Thermally Driven Photonic Actuator Based on Silica Opal Photonic Crystal with Liquid Crystal Elastomer

Defect Structures in Colloidal Crystals and Their Effect on Grating Diffraction Structural Color

Colloidal crystals of micrometer-sized colloids create prismatic structural colors through the grating diffraction of visible light. Here, we develop design rules to engineer such structural color by specifically accounting for the effect of crystal defects. The local quality and grain size of the colloidal structure are varied by performing self-assembly in the presence of a direct current (DC) electric field. The deposition, self-assembly, and crystallization of these colloids results in polycrystals of variable size, as controlled by the dissolved ion concentration (from 0.01 to 10 mM) and the applied electric current (from 1.6-310 μA/cm2). Under these operating conditions, the global 6-fold crystal bond order parameter (ψ6) of the self-assembled crystals varies from 0.45 ± 0.05 to 0.95 ± 0.01 and the crystal grain number density varies from about 5 to 100 per 0.01 mm2. We find that the grating diffraction structural color intensity of these self-assembled materials is strongly correlated with the crystal quality and grain number, with the diffraction efficiency varying by a factor of ∼2.5 over the range of ψ6 probed. Molecular dynamics (MD) simulation of the electrophoretic deposition reproduces the kinetics of the self-assembly as well as the final structures. It also extends the number and range of deposition conditions probed, thereby creating a library that can be used to study the relationship between defect properties and the grating diffraction structural color. Applying the finite-difference time domain (FDTD) method to solve for light-material interactions in the MD simulated structures yields calculated spectra that agree with experimental observations. The analysis also identifies a design trade-off between diffraction intensity and azimuthal uniformity as order parameter and grain density are varied, thereby demonstrating that the grating diffraction structural color of self-assembled crystals of micrometer-sized colloidal spheres may be controlled by means of their local crystal quality and polycrystallinity.

Light-powered self-scrolling liquid crystal elastomer crane

Traditional liquid crystal elastomer (LCE)-based machines are constrained by the need for complex controllers and large power supplies, which limits their applicability in microrobots and other small-scale machines. In this paper, we propose a light-powered self-scrolling LCE crane, which is capable of self-scrolling to lift weights under steady light. Based on a dynamic LCE model, we derive the lateral curvature of the LCE crane and the driving moment for steady scrolling. By numerically solving the equilibrium equations, we found that the driving moment for the self-scrolling is originated from the uneven distribution of the LCE rod in the horizontal direction caused by light. The angular velocity of the self-scrolling depends on five system parameters: heat flux, coefficient of heat transfer, support spacing, weight mass, and scrolling friction coefficient. Through experimental comparative analysis, the results are consistent with the numerical simulation. The light-powered self-scrolling LCE crane device proposed in this paper features a simple structure, consistent horizontal illumination, and a compact light irradiation area. It advances the understanding of self-sustaining structures utilizing active materials and offers valuable insight into the potential applications of light-responsive LCEs in self-driven devices, medical instruments, robotics, sensors, and the energy sector.

Patterned Photonic Actuators with Dynamic Shape-Morphing and Color-Changing Capabilities Fabricated by Athermal Embossing Technology

Stimuli-responsive patterned photonic actuators, characterized by their patterned nano/microscale structures and capacity to demonstrate synergistic color changes and shape morphing in response to external stimuli, have attracted intense scientific attention. However, traditional patterned photonic actuator systems still face limitations such as cumbersome and time-consuming preparation processes and small-scale deformations. Herein, we introduce a facile approach involving an athermal embossing technique to rapidly fabricate patterned photonic actuators based on near-infrared (NIR) light-responsive liquid crystal elastomers. The resulting patterned photonic actuators demonstrate remarkable features, including brilliant angle-dependent structural color, complex three-dimensional actuation, and good color durability under NIR light stimulation. As illustrative demonstrations of the proof-of-concept, we fabricate two light-fuelled patterned photonic soft actuators: a butterfly-inspired actuator that can produce wing-flapping dynamic changes in structural color, and an origami crane-shaped actuator with shape memory, structural color information storage, and dynamic display properties. This strategy provides distinct insights into the design and fabrication of various patterned photonic soft robotic devices and intelligent actuators.

Untethered soft actuators for soft standalone robotics

Soft actuators produce the mechanical force needed for the functional movements of soft robots, but they suffer from critical drawbacks since previously reported soft actuators often rely on electrical wires or pneumatic tubes for the power supply, which would limit the potential usage of soft robots in various practical applications. In this article, we review the new types of untethered soft actuators that represent breakthroughs and discuss the future perspective of soft actuators. We discuss the functional materials and innovative strategies that gave rise to untethered soft actuators and deliver our perspective on challenges and opportunities for future-generation soft actuators.

Modeling of a light-fueled self-paddling boat with a liquid crystal elastomer-based motor

Active materials possess unique properties of being able to respond autonomously to external stimuli, yet realizing and regulating the motion behavior of active machines remains a major challenge. Conventional control approaches, including sensor control and external device control, are both complex and difficult to implement. In contrast, active materials-based self-oscillators offer distinct properties such as periodic motion and ease of regulation. Inspired by paddle boats, we have proposed a conceptual light-fueled self-paddling boat with a photothermally responsive liquid crystal elastomer (LCE)-based motor that operates under steady illumination and incorporates an LCE fiber. Based on the well-established dynamic LCE model and rotation dynamics, the dynamic equations for governing the self-paddling of the LCE-steered boat are derived, and the driving torque of the LCE-based motor and the paddling velocity of the LCE-steered boat are formulated successively. The numerical results show that two motion modes of the boat under steady illumination: the static mode and the self-paddling mode. The self-paddling regime arises from the competition between the light-fueled driving torque and the frictional torque. Moreover, the critical conditions required to trigger the self-paddling are quantitatively examined as well as the significant system parameters affecting the driving torque, angular velocity, and paddling velocity. The proposed conceptual light-fueled self-paddling LCE-steered boat exhibits benefits including customizable size and being untethered and ambient powered, which provides valuable insights into the design and application of micromachines, soft robotics, energy harvesters, and beyond.

