Actuation performance</th><th>Actuation mechanism</th><th>Factors to control actuation kinetics</th><th>Refs.</th></tr></thead><tbody><tr><td>dielectric LCE</td><td>37.5</td><td>contraction strain 5%</td><td>Maxwell stress</td><td>dielectric constant, elastic modulus and thickness</td><td>[44]</td></tr><tr><td>dielectric LCE</td><td>31.25</td><td>voxelated 3D deformation</td><td>Maxwell stress</td><td>dielectric constant, elastic modulus and thickness</td><td>[44]</td></tr><tr><td>dielectric LCE</td><td>116.7</td><td>contraction/expansion strain 17.5%/20%</td><td>Maxwell stress</td><td>dielectric constant, elastic modulus and thickness</td><td>[45]</td></tr><tr><td>dielectric LCE</td><td>166.7</td><td>voxelated 3D deformation</td><td>Maxwell stress</td><td>dielectric constant, elastic modulus and thickness</td><td>[45]</td></tr><tr><td>dielectric LCE</td><td>20</td><td>expansion strain 88%</td><td>Maxwell stress</td><td>soft elasticity</td><td>[46]</td></tr><tr><td>dielectric LCE</td><td>8</td><td>bending deformation</td><td>Maxwell stress</td><td>dynamic network rearrangement</td><td>[50]</td></tr><tr><td>nematic LCE swollen by nematic solvents</td><td>≈0.005–0.1</td><td>expansion strain ≈1%–20%</td><td>electrostriction effect with N phase</td><td>elastic modulus</td><td>[64]</td></tr><tr><td>nematic LCE swollen by nematic solvents</td><td>0.5</td><td>expansion strain</td><td>electrostriction effect with N phase</td><td>elastic modulus</td><td>[67]</td></tr><tr><td>LCE charged with SWNT</td><td>1.2</td><td>voxelated 3D deformation</td><td>electrostriction effect with N phase</td><td>dipole rotational activation energy</td><td>[69]</td></tr><tr><td>LCE charged with SWNT</td><td>1.4</td><td>contraction strain 4%</td><td>electrostriction effect with N phase</td><td>dielectric constant</td><td>[70]</td></tr><tr><td>polydomain nematic LCE crosslinked in the isotropic phase</td><td>25</td><td>expansion strain 35%</td><td>electrostriction effect with N phase and Maxwell stress</td><td>softness of director rotation</td><td>[71]</td></tr><tr><td>LCN</td><td>7.5</td><td>surface oscillation deformation 6%</td><td>electrostriction effect with N phase</td><td>viscoelastic deformation</td><td>[72]</td></tr><tr><td>chiral smectic A LCE made of side-chain polysiloxane</td><td>1.5</td><td>contraction strain 4%</td><td>electroclinic effect</td><td>transverse dipole</td><td>[76]</td></tr><tr><td>side-chain polysiloxane LCE with SmA*</td><td>3.0</td><td>contraction strain 1%</td><td>electroclinic effect</td><td>transverse dipole</td><td>[77]</td></tr><tr><td>chiral smectic A LCE made of side-chain polyacrylate</td><td>16.7</td><td>contraction strain</td><td>electroclinic effect</td><td>transverse dipole</td><td>[78]</td></tr><tr><td>chiral smectic A LCE made of side-chain polyacrylate</td><td>±16</td><td>twisting 1%</td><td>electroclinic effect</td><td>transverse dipole</td><td>[79]</td></tr></tbody></table>

<figure><img src="image_14.png" /><figcaption>**Figure 9.** Ionic EAP by electrochemical effect. a) Dominant mechanism for volume change in conjugated polymers. (1) Oxidized state of the polymer where positive charges on the polymer backbone are compensated by possibly solvated anions. (2) Reduced state where the small and mobile anions are expelled (Equation 5), causing contraction. (3) Reduced state where the anions are large and immobile. Consequently, solvated cations are incorporated to maintain charge neutrality (Equation 6), causing expansion. b) Schematic illustration of the principle of electroactive bending deformation for the trilayer ionic electroactive device (i-EAD).</figcaption></figure>
<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /><figcaption>**Figure 10.** a) iLCE precursor containing IL (HMIM-PF₆). b) Surface-enforced method for monodomain ionic-LCE (iLCE) preparation. c) Illustration of iLCEs with different modes of molecular alignments (planar, homeotropic, hybrid, and isotropic) and the bending performance of trilayer ionic electroactive device (i-EAD) containing these iLCEs. d) Schematic presentation of an i-EAD with iLCE as central membrane. e) Electro-actuated bending deformation obtained by i-EAD device in (d).[87] Reproduced from ref. [87]. Copyright (2019), with permission from Wiley-VCH. The bending strain difference in the actuator was calculated using the equation: $\Delta\epsilon = \frac{\Delta L}{L} = \frac{2\bar{d}\omega}{\bar{d}^2+L^2}$, where $\Delta L$ is the length difference between the expanding side and the contracting side of the trilayer i-EAD, and $\omega$ is the thickness of the i-EAD.[93]</figcaption></figure>

(ILs) are often used as ionic conducting membrane.[85,86] This soft i-EAD can operate in open-air and is able to convert either electrical stimulation into reversible and large bending deformations under low voltage (e.g., ≈2 V for a trilayer thickness of 200 µm) (actuator) or convert mechanical stimulation into electrical signal (sensor).

Recently, ionic liquids were incorporated into LCE to obtain ionic conducting LCE (iLCE) membrane.[87–89] Jákli et al.[87] first incorporated ILs directly into monodomain LCE as shown in **Figure 10a,b**. iLCE precursor was composed of monofunctional acrylate monomer M1, bifunctional crosslinker M2 (RM82), photo initiator Irgacure 651 and IL (HMIM-PF₆). Surface-enforced method was used to align the iLCE precursor with different alignment modes (Figure 10c) followed by photopolymerization/crosslinking. The obtained iLCE membrane was then sandwiched by two thin layer electrodes made of conductive PEDOT:PSS (Figure 10d). Like classical i-EAD, PEDOT:PSS/iLCE/PEDOT:PSS thus constructed could effectively perform bending deformation under DC voltages or low-frequency AC (± 2 V) with the maximum bending strain difference of 0.3% in hybrid actuator (Figure 10e) and could also work as bending sensor to output electrical signal.[87,90–92] In sum-

mary, iLCE-containing i-EAD can be actuated by low (2 V) AC or DC voltages providing bending strains comparable to the classical i-EADs with several novel and superior properties, such as the alignment-dependent bending performance, which offers the possibility to prepare i-EAD with patterned actuation.

