•The strain and stress concentration behaviors of a monodomain LCE sheet with an elliptical hole subjected to uniaxial stretches are studied using finite element methods.•Liquid crystal elastomers ...exhibit severer stress and strain concentration than neoHookean, and such concentration is further aggravated with the sharpening of the elliptical hole.•The location of maximal strain deviates slightly from that of maximal stress when the applied stretches are small. The two locations merge together as the elliptical holes become sharper or the applied stretches become larger.•Director reorientations and resulting spontaneous strains around the hole edge are the main causes of these unusual stress and strain concentration behaviors.
Liquid crystal elastomers (LCEs) are a kind of soft materials which couple the high elastic deformability with the unique properties of liquid crystals. Such combination endows notched LCEs unusual stress and strain concentration behaviors, which is preliminary discussed by us in the case of a large sheet with a centralized circular hole. Here we extend our study from circular holes to more generalized, elliptical holes. We investigate the stress and strain fields around a centralized elliptical hole in a LCE sheet subjected to uniaxial stretches by finite element methods. The effects of the shape factor, defined as the ratio of the minor-to-major axis of the ellipse, are mainly concerned. Compared with common elastomers like neoHookean, LCEs get higher stress and strain concentration factors (SCFs), and the location of maximal strain deviates slightly from that of maximal stress. With the decrease of the shape factor (a sharper ellipse) the SCFs of LCEs rise much severer than those of neoHookean, and the deviation is diminishing. In addition, as the stretch increases, the region near the root is getting close to an approximately uniaxial stress state, and the high strain region will extend non-monotonously along the hole edge, which differs from a monotonous extension of the high stress region. All the unusual behaviors of LCEs above are attributed to the spontaneous strain induced by director rotation. Attempts to correlate stress and strain concentration behaviors to the spontaneous strain are made in this article.
Liquid crystal elastomer (LCE) fibers are capable of large and reversible deformations, making them an ideal artificial muscle. However, limited to stimulating source and structural design, current ...LCE fibers have not yet achieved both large contraction ratio and fast contraction rate to perform the intense motion. In this work, electrothermal‐responsive liquid metal (LM) containing LCE (LM‐LCE) fibers is reported. By introducing flexible liquid metal, LM‐LCE fibers retain deformability with a large contraction ratio similar to that of pure LCE fibers and are endowed with electrical responsiveness. Applying precisely controlled electrical stimulation, the contraction ratio and rate of LM‐LCE fibers can be programmed by adjusting voltage value and pulse time. Under electrical stimulation at 1.25 V cm−1, 0.1 s, LM‐LCE fibers can produce over 40% contraction ratio at an ultrafast contraction rate of up to 280% s−1. Furthermore, LM‐LCE fibers mimic human triceps muscle and can conduct precise ball shooting. LM‐LCE fibers with excellent contraction ratio and rate extend their functionality as artificial muscles to perform intense movements and are expected to enrich the challenging applications of soft robots.
An electrothermal‐responsive liquid metal containing liquid crystal elastomer (LM‐LCE) fiber is developed and generated high contraction over 40% with ultrafast contraction rate over 284% s−1 under pulsed voltage. By mimicking human triceps muscle, the fiber actuates an artificial arm to conduct precise ball shooting. It extends the functionality of LCE as artificial muscles to perform intense motions and to actuate soft robots undertaking versatile tasks.
Multicolor Switching
In article number 2302456, Su Seok Choi and co‐workers investigate biomimetic multicolor separation control of mechanochromic switching using electrically stretchable chiral ...liquid crystal elastomers (CLCEs). With advanced engineering of electrically stretchable CLCEs, various multicolor switching devices are experimentally verified. A chameleon‐like synthetic multicolor photonic skin control is also reported, with invisibility switching using electrically stretchable CLCEs. The conceived multicolor CLCEs are also validated with camouflage information control using color pixels reconfiguration and invisibility switching.