Smart colloidal photonic crystal sensors

Smart colloidal photonic crystals (PCs) with stimuli-responsive periodic micro/nano-structures, photonic bandgaps, and structural colors have shown unique advantages (high sensitivity, visual readout, wireless characteristics, etc.) in sensing by outputting diverse structural colors and reflection signals. In this review, smart PC sensors are summarized according to their fabrications, structures, sensing mechanisms, and applications. The fabrications of colloidal PCs are mainly by self-assembling the well-defined nanoparticles into the periodical structure (supersaturation-, polymerization-, evaporation-, shear-, interaction-, and field-induced self-assembly process). Their structures can be divided into two groups: closely packed and non-closely packed nano-structures. The sensing mechanisms can be explained by Bragg's law, including the change in the effective refractive index, lattice constant, and the order degree. The sensing applications are detailly introduced according to the analytes of the target, including solvents, vapors, humidity, mechanical force, temperature, electrical field, magnetic field, pH, ions/molecules, and so on. Finally, the corresponding challenges and the future potential prospects of artificial smart colloidal PCs in the sensing field are discussed.

Practical Preparation of Elastomer-Immobilized Nonclose-Packed Colloidal Photonic Crystal Films with Various Uniform Colors

Colloidal photonic crystals, which are three-dimensional periodic structures of monodisperse submicron-sized particles, are expected to be suitable for novel photonic applications and color materials. In particular, nonclose-packed colloidal photonic crystals immobilized in elastomers exhibit significant potential for use in tunable photonic applications and strain sensors that detect strain based on color change. This paper reports a practical method for preparing elastomer-immobilized nonclose-packed colloidal photonic crystal films with various uniform Bragg reflection colors using one kind of gel-immobilized nonclose-packed colloidal photonic crystal film. The degree of swelling was controlled by the mixing ratio of the precursor solutions, which used a mixture of solutions with high and low affinities for the gel film as the swelling solvent. This facilitated color tuning over a wide range, enabling the facile preparation of elastomer-immobilized nonclose-packed colloidal photonic crystal films with various uniform colors via subsequent photopolymerization. The present preparation method can contribute to the development of practical applications of elastomer-immobilized tunable colloidal photonic crystals and sensors.

Thermochromic Photonic Crystal Microspheres with Uniform Color Display and Wide Coloration Range

Thermochromic microspheres based on poly(N-isopropylacrylamide) attract much attention in detection and sensor due to the noticeable color response and fast response rate. However, some issues such as uneven color display and narrow coloration range still limit their practical applications. Herein, novel thermochromic microspheres with homogeneous color displays and wide thermochromic range are designed by combining the microfluidic technology with the magnetically-induced self-assembly technique and copolymerizing acrylamide (AM) with N-isopropylacrylamide. The photonic crystal structure with especially even colors is fast and conveniently constructed by magnetic assembly. The addition of AM makes the microspheres more hydrophilic and thus leading to a broader coloration range. The relationship between the structural color display and both the microstructures of photonic crystals and the thermo-responsive properties of gel matrix are elucidated. The detectable temperature of microspheres rises to as high as 60°C, and displays bright iridescent color variations from orange to blue-violet in the heating process. Importantly, their shrinking or swelling equilibrium can be reached in 80 and 105 s. Such microspheres are successfully used to visually indicate the appropriate temperature of enzymatic reaction, and have great potential in practical applications such as visual temperature detection and efficiency monitoring of chemical reactions.

Bioinspired Colloidal Photonic Composites: Fabrications and Emerging Applications

Organisms in nature have evolved unique structural colors and stimuli-responsive functions for camouflage, warning, and communication over millions of years, which are essential to their survival in harsh conditions. Inspired by these characteristics, colloidal photonic composites (CPCs) composed of colloidal photonic crystals embedded in the polymeric matrix are artificially prepared and show great promise in applications. This review focuses on the summary of building blocks, i.e., colloidal particles and polymeric matrices, and constructive strategies from the perspective of designing CPCs with robust performance and specific functionality. Furthermore, their state-of-the-art applications are also discussed, including colorful coatings, anti-counterfeiting, and regulation of photoluminescence, especially in the field of visualized sensing. Finally, current challenges and potential for future developments in this field are discussed. The purpose of this review is not only to clarify the design principle for artificial CPCs but also to serve as a roadmap for the exploration of next-generation photonic materials.

Liquid crystal-based structural color actuators

Animals can modify their body shape and/or color for protection, camouflage and communication. This adaptability has inspired fabrication of actuators with structural color changes to endow soft robots with additional functionalities. Using liquid crystal-based materials for actuators with structural color changes is a promising approach. In this review, we discuss the current state of liquid crystal-based actuators with structural color changes and the potential applications of these structural color actuators in soft robotic devices.

Photothermal-Driven Liquid Crystal Elastomers: Materials, Alignment and Applications

Liquid crystal elastomers (LCEs) are programmable deformable materials that can respond to physical fields such as light, heat, and electricity. Photothermal-driven LCE has the advantages of accuracy and remote control and avoids the requirement of high photon energy for photochemistry. In this review, we discuss recent advances in photothermal LCE materials and investigate methods for mechanical alignment, external field alignment, and surface-induced alignment. Advances in the synthesis and orientation of LCEs have enabled liquid crystal elastomers to meet applications in optics, robotics, and more. The review concludes with a discussion of current challenges and research opportunities.

