Cephalopod-Inspired Chromotropic Ionic Skin with Rapid Visual Sensing Capabilities to Multiple Stimuli

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.

Cephalopod-Inspired Nanomaterials for Optical and Thermal Regulation: Mechanisms, Applications and Perspectives

The manipulation of interactions between light and matter plays a crucial role in the evolution of organisms and a better life for humans. As a result of natural selection, precise light-regulatory systems of biology have been engineered that provide many powerful and promising bioinspired strategies. As the "king of disguise", cephalopods, which can perfectly control the propagation of light and thus achieve excellent surrounding-matching via their delicate skin structure, have made themselves an exciting source of inspiration for developing optical and thermal regulation nanomaterials. This review presents cutting-edge advancements in cephalopod-inspired optical and thermal regulation nanomaterials, highlighting the key milestones and breakthroughs achieved thus far. We begin with the underlying mechanisms of the adaptive color-changing ability of cephalopods, as well as their special hierarchical skin structure. Then, different types of bioinspired nanomaterials and devices are comprehensively summarized. Furthermore, some advanced and emerging applications of these nanomaterials and devices, including camouflage, thermal management, pixelation, medical health, sensing and wireless communication, are addressed. Finally, some remaining but significant challenges and potential directions for future work are discussed. We anticipate that this comprehensive review will promote the further development of cephalopod-inspired nanomaterials for optical and thermal regulation and trigger ideas for bioinspired design of nanomaterials in multidisciplinary applications.

Fundamental Aspects of Stretchable Mechanochromic Materials: Fabrication and Characterization

Mechanochromic materials provide optical changes in response to mechanical stress and are of interest in a wide range of potential applications such as strain sensing, structural health monitoring, and encryption. Advanced manufacturing such as 3D printing enables the fabrication of complex patterns and geometries. In this work, classes of stretchable mechanochromic materials that provide visual color changes when tension is applied, namely, dyes, polymer dispersed liquid crystals, liquid crystal elastomers, cellulose nanocrystals, photonic nanostructures, hydrogels, and hybrid systems (combinations of other classes) are reviewed. For each class, synthesis and processing, as well as the mechanism of color change are discussed. To enable materials selection across the classes, the mechanochromic sensitivity of the different classes of materials are compared. Photonic systems demonstrate high mechanochromic sensitivity (Δnm/% strain), large dynamic color range, and rapid reversibility. Further, the mechanochromic behavior can be predicted using a simple mechanical model. Photonic systems with a wide range of mechanical properties (elastic modulus) have been achieved. The addition of dyes to photonic systems has broadened the dynamic range, i.e., the strain over which there is an optical change. For applications in which irreversible color change is desired, dye-based systems or liquid crystal elastomer systems can be formulated. While many promising applications have been demonstrated, manufacturing uniform color on a large scale remains a challenge. Standardized characterization methods are needed to translate materials to practical applications. The sustainability of mechanochromic materials is also an important consideration.

Colorimetric sensing for translational applications: from colorants to mechanisms

Colorimetric sensing offers instant reporting via visible signals. Versus labor-intensive and instrument-dependent detection methods, colorimetric sensors present advantages including short acquisition time, high throughput screening, low cost, portability, and a user-friendly approach. These advantages have driven substantial growth in colorimetric sensors, particularly in point-of-care (POC) diagnostics. Rapid progress in nanotechnology, materials science, microfluidics technology, biomarker discovery, digital technology, and signal pattern analysis has led to a variety of colorimetric reagents and detection mechanisms, which are fundamental to advance colorimetric sensing applications. This review first summarizes the basic components (e.g., color reagents, recognition interactions, and sampling procedures) in the design of a colorimetric sensing system. It then presents the rationale design and typical examples of POC devices, e.g., lateral flow devices, microfluidic paper-based analytical devices, and wearable sensing devices. Two highlighted colorimetric formats are discussed: combinational and activatable systems based on the sensor-array and lock-and-key mechanisms, respectively. Case discussions in colorimetric assays are organized by the analyte identities. Finally, the review presents challenges and perspectives for the design and development of colorimetric detection schemes as well as applications. The goal of this review is to provide a foundational resource for developing colorimetric systems and underscoring the colorants and mechanisms that facilitate the continuing evolution of POC sensors.

Cephalopod-Inspired Chemical-Gated Hydrogel Actuation Systems for Information 3D-Encoding Display

Cephalopods evolve the acetylcholine-gated actuation control function of their skin muscles, which enables their dynamic/static multimode display capacities for achieving perfectly spatial control over the colors/patterns on every inch of skin. Reproduction of artificial analogs that exhibit similar multimodal display is essential to reach advanced information three-dimensional (3D) encoding with higher security than the classic 2D-encoding strategy, but remains underdeveloped. The core difficulty is how to replicate such chemical-gated actuation control function into artificial soft actuating systems. Herein, this work proposes to develop azobenzene-functionalized poly(acrylamide) (PAAm) hydrogel systems, whose upper critical solution temperature (UCST) type actuation responsiveness can be intelligently programmed or even gated by the addition of hydrophilic α-cyclodextrin (α-CD) molecules for reversible association with pendant azobenzene moieties via supramolecular host-guest interactions. By employing such α-CD-gated hydrogel actuator as an analogue of cephalopods' skin muscle, biomimetic mechanically modulated multicolor fluorescent display systems are designed, which demonstrate a conceptually new α-CD-gated "thermal stimulation-hydrogel actuation-fluorescence output" display mechanism. Consequently, high-security 3D-encoding information carriers with an unprecedented combination of single-input multiple-output, dynamic/static dual-mode and spatially controlled display capacities are achieved. This bioinspired strategy brings functional-integrated features for artificial display systems and opens previously unidentified avenues for information security.

