Liquid crystalline elastomers as actuators and sensors

Multimaterial Printing of Liquid Crystal Elastomers with Integrated Stretchable Electronics

Liquid crystal elastomers (LCEs) have grown in popularity in recent years as a stimuli-responsive material for soft actuators and shape reconfigurable structures. To make these material systems electrically responsive, they must be integrated with soft conductive materials that match the compliance and deformability of the LCE. This study introduces a design and manufacturing methodology for combining direct ink write (DIW) 3D printing of soft, stretchable conductive inks with DIW-based "4D printing" of LCE to create fully integrated, electrically responsive, shape programmable matter. The conductive ink is composed of a soft thermoplastic elastomer, a liquid metal alloy (eutectic gallium indium, EGaIn), and silver flakes, exhibiting both high stretchability and conductivity (order of 10⁵ S m^-1). Empirical tuning of the LCE printing parameters gives rise to a smooth surface (<10 μm) for patterning the conductive ink with controlled trace dimensions. This multimaterial printing method is used to create shape reconfigurable LCE devices with on-demand circuit patterning that could otherwise not be easily fabricated through traditional means, such as an LCE bending actuator able to blink a Morse code signal and an LCE crawler with an on/off photoresistor controller. In contrast to existing fabrication methodologies, the inclusion of the conductive ink allows for stable power delivery to surface mount devices and Joule heating traces in a highly dynamic LCE system. This digital fabrication approach can be leveraged to push LCE actuators closer to becoming functional devices, such as shape programmable antennas and actuators with integrated sensing.

Uni- and Bidirectional Rotation and Speed Control in Chiral Photonic Micromotors Powered by Light

Liquid crystalline polymers are attractive materials for untethered miniature soft robots. When they contain azo dyes, they acquire light-responsive actuation properties. However, the manipulation of such photoresponsive polymers at the micrometer scale remains largely unexplored. Here, uni- and bidirectional rotation and speed control of polymerized azo-containing chiral liquid crystalline photonic microparticles powered by light is reported. The rotation of these polymer particles is first studied in an optical trap experimentally and theoretically. The micro-sized polymer particles respond to the handedness of a circularly polarized trapping laser due to their chirality and exhibit uni- and bidirectional rotation depending on their alignment within the optical tweezers. The attained optical torque causes the particles to spin with a rotation rate of several hertz. The angular speed can be controlled by small structural changes, induced by ultraviolet (UV) light absorption. After switching off the UV illumination, the particle recovers its rotation speed. The results provide evidence of uni- and bidirectional motion and speed control in light-responsive polymer particles and offer a new way to devise light-controlled rotary microengines at the micrometer scale.

A cold-responsive liquid crystal elastomer provides visual signals for monitoring a critical temperature decrease

Critical temperature indicators have been extensively utilized in various fields, ranging from healthcare to food safety. However, the majority of the temperature indicators are designed for upper critical temperature monitoring, indicating when the temperature rises and exceeds a predefined limit, whereas stringently demanded low critical temperature indicators are scarcely developed. Herein, we develop a new material and system that monitor temperature decrease, e.g., from ambient temperature to the freezing point, or even to an ultra-low temperature of -20 °C. For this purpose, we create a dynamic membrane which can open and close during temperature cycles from high temperature to low temperature. This membrane consists of a gold-liquid crystal elastomer (Au-LCE) bilayer structure. Unlike the commonly used thermo-responsive LCEs which actuate upon temperature rise, our LCE is cold-responsive. This means that geometric deformations occur when the environmental temperature decreases. Specifically, upon temperature decrease the LCE creates stresses at the gold interface by uniaxial deformation due to expansion along the molecular director and shrinkage perpendicular to it. At a critical stress, optimized to occur at the desired temperature, the brittle Au top layer fractures, which allows contact between the LCE and material on top of the gold layer. Material transport via cracks enables the onset of the visible signal for instance caused by a pH indicator substance. We apply the dynamic Au-LCE membrane for cold-chain applications, indicating the loss of the effectiveness of perishable goods. We anticipate that our newly developed low critical temperature/time indicator will be shortly implemented in supply chains to minimize food and medical product waste.

Bio-Inspired Soft-Rigid Hybrid Smart Artificial Muscle Based on Liquid Crystal Elastomer and Helical Metal Wire

Artificial muscles are of significant value in robotic applications. Rigid artificial muscles possess a strong load-bearing capacity, while their deformation is small; soft artificial muscles can be shifted to a large degree; however, their load-bearing capacity is weak. Furthermore, artificial muscles are generally controlled in an open loop due to a lack of deformation-related feedback. Human arms include muscles, bones, and nerves, which ingeniously coordinate the actuation, load-bearing, and sensory systems. Inspired by this, a soft-rigid hybrid smart artificial muscle (SRH-SAM) based on liquid crystal elastomer (LCE) and helical metal wire is proposed. The thermotropic responsiveness of the LCE is adopted for large reversible deformation, and the helical metal wire is used to fulfill high bearing capacity and electric heating function requirements. During actuation, the helical metal wire's resistance changes with the LCE's electrothermal deformation, thereby achieving deformation-sensing characteristics. Based on the proposed SRH-SAM, a reconfigurable blazed grating plane and the effective switch between attachment and detachment in bionic dry adhesion are accomplished. The SRH-SAM opens a new avenue for designing smart artificial muscles and can promote the development of artificial muscle-based devices.

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 1360 W kg^-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.

Modeling and Control Strategies for Liquid Crystal Elastomer-Based Soft Robot Actuator

Liquid crystal elastomer is a type of soft material with unique physical and chemical properties that offer a variety of possibilities in the growing field of soft robot actuators. This type of material is able to exhibit large, revertible deformation under various external stimuli, including heat, electric or magnetic fields, light, etc., which may lead to a wide range of different applications such as bio-sensors, artificial muscles, optical devices, solar cell plants, etc. With these possibilities, it is important to establish modeling and control strategies for liquid crystal elastomer-based actuators, to obtain the accurate prediction and description of its physical dynamics. However, so far, existing studies on this type of the actuators mainly focus on material properties and fabrication, the state of art on the modeling and control of such actuators is still preliminary. To gain a better understanding on current studies of the topic from the control perspective, this review provides a brief collection on recent studies on the modeling and control of the liquid crystal elastomer-based soft robot actuator. The review will introduce the deformation mechanism of the actuator, as well as basic concepts. Existing studies on the modeling and control for the liquid crystal elastomer-based actuator will be organized and introduced to provide an overview in this field as well as future insights.