Pigmented Structural Color Actuators Fueled by Near-Infrared Light

Cuttlefish can modify their body shape and both their pigmentary and structural colors for protection. This adaptability has inspired the development of appearance-changing polymers such as structural color actuators, although in most cases, the original shape has been confined to being flat, and pigmented structural color actuators have not yet been reported. Here, we have successfully created a pigmented structural color actuator using a cholesteric liquid crystal elastomer with a lower actuation temperature where both actuation and coloration (structural and pigmental) are tunable with temperature and NIR light. The shape, structural color, and absorption of the NIR-absorbing dye pigment of the actuator all change with temperature. Light can be used to trigger local in-plane bending actuation in flat films and local shape changes in a variety of 3D-shaped objects. A cuttlefish mimic that can sense light and respond by locally changing its appearance was also made to demonstrate the potential of pigmented structural color actuators for signaling and camouflage in soft robotics.

Biomimetic Janus photonic soft actuator with structural color self-reporting

Soft actuators with variable signal/color play an important role in the fields of targeted locomotion, artificial phototropism, drug screening, cargo transportation, and interactive sensing. The ability to achieve rapid response, large curvature, wide bending angle, and full-color display continues to be an unresolved challenge for artificial actuating materials. Inspired by the angle-dependent structural color of broad-tailed hummingbird and the Janus wettability of the lotus leaf, a Janus photonic soft actuator (JPSA) was fabricated by integrating an underwater super-oleophilic copper micro-nano array and oil-phobic inverse opal through a Laplace channel. The JPSA exhibits unidirectional permeability to underwater oil droplets. Attractively, with the combination of a swellable super-oleophilic surface and photonic crystals, JPSAs were endowed with oil-controlled reversible bending behavior with self-reporting angle-dependent color indication. We described for the first time the directional actuating mechanism induced by underwater oil unidirectional penetration and revealed the corresponding actuating kinetics and the inner-stress distribution/transfer by using structural color. As an extension of such theory, a rapid responsive JPSA with a wide bending angle and full-color self-reporting is further fabricated. This work provides an efficient strategy for oil directional transportation and separation in aqueous media and inspires the fabrication of a soft actuator/sensor with structural color self-reporting.

Colloidal Self-Assembly Approaches to Smart Nanostructured Materials

Colloidal self-assembly refers to a solution-processed assembly of nanometer-/micrometer-sized, well-dispersed particles into secondary structures, whose collective properties are controlled by not only nanoparticle property but also the superstructure symmetry, orientation, phase, and dimension. This combination of characteristics makes colloidal superstructures highly susceptible to remote stimuli or local environmental changes, representing a prominent platform for developing stimuli-responsive materials and smart devices. Chemists are achieving even more delicate control over their active responses to various practical stimuli, setting the stage ready for fully exploiting the potential of this unique set of materials. This review addresses the assembly of colloids into stimuli-responsive or smart nanostructured materials. We first delineate the colloidal self-assembly driven by forces of different length scales. A set of concepts and equations are outlined for controlling the colloidal crystal growth, appreciating the importance of particle connectivity in creating responsive superstructures. We then present working mechanisms and practical strategies for engineering smart colloidal assemblies. The concepts underpinning separation and connectivity control are systematically introduced, allowing active tuning and precise prediction of the colloidal crystal properties in response to external stimuli. Various exciting applications of these unique materials are summarized with a specific focus on the structure-property correlation in smart materials and functional devices. We conclude this review with a summary of existing challenges in colloidal self-assembly of smart materials and provide a perspective on their further advances to the next generation.

Photonic artificial muscles: from micro robots to tissue engineering

Light responsive shape-changing polymers are able to mimic the function of biological muscles accomplishing mechanical work in response to selected stimuli. A variety of manufacturing techniques and chemical processes can be employed to shape these materials to different length scales, from centimeter fibers and films to 3D printed micrometric objects trying to replicate biological functions and operations. Controlled deformations shown to mimick basic animal operations such as walking, swimming or grabbing objects, while also controlling the refractive index and the geometry of devices, opens up the potential to implement tunable optical properties. Another possibility is that of combining artificial polymers with cells or biological tissue (such as intact cardiac trabeculae) with the aim to improve tissue formation in vitro or to support the mechanical function of damaged biological muscles. Such versatility is afforded by chemistry. New customized liquid crystalline monomers are presented here that modulate material properties for different applications. The role of synthetic material composition is highlighted as we demonstrate how using apparently similar molecular formulations, that liquid crystalline polymers can be adapted to different technological and medical challenges.

Mesomeric Effects of Azobenzene Bearing Natural Product-Based Molecules for Liquid Crystal Materials: An Overview

Latest progress in the liquid crystal (LC) field related to azo molecules incorporated into natural product- based moieties for the improvement of LC texture and mesomeric phases has received great interest among researchers. A LC containing natural product-based moieties i.e. menthol, kojic acid, cholesterol and chalcone with stable azo and azobenzene scaffolds with specific optical tunability, has been widely used in photo-active materials such as Liquid Crystal Display (LCD), LC films, smart windows and other devices. This review discusses the influence of azobenzene, a renowned photo-responsive and stable LC scaffold, in mesogenic phases due to photo-isomerization and optical switching. The incorporation of mesomeric phases of natural product moieties to azo molecules has improved the properties of LC, i.e, from the nematic phase to the smectic phase with proper magnetic field alignment. Natural product-based LC can be useful in numerous applications, especially practical electronic or optic devices such as optical image storage, display devices, solar cells, optical switching.

Toward Application of Liquid Crystalline Elastomer for Smart Robotics: State of the Art and Challenges

Liquid crystalline elastomers (LCEs) are lightly crosslinked polymers that combine liquid crystalline order and rubber elasticity. Owing to their unique anisotropic behavior and reversible shape responses to external stimulation (temperature, light, etc.), LCEs have emerged as preferred candidates for actuators, artificial muscles, sensors, smart robots, or other intelligent devices. Herein, we discuss the basic action, control mechanisms, phase transitions, and the structure-property correlation of LCEs; this review provides a comprehensive overview of LCEs for applications in actuators and other smart devices. Furthermore, the synthesis and processing of liquid crystal elastomer are briefly discussed, and the current challenges and future opportunities are prospected. With all recent progress pertaining to material design, sophisticated manipulation, and advanced applications presented, a vision for the application of LCEs in the next generation smart robots or automatic action systems is outlined.

