Liquid Crystal Elastomer Films Research Articles

A review of humidity-Responsive Actuation in Liquid Crystal elastomer films: Controlling reversibility and irreversibility through hydrogen bonding

Humidity-responsive actuation refers to the phenomenon where certain materials alter their shape, size, or color in response to fluctuations in humidity levels. This behavior is reminiscent of natural processes observed in plants, where seeds and flowers react to moisture. The potential applications of humidity-responsive materials span various fields, including soft robotics, smart sensors, biomimetic devices, and anti-counterfeiting measures. Liquid Crystal Elastomers (LCEs)are particularly noteworthy in this context, as they possess a unique combination of properties derived from both liquid crystals and elastomers. These materials exhibit significant changes in shape in response to external stimuli such as temperature, light, electric fields, and humidity. As a result, this paper focuses on the humidity-responsive properties of Liquid Crystal Elastomer (LCE) films, with a specific emphasis on the role of hydrogen bonding in this process. And this review covers the synthesis methods, underlying mechanisms, performance characteristics, challenges, and future prospects of humidity-responsive Liquid Crystal Elastomer (LCE) films.

Surface wrinkling of a hyperelastic half-space coated by a liquid crystal elastomer film

We consider the stability of a hyperelastic substrate coated by a liquid crystal elastomer film and subjected to compressive forces. In this problem, the liquid crystal elastomer directors are free to evolve and this possible variation needs to be included in the stability analysis. We consider the case where the initial directors are aligned either in the horizontal or in the vertical direction and obtain an exact bifurcation condition for surface wrinkling. We show that director reorientation increases both the critical compressive strain and the critical wavenumber, hence stabilizing the material. In the small wavenumber limit we carry out an asymptotic analysis and obtain analytical solutions for the critical stretch and the critical wavenumber, which can be useful in applications.

Phase patterning of liquid crystal elastomers by laser-induced dynamic crosslinking.

Liquid crystal elastomers hold promise in various fields due to their reversible transition of mechanical and optical properties across distinct phases. However, the lack of local phase patterning techniques and irreversible phase programming has hindered their broad implementation. Here we introduce laser-induced dynamic crosslinking, which leverages the precision and control offered by laser technology to achieve high-resolution multilevel patterning and transmittance modulation. Incorporation of allyl sulfide groups enables adaptive liquid crystal elastomers that can be reconfigured into desired phases or complex patterns. Laser-induced dynamic crosslinking is compatible with existing processing methods and allows the generation of thermo- and strain-responsive patterns that include isotropic, polydomain and monodomain phases within a single liquid crystal elastomer film. We show temporary information encryption at body temperature, expanding the functionality of liquid crystal elastomer devices in wearable applications.

In situ Light‐Writable Orientation Control in Liquid Crystal Elastomer Film Enabled by Chalcones

AbstractLiquid crystal elastomers (LCEs) are stimulus‐responsive materials with intrinsic anisotropy. However, it is still challenging to in situ program the mesogen alignment to realize three‐dimensional (3D) deformations with high‐resolution patterned structures. This work presents a feasible strategy to program the anisotropy of LCEs by using chalcone mesogens that can undergo a photoinduced cycloaddition reaction under linear polarized light. It is shown that by controlling the polarization director and the irradiation region, patterned alignment distribution in a freestanding LCE film can be created, which leads to complex and reversible 3D shape‐morphing behaviors. The work demonstrates an in situ light‐writing method to achieve sophisticated topography changes in LCEs, which has potential applications in encryption, sensors, and beyond.

In situ Light-Writable Orientation Control in Liquid Crystal Elastomer Film Enabled by Chalcones.

Liquid crystal elastomers (LCEs) are stimulus-responsive materials with intrinsic anisotropy. However, it is still challenging to in situ program the mesogen alignment to realize three-dimensional (3D) deformations with high-resolution patterned structures. This work presents a feasible strategy to program the anisotropy of LCEs by using chalcone mesogens that can undergo a photoinduced cycloaddition reaction under linear polarized light. It is shown that by controlling the polarization director and the irradiation region, patterned alignment distribution in a freestanding LCE film can be created, which leads to complex and reversible 3D shape-morphing behaviors. The work demonstrates an in situ light-writing method to achieve sophisticated topography changes in LCEs, which has potential applications in encryption, sensors, and beyond.