In the work of Jákli et al.[87] only the anisotropic mesogen alignment was explored to develop i-EAD with novel properties, while the LCP conformation change was not involved. In the recent work of our group,[88,94] we studied especially the LCP conformation change in new iLCE-containing i-EADs under electric field. Our initial objective is to introduce both bending and contractile deformations in the same device by controlling the frequency of the low-voltage stimulation. The contractile deformations are realized by electrothermal effects that heat the i-EADs to N-I phase transition to induce sample contraction, which will be discussed in detail in the next Section 4. **eLCEs by electrothermal effects.** Here, we present first their bending actuation and sensing performance. The designs of the iLCE and electrodes in the trilayer i-EAD have been thoroughly reconsidered. New formulations of ECP electrodes and the iLCE involving polyelectrolytes and aligned main-chain LCPs were proposed.
<figure><img src="image_19.png" /><figcaption>**Figure 11.** a) Chemical structures of the components to make the main-chain ion-conducting LCE film.[88] b) Chemical structures of formulated PEDOT:PSS electrodes.[88,94] Reproduced from ref.[88] Copyright (2024), with permission from Wiley-VCH. c) Chemical structures of the components to make the i-IPN-LCE film with interpenetrating ionogel and LCE networks.[94] d) The trilayer i-EAD-IPN-LCE as electroactive bending actuator. The orange central layer represents the i-IPN-LCE film (n indicating the nematic director), while the two black side layers represent the formulated PEDOT:PSS electrodes. It can mimic the Golf shot to kick a ball (top view).[94] e) The trilayer i-EAD-IPN-LCE as electroactive bending sensor with Open Circuit Voltage response to the mechanical bending.[94] f) Schematic illustration of the principle of electroactive bending.[94] g) The trilayer i-EAD-IPN-LCE as electroactive linear actuator.[94] Reproduced from ref.[94] Copyright (2024), with permission from Wiley-VCH.</figcaption></figure>

To construct the iLCE central membrane, poly(ethylene glycol)diacrylate (PEGDA) was first added as co-monomer of RM82 for the affinity between PEG chain and the ILs, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI) (Figure 11a). The iLCE membrane of main-chain LCP with high anisotropy of chain conformation was then prepared by a two-stage polymerization/crosslinking and mechanical stretching alignment (Yakacki procedure).[47] In the electrodes, PEG network was also introduced into the commercial PEDOT:PSS by adding PEG as side chains and crosslinkers. After curing, the softness of electrodes and their compatibility with the central LC ionogel increased (Figure 11b). By applying a voltage of ±2 V at 0.1 Hz on this i-EAD-LCE,[88] a bending strain difference of 0.80% is obtained. However, the function of sensor has not been observed in this i-EAD-LCE.

In order to improve the mechanical performance of the i-EAD, we have further elaborated an interpenetrating LCE and ionogel (i-IPN-LCE) (Figure 11c) to replace the simple network of ion-conducting LCE as the central membrane and a new device i-EAD-IPN-LCE was obtained.[94] As shown in Figure 11c, the interpenetrating polymer networks (IPN) were prepared via two orthogonal polymerization methods. The polyurethane LCE was obtained by the polyaddition of the diol groups in the

RM82-like LC monomer with the tri-isocyanate groups in the crosslinker Desmodur N3300 catalyzed by dibutyltin dilaurate (DBTDL), while the polyacrylate ionogel was formed by free-radical photo-polymerization/crosslinking of PEGA and PEGDA in the presence of ILs. This i-EAD-IPN-LCE can function not only as electroactive bending actuator, but also as a bending sensor (Figure 11d,e).[94] As a bending actuator, its bending strain difference $\Delta\epsilon$ is 1.18% with 3 mN output force (Figure 11d). As a bending sensor, a 0.4 mV of output signals in Open-Circuit-Voltage can be periodically produced with a periodic bending strain difference of 9% under a periodic force of 100 mN applied at 3 mm from the fixed side of the sample film (Figure 11e).

The principle of bending deformation is again associated with ionic motion caused by the redox reactions (electrochemical effects, Equation 6). As shown in Figure 11f, typically, during the application of a potential difference (DC stimulation) between the two PEDOT:PSS electrodes, the reduction (cathode) of the PEDOT$^{(x)+}$ into PEDOT$^{(x-y)+}$ will promote the insertion of cations like EMIM$^+$ to keep the charge neutrality and the volume increase of the electrode. Concomitantly the oxidation (anode) of PEDOT$^{(x)+}$ into PEDOT$^{(x+y)+}$ will induce the expulsion of cations and contraction of the electrode. The expulsion of cations
instead of the insertion of anions originates from the higher mobility of EMIM⁺ cations versus TFSI⁻ anions. The combination of cathode-expansion and anode-shrinkage results in the trilayer film bending toward anode side. By contrast, in the sensor mode, when applying mechanical bending force, the bending deformation creates a pressure gradient through the thickness of the ionic membrane from the compressed side to expanded side. Consequently, a pressure-induced flux of the electrolytes will promote the transient separation of ionic charges with cations being faster than the anions and generate an open circuit voltage (OCV) variation as electrical signal output.[95-97]

## 4. eLCEs by Electrothermal Effects

Joule heating is a well-established phenomenon in which the passage of an electric current through a conductor with finite resistance generates heat. Because LCEs are intrinsically dielectric and exhibit low electrical conductivity, conductive fillers are typically incorporated to induce Joule heating and thereby enable electrothermal actuation. An alternative, though less commonly explored, strategy to achieve electrothermal effects involves integrating a thermoelectric device (TED) based on the Peltier or Seebeck effect into the LCE matrix, for instance using n-type and p-type bismuth telluride (Bi₂Te₃) semiconductors.[98]

Electrothermally active eLCEs can undergo a liquid crystal–to-isotropic phase transition driven by temperature increases under an applied electric field, resulting in reversible and programmable macroscopic shape transformations. In this review, we primarily focus on electrothermal actuation arising from Joule heating, the most widely adopted mechanism. Joule heating in LCEs can be realized through the incorporation of either ion-conducting or electron-conducting components. Following Section 3, we first examine LCE systems incorporating ion-conducting materials.

### 4.1. Electrothermal Effects Achieved by Incorporation of Ion-Conducting Materials

In the system i-EAD-LCE developed by our group,[88] by applying an AC voltage of ±6 V at 10 Hz the ionic current-induced Joule heating effectively triggers the muscle-like linear contraction with 20% strain without load. With load the i-EAD-LCE can lift a weight of 270 times its own weight, while keeping 20% strain and affording 5.38 kJ·m⁻³ work capacity. However, the i-EAD-LCE has a Young's modulus of only 1.63 MPa and is too soft to measure the actuation stress by any mechanical stress/strain sensor. Note that mammalian skeletal muscles have much higher Young's modulus (10–60 MPa) and actuation stress (0.1–0.35 MPa).

