Electroactive Soft Photonic Devices for the Synesthetic Perception of Color and Sound

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.

Interactive Deformable Colored Sound Display Achieved with Electrostrictive Fluoropolymer and Halide Perovskite

The association of color and sound helps human cognition through a synergetic effect like intersensory facilitation. Although soft human-machine interfaces (HMIs) providing unisensory expression have been widely developed, achieving synchronized optic and acoustic expression in one device system has been relatively less explored. It is because their operating principles are different in terms of materials, and implementation has mainly been attempted through structural approaches. Here, a deformable sound display is developed that generates multiple colored lights with large sound at low input voltage. The device is based on alternating-current electroluminescence (ACEL) covered with perovskite composite films. A sound wave is created by a polymer matrix of the ACEL, while simultaneously, various colors are produced by the perovskite films and the blue electroluminescence (EL) emitted from the phosphors in the ACEL. By patterning different colored perovskite films onto the ACELs, associating the color and the sound is successfully demonstrated by a piano keyboard and a wearable interactive device.

Omnidirectional color wavelength tuning of stretchable chiral liquid crystal elastomers

Wavelength-tunable structural colors using stimuli-responsive materials, such as chiral liquid crystals (CLCs), have attracted increasing attention owing to their high functionality in various tunable photonic applications. Ideally, on-demand omnidirectional wavelength control is highly desirable from the perspective of wavelength-tuning freedom. However, despite numerous previous research efforts on tunable CLC structural colors, only mono-directional wavelength tuning toward shorter wavelengths has been employed in most studies to date. In this study, we report the ideally desired omnidirectional wavelength control toward longer and shorter wavelengths with significantly improved tunability over a broadband wavelength range. By using areal expanding and contractive strain control of dielectric elastomer actuators (DEAs) with chiral liquid crystal elastomers (CLCEs), simultaneous and omnidirectional structural color-tuning control was achieved. This breakthrough in omnidirectional wavelength control enhances the achievable tuning freedom and versatility, making it applicable to a broad range of high-functional photonic applications.

Full-Color Generation via Phototunable Mono Ink for Fast and Elaborate Printings

Unlike pigment-based colors, which are determined by their molecular structure, diverse colors can be expressed by a regular arrangement of nanomaterials. However, existing techniques for constructing such nanostructures have struggled to combine high precision and speed, resulting in a narrow gamut, and prolonged color fabrication time. Here, this work reports a phototunable mono ink that can generate a wide range of colors by controlling regularly arranged nanostructure. Core-shell growth controlled by polymerization time precisely regulates the distance between arranged particles at a nanometer-scale, enabling the generation of various colors. Moreover, the wide and thin arrangement induces constrained out-of-plane growth, thus facilitating the intricate color generation at the desired location via photopolymerization. Upon terminating polymerization by oxygen gas, the generated colors are readily fixed and kept stable. Utilizing programmed ultraviolet illumination, large-scale and high-resolution (≈1 µm) full-color printings are demonstrated at high speed (100 mm2 s-1 ).

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.

Electrically responsive photonic crystals with bistable states for low-power electrophoretic color displays

Electrically responsive photonic crystals are promising materials for electrophoretic color displays with better brightness and color saturation. However, electric field must always be applied to maintain the specific colors, which brings concerns about the power consumption and signal stability and reversibility. Here, we show an electrically responsive photonic crystal with two stable states at 0 V, which are the colored state or the colorless state with ordered or disordered particle arrangement. The color state can be reversibly switched by applying a short-time electrical field, just like in the case of commercial electrophoretic ink. With optimized recipe and electric field, the photonic crystals encapsulated in the prototype display panel are proved to have potentials in high resolution, multi-color, and greyscale display, which lays down a firm basis for reflective displays with low power consumption and good visibility.

Materials development in stretchable iontronics

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

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.

Voltage-tunable elastomer composites that use shape instabilities for rapid structural color changes

Structurally colored materials can switch colors in response to external stimuli, which makes them potentially useful as colorimetric sensors, dynamic displays, and camouflage. However, their applications are limited by the angular dependence, slow response, and absence of synchronous control in time and space. In addition, out-of-plane deformation from shape instability easily occurs in photonic films, leading to inhomogeneous colors in photonic-crystal materials. To address these challenges, we combine structurally colored photonic glasses and dielectric elastomer actuators. We use an external voltage signal to tune color changes quickly (much less than 0.1 s). The photonic glassses produce colors with low angular dependence, so that their colors are homogeneous even when they become curved due to voltage-triggered instabilities (buckling or wrinkling). As proof of concept, we present a pixelated display in which segments can be independently and rapidly turned on and off. This wide-angle, instability-tolerant, color-changing platform could be used in next-generation soft and curved color displays, camouflage with both shape and color changes, and multifunctional sensors.