Biomimetic Photonic Elastomer Exhibiting Stress/Moisture Reconfigurable Wrinkle-Lattice for Reversible Deformation Information Storage

Photonic elastomers, capable of converting imperceptible deformations into visible colors, show significant potential in smart materials. However, instantaneous deformation is arduous to record accurately due to the disappearance of optical information after deformation recovery. Herein, inspired by the folding structures of iridocytes in cephalopods, a stress- and moisture-triggered wrinkling and erasure effect is proposed to be introduced in the construction of a photonic elastomer. Implemented in a dual-network polymer framework with modulatable locking, it allows for reversible deformation storage. The photonic elastomer comprises a surface one-dimensional photonic crystal (1DPC) and a poly(dimethylsiloxane) (PDMS) substrate. The deformed 1DPC lattice transforms into a wrinkled state due to a substrate deformation mismatch, preserving strain-induced structural color information through interchain hydrogen bonding and crystalline shape-locking in dual-network polymers. Reading the color provides multidimensional information about the instantaneous deformation degree and distribution. Moreover, the moisture-induced shape-memory feature of the 1DPC can be triggered with a minute amount of water, like fingertip perspiration or humidity change (35% to 80%), to restore the original color. This stress/moisture-responsive photonic elastomer, with its dynamically reconfigurable wrinkle-lattice, holds great promise for applications in mechanical sensing, inkless writing, and anticounterfeiting, significantly enhancing the versatility of photonic materials.

Ultrasensitive Biomimetic Skin with Multimodal and Photoelectric Dual-Signal Sensing

Mimicking biological skin enabling direct, intelligent interaction between users and devices, multimodal sensing with optical/electrical (OE) output signals is urgently required. Owing to this, this work aims to logically design a stretchable OE biomimetic skin (OE skin), which can sensitively sense complex external stimuli of pressure, strain, temperature, and localization. The OE skin consists of elastic thin polymer-stabilized cholesteric liquid crystal films, an ion-conductive hydrogel layer, and an elastic protective membrane formed with thin polydimethylsiloxane. The as-designed OE skin exhibits customizable structural color on demand, good thermochromism, and excellent mechanochromism, with the ability to extend the full visible spectrum, a good linearity of over 0.99, fast response speed of 93 ms, and wide temperature range of 119 °C. In addition, the conduction resistance variation of ion-conductive hydrogel exhibits excellent sensing capabilities under pressure, stretch, and temperature, endowing a good linearity of 0.99998 (stretching from 0 to 150%) and high thermal sensitivity of 0.86% per °C. Such an outstanding OE skin provides design concepts for the development of multifunctional biomimetic skin used in human-machine interaction and can find wide applications in intelligent wearable devices and human-machine interactions.

Hydrochromic film for dynamic information storage using cellulose nanofibers and silica nanoparticles

A hydrochromic composite film was fabricated by incorporating silica nanoparticles (SiNPs) with cellulose nanofibers (CNFs). The CNF/SiNP composite film underwent a reversible change in transparency in response to external moisture variation. The CNFs improved the dimensional stability of the CNF/SiNP composite film and induced morphological differences in SiNP agglomerates, which control the water vapor condensation in a porous film. The condensed water in the pores reduced the difference in refractive index over the CNF/SiNP film, enhancing its transparency. The selective transparency of the composite film was challenged by printing CNF/SiNP inks at different composition ratios. The differing susceptibility of the printed patterns to moisture provided selective transparency at specific patterns, which can store dynamic information such as QR or numerical codes by simple water vapor adsorption and desorption.

Generating dual structurally and functionally skin-mimicking hydrogels by crosslinking cell-membrane compartments

The skin is intrinsically a cell-membrane-compartmentalized hydrogel with high mechanical strength, potent antimicrobial ability, and robust immunological competence, which provide multiple protective effects to the body. Methods capable of preparing hydrogels that can simultaneously mimic the structure and function of the skin are highly desirable but have been proven to be a challenge. Here, dual structurally and functionally skin-mimicking hydrogels are generated by crosslinking cell-membrane compartments. The crosslinked network is formed via free radical polymerization using olefinic double bond-functionalized extracellular vesicles as a crosslinker. Due to the dissipation of stretching energy mediated by vesicular deformation, the obtained compartment-crosslinked network shows enhanced mechanical strength compared to hydrogels crosslinked by regular divinyl monomers. Biomimetic hydrogels also exhibit specific antibacterial activity and adequate ability to promote the maturation and activation of dendritic cells given the existence of numerous extracellular vesicle-associated bioactive substances. In addition, the versatility of this approach to tune both the structure and function of the resulting hydrogels is demonstrated through introducing a second network by catalyst-free click reaction-mediated crosslinking between alkyne-double-ended polymers and azido-decorated extracellular vesicles. This study provides a platform to develop dual structure- and function-controllable skin-inspired biomaterials.

Bio-inspired Double Angstrom-Scale Confinement in Ti-deficient Ti0.87 O2 Nanosheet Membranes for Ultrahigh-performance Osmotic Power Generation

Osmotic power, a clean energy source, can be harvested from the salinity difference between seawater and river water. However, the output power densities are hampered by the trade-off between ion selectivity and ion permeability. Here we propose an effective strategy of double angstrom-scale confinement (DAC) to design ion-permselective channels with enhanced ion selectivity and permeability simultaneously. The fabricated DAC-Ti0.87 O2 membranes possess both Ti atomic vacancies and an interlayer free spacing of ≈2.2 Å, which not only generates a profitable confinement effect for Na+ ions to enable high ion selectivity but also induces a strong interaction with Na+ ions to benefit high ion permeability. Consequently, when applied to osmotic power generation, the DAC-Ti0.87 O2 membranes achieved an ultrahigh power density of 17.8 W m-2 by mixing 0.5/0.01 M NaCl solution and up to 114.2 W m-2 with a 500-fold salinity gradient, far exceeding all the reported macroscopic-scale membranes. This work highlights the potential of the construction of DAC ion-permselective channels for two-dimensional materials in high-performance nanofluidic energy systems.

