Mechanochromic and thermochromic shape memory photonic crystal films based on core/shell nanoparticles for smart monitoring

Polymerization-induced highly brilliant and color-recordable mechanochromic photonic gels for ink-free patterning

Mechanochromic photonic crystals (MPCs) are extremely attractive since they can adjust their structural color by forces. However, the poor color saturation and color-recordability of conventional MPCs significantly limit their practical applications. Herein, a highly brilliant and color-recordable MPC gel (MPCG) has been fabricated by photopolymerizing the liquid photonic crystals with silica particles non-closely packed in acrylate, dichlorobenzene, and oleylamine. Photopolymerization induces elastic gradient non-close-packing structures and thus broad photonic bandgaps (>100 nm), resulting in 1) high color saturation despite possessing a small refractive index contrast (0.06), 2) remarkable mechanochromic properties, including a large wavelength tuning range (228 nm), fast responsiveness (8.8-10.3 nm/ms), and high sensitivity (4.4 nm/kPa), and 3) unconventional color-recordable properties. MPCGs were experimentally proved to be ideal rewritable papers for constructing multicolor and high-resolution patterns in an ink-free way, difficult for traditional MPC-based units. The unique working mechanism of polymerization-induced phase separation and thus continuous swelling and gelation, and precise design of materials and structures are the keys to MPCGs' characteristics. This study paves a new way for constructing advanced stimulus-responsive photonic structures and will promote their applications in printing, display, anti-counterfeiting, etc.

Flexible self-supporting photonic crystals: Fabrications and responsive structural colors

Photonic crystals (PCs) play an increasingly significant role in anti-counterfeiting, sensors, displays, and other fields due to their tunable structural colors produced by light manipulation of photonic stop bands. Flexible self-supporting photonic crystals (FSPCs) eliminate the requirement for conventional structures to rely on the existence of hard substrates, as well as the problem of poor mechanical qualities caused by the stiffness of the building blocks. Meanwhile, diverse production techniques and materials provide FSPCs with varied stimulus-responsive color-changing capacities, thus they have received an abundance of focus. This review summarizes the preparation strategies and variable structural colors of FSPCs. First, a series of preparation strategies by integrating polymers with PCs are summarized, including assembly of colloidal spheres on flexible substrates, polymer packaging, polymer-based direct assembly, nanoimprinting, and 3D printing. Subsequently, variable structural colors of FSPCs with different stimulations, such as viewing angle, chemical stimulation (solvents, ions, pH, biomolecules, etc.), temperature, mechanical/magnetic stress, and light, are described in detail. Finally, the outlook and challenges regarding FSPCs are presented, and several potential directions for their fabrication and application are discussed. It's believed that FSPCs will be a valuable platform for advancing the practical implementation of optical metamaterials.

A Simply Synthesized Shaking-induced Small Molecule System with Repeatable and Instantaneous Discoloration Response

Force-related discoloration materials are highly valuable because of their characteristics of visualization, easy operation, and environment friendliness. Most force-related discoloration materials focus on polymers and depend on bond scission, which leads to insensitivity and unrecoverable. Small-molecule systems based on well-defined molecular structures and simple composition with high sensitivity would exhibit considerable mechanochromic potential. However, to date, researches about force-related discoloration materials based on small molecule solution remain limited and are rarely reported. In this study, we developed a repeatable and instantaneous discoloration small molecule solution system by simple one-pot synthesis method. It exhibited an instantaneous chromic change from yellowish to dark green under shaking and reverting back to yellow within 1 minute after removal of the shaking. Experimental results confirmed that the discoloration mechanism is attributed to the oscillation accelerating the production of unstable ortho-OH phenoxyl radical. The newly developed shaking-induced discoloration small molecule system (SDSMS) promises in field of mechanical force sensing and optical encryption.