The new device i-EAD-IPN-LCE developed by us with an interpenetrating LCE and ionogel (i-IPN-LCE) as central membrane shows the improved mechanical performance.[94] Its Young's modulus, actuation stress and actuation strain are 51.6 MPa, 0.14 MPa and 9%, respectively, reaching skeletal muscles' values. By applying an AC voltage of ±10 V at 10 Hz the ionic current-induced Joule heating triggers the linear contraction with 9% strain without load. This maximum contraction ratios (9%) are

confirmed when repeating four electro-actuation (EA). The heating rate reaches 250 °C min⁻¹ in the first EA and 120 °C min⁻¹ in the fourth EA. The very fast actuation of the i-EAD-IPN-LCE is promising for potential applications. As shown in Figure 11g, the i-EAD-IPN-LCE film can lift a weight 76 times heavier than its own mass by doing a work of 6.3 × 10⁻⁵ J with a work capacity of 1.12 kJ m⁻³. This work capacity is in the same order of magnitude as that of mammalian skeletal muscles (8 kJ m⁻³). Certainly, the applied potential of ±10 V is still too high for the device. Work is in progress to develop new LCE membrane that has lower N-I phase transition temperature and thus needs lower electrical potential to activate.

### 4.2. Electrothermal Effects Achieved by Incorporation of Electron-Conducting Materials

The commonly used electron-conducting materials can be further classified into carbon-based, MXene-based and metal-based materials.

#### 4.2.1. eLCEs by Electrothermal Effects of Carbon-Based Materials

The first eLCEs by Joule heating used carbon-based materials as electron conductors, which include graphite, carbon black (CB) and carbon nanotubes (CNTs).

**Graphite-based eLCEs:** Shahinpoor and Finkelmann[99,100] are the first researchers to report on the electrothermal effect in composites of monodomain nematic LCEs and a conducting material. They used graphite fibers as electron-conductors. As shown in Figure 12a, the nematic LCEs here were obtained via a hydrosilylation reaction of the monofunctional side chain mesogen 2 and the bifunctional LC main chain polyether 3 with poly(methylhydrogensiloxane) 1.[101] To obtain a monodomain LCE, the weakly crosslinked network after gelation was mechanically loaded at about 103 N m⁻², and the hydrosilylation reaction was completed under load (so called “Finkelmann procedure”).[11] The graphite fibers (10 micron in diameter) were then sewn in a zigzag configuration into the LCE sample of dimensions 32 mm x 7 mm x 0.4 mm.[99] By Joule heating through electrical activation, the composite sample could undergo 55% reversible contraction strain (ΔL/L₀) in a few seconds and exhibiting appreciable force.

Graphite powders and nanoparticles have also been used to prepare LCE composites.[100,102] Li et al.[102] reported a photonic crystal with electrothermally driven structural color change made of LCE-based inverse opaline film (2 mm in thickness) combined with a spin-coated thin layer of graphite nanoparticles. The LCE here is based on end-on side-chain polyacrylate. The alignment of LCE was achieved through LC monomer alignment by surface-enforced method and fixed by subsequent polymerization/crosslinking. When voltage is applied on the graphite-coated LCE inverse opaline film, the heat produced by the graphite layer induces the deformation of the inverse opaline film and provokes the structural color change.

**Carbon black (CB)-based eLCEs:** Chamber et al.[103–105] prepared a surface integrated CB-eLCE using side-chain polysiloxane LCE obtained via a hydrosilylation reaction (see Figure 12b)
<figure><img src="image_20.png" /><figcaption>(b)</figcaption></figure>

<figure><img src="image_21.png" /><figcaption>(c)</figcaption></figure>

<figure><img src="image_22.png" /><figcaption>**Figure 12.** a) Synthesis of the nematic LCE composed of both side-chain and main-chain polysiloxane via hydrosilylation reaction.[101] b) Composition of side-chain polysiloxane LCE.[105] c) A two-step method to produce CB-eLCE materials with improved CB incorporation into LCE.[106] Reproduced from ref.[106] Copyright (2016), with permission from American Chemical Society.</figcaption></figure>

with the "Finkelmann procedure."[11] The monodomain LCE swollen by toluene was placed in a solvent (e.g., toluene) containing a CB dispersion. CB particles were adsorbed onto the surface of the swollen LCE. After deswelling, a 3 mm-thick layer rich in CB particles was formed at the LCE surfaces. This layer allows the electrothermal actuation of CB-eLCE with a contraction strain of 33%. The large changes in geometrical shape both in contraction and expansion can be cycled beyond 10 k cycles.

Later, Jacot and Verduzco[106] improved the CB incorporation into LCE using a two-step method to produce CB-eLCE materials of dimensions of 10 mm × 5 mm × 1 mm with "bulk + surface" CB (Figure 12b,c). These CB-eLCEs exhibit rapid (response times as fast at 0.6 s), large-amplitude (contraction strain up to 30%), and fully reversible shape changes (stable to over 5000 cycles) under externally applied voltages (5–40 V). The electromechanical response of the material can be adjusted by changing the electrical potential and the composition of CB-eLCE. Interestingly, when this CB-eLCE was used as responsive substrates to culture neonatal rat ventricular myocytes (NRVM), NRVM remained viable on both stimulated and static CB-eLCE substrates.

In contrast to the approach of Jacot and Verduzco,[106] Yang et al.[107] prepared a bimorph device by glueing CB powder layer to LCE layer using a conductive adhesive. The bilayer LCE/CB film has dimensions of 4.5 cm × 1.0 cm × 0.45 mm, where the thickness of LCE is 0.3 mm and CB film 0.15 mm. For LCE preparation, authors used one of the famous reactive monomers RM257 (Figure 13a) and previously published thiol-acrylate Michael addition and free radical acrylate polymerization coupled with mechanical alignment (Yakacki procedure).[47] Under DC power, the bilayer LCE/CB film can perform a fast-bending deformation toward the LCE layer side because of the LCE layer contraction induced by Joule heating up to $T_{NI}$, while CB layer keep the same length (Figure 13b).[108] Yang et al.[107] then designed a cylindrical bilayer electroactive actuator consisting of an LCE circular ring as the inner layer and several U-shaped CB conductive regions stuck on the outer surface of the LCE ring as the outer layer. The bending deformations become asymmetrical along the circumference; therefore, on the conductive copper track, Joule heating of the U-shaped CB layer can result in a rolling movement of the cylinder. This cylindrical actuator without wires can perform

a continuous rolling with a rate of 1.6 mm s⁻¹ under 50 V DC power (Figure 13c). This CB-eLCE cylinder is an amazing example of smart actuators free of electric wires for electric transportation and devices.

**Carbon nanotube (CNT)-based eLCEs:** Forest-drawn CNT sheets[108] consist of highly aligned carbon nanotubes (CNTs) arrays that present both chemical and geometrical anisotropy with nanogrooves. Therefore, they are suitable aligning layers for small molecule liquid crystals. In the LCE/CNT composites reported by Ware et al.[109] CNT sheets played double roles: they were used as control-surface to uniaxially orient LCE precursors for preparation of monodomain LCE and as electric conducting materials to introduce the Joule heating for electrothermal actuation. **Figure 14a** shows the LCE precursors with another famous reactive monomer RM82 and the procedure of LCE/CNT composite preparation using previously published aza-Michael addition reaction and free radical acrylate polymerization coupled with surface-enforced alignment (White procedure).[18] The uniaxially aligned composite film LCE/CNT$_{5+5}$ (fabricated with 5 layers of CNT sheets on each side of LCE) was obtained (Figure 14b). Then LCE/CNT composite film was twisted to form the yarn. Under a DC voltage input of 15.1 V cm⁻¹, the LCE/CNT yarn (diameter of ≈200 µm, with a resisting load of 840 kPa) provides a stroke strain of 12%, which is four times that of the LCE/CNT film without twisting (Figure 14c). The work capacity of this twisted yarn actuator is of 97 kJ m⁻³ being about 3.6 times higher than that of LCE/CNT film without twisting and 2.5 times higher than that of mammalian muscles. This eLCE yarn fiber may find applications in soft robotic actuators.