Magnetically Assembled Photonic Crystal Gels with Wide Thermochromic Range and High Sensitivity

Thermochromic poly(N-isopropyl acrylamide) (PNIPAM) photonic crystal gels based on 1D magnetically assembling colloidal nanocrystal clusters have attracted much attention due to its convenient preparation process, striking color response, and good mechanical strength. However, there remain challenges to broaden the thermochromic range and improve the sensitivity for their iridescent color display. Here, a PNIPAM photonic gel with wide thermochromic range and high sensitivity is prepared by using four-arm star poly(ethylene glycol) acrylamide (PEGAAm) as cross-linker at appropriately reduced magnetic field strength as well as cross-linker content. PEGAAm improves the homogeneity of the microstructure in PNIPAM photonic gel and thus maintains the structure colors at a wide temperature range from room temperature to 44 °C. The reduced magnetic field strength of 70 Gs and low cross-linker content (the molar ratio of monomer to cross-linker of 300:1) lead to a large initial lattice spacing of the photonic gel and thus wide diffraction wavelength migration of 194 nm. This optimized PNIPAM gel exhibits vivid iridescent colors from orange-red to indigo blue as temperature increases from 20 to 44 °C with satisfactory repeatability. Therefore, it may be an ideal candidate for temperature sensors and displays with utility and accuracy such as low-temperature burns.

Recent advances in colour-tunable soft actuators

In nature, some creatures have the capability to change shapes to adapt to ever-changing environments, which greatly inspire researchers to develop soft actuators. To endow soft actuators with capabilities to interact with environment and integrate more feedbacks is of great significance. Colour-tunable soft actuators that provide colour change feedbacks have therefore attracted extensive attention. Based on either chemical-colour or structural-colour based materials, a variety of colour-tunable soft actuators enabling shape deformations (or locomotion) and colour changes have been prepared and hold promise for applications in soft robotics and biomedical devices. This review summarizes the recent advances of colour-tunable soft actuators, with emphasis on their colour-change mechanisms and highlighting their applications. Existing challenges and future perspectives on colour-tunable soft actuators are presented.

Wettability-Controlled Directional Actuating Strategy Based on Bilayer Photonic Crystals

Although the water-triggered bending behavior of bilayer films has been a wide concerned, there are few reports on wettability-controlled directional actuators with visible color changes. Using photonic crystals as carriers, bilayer directional bending structural color actuators were prepared based on the hydrophilic difference. Top inverse opal with strong hydrophilicity can promote water penetration and strengthen the effect of swelling. While, bottom inverse opal with weak hydrophilicity can inhibit water penetration and weaken the effect of swelling. When the bilayer structure is immersed in water, its wettability differences will produce different optical responses for visualization and will bring different swelling performances, resulting in directional bending. Infiltration differences are visualized as structural color red shifts or transparency. The mechanism of the design involves optical diffractions in the fabricated periodic nanostructures, differences in the surface wettability and swelling rate, uses the infiltration and capillary evaporation of water to realize the spectral diversity of reflectance, and the enhancement of bending by gradient infiltration. This work deeply analyzes the improvement of the photonic crystal structure on the optical and bending performance of the wettability-controlled actuator, provides a basic model for the design of bionic components, and opens an idea for the combination of bilayer photonic crystals and actuators.

Diphylleia grayi-Inspired Intelligent Hydrochromic Adhesive Film

Diphylleia grayi-inspired hydrochromic nano/microstructured films have received much attention for its promising smart hydrochromic applications owing to their simple and low-cost but energy-effective strategy. A new type of water-switchable glazing film patterned with various nano/micro air-hole inverse opal arrays is introduced by selectively removing nano/microsphere polystyrene arrays embedded in the surface of polydimethylsiloxane (PDMS) films. Using the significant contrast ratio of the bleaching and the scattering states, we have optimized the switching properties of Mie scattered patterns. As a result, we obtained a single inverse opal layer-embedded PDMS adhesive film with hexagonally close-packed 1 μm air-hole arrays as an optimum scattered film. The differences of diffusive transmittance and optical haze values between the dry and the wet states of the best scattered film reached 44.93% (ΔT_D.T = 59.11-14.18%) and 54.88% (ΔH = 69.42-14.54%), respectively. In addition, using the best-optimized inverse opal layer-embedded PDMS film, we fabricated a perfectly imitated Diphylleia grayi structure for camouflage application and an intelligent hydrochromic window device. The dynamic water modulation of the scattered opaque and nonscattered transparent state of the inverse opal-patterned PDMS adhesive film can provide an advanced platform structure in the area of hydrochromic technology for smart windows, camouflage, and clear umbrellas for rainy days.

Processing advances in liquid crystal elastomers provide a path to biomedical applications

Liquid crystal elastomers (LCEs) are a class of stimuli-responsive polymers that undergo reversible shape-change in response to environmental changes. The shape change of LCEs can be programmed during processing by orienting the liquid crystal phase prior to crosslinking. The suite of processing techniques that has been developed has resulted in a myriad of LCEs with different shape-changing behavior and mechanical properties. Aligning LCEs via mechanical straining yields large uniaxial actuators capable of a moderate force output. Magnetic fields are utilized to control the alignment within LCE microstructures. The generation of out-of-plane deformations such as bending, twisting, and coning is enabled by surface alignment techniques within thin films. 4D printing processes have emerged that enable the fabrication of centimeter-scale, 3D LCE structures with a complex alignment. The processing technique also determines, to a large extent, the potential applications of the LCE. For example, 4D printing enables the fabrication of LCE actuators capable of replicating the forces generated by human muscles. Employing surface alignment techniques, LCE films can be designed for use as coatings or as substrates for stretchable electronics. The growth of new processes and strategies opens and strengthens the path for LCEs to be applicable within biomedical device designs.