Robust and Durable Photonic Crystal with Liquid-Repellent Property for Self-Cleaning Coatings and Structural Colored Textiles

Photonic crystal coatings with unique structural colors and self-cleaning properties have been providing an efficient way for substrate coloration. However, the enhancement of the robustness and durability of structural colored coatings to meet the requirements in diverse environments remains a challenging task. Here, to realize the application of photonic crystal films under various kinds of conditions, we present a poly(fluoroalkyl acrylate)-based colloidal photonic crystal (fCPC) coating. Fluorinated core-interlayer-shell (FCIS) colloidal particles of polystyrene (PS) core, poly(methyl methacrylate) (PMMA) interlayer, and poly(fluoroalkyl acrylate-ethyl acrylate-butyl acrylate) (P(FA-EA-BA)) shell copolymers have been first prepared by a stepwise emulsion polymerization. fCPCs with self-supporting property, reprocessing ability, friction resistance, as well as excellent wettability and liquid-repellent properties are successfully obtained via the bending-induced ordering technique (BIOT). When applied in antifouling applications, the fCPC film exhibits resistance against various oil and inorganic liquids. Furthermore, the fCPC coatings demonstrate their durability under outdoor conditions by maintaining stable color appearances during rainy and sunny conditions. Additionally, an electronic product adhered with the fCPC coatings is presented, which exhibits a surface that remains clean even after prolonged usage in comparison to the conventional CPC coating. Structural colored textiles with enhanced stability and functionalized liquid-repellent properties are achieved through a one-step process using FCIS particles. Therefore, the developed self-cleaning and comprehensive fCPC coatings capable of withstanding diverse conditions may open up new avenues for the advancement of structural coloration in decoration, vehicle, textile, and building.

Multicolor recordable and erasable photonic crystals based on on-off thermoswitchable mechanochromism toward inkless rewritable paper

Mechanochromic photonic crystals are attractive due to their force-dependent structural colors; however, showing unrecordable color and unsatisfied performances, which significantly limits their development and expansion toward advanced applications. Here, a thermal-responsive mechanochromic photonic crystal with a multicolor recordability-erasability was fabricated by combining non-close-packing mechanochromic photonic crystals and phase-change materials. Multicolor recordability is realized by pressing thermal-responsive mechanochromic photonic crystals to obtain target colors over the phase-change temperature followed by fixing the target colors and deformed configuration at room temperature. The stable recorded color can be erased and reconfigured by simply heating and similar color-recording procedures respectively due to the thermoswitchable on-off mechanochromism of thermal-responsive mechanochromic photonic crystals along with solid-gel phase transition. These thermal-responsive mechanochromic photonic crystals are ideal rewritable papers for ink-freely achieving multicolor patterns with high resolution, difficult for conventional photonic papers. This work offers a perspective for designing color-recordable/erasable and other stimulus-switchable materials with advanced applications.

Preparation of thermochromic ink from anthocyanidin-encapsulated alginate nanoparticles for anticounterfeiting applications

In order to make commercial products less vulnerable to counterfeiting, thermochromic inks have proven to be a viable authentication strategy. Herein, we developed a thermochromic ink for authentication by combining an anthocyanidin (ACYD) extract with alginate (ALG). To increase the anthocyanidin/alginate ink stability, a mordant (ferrous sulfate) was employed to tie up the anthocyanidin biomolecules with alginate. ACYD was extracted from red-cabbage and then immobilized into alginate to serve as an environmentally friendly spectroscopic probe. Thermochromic composite inks (ACYD@ALG) were made by adjusting the content of anthocyanidin. A homogenous blue film (608 nm) was printed on a paper surface and investigated by the CIE Lab coordinate system. The blue color transformed into reddish (477 nm) when heated from 35°C to 65°C. Nanoparticles (NPs) of anthocyanidin/mordant (ACYD/M) were examined for their size and morphology to indicate diameters of 80-90 nm, whereas the ACYD/M-encapsulated alginate nanoparticles showed diameters of 120-150 nm. Multiple analytical techniques were utilized to examine the printed papers. The mechanical and rheological performance of both stamped sheets and ink fluid were explored. The cytotoxicity and antimicrobial efficacy of ink (ACYD@ALG) were investigated.

Metal-organic framework photonic crystals with bidisperse particles-based brilliant structural colors and high optical transparency for elaborate anti-counterfeiting

Photonic crystals (PCs) have attracted great interest and wide applications in displays, printing, anti-counterfeiting, etc. However, two main challenges significantly hinder their applications: 1) the tradeoff between high optical transparency across the whole visible range and brilliant colors requiring a large refractive index contrast (Δn), and 2) the way of regulating structural colors by altering tens of different sizes. To address these issues, a new type of metal-organic framework (MOF)-based transparent photonic crystal (TPC) has been fabricated through self-assembling MOF particles into three-dimensional ordered structures which were then infiltrated by polydimethylsiloxane (PDMS). Compared to conventional PCs, these TPCs exhibit 1) both brilliant forward iridescent structural colors and high transmittance (>75 %) across the whole visible spectra range, and 2) conveniently adjustable colors based on bidisperse particles. The unique color-generating mechanism of the light diffraction by each plane lattice and the small Δn between MOF particles and PDMS are the keys to TPCs' characteristics. Moreover, the prepared invisible anti-counterfeit labels can reversibly hide-reveal patterns with elaborate and exchangeable color contrast in a non-destructive way, showing potential applications in anti-counterfeiting, information encryption, and optical devices.