Recently, Lv et al.[110] designed a CNT-based Joule heater with high performance, which is composed of three layers of CNT including two aligned CNT layers sandwiching a random CNT layer (Figure 15). This thin hierarchically structured carbon nanotube (HS-CNT) film (0.18–2.96 mm thick) is connected to a pre-stretched loosely crosslinked LCE film through lamination followed by a second photo-crosslinking. This HS-CNT possesses networks with angulated scissor-like microstructures that can deform concurrently with the driven LCE, and do not bring mechanical constraints to attenuate the shape-changing performance of the LCE. This HS-CNT/LCE can perform
<figure><img src="image_23.png" /><figcaption>**Figure 13.** a) Major components for monodomain LCE preparation via Yakacki procedure.[47] Reproduced from ref.[47] Copyright (2015), with permission from Royal Society of Chemistry. b) Under DC power, the bilayer LCE/CB film could perform a fast-bending deformation toward the LCE layer side.[107] Reproduced from ref.[107] Copyright (2020), with permission from American Chemical Society. c) A cylindrical actuator made of bilayer LCE/CB performs a continuous rolling on the conductive copper track.[107] Reproduced from ref.[107] Copyright (2020), with permission from American Chemical Society.</figcaption></figure>

omnidirectional large-scale stretchability/deformation without compromising electro-thermal stability and mechanical output power. It demonstrates large strain (40%), and remarkable conductive robustness ($R/R_0 < 1.03$ under actuation), excellent Joule heating efficiency (≈233 °C at 4 V), and high load-bearing capacity ($R/R_0 < 1.15$ at 4000 times its weight loaded). This CNT Joule heater design would have technological impacts on applications of eLCE in soft robotics and wearable flexible electronics.

### 4.2.2. eLCEs by Electrothermal Effects of MXene

MXene is a new family of 2D materials and produced by selectively etching out the main-group sp elements A by HF acid from a MAX phase, where M represents early d-block transition metals, A main-group sp elements (predominantly IIIA or IVA), and X either C or both C and N atoms.[111] Among MXene, $Ti_3C_2Tx$ is the first compound reported in 2011.[112] where Tx are functional groups (–OH, –F, –O, and –Cl) terminating the M surface. $Ti_3C_2Tx$ is attractive by its lightweight, solution processability, ≈99% light-to-heat conversion efficiency in the near infrared range (NIR) and significantly high electrical conductivity.[83] Therefore, MXene combined with LCE can endower LCE composites with high photothermal and electrothermal features. Wie et al.[113] reported $Ti_3C_2Tx$/LCE bilayer with a superior electrical conductivity of ≈5300 $S cm^{-1}$. $Ti_3C_2Tx$/LCE bilayer with planar/homeotropic hybrid alignment of LCE can bend

and twist, respectively, by electrothermal effect according to the offset angle determined as the angle between the nematic director and the long axis of the $Ti_3C_2Tx$/LCE bilayer film. Upon voltage of 3.5 V for 14 s, $Ti_3C_2Tx$/LCE bilayer can bend with a maximum bending angle of 697° at offset angle of 0° or directionally twist at offset angle of ± 60°. Interestingly, the $Ti_3C_2Tx$/LCE bilayer exhibits a strong interfacial adhesion induced by hydrogen bonding between functional groups (–OH, –F, O, and –Cl) at M surface of $Ti_3C_2Tx$ and LCE and presents structural durability after 1000 cycles of bending deformation. Wei et al.[114] reported PDA-MXene/LCE fiber, in which $Ti_3C_2Tx$ solution modified by polydopamine (PDA) was uniformly coated on the surface of the plasma treated LCE fibers (Figure 16). PDA-modified MXene effectively inhibits oxygen penetration and allows a stable interface combination between the PDA/MXene layer and LCE fiber layer. The PDA-MXene/LCE microfibers thus designed can generate large actuation strains (greater than 50%) within a fraction of 0.4 s, fueled by low direct-current (DC) voltage (4 V). The MXene-based eLCE represents a new class of composite materials for the manufacture of soft robotics and soft intelligent optoelectronic devices.

### 4.2.3. eLCEs by Electrothermal Effects of Metal-Based Materials

Metal-based electrothermal-responsive eLCEs have been widely reported due to the good electric conductivity and the
<figure><img src="image_24.png" /></figure>

<figure><img src="image_25.png" /></figure>

<figure><img src="image_26.png" /><figcaption>**Figure 14.** a) Preparation of the uniaxially aligned composite film LCE/CNT using cells made of forest-drawn CNT sheets and via White procedure.[18,109] b) The LCE/CNT composite film and its twisted form (the LCE/CNT yarn).[109] c) Actuation performance of the LCE/CNT film and yarn under a DC voltage input of 15.1 V cm⁻¹.[109] Reproduced from ref.[109] Copyright (2019), with permission from Wiley-VCH.</figcaption></figure>

electrochemical stability of metallic materials embedded inside, which include metal heating wires and liquid metals (LM).

Metal wire-based eLCEs:

### (1) Platinum wires

The first example of metal wire-based eLCE was reported by Schuhladen and Zappe et al.[115] An iris-like electroactive tunable aperture was built by incorporating S-shape stretchable polyimide-coated platinum heater wires inside a nematic LCE (Figure 17a). The iris-like LCE (diameter 12 mm, thickness 0.3 mm) was prepared by embedding heater wires in a side-on nematic acrylate monomer mixtures (Figure 17b) and then radially orienting nematogens using magnetic fields and finally photo-polymerizing/crosslinking to get elastomer (Keller procedure).[116,117] The LCE radially contracts to open the middle orifice (pupil-like) when a 50 V electric field is applied to the resistive heaters. The opening of the iris takes 7 s, while the closing only takes 3 s, probably due to unequal active surface areas for heating and cooling. This tunable aperture may have significant potential in miniaturized photographic optics; however, the operating temperature is high because the nematic-isotropic phase transition ($T_{NI}$) is ≈120 °C for the used LCE.[116]