A wireless fluorescent flexible force sensor based on aggregation-induced emission doped liquid crystal elastomers.

Flexible strain sensors have drawn a lot of interest in various applications including human mobility tracking, rehabilitation/personalized health monitoring, and human-machine interaction, but suffer from interference of electromagnetic (EM). To overcome the EM interference, flexible force sensors without sensitive electronic elements have been developed, with drawbacks of bulky modules that hinders their applications in remote measurement with power-free environment. Therefore, it is highly desirable to fabricate a compact wireless flexible force sensor but it is still a challenge. Here, we demonstrate a fluorescent flexible force sensor based on aggregation-induced emission (AIE) doped liquid crystal elastomer (LCE) experimentally. The proposed force sensor film can be used to measure force through the variation of fluorescent intensity induced by the extension or contraction of LCE film, which leads to reduce or increase of the aggregation degree of AIE molecules within. This compact wireless force sensor features lightweight, low-cost, high flexibility, passivity and anti-EM interference, which also enables the naked eye observation. The proposed sensor provides inspiration and a platform for a new concept of non-contact detection, showing application potential in human-friendly interactive electronics and remote-control integration platform.

Lifting, Loading, and Buckling in Conical Shells.

Liquid crystal elastomer films that morph into cones are strikingly capable lifters. Thus motivated, we combine theory, numerics, and experiments to reexamine the load-bearing capacity of conical shells. We show that a cone squashed between frictionless surfaces buckles at a smaller load, even in scaling, than the classical Seide-Koiter result. Such buckling begins in a region of greatly amplified azimuthal compression generated in an outer boundary layer with oscillatory bend. Experimentally and numerically, buckling then grows subcritically over the full cone. We derive a new thin-limit formula for the critical load, ∝t^{5/2}, and validate it numerically. We also investigate deep postbuckling, finding further instabilities producing intricate states with multiple Pogorelov-type curved ridges arranged in concentric circles or Archimedean spirals. Finally, we investigate the forces exerted by such states, which limit lifting performance in active cones.

Liquid crystal elastomer film actuators with anti-strain robustness

Thermotropic monodomain liquid crystal elastomers (mLCEs) exhibit reversible contractile actuation properties along the collimated direction of mesogens when stimulated by external temperature, similar to those of biological skeletal muscle, and have great potential applications in flexible actuators and artificial muscles. However, the existence of flexible chain segments causes mLCEs to elongate under a small load, making calibration under different loads necessary when they are used as an actuator. In this study, a physical crosslinking point between flexible segments is provided through the introduction of designed long-chain molecules (LHS) containing amide bonds on the main chain of LCEs. The 5% LHS-mLCEs with enhanced anti-strain robustness achieved essentially constant original length over a range of loads (0–0.25 MPa) and maintained a reversible contraction ratio of 39.4%, comparable to biological skeletal muscle. Furthermore, a nonreconfigurable topological structure of a metal electrothermal layer with lossless compression properties was designed and prepared on the surface of mLCEs. The film actuators exhibit higher frequency oscillatory actuation (0.1 Hz 9.2%; 0.2 Hz 6.3%; 0.5 Hz 2%) relative to previously reported actuators, benefitting from the optimized heat dissipation structure of LCEs/Ag/air. This study provides valuable references and ideas for applying thermotropic mLCEs in practice.

Fully (Re)configurable Interactive Material through a Switchable Photothermal Charge Transfer Complex Gated by a Supramolecular Liquid Crystal Elastomer Actuator.

Charge transfer complexes (CTCs) based on self-assembled donor and acceptor molecules allow light absorption of significantly redshifted wavelengths to either the donor or acceptor. In this work, we demonstrate a CTC embedded in a hydrogen-bonded liquid crystal elastomer (LCE), which in itself is fully reformable and reprocessable. The LCE host acts as a gate, directing the self-assembly of the CTC. When hydrogen bonding is present, the CTC behaves as a near-infrared (NIR) dye allowing photothermal actuation of the LCE. The CTC can be disassembled in specific regions of the LCE film by disrupting the hydrogen bond interactions, allowing selective NIR heating and localized actuation of the films. The metastable non-CTC state may persist for weeks or can be recovered on demand by heat treatment. Besides the CTC variability, the capability of completely reforming the shape, color, and actuation mode of the LCE provides an interactive material with unprecedented application versatility.