Scalable production of structurally colored composite films by shearing supramolecular composites of polymers and colloids

Structurally colored composite films, composed of orderly arranged colloids in polymeric matrix, are emerging flexible optical materials, but their production is bottlenecked by time-consuming procedures and limited material choices. Here, we present a mild approach to producing large-scale structurally colored composite films by shearing supramolecular composites composed of polymers and colloids with supramolecular interactions. Leveraging dynamic connection and dissociation of supramolecular interactions, shearing force stretches the polymer chains and drags colloids to migrate directionally within the polymeric matrix with reduced viscous resistance. We show that meter-scale structurally colored composite films with iridescence color can be produced within several minutes at room temperature. Significantly, the tunability and diversity of supramolecular interactions allow this shearing approach extendable to various commonly-used polymers. This study overcomes the traditional material limitations of manufacturing structurally colored composite films by shearing method and opens an avenue for mildly producing ordered composites with commonly-available materials via supramolecular strategies.

2D Information Security System Based on Polyurethane Inverse Photonic Glass Structure

Information security has become a major global problem in recent years. Thus, people continue to exert much effort in developing new information security technologies based on encryption and storage. In this study, a 2D information security technology based on polyurethane optical devices with inverse photonic glass structure (PU-IPG) is introduced. Based on 1) the swelling and plasticizing effects of various solvents on PU-IPG and 2) the capillary force that can produce geometric deformation on micro/nanostructures when solvents evaporate, a 2D information security system with two modules of decryption (structural color information display) and anticounterfeiting (structural color transformation) is successfully constructed. The spraying method adopted can be simple and fast and can provide a large area to build photonic glass templates, which greatly improves the capacity and category of information in the encryption system. The prepared PU-IPG optical devices can produce large-area multicolor output capability of information. These devices also have excellent mechanical properties, strong cycle stability, environmental friendliness, and low price. Therefore, the preparation strategy has great reference value and application prospects in the field of information security.

A Rule for Response Sensitivity of Structural-Color Photonic Colloids

To mimic natural photonic crystals having color regulation capacities dynamically responsive to the surrounding environment, periodic assembly structures have been widely constructed with response materials. Beyond monocomponent materials with stimulus responses, binary and multiphase systems generally offer extended color space and complex functionality. Constructing a rule for predicting response sensitivity can provide great benefits for the tailored design of intelligently responsive photonic materials. Here, we elucidate mathematical relationships between the response sensitivity of dynamically structural-color changes and the location distances of photonic co-phases in three-dimensional Hansen space that can empirically express the strength of their interaction forces, including dispersion force, polarity force, and hydrogen bonding. Such an empirical rule is proven to be applicable for some typical alcohols, acetone, and acetic acid regardless of their molecular structures, as verified by angle resolution spectroscopy, in situ infrared spectroscopy, and molecular simulation. The theoretical method we demonstrate provides rational access to custom-designed responsive structural coloration.

Stimulus-responsive nonclose-packed photonic crystals: fabrications and applications

Stimulus-responsive photonic crystals (PCs) possessing unconventional nonclosely packed structures have received growing attention due to their unique capability of mimicking the active structural colors of natural organisms (for example, chameleons' mechanochromic properties). However, there is rarely any systematic review regarding the progress of nonclose-packed photonic crystals (NPCs), involving their fabrication, working mechanisms, and applications. Herein, a comprehensive review of the fundamental principles and practical fabrication strategies of one/two/three-dimensional NPCs is summarized from the perspective of designing nonclose-packed structures. Subsequently, responsive NPCs with exciting functions and working mechanisms are sorted and delineated according to their diverse responses to physical (force, temperature, magnetic, and electric fields), chemical (ions, pH, vapors, and solvents), and biological (glucose, organophosphate, creatinine, and bacteria) stimuli. We then systematically introduced and discussed the applications of NPCs in sensors, printing, anticounterfeiting, display, optical devices, etc. Finally, the current challenges and development prospects for NPCs are presented. This review not only concludes the design principle for NPCs but also provides a significant basis for the exploration of next-generation NPCs.

