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Moisture responsive soft actuators are receiving increasing attention due to their unique potential in reducing external energy dependence and carbon footprint. For the conventional moisture responsive soft actuators, their bending deformation under moisture stimulation is usually bidirectional, and the orientation of the bending axis is random. Achieving a moisture responsive monolithic actuator with controllable unidirectional deformation remains a challenge. Here, a Ti3C2Tx MXene/carboxymethyl chitosan composite film actuator with thickness gradient along length direction is fabricated via a vacuum‐assisted “tilt‐filtration” approach. The actuator exhibits a “diode‐like” controllable unidirectional deformation behavior under moisture gradient, and its deformation direction is strictly correlated to its thickness gradient direction and moisture source direction. Based on this highly correlated actuation behavior with internal structural asymmetry, a self‐sustained oscillator under a constant moisture gradient is achieved. Besides, various multifunctional applications based on this actuator driven by human metabolism are also demonstrated, including non‐contact switch with unidirectional conductivity, intelligent keyboard for non‐contact character input, biomimetic crawling robot, wearable intelligent thermal management clothing, and a self‐powered respiratory sensor. This work paves the way for the realization of moisture responsive soft actuators with unidirectional controllable deformation, and further promotes the development of sustainable intelligent materials in soft robotics and electronics. A MXene‐based actuator with gradient thickness is fabricated by a novel and simple vacuum‐assisted “tilt‐filtration” approach. The actuator exhibits a “diode‐like” controllable unidirectional deformation behavior under moisture gradient, and its deformation direction is strictly correlated to its thickness gradient direction and moisture source direction. Based on this actuator, a series of versatile applications driven by human metabolism are demonstrated.

Liquid metals based on gallium alloy are of potential use in the development of soft and stretchable electronics due to their intrinsic fluidity and high conductivity. However, it is challenging to build three-dimensional circuits using liquid metals, which limits the complexity and integration of the resulting devices. Here we show that a gallium–indium alloy can be used to fabricate flexible electronics with three-dimensional circuits by exploiting the solid–liquid phase transition and plastic deformation of the liquid metal. Solid but plastically deformable alloy wires are shaped into circuits at low temperatures (under 15 °C) and encapsulated in an elastomer, before being heated above their melting temperature. Subsequently, the supercooling effect allows the alloy to maintain a liquid state at a wide range of temperatures, including below the melting point. We use the technique to fabricate high-sensitivity strain sensors, three-dimensional interconnect arches for integrating an array of light-emitting diodes, and a three-dimensional wearable sensor and multilayer flexible circuit board for monitoring finger motion. Three-dimensional liquid metal structures can be created by manipulating ductile gallium–indium alloy wires that are then encapsulated in an elastomer and heated to recover their fluidity, and can remain in a liquid state for a range of temperatures due to a supercooling effect.

Lead halide perovskites have demonstrated outstanding performance in photovoltaics, photodetectors, radiation detectors and light-emitting diodes. However, the electromechanical properties, which are the main application of inorganic perovskites, have rarely been explored for lead halide perovskites. Here, we report the discovery of a large electrostrictive response in methylammonium lead triiodide (MAPbI3) single crystals. Under an electric field of 3.7 V µm−1, MAPbI3 shows a large compressive strain of 1%, corresponding to a mechanical energy density of 0.74 J cm−3, comparable to that of human muscles. The influences of piezoelectricity, thermal expansion, intrinsic electrostrictive effect, Maxwell stress, ferroelectricity, local polar fluctuation and methylammonium cation ordering on this electromechanical response are excluded. We speculate, using density functional theory, that electrostriction of MAPbI3 probably originates from lattice deformation due to formation of additional defects under applied bias. The discovery of large electrostriction in lead iodide perovskites may lead to new potential applications in actuators, sonar and micro-electromechanical systems and aid the understanding of other field-dependent material properties.