Photocontrollable Elongation Actuation of Liquid Crystal Elastomer Films with Well-Defined Crease Structures.

Although liquid crystal elastomers (LCEs) have demonstrated various applications in artificial muscles and soft robotics, their inherent flexibility and orientation-dependent forces limit their functions. For instance, LCEs can sustain a high actuation force when they contract but cannot elongate to drive loads with large displacements. In this study, it is demonstrated that photocontrollable elongation actuation with a large strain can be achieved in polydomain LCEs by programming the crease structures in a well-defined order to couple the actuation forces. Efficient photoactuation without overheating-induced damage to the materials is favored, based on the well-designed photosensitive molecular switch crosslinker via the synergy of photochemical and photothermal effects. The LCE actuator can jack up heavy loads, elongate freely, and contract back to manipulate distant objects. Theoretical analysis based on a finite element simulation of the deformation energy during the actuation process reveals a trade-off between the abilities of jacking-up and withstanding load. More importantly, this study simplifies the design of a single material with functions inherent only in other soft robotic devices based on the assembly of multiplemodules, thus providing a design strategy for surpassing instinctive properties of conventional soft materials to expand the functions of soft robotics.

Self-Sustained Oscillation of Electrothermally Responsive Liquid Crystal Elastomer Film in Steady-State Circuits.

Self-excited oscillations have the advantages of absorbing energy from a stable environment and Self-control; therefore, Self-excited motion patterns have broader applications in micro devices, autonomous robots, sensors and energy-generating devices. In this paper, a Self-sustained curling liquid crystal elastomer (LCE) film-mass system is proposed on the basis of electrothermally responsive materials, which can realize Self-oscillation under a steady-state current. Based on the contact model and dynamic LCE model, a nonlinear dynamics model of LCE film in steady-state circuits is developed and numerical calculations are carried out using the Runge-Kutta method. Through numerical calculations, it is demonstrated that LCE film-mass systems have two motion patterns in steady-state circuits: namely, a Self-oscillation pattern and a stationary pattern. Self-sustained curling of LCE film originates from the fact that the energy absorbed by the system exceeds the energy dissipated due to the damping effect. In addition, the critical conditions for triggering Self-oscillation and the effects of several key dimensionless system parameters on the amplitude and period of Self-oscillation are investigated in detail. Calculation results show that the height of electrolyte solution, gravitational acceleration, elastic modulus of LCE film, limit temperature, curvature coefficient, thermal shrinkage coefficient and damping factor all have a modulating effect on the amplitude and period of Self-oscillation. This research may deepen the understanding of Self-excited oscillation, with promising applications in energy harvesting, power generation, monitoring, soft robotics, medical devices, and micro and nano devices.

Principle of virtual power and drilling degrees of freedom for dynamic modeling of the behavior of liquid crystal elastomer films

In this work, we aim to model the reorientation process of mesogens in nematic liquid crystal elastomers within the context of dynamics. We consider a continuum model with separate mappings for the deformation of the monolithic material and the orientation of the nematic director, where the latter describes the inclination of the mesogens. We achieve the inextensibility of the nematic director through the introduction of drilling degrees of freedom. We combine this approach with the application of the principle of virtual power and a mixed finite element formulation, in order to formulate distinct momentum and angular momentum balance laws for the two separate mappings. Furthermore, we include in our continuum model a volume load and a surface load associated only with the orientation mapping. We show in the presented three numerical examples that our formulation enables the fulfillment of all momentum and angular momentum balance laws.

Liquid Crystal Elastomer Based Dexterous Artificial Motor Unit.