### (2) Copper and Ni/Cu wires

Cai et al.[118] used copper instead of platinum as metal. Similar S-shape stretchable polyimide-coated copper heater wires were sandwiched between two loosely crosslinked LCE films (thickness 1 mm) and compressed. The tubular shape was formed by rolling the sandwich-like structure and stretching it to 180% of its initial length for the alignment of LC polymers. The LCE tubular actuator (Figure 17c) was finally obtained by the second photo-crosslinking to fix the alignment. Authors used the monomer RM257 (Figure 13a) and Yakacki procedure for LCE preparation.[45] The LCE phase transition $T_{NI}$ is ≈80 °C and the linear actuation strain from 7% to 40% can be obtained with the operating temperature from 40 to 90 °C. Multimodal actuations including linear contraction and multidirectional bending are achieved by applying electrical potential (3.0 V) to all the three heating wires or one of the three heating wires as shown in Figure 17c. Using multiple tubular actuators, they also built a multifunctional soft gripper and an untethered soft robotic.[118] The most specular example of metal-wire-based eLCEs is the system mimicking elephant trunk reported recently (Figure 17d).[119] Selective and sequential actuation of elephant trunk inspired multimodal deformation was achieved by this eLCE fibers.[89] Other interesting geometries of eLCE include soft actuators weaved from LCE fibers containing helical Ni/Cr wire using traditional rope artistry reported by Xu et al.[120] These
<figure><img src="image_27.png" /><figcaption>**Figure 15.** The omnidirectionally deformable electrothermal eLCE based on hierarchically structured carbon nanotube (HS-CNT).[110] a) Schematic diagram of the HS-CNT-LCE film with CNT trilayers including upper and lower aligned CNT layers and an intermediate random CNT layer. b) The relative thickness of aligned and random CNT layers can be adjusted by raw CNT dispersion concentrations. c) Chemical structures of main components to prepare LCE by Yakacki procedure.[47] d) Schematic illustration of the deformation of HS-CNT/LCE under applied forces. i) Variation in resistance of the HS-CNT/LCE film under various stretch ratios and stretching angles (0°, 45°, and 90°). ii) The strain/stress curve of the LCE film and HS-CNT/LCE film. Reproduced from ref.[110] Copyright (2024), with permission from Royal Society of Chemistry.</figcaption></figure>

weaved fiber soft actuators, like the double twisting, circular four-strand, orthogonal or diagonal weaving actuator, can be readily assembled in various soft robots to perform multiple functionalities, including surface shape programmability, biomimetic blood pumping inspired by cardiac muscle, and versatile locomotion modes such as crawling and swimming.

Ahn et al.[121] incorporated multiple thiourea bonds into the LCE by choosing the poly(ether-thiourea) comprising a triethylene glycol spacer (LCE-TUEG) as dynamic crosslinker (Figure 17e). Here, thio-acrylate Michael addition was used for chain extension and aza-Michael addition was used to ensure crosslinking. The thiourea groups can form hydrogen bonding between polymer chains, which enable considerable enhancement of the mechanical properties of LCE. The thiourea bonds are dynamic covalent bonds and can undergo dissociation at high temperatures, resulting in network rearrangement to permit melt reprocessing, shape reprogramming, welding, and self-healing of LCE. Then, a Ni/Cr resistive wire was wound around a polydomain LCE film that was further sandwiched between two identical polydomain LCE films (but without metal wires) as outer layers. The final monodomain eLCE was obtained by sequential welding and mechanical alignment of the three-layers system. Various reversible actuations like hand gestures were produced through selective electrothermal stimulation of each ar-

tificial finger.[121] The ability to reprocess, reprogram, and recycle complex actuating object of LCEs by exchangeable dynamic covalent bonds will be beneficial for the development of sustainable soft robotics and actuators.

### (3) Silver wires

Zhu et al.[122] constructed a thermal bimorph actuator composed of LCE, silver nanowire networks (AgNWs) and CB-doped polydimethylsiloxane (PDMS). The patterned silver nanowire networks (AgNWs) are half-embedded below the top surface of PDMS/CB composite; the LCE is made of the same formulation as in Figure 13a.[47] This bimorph actuator can function as a caterpillar-inspired crawling robot with multiple crawling modes enabled by programmable joule heating. Shao and Tian et al.[123] used CNTs and polyimide (PI) film instead of CB and PDMS to construct the similar electrothermal bimorph. CNTs-AgNWs were embedded in PI film with an interconnected mesh-like structure providing improved mechanical properties and Joule heating. Similar bimorph actuator was also reported earlier by Yu et al.[124] using stretchable polyimide-coated Au wires as heaters.

Shao et al.[125] fabricated a twisted yarn actuator with the skin-core structure by concentrically twisting several LCE fibers around a flexible silver-plated polyamide strand. Again, the
<figure><img src="image_28.png" /><figcaption>**Figure 16.** Schematic illustration of the preparation process of the PDA@MXene/LCE fiber actuator.[114] a) Main components and chemical structure of spinning solutions. b) Preparation of dopamine-modified MXene sheets. c) Manufacturing process of the PDA@MXene/LCE fiber actuator. Reproduced from ref.[114] Copyright (2023), with permission from Royal Society of Chemistry.</figcaption></figure>

monomer RM257 (Figure 13a) and Yakacki procedure was used for LCE fiber preparation.[47] The twisting here played the role of stretching for LCP alignment. The yarn actuator can produce a peak torque of 33.4 N m kg⁻¹ which outperforms reported commercial rotating motors and a specific power density of 21.2 W kg⁻¹ that is comparable to the human skeletal muscle. Electrical soft rotors, a flexible lifter and a smart crawling fabric have been successfully constructed based on the twisted yarn eLCE.

**Liquid metal-based eLCEs:** Liquid metal (LM), especially liquid gallium and its alloys like eutectic gallium indium (EGAln), is a class of emerging multifunctional materials because of its liquid state at room temperature, high conductivity, negligible vapor pressure, and low toxicity. The applications of LMs are closely related to their surface oxidation. The oxide layer at the LM surface can helpfully stabilize the LM shape and decrease its apparent surface tension, which are important for applications in circuit printing and additive manufacturing.[126] A series of eLCEs incorporating LM as the Joule heater have been reported and these LM-based eLCEs exhibit multifunctionality such as crawler, bending, buckling and rotation, because the liquid inclusions can freely deform with the surrounding LCE matrix.[127–145] We can list five types of LM as Joule heater in LCE matrix: (1) LM droplets, (2) LM central wires inside LCE fibers, (3) LM-containing conductive inks for circuit printing on LCEs.

## 1) LM droplets

EGAln microdroplets are embedded in LCE,[127–129] where electrical conductivity in the LM-LCE composite can be induced by mechanical sintering that forms percolating LM pathways,[130] and reconfigured when conductive traces are mechanically damaged (Figure 18a).[127] By using the LC monomer RM82 and

modified Yakacki procedure[47] for LCE preparation (Figure 18b), Shankar et al.[128] investigated the effect of the structure of chain extender and the composition of monomer mixture on the thermomechanical properties of LM-droplets-LCE composite. Majidi et al.[129] showed the actuator properties of LM-droplets-LCE composite could also be tuned by controlling the size of LM droplets through simple processing techniques.