Dynamically Tunable Optical Cavities with Embedded Nematic Liquid Crystalline Networks

Tunable metal-insulator-metal (MIM) Fabry-Pérot (FP) cavities that can dynamically control light enable novel sensing, imaging and display applications. However, the realization of dynamic cavities incorporating stimuli-responsive materials poses a significant engineering challenge. Current approaches rely on refractive index modulation and suffer from low dynamic tunability, high losses, and limited spectral ranges, and require liquid and hazardous materials for operation. To overcome these challenges, a new tuning mechanism employing reversible mechanical adaptations of a polymer network is proposed, and dynamic tuning of optical resonances is demonstrated. Solid-state temperature-responsive optical coatings are developed by preparing a monodomain nematic liquid crystalline network (LCN) and are incorporated between metallic mirrors to form active optical microcavities. LCN microcavities offer large, reversible and highly linear spectral tuning of FP resonances reaching wavelength-shifts up to 40 nm via thermomechanical actuation while featuring outstanding repeatability and precision over more than 100 heating-cooling cycles. This degree of tunability allows for reversible switching between the reflective and the absorbing states of the device over the entire visible and near-infrared spectral regions, reaching large changes in reflectance with modulation efficiency ΔR = 79%.

Magnetic-responsive Covalent Adaptable Networks

Covalent adaptable networks (CANs) are reprocessable polymers whose structural arrangement is based on the recombination of dynamic covalent bonds. Composite materials prepared by incorporating magnetic particles into CANs attract much attention due to their remote and precise control, fast response speed, high biological safety and strong penetration of magnetic stimuli. These properties often involve magnetothermal effect and direct magnetic-field guidance. Besides, some of them can also respond to light, electricity or pH values. Thus, they are favorable for soft actuators since various functions are achieved such as magnetic-assisted self-healing (heating or at ambient temperature), welding (on land or under water), shape-morphing, and so on. Although magnetic CANs just start to be studied in recent two years, their advances are promised to expand the practical applications in both cutting-edge academic and engineering fields. This review aims to summarize recent progress in magnetic-responsive CANs, including their design, synthesis and application.

Continuous Flow Microfluidic Production of Arbitrarily Long Tubular Liquid Crystal Elastomer Peristaltic Pump Actuators

While liquid crystal elastomers (LCEs) are ideal materials for soft-robotic actuators, filling the role of muscle and shape-defining material simultaneously, it is non-trivial to give them ground state shapes beyond simple sheets or fibers. Here tubular LCE actuators scalable to arbitrary length are produced using a continuous three-phase coaxial flow microfluidic process. By pumping an oligomeric precursor solution between inner and outer aqueous phases in a cylindrically symmetric nested capillary set-up, and by reducing the interfacial tension to negligible values using surfactants adapted to each phase, the tubular liquid flow is stabilized over distances more than 200 times the diameter or 2000 times the thickness. In situ photocrosslinking of the middle phase turns it into an LCE network that is flow-aligned by the shear gradient over the phase. The reversible actuation of the tubes upon heating yields a reduction of the interior space, pumping out enclosed fluid, and the relaxation upon cooling leads to the fluid being sucked back in. By moving a local heat source along the tube, it acts as a peristaltic pump. It is proposed that the tubes could, pending functionalization for light-triggered actuation, function as active synthetic vasculature in biological contexts.

Wireless electrochemical actuation of soft materials towards chiral stimuli

Different areas of modern chemistry, require wireless systems able to transfer chirality from the molecular to the macroscopic event. The ability to recognize the enantiomers of a chiral analyte is highly desired, since in the majority of cases such molecules present different physico-chemical properties that could lead, eventually, to dangerous or harmful interactions with the environment or the human body. From an electrochemical point of view, enantiomers have the same electrochemical behavior except when they interact in a chiral environment. In this Feature Article, different approaches for the electrochemical recognition of chiral information based on the actuation of conducting polymers are described. Such a dynamic behavior of π-conjugated materials is based on an electrochemically induced shrinking/swelling transition of the polymeric matrix. Since all the systems, described so far in the literature, are achiral and require a direct connection to a power supply, new strategies will be presented in the manuscript, concerning the implementation of chirality in electrochemical actuators and their use in a wireless manner through bipolar electrochemistry. Herein, the synergy between the wireless unconventional actuation and the outstanding enantiorecognition of inherent chiral oligomers is presented as an easy and straightforward read out of chiral information in solution. This approach presents different advantages in comparison to classic electrochemical systems such as its wireless nature and the possible real-time data acquisition.

A multi-stimulus-responsive bionic fish microrobot for remote intelligent control applications

In nature, all creatures have their unique characteristics that allow them to adapt to the complex and changeable living environments. In recent years, bionic fish has received increased attention from the research community, and many fish-like microrobots driven by the Marangoni effect have been developed. They are generally characterized by easy operation and rapid driving. However, traditional fish-like microrobots can only be driven by a single stimulus and move on two-dimensional (2D) gas-liquid interfaces, which greatly limits their ability in obstacle avoidance and transportation. In this article, we propose a multi-stimulus-responsive bionic fish microrobot, which is made of temperature-responsive hydrogel poly(N-isopropylacrylamide) (pNIPAM). This microrobot is impregnated with carbon nanotubes (CNTs) and Fe₃O₄ and therefore has magnetic and photothermal conversion properties. Under the action of optical, magnetic or ethanol molecules, the microrobot can perform complex programmable translational motion on 2D surfaces and controllable rising and sinking, while realizing motion simulation and obstacle avoidance. The microrobot is expected to be used for a wide range of applications in intelligent control systems.