Despite the great advancement in designing diverse soft robots, they are not yet as dexterous as animals in many aspects. One challenge is that they still lack the compact design of an artificial motor unit with a great comprehensive performance that can be conveniently fabricated, although many recently developed artificial muscles have shown excellent properties in one or two aspects. Herein, an artificial motor unit is developed based on gold-coated ultrathin liquid crystal elastomer (LCE) film. Subject to a voltage, Joule heating generated by the gold film increases the temperature of the LCE film underneath and causes it to contract. Due to the small thermal inertial and electrically controlling method of the ultrathin LCE structure, its cyclic actuation speed is fast and controllable. It is shown that under electrical stimulation, the actuation strain of the LCE-based motor unit reaches 45%, the strain rate reaches 750%/s, and the output power density is as high as 1360Wkg-1 . It is further demonstrated that the LCE-based motor unit behaves like an actuator, a brake, or a nonlinear spring on demand, analogous to most animal muscles. Finally, as a proof-of-concept, multiple highly dexterous artificial neuromuscular systems are demonstrated using the LCE-based motor unit.

Light-induced wrinkling instability of confined annulus liquid crystal elastomer film.

Authors of previous studies have focused on wrinkling phenomena in thin films with relatively large stiffness under mechanical loadings. A liquid crystal elastomer (LCE) is a smart soft material sensitive to various environmental stimuli. In this letter, we investigate the wrinkling instability of a confined LCE film with different inner hole sizes induced by light illumination. The LCE film is clamped at the outer edge and has a free inner edge. Based on linear perturbation analysis, we obtain the critical condition for the onset of wrinkling instability. Whether the film will remain flat or bifurcate into a wrinkled state is determined by the value of expansion coefficient, light intensity, inner hole size, and thickness of the LCE film. We find that a film with a bigger inner hole size corresponds to a large wrinkle number and show that the expansion coefficient of a LCE and light intensity can be used to adjust the wrinkling instability behavior and time consumption for the appearance of wrinkles.

Preparation and characterisation of liquid crystal elastomer films with selective reflectivity

ABSTRACT In this paper, the liquid crystal elastomer films (A’BG series) have been prepared by grafting polymerisation, using the 4-allyloxybenzoic acid cholesteryl ester (MB) as liquid crystal monomer, dodecane-1,12-diyl bis(undec-10-enoate) (MG) as flexible cross-linker and polymethyl hydrosiloxane (containing 35 Si-H) as the main chain. The A’BG series films are cholesteric liquid crystal elastomer films (LCEs) with a liquid crystal region above 100°C. With the crosslinker content decreasing, A’BG-2 and A’BG-3 display selective reflection in UV region and visible region, respectively. The selective reflection wavelength of A’BG-3 occurs blue shift with the temperature increasing. The glass transition temperatures (Tg) of the A’BG series films are 16.4–40.8°C and A’BG series films display shape memory properties near room temperature. The A’BG series elastomer films have a high thermal stability and the temperatures of the 5% mass loss are above 300°C. The backbone siloxane structures in the A’BG series elastomer films are highly stable, and the -COOChol group is relatively easy to decompose.

Circularly Polarized Light‐driven Liquid Crystal Elastomer Actuators

AbstractPhoto‐responsive liquid crystal elastomer (LCE) actuator is of paramount significance for emerging applications in soft robotics, biomimetic devices, and adaptive optical devices. However, to achieve circularly polarized light‐driven LCE actuator, which can provide an additional degree of freedom of light in the control method, is still a challenge. Herein, a circularly polarized light‐driven LCE actuator is demonstrated. The actuator is fabricated by bonding a cholesteric liquid crystal (CLC) film, which is capable of selectively reflecting and transmitting circularly polarized light, with a photothermal agent doped LCE film in splay configuration as a bilayer structure. An electrically controlled optical system with a fast switching ability of circularly polarized light's handedness has been developed to study the circularly polarized light‐response bending property of the proposed actuator under various periodic driven voltage schemes. A crawler driven by circularly polarized light based on CLC‐LCE actuator is demonstrated experimentally with different moving modes. The application of circularly polarized light in the LCE actuator driving provides an additional controlling degree of freedom for photo‐responsive LCE actuators besides the wavelength, spatial distribution, and linear polarization of light, showing wide application prospects in soft robotics, biomimetic technologies, and adaptive optical devices.