Structural Design of Intelligent Reversible Two-Way Structural Color Films

Structural color always shows a reversible switch between reflection and transmission states when viewed from different angles, attracting increasing attention in display applications. However, this switching between reflection and transmission states of structural color suffers from the inherent lack of autonomous regulation, which is unmanageable in the case of different application scenarios. Here, we design an intelligent two-way structural color film which can reversibly change its color when applied with an extra stimulation such as voltage, heat signal, or light. A special structural feature contains a traditional photonic crystal film of polystyrene (PS) microspheres assembled by smart windows. Remarkably, our structural color film shows a prominent polarization sensitivity, and the angle dependence of the structural color broadens the gamut of display color demonstrated by both finite element theoretical analysis and experimental observation. Prospectively, this hierarchically designed film provides a promising pathway toward next-generation multicolor displays and smart windows.

Chameleon-Inspired Mechanochromic Photonic Elastomer with Brilliant Structural Color and Stable Optical Response for Human Motion Visualization

Flexible and stretchable electronic devices are indispensable parts of wearable devices. However, these electronics employ electrical transducing modes and lack the ability to visually respond to external stimuli, restricting their versatile application in the visualized human-machine interaction. Inspired by the color variation of chameleons' skin, we developed a series of novel mechanochromic photonic elastomers (PEs) with brilliant structural colors and a stable optical response. Typically, these PEs with a sandwich structure were prepared by embedding PS@SiO₂ photonic crystals (PCs)within the polydimethylsiloxane (PDMS) elastomer. Benefiting from this structure, these PEs exhibit not only bright structural colors, but also superior structural integrity. Notably, they possess excellent mechanochromism through lattice spacing regulation, and their optical responses are stably maintained even when suffering from 100 stretching-releasing cycles, showing superior stability and reliability and excellent durability. Moreover, a variety of patterned PEs were successfully obtained through a facile mask method, which provides great inspiration to create intelligent patterns and displays. Based on these merits, such PEs can be utilized as visualized wearable devices for detecting various human joint movements in real time. This work offers a new strategy for realizing visualized interactions based on PEs, showing huge application prospects in photonic skins, soft robotics, and human-machine interactions.

Chameleon-inspired active tunable structural color based on smart skin with multi-functions of structural color, sensing and actuation

Tunable structural color has many potential applications in artificial camouflage, mechanical sensors, etc. Despite the extensive efforts to develop efficient tunable structural color, there is still a wide gap between the existing "passive" tuning methods and the "active" strategy found on organisms such as chameleons that can change color according to the environment. Inspired by the active tunable color system of chameleons, we propose a smart skin comprising a nanoscale hole array of photonic crystals, carbon nanotube coatings, and liquid crystal elastomers, to integrate multiple functions, i.e., structural color tunability, sensing, and actuation, in one structure. The smart skin was further coupled with an image acquisition unit (which mimics eyes to obtain colors from the environment) and a controller (which mimics the brain to process the signals transmitted from the image acquisition unit to the smart skin), to construct an active tunable structural color system. The proposed system autonomously modulates the color according to the environmental color. To validate the color tuning, color scanning from red to green to blue or vice versa is demonstrated in this work, which could certainly open up new paths to create active tunable structural color systems, and thus, push the development of structural color-based devices and systems.

Butterfly-Inspired Tri-State Photonic Crystal Composite Film for Multilevel Information Encryption and Anti-Counterfeiting

Counterfeiting is a worldwide issue and has long troubled legitimate businesses, while nowadays anti-counterfeiting materials and technology are still insufficient to combat the escalating counterfeit behaviors. Inspired by hindwing structure of Troides magellanus, a new kind of anti-counterfeiting material taking advantage of both physical and chemical structures to display multiple optical states is prepared. The chemical units (luminescent lanthanide) are blended with physical units (monodispersed colloidal particles) and mediating molecules, which are then assembled into a photonic crystal structure at room temperature in less than 10 s through a new assembly technique called molecule-mediated shear-induced assembly technique (MSAT). The as-prepared photonic crystal films feature three unique optical states, each displaying structural, fluorescent, and phosphorescent color under different lighting conditions, which integrates colors from both physical and chemical origins. Furthermore, by incorporating different luminescent materials into different parts of the photonic crystal pattern, a high-level information encryption system is designed to be capable of carrying three distinct types of information. Thanks to this powerful tool of MSAT, it is now possible to assemble different-sized, even irregular non-spherical units with monodispersed spherical units into high-quality photonic crystal films, which provides easy access to incorporating new features into photonic crystal systems.