Electroactive Polymer-Based Soft Actuator with Integrated Functions of Multi-Degree-of-Freedom Motion and Perception

Soft actuators have received extensive attention in the fields of soft robotics, biomedicine, and intelligence systems owing to their advantages of pliancy, silence, and essential safety. However, most existing soft actuators have only single actuation elements and lack sensing. Therefore, it is difficult for them to perform complex motions with multiple degrees of freedom (multi-DOFs) and high precision. This article reports a miniature columnar dielectric elastomer actuator (DEA) with multi-DOF actuation and sensing, which was fabricated with an electroactive polymer acrylic film (Very High Bond [VHB] acrylic film by 3M Company) and carbon black grease electrodes. The arrangement of the simulation electrodes on the VHB was optimized to realize multi-DOF actuation, and the sensing electrodes were configured on the outer part of the DEA to realize real-time sensing. The results showed that the soft actuator can achieve all-round actuation through the selective power of the stimulation electrodes with a controllable voltage. The maximum bending angle and axial strain of the actuator reached 50° and 13%, respectively. Moreover, the deformation modes, direction, and quantity could be precisely measured using the integrative sensing function. In addition, to demonstrate the advantages of the proposed actuator, a manipulator with multiple actuators was designed and controlled to realize different actions of screwing and grasping with sensing. This research is useful not only for the design of multifunctional soft actuators but also for the development of soft robots with flexible, complex, and precisely controllable motions.

Localized Photoactuation of Polymer Pens for Nanolithography

Localized actuation is an important goal of nanotechnology broadly impacting applications such as programmable materials, soft robotics, and nanolithography. Despite significant recent advances, actuation with high temporal and spatial resolution remains challenging to achieve. Herein, we demonstrate strongly localized photoactuation of polymer pens made of polydimethylsiloxane (PDMS) and surface-functionalized short carbon nanotubes based on a fundamental understanding of the nanocomposite chemistry and device innovations in directing intense light with digital micromirrors to microscale domains. We show that local illumination can drive a small group of pens (3 × 3 over 170 μm × 170 μm) within a massively two-dimensional array to attain an out-of-plane motion by more than 7 μm for active molecular printing. The observed effect marks a striking three-order-of-magnitude improvement over the state of the art and suggests new opportunities for active actuation.

Smart Nematic Liquid Crystal Polymers for Micromachining Advances

The miniaturization of tools is an important step in human evolution to create faster devices as well as precise micromachines. Studies around this topic have allowed the creation of small-scale objects capable of a wide range of deformation to achieve complex tasks. Molecular arrangements have been investigated through liquid crystal polymer (LCP) to program such a movement. Smart polymers and hereby liquid crystal matrices are materials of interest for their easy structuration properties and their response to external stimuli. However, up until very recently, their employment at the microscale was mainly limited to 2D structuration. Among the numerous issues, one concerns the ability to 3D structure the material while controlling the molecular orientation during the polymerization process. This review aims to report recent efforts focused on the microstructuration of LCP, in particular those dealing with 3D microfabrication via two-photon polymerization (TPP). Indeed, the latter has revolutionized the production of 3D complex micro-objects and is nowadays recognized as the gold standard for 3D micro-printing. After a short introduction highlighting the interest in micromachines, some basic principles of liquid crystals are recalled from the molecular aspect to their implementation. Finally, the possibilities offered by TPP as well as the way to monitor the motion into the fabricated microrobots are highlighted.

Merging the Interfaces of Different Shape-Shifting Polymers Using Hybrid Exchange Reactions

Sophisticated shape-shifting structures and integration of advanced functions often call for different-chemistry-based polymers (such as epoxy and polyurethane) in a unified system. However, permanent cross-links pose crucial obstacles to be seamless. Here, merging interfaces via hybrid exchange reactions among different dynamic covalent bonds (including ester, urethane, thiourethane, boronic-ester, and oxime-ester linkages) is proposed, breaking the long-lasting restriction that these widely used bonds only undergo self-exchange reactions. Model compound studies are conducted to verify that hybrid exchange reactions occur. As demonstrations, different liquid crystal elastomers are tenaciously joined into coherent assemblies, with the desired biomimetic structures (e.g., flying fish containing stiff and flexible parts) and rare deformation modes (e.g., flower blooming upon both heating and cooling). Besides connecting polymers, hybrid exchange reactions also facilitate the creation of new materials through cross-fusion of different polymers. In addition to the polymers used in this work, hybrid exchange reactions can be adapted to other polymers based on similar mechanisms and beyond. Besides shape-shifting-related areas (e.g., soft robots, flexible electronics, and biomedical devices), it may also foster innovation in other fields involving general polymers, as well as promote deeper understanding of dynamic covalent chemistry.

Target and suspect analysis of liquid crystal monomers in soil from different urban functional zones

Recent studies have reported the occurrence of liquid crystal monomers (LCMs) in sediment, indoor dust, hand wipes, and human serum samples; however, information regarding their contamination status in soil is currently unavailable. The concentrations of 39 target LCMs were determined in n = 96 surface soil samples collected from five different urban functional zones including agricultural, scenic, industrial, commercial, and residential zones. We observed that 76 of 96 surface soil samples contained at least 19, 13, 16, 19, and 14 of the 39 target LCMs that were detectable in samples from agricultural, scenic, industrial, commercial, and residential zones, respectively. The LCMs in the samples from the agricultural zone exhibited the highest mean concentrations of 12.9 ng/g dry weight (dw), followed by those from commercial (5.23 ng/g dw), residential (3.30 ng/g dw), industrial (2.48 ng/g dw), and scenic zones (0.774 ng/g dw). Furthermore, strong and statistically significant (p < 0.05) correlations were observed for several pairs of LCMs (3cH2B vs. 5bcHdFB in the agricultural zone; 5bcHdFB vs. 2bcHdFB, 5bcHdFB vs. 3cH2B in the commercial zone; 5bcHdFB vs. 2bcHdFB in the industrial zone), indicating that they might have similar commercial applications and sources. Based on a newly established database containing 1173 LCMs, suspect screening was applied to discover other LCMs in these 96 soil samples using gas chromatograph coupled with quadrupole-time-of-flight mass spectrometry (GC-QTOF/MS). We tentatively identified 51 LCM formulas with 69 chemical structures. Collectively, this study provides the first evidence for the occurrence of LCMs in soil samples, and suggests that LCMs could be widely distributed across all five urban functional zones.