Stimuli-responsive functional materials for soft robotics

Functional materials have spurred the advancement of soft robotics with the potential to perform safe interactions and adaptative functions in unstructured environments. The responses of functional materials under external stimuli lend themselves to programmable actuation and sensing, opening up new possibilities of robot design with built-in mechanical intelligence and unlocking new applications. Here, we review the development of stimuli-responsive functional materials particularly used for soft robotic systems. This review covers five representative types of soft stimuli-responsive functional materials, namely (i) dielectric elastomers, (ii) hydrogels, (iii) shape memory polymers, (iv) liquid crystal elastomers, and (v) magnetic materials, with focuses on their inherent material properties, working mechanisms, and design strategies for actuation and sensing. We also highlight the state-of-the-art applications of soft stimuli-responsive functional materials in locomotion robots, grippers and sensors. Finally, we summarize the current challenges and map out future trends for engineering next-generation functional materials for soft robotics.

Full-color natural rubber latex with a photonic nanostructure composite

As environmental problems increase, there is an urgent demand for new eco-friendly materials. Natural rubber latex (NRL) is a natural material extracted from rubber trees. But its dyeing process with chemical dyes might result in contamination and environmental degradation. Here, NRL is composited with a photonic crystal (PhC) structure by spin coating for the first time. The polymethyl methacrylate (PMMA) photonic nanostructure has been embedded into NRL to give it colors and provide it with optical functionalities. Colors of the composite could be designed and controlled by the sizes of the nanocolloids from 180 nm to 295 nm. The colors have strong stability under external stretching. The 3D natural rubber latex photonic crystal (NRLPC) is used as a responsive material to detect volatile organic compounds (VOCs) including formaldehyde, acetone, toluene, xylene and styrene. With its visual color appearance, biocompatibility and flexibility, NRLPC has promising potential in various sensing applications.

Thermal Responsive Photonic Crystal Achieved through the Control of Light Path Guided by Phase Transition

Responsive photonic crystal is widely considered in the field of anti-counterfeiting and information encryption because of their structural color changes caused by external stimulation. However, the response signal is usually achieved by adjusting the periodic lattice constant based on Bragg's law with volume changes. Thus, it is a great challenge to achieve the response of photonic crystals by non-array parameter control. Herein, novel thermal responsive photonic crystal (TRPC) with low angle dependent structural color is fabricated by introducing poly(ethylene glycol) into the structure of low angle dependent SnO₂ inverse opal. The response is achieved through the control of light path guided by phase transition and the significant volume change caused by the change of traditional array parameters can be effectively avoided. Meanwhile, the low angle dependent structural color of TRPC can effectively reduce the interference of observation angle change to response signal caused by external thermal stimulation. Patterned responsive photonic crystals with temperature gradient response are easily obtained by combining confinement self-assembly and direct template method, and the patterns can be presented and hidden by the control of light path, showing great potential in anti-counterfeiting and information encryption fields.

A controllable oil-triggered actuator with aligned microchannel design for implementing precise deformation

Soft actuators with the integration of facile preparation, rapid actuation rate, sophisticated motions, and precise control over deformation for application in complex devices are still highly desirable. Inspired by the aligned structures of moisture responsive pineal scales, an oil-triggered Janus actuator composed of a smooth hydrophobic surface and a superhydrophobic surface with aligned microchannels by simple laser etching was fabricated successfully, which can deform into various desirable shapes and recover to the original shape when triggered by oil and ethanol molecules. The aligned microchannel design causes different oil spread distances in the longitudinal and transverse directions, resulting in orientation-controlled bending and twisting with large-scale displacement. By changing the orientations of the patterned microchannels, one-dimensional folding deformation, twisting, rolling curling and object-inspired architectures can be facilely programmed. The reversible oil-triggered actuator will inspire more attractive applications such as in vivo surgery, biomimetic devices, energy harvesting systems and soft robotics.

Synthesis of High-Performance Photonic Crystal Film for SERS Applications via Drop-Coating Method

Silica nanospheres with a well-controlled particle size were prepared via a nucleation-to-growth synthesis process. A facile method is proposed for improving the self-assembly behavior of silica colloidal particles in droplet coatings by the simple controlling of the drying temperature. It is shown that a periodically arranged, opal-structured, photonic crystal film with a large area of approximately 4.0 cm2 can be prepared, even when the particle size is up to 840 nm. When the band gap of the silica photonic crystals falls in the visible-light region, the crystals exhibit distinct structural colors. Moreover, the wavelength of the reflected light increases with an increasing particle size of silica. When the photonic band gap overlaps the wavelength of the laser source, the overall Raman spectrum intensity is significantly enhanced. Accordingly, the proposed nucleation-to-growth process and drop-coating method provides a cheap and simple approach for the manufacture of uniform sized silica and surface-enhanced Raman scattering substrates, respectively.

Controllable Structural Colored Screen for Real-Time Display via Near-Infrared Light

Patterned colloidal crystals with stimuli-responsive materials provide sensitive and versatile means for investigating the varying ambiance of heat, light, electricity, magnetism, and stress. However, it remains a challenge to integrate stimuli-responsive materials with colloidal crystals by a simple and efficient method, thus restricting them from being used in general applications. Inspired from chameleons, we present a facile yet high-quality approach for the fabrication of the assembly of colloidal nanoparticles based on the hydrophilic-modified thermosensitive films. Various kinds of integral thermosensitive structural colored (TSSC) films are simply prepared in a high-quality screen on a large scale, with low cost, angle independence, and excellent flexibility. Simply turning on the near-infrared (NIR) laser brings heat to the irradiated region to increase the temperature. Integration of the multi-colored photonic bandgap (PBG) of the thermal-sensitive colloidal crystal and flexible anti-counterfeit labels into the NIR light exciting screens can change the intensity of PBG obviously. This advanced technology not only provides an efficient strategy for the preparation of colloidal crystal but also demonstrates a highly thermosensitive structural colored screen that has great prospect for information storage, anticounterfeiting, and real-time display materials.