Hydration Activates Dual-Confined Shape-Memory Effects of Cold-Reprogrammable Photonic Crystals

Shape-memory photonic crystals (SMPCs) transform the nanoscale deformation of copolymers into structural color through an undifferentiated response to stimuli; however, activatable selective responses are extremely rare. Herein, activatable dual confined shape-memory effects (CSMEs) derived from the remodeling of the interchain hydrogen bonds (H-bonds) in cold-programmable SMPCs are revealed. The first level is the water-triggered reconstruction of interchain H-bonds, which can activate/lock the collapsed skeleton, showing shape recovery/retention in response to ethanol vapor. The second level is the pressure-induced reorganization of interchain H-bonds that results in the recovered skeleton being locked even when exposed to ethanol vapor or water, while the background porous structure can switch between collapse and recovery. Dual CSMEs result from the Laplace pressure difference and the binding effect of interchain H-bonds in the skeleton according to insights of swelling, in situ deformation tracking, multidimensional infrared spectra, and water wetting/evaporation simulations. The signal interference, source code extraction, and color enhancement of structurally colored patterns can be implemented using CSMEs. This work opens up a new method for fabricating activatable responsive structural color and contributes to the expansion of nanophotonic technology in water printing, erasable watermarks, signal amplifiers, and information coding.

Review on Mechanoresponsive Smart Windows: Structures and Driving Modes

The growing awareness about the global energy crisis and extreme weather from global warming drives the development of smart windows market. Compared to conventional electrochromic, photochromic, or thermochromic smart windows, mechanoresponsive smart windows present advantages of simple construction, low cost, and excellent stability. In this review, we summarize recent developments in mechanoresponsive smart windows with a focus on the structures and properties. We outline the categories and discuss the advantages and disadvantages. Especially, we also summarize six unconventional driving modes to generate mechanical strain, including pneumatic, optical, thermal, electric, magnetic, and humidity modes. Lastly, we provide practical recommendations in prospects for future development. This review aims to provide a useful reference for the design of novel mechanoresponsive smart windows and accelerate their practical applications.

Deformable Photonic Crystals Based on Chiral Liquid Crystals with Thermal-Mediative Shape Memory Effect

We propose a deformable photonic crystal that exhibits the thermal-mediative shape memory effect. The chiral liquid crystalline polymeric scaffold, which produces the structural colors from a helical twist of the liquid crystal director, is prepared through phase-stabilization of a reactive mesogen in a small molecular chiral liquid crystal (CLC), polymerization, and removal of the CLC. The prepolymer of polyurethane acrylate (PUA) is then infiltrated in the prepared scaffold and subsequently photo-polymerized to form a CLC-PUA composite film. Upon compression, this film shows the blue shift of the structural color and retains this color-shift as released from compression. As the temperature increases, the color is recovered to a pristine state. The concept proposed in this study will be useful for designing mechanochromic soft materials.

Color-Tuning Mechanism of Electrically Stretchable Photonic Organogels

In contrast to nano-processed rigid photonic crystals with fixed structures, soft photonic organic hydrogel beads with dielectric nanostructures possess advanced capabilities, such as stimuli-responsive deformation and photonic wavelength color changes. Recenlty, advanced from well-investigated mechanochromic method, an electromechanical stress approach is used to demonstrate electrically induced mechanical color shifts in soft organic photonic hydrogel beads. To better understand the electrically stretchable color change functionality in such soft organic photonic hydrogel systems, the electromechanical wavelength-tuning mechanism is comprehensively investigated in this study. By employing controllable electroactive dielectric elastomeric actuators, the discoloration wavelength-tuning process of an electrically stretchable photonic organogel is carefully examined. Based on the experimental in-situ response of electrically stretchable nano-spherical polystyrene hydrogel beads, the color change mechanism is meticulously analyzed. Further, changes in the nanostructure of the symmetrically and electrically stretchable organogel are analytically investigated through simulations of its hexagonal close-packed (HCP) lattice model. Detailed photonic wavelength control factors, such as the refractive index of dielectric materials, lattice diffraction, and bead distance in an organogel lattice, are theoretically studied. Herein, the switcing mechanism of electrically stretchable mechanochromic photonic organogels with photonic stopband-tuning features are suggested for the first time.