Automated photo-aligned liquid crystal elastomer film fabrication with a low-tech, home-built robotic workstation

Laboratory procedures are often considered so unique that automating them is not economically justified – time and resources invested in designing, building and calibrating the machines are unlikely to pay off. This is particularly true if cheap labour force (technicians or students) is available. Yet, with increasing availability and dropping prices of many off-the-shelf components such as motorised stages, grippers, light sources (LEDs and lasers), detectors (high resolution, fast cameras), as well as user-friendly programmable microprocessors, many of the repeatable tasks may soon be within reach of either custom-built or universal lab robots. Building on our previous work on fabrication, characterization and applications of light-responsive liquid crystal elastomers (LCEs) in micro-robotics and micro-mechanics, in this paper we present a robotic workstation that can make LCE films with arbitrary molecular orientation. Based on a commercial 3D printer, the RoboLEC (Robot for LCE fabrication) performs precision component handling, structured light illumination, liquid dispensing and UV-triggered polymerization, within a four-hour-long procedure. Thus fabricated films with patterned molecular orientation are compared to the same, but handmade, structures.

Synthesis and characterization of cholesteric liquid crystal elastomer films

A series of liquid crystal elastomer films (the PBC series) have been synthesized by grafting liquid crystal (LC) monomer 4-allyloxybenzoic acid cholesteryl ester (MB) and crosslinking agent [1,1'-biphenyl]-4,4'-diyl bis(undec-10-enoate) (MC) onto polymethylhydrosiloxane catalyzed by hexachloroplatinic acid at 60 °C. The PBC series are cholesteric liquid crystal elastomers (LCEs). The glass transition temperatures (Tg) of the PBC series are between 23-33 °C, and the LC range is above 200 °C. The temperatures of 5% weight loss of the PBC series are above 300 °C, which shows good thermal stability. The PBC series display good orientation. PB3C3 shows a good ability of shape recovery above Tg.

Liquid Crystal Elastomer Based Thermal Microactuators and Photothermal Microgrippers Using Lateral Bending Beams

AbstractThe remarkable properties of liquid crystal elastomers (LCE) for spatially controlled actuation are utilized to perform complex tasks such as to mimic versatile motion of living organisms and cilia in soft robotics. As opposed to the common out‐of‐plane bending in flat LCE films, many planar micro‐electro‐mechanical and microfluidic systems typically operate based on the lateral movement of the functional parts. The challenge of implementing the in‐plane bending of LCE actuators is often associated with the limited capability for miniaturization of the existing techniques, along with the requirement for post‐fabrication assembly. In this paper a method for fabrication of thermally actuated LCE beams for in‐plane bending applications, which is based on direct laser writing and micromolding is reported. Mesogenic alignment is modified near the sidewalls of the channel structures patterned with vertical microgrooves. The method allows monolithic fabrication of LCE‐based microgrippers for micromanipulation and microassembly applications in life sciences and in manufacture of microsystems. It is shown that the photothermally driven microgrippers can perform soft grasping of micro‐objects, providing large gripping strokes with relatively low actuation stimuli. The fabrication method offers more design opportunities for LCE‐based microactuators and a useful route toward realization of gripping and cargo transportation functionality in microrobotics.

Effects of network structure of main-chain liquid crystal elastomer on its thermal actuation performance

The effect of the network structure on the thermal actuation of the main-chain liquid crystal elastomers (LCEs) was studied by controlling the strand length and crosslinking density of the network. The LCEs were prepared with the thiol-terminated oligomers synthesized via a thiol-acrylate Michael addition reaction between dithiol and diacrylate monomers. The LCE network structure was formed by crosslinking the oligomers using the pentaerythritol tetraacylate crosslinkers and coupling between the thiol end groups in the oligomer to form disulfide linkages. The monodomain LCE (MLCE) films were fixed by the dynamic exchange reaction between disulfide linkages through ultraviolet irradiation after drawing the polydomain LCE films. The maximum degree of actuation was obtained when 50% of the available thiol groups were crosslinked with the acrylate groups of the crosslinkers. The degree of actuation increased with the oligomer length for sufficiently high crosslinking density. However, the degree of actuation decreased with the oligomer length after reaching the maximum especially for low crosslinker density because crosslinking with long oligomers did not provide enough crosslinking sites and disulfide linkages. Thus, the oligomer length and crosslinking density of the MLCE are important control factors during LCE preparation to obtain the best actuation performance of the MLCE film.