Stretchable organic light-emitting diodes (OLEDs) have emerged as promising optoelectronic devices with exceptional degree of freedom in form factors. However, stretching OLEDs often results in a reduction in the geometrical fill factor (FF), that is the ratio of an active area to the total area, thereby limiting their potential for a broad range of applications. To overcome these challenges, we propose a three-dimensional (3D) architecture adopting a hidden active area that serves a dual role as both an emitting area and an interconnector. For this purpose, an ultrathin OLED is first attached to a 3D rigid island array structure through quadaxial stretching for precise, deformation-free alignment. A portion of the ultrathin OLED is concealed by letting it ‘fold in’ between the adjacent islands in the initial, non-stretched condition and gradually surfaces to the top upon stretching. This design enables the proposed stretchable OLEDs to exhibit a relatively high FF not only in the initial state but also after substantial deformation corresponding to a 30% biaxial system strain. Moreover, passive-matrix OLED displays that utilize this architecture are shown to be configurable for compensation of post-stretch resolution loss, demonstrating the efficacy of the proposed approach in realizing the full potential of stretchable OLEDs. The reduction in geometrical fill factor in stretchable organic light-emitting diodes (OLEDs) limits their potential for applications. Here, authors report a 3D architecture adopting a hidden active area as both emitting area and interconnector, realizing OLEDs with high post-stretch fill factor.

The seamless integration of rigid/flexible electronic components into stretchable substrates is imperative for the realization of reliable stretchable electronics. However, the transition from flexible-to-stretchable substrates presents inherent challenges in interfacial behavior, predominantly arising from disparities in elastic moduli, thereby rendering their integration arduous for practical deployment. Here, we introduce a bioinspired interface-engineered flexible island (BIEFI), which effectively facilitates the creation of highly stretchable electronics by optimizing the interface with flexible mechanical interlocking mechanisms, resilient to physical deformations. Various electronic components, such as light-emitting diodes (LEDs) and solar cells, are affixed onto the flexible island, showcasing its versatility as a robust platform for rigid components while ensuring the entire substrate maintains high stretchability. Additionally, a smart workout monitoring system is demonstrated by integrating a resistance band with a flexible-to-stretchable platform. This approach seamlessly integrates a wide range of rigid, flexible, and stretchable components, ensuring durability under diverse physical deformations. The transition from flexible-to-stretchable substrates is limited by challenges in interfacial behavior. Here, the authors present an interface design with flexible mechanical interlocking mechanisms, resilient to physical deformations for flexible electronics.

Recently, significant efforts have been directed at overcoming the limitations of conventional rigid optoelectronic devices, particularly their poor mechanical stability under bending, folding, and stretching deformations. One of major approaches for rendering optoelectronic devices mechanically deformable is to replace the conventional electronic/optoelectronic materials with functional nanomaterials or organic materials that are intrinsically flexible/stretchable. Further, advanced device designs and unconventional fabrication methods have also contributed to the development of soft optoelectronic devices. Accordingly, new devices such as bio-inspired curved image sensors, wearable light emitting devices, and deformable bio-integrated optoelectronic devices have been developed. In this review, recent progress in the development of soft optoelectronic materials and devices is outlined. First, various materials such as nanomaterials, organic materials, and their hybrids that are suitable for developing deformable photodetectors, are presented. Then, the nanomaterials and organic/polymeric materials that are applicable in deformable light-emitting diodes are described. Finally, representative system-level applications of flexible and stretchable photodetectors and light-emitting diodes are reviewed, and future prospects are discussed.

Curvy imagers that can adjust their shape are of use in imaging applications that require low optical aberration and tunable focusing power. Existing curvy imagers are either flexible but not compatible with tunable focal surfaces, or stretchable but with low resolution and pixel fill factors. Here, we show that curvy and shape-adaptive imagers with high pixel fill factors can be created by transferring an array of ultrathin silicon optoelectronic pixels with a kirigami design onto curvy surfaces using conformal additive stamp printing. An imager with a 32 × 32-pixel array exhibits a fill factor, before stretching, of 78% and can maintain its electrical performance under 30% biaxial strain. We also develop an adaptive imager that can achieve focused views of objects at different distances by combining a concave-shaped imager printed on a magnetic rubber composite with a tunable lens. Adaptive optical focus is achieved by tuning both the focal length of the lens and the curvature of the imager, allowing far and near objects to be imaged with low aberration. Curvy and shape-adaptive imagers with high pixel fill factors and tunable focusing power can be created by transferring an array of ultrathin silicon optoelectronic pixels with a kirigami design onto curvy surfaces using conformal additive stamp printing.