Mechanochromic and ionic conductive cholesteric liquid crystal elastomers for biomechanical monitoring and human-machine interaction

Cholesteric liquid crystal elastomers (CLCEs) that combine rubbery elasticity with structural colour from self-assembled helical nanostructures are of paramount importance for diverse applications such as biomimetic skins, adaptive optics and soft robotics. Despite great advances, it is challenging to integrate electrical sensing and colour-changing characteristics in a single CLCE system. Here, we report the design and synthesis of an ionic conductive cholesteric liquid crystal elastomer (iCLCE) through in situ Michael addition and free-radical photopolymerization of CLCE precursors on silane-functionalized polymer ionic liquid networks, in which robust covalent chemical bonding was formed at the interface. Thanks to superior mechanochromism and ionic conductivity, the resulting iCLCEs exhibit dynamic colour-changing and electrical sensing functions in a wide range upon mechanical stretching, and can be used for biomechanical monitoring during joint bending. Importantly, a capacitive elastomeric sensor can be constructed through facilely stacking iCLCEs, where the optical and electrical dual-signal reporting performance allows intuitive visual localization of pressure intensity and distribution. Moreover, proof-of-concept application of the iCLCEs has been demonstrated with human-interactive systems. The research disclosed herein can provide new insights into the development of bioinspired somatosensory materials for emerging applications in diverse fields such as human-machine interaction, prostheses and intelligent robots.

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.

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.

Mechanochromic and Conductive Chiral Nematic Nanostructured Film for Bioinspired Ionic Skins

Chameleon skin is naturally adaptive and can sense environmental changes and transform sensing into bioelectrical and optical signals by manipulating ion transduction and photonic nanostructures. The increasing interest in mimicking biological skins has considerably promoted the development of advanced photonic materials with an increasing ionic conductivity. Herein, we report the judicious design and fabrication of a bioinspired mechanochromic chiral nematic nanostructured film with good ionic conductivity by infiltrating fluorine-rich ionic liquids (FILs) into a swollen self-assembled cellulose nanocrystal (CNC) film with helical nanoarchitectures. Notably, the introduction of 2-hydroxyethyl acrylate considerably enhances the compatibility of hydrophobic FILs and hydrophilic CNCs. The resulting FIL-CNC nanostructured films exhibited excellent mechanochromism, good ionic conductivity, and outstanding optical/electrical dual-signal sensing performance when used as a bioinspired ionic skin for real-time monitoring of human motions. Owing to the integration of FILs, the underwater stability of the chiral liquid crystal nanostructures of CNCs was significantly enhanced. Notably, underwater contact/contactless sensing modes and encrypted information transmission have been achieved with the FIL-CNC nanostructured film. This study can offer great insights for the advancement of biomimetic multifunctional artificial skins and emerging interactive devices, which can find important applications in wearable iontronics, human-machine interactions, and intelligent robots.

Self-regulating bioinspired supramolecular photonic hydrogels based on chemical reaction networks for monitoring activities of enzymes and biofuels

Inspired by the way many living organisms utilize chemical/biological reactions to regulate their skin and respond to stimuli in the external environment, we have developed a self-regulating hydrogel design by incorporating chemical reaction networks (CRNs) into biomimetic photonic crystal hydrogels. In this hydrogel system, we used host-guest supramolecular non-covalent bonds between beta-cyclodextrin (β-CD) and ferrocene (Fc) as partial crosslinkers and designed a CRN involving enzyme-fuel couples of horseradish peroxidase (HRP)/H₂O₂ and glucose oxidase (GOD)/d-glucose, by which the responsive hydrogel was transformed into a glucose-driven self-regulating hydrogel. Due to the biomimetic structural color in the hydrogel, the progress of the chemical reaction was accompanied by a change in the color of the hydrogel. Based on this principle, the designed supramolecular photonic hydrogel (SPH) can not only achieve naked-eye detection of H₂O₂ and glucose concentrations with the assistance of a smartphone but also monitor the reactions of HRP and GOD enzymes and determine their activity parameters. The sensitivity and stability of the sensor have been proven. In addition, due to the reversibility of the chemical reaction network, the sensor can be reused, thus having the potential to serve as a low-cost point-of-care sensor. The SPH was ultimately used to detect glucose in human plasma and H₂O₂ in liver tumor tissue. The results are comparable with commercial assay kits. By redesigning the chemical reaction network in the hydrogel, it is expected to be used for detecting other enzymes or fuels.

Bioinspired HOF-based luminescent skin sensor with triple mechanochromism responses for the recognition and collection of human biophysical signals

It is promising to develop luminescent force-sensitive bionic skin sensor (FBSS) with multiple mechanochromism (MC) responses for the sensing and collection of human biophysical signals. Herein, four bilayer hydrogen-bonded organic framework (HOF)-based polydimethylsiloxane (PDMS) elastic films (1-4) with full-color MC responses are fabricated by doping various luminescent HOF materials (HOF-TJ-1, R, Y, and G) into different PDMS layers. 1-4 as luminescent FBSSs possess stretched-, bended-, and pressed-MC properties. In stretching and bending processes, the color-switching performances of 1-4 are from initial blue to red, yellow, and green, and 4 exhibits an increased blue emission. Under pressing strain of 0-40%, the blue emission of 1-4 is strengthened. In stretched-, bended-, and pressed-MC processes, 1-4 show ultralow detection limits and excellent recyclability with 6000, 20 000, and 10 000 times, respectively. The MC mechanisms in stretched, bent, and pressed processes are investigated in depth via finite element simulation. Moreover, three intelligent applications, such as finger motion sensing, hand gesture recognition, and tactile-enhanced palm information collection have been well realized based on luminescent MC responses of 1. This work provides the facile synthetic method to fabricate full-color HOF-based luminescent FBSSs, greatly expands the types of luminescent MC responses, and enriches the applications of luminescent FBSS.