Smectic C Self-Assembly in Mesogen-Free Liquid Crystalline Chiral Polyethers with Sulfonylated Methyl-Branched Side Chains

To realize advanced electrical applications for ferroelectric liquid crystalline polymers, high spontaneous polarization (P_s ) is highly desired. However, current ferroelectric liquid crystalline polymers usually exhibit a low P_s . In this work, mesogen-free, chiral polyethers containing sulfonylated methyl-branched alkyl side chains with a (CH₂ )₃ O spacer between the sulfonyl and the branched alkyl groups are designed and synthesized. In contrast to the linear n-alkyl side chains, the methyl-branched alkyl side chains induce chain tilting in the smectic layers. When double chirality exists in both the main chain and the side chains, a crystalline structure is observed after mechanical stretching. Intriguingly, when single chirality exists in either the backbone or the side chains, a liquid crystalline smectic C phase is obtained. The electric displacement-electric field study, however, does not show typical ferroelectric switching, although the dielectric constants are relatively high for these liquid crystalline polymers. This is likely because the dipole-dipole interactions among neighboring sulfonyl groups along the main chain are so strong that the ferroelectric switching is hindered in the samples. For the future work, it is desired to weaken the dipole-dipole interaction to achieve ferroelectricity in these mesogen-free liquid crystalline polymers.

Raw Materials, Technology, Healthcare Applications, Patent Repository and Clinical Trials on 4D Printing Technology: An Updated Review

After the successful commercial exploitation of 3D printing technology, the advanced version of additive manufacturing, i.e., 4D printing, has been a new buzz in the technology-driven industries since 2013. It is a judicious combination of 3D printing technologies and smart materials (stimuli responsive), where time is the fourth dimension. Materials such as liquid crystal elastomer (LCE), shape memory polymers, alloys and composites exhibiting properties such as self-assembling and self-healing are used in the development/manufacturing of these products, which respond to external stimuli such as solvent, temperature, light, etc. The technologies being used are direct ink writing (DIW), fused filament fabrication (FFF), etc. It offers several advantages over 3D printing and has been exploited in different sectors such as healthcare, textiles, etc. Some remarkable applications of 4D printing technology in healthcare are self-adjusting stents, artificial muscle and drug delivery applications. Potential of applications call for further research into more responsive materials and technologies in this field. The given review is an attempt to collate all the information pertaining to techniques employed, raw materials, applications, clinical trials, recent patents and publications specific to healthcare products. The technology has also been evaluated in terms of regulatory perspectives. The data garnered is expected to make a strong contribution to the field of technology for human welfare and healthcare.

Regression analysis for predicting the elasticity of liquid crystal elastomers

It is highly desirable but difficult to understand how microscopic molecular details influence the macroscopic material properties, especially for soft materials with complex molecular architectures. In this study we focus on liquid crystal elastomers (LCEs) and aim at identifying the design variables of their molecular architectures that govern their macroscopic deformations. We apply the regression analysis using machine learning (ML) to a database containing the results of coarse grained molecular dynamics simulations of LCEs with various molecular architectures. The predictive performance of a surrogate model generated by the regression analysis is also tested. The database contains design variables for LCE molecular architectures, system and simulation conditions, and stress-strain curves for each LCE molecular system. Regression analysis is applied using the stress-strain curves as objective variables and the other factors as explanatory variables. The results reveal several descriptors governing the stress-strain curves. To test the predictive performance of the surrogate model, stress-strain curves are predicted for LCE molecular architectures that were not used in the ML scheme. The predicted curves capture the characteristics of the results obtained from molecular dynamics simulations. Therefore, the ML scheme has great potential to accelerate LCE material exploration by detecting the key design variables in the molecular architecture and predicting the LCE deformations.

New luminescent ordered liquid crystalline molecules with a 3-cyano-2-pyridone core unit

The self-organized states of liquid crystals (LCs) have recently received a lot of attention because of their various applications, especially in the fields of electron transport materials and optoelectronic devices. In most of these applications, molecules containing a donor-acceptor skeleton have been widely employed and generally these molecular frameworks have been designed mainly on the basis of the donor-acceptor concept. Inspired from this concept, we synthesized a series of new donor-acceptor based luminescent molecules, i.e. 4,6-bis(4-alkyloxyphenyl)-2-oxo-1,2-dihydropyridine-3-carbonitrile (Pn-series), carrying variable alkoxy chains [i.e. n = 8, 10, 12, 14, 16]. The structures of the synthesized molecules were confirmed by various spectral analyses. Further, their absorption and emission studies indicated that these molecules show blue light emitting properties. Moreover, the experimentally obtained optical band gap was analogous to the theoretical band gap calculated from the DFT study. The first two members of the Pn-series (n = 8 and 10) are non-mesogens. As the alkyl chain length increased to n = 12 and n = 14, the smectic C phase appeared along with an additional low temperature ordered lamellar phase. When n = 16, the smectic C phase disappeared and the compound exhibited only an ordered lamellar phase. This ordered lamellar phase is mainly due to the face to face alignment which makes these molecules potential candidates for electron transport materials and optoelectronic devices.

