Bioinspired Mechanochromic Liquid Crystal Materials: from Fundamentals to Functionalities and Applications.

Inspired by intriguing color changeable ability of natural animals, the design and fabrication of artificial mechanochromic materials capable of changing colors upon stretching or pressing have attracted intense scientific interest. Liquid crystal (LC) is a self-organized soft matter with anisotropic molecular alignment. Due to the sensitivity to various external stimulations, LC has been considered as an emerging and appealing responsive building block to construct intelligent materials and advanced devices. Recently, mechanochromic LC materials have becoming a hot topic in multi fields from flexible artificial skins to visualized sensors and smart biomimetic devices. In this review, the recent progress of mechanochromic LCs is comprehensively summarized. Firstly, the mechanism and functionalities of mechanochromic LC is introduced, followed by preparation of various functional materials based on mechanochromic LCs. Then the applications of mechanochromic LCs are provided. Finally, the conclusion and outlooks of this field is given. This overview is hoped to provide inspiration in fabrication of advanced functional soft materials for scientists and engineers from multidisciplines including materials science, elastomers, chemistry and physical science. This article is protected by copyright. All rights reserved.

Annual Energy-Saving Smart Windows with Actively Controllable Passive Radiative Cooling and Multimode Heating Regulation.

Smart windows with radiative heat management capability using the sun and outer space as zero-energy thermodynamic resources have gained prominence, demonstrating a minimum carbon footprint. However, realizing on-demand thermal management throughout all seasons while reducing fossil energy consumption remains a formidable challenge. Herein, an energy-efficient smart window that enables actively tunable passive radiative cooling (PRC) and multimode heating regulation is demonstrated by integrating the emission-enhanced polymer-dispersed liquid crystal (SiO2@PRC PDLC) film and a low-emission layer deposited with carbon nanotubes. Specifically, this device can achieve a temperature close to the chamber interior ambient under solar irradiance of 700 W m-2, as well as a temperature drop of 2.3 °C at sunlight of 500 W m-2, whose multistage PRC efficiency can be rapidly adjusted by a moderate voltage. Meanwhile, synchronous cooperation of passive radiative heating (PRH), solar heating (SH), and electric heating (EH) endows this smart window with the capability to handle complicated heating situations during cold weather. Energy simulation reveals the substantial superiority of this device in energy savings compared with single-layer SiO2@PRC PDLC, normal glass, and commercial low-E glass when applied in different climate zones. This work provides a feasible pathway for year-round thermal management, presenting a huge potential in energy-saving applications.

Knotted Artificial Muscles for Bio-Mimetic Actuation under Deepwater.

Muscles featuring high frequency and high stroke linear actuation are essential for animals to achieve superior maneuverability, agility, and environmental adaptability. Artificial muscles are yet to match their biological counterparts, due to inferior actuation speed, magnitude, mode, or adaptability. Inspired by the hierarchical structure of natural muscles, artificial muscles are created that are powerful, responsive, robust, and adaptable. The artificial muscles consist of knots braided from 3D printed liquid crystal elastomer fibers and thin heating threads. The unique hierarchical, braided knot structure offers amplified linear stroke, force rate, and damage-tolerance, as verified by both numerical simulations and experiments. In particular, the square knotted artificial muscle shows reliable cycles of actuation at 1Hz in 3000m depth underwater. Potential application is demonstrated by propelling a model boat. Looking ahead, the knotted artificial muscles can empower novel biomedical devices and soft robots to explore various environments, from inside human body to the mysterious deep sea.

Dynamically Modulating the Dissymmetry Factor of Circularly Polarized Organic Ultralong Room-Temperature Phosphorescence from Soft Helical Superstructures.