Nacre-inspired layered composite gels with broad tunable mechanical strength and structural color for stress visualization

The brick-and-mortar architecture of nacre shells brings radiant structural colors, high toughness, and strength, inspiring numerous designs for structural and optical materials. However, constructing structural color is not always easy, especially among soft materials where aligning components against random and dynamically active environments is generally difficult. Here, we propose a composite organohydrogel capable of visualizing multiple levels of stress, featuring broad tunable mechanical properties, dynamic mechanochromism, deep low working temperatures, and anti-drying attributes. In the composite gels, the intercalation between α-zirconium phosphate (α-ZrP) nanoplates and poly-(diacetone acrylamide-co-acrylamide) is induced by shear-orientation-assisted self-assembly followed by solvent replacement. The highly tailorable (from ∼780 nm to ∼445 nm) range of colors was achieved by regulating the concentration of α-ZrP and glycerol inside the matrix. With the help from glycerol, the composite gels exhibited long-term stability (7 d) in the arid condition and remarkable low-temperature tolerance (-80 °C). The extraordinary mechanical property (compressive strength up to 119 MPa) of composite gels is achieved by the assembled α-ZrP plates with a small aspect ratio, high negative charge repulsion, and abundant hydrogen bonding sites. As a result, the mechanochromic sensor based on the composite gel enjoys a wide range of stress detection (0-1862 KPa). This study provides a new strategy for constructing high strength structural-colored gels, opening up opportunities for sensitive yet strong mechanochromic sensors in extreme environments.

Recent Advances in Bioinspired Hydrogels: Materials, Devices, and Biosignal Computing

The remarkable ability of biological systems to sense and adapt to complex environmental conditions has inspired new materials and novel designs for next-generation wearable devices. Hydrogels are being intensively investigated for their versatile functions in wearable devices due to their superior softness, biocompatibility, and rapid stimulus response. This review focuses on recent strategies for developing bioinspired hydrogel wearable devices that can accommodate mechanical strain and integrate seamlessly with biological systems. We will provide an overview of different types of bioinspired hydrogels tailored for wearable devices. Next, we will discuss the recent progress of bioinspired hydrogel wearable devices such as electronic skin and smart contact lenses. Also, we will comprehensively summarize biosignal readout methods for hydrogel wearable devices as well as advances in powering and wireless data transmission technologies. Finally, current challenges facing these wearable devices are discussed, and future directions are proposed.

Synergetic Lithium and Hydrogen Bonds Endow Liquid-Free Photonic Ionic Elastomer with Mechanical Robustness and Electrical/Optical Dual-Output

Photonic ionic elastomers (PIEs) capable of multiple signal outputs are intriguing in flexible interactive electronics. However, fabricating PIEs with simultaneous mechanical robustness, good ionic conductivity, and brilliant structure color still remains challenging. Here, the limitations are broken through introducing the synergistic effect of lithium and hydrogen bonds into an elastomer. In virtue of lithium bonding between lithium ions and carbonyl groups in the polymer matrix as well as hydrogen bonding between silanol on the surface of silica nanoparticles (SiNPs) and ether groups along polymer chains, the PIEs demonstrate mechanical strength up to 4.3 MPa and toughness up to 8.6 MJ m^-3 . Meanwhile, the synchronous electrical and optical output under mechanical strains can be achieved in the PIEs with the presence of dissociated ions contributed by lithium bond and non-close-packed SiNPs stabilized by the hydrogen bond. Moreover, due to their liquid-free nature, the PIEs exhibit extraordinary stability and durability, which can withstand extreme conditions including both high and low temperatures as well as high humidity. This work provides a promising molecular engineering route to construct high-performance photonic ionic conductors toward advanced ionotronic applications.

Dual-Mode Fiber Strain Sensor Based on Mechanochromic Photonic Crystal and Transparent Conductive Elastomer for Human Motion Detection

As an important component of wearable and stretchable strain sensors, dual-mode strain sensors can respond to deformation via optical/electrical dual-signal changes, which have important applications in human motion monitoring. However, realizing a fiber-shaped dual-mode strain sensor that can work stably in real life remains a challenge. Here, we design an interactive dual-mode fiber strain sensor with both mechanochromic and mechanoelectrical functions that can be applied to a variety of different environments. The dual-mode fiber is produced by coating a transparent elastic conductive layer onto photonic fiber composed of silica particles and elastic rubber. The sensor has visualized dynamic color change, a large strain range (0-80%), and a high sensitivity (1.90). Compared to other dual-mode strain sensors based on the photonic elastomer, our sensor exhibits a significant advantage in strain range. Most importantly, it can achieve reversible and stable optical/electrical dual-signal outputs in response to strain under various environmental conditions. As a wearable portable device, the dual-mode fiber strain sensor can be used for real-time monitoring of human motion, realizing the direct interaction between users and devices, and is expected to be used in fields such as smart wearable, human-machine interactions, and health monitoring.

Mechanochromic Optical/Electrical Skin for Ultrasensitive Dual-Signal Sensing

Following earlier research efforts dedicated to the realization of multifunctional sensing, recent developments of artificial skins endeavor to go beyond human sensory functions by integrating interactive visualization of strain and pressure stimuli. Inspired by the microcracked structure of spider slit organs and the mechanochromic mechanism of chameleons, this work aims to design a flexible optical/electrical skin (OE-skin) capable of responding to complex stimuli with interactive feedback of human-readable structural colors. The OE-skin consists of an ionic electrode combined with an elastomer dielectric layer, a chromotropic layer containing photonic crystals and a conductive carbon nanotube/MXene layer. The electrode/dielectric layers function as a capacitive pressure sensor. The mechanochromic photonic crystals of ferroferric oxide-carbon magnetic arrays embedded in the gelatin/polyacrylamide stretchable hydrogel film perceive strain and pressure stimuli with bright color switching outputs in the full visible spectrum. The underlying microcracked conductive layer is devoted to ultrasensitive strain sensing with a gauge factor of 191.8. The multilayered OE-skin delivers an ultrafast, accurate response for capacitive pressure sensing with a detection limit of 75 Pa and long-term stability of 5000 cycles, while visualizing complex deformations in the form of high-resolution spatial colors. These findings offer deep insights into the rational design of OE-skins as multifunctional sensing devices.