Photo-Programmed Deformations in Rigid Liquid Crystalline Polymers Triggered by Body Temperature

Deformations triggered by body heat are desirable in the context of shape-morphing applications because, under the majority of circumstances, the human body maintains a higher temperature than that of its surroundings. However, at present, this bioenergy-triggered action is primarily limited to soft polymeric networks. Thus, herein, the programming of body temperature-triggered deformations into rigid azobenzene-containing liquid crystalline polymers (azo-LCPs) with a glass-transition temperature of 100 °C is demonstrated. To achieve this, a mechano-assisted photo-programming strategy is used to create a metastable state with room-temperature stable residual stress, which is induced by the isomerization of azobenzene. The programmed rigid azo-LCP can undergo large-amplitude body temperature-triggered shape changes within minutes and can be regenerated without any performance degradation. By changing the programming photomasks and irradiation conditions employed, various 2D to 3D shape-morphing architectures, including folded clips, inch-worm structures, spiral structures, and snap-through motions are achieved. When programmed with polarized light, the proposed strategy results in domain-selective activation, generating designed characteristics in multi-domain azo-LCPs. The reported strategy is therefore expected to broaden the applications of azo-LCPs in the fields of biomedical and flexible microelectronic devices.

Human-muscle-inspired single fibre actuator with reversible percolation

Artificial muscles are indispensable components for next-generation robotics capable of mimicking sophisticated movements of living systems. However, an optimal combination of actuation parameters, including strain, stress, energy density and high mechanical strength, is required for their practical applications. Here we report mammalian-skeletal-muscle-inspired single fibres and bundles with large and strong contractive actuation. The use of exfoliated graphene fillers within a uniaxial liquid crystalline matrix enables photothermal actuation with large work capacity and rapid response. Moreover, the reversible percolation of graphene fillers induced by the thermodynamic conformational transition of mesoscale structures can be in situ monitored by electrical switching. Such a dynamic percolation behaviour effectively strengthens the mechanical properties of the actuator fibres, particularly in the contracted actuation state, enabling mammalian-muscle-like reliable reversible actuation. Taking advantage of a mechanically compliant fibre structure, smart actuators are readily integrated into strong bundles as well as high-power soft robotics with light-driven remote control.

Alignment Control of Smectic Layer Structures in Liquid-Crystalline Polymers by Photopolymerization with Scanned Slit Light

Photoalignment control of hierarchical structures is a key process to enhance the properties of optical and mechanical materials. We developed an in situ molecular alignment method, where photopolymerization with the scanned slit light causes molecular flow, leading to two-dimensional precise alignment of molecules over large areas; however, the alignment control has been explored only on a molecular scale. In this study, we demonstrate this photopolymerization-induced molecular flow, enabling mesoscopic alignment of smectic layer structures composed of anisotropic molecules. Side-chain liquid-crystalline polymers were obtained from two different monomers with or without alkyl spacers by photopolymerization with one-dimensionally scanned slit light. The polymer with an alkyl spacer displayed mesogens aligned parallel to the scanning direction, while the polymer with no alkyl spacer resulted in perpendicular alignment of mesogens to the scanning direction, regulated by the alignment of the polymer main chain along the light scanning direction. Moreover, the polymerization with the scanned light aligned not only the mesogens but also mesoscopic smectic layer structures over large areas, depending on the structure and scanning pattern of light. We envision that such a simple polymerization technique could become a powerful and versatile alignment platform of anisotropic materials in a wide range of scales.

Amplifying Molecular Scale Rotary Motion: The Marriage of Overcrowded Alkene Molecular Motor with Liquid Crystals

Design and fabrication of macroscopic functional devices by molecular engineering is an emerging and effective strategy in exploration of advanced materials. Photoresponsive overcrowded alkene-based molecular motor (OAMM) is considered as one of the most promising molecular machines due to the unique rotary motion driven by light with high temporal and spatial precision. Amplifying the molecular rotary motions into macroscopic behaviors of photodirected systems links the molecular dynamics with macroscopic motions of materials, providing new opportunities to design novel materials and devices with a bottom-up strategy. In this review, recent developments of the light-responsive liquid crystal system triggered by OAMM will be summarized. The mechanism of amplification effect of liquid crystal matrix will be introduced first. Then progress of the OAMM-driven liquid crystal materials will be described including light-controlled photonic crystals, texture-tunable liquid crystal coating and microspheres, photoactuated soft robots, and dynamic optical devices. It is hoped that this review provides inspirations in design and exploration of light-driven soft matters and novel functional materials from molecular engineering to structural modification.

Electroactive Soft Actuators Based on Columnar Ionic Liquid Crystal/Polymer Composite Membrane Electrolytes Forming 3D Continuous Ionic Channels

Here, we report low-voltage-driven fast-response nanostructured columnar ionic liquid crystal/polymer composite actuators that form three-dimensional continuous ion channels. A three-component self-assembly of a zwitterionic rod-like molecule (49.5 wt %), an ionic liquid (27.5 wt %), and poly(vinyl alcohol) (23.0 wt %) provided a free-standing stretchable membrane electrolyte. The dissociated ions can move through a continuous 3D ionophilic matrix surrounding the hydrophobic columns formed by the hexagonally organized rod-mesogens. Three-layer actuators composed of the electrolyte film sandwiched between two conductive polymer film electrodes of doped polythiophene exhibited a bending motion with 0.32% strain and moved 2 mm within 220 ms under 1 V at 0.1 Hz in 70% relative humidity due to the formation of electric double layers at the soft solid electrolyte/electrode interfaces. The bending strain of the columnar nanostructured actuator is comparable to those of polymer iongel actuators and block polymer actuators containing 25-80 wt % of ionic liquids. It is noteworthy that a small number of ions organized into the 3D nanochannels can generate the large bending deformation, which can contribute to reduce the risk of leakage of ions and the production cost. In addition, we have demonstrated a low-voltage-driven deformable mirror actuator that is expected to be applied to optical devices.