## 2) LM central wires inside LCE fibers

Several groups developed LM-LCE fibers with LM core and LCE shell that can produce both ultrafast contraction rate and large contraction ratio under electrical stimulation. These LM-LCE fibers can be fabricated via core–shell 3D printing (Figure 18c)[131–133] or other technics.[132,135] Recently, Yang et al.[136] used microfluidic technique and two types of liquid metals (LMs), Hg and Ga, to produce programmable and rewritable Joule heating circuit. In this method, Hg and Ga are injected into an LCE hollow fiber to form the driven circuit. The injection is controlled by a microfluidic pump to get the designed distribution of LMs. By the same pump, LMs can be renewed by injecting fresh LMs. Due to the different electrothermal effects of the two LMs, the fiber sections containing Hg with a higher resistivity will contract more than those containing Ga with a lower resistivity. Therefore, a specific driven circuit inside one LCE fiber can result in an electroactive multimodal deformation (Figure 18d).

## 3) LM-containing conductive inks for circuit printing on LCEs

The LM-containing circuits obtained by ink printing on LCEs can also function as stretchable Joule heating elements that deliver heat to the LCE to induce the phase transition.[137–141]
<figure><img src="image_29.png" /></figure>

<figure><img src="image_30.png" /></figure>

<figure><img src="image_31.png" /><figcaption>**Figure 17.** a) LCE iris device with integrated platinum wire heaters and its real-time images showing different apertures tuned by electric field.[115] Reproduced from ref.[115] Copyright (2014), with permission from Wiley-VCH. b) LC monomer mixture for LCE preparation in (a). c) Operation principle of tubular LCE actuators. Bending motion can be realized by applying electrical potential to one of the three heating wires (heating wire in red color); homogeneous contraction can be obtained by applying the electrical potential to all heating wires.[118] Reproduced from ref.[118] Copyright (2019), with permission from American Association for the Advancement of Science. d) Copper-wire-based eLCEs to mimic elephant trunk.[119] Reproduced from ref.[119] Copyright (2024), with permission from Wiley-VCH. e) Synthesis of LCE-TUEG with multiple thiourea dynamic covalent bonds by choosing the poly(ether-thiourea) comprising a triethylene glycol spacer (TUEG4) as crosslinker.[121] Reproduced from ref.[121] Copyright (2022), with permission from Wiley-VCH.</figcaption></figure>

For example, Liu et al. first added magnetic Ni microparticles (5 wt%) into LM, then created LM circuits by magnetic printing on the LCE surface, which was finally covered by silicone (Figure 18e).[138] Others used the conductive biphasic ink with polystyrene-isoprene-styrene (SIS) matrix (EGAln-AgSIS)[142–145] that has high stretchability of more than 600% and high conductivity of 10⁵ S m⁻¹. By direct ink writing (DIW), different patterns of circuits are printed directly on the LCE substrate with high adhesion (Figure 18f).

### 4.2.4. eLCEs with Printed Electrodes from Conductive Inks as Joule Heater

Conductive ink is generally composed of metal or carbon particles or conductive polymers linked to each other by solving agents. Conductive inks based on metal particles have the best

conductivity. After the evacuation of solvents by drying or curing, the inks based on silver particles have a conductivity of ≈10⁵ S m⁻¹ (compared to 59 × 10⁷ S m⁻¹ for bulk copper). Organic inks based on graphene and carbon nanotubes have conductivities of ≈10²–10³ S m⁻¹. Conductive inks are largely applied to flexible electronics. Electrodes printed from them can also be used as Joule heater.

Liu et al.[146] developed Mimosa-leave-mimic interactive systems composed of LCE film and printed actuating and sensing electrodes (Figure 19). Actuating electrodes are printed from a mixture of two commercial inks (50 wt% stretchable silver conductor paste PE874 and 50 wt% stretchable carbon conductor paste PE671), while sensing electrodes are printed from another silver paste, Elepaste NP1. By engineering LC molecular orientation, either parallel or perpendicular to the sensing electrode, two binary electronic logic operations are demonstrated, namely self-propelled gate (SPG) and self-terminated gate (STG). SPG

<figure><img src="image_32.png" /></figure>
<figure><img src="image_33.png" /><figcaption>**Figure 18.** a) Illustrations of LM microdroplets embedding in LCE and photographs of autonomous self-configuring electrical conductivity upon puncturing through the traces.[127] Reproduced from ref.[127] Copyright (2019), with permission from National Academy of Sciences. b) Composition of mixtures to prepare LCEs using thiol-acrylate Michael addition between RM82 and various mole ratios of a range of chain extenders (GDMP, BDMT, EDDT and BMBP) with crosslinker TATATO.[128] Reproduced from ref.[128] Copyright (2021), with permission from Royal Society of Chemistry. c) Schematic illustration of core-shell 3D printing of LM-LCE fibers composed of LM core surrounded by LCE shell, whose director is aligned along the print path.[134] Reproduced from ref.[134] Copyright (2021), with permission from Wiley-VCH. d) Injecting LMs into the LCE hollow fiber to form driven circuits (left, the arrow indicates the flow direction of LMs), and an LCE artificial muscle fiber containing LMs before and after the electrical trigger (right).[136] Reproduced from ref.[136] Copyright (2024), with permission from American Chemical Society. e) Schematic illustration of magnetic printing to get LM circuit containing magnetic Ni microparticles into the LCE film.[138] Reproduced from ref.[138] Copyright (2021), with permission from American Chemical Society. f) Printing of the LCE and EGAln-Ag-SIS ink. The conductive ink is patterned directly on top of an LCE substrate.[144] Reproduced from ref.[144] Copyright (2023), with permission from American Chemical Society.</figcaption></figure>

initiates signal transmission and establishes a positive feedback loop, while STG stops signal transmission and enforces a negative feedback loop. The artificial Mimosa with SPG exhibits a sequential folding response upon human touch by coupling closed-loop feedback and cascading transmission mechanisms across multiple SPGs. Here the aligned LCE is prepared using the

DIW method through the rheological alignment of an acrylate-terminated main chain oligomer synthesized via a Michael addition reaction followed by photo-crosslinking to lockdown the orientation.[45,117–119] The rheological alignment relies on shear forces and elongational flow to align the LC oligomer chains parallel to the flow direction. As LC monomers, Molecule 2 with only
<figure><img src="image_34.png" /><figcaption>**Figure 19.** Mimosa-leave-mimicking eLCE.[146] a) Left, schematic of the response of Mimosa pudica Linn's leaves upon human touch. Right, a printed adaptive electronic unit mimicking a Mimosa leaflet is demonstrated on a human finger. The background image shows the birefringence color of the printed LCE film under the polarized optical microscope. Scale bar, 300 µm. b) Illustration of the direct ink writing method to fabricate LCE. Enlarged images show the principle of i) a self-propelled gate (SPG) by the closure of the gap in the conductor during LCE heating and ii) a self-terminated gate (STG) by interrupting the conductor by heating. Red arrows mark the direction of internal stress. T<sub>th</sub> denotes the threshold temperature. Reproduced from ref.[146] Copyright (2024), with permission from Springer Nature.</figcaption></figure>

two phenyl rings is introduced to the classical RM 257 (Molecule 1 with three phenyl rings) to lower the phase transition temperature due to its weak π-π interactions.[120–122] This LC monomer mixture allows to adjust the phase transition to ensure that the final eLCE with a low threshold temperature (T<sub>th</sub>) can respond to a healthy adult finger temperature range between 20 and 37 °C. These kinds of designs offer opportunities for next-generation soft robotics to adapt to unpredictable natural environments.