Achieving circularly polarized organic ultralong room-temperature phosphorescence (CP-OURTP) with a high luminescent dissymmetry factor (glum ) is crucial for diverse optoelectronic applications. In particular, dynamically controlling the dissymmetry factor of CP-OURTP can profoundly advance these applications, but it is still unprecedented. This study introduces an effective strategy to achieve photoirradiation-driven chirality regulation in a bilayered structure film, which consists of a layer of soft helical superstructure incorporated with a light-driven molecular motor and a layer of room-temperature phosphorescent (RTP) polymer. The prepared bilayered film exhibits CP-OURTP with an emission lifetime of 805 ms and a glum value up to 1.38. Remarkably, the glum value of the resulting CP-OURTP film can be reversibly controlled between 0.6 and 1.38 over 20 cycles by light irradiation, representing the first example of dynamically controlling the glum in CP-OURTP.

Electrostatically Powered Multimode Liquid Crystalline Elastomer Actuators.

Soft actuators based on liquid crystalline elastomers (LCEs) are captivating significant interest because of their unique properties combining the programmable liquid crystalline molecular order and elasticity of polymeric materials. For practical applications, the ability to perform multimodal shape changes in a single LCE actuator at a subsecond level is a bottleneck. Here, we fabricate a monodomain LCE powered by electrostatic force, which enables fast multidirectional bending, oscillation, rotation, and complex actuation with a high degree of freedom. By tuning the dielectric constant and resistivity in LCE gels, a complete cycle of oscillation and rotation only takes 0.1 s. In addition, monodomain actuators exhibit anisotropic actuation behaviors that promise a more complex deployment in a potential electromechanical system. The presented study will pave the way for electrostatically controllable isothermal manipulation for a fast and multimode soft actuator.

Shape-Programmable Liquid-Crystalline Polyurethane-Based Multimode Actuators Triggered by Light-Driven Molecular Motors.

In recent years, light-driven soft actuators have been rapidly developed as enablers in the fabrication of artificial robots and biomimetic devices. However, it remains challenging to amplify molecular isomerization to multiple modes of macroscopic actuation with large amplitude and complex motions. Here, a strategy is reported to build a light-responsive liquid-crystalline polyurethane elastomer by phototriggered overcrowded alkene-based molecular motors. A trifunctional molecular motor modified with an ethylene glycol spacer on the rotor and stator functions as a crosslinker and unidirectional stirrer that amplifies molecular motion into macroscopic movement. The shape-programmable polymeric film presents superior mechanical properties and characteristic shape-memory effect. Furthermore, diverse modes of motions including bending, unwinding, and contracting with tunable actuation speed over a wide range are achieved. Such research is hoped to pave a new way for the design of advanced light-responsive soft actuators and robots.

Bioinspired humidity-responsive liquid crystalline materials: from adaptive soft actuators to visualized sensors and detectors.

Inspired by nature, humidity-responsive materials and devices have attracted significant interest from scientists in multiple disciplines, ranging from chemistry, physics and materials science to biomimetics. Owing to their superiorities, including harmless stimulus and untethered control, humidity-driven materials have been widely investigated for application in soft robots, smart sensors and detectors, biomimetic devices and anticounterfeiting labels. Especially, humidity-responsive liquid crystalline materials are particularly appealing due to the combination of programmable and adaptive liquid crystal matrix and humidity-controllability, enabling the fabrication of advanced self-adaptive robots and visualized sensors. In this review, we summarize the recent progress in humidity-driven liquid crystalline materials. First, a brief introduction of liquid crystal materials, including liquid crystalline polymers, cholesteric liquid crystals, blue-phase liquid crystals and cholesteric cellulose nanocrystals is provided. Subsequently, the mechanisms of humidity-responsiveness are presented, followed by the diverse strategies for the fabrication of humidity-responsive liquid crystalline materials. The applications of humidity-driven devices will be presented ranging from soft actuators to visualized sensors and detectors. Finally, we provide an outlook on the development of humidity-driven liquid crystalline materials.

Graphene/Cholesteric Liquid-Crystal-Based Electro-Driven Thermochromic Light Modulators toward Wide-Gamut Dynamic Light Color-Tuning-Related Applications.