Three-dimensional thermochromic liquid crystal elastomer structures with reversible shape-morphing and color-changing capabilities for soft robotics

Functional structures with reversible shape-morphing and color-changing capabilities are promising for applications including soft robotics and biomimetic camouflage devices. Despite extensive studies, there are few reports on achieving both reversible shape-switching and color-changing capabilities within one structure. Here, we report a facile and versatile strategy to realize such capabilities via spatially programmed liquid crystal elastomer (LCE) structures incorporated with thermochromic dyes. By coupling the shape-changing behavior of LCEs resulting from the nematic-to-isotropic transition of liquid crystals with the color-changing thermochromic dyes, 3D thermochromic LCE structures change their shapes and colors simultaneously, which are controlled by the nematic-isotropic transition temperature of LCEs and the critical color-changing temperature of dyes, respectively. Demonstrations, including the simulated blooming process of a resembled flower, the camouflage behavior of a "butterfly"/"chameleon" robot in response to environmental changes, and the underwater camouflage of an "octopus" robot, highlight the reliability of this strategy. Furthermore, integrating micro-ferromagnetic particles into the "octopus" thermochromic LCE robot allows it to respond to thermal-magnetic dual stimuli for "adaptive" motion and diverse biomimetic motion modes, including swimming, rolling, rotating, and crawling, accompanied by color-changing behaviors for camouflage. The reversibly reconfigurable and color-changing thermochromic LCE structures are promising for applications including soft camouflage robots and multifunctional biomimetic devices.

Materials development in stretchable iontronics

Stretchable iontronics have recently been developed as an ideal interface to promote the interaction between humans and devices. Since the materials that use ions as charge carriers are typically transparent and stretchable, they have been used to fabricate devices with diverse functions with intrinsic transparency and stretchability. With the development of device design, material design has also been investigated to mitigate the issues associated with ionic materials, such as their weak mechanical properties, poor electrical properties, or poor environmental stabilities. In this review, we describe the recent progress on the design of materials in stretchable iontronics. By classifying stretchable ionic materials into three types of components (ionic conductors, ionic semiconductors, and ionic insulators), the issues each component has and the strategies to solve them are introduced, specifically in terms of molecular interactions. We then discuss the existing hurdles and challenges to be handled and shine light on the possibilities and opportunities from the insight of molecular interactions.

Confinement-Induced Fabrication of Liquid Crystalline Polymeric Fibers

In aqueous media, liquid crystalline droplets typically form spherical shapes in order to minimize surface energy. Recently, non-spherical geometry has been reported using molecular self-assembly of surfactant-stabilized liquid crystalline oligomers, resulting in branched and randomly oriented filamentous networks. In this study, we report a polymerization of liquid crystalline polymeric fibers within a micro-mold. When liquid crystal oligomers are polymerized in freely suspended aqueous media, curvilinear and randomly networked filaments are obtained. When reactive liquid crystalline monomers are oligomerized in a micro-channel, however, highly aligned linear fibers are polymerized. Within a top-down microfabricated mold, a bottom-up molecular assembly was successfully achieved in a controlled manner by micro-confinement, suggesting a unique opportunity for the programming architecture of materials via a hybrid approach.

Metallic Mimosa pudica: A 3D biomimetic buckling structure made of metallic glasses

Metallic Mimosa pudica, a three-dimensional (3D) biomimetic structure made of metallic glass, is formed via laser patterning: Blooming, closing, and reversing of the metallic M. pudica can be controlled by an applied magnetic field or by manual reshaping. An array of laser-crystallized lines is written in a metallic glass ribbon. Changes in density and/or elastic modulus due to laser patterning result in an appropriate size mismatch between the shrunken crystalline regions and the glassy matrix. The residual stress and elastic distortion energy make the composite material to buckle within the elastic limit and to obey the minimum elastic energy criterion. This work not only provides a programming route for constructing buckling structures of metallic glasses but also provides clues for the study of materials with automatic functions desired in robotics, electronic devices, and, especially, medical devices in the field of medicine, such as vessel scaffolds and vascular filters, which require contactless expansion and contraction functions.

Amphiphilic Liquid Crystalline Polymer Micelles That Exhibit a Phase Transition at Body Temperature

Liquid crystalline polymers (LCPs), which exhibit unique structures and properties intermediate between those of liquids and solids, are widely utilized as functional and advanced materials for fabricating optical devices and high-performance fibers. This utility stems from their ability to abruptly change their organized structures and mobilities at their liquid crystalline-isotropic phase transition temperatures, similar to the properties of biological membranes. Despite these numerous potential applications of LCPs, no study on their use in medical applications such as drug delivery has been reported. In the present study, we synthesized amphiphilic side-chain LCPs (LCP-g-OEGs, where OEG is oligo(ethylene glycol)) for medical applications, where the LCP-g-OEGs undergo a nematic-isotropic phase transition at body temperature. The LCP-g-OEGs formed micelles with a diameter of approximately 130 nm in aqueous media. The micelles were stable and did not dissociate in aqueous media even when the temperature exceeded the nematic-isotropic phase transition temperature (T_NI). Although the release of a dye as a model drug from micelles was suppressed at temperatures lower than T_NI, their dye release was drastically enhanced at temperatures higher than T_NI. The LCP-g-OEG micelles regulated dye release reversibly in accordance with stepwise changes in temperature, without undergoing dissociation, differing from the behavior of standard temperature-responsive micelles. The temperature-responsive dye release behavior is induced by dramatic changes in their well-organized and dynamic structures as a result of the nematic-isotropic phase transition. These results demonstrate that the LCP-g-OEG micelles have a lot of medical applications as reversibly stimuli-responsive drug carriers.

Electrically driven liquid crystal network actuators

Soft actuators based on liquid crystal networks (LCNs) have aroused great scientific interest for use as stimuli-controlled shape-changing and moving components for robotic devices due to their fast, large, programmable and solvent-free actuation responses. Recently, various LCN actuators have been implemented in soft robotics using stimulus sources such as heat, light, humidity and chemical reactions. Among them, electrically driven LCN actuators allow easy modulation and programming of the input electrical signals (amplitude, phase, and frequency) as well as stimulation throughout the volume, rendering them promising actuators for practical applications. Herein, the progress of electrically driven LCN actuators regarding their construction, actuation mechanisms, actuation performance, actuation programmability and the design strategies for intelligent systems is elucidated. We also discuss new robotic functions and advanced actuation control. Finally, an outlook is provided, highlighting the research challenges faced with this type of actuator.