Portable Comestible-Liquid Quality Test Enabled by Stretchable and Reusable Ion-Detection Photonic Papers

Currently, there have been widespread investigation conducted into responsive photonic crystal hydrogels (RPCHs) characterized by high selectivity and sensitivity for colorimetric indicators and physical/chemical sensors. In spite of this, it remains challenging to use RPCHs for sensing due to their limited mechanical property and molding capability. In the present study, a double-network structure is proposed to design highly stretchable, sensitive, and reusable ion-detection photonic papers (IDPPs) for assessing the quality of visual and portable comestible liquids (e.g., soy sauce). It is constructed by integrating polyacrylamide and poly-methacryloxyethyl trimethyl ammonium chloride with highly ordered polystyrene microspheres. The double-network structure improves the mechanical properties of IDPPs with their elongation at break increasing from 110 to 1600%. Meanwhile, the optical properties of photonic crystals are retained. The IDPPs achieve a fast ion response by applying control on the swelling behavior of the hydration radius of the counter ions through ion exchange. Given a certain concentration range (0.01-0.10 M), chloride ions can be detected fast (3-30 s) by exchanging ions with a small hydration radius through an IDPP, which is clearly observable. Due to the improvement of mechanical properties and the reversible exchange of ions derived from IDPPs, their reusability is significantly enhanced (>30 times). Characterized by a simple operation, high durability, and excellent sustainability, these IDPPs are promising for practical application in food security and human health assessment.

Multiplex Identification of Post-Translational Modifications at Point-of-Care by Deep Learning-Assisted Hydrogel Sensors

Multiplex detection of protein post-translational modifications (PTMs), especially at point-of-care, is of great significance in cancer diagnosis. Herein, we report a machine learning-assisted photonic crystal hydrogel (PCH) sensor for multiplex detection of PTMs. With closely-related PCH sensors microfabricated on a single chip, our design achieved not only rapid screening of PTMs at specific protein sites by using only naked eyes/cellphone, but also the feasibility of real-time monitoring of phosphorylation reactions. By taking advantage of multiplex sensor chips and a neural network algorithm, accurate prediction of PTMs by both their types and concentrations was enabled. This approach was ultimately used to detect and differentiate up/down regulation of different phosphorylation sites within the same protein in live mammalian cells. Our developed method thus holds potential for POC identification of various PTMs in early-stage diagnosis of protein-related diseases.

Fluorescent double network ionogels with fast self-healability and high resilience for reliable human motion detection

Fascinating properties are displayed by high-performance ionogel-based flexible strain sensors, thereby gaining increasing attention in various applications ranging from human motion monitoring to soft robotics. However, the integration of excellent properties such as optical and mechanical properties and satisfactory sensing performance for one ionogel sensor is still a challenge. In particular, fatigue-resistant and self-healing properties are essential to continuous sensing. Herein, we design a flexible ion-conductive sensor based on a multifunctional ionogel with a double network using polyacrylamide, amino-modified agarose, 1,3,5-benzenetricarboxaldehyde and 1-ethyl-3-methylimidazolium chloride. The ionogel exhibits comprehensive properties including high transparency (>95%), nonflammability, strong adhesion and good temperature tolerance (about -96 to 260 °C), especially adaptive for extreme conditions. The dynamic imine bonds and abundant hydrogen bonds endow the ionogel with excellent self-healing capability, to realize rapid self-repair within minutes, as well as good mechanical properties and ductility to dissipate input energy and realize high resilience. Notably, unexpected fluorescence has been observed for the ionogel because of the gelation-induced emission phenomenon. Flexible strain sensors prepared directly from ionogels can sensitively monitor and differentiate various human motions, exhibiting a fast response time (38 ms), high sensitivity (gauge factor = 3.13 at 800% strain), good durability (>1000 cycles) and excellent stability over a wide temperature range (-30 to 80 °C). Therefore, the prepared ionogel as a high-performance flexible strain sensor in this study shows tremendous potential in wearable devices and soft ionotronics.

Heterogeneous Structural Color Conductive Photonic Organohydrogel Fibers with Alternating Single and Dual Networks

Intelligent interactive electronic devices can dynamically respond to and visualize various stimuli, promoting the rapid development of flexible electronics. In this paper, an alternating single- and dual-network design strategy was developed for ingeniously constructing an interactive electronic fiber sensor with heterogeneous structural color (HSCEF sensor). The resulting sensor can rapidly output the synchronous electrical and optical dual signals under strain by adjusting the transport distance of conductive ions and the lattice spacing of the photonic crystal (∼200 ms). Meanwhile, the addition of low-freezing-point glycerol endowed the HSCEF sensor with excellent low-temperature tolerance (-25 °C) and cyclic stability. Notably, benefiting from the alternating single- and dual-network structure, the HSCEF sensor exhibits attractive heterogeneous structural color, which achieves colorimetric changes in the full visible light region with high mechanochromic sensitivity (2.25 nm %^-1) and large wavelength shift (Δλ ∼ 225 nm). An intelligent wearable interactive sensor is finally used for real-time dynamic detection of joint movements, realizing precise resolution of different amplitudes. This work provides a general strategy to transform conventional photonic gels into heterogeneous structural color ones, and the developed new interactive sensor with rich optical information could be further used for visual health and exercise monitoring, intelligent soft robotics, wearable sensors, etc.