Digital Programming Graphene Oxide Liquid Crystalline Hybrid Hydrogel by Shearing Microlithography

Designing anisotropic architectures enables the creation of soft materials with rich properties and functions to artificially simulate the evolutionary diversity of biology. In the important liquid crystalline hybrid (LCH) hydrogels, free manipulation of liquid crystalline order in high accuracy and efficiency has been long pursued to design properties and functions but remains a challenge. Here, we realize digital programing LC order in graphene oxide LCH hydrogels in high size resolution (∼20 μm) and efficiency by using shearing microlithography. The localized shear-induced LC order organization is immobilized by cross-linking gelation, and we prepare graphene oxide LCH hydrogels with digital programmed patterns in a large area. The shearing order generates a vertical alignment of graphene oxide sheets in hydrogels and a considerable mechanical anisotropy controlled by the shearing angle and interval spacing. By diversely organizing geometry of LC order, the mechanical response behaviors of LCH hydrogels are designed to exhibit localized deformations, steered cracking, and programmable swelling actuations. Our work offers a versatile avenue to scalably digital program LCH hydrogels in a high efficiency and accuracy. The digital designed hydrogel promises wide uses in actuators, bioscaffolds, biomimetic materials, and soft designer materials.

Programmable 3D Shape-Change Liquid Crystalline Elastomer Based on a Vertically Aligned Monodomain with Cross-link Gradient

A liquid crystalline elastomer (LCE) as a kind of stimuli-responsive materials, which can be fabricated to present the three-dimensional (3D) change in shape, shows a wide range of applications. Herein, we propose a simple and robust way to prepare a 3D shape-change actuator based on gradient cross-linking of the vertically aligned monodomain of liquid crystals (LCs). First, gold nanoparticles grafted by liquid crystalline polymers (LCPs) are used to induce the homeotropic orientation of the LC monomer and cross-linkers. Then, photopolymerization under UV irradiation is carried out, which can result in the LCE film with a cross-link gradient. Different from the typical LCEs with homogenous alignment that usually show the shape change of extension/contraction, the obtained vertically aligned LCE film exhibits excellent bendability under a thermal stimulus. The nanoindentation experiment demonstrates that the deformation of LCE films comes from the difference in Young's modulus on two sides of the thin film. Simply scissoring the thin film can prepare the samples with different bending angles under the fixed length. Moreover, using a photomask to pattern the film during photopolymerization can realize the complex 3D deformation, such as bend, fold, and buckling. Further, the patterned LCE film doped with multiwalled carbon nanotubes modified by LCPs (CNT-PDB) can act as a light-fueled microwalker with fast crawl behavior.

An ultra-large deformation bidirectional actuator based on a carbon nanotube/PDMS composite and a chitosan film

Actuating materials can convert external stimuli (humidity, light, electricity, etc.) into mechanical energy and realize multiple forms of movements. However, a majority of current actuating materials are driven by a single stimulus with a small degree of actuation and rough control which is unfavorable for practical applications. Here, a new type of bidirectional actuating material based on carbon nanotube/PDMS composites and chitosan films is proposed. Thanks to the robust mechanical support by PDMS, due to the ultra-large water capacity in between chitosan chains and strong near-infrared light absorption by carbon nanotube layers, the actuator can be driven by humidity and light for an ultra-large actuation curvature (3.91 cm^-1 in humidity actuation, 3.84 cm^-1 in light actuation). The well-established light power-curvature, relative humidity-curvature profiles and a fine mechanic modelling of the actuator show the possibility of controlling the actuator's bending. A lab application as a cargo-moving device preliminarily demonstrates a robust mechanical functionality of this actuator with a low body weight.

Dyeing and Functionalization of Wearable Silk Fibroin/Cellulose Composite by Nanocolloidal Array

A wearable silk fibroin/cellulose composite is reported. It is structurally dyed and functionalized by embedding three-dimensional (3D) or two-dimensional poly(methyl methacrylate) and polystyrene nanocolloidal arrays to form opal and inverse opal silk methylcellulose photonic crystal films (SMPCF). The brilliant color of SMPCF is utilized for naked-eye detection of humidity and a trace amount (0.02%) of H₂O content in organic solvents. Volatile organic compounds gases of 5 types were detected. By alternately exposed to organic solvents of methanol, acetonitrile, acetone, ethanol, isopropanol, n-butanol, carbon tetrachloride, and toluene, 3D inverse opal SMPCF displayed an excellent sensing performance with instantaneously color changes from green to red. The organic solvent sensitive SMPCF are wearable by integrated into a rubber glove. This composite has the potential to be used in wearable real-time sensing materials.

Coherent poly propagation materials with 3-dimensional photonic control over visible light

The purpose of the present research was to identify and examine materials demonstrating a previously undiscovered property of coherent poly propagation (CPP). The materials were amorphous silicates as natural precious opals. CPP enabled three-dimensional photonic control over mono and polychromatic visible light wavelengths. CPP caused coherent diffraction of incident poly and monochromatic light. Apart from the iconic play-of-color of precious opal, CPP specimens demonstrated diffractive photonic demultiplexing and/or upconversion and/or downconversion of incident light with strong photonic coherence such that the shape of the incident light source was propagated over three dimensions over multiple visible frequencies. CPP events manifested as each specimen was rocked under the incident light. Additionally, the specimens demonstrated atypical control over internally reflected and transmitted light. The specimens applied axial rotational symmetry over the incident light. Amorphous materials would be expected to exert no symmetry control. CPP and rotational properties occurred in isolation from exogenous thermal, photonic and electrical influences. Furthermore, several non-destructive analytical instruments were employed, such as: spectrophotometer, polariscope and refractometer. The analytical methods revealed unusual behaviors of these specimens. The application of materials demonstrating three-dimensional photonic control will have far-reaching implications for many industries, including: photonic wavelength demultiplexing, fiber optics, imaging, microscopy, projections, security, cryptography, computers and communications.