Double-Network Hydrogel for Stretchable Triboelectric Nanogenerator and Integrated Electroluminescent Skin with Self-Powered Rapid Visual Sensing

Bio-inspired design plays a very significant role in adapting biological models to technical applications of flexible electronics. The flexible, stretchable, and portable electrode is one of the key technical challenges in the field. Inspired by the responses to mechanical stimuli of natural plants, we designed a highly transparent (over 95%), stretchable (over 1500%), and biocompatible electrode material by using polymerized double-network hydrogel for fabricating a triboelectric nanogenerator (SH-TENG). The SH-TENG can convert tiny mechanical stretching from human movements directly into electrical energy, and is capable of lighting up to 50 LEDs. Benefiting from bio-inspired design, the coplanar, non-overlapping electrode structure breaks through the limitations of conventional electrodes in wearable devices and overcomes the bottleneck of transparent materials. Furthermore, a self-powered raindrop visual sensing system was realized, which can perform quasi-real-time rainfall information monitoring and increase rainfall recognition ability of vehicle automatic driving systems. This study provides a novel strategy for making next-generation stretchable electronic devices and flexible visual sensing systems.

Water-Saturated Ion Gel for Humidity-Independent High Precision Epidermal Ionic Temperature Sensor

Although ion gels are attractive sensing materials for deformable epidermal sensors or implantable devices, their sensing performances are highly affected by environmental humidity change, so that their sensing reliability cannot be secured. This study proposes a new concept of maintaining the high-precision temperature sensing performance of highly deformable ion gel sensors. In this approach, a hydrophobic ion gel sensing layer is kept water-saturated by attaching a hydrogel layer, rather than attempting to completely block water penetration. This study performs experimental and theoretical investigation on water concentration in the ion gel, using the analysis of mass transportation at the interface of the ion gel and the hydrogel. By using the charge relaxation time of the ionic molecules, the temperature sensor is not affected by environmental humidity in the extreme range of humidity (30%-100%). This study demonstrates a highly deformable on-skin temperature sensor which shows the same performance either in water or dry state and while exercising with large strains (ε = 50%).

Structural color printing via polymer-assisted photochemical deposition

Structural color printings have broad applications due to their advantages of long-term sustainability, eco-friendly manufacturing, and ultra-high resolution. However, most of them require costly and time-consuming fabrication processes from nanolithography to vacuum deposition and etching. Here, we demonstrate a new color printing technology based on polymer-assisted photochemical metal deposition (PPD), a room temperature, ambient, and additive manufacturing process without requiring heating, vacuum deposition or etching. The PPD-printed silver films comprise densely aggregated silver nanoparticles filled with a small amount (estimated <20% volume) of polymers, producing a smooth surface (roughness 2.5 nm) even better than vacuum-deposited silver films (roughness 2.8 nm) at ~4 nm thickness. Further, the printed composite films have a much larger effective refractive index n (~1.90) and a smaller extinction coefficient k (~0.92) than PVD ones in the visible wavelength range (400 to 800 nm), therefore modulating the surface reflection and the phase accumulation. The capability of PPD in printing both ultra-thin (~5 nm) composite films and highly reflective thicker film greatly benefit the design and construction of multilayered Fabry-Perot (FP) cavity structures to exhibit vivid and saturated colors. We demonstrated programmed printing of complex pictures of different color schemes at a high spatial resolution of ~6.5 μm by three-dimensionally modulating the top composite film geometries and dielectric spacer thicknesses (75 to 200 nm). Finally, PPD-based color picture printing is demonstrated on a wide range of substrates, including glass, PDMS, and plastic, proving its broad potential in future applications from security labeling to color displays.

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.