Advances in 4D printing of liquid crystalline elastomers: materials, techniques, and applications

Liquid crystalline elastomers (LCEs) are polymer networks exhibiting anisotropic liquid crystallinity while maintaining elastomeric properties. Owing to diverse polymeric forms and self-alignment molecular behaviors, LCEs have fascinated state-of-the-art efforts in various disciplines other than the traditional low-molar-mass display market. By patterning order to structures, LCEs demonstrate reversible high-speed and large-scale actuations in response to external stimuli, allowing for close integration with 4D printing and architectures of digital devices, which is scarcely observed in homogeneous soft polymer networks. In this review, we collect recent advances in 4D printing of LCEs, with emphases on synthesis and processing methods that enable microscopic changes in the molecular orientation and hence macroscopic changes in the properties of end-use objects. Promising potentials of printed complexes include fields of soft robotics, optics, and biomedical devices. Within this scope, we elucidate the relationships among external stimuli, tailorable morphologies in mesophases of liquid crystals, and programmable topological configurations of printed parts. Lastly, perspectives and potential challenges facing 4D printing of LCEs are discussed.

Electroactive Polymer-Based Composites for Artificial Muscle-like Actuators: A Review

Unlike traditional actuators, such as piezoelectric ceramic or metallic actuators, polymer actuators are currently attracting more interest in biomedicine due to their unique properties, such as light weight, easy processing, biodegradability, fast response, large active strains, and good mechanical properties. They can be actuated under external stimuli, such as chemical (pH changes), electric, humidity, light, temperature, and magnetic field. Electroactive polymers (EAPs), called ‘artificial muscles’, can be activated by an electric stimulus, and fixed into a temporary shape. Restoring their permanent shape after the release of an electrical field, electroactive polymer is considered the most attractive actuator type because of its high suitability for prosthetics and soft robotics applications. However, robust control, modeling non-linear behavior, and scalable fabrication are considered the most critical challenges for applying the soft robotic systems in real conditions. Researchers from around the world investigate the scientific and engineering foundations of polymer actuators, especially the principles of their work, for the purpose of a better control of their capability and durability. The activation method of actuators and the realization of required mechanical properties are the main restrictions on using actuators in real applications. The latest highlights, operating principles, perspectives, and challenges of electroactive materials (EAPs) such as dielectric EAPs, ferroelectric polymers, electrostrictive graft elastomers, liquid crystal elastomers, ionic gels, and ionic polymer–metal composites are reviewed in this article.

Thiol-acrylate side-chain liquid crystal elastomers

The Michael addition 'click' chemistry was used to graft acrylate-terminated mesogenic groups onto the polysiloxane backbone polymer chain with thiol functional groups, with a constant 15% fraction of diacrylate reacting monomers as crosslinkers. Three different types of mesogens were used, and also their 50 : 50 mixtures, and in all cases we have obtained the smectic-A phase of the resulting liquid crystalline elastomer. Using X-ray diffraction, calorimetry and dynamic mechanical analysis, we investigated the relationship between the molecular structure of mesogenic side groups and the structure and properties of the elastomers. The shape-memory of smectic elastomers was verified. The unusual features were the semi-crystalline nature of elastomers with non-polar mesogens and the clear role of side-by-side rod dimerization of polar mesogens leading to a higher smectic layer spacing. We investigated the evolution of the smectic alignment on uniaxial stretching along the layer normal and identified two distinct ways in which the elastomer responds: the coarsened Helfrich-Hurault zig-zag layer texture and the large-scale stripe domains of uniform layer rotation in the systems with lower order parameter and the associated layer constraints.

Locally controllable magnetic soft actuators with reprogrammable contraction-derived motions

Reprogrammable magneto-responsive soft actuators capable of working in enclosed and confined spaces and adapting functions under changing situations are highly demanded for new-generation smart devices. Despite the promising prospect, the realization of versatile morphing modes (more than bending) and local magnetic control remains challenging but is crucial for further on-demand applications. Here, we address the challenges by maximizing the unexplored potential of magnetothermal responsiveness and covalent adaptable networks (CANs) in liquid crystalline elastomers (LCEs). Various magneto-actuated contraction-derived motions that were hard to achieve previously (e.g., bidirectional shrinkage and dynamic 3D patterns) can be attained, reprogrammed, and assembled seamlessly to endow functional diversity and complexity. By integration of LCEs with different magneto-responsive threshold values, local and sequential magnetic control is readily realized. Many magnetic actuation portfolios are performed by rationally imputing "logic switch" sequences. Meanwhile, our systems exhibit additional favorable performances including stepwise magnetic controllability, multiresponsiveness, self-healing, and remolding ability.

From passive to emerging smart silicones

Amassing remarkable properties, silicones are practically indispensable in our everyday life. In most classic applications, they play a passive role in that they cover, seal, insulate, lubricate, water-proof, weather-proof etc. However, silicone science and engineering are highly innovative, seeking to develop new compounds and materials that meet market demands. Thus, the unusual properties of silicones, coupled with chemical group functionalization, has allowed silicones to gradually evolve from passive materials to active ones, meeting the concept of “smart materials”, which are able to respond to external stimuli. In such cases, the intrinsic properties of polysiloxanes are augmented by various chemical modifications aiming to attach reactive or functional groups, and/or by engineering through proper cross-linking pattern or loading with suitable fillers (ceramic, magnetic, highly dielectric or electrically conductive materials, biologically active, etc.), to add new capabilities and develop high value materials. The literature and own data reflecting the state-of-the art in the field of smart silicones, such as thermoplasticity, self-healing ability, surface activity, electromechanical activity and magnetostriction, thermo-, photo-, and piezoresponsivity are reviewed.