Light filters are ubiquitous in projection and display techniques, illumination engineering, image sensing, photography, etc., while those enabling wide-gamut dynamic light color tuning are still lacking. Herein, by combining the electro-heating capability of graphene and unique optical properties (thermochromism and circular dichroism) of small-molecule-weight cholesteric liquid crystal (ChLC), a brand-new thermochromic light modulator is constructed as actively tunable color filter. Transparent graphene/glass hybrid with reasonably high conductivity serves both as a high-performance heater for actuating the thermochromism of temperature-responsive ChLC and as neutral light attenuator for brightness control. Thanks to the temperature- and polarization-dependent spectral properties of the ChLC, widely tunable hue and saturation properties of transmission light color are achieved, respectively. Several intriguing applications, e.g., color-variable smart windows for backlight color tuning and color-variable filters for photography, are also demonstrated. This work hereby provides new paradigms for promoting the applications of graphene/ChLC-based light modulators in next-generation light-management-related scenarios.

Orthogonally Integrating Programmable Structural Color and Photo-Rewritable Fluorescence in Hydrazone Photoswitch-bonded Cholesteric Liquid Crystalline Network.

Design and fabrication of advanced security label showing superior performance in data encryption has attracted tremendous scientific interests. Especially, multifunctional optical labels capable of storing distinct information in different modes are highly demanded. Here, a fluorescent cholesteric liquid crystalline network (CLCN) film with programmable visible patterns and photo-rewritable fluorescent patterns was designed and prepared. The visible patterns were fixed by helical network and the colors of visible patterns were tunable by changing relative humidity (RH). The fluorescent patterns originated from dynamic isomerization of fluorescent hydrazones, exhibiting highly thermal stability and switching-ability controlled by light. The orthogonal construction of visible and fluorescent pattern enabled the novel CLCN to encrypt distinct information in reflective mode and in emissive mode, indicating its potential in anti-counterfeiting and information encryptions.

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.

Freestanding Helical Nanostructured Chiro-Photonic Crystal Film and Anticounterfeiting Label Enabled by a Cholesterol-Grafted Light-Driven Molecular Motor.

Design and fabrication of freestanding chiro-photonic crystal film with the ability to change color over the whole visible light spectrum is appealing for anticounterfeiting technology and smart labels. Utilizing a newly synthesized light-responsive molecular motor functionalized with cholesterol (chol-MM) on the rotor, novel light-controlled photonic crystal is prepared by doping the novel chol-MM into liquid crystals (LCs). Thanks to the liquid crystalline cholesterol substituent, the chol-MM can be triggered by visible light (420 nm). At the same time, the miscibility of chol-MM in LC matrix is significantly enhanced. Integrating the chol-MM with thermochromic hydrogen-bonded LC matrix, thermal and light dual-responsive cholesteric LC (CLC) material is prepared, in which the nanoscale helical pitch is tunable by photo-induced molecular motions of chol-MM. More importantly, utilizing UV-initiated polymerization of the visible light-modulated CLC material, structural colored photonic crystal films with arbitrary colorful patterns are fabricated. Such freestanding helical nanostructured labels have potential in the application of encrypted communication and anticounterfeiting.

Reprogrammable Humidity-Driven Liquid Crystalline Polymer Actuator Enabled by Dynamic Ionic Bonds.

Liquid crystalline polymer (LCP) is a promising candidate in the design and fabrication of intelligent soft materials due to the combination of programmable anisotropy and elasticity. Here, a novel strategy to fabricate reprogrammable humidity-responsive LCP materials enabled by dynamic ionic cross-links were put forward. The prepared LCP film deforms reversibly with the change of relative humidity (RH). However, the humidity responsivity loses after soaking the film into CaCl2 solution because of the lock of hygroscopic groups by the formed ionic bonds. By selectively cross-linking specific regions of the LCP film, distinctive humidity-driven motions of the film could be realized. More interestingly, by the EDTA-2K solution treatment, ionic cross-links can be interrupted, leading the LCP film responsive to humidity again. Thanks to feasibly removable ionic cross-links, the humidity-directed soft actuator was totally reprogrammable. The behavior of the novel actuator could be manipulated by either the mesogens alignment or the spatially ionic treatment, providing a feasible but robust strategy to fabricate complex humidity-driven soft robots.