Structural Color Ionic Hydrogel Patches for Wound Management

Ionic hydrogels have attracted extensive attention because of their wide applicability in electronic skins, biosensors, and other biomedical areas. Tremendous effort is dedicated to developing ionic hydrogels with improved detection accuracy and multifunctionality. Herein, we present an inverse opal scaffold-based structural color ionic hydrogel with the desired features as intelligent patches for wound management. The patches were composed of a polyacrylamide-poly(vinyl alcohol)-polyethylenimine-lithium chloride (PAM-PVA-PEI-LiCl) inverse opal scaffold and a vascular endothelial growth factor (VEGF) mixed methacrylated gelatin (GelMA) hydrogel filler surface. The scaffold imparted the composite patches with brilliant structural color, conductive property, and freezing resistance, while the VEGF-GelMA surface could not only prevent the ionic hydrogel from the interference of complex wound conditions but also contribute to the cell proliferation and tissue repair in the wounds. Thus, the hydrogel patches could serve as electronic skins for in vivo wound healing and monitoring with high accuracy and reliability. These features indicate that the proposed structural color ionic hydrogel patches have great potential for clinical applications.

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.

Touch-Responsive Hydrogel for Biomimetic Flytrap-Like Soft Actuator

Stimuli-responsive hydrogel is regarded as one of the most promising smart soft materials for the next-generation advanced technologies and intelligence robots, but the limited variety of stimulus has become a non-negligible issue restricting its further development. Herein, we develop a new stimulus of "touch" (i.e., spatial contact with foreign object) for smart materials and propose a flytrap-inspired touch-responsive polymeric hydrogel based on supersaturated salt solution, exhibiting multiple responsive behaviors in crystallization, heat releasing, and electric signal under touch stimulation. Furthermore, utilizing flytrap-like cascade response strategy, a soft actuator with touch-responsive actuation is fabricated by employing the touch-responsive hydrogel and the thermo-responsive hydrogel. This investigation provides a facile and versatile strategy to design touch-responsive smart materials, enabling a profound potential application in intelligence areas.

Mimicking Color-Changing Organisms to Enable the Multicolors and Multifunctions of Smart Fluorescent Polymeric Hydrogels

Fluorescent polymer hydrogels (FPHs) are of significant interest for diverse emerging applications such as visualized sensing, smart display, camouflaging skins, soft actuators/robots, because they can synergize the features of classic fluorescent polymers and hydrogels. With great efforts in the past decades, the major challenge in this field has been believed to be not whether a given FPH of interest can be prepared but how to fabricate robust FPHs with multicolor tunability and multifunctional synergy. Such materials will conceptually minimize the contribution of passive materials to the mass and size of the final system, holding great potential to facilitate multiple applications. To this end, one promising way is to learn from the Nature that has superb capability to forge delicate or sometimes beyond-imagination materials. Chameleons and cephalopods serve as typical examples, which are famous for not only diverse skin color adaptability under changing environmental demands, but also synergistic skin color and body gesture changes to communicate, warn, camouflage, etc. Biological studies revealed their structural color-changing capacity derives from different types of skin chromatophores and their rational multilayer arrangement in under-skin tissues. Besides, their superb ability to heterogeneously integrate soft tissues with disparate functions into topology-optimized architectures has led to various multifunctional performances. Such natural strategies, if replicated and implemented in artificial systems, would significantly benefit and advance the development of robust FPHs for various applications.In this Account, we summarizes the key advances of smart FPHs mainly achieved by our groups. We start by introducing the unique hierarchical multilayer structures of skin chromatophores in structural color-changing reptiles, followed by an in-depth discussion on how a rational integration of bioinspiration and man-made design makes it possible to largely expand the fluorescence color-changing range of smart FPHs to almost cover the whole visible spectrum. Then, to closely mimic the multifunctional behaviors of chameleons and cephalopods, we further develop efficient strategies to introduce supramolecular interactions or heterogeneously integrating smart FPHs with other soft materials with disparate functions, producing a number of multifunctional fluorescent polymeric hydrogel systems. These robust FPHs can find many frontier applications, including bioinspired synergistic color/shape switchable hydrogel actuators/robots, smart systems with on-demand fluorescent patterning capacities for displaying or information encryption, as well as robust chemosensors for important food or environmental analytes. We expect that the discussion presented in this Account would promote better understanding of the discoloration systems in nature, and advance the development of bioinspired color-changing materials.

Emerging Iontronic Sensing: Materials, Mechanisms, and Applications

Iontronic sensors represent a novel class of soft electronics which not only replicate the biomimetic structures and perception functions of human skin but also simulate the mechanical sensing mechanism. Relying on the similar mechanism with skin perception, the iontronic sensors can achieve ion migration/redistribution in response to external stimuli, promising iontronic sensing to establish more intelligent sensing interface for human-robotic interaction. Here, a comprehensive review on advanced technologies and diversified applications for the exploitation of iontronic sensors toward ionic skins and artificial intelligence is provided. By virtue of the excellent stretchability, high transparency, ultrahigh sensitivity, and mechanical conformality, numerous attempts have been made to explore various novel ionic materials to fabricate iontronic sensors with skin-like perceptive properties, such as self-healing and multimodal sensing. Moreover, to achieve multifunctional artificial skins and intelligent devices, various mechanisms based on iontronics have been investigated to satisfy multiple functions and human interactive experiences. Benefiting from the unique material property, diverse sensing mechanisms, and elaborate device structure, iontronic sensors have demonstrated a variety of applications toward ionic skins and artificial intelligence.