Different eLCEs driven by electrothermal effects are summarized in Table 2.

**5. Concluding Remarks**

Artificial muscles based on electroactive polymers (EAPs),[99,100] which convert electrical energy into mechanical energy, have been extensively investigated over the past decades due to their promising applications. These smart materials include ionic EAPs (e.g., conducting polymers such as polypyrrole or polyani-

line, and ionic polymer-metal composites) and electronic EAPs (e.g., dielectric elastomers, ferroelectric polymers, and electrostrictive polymers). Traditionally, eLCEs have been classified as electronic EAPs exhibiting electromechanical effects. In fact, the scope of eLCEs is broader, encompassing electromechanical, electrochemical, and electrothermal systems, with the latter two also including ionic types.

As discussed in Section 2, electromechanical actuation in LCEs can occur through three distinct mechanisms. Aligned nematic LCEs act as anisotropic dielectric elastomers that expand anisotropically in the plane perpendicular to the electric field via Maxwell stress. LCEs with classical nematic (N) and ferroelectric nematic (N<sub>F</sub>) phases also respond to electric fields through electrostriction, inducing mesogen reorientation. Smectic LCEs with SmA* phases can undergo shear deformation parallel to the smectic layers due to the electroclinic effect. eLCEs exhibiting electromechanical effects feature fast response and large strain (except SmA* LCEs, which show smaller strain) and can operate at room temperature. However, they require high activation fields (typically 10–10<sup>2</sup> V µm<sup>-1</sup>), which pose safety concerns and risk electrical breakdown. Nevertheless, the recently discovered N<sub>F</sub> phase may revolutionize this scenario, as the electric field required to reorient N<sub>F</sub> molecules is orders of magnitude lower than that for N molecules. Although no eLCEs with N<sub>F</sub> phase have yet been reported, their development appears imminent.

Section 3 discussed eLCEs utilizing electrochemical effects, which operate under very low voltages (<10 V). These eLCEs are based on typical ionic electroactive devices (i-EADs) with a trilayer structure, consisting of an ion-conducting LCE (iLCE) membrane sandwiched between two soft conducting polymer electrodes. The iLCE combines LCEs with ionic liquids, while conducting polymer belongs to ionic EAP. These trilayer devices enable bending actuation and sensing, with performance dependent on the LC molecular alignment.

As described in Section 4, electrothermal eLCEs operate through Joule heating generated by ionic or electronic conductive media integrated into the LCE system. These eLCEs can function under low voltages (1–10<sup>2</sup> V) even in thick, macroscopic samples, while maintaining the key properties of thermotropic LCEs including large and reversible muscle-like strain, tunable processability, and high programmability. Thus, electrothermal eLCEs are promising active soft materials for artificial muscles. Their main challenge lies in designing efficient compliant heaters. Various ion- and electron-conducting materials have been explored as Joule heaters and are integrated with LCEs.

Ion-conducting materials, such as ionic liquids (ILs), enable electrothermal actuation in ionic eLCEs with the same trilayer i-EAD structure used for electrochemical devices. Joule heating arises from ion motion under alternating current (10 Hz, 5–10 V), driving the LCE through the N–I phase transition. Combining LCEs and i-EADs enables both linear and bending actuation under low voltages (≤10 V) field of different frequencies. The ionic eLCE i-EAD-IPN-LCE, featuring an interpenetrating LCE/ionogel network (i-IPN-LCE) as the central membrane, exhibits mechanical performance comparable to mammalian skeletal muscle and can also function as a bending sensor. However, the mechanical fatigue of the polymer electrodes employed continues to pose a challenge.
<table><caption>**Table 2.** Summary of eLCEs driven by electrothermal effects.</caption><thead><tr><th>eLCEs actuators (LCE-conductive mediums)</th><th>Electrical field</th><th>Actuation frequency [Hz]</th><th>Actuation temperature, heating time required</th><th>Heating rate [°C s<sup>-1</sup>]</th><th>Actuation performance</th><th>Actuation stress [MPa]</th><th>Work capacity [kJ m<sup>-3</sup>]</th><th>Refs.</th></tr></thead><tbody><tr><td>LCE-IL</td><td>±2 V</td><td>0.1</td><td>25 °C, 3.5 s</td><td></td><td>bending strain difference 0.3%</td><td></td><td></td><td>[87]</td></tr><tr><td>LCE-IL/PEDOT:PSS</td><td>±2 V</td><td>0.1</td><td>23 °C, 10 s</td><td></td><td>bending strain difference 0.8%</td><td></td><td></td><td>[88]</td></tr><tr><td>LCE-IL/PEDOT:PSS</td><td>±6 V</td><td>10</td><td>134 °C, 74 s</td><td>1.50</td><td>contraction strain 20%</td><td></td><td>5.38</td><td>[88]</td></tr><tr><td>LCE-IL/PEDOT:PSS</td><td>±5 V</td><td>0.1</td><td>20 °C, 10 s</td><td></td><td>bending strain difference 1.18%</td><td></td><td></td><td>[94]</td></tr><tr><td>LCE-IL/PEDOT:PSS</td><td>±10 V</td><td>10</td><td>127 °C, 25 s</td><td>4.28</td><td>contraction strain 9%</td><td>0.14</td><td>1.12</td><td>[94]</td></tr><tr><td>LCE-CNT film</td><td>15.1 V cm<sup>-1</sup></td><td></td><td>150 °C, 5 s</td><td>25</td><td>contraction strain 3%</td><td></td><td>27</td><td>[109]</td></tr><tr><td>LCE-CNT yarn</td><td>15.1 V cm<sup>-1</sup></td><td></td><td>150 °C, 5 s</td><td>25</td><td>contraction strain 12%</td><td></td><td>97</td><td>[109]</td></tr><tr><td>LCE-CNT</td><td>4 V</td><td>0.05</td><td>233 °C, 11 s</td><td>22</td><td>contraction strain 40%</td><td>1.1</td><td></td><td>[110]</td></tr><tr><td>LCE-MXene</td><td>3.5 V</td><td></td><td>119.2 °C, 60 s</td><td>1.65</td><td>bending 697°</td><td></td><td></td><td>[113]</td></tr><tr><td>LCE-PDA-MXene</td><td>4 V</td><td></td><td>122.8 °C, 10 s</td><td>9.78</td><td>contraction strain 45%</td><td></td><td></td><td>[114]</td></tr><tr><td>LCE-copper wire</td><td>3 V</td><td></td><td>120 °C, 30 s</td><td>3.3</td><td>contraction strain 41%</td><td>0.35</td><td>150</td><td>[118]</td></tr><tr><td>LCE fiber-nickel/chrome alloy wire</td><td>0.3 A</td><td>1</td><td>156.3 °C, 6 s</td><td>22</td><td>contraction strain 43%</td><td></td><td></td><td>[120]</td></tr><tr><td>LCE-Ni/Cr wire</td><td>2.2 W</td><td></td><td>126 °C, 90 s</td><td>1.1</td><td>contraction strain 27%</td><td></td><td>56.6</td><td>[121]</td></tr><tr><td>LCE-CNTs/AgNWs</td><td>6.5 V</td><td></td><td>189 °C, 9 s</td><td>18.7</td><td>bending 180°</td><td>0.46</td><td>9.97</td><td>[123]</td></tr><tr><td>LCE-CB/Cr/Au/PI wire</td><td>8.2 V</td><td></td><td>80 °C</td><td></td><td>contraction strain 24%</td><td></td><td></td><td>[124]</td></tr><tr><td>LCE-silver plated polyamide strand yarn</td><td>0.18 A</td><td></td><td>105 °C, 20 s</td><td>17.5</td><td>contraction strain 19.7</td><td></td><td></td><td>[125]</td></tr><tr><td>LCE-LM droplets</td><td>2 V</td><td></td><td>67.9 °C, 10 s</td><td>4.66</td><td>contraction strain 12%</td><td></td><td></td><td>[128]</td></tr><tr><td>LCE fiber-LM</td><td>2.4 V</td><td></td><td>140 °C, 10 s</td><td>11.5</td><td>contraction strain 26%</td><td>0.6</td><td>417</td><td>[131]</td></tr><tr><td>LCE fiber-LM</td><td>8.9 W cm<sup>-3</sup>,</td><td></td><td>80 °C, 15 s</td><td>3.67</td><td>contraction strain 12%</td><td></td><td></td><td>[133]</td></tr><tr><td>LCE fiber-LM</td><td>40 mW mm<sup>-2</sup></td><td></td><td>179 °C, 150 s</td><td>1.02</td><td>contraction strain 21%</td><td></td><td></td><td>[134]</td></tr><tr><td>LCE-LM circuits</td><td>2 V</td><td></td><td>78 °C, 5 s</td><td>10.6</td><td>bending 350°</td><td>0.375</td><td></td><td>[140]</td></tr><tr><td>LCE-LM circuits</td><td>0.8 A</td><td></td><td>78 °C, 20 s</td><td>1.65</td><td>Bending 60°</td><td></td><td></td><td>[141]</td></tr></tbody></table>