Wearable displays require mechanical deformability to conform to the skin, as well as long-term stability, multicolour emission and sufficient brightness to enable practically useful applications. However, endowing a single device with all the features remains a challenge. Here we present a rational material design strategy and simple device-manufacturing process for skin-conformable perovskite-based alternating-current electroluminescent (PeACEL) devices. These devices exhibit a narrow emission bandwidth (full-width at half-maximum, <37 nm), continuously tuneable emission wavelength (468–694 nm), high stretchability (400%) and adequate luminance (>200 cd m −2 ). The approach leverages a new class of perovskite zinc sulfide (PeZS) phosphors, consisting of ZnS phosphors coated with perovskite nanoparticles for electrical excitation via total intraparticle energy transfer. This strategy results in pure red and green emissions and expands the colour gamut of powder-based ACEL devices by 250%. Moreover, our processing technique facilitates the integration of PeACEL displays with wearable electronics, enabling applications in dynamic interactive displays and visual real-time temperature monitoring. These PeACEL displays offer new routes in flexible electronics and hold potential for the development of efficient artificial skins, robotics and biomedical monitoring devices. Perovskite zinc sulphide phosphors in perovskite-based alternating-current electroluminescent devices are employed as skin-wearable devices with high stretchability, monochromaticity and power efficiency.

Low-dimensional perovskites have—in view of their high radiative recombination rates—shown great promise in achieving high luminescence brightness and colour saturation. Here we investigate the effect of electron–phonon interactions on the luminescence of single crystals of two-dimensional perovskites, showing that reducing these interactions can lead to bright blue emission in two-dimensional perovskites. Resonance Raman spectra and deformation potential analysis show that strong electron–phonon interactions result in fast non-radiative decay, and that this lowers the photoluminescence quantum yield (PLQY). Neutron scattering, solid-state NMR measurements of spin–lattice relaxation, density functional theory simulations and experimental atomic displacement measurements reveal that molecular motion is slowest, and rigidity greatest, in the brightest emitter. By varying the molecular configuration of the ligands, we show that a PLQY up to 79% and linewidth of 20 nm can be reached by controlling crystal rigidity and electron–phonon interactions. Designing crystal structures with electron–phonon interactions in mind offers a previously underexplored avenue to improve optoelectronic materials' performance.

Textiles that are capable of harvesting biomechanical energy via triboelectric effects are of interest for self-powered wearable electronics. Fabrication of conformable and durable textiles with high triboelectric outputs remains challenging. Here we propose a washable skin-touch-actuated textile-based triboelectric nanogenerator for harvesting mechanical energy from both voluntary and involuntary body motions. Black phosphorus encapsulated with hydrophobic cellulose oleoyl ester nanoparticles serves as a synergetic electron-trapping coating, rendering a textile nanogenerator with long-term reliability and high triboelectricity regardless of various extreme deformations, severe washing, and extended environmental exposure. Considerably high output (~250–880 V, ~0.48–1.1 µA cm −2 ) can be attained upon touching by hand with a small force (~5 N) and low frequency (~4 Hz), which can power light-emitting diodes and a digital watch. This conformable all-textile-nanogenerator is incorporable onto cloths/skin to capture the low output of 60 V from subtle involuntary friction with skin, well suited for users’ motion or daily operations. Incorporation of triboelectric nanogenerators into textiles is attractive for self-powered wearable electronics. Here the authors employ black phosphorus with a hydrophobic coating in a durable, washable, and air permeable textile-based device that converts biomechanical motion into electricity.