3D printed, stimuli-responsive materials that reversibly actuate between programmed shapes are promising for applications ranging from biomedical implants to soft robotics. However, current 3D ...printing of reversible actuators significantly limits the range of possible shapes and/or shape responses because they couple the print path to mathematically determined director profiles to elicit a desired shape change. Here, we report a reactive 3D-printing method that decouples printing and shape-programming steps, enabling a broad range of complex architectures and virtually any arbitrary shape changes. This method involves first printing liquid crystal elastomer (LCE) precursor solution into a catalyst bath, producing complex architectures defined by printing. Shape changes are then programmed through mechanical deformation and UV irradiation. Upon heating and cooling, the LCE reversibly shape-shifts between printed and programmed shapes, respectively. The potential of this method was demonstrated by programming a variety of arbitrary shape changes in a single printed material, producing auxetic LCE structures and symmetry-breaking shape changes in LCE sheets.
Untethered twist fibers do not require end‐anchoring structures to hold their twist orientation and offer simple designs and convenient operation. The reversible responsiveness of these fibers allows ...them to generate torque and rotational deformation continuously upon the application of external stimuli. The fibers therefore have potential in rotating microengines. In practical applications, high torque and rotational deformation are desirable to meet work capacity requirements. However, the simultaneous endowment of reversible responsiveness and high rotational performance to untethered twist fibers remains a challenge. In this study, a liquid crystal elastomer twist fiber (LCETF) is designed and developed with a fixed twisting alignment of mesogens to provide untethered and reversible responsiveness. Outstanding rotational performance can be achieved when the mesogenic orientation is disrupted through heat triggering. Owing to the significant intrinsic contractile ratio of the LCE material, the rotational deformation of the LCETF can reach 243.6° mm−1. More importantly, the specific torque can reach 10.1 N m kg−1, which exceeds previously reported values. In addition, the LCETF can be exploited in a rotating microengine to convert heat into electricity with an induction voltage as high as 9.4 V. This work broadens the applications of LCEs for energy harvesters, micromachines, and soft robots.
Untethered twist fibers with reversible responsiveness have shown great potential as rotating microengine, of which high torque and high rotational deformation are desirable. Herein, liquid crystal elastomer twist fiber (LCETF) with fixed twisting orientation is developed, demonstrating outstanding rotational performance due to its significant intrinsic deformation. The obtained LCETF can function as a rotating microengine in an electricity generator.
The integration of soft, stimuli‐responsive materials in robotic systems is a promising approach to introduce dexterous and delicate manipulation of objects. Electrical control of mechanical response ...offers many benefits in robotic systems including the availability of this energy input, the associated response time, magnitude of actuation, and opportunity for self‐regulation. Here, a materials chemistry is detailed to prepare liquid crystal elastomers (LCEs) with a 14:1 modulus contrast and increase in dielectric constant to enhance electromechanical deformation. The inherent modulus contrast of these LCEs (when coated with compliant electrodes) directly convert an electric field to a directional expansion of 20%. The electromechanical response of LCE actuators is observed upon application of voltage ranging from 0.5 to 6 kV. The deformation of these materials is rapid, reaching strain rates of 18% s−1. Upon removal of the electric field, little hysteresis is observed. Patterning the spatial orientation of the nematic director of the LCEs results in a 2D–3D shape transformation to a cone 8 mm in height. Individual and sequential addressing of an array of LCE actuators is demonstrated as a haptic surface.
Stimuli‐responsive materials are compelling approaches to advance the capabilities of soft robots as artificial muscles. A material chemistry is reported to prepare liquid crystal elastomers with a fivefold increase in directional actuation response to electric fields. Electromechanical deformation of liquid crystal elastomers is shown to be rapid and locally programmable.