Highly Transparent, Stretchable, and Conducting Ionoelastomers Based on Poly(ionic liquid)s

The rapid development of soft electronics has revitalized the research of conducting elastomers. However, the design of conducting elastomers having high stretchability and good transparency still remains a considerable challenge. In this study, we develop a highly transparent, stretchable, and conducting ionoelastomer based on a poly(ionic liquid) in which cations are fixed to a stretchable elastomeric network and counter anions are mobile. The ionoelastomer solves the dilemma of simultaneous transparency and stretchability in the design of traditional conducting elastomers, possessing good transparency (96%) with an extraordinarily high stretchability, up to a limiting strain of 1460%. Moreover, this novel material is completely nonvolatile and nonhygroscopic, endowing the ionoelastomer with highly stable thermal, environmental, electrochemical, and mechanoelectrical properties. An underwater sensor based on the ionoelastomer is developed with good performance in an aqueous environment. Also, a transparent dielectric elastomer actuator (DEA) is demonstrated using the ionoelastomer. It is believed that the ionoelastomer would pave the way to develop exceptional conducting elastomers toward next-generation soft electronics.

Recent Progress in Artificial Muscles for Interactive Soft Robotics

Artificial muscles are the core components of the smart and interactive soft robotic systems, providing the capabilities in shape morphing, manipulation, and mobility. Intense research efforts in the development of artificial muscles are based on the dielectric elastomer actuators, pneumatic actuators, electrochemical actuators, soft magnetic actuators, and stimulus responsive polymers. Recent progress has presented artificial muscles with impressive specific power output exceeding that of the natural muscles, dexterous shape morphing behavior that can be programmed and reconfigured, and exceptional high maneuverability to traverse surfaces with obstacles and different textures. Here, a succinct and critical summary is provided on the materials and strategies that have contributed to the important advancement of the artificial muscles in recent research. On that basis, the exciting opportunities are discussed in the integration of soft electronic devices with artificial muscles to enable smart and interactive soft robotic systems.

Perovskite Nanosheet Hydrogels with Mechanochromic Structural Color

Structural color colloidal sols of perovskite nanosheets were synthesized and were immobilized in a polymer hydrogel film by in situ photopolymerization, leading to a novel mechanochromic material. Visible absorption spectroscopy, polarized optical microscopy and small-angle X-ray scattering revealed that the nanosheets are aligned parallel to the film surface with the periodic distance of up to ca. 300 nm, giving the structural color tunable over full color range. The present structural color gel showed reversible mechanochromic response that detects weak stress of 1 kPa with the quick response time less than 1 ms as well as high mechanical toughness (compressive breaking stress of up to 3 MPa). These excellent properties are suitable for applications for mechano-sensors and displays.

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.

High-purity reflective color filters based on thin film cavities embedded with an ultrathin Ge2Sb2Te5 absorption layer

A thin film cavity formed by stacking metal-insulator-metal (MIM) continuous layers is of significant interest as a lithography-free and scalable color-filtering structure. Such a cavity can selectively transmit a certain frequency range of incident light, thus producing vivid transmission colors. However, the generation of reflection colors with high purity and reflectivity is a challenge because a cavity in reflection mode resonantly absorbs a narrow range of wavelengths and reflects the remaining spectrum. This study shows that highly pure and reflective colors can be obtained by embedding an ultrathin Ge₂Sb₂Te₅ layer within the cavity. Because the MIM structure exhibits a nonuniform intensity distribution across the insulator layer, the approach is to place the Ge₂Sb₂Te₅ layer in a high-intensity region within the insulator and thereby create another absorption band in addition to the cavity resonance mode. When combined with the refractive-index engineering of the metal layer, this approach leads to red, green, and blue colors having a bandwidth of ∼100 nm and a reflection efficiency of 90%. The results of the study may be effectively utilized in numerous applications, including reflective color filters, colorimetric sensors, and surface decorations.

Highly Stretchable and Transparent Ionic Conductor with Novel Hydrophobicity and Extreme-Temperature Tolerance

Highly stretchable and transparent ionic conducting materials have enabled new concepts of electronic devices denoted as iontronics, with a distinguishable working mechanism and performances from the conventional electronics. However, the existing ionic conducting materials can hardly bear the humidity and temperature change of our daily life, which has greatly hindered the development and real-world application of iontronics. Herein, we design an ion gel possessing unique traits of hydrophobicity, humidity insensitivity, wide working temperature range (exceeding 100°C, and the range covered our daily life temperature), high conductivity (10⁻³~10⁻⁵ S/cm), extensive stretchability, and high transparency, which is among the best-performing ionic conductors ever developed for flexible iontronics. Several ion gel-based iontronics have been demonstrated, including large-deformation sensors, electroluminescent devices, and ionic cables, which can serve for a long time under harsh conditions. The designed material opens new potential for the real-world application progress of iontronics.