Photonic actuators with predefined shapes

Developing actuators with multi-responsibility, large deformation, and predefined shapes is critical for the application of actuators in the field of artificial intelligence. Herein, we report the preparation of a new type of unimorph actuators containing phenol-formaldelyde resin (PFR) and graphene oxide (GO) using the chiral nematic structure of cellulose nanocrystals (CNCs) as the template. The so-obtained PFR/GO films have a unimorph structure with an asymmetric distribution of GO across the film. They exhibit synchronous responses of both photonic properties and actuation to humidifying/dehumidifying. Moreover, PFR/GO films can be forged into desired shapes by aldehyde treatment, and thereby are able to produce complex movements. In addition, the objects with predetermined shapes show good shape recovery capability upon many wetting-drying cycles, especially through the treatment with formaldehyde. A mechanism model for shape predetermination by aldehyde treatment is suggested based on experimental details. By further designing the predetermined shapes and patterns, such PFR/GO actuators may hold great promise for smart actuation devices of highly complex movements.

Janus Soft Actuators with On-Off Switchable Behaviors for Controllable Manipulation Driven by Oil

Soft actuators have tremendous applications in diverse fields. Facile preparation, rapid actuation, and versatile actions are always pursued when developing new types of soft actuators. In this paper, we present a facile method integrating laser etching and mechanical cutting to prepare Janus actuators driven by oil. A Janus film with superhydrophobic and hydrophobic sides was fabricated successfully. By cutting the functional layer at the desired positions, a number of quintessential oil-driven soft devices were demonstrated. Furthermore, Janus actuators with surfaces of different wettabilities exhibited different swelling behaviors, and different media manifested different surface extensions; thus, these actuators are promising candidates for soft actuators and also realized on-off switchability between an oil/water mixture and ethanol. This study offers novel insight into the design of soft actuators, and this insight may be helpful for developing an oil-driven soft actuator that can be operated like a human finger to manipulate any object and extending stimuli-responsive applications for soft robotics.

Thermal and stress tension dual-responsive photonic crystal nanocomposite hydrogels

Easily prepared dual-responsive optical nanocomposite hydrogel (ONH) sensors which are responsive to tension and temperature are reported in which polymethyl methacrylate (PMMA) colloidal arrays were embedded into the hydrogels to obtain an optical response. Because of the band gap in the photonic crystal (PhC), the bright color of ONHs can be tuned by an external stimulus according to Bragg’s law. Thermosensitive N-isopropyl acrylamide (NiPAm) is added to the gel system, and by controlling NiPAm content and temperature, the contraction of the dual-response ONHs and the structural color response in the visible light range can change accordingly. Meanwhile, the temperature responses can be repeated more than seven times. Owing to the high biocompatibility, the excellent temperature response and the good mechanical strength of the ONHs, such optical biosensors have wide application in the biological field as an external stimulus sensor for implantable sensors, intracorporeal pressure measurement, and body temperature detection.

Easily prepared dual-responsive optical nanocomposite hydrogel (ONH) sensors which are responsive to tension and temperature are reported.

A NIR light-triggered pyroelectric-dominated generator based on a liquid crystal elastomer composite actuator for photoelectric conversion and self-powered sensing

Transducing light into electricity in photoactive materials and composites is especially attractive for light sensing and light energy harvesting. Here, we present a near-infrared (NIR) light-triggered pyroelectric-based generator by integrating a photoresponsive composite actuator composed of a liquid crystal elastomer (LCE) and graphene-doped poly(dimethylsiloxane) (PDMS) into a polyvinylidene fluoride (PVDF) film, which can effectively convert photothermal and mechanical energy into electricity. Notably, a NIR light photothermal-triggered pyroelectric effect leads to outstanding electric output performance resulting from the large temperature fluctuation induced by the contact and separation between the LCE-based composite actuator and PVDF film upon turning on or off the NIR illumination. In addition, the photothermal pyroelectric property arising from the thermal fluctuations makes the hybrid generator highly suitable as a self-powered NIR light and temperature sensor. This light-driven LCE-based hybrid generator opens a new opportunity for developing novel power generators and active sensors.

Reversible solvent-sensitive actuator with continuous bending/debending process from liquid crystal elastomer-colloidal material

A reversible solvent-sensitive actuator with a continuous bending/debending process is fabricated by over-infiltration of liquid crystal monomers into a colloidal template and subsequent photopolymerization. The fabricated actuator exhibits a maximum bending angle of 1080° in 1.58 s in dichloromethane, accompanied with successive debending in 0.32 s. The behavior of the actuator can be modulated by changing the solvent type, film thickness/length and molar ratio of A6OCB/C6M. This study will provide an important experimental and theoretical basis for the development of novel actuators.

Dual-Responsive SPMA-Modified Polymer Photonic Crystals and Their Dynamic Display Patterns

Light and electrothermal responsive polymer photonic crystals (PCs) modified with 1'-acryloyl chloride-3',3'-dimethyl-6-nitro-spiro(2H-1-benzopyran-2,2'-indoline) (SPMA) are proposed, and their dynamic display patterns are achieved through the combination of the SPMA-modified PCs and a patterned graphite layer. These PCs exhibit fluorescence under UV light irradiation because of the isomerization of the SPMA, which is restricted in the shell of the polymer colloidal spheres. After a voltage is applied to the patterned graphite layer, the fluorescence of PCs in the specific area disappears, and dynamic display patterns are obtained. Under UV light irradiation, the PCs change from the "partial-fluorescence" state to the initial "fluorescence" state, and the patterns disappear. Using this technique, the PC pattern "M L N" on the glass substrate and PC patterns from "0" to "9" on the paper substrate are fabricated. Thus, these dual-responsive PCs have potential applications in information recording, anticounterfeiting, dynamic display, and photoelectric devices.