Electron-conducting Joule heaters include carbon nanotubes, metal wires, liquid metals, and printed conductive inks. Their structural design (e.g., spiral or serpentine patterns) and integration with various LCE geometries (films, rods, twisted fibers, or knitted/weaved fabrics) critically influence actuator performance. Representative examples include omnidirectionally deformable eLCEs with hierarchically structured CNTs, elephant-trunk-inspired actuators incorporating copper-wire networks, and mimosa-leaf-inspired interactive devices combining LCE films with printed actuating/sensing electrodes. LCEs are increasingly utilized in additive manufacturing, enabling the transition from 3D printing to 4D printing. Among various additive manufacturing techniques, direct ink writing (DIW) is particularly well suited for fabricating electrothermal eLCEs, as conductive media can be co-printed with LCE precursors in customized patterns. Heat accumulation remains a significant challenge due to the low thermal conductivity of LCEs, leading to slow cooling compared to heating. Potential strategies include (1) using conductive fillers with high thermal conductivity (e.g., CNTs or MXenes) and (2) developing microscale

eLCEs, such as LCE micropillars, which dissipate heat more efficiently.

Analysis of reported eLCEs indicates that significant challenges remain before their widespread use as artificial muscles or actuators. Although each eLCE type reviewed here demonstrates progress and niche potential, high-performance eLCE actuators must still be developed. Future systems should exhibit: (1) low or suitable driving voltages; (2) stable, repeatable electroactive output; (3) electrode materials mechanically compatible with LCEs, avoiding premature fatigue; (4) low transition temperatures in the case of electrothermal eLCEs (especially $T_{NI} < 45$ °C for body-contact applications); and (5) well-integrated Joule heaters that enhance actuation strength without degrading shape programmability. Incorporating sensing capabilities into material design will also be crucial, though few examples exist so far.

In general, the performance of eLCEs arises from a complex interplay of material and device parameters. The mesogen structure and LC order parameter dictate the attainable actuation strain and dielectric anisotropy. Network architecture, particularly crosslink density and topology, balances actuation
strain against stress generation. The degree of mesogen alignment (monodomain versus polydomain) determines anisotropy, efficiency, and reproducibility. Electrical actuation strategies define the required operating field strengths, response dynamics, and long-term stability, while LC phase transitions define the operational window and response kinetics. Geometric factors such as film thickness, aspect ratio, and patterned alignment modulate mechanical output, whereas viscoelastic relaxation limits actuation speed and frequency. Ultimately, device reliability depends on fatigue resistance and electrode-polymer interfacial stability. Optimization therefore requires navigating intrinsic trade-offs among strain and stress, voltage and stability, deformation amplitude and response speed, as well as alignment precision and manufacturability. Finally, we anticipate that the development of high-performance eLCEs will significantly advance their practical applications in soft robotics and wearable technologies.

## Acknowledgements

This work was financially supported by the French National Research Agency (ANR-22-CE06-0023, AS-LCE).

## Conflict of Interest

The authors declare no conflict of interest.

## Keywords

electroactive liquid crystal elastomers, electromechanical effects, electrochemical effects, electrothermal effects, soft actuators

Received: July 21, 2025

Revised: October 27, 2025

Published online:

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<figure><img src="image_35.png" /></figure>

Yakui Deng is currently a postdoctoral fellow in the Institut de Recherche de Chimie Paris, UMR8247, CNRS, Chimie ParisTech, Université Paris Sciences & Lettres. He obtained his Ph.D. degree in Chimie ParisTech-PSL in 2024 under supervision of Dr. Min-Hui Li. His research is focused on the electroactive soft actuators and sensors by liquid crystal elastomers.

<figure><img src="image_36.png" /></figure>

Min-Hui Li is a research director (DR1-CNRS) in the Institut de Recherche de Chimie Paris, UMR8247, CNRS, Chimie ParisTech, Université Paris Sciences & Lettres. She obtained her Ph.D. degree in Université Pierre et Marie Curie in 1993 under supervision of Dr. Jean-Pierre Cotton (CEA, Saclay). Her research is focused on the liquid crystal polymers and elastomers, polymer self-assemblies and polymer nanoparticles for photocatalysis.