Flexible electronics, as an emerging and exciting research field, have raised great concerns on how to make flexible electronic materials that offer both durability and high performance at strained states. With the advent of on-body wearable and implantable electronics as well as increasing demands for human-friendly intelligent soft robots, highly flexible functional materials, especially stretchable electrodes, are receiving enormous efforts from both academic and industrial communities. Among different deformation modes, stretchability is the most demanding and challenging one. This review focuses on the latest advances in stretchable transparent electrodes based on a new design strategy known as kirigami (the art of paper cutting) and investigates the recent progress on novel applications, including skin-like electronics, implantable biodegradable devices as well as bioinspired soft robotics. By comparing the optoelectrical and mechanical properties of different electrode materials, some of the most important outcomes with comments on their merits and demerits are raised. Key design considerations in terms of geometries, substrates and adhesion are also discussed, offering insights into the universal strategies for engineering stretchable electrodes with basically any material. It is suggested that highly stretchable and biocompatible electrodes will greatly boost the development of next-generation intelligent life-like electronics.Received: ((will be filled in by the editorial staff)) Revised: ((will be filled in by the editorial staff))

Flexible electronic skins (e-skins) with high sensitivity and broad-range pressure sensing are highly desired in artificial intelligence, and humanmachine interaction. Capacitive-type e-skins have a simple configuration, but the change in dimensions of the dielectric layer is often quite limited, although introducing surface microstructures might improve the sensitivity in some extent. Moreover, such surface structures typically require costly microfabrication methods to fabricate. Here, a low-cost microstructured ionic gel (MIG) with uniform cone-like surface microstructures for highperformance capacitive e-skins is reported. The MIG film is templated from a Calathea zebrine leaf using soft lithography, and sandwiched by two flexible electrodes. The device exhibits a low limit of detection down to 0.1 Pa, a ultrahigh sensitivity of 54.31 kPa −1 in the low pressure regime (<0.5 kPa), and the sensitivity keeps larger than 1 kPa −1 over a broad-range pressure from 0.1 Pa to 115 kPa. Electric double layers (EDL) form on both the top and bottom interfaces, and the area of EDL of the rough interface increases as the cones are compressed. Such ionic skins with biomimetic gel templated Calathea zebrine leaf allow for sensitive tactile sensing in the applications of human-machine interaction.

Silver nanowire (AgNW) films have been studied as the most promising flexible transparent electrodes for flexible photoelectronics. The wire-wire junction resistance in the AgNW film is a critical parameter to the electrical performance, and several techniques of nanowelding or soldering have been reported to reduce the wire-wire junction resistance. However, these methods require either specific facilities, or additional materials as the "solder", and often have adverse effects to the AgNW film or substrate. In this study, we show that at the nanoscale, capillary force is a powerful driving force that can effectively cause self-limited cold welding of the wire-wire junction for AgNWs. The capillary-force-induced welding can be simply achieved by applying moisture on the AgNW film, without any technical support like the addition of materials or the use of specific facilities. The moisture-treated AgNW films exhibit a significant decrease in sheet resistance, but negligible changes in transparency. We have also demonstrated that this method is effective to heal damaged AgNW films of wearable electronics and can be conveniently performed not only indoors but also outdoors where technical support is often unavailable. The capillary-force-based method may also be useful in the welding of other metal NWs, the fabrication of nanostructures, and smart assemblies for versatile flexible optoelectronic applications.

Nature has long offered human beings with useful materials. Herein, plant materials including flowers and leaves have been directly used as the dielectric material in flexible capacitive electronic skin (e-skin), which simply consists of a dried flower petal or leaf sandwiched by two flexible electrodes. The plant material is a 3D cell wall network which plays like a compressible metamaterial that elastically collapses upon pressing plus some specific surface structures, and thus the device can sensitively respond to pressure. The device works over a broad-pressure range from 0.6 Pa to 115 kPa with a maximum sensitivity of 1.54 kPa , and shows high stability over 5000 cyclic pressings or bends. The natural-material-based e-skin has been applied in touch sensing, motion monitoring, gas flow detection, and the spatial distribution of pressure. As the foam-like structure is ubiquitous in plants, a general strategy for a green, cost-effective, and scalable approach to make flexible e-skins is offered here.