Transparent Polymer Opal Thin Films with Intense UV Structural Color

We report on shear-ordered polymer photonic crystals demonstrating intense structural color with a photonic bandgap at 270 nm. Our work examines this UV structural color, originating from a low refractive index contrast polymer composite system as a function of the viewing angle. We report extensive characterization of the angle-dependent nature of this color in the form of 'scattering cones', which showed strong reflectivity in the 275-315 nm range. The viewing range of the scattering was fully quantified for a number of planes and angles, and we additionally discuss the unique spectral anisotropy observed in these structures. Such films could serve as low-cost UV reflection coatings with applications in photovoltaics due to the fact of their non-photobleaching and robust mechanical behavior in addition to their favorable optical properties.

Scalable and Sensitive Humidity-Responsive Polymer Photonic Crystal Films for Anticounterfeiting Application

In this study, we fabricate a new kind of ultrasensitive humidity-responsive photonic crystal (HPC) films based on emulsion polymerization and the open mill and bending-induced ordering technique (OM-BIOT) method, which is simple and scalable. The HPC film senses relative humidity (RH) from 9 to 98% for the polymer matrix swells up in high RH and shrinks in low RH, leading to a large reflectance shift (81 nm) and distinct color change. Based on the double-peak reflective spectra of the HPC film, we confirm the gradient swelling hypothesis and find that the thickness is another important factor for controlling the sensitivity and response rate of the HPC film. Except for static humidity, the HPC film can also respond to the dynamic humid flow of blowing and polar solvents, which broadens its application potential. This kind of HPC film shows a vivid structural color, and the humidity-responsive behavior is quick, distinct, energy-free, and reversible, having a great prospect for anticounterfeiting application.

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.

Chameleon-Inspired Brilliant and Sensitive Mechano-Chromic Photonic Skins for Self-Reporting the Strains of Earthworms

The skins of chameleons have attracted growing interest because they have sensitive mechano-chromic properties and bright colors due to the large surface-to-surface distances (D_s-s) between neighboring particles and contrast of the refractive index (Δn), respectively. Inspired by these, artificial mechano-chromic photonic skins (MPSs) mimicking those of chameleons were fabricated by the large Δn and D_s-s. The fabrication is considerably simple and efficient based on the self-assembly strategy using commercial chemicals and materials. The reflectance of MPSs depends on the value of Δn, which can be greatly increased to 70% with a Δn of 0.035, leading to their brilliant colors. Because of the large D_s-s, the MPSs possess outstanding mechano-chromic performances, including a large maximal (Δλ = 205 nm) and effective (Δλ_e = 184 nm) tuning range of the reflection wavelength, high sensitivity (368), fast responsiveness (2.2 nm/ms), good stabilities (>1 year), and reversibility (>100 times). Based on these advantages, MPSs have been used for self-reporting the strain of earthworms by outputting diverse colors during the peristaltic process, indicating the great potential of the MPSs as visual sensors and optical coatings.

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.

Magneto-sensitive photonic crystal ink for quick printing of smart devices with structural colors

In this paper, we report a facile strategy to combine magneto-responsive photonic crystal (MRPC) ink with 3D printing technology. The building blocks of MRPC are based on Fe₃O₄ magnetic nanoparticle clusters (MNCs) with uniform and tunable size. The MNC dispersion is able to change its photonic band gap from red to blue as the external magnetic field strength is increased. The magneto-responsive photonic crystal ink can be readily obtained by taking advantage of an ethylene glycol (EG)-in-oil emulsion with a reinforced silicone rubber prepolymer as the outer phase. Using the well-designed formula, the MNC dispersion can be well-preserved inside the emulsion droplets of the ink, maintaining its original contactless magnetic field response. As a proof of concept, custom quick response code and butterfly patterns were successfully printed and showed vivid and tunable color as a function of the external magnetic field strength with good repeatability.