Humidity-Responsive Photonic Crystals with pH and SO2 Gas Detection Ability Based on Cholesteric Liquid Crystalline Networks.

Dynamic photonic crystals with tunable structural colors have been a hot topic in the research of anticounterfeiting devices, decoration, and detection. In this work, we prepared cholesteric liquid crystalline network (CLCN)-based photonic crystals that present humidity- and SO2 gas-responsive behaviors. The covalently cross-linked CLCN film presents humidity-responsive color changes due to the swelling/deswelling of the matrix under different humidity conditions. When treating the CLCN film with SO2 gas, the carboxylic salt converted to the acid and the film was not able to respond to the humidity change anymore. The mechanism of the SO2 gas-gated humidity responsiveness of the CLCN film was characterized. It was found that the acidic gas caused changes of pH, resulting in the conversion of the salt to acid and alteration of the surface property. The influence of concentration of SO2 gas and pH on humidity responsiveness of the CLCN film was investigated. We hope that this method provides inspirations for the design and fabrication of visualized pH and acidic gas detectors.

Tunable Circularly Polarized Luminescence with a High Dissymmetry Factor Emitted from Luminogen-Bonded and Electrically Controlled Polymer-Stabilized Cholesteric Liquid Crystals.

Circularly polarized luminescence (CPL)-active materials with high dissymmetry factor (glum) values show great potential in photonic devices. In this study, electric-field-driven systems with tunable CPL signals are successfully fabricated based on polymer-stabilized cholesteric liquid crystal (PSChLC) doping by fluorescent molecules. By constructing a gradient helical superstructure of PSChLCs, distinctive CPL emission from two sides of a single sample is realized, in which the |glum| values were measured to be 0.6 and 1.5, respectively. Herein, we discussed the possible mechanism of this phenomenon. In addition, an applied electric field could broaden the reflection bandwidth of PSChLCs from 150 to 500 nm, covering the whole visible light region. Furthermore, this electric field-induced behavior leads to the variation of CPL signals and corresponding glum values, indicating the potential of the novel materials in the design and preparation of CPL-emitting devices with electrically tunable CPL intensity and glum.

Remotely Controlling Drug Release by Light-Responsive Cholesteric Liquid Crystal Microcapsules Triggered by Molecular Motors.

Stimuli-responsive smart nanocarriers are an emerging class of materials applicable in fields including drug delivery and tissue engineering. Instead of constructing responsive polymer shells to control the release and delivery of drugs, in this work, we put forward a novel strategy to endow the internal drugs with light responsivity. The microcapsule consisted of molecular motor (MM)-doped cholesteric liquid crystals (CLCs) and drugs. The drug in gelatin-gum arabic microcapsules can protect the carried drugs for a long time with a low release speed totally resulting from drug diffusion. Under UV light, the MM isomerizes and the chirality changes, inducing the alteration of the superstructure of the CLCs. In this process, the cooperative molecular disturbance accelerates the diffusion of the drugs from the microcapsule core to the outside. As a result, thanks to the cooperative effect of liquid crystalline mesogens, molecular-scale geometric changes of motors could be amplified to the microscale disturbance of the self-organized superstructure of the CLCs, resulting in the acceleration of the drug release. This method is hoped to provide opportunities in the design and fabrication of novel functional drug delivery systems.

Photochemically and Photothermally Controllable Liquid Crystalline Network and Soft Walkers.