Ionic spiderwebs

Spiders use adhesive, stretchable, and translucent webs to capture their prey. However, sustaining the capturing capability of these webs can be challenging because the webs inevitably invite contamination, thus reducing its adhesion force. To overcome these challenges, spiders have developed strategies of using webs to sense prey and clean contaminants. Here, we emulate the capturing strategies of a spider with a single pair of ionic threads based on electrostatics. Our ionic spiderwebs completed consecutive missions of cleaning contamination on itself, sensing approaching targets, capturing those targets, and releasing them. The ionic spiderwebs demonstrate the importance of learning from nature and push the boundaries of soft robotics in an attempt to combine mutually complementary functions into a single unit with a simple structure.

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.

Highly Stretchable Sound-in-Display Electronics Based on Strain-Insensitive Metallic Nanonetworks

The growing importance of human-machine interfaces and the rapid expansion of the internet of things (IoT) have inspired the integration of displays with sound generation systems to afford stretchable sound-in-display devices and thus establish human-to-machine connections via auditory system visualization. Herein, the synchronized generation of sound and color is demonstrated for a stretchable sound-in-display device with electrodes of strain-insensitive silver nanowires (AgNWs) and emissive layers of field-induced inorganic electroluminescent (EL) phosphors. In this device, EL phosphors embedded in a dielectric elastomer actuator (DEA) emit light under alternating-current bias, while audible sound waves are simultaneously generated via DEA actuation along with input sound signals. The electroluminescence and sound-generation performances of the fabricated device are highly robust and reliable, being insensitive to stretch-release cycling because of the presence of the AgNW stretchable electrodes. The presented principle of integrating light emission and acoustic systems in a single stretchable device can be further expanded to realize sound-in-display electronics for IoT and human-machine interface applications.

Highly Invisible Photonic Crystal Patterns Encrypted in an Inverse Opaline Macroporous Polyurethane Film for Anti-Counterfeiting Applications

Invisible photonic crystal (PC) pattern with encrypted and discoverable information is potentially useful for anti-counterfeiting labels, but it is still a big challenge to realize strict invisibility, fast response, and convenient triggering. Here, a new kind of soaking-revealed invisible PC pattern is fabricated by the regional coating of "ethylene glycol-ethanol" ink on a collapsed inverse opaline macroporous polyurethane (IOM-PU) film, followed by a quick thermal treatment. During the above process, wet heating retains the collapsed but recoverable IOM structure, but dry heating disables the recovery of ordered IOM structure due to the adhesion of macropore walls, which render the "pattern" and the "background" with different optical responses to the solvent. In the dry state, the pattern was invisible because both the collapsed IOM-PU film and the adhesive PU film are colorless and transparent. Once the sample is soaked in ethanol-water mixtures, the invisible pattern appears immediately because only the "wet-heated" region recovers the ordered macroporous structure and shows color, which forms a significant contrast in color to the "dry-heated" region. Compared to the previously invisible PC pattern, the current material has many superior properties, such as high invisibility, large color contrast in showing, excellent recyclability, and good toughness in bending and stretching.

Stretchable and reflective displays: materials, technologies and strategies

Displays play a significant role in delivering information and providing visual data across all media platforms. Among displays, the prominence of reflective displays is increasing, in the form of E-paper, which has features distinct from emissive displays. These unique features include high visibility under daylight conditions, reduced eye strain and low power consumption, which make them highly effective for outdoor use. Furthermore, such characteristics enable reflective displays to achieve high synergy in combination with wearable devices, which are frequently used for outdoor activities. However, as wearable devices must stretch to conform to the dynamic surfaces of the human body, the issue of how to fabricate stretchable reflective displays should be tackled prior to merging them with wearable devices. In this paper, we discuss stretchable and reflective displays. In particular, we focus on reflective displays that can be divided into two types, passive and active, according to their responses to stretching. Passive displays, which consist of dyes or pigments, exhibit consistent colors under stretching, while active displays, which are based on mechanochromic materials, change their color under the same conditions. We will provide a comprehensive overview of the materials and technologies for each display type, and present strategies for stretchable and reflective displays.