Leveraging liquid crystal elastomers (LCEs) to realize scalable fabrication of high‐performing fibrous artificial muscles is of particular interest because these active soft materials can provide ...large, reversible, programmable deformations upon environmental stimuli. High‐performing fibrous LCEs require the used processing technology to enable shaping LCEs into micro‐scale fine fibers as thin as possible while achieving macroscopic LC orientation, which however remains a daunting challenge. Here, a bioinspired spinning technology is reported that allows for continuous, high‐speed production (fabrication speed up to 8400 m h−1) of thin and aligned LCE microfibers combined with rapid deformation (actuation strain rate up to 810% s−1), powerful actuation (actuation stress up to 5.3 MPa), high response frequency (50 Hz), and long cycle life (250 000 cycles without obvious fatigue). Inspired by liquid crystalline spinning of spiders that takes advantage of multiple drawdowns to thin and align their dragline silks, internal drawdown via tapered‐wall‐induced‐shearing and external drawdown via mechanical stretching are employed to shape LCEs into long, thin, aligned microfibers with the desirable actuation performances, which few processing technologies can achieve. This bioinspired processing technology capable of scalable production of high‐performing fibrous LCEs would benefit the development of smart fabrics, intelligent wearable devices, humanoid robotics, and other areas.
A bioinspired spinning technology is reported that allows for continuous, high‐speed production of thin and aligned liquid crystal elastomer (LCE) microfibers combined with rapid deformation, powerful actuation, high response frequency, and long cycle life. This processing technology capable of scalable production of high‐performing fibrous LCEs would benefit the development of smart fabrics, intelligent wearable devices, humanoid robotics, and other areas.
Multi‐stimuli‐responsive soft materials introduce a new dimension in the design and control of soft machines, enhancing their use in complex applications. In article number 2100336, Metin Sitti and ...co‐workers report a bimorph material with uncoupled integration of magnetic‐responsive elastomer and liquid crystal elastomer. This material enables heterogeneous material properties and local addressability for independent control of separate parts within a single device, enhancing its physical intelligence. The illustrated legged milli‐robot showcases one example of the robotic application of the reported material.
Bio‐Inspired Soft‐Rigid Hybrid Smart Artificial Muscle
In article number 2206342, Hongmiao Tian and co‐workers propose a soft‐rigid hybrid smart artificial muscle (SRH‐SAM), which ingeniously ...coordinates the actuation, load‐bearing, and sensory systems. Based on the SRH‐SAM, an advanced approach for a reconfigurable blazed grating plane and a new strategy for controllable attachment/detachment transformation in the field of bionic dry adhesion are completed. This study exhibits a promising prospect for the development of artificial muscles.
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•New conjugated polymers with high photothermal conversion efficiency (52.7%) and photostability were synthesized.•Light-responsive conjugated polymers/liquid crystal photoresists ...with a room-temperature liquid crystal phase were developed for direct laser writing via two-photon polymerization.•The incorporation of the 0.3 wt% conjugated polymers could lower the nematic-to-isotropic temperature of the LC photoresists to 48.0 °C.•A large actuation strain (25.0%) of the 4D printed microactuator was achieved under the stimulation of near-infrared light with 808 nm wavelength.
4D printed photo-triggered liquid crystal elastomers (LCEs) microactuators by direct laser writing via two-photon polymerization (DLW-TPP) have attracted increasing attention due to their manipulation flexibility, reversible and rapid actuation capabilities. However, their development is hampered by the lack of room-temperature printable liquid crystal (LC) photoresists. Here, we developed new light-responsive LC photoresists by incorporating novel conjugated polymers (CPs) as photothermal agents for the DLW-TPP technology. The CPs displayed a remarkable photothermal effect and effectively avoided the aggregation problems that always happened for inorganic nanoparticles in photoresists. Moreover, the CPs incorporation lowered the nematic-to-isotropic temperature of the LC photoresists which is beneficial for room-temperature DLW-TPP. The printing parameters, including laser power and scanning speed, were investigated using the developed LC photoresists. It was found the range of printing parameters decreased with the increase of the CPs loading fraction from 0.1 to 0.5 wt%, which was attributed to the high photothermal conversion efficiency (52.7%). A well-defined CPs/LCEs microactuator with CPs as low as 0.3 wt% was printed, which could achieve a large 25.0% actuation strain in 5 s upon near-infrared (NIR) light stimulation. It could be used for thriving soft micro-robotics and micro-membranes with controllable separation capabilities.
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