Recent Advances in Sensing Applications of Molecularly Imprinted Photonic Crystals

Photonic crystals (PhCs) with a brightly colored structure are novel materials and are widely used in chemical and biological sensing. Combining PhCs with molecular imprinting technology (MIT), the molecularly imprinted PhC (MIPC) sensors are fabricated, which can specifically recognize the target molecules. Aside from high sensitivity and selectivity, the MIPC sensors could recognize the naked eye detection because of its optical properties. In this review, an overview of recent advances in sensing applications of MIPC sensors including the responsive mechanisms, application in environmental monitoring, and the application to human health were illustrated. The MIPC sensors all responded to the analytes specifically and also showed high sensitivity in real samples, which provided a method to realize the rapid, convenient, naked eye, and real-time detection. Furthermore, the current limitations and potential future directions of MIPC sensors were also discussed.

Environmentally Stable Chiral-Nematic Liquid-Crystal Elastomers with Mechano-Optical Properties

Chiral-nematic liquid crystal (N* LC) elastomers exhibit mechano-optical responsive behavior. However, practical sensor applications have been limited by the intrinsic sensitivity of N* LC elastomers to environmental conditions, such as temperature. Although densely cross-linked LC network polymers exhibit high thermal stability, they are not proper for the mechanical sensor due to high glass transition temperatures and low flexibility. To overcome these issues, we focused on enhancing thermal stability by introducing noncovalent cross-linking sites via intermolecular interactions between LC molecules bonded to the polymer network. N* LC elastomers with a cyanobiphenyl derivative as a side-chain mesogen exhibited mechano-optical responsive behavior, with a hypsochromic shift of the reflection peak wavelength under an applied tensile strain and quick shape and color recovery owing to high elasticity. Notably, the N* LC elastomers showed high resistance to harsh environments, including high temperatures and various solvents. Interactions, such as π–π stacking and dipole–dipole interactions, between the cyanobiphenyl units can act as weak cross-links, thus improving the thermal stability of the LC phase without affecting the mechano-optical response. Thus, these N* LC elastomers have great potential for the realization of practical mechano-optical sensors.

A Review of Shape Memory Polymers and Composites: Mechanisms, Materials, and Applications

Over the past decades, interest in shape memory polymers (SMPs) has persisted, and immense efforts have been dedicated to developing SMPs and their multifunctional composites. As a class of stimuli-responsive polymers, SMPs can return to their initial shape from a programmed temporary shape under external stimuli, such as light, heat, magnetism, and electricity. The introduction of functional materials and nanostructures results in shape memory polymer composites (SMPCs) with large recoverable deformation, enhanced mechanical properties, and controllable remote actuation. Because of these unique features, SMPCs have a broad application prospect in many fields covering aerospace engineering, biomedical devices, flexible electronics, soft robotics, shape memory arrays, and 4D printing. Herein, a comprehensive analysis of the shape recovery mechanisms, multifunctionality, applications, and recent advances in SMPs and SMPCs is presented. Specifically, the combination of functional, reversible, multiple, and controllable shape recovery processes is discussed. Further, established products from such materials are highlighted. Finally, potential directions for the future advancement of SMPs are proposed.

Mechanochromism in Structurally Colored Polymeric Materials

Mechanochromic effects in structurally colored materials are the result of deformation-induced changes to their ordered nanostructures. Polymeric materials which respond in this way to deformation offer an attractive combination of characteristics, including continuous strain sensing, high strain resolution, and a wide strain-sensing range. Such materials are potentially useful for a wide range of applications, which extend from pressure-sensing bandages to anti-counterfeiting devices. Focusing on the materials design aspects, recent developments in this field are summarized. The article starts with an overview of different approaches to achieve mechanochromic effects in structurally colored materials, before the physical principles governing the interaction of light with each of these materials types are summarized. Diverse methodologies to prepare these polymers are then discussed in detail, and where applicable, naturally occurring materials that inspired the design of artificial systems are discussed. The capabilities and limitations of structurally colored materials in reporting and visualizing mechanical deformation are examined from a general standpoint and also in more specific technological contexts. To conclude, current trends in the field are highlighted and possible future opportunities are identified.