A High-Fidelity Preparation Method for Liquid Crystal Elastomer Actuators

Three-dimensional (3D) structural actuators based on monodomain liquid crystal elastomers (mLCEs) show a wide range of potential applications. A direct ink writing technique has been developed to print LCE structures. It is still a challenge to print high-precision 3D-mLCE actuators. Here, a method of wet 3D printing combined with freeze-drying is proposed. The coagulation bath is designed to restrain the nascent fiber disturbance of the capillary wave and weight by adjusting the ink viscosity and printing speed to control the LC molecular order, enabling uniform (B = 1.02) fibers with a high degree of orientational alignment (S = 0.45) of the mesogens. Furthermore, dynamic disulfide bond formation was used as the cross-linking point, which can allow the LCE network structure to be continuously cured to ensure adjacent layers are effectively bonded and, in combination with freeze-drying, produce the 3D-mLCE actuators of fidelity architecture (98.37 vol %) by printing. The actuators have excellent actuating strain (45.12%), and the dynamic disulfide bond makes them programmable. Finally, a printed bionic starfish and a printed bionic hand can easily grab regular and irregular objects. This work provides a feasible scheme for fabricating complex 3D-mLCEs with reversible changes in shape.

Formation of rolls from liquid crystal elastomer bistrips

Formation of desired three-dimensional (3D) shapes from flat thin sheets with programmed non-uniform deformation profiles is an effective strategy to create functional 3D structures. Liquid crystal elastomers (LCEs) are of particular use in programmable shape morphing due to their ability to undergo large, reversible, and anisotropic deformation in response to a stimulus. Here we consider a rectangular monodomain LCE thin sheet divided into one high- and one low-temperature strip, which we dub a 'bistrip'. Upon activation, a discontinuously patterned, anisotropic in-plane stretch profile is generated, and induces buckling of the bistrip into a rolled shape with a transitional bottle neck. Based on the non-Euclidean plate theory, we derive an analytical model to quantitatively capture the formation of the rolled shapes from a flat bistrip with finite thickness by minimizing the total elastic energy involving both stretching and bending energies. Using this analytical model, we identify the critical thickness at which the transition from the unbuckled to buckled configuration occurs. We further study the influence of the anisotropy of the stretch profile on the rolled shapes by first converting prescribed metric tensors with different anisotropy to a unified metric tensor embedded in a bistrip of modified geometry, and then investigating the effect of each parameter in this unified metric tensor on the rolled shapes. Our analysis sheds light on designing shape morphing of LCE thin sheets, and provides quantitative predictions on the 3D shapes that programmed LCE sheets can form upon activation for various applications.

Advanced Functional Liquid Crystals

Liquid crystals have been intensively studied as functional materials. Recently, integration of various disciplines has led to new directions in the design of functional liquid-crystalline materials in the fields of energy, water, photonics, actuation, sensing, and biotechnology. Here, recent advances in functional liquid crystals based on polymers, supramolecular complexes, gels, colloids, and inorganic-based hybrids are reviewed, from design strategies to functionalization of these materials and interfaces. New insights into liquid crystals provided by significant progress in advanced measurements and computational simulations, which enhance new design and functionalization of liquid-crystalline materials, are also discussed.

A Humidity-Powered Soft Robot with Fast Rolling Locomotion

A range of soft robotic systems have recently been developed that use soft, flexible materials and respond to environmental stimulus. The greatest challenge in their design is the integration of the actuator, energy sources, and body of robots while achieving fast locomotion and well-defined programmable trajectories. This work presents such a design that operates under constant conditions without the need for an externally modulated stimulus. By using a humidity-sensitive agarose film and overcoming the isotropic and random bending of the film, the robot, which we call the Hydrollbot, harnesses energy from evaporation for spontaneous and continuous fast self-rolling locomotion with a programmable trajectory in a constant-humidity environment. Moreover, the geometric parameters of the film were fine-tuned to maximize the rolling speed, and the optimised hydrollbot is capable of carrying a payload up to 100% of its own weight. The ability to self-propel fast under constant conditions with programmable trajectories will confer practical advantages to this robot in the applications for sensors, medical robots, actuation, etc.

Beating of a Spherical Liquid Crystal Elastomer Balloon under Periodic Illumination

Periodic excitation is a relatively simple and common active control mode. Owing to the advantages of direct access to environmental energy and controllability under periodic illumination, it enjoys broad prospects for application in soft robotics and opto-mechanical energy conversion systems. More new oscillating systems need to be excavated to meet the various application requirements. A spherical liquid crystal elastomer (LCE) balloon model driven by periodic illumination is proposed and its periodic beating is studied theoretically. Based on the existing dynamic LCE model and the ideal gas model, the governing equation of motion for the LCE balloon is established. The numerical calculations show that periodic illumination can cause periodic beating of the LCE balloon, and the beating period of the LCE balloon depends on the illumination period. For the maximum steady-state amplitude of the beating, there exists an optimum illumination period and illumination time rate. The optimal illumination period is proved to be equivalent to the natural period of balloon oscillation. The effect of system parameters on beating amplitude are also studied. The amplitude is mainly affected by light intensity, contraction coefficient, amount of gaseous substance, volume of LCE balloon, mass density, external pressure, and damping coefficient, but not the initial velocity. It is expected that the beating LCE balloon will be suitable for the design of light-powered machines including engines, prosthetic blood pumps, aircraft, and swimmers.

Optical manipulation and assembly of micro/nanoscale objects on solid substrates

Many light-based technologies have been developed to manipulate micro/nanoscale objects such as colloidal particles and biological cells for basic research and practical applications. While most approaches such as optical tweezers are best suited for manipulation of objects in fluidic environments, optical manipulation on solid substrates has recently gained research interest for its advantages in constructing, reconfiguring, or powering solid-state devices consisting of colloidal particles as building blocks. Here, we review recent progress in optical technologies that enable versatile manipulation and assembly of micro/nanoscale objects on solid substrates. Diverse technologies based on distinct physical mechanisms, including photophoresis, photochemical isomerization, optothermal phase transition, optothermally induced surface acoustic waves, and optothermal expansion, are discussed. We conclude this review with our perspectives on the opportunities, challenges, and future directions in optical manipulation and assembly on solid substrates.