A true color palette: binary metastable photonic pigments

Different from the traditional concept that binary photonic crystals can only reproduce mixed colors due to the simple superposition of the photonic band gaps, precisely addressable "true colors" obtained from volume fraction deviation of binary photonic crystals with metastable structures are reported here. Inspired by the mussels' adhesion and longhorn beetles' photonic scales, a binary metastable amorphous photonic crystal was obtained by enhancing the driving forces and customizing the surface roughness of building blocks to regulate the thermodynamic and dynamic factors simultaneously. By controlling the volume fraction of two building blocks, the tunable photonic bandgap varies linearly in the visible region. Furthermore, the "true violet" that cannot be obtained by conventional color mixing is reproduced with the particular ultraviolet characteristics of red photonic pigment's metastable structures, which complement the palette effect of "true colors". Meanwhile, due to the self-adhesion and post-modification of building blocks, the stability of photonic pigments is further improved. The binary photonic pigments not only solve the dilemma of mixed colors, but also realize the tunability and multiplicity of "true colors", offering a new choice for the color palette of the world.

van der Waals Semiconductor Empowered Vertical Color Sensor

Biomimetic artificial vision is receiving significant attention nowadays, particularly for the development of neuromorphic electronic devices, artificial intelligence, and microrobotics. Nevertheless, color recognition, the most critical vision function, is missed in the current research due to the difficulty of downscaling of the prevailing color sensing devices. Conventional color sensors typically adopt a lateral color sensing channel layout and consume a large amount of physical space, whereas compact designs suffer from an unsatisfactory color detection accuracy. In this work, we report a van der Waals semiconductor-empowered vertical color sensing structure with the emphasis on compact device profile and precise color recognition capability. More attractive, we endow color sensor hardware with the function of chromatic aberration correction, which can simplify the design of an optical lens system and, in turn, further downscales the artificial vision systems. Also, the dimension of a multiple pixel prototype device in our study confirms the scalability and practical potentials of our developed device architecture toward the above applications.

Toward High Sensitivity: Perspective on Colorimetric Photonic Crystal Sensors

The sensitivity of colorimetric photonic crystal (PC) sensors have been significantly improved with the advancement of deformable structural color materials, structures design, sensing signal analysis methods, and fabrication strategies. In this perspective, the strategies toward high-sensitivity colorimetric PC sensors are discussed, from the perspectives of molecular design, single sensor construction, and multisensor assembly, which include incorporation of flexible polymer chains, construction of strong sensor-analyte interactions, incorporation of more soft materials, construction of stimuli-angle/orientation relationship, design of colorimetric sensors in series, and assembly of colorimetric PC sensors in parallel. Based on these strategies, progress of high-sensitivity colorimetric PC sensors in recent years is summarized, in terms of mechano-sensors and chemo-/biosensors. Specifically, PC based optical-electrical dual-signal sensing devices are included. Finally, the future development and challenges of high-sensitivity colorimetric PC sensors are presented, in regards to deformable properties, optical properties, analysis methods, and fabrication strategies.

Self-Powered Multifunction Ionic Skins Based on Gradient Polyelectrolyte Hydrogels

Human skin is the largest organ, and it can transform multiple external stimuli into the biopotential signals by virtue of ions as information carriers. Ionic skins (i-skins) that can mimic human skin have been extensively explored; however, the limited sensing capacities as well as the need of an extra power supply significantly restrict their broad applications. Herein, we develop self-powered humanlike i-skins based on gradient polyelectrolyte membranes (GPMs) that can directly and accurately perceive multiple stimuli. Prepared by a hydrogel-assisted reaction-diffusion method, the GPMs exhibit gradient-distributed charged groups across polymer networks, enabling one to generate a thickness-dependent and thermoresponsive self-induced potential in a hydrated situation and in a humidity-sensitive self-induced potential in a dehydrated/dried situation, respectively. Consequently, the GPM-based i-skins can precisely detect pressure, temperature, and humidity in a self-powered manner. The coupling of mechano-electric and thermo-electric effects inherent in GPMs provides a general strategy for developing innovative self-powered ion-based perception systems.

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.

Hierarchical Response Network Boosts Solvent-Free Ionic Conductive Elastomers with Extreme Stretchability, Healability, and Recyclability for Ionic Sensors

The construction of solvent-free ionic conductive elastomers with high mechanical stretchability and large dynamic reversibility of chain segments is highly desired yet challenging. Here, a hierarchical response network strategy is presented for preparing highly stretchable yet mechanical robust ionic conductive elastomer composites (ICECs), among which poly(ethylene oxide) (PEO) microcrystalline serves as a physical cross-linking site providing high mechanical strength and elasticity, while dense hydrogen bonds endow superior mechanical toughness and dynamic reversibility. Due to the formation of the hierarchical response network, the resultant ICECs exhibit intrinsically high stretchability (>1500%), large tensile strength (∼2.1 MPa), and high fracture toughness (∼28 MJ m^-3). Intriguingly, due to the high reversibility of hydrogen-bonded networks, the ICECs after being crushed are capable of healing and recycling by simple hot-pressing for multiple cycles. Moreover, the ICECs are dissolvable under an alkaline condition and easily regenerated in an acid solution for manifold cycles. Importantly, the healed, recycled, and regenerated ICECs are capable of maintaining their initial mechanical elasticity and ionic conducting performance. Due to the integration of high stretchability, fatigue resistance, and ionic conductivity, the ICECs can readily work as a stretchable ionic conductor for skin-inspired ionic sensors for real-time and accurately sensing complex human motions. This study thus provides a promising strategy for the development of healable and renewable ionic sensing materials with high stretchability and mechanical robustness, demonstrating great potential in soft ionotronics.

Ultrastretchable, Adhesive, Fast Self-Healable, and Three-Dimensional Printable Photoluminescent Ionic Skin Based on Hybrid Network Ionogels

Developing multifunctional stretchable ionic skin (I-Skin) to mimic the sensations of the human skin is of great interest and shows promising potential in wearable sensors and human-machine interfaces (HMIs). However, common ionogels prepared with small-molecule cross-linkers and single networks can hardly satisfy the requirements of adjustable mechanical properties, strong adhesion, fast self-healability, and good stability in extreme environments. Herein, an ultrastretchable (>10,000%), ultrastrong adhesive (>6.8 MPa), ultrafast self-healable (10 s), high thermally stable (-60 to 250 °C), and three-dimensional (3D)-printable photoluminescent ionogel with shape memory properties has been designed. The ionogel consists of hyperbranched polymer covalent-cross-linked poly(zwitterionic ionic liquid)-co-poly(acrylic acid) and multiple dynamic bonding cross-linked networks. The excellent performance of the ionogel-based high-stretchable strain sensor and the triboelectric nanogenerator (TENG)-based self-powered touch sensor is further demonstrated over a wide temperature range (-40 to 150 °C). More importantly, ionogel-based I-Skin can work as an HMI for human gesture recognition and real-time wireless control of robots under extreme vacuum conditions and can also self-heal immediately along with function recovery after mechanical damage.