The spin and orbital angular momentum (SAM and OAM) of light is providing a new gateway toward high capacity and robust optical communications. While the generation of light with angular momentum is well studied in linear optics, its further integration into nonlinear optical devices will open new avenues for increasing the capacity of optical communications through additional information channels at new frequencies. However, it has been challenging to manipulate the both SAM and OAM of nonlinear signals in harmonic generation processes with conventional nonlinear materials. Here, we report the generation of spin-controlled OAM of light in harmonic generations by using ultrathin photonic metasurfaces. The spin manipulation of OAM mode of harmonic waves is experimentally verified by using second harmonic generation (SHG) from gold meta-atom with 3-fold rotational symmetry. By introducing nonlinear phase singularity into the metasurface devices, we successfully generate and measure the topological charges of spin-controlled OAM mode of SHG through an on-chip metasurface interferometer. The nonlinear photonic metasurface proposed in this work not only opens new avenues for manipulating the OAM of nonlinear optical signals but also benefits the understanding of the nonlinear spin-orbit interaction of light in nanoscale devices.

Flexible pressure sensors are essential components for soft electronics by providing physiological monitoring capability for wearables and tactile perceptions for soft robotics. Flexible pressure sensors with reliable performance are highly desired yet challenging to construct to meet the requirements of practical applications in daily activities and even harsh environments, such as high temperatures. This work describes a highly sensitive and reliable capacitive pressure sensor based on flexible ceramic nanofibrous networks with high structural elasticity, which minimizes performance degradation commonly seen in polymer‐based sensors because of the viscoelastic behavior of polymers. Such ceramic pressure sensors exhibit high sensitivity (≈4.4 kPa−1), ultralow limit of detection (<0.8 Pa), fast response speed (<16 ms) as well as low fatigue over 50 000 loading/unloading cycles. The high stability is attributed to the excellent mechanical stability of the ceramic nanofibrous network. By employing textile‐based electrodes, a fully breathable and wearable ceramic pressure sensor is demonstrated for real‐time health monitoring and motion detection. Owing to the high‐temperature resistance of ceramics, the ceramic nanofibrous network sensor can function properly at temperatures up to 370 °C, showing great promise for harsh environment applications.

In article number https://doi.org/10.1002/adfm.201802343, Chuan Fei Guo and co‐workers report a new approach to artificial skin based on the unique leaf structure of Calathea zebrine. Fast response time, a very low limit of detection and good sensitivity promise application of the artificial skin in human–machine interaction, motion detection, and artificial intelligence.

We report stretchable metal-mesh transparent electrodes (TEs) with excellent electrical conductivity (<2 Ω/sq) and optical transparency (>80%) under up to 55% strain. The figures of merit on these electrodes, as defined as the ratio between electrical conductivity and optical conductivity, are among the highest reported for stretchable TEs under moderate stretching. Moreover, we demonstrate their application in a stretchable electroluminescent (EL) light-emitting film as top and bottom electrodes. EL lighting devices require low-resistance electrodes to unleash their potential for large-area low-power-consumption applications, in which our highly conductive and transparent stretchable TEs provide an edge on other competitor approaches. Importantly, our stretchable metal-mesh electrodes are fabricated through a vacuum-free solution-processed approach that is scalable for cost-effective mass production. We also investigate the fracture and fatigue mechanisms of stretchable metal-mesh electrodes with various mesh patterns and observe different behaviors under one-time and cyclic stretching conditions. Our solution-processed fabrication method, failure mechanism investigation, and device demonstration for metal-mesh stretchable TEs will facilitate the adoption of this promising high-performance approach in stretchable and wearable electronics applications.