Developing intelligent soft robots capable to perform various responses to different stimulations has been a hot topic in recent years. Liquid crystalline networks (LCNs) have been considered as one of the most promising candidates in the fabrication of soft actuators because of the combination of elasticity of the polymer and anisotropy of the liquid crystals. However, the design and fabrication of advanced LCN materials with outstanding performances and multiple responsivities is highly demanded but still a challenge. In this work, a NIR-UV dual light-responsive LCN actuator was prepared by selectively coating a polydopamine (PDA) layer on an azobenzene-doped LCN film. This actuator presents UV responsivity in the uncoated region because of the photochemical isomerization of azobenzene and NIR sensitivity in the PDA-coated region originated from the striking photothermal effect. Thanks to the reprogrammable PDA coating, this dual-responsive LCN actuator was totally reprogrammable by coating and washing the PDA layer repeatedly. Based on the novel soft actuator, an artificial car that can imitate the switch of the "forward gear" to "neutral gear" of a real car was prepared. In normal mode, the actuator can move forward under NIR irradiation. After UV light excitation, the actuator cannot move under the same NIR irradiation, just like the car with the level in neutral. This novel actuator may provide inspirations for the fabrication of light-driven functional devices and soft actuators.

Multiple Anti-Counterfeiting Composite Film Based on Cholesteric Liquid Crystal and QD Materials.

A composite film with multiple anti-counterfeiting features was demonstrated by superposing quantum dots (QDs) polymer matrix (film A) and cholesteric liquid crystal film (film B) together. The first-line and second-line anti-counterfeiting characteristics were successfully implemented by employing thermochromic, angular photochromic, and circularly polarized discoloration of film B, respectively. By initiatively utilizing the different relative positions between the fluorescence emission peak (λem) of film A and the central selective reflection wavelength (λm) of film B at different temperatures, which resulted in changes in the fluorescence spectra or the different presence of latent patterns, the most important third-line anti-counterfeiting feature was successfully achieved.

Near-Infrared Photodriven Self-Sustained Oscillation of Liquid-Crystalline Network Film with Predesignated Polydopamine Coating.

Movement is one of the vital features of living systems, and remote control of bioinspired soft robotic systems in a precise, contactless and harmless way is extremely desirable but challenging. A near-infrared (NIR) photodriven polymeric oscillator is designed and fabricated by selectively coating a mussel-inspired polydopamine (PDA) polymer layer on the surface of splay-aligned liquid crystalline network (LCN) film. The oscillating motions of the LCN oscillators can be facilely manipulated by tuning light intensity and film thickness. More importantly, the programmability of the PDA coating enables the oscillating behaviors of LCN film to be predesignated and finely adjusted by coating the film with PDA locally and repeatedly. The self-oscillating movement mechanism can be attributed to the temperature oscillation at the PDA-coated LCN film since it is alternatively exposed and sheltered to the NIR-light irradiations. Owing to over 50% NIR irradiation in solar spectrum, PDA-coated film is found to oscillate upon exposure of focused sunlight, presenting great potential in fabrication of solar power generation devices. This provides a versatile strategy to fabricate NIR-light-actuated polymeric oscillators, providing inspirations in the development of biological soft robots and advanced biomimetic devices.

Stimuli-Directed Dynamic Reconfiguration in Self-Organized Helical Superstructures Enabled by Chemical Kinetics of Chiral Molecular Motors.

Dynamic controllability of self-organized helical superstructures in spatial dimensions is a key step to promote bottom-up artificial nanoarchitectures and functional devices for diverse applications in a variety of areas. Here, a light-driven chiral overcrowded alkene molecular motor with rod-like substituent is designed and synthesized, and its thermal isomerization reaction exhibits an increasing structural entropy effect on chemical kinetic analysis in anisotropic achiral liquid crystal host than that in isotropic organic liquid. Interestingly, the stimuli-directed angular orientation motion of helical axes in the self-organized helical superstructures doped with the chiral motors enables the dynamic reconfiguration between the planar (thermostationary) and focal conic (photostationary) states. The reversible micromorphology deformation processes are compatible with the free energy fluctuation of self-organized helical superstructures and the chemical kinetics of chiral motors under different conditions. Furthermore, stimuli-directed reversible nonmechanical beam steering is achieved in dynamic hidden periodic photopatterns with reconfigurable attributes prerecorded with a corresponding photomask and photoinduced polymerization.