Responsive Janus Structural Color Hydrogel Micromotors for Label-Free Multiplex Assays

Micromotors with self-propelling ability demonstrate great values in highly sensitive analysis. Developing novel micromotors to achieve label-free multiplex assay is particularly intriguing in terms of detection efficiency. Herein, structural color micromotors (SCMs) were developed and employed for this purpose. The SCMs were derived from phase separation of droplet templates and exhibited a Janus structure with two distinct sections, including one with structural colors and the other providing catalytic self-propelling functions. Besides, the SCMs were functionalized with ion-responsive aptamers, through which the interaction between the ions and aptamers resulted in the shift of the intrinsic color of the SCMs. It was demonstrated that the SCMs could realize multiplex label-free detection of ions based on their optical coding capacity and responsive behaviors. Moreover, the detection sensitivity was greatly improved benefiting from the autonomous motion of the SCMs which enhanced the ion-aptamer interactions. We anticipate that the SCMs can significantly promote the development of multiplex assay and biomedical fields.

Electrochemistry-Induced Improvements of Mechanical Strength, Self-Healing, and Interfacial Adhesion of Hydrogels

Hydrogels have demonstrated great potential in biomedical and engineering areas. To improve the physical performance, development of efficient physical/chemical protocols is essential. Herein, an electrochemistry functionalization strategy that is capable of enabling the functional improvements of hydrogel is reported. The electrochemistry functionalization is demonstrated on a hydrogel model of polyacrylamide (PAAm)@κ-carrageenan. The electrochemistry reaction generates metal ions (Fe³⁺ ) that migrate and coordinate with the sulfate groups of κ-carrageenan resulting in the prominent function improvements. In comparison with untreated PAAm@κ-carrageenan hydrogel, it can improve the mechanical strength by 7.37 times, and can increase the interfacial adhesion energy of the hydrogel on a glass surface from 0 to 1400 J m^-2 , stronger than the bonding strength of tendons (adhesion energy: ≈800 J m^-2 ). Two pieces of hydrogel strips integrate into an intact structure by the electrochemistry functionalization, where the healing efficiency reaches 100% in comparison to the untreated hydrogel. The most significant development is that it enables functional patterning on the hydrogel by the electrode assembly, which provides the hydrogel with modular sensitivity to external pressure. Therefore, it can be a general protocol for rapid generation of multifunctional hydrogels for biomedical and engineering developments.

Bioinspired Photonic Ionogels as Interactively Visual Ionic Skin with Optical and Electrical Synergy

With the ever-growing demands for flexible smart interactive electronics, it remains highly desirable yet challenging to design and fabricate interactive ionic skin with multiple signal synergistic outputs. Herein, high-performance photonic ionogels (PIGs) with excellent stability and synergy sensitivity are designed by locking a non-volatile and non-hygroscopic ionic liquid (IL), that is, 1-ethyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)imide ([EMIm][TFSI]), into photonic elastomers based on polymer networks of poly(ethylene glycol) phenyl ether acrylate (PEGPEA). Through manipulating the degree of crosslinking, PIGs exhibit high sensitivity that can output distinct and intuitive color change in visual with the mechanochromic sensitivity of ≈1.76 nm per percent strain and clear electrical signal with the gauge factor of 1, in response to a tiny stretch of millimeter scale. Thanks to the stable photonic elastomers and IL employed, the PIGs developed in this study exhibit good performance under harsh and complex environmental conditions, including high/low temperature (from -35 °C to 100 °C), dry/wet air, and high vacuum. This study provides a novel strategy for developing integrated, stable, and multifunctional photonic ionogels for ionic skin sensors and flexible interactive devices with synergistically optical and electrical output.

Multi-solvent large stopband monitoring based on the insolubility/superoleophilicity of PEDOT inverse opals

Monitoring and post-processing of organic solvents are important for environmental protection. Challenges remain in the development of a universal material which can detect any solvent with a large stopband shift and show excellent stability. Herein, we demonstrate a poly 3,4-ethylenedioxythiophene inverse opal (PEDOT-IO) with a large stopband shift toward various solvents based on the insolubility/superoleophilicity properties. The PEDOT-IO film was fabricated by the potentiostatic polymerization of 3,4-ethylene dioxythiophene using a three-electrode system, infiltrating the interstices of the photonic crystal template with PEDOT and subsequently removing the template. The surface of the PEDOT-IO film presented a composite structure: interconnected pores and hollow shells. When the solvent was introduced into the voids of PEDOT-IO film, the effective refractive index (n) of the whole sample increased due to the replacement of air with the solvent, and the pores and hollow shells showed different degrees of swelling. The synergistic effect of increased n and volume expansion contributed to a large redshift of the stopband of the PEDOT-IO film. PEDOT-IO film exhibited excellent resistance to various solvents and high/low temperature. This work further enriches the application of conductive polymers in solvent-responsive PC sensors and provides a novel means of creating PC-based optical materials and devices.

Highly synergistic, electromechanical and mechanochromic dual-sensing ionic skin with multiple monitoring, antibacterial, self-healing, and anti-freezing functions

A novel electromechanical and mechanochromic dual-sensing ionic skin (DSI-skin) with multiple biological functions is achieved by mimicking biological skin.