Humidity- and light-driven actuators based on carbon nanotube-coated paper and polymer composite

Multi-responsive actuators driven by different stimuli (e.g. light, humidity, electricity) have attracted intense attention recently for the advantages of being used in various environments and show enormous actuation. In this work, we propose humidity- and light-driven actuators based on carbon nanotube (CNT)-coated paper and a biaxially oriented polypropylene (BOPP) composite. The CNT-paper/BOPP actuator shows large bending actuation when driven by humidity change (curvature of 1.2 cm-1) and near infrared (NIR) light irradiation (curvature up to 1.6 cm-1). The great actuation performances outperform most other paper-based actuators. Finally, a smart gripper, of which the initial opening width can be enlarged, is fabricated on the basis of the CNT-paper/BOPP actuators. By utilizing the bidirectional bending motion of the actuator, the opening width of the gripper can increase to a width that is 4 times larger than its initial width, so as to grasp a large object. The gripper is also able to raise and move an object that is 20 times heavier than one actuator of the gripper. We assume that this new type of actuator has great potential in artificial muscle, soft robotics and biomimetic applications.

Infrared actuation-induced simultaneous reconfiguration of surface color and morphology for soft robotics

Cephalopods, such as cuttlefish, demonstrate remarkable adaptability to the coloration and texture of their surroundings by modulating their skin color and surface morphology simultaneously, for the purpose of adaptive camouflage and signal communication. Inspired by this unique feature of cuttlefish skins, we present a general approach to remote-controlled, smart films that undergo simultaneous changes of surface color and morphology upon infrared (IR) actuation. The smart film has a reconfigurable laminated structure that comprises an IR-responsive nanocomposite actuator layer and a mechanochromic elastomeric photonic crystal layer. Upon global or localized IR irradiation, the actuator layer exhibits fast, large, and reversible strain in the irradiated region, which causes a synergistically coupled change in the shape of the laminated film and color of the mechanochromic elastomeric photonic crystal layer in the same region. Bending and twisting deformations can be created under IR irradiation, through modulating the strain direction in the actuator layer of the laminated film. Furthermore, the laminated film has been used in a remote-controlled inchworm walker that can directly couple a color-changing skin with the robotic movements. Such remote-controlled, smart films may open up new application possibilities in soft robotics and wearable devices.

Thermo-, photo-, and mechano-responsive liquid crystal networks enable tunable photonic crystals

Tunable photonic crystals exhibiting optical properties that respond reversibly to external stimuli have been developed using liquid crystal networks (LCNs) and liquid crystal elastomers (LCEs). These tunable photonic crystals possess an inverse opal structure and are photo-responsive, but circumvent the usual requirement to contain dye molecules in the structure that often limit their applicability and cause optical degradation. Herein, we report tunable photonic crystal films that reversibly tune the reflection peak wavelength under thermo-, photo- and mechano-stimuli, through bilayering a stimuli-responsive LCN including azobenzene units with a colourless inverse opal film composed of non-responsive, flexible durable polymers. By mechanically deforming the azobenzene containing LCN via various stimuli, the reflection peak wavelength from the bilayered film assembly could be shifted on demand. We confirm that the reflection peak shift occurs due to the deformation of the stimuli-responsive layer propagating towards and into the inverse opal layer to change its shape in response, and this shift behaviour is repeatable without optical degradation.

Reprogrammable, Reprocessible, and Self-Healable Liquid Crystal Elastomer with Exchangeable Disulfide Bonds

A liquid crystal elastomer (LCE) can be regarded as an integration of mesogenic molecules into a polymer network. The LCE can generate large mechanical actuation when subjected to various external stimuli. Recently, it has been extensively explored to make artificial muscle and multifunctional devices. However, in the commonly adopted two-step crosslinking method for synthesizing monodomain LCEs, the LCE needs to be well-cross-linked in the first step before stretching, which increases the disorder of mesogenic molecules in the final state of the LCE and makes it very challenging to fabricate the LCE of complex shapes. In this article, we developed a new LCE with disulfide bonds, which can be reprogrammed from the polydomain state to the monodomain state either through heating or UV illumination, owing to the rearrangement of the polymer network induced by the metathesis reaction of disulfide bonds. In addition, the newly developed LCE can be easily reprocessed and self-healed by heating. Because of the excellent reprogrammability as well as reprocessability of the LCE, we further fabricated LCE-based active micropillar arrays through robust imprint lithography, which can be hardly achieved using the LCE prepared previously. Finally, we showed an excellent long-term durability of the newly developed LCE.

Photothermally Driven Refreshable Microactuators Based on Graphene Oxide Doped Paraffin

Actuators based on phase change materials (paraffin) can simultaneously produce large stroke length and large force due to thermal expansion, but the low thermal conductivity of paraffin requires high power input and long actuation time. The graphene oxide (GO) doped paraffin dynamic actuator addresses the key challenges in the design of thermal phase change actuators: Thermal conductivity and light absorbing are increased, and the response time is reduced compared to the standard phase change actuator designed with metal heating resistors. The thermal properties of GO-paraffin composites with varied loading amount are characterized to confirm the optimal loading amount of 1.0%. A multicell phase change actuator was integrated into a digital micromirror controlled optical system. A series of photothermally driven refreshable patterns were generated and confirmed with infrared imaging.

Vapor sensing with color-tunable multilayered coatings of cellulose nanocrystals

Colloidal cellulose nanocrystals were LBL deposited to form firmly-stacked optical coatings in which the nanorods regulated their head-to-tail association and aligned in the axial-centrifuged direction. The periodically transition from blue to orange of reflected colors was tunable via deposition layer adjustment. While the sensing coating was exposed to vapors of NH₃^.H₂O, H₂O, HCl and HAc, respectively, the color variation in the response process was irreversible at room temperature and highly dependent on vapor diffusion and chemical interface interaction. Consequently, HAc vapor presented the longest sensing transition of wavelength, whereas the alkaline NH₃^.H₂O displays a less recovery ratio than HAc and H₂O at room temperature. Under heating at 50°C, the sensed coatings could mostly be restored to their original state except HCl-etched one. Therefore, the naked-eyed qualitative detectability of vapors by nanocellulose could be realized by the divergence in color shift which is of great importance in chemical sensors.