Transparent and UV Blocking Structural Colored Hydrogel for Contact Lenses

Usually, materials with perfect structures possess excellent properties, but it is not always the case. Here, a new approach is reported to construct structural colored hydrogel films with excellent ultraviolet (UV) blocking performance for contact lenses. The theoretical simulation predicts that with perfect periodic structures, the hydrogel films can strongly reflect incident light in a narrow visible wavelength range and thus exhibit extraordinarily brilliant colors. However, such hydrogel films cannot effectively block UV light. By slightly breaking the structural periodicity (quasi-periodic structure), strong diffuse scattering or pseudoabsorption of light can occur for all of the wavelengths shorter than a structural characteristic length, leading to perfect UV blocking. According to the theoretical prediction, a structural colored hydrogel film with nearly periodic polystyrene sphere arrays in poly(hydroxyethyl methacrylate) hydrogel matrix is fabricated; this hydrogel film possesses brilliant colors and perfect UV blocking, and the core particle composition and size have been investigated in detail for the optimized properties of contact lenses. Meanwhile, the cell proliferation assay proves the cytocompatibility of the hydrogel for real application. Regarding its unique optical characteristics, the as-prepared structural colored hydrogel shows great promise in the fields of UV-protective equipment, medical device, soft robot, sensor, and so on.

Modern evolution of paper-based analytical devices for wearable use: from disorder to order

Paper devices have attracted great attention for their rapid development in multiple fields, such as life sciences, biochemistry, and materials science. When manufacturing paper chips, flexible materials, such as cellulose paper or other porous flexible membranes, can offer several advantages in terms of their flexibility, lightweight, low cost, safety and wearability. However, traditional cellulose paper sheets with chaotic cellulose fiber constitutions do not have special structures and optical characteristics, leading to poor repeatability and low sensitivity during biochemical sensing, limiting their wide application. Recent evidence showed that the addition of ordered structure provides a promising method for manufacturing intelligent flexible devices, making traditional flexible devices with multiple functions (microfluidics, motion detection and optical display). There is an urgent need for an overall summary of the evolution of paper devices so that readers can fully understand the field. Hence, in this review, we summarized the latest developments in intelligent paper devices, starting with the fabrication of paper and smart flexible paper devices, in the fields of biology, chemistry, electronics, etc. First, we outlined the manufacturing methods and applications of both traditional cellulose paper devices and modern smart devices based on pseudopaper (order paper). Then, considering different materials, such as cellulose, nitrocellulose, nature sourced photonic crystals (photonic crystals sourced from nature directly) and artificial photonic crystals, we summarized a new type of smart flexible device containing an ordered structure. Next, the applications of paper devices in biochemical sensing, wearable sensing, and cross-scale sensing were discussed. Finally, we summarized the development direction of this field. The aim of this review is to take an integral cognition approach to the development of smart flexible paper devices in multiple fields and promote communications between materials science, biology, chemistry and electrical science.

Mechanochromic and Thermochromic Sensors Based on Graphene Infused Polymer Opals

High quality opal-like photonic crystals containing graphene are fabricated using evaporation-driven self-assembly of soft polymer colloids. A miniscule amount of pristine graphene within a colloidal crystal lattice results in the formation of colloidal crystals with a strong angle-dependent structural color and a stop band that can be reversibly shifted across the visible spectrum. The crystals can be mechanically deformed or can reversibly change color as a function of their temperature, hence their sensitive mechanochromic and thermochromic response make them attractive candidates for a wide range of visual sensing applications. In particular, it is shown that the crystals are excellent candidates for visual strain sensors or integrated time-temperature indicators which act over large temperature windows. Given the versatility of these crystals, this method represents a simple, inexpensive, and scalable approach to produce multifunctional graphene infused synthetic opals and opens up exciting applications for novel solution-processable nanomaterial based photonics.

Study on the Twisted and Coiled Polymer Actuator with Strain Self-Sensing Ability

Twisted and coiled polymer actuators (TCAs) are a kind of efficient artificial muscles, which have good prospects for application in soft robots, bionic devices, and biological, medical, and other high-tech fields. However, the inability to dynamically sense and adjust the strain of the actuator will lead to uncertainty in the accuracy of deformation and strain, resulting in imprecise target action. Herein, TCAs with strain self-sensing ability (TCASA) are prepared by integrating the stretchable optomechanical film (SOMF) sensors into TCAs, which provides a simple strategy for dynamical strain sensing. These SOMFs have a wide range of color changes during deformation of TCAs, and the strain is perceived by observing the color change according to the corresponding relationship between color change and strain. Furthermore, the proposed TCASA maintain excellent cycling stability of strain self-sensing during cyclic tests (200 cycles) and excellent strain self-sensing performance to perform strain control compared to TCAs without SOMFs. The results indicate that the proposed structure is a promising soft actuator with excellent strain self-sensing ability, which is well suited for soft robots, bionic devices, biological and medical fields, smart wearable technologies, and so forth, especially when controlled, repetitive deformations are required.