Skin‐Like Stretchable Fuel Cell Based on Gold‐Nanowire‐Impregnated Porous Polymer Scaffolds

Skin‐like energy devices can be conformally attached to the human body, which are highly desirable to power soft wearable electronics in the future. Here, a skin‐like stretchable fuel cell based on ultrathin gold nanowires (AuNWs) and polymerized high internal phase emulsions (polyHIPEs) scaffolds is demonstrated. The polyHIPEs can offer a high porosity of 80% yet with an overall thickness comparable to human skin. Upon impregnation with electronic inks containing ultrathin (2 nm in diameter) and ultrahigh aspect‐ratio (>10 000) gold nanowires, skin‐like strain‐insensitive stretchable electrodes are successfully fabricated. With such designed strain‐insensitive electrodes, a stretchable fuel cell is fabricated by using AuNWs@polyHIPEs, platinum (Pt)‐modified AuNWs@polyHIPEs, and ethanol as the anode, cathode, and fuel, respectively. The resulting epidermal fuel cell can be patterned and transferred onto skin as “tattoos” yet can offer a high power density of 280 µW cm^?2 and a high durability (>90% performance retention under stretching, compression, and twisting). The results presented here demonstrate that this skin‐thin, porous, yet stretchable electrode is essentially multifunctional, simultaneously serving as a current collector, an electrocatalyst, and a fuel host, indicating potential applications to power future soft wearable 2.0 electronics for remote healthcare and soft robotics.     

Metasurface Enabled Wide‐Angle Fourier Lens

Fourier optics, the principle of using Fourier transformation to understand the functionalities of optical elements, lies at the heart of modern optics, and it has been widely applied to optical information processing, imaging, holography, etc. While a simple thin lens is capable of resolving Fourier components of an arbitrary optical wavefront, its operation is limited to near normal light incidence, i.e., the paraxial approximation, which puts a severe constraint on the resolvable Fourier domain. As a result, high‐order Fourier components are lost, resulting in extinction of high‐resolution information of an image. Other high numerical aperture Fourier lenses usually suffer from the bulky size and costly designs. Here, a dielectric metasurface consisting of high‐aspect‐ratio silicon waveguide array is demonstrated experimentally, which is capable of performing 1D Fourier transform for a large incident angle range and a broad operating bandwidth. Thus, the device significantly expands the operational Fourier space, benefitting from the large numerical aperture and negligible angular dispersion at large incident angles. The Fourier metasurface will not only facilitate efficient manipulation of spatial spectrum of free‐space optical wavefront, but also be readily integrated into micro‐optical platforms due to its compact size.     

Cryo‐Transferred Ultrathin and Stretchable Epidermal Electrodes

A simple cryo‐transfer method to fabricate ultrathin, stretchable, and conformal epidermal electrodes based on a combination of silver nanowires (AgNWs) network and elastomeric polymers is developed. This method can temporarily enable the soft elastomers with much higher elastic modulus and dimensional contraction through exploiting their glass‐transition behaviors. During this process, a much higher Von Mises stress can be loaded on AgNWs than usual, and the generated strong grip force can facilitate the complete transfer of AgNWs. Afterward, the thawed AgNWs and elastomer composites quickly recover to their soft state at room temperature. The obtained ultrathin and soft electrode with a thickness of 8.4 µm and transmittance of 90.8% at a sheet resistance of 13.2 Ω sq^?1 can tolerate a stretching strain of 70% and 50 000 repeated bending cycles, which meets rigorous requirements of epidermal applications. The as‐prepared epidermal electrodes are effective and comfortable for electrophysiological signal monitoring, and while showing excellent performance exceeding the commercialized gel electrodes.     

Giant Poisson's Effect for Wrinkle‐Free Stretchable Transparent Electrodes

The next generation of flexible electronics will require highly stretchable and transparent electrodes, many of which consist of a relatively stiff metal network (or carbon materials) and an underlying soft substrate. Typically, such a stiff–soft bilayer suffers from wrinkling or folding when subjected to strains, causing high surface roughness and seriously deteriorated optical transparency. In this work, a network with a giant effective Poisson's ratio on a soft substrate is found to be under biaxial tension upon deformation, and thus does not wrinkle or fold, but maintains smooth surfaces and high transparency. Soft tactile sensors employing such network electrodes exhibit high transparency and low fatigue over many stretching cycles. Such a giant Poisson's ratio has the same effect in other systems. This work offers a new understanding of surface instabilities and a general strategy to prevent them not only in flexible electronics, but also in other materials and mechanical structures that require flat surfaces.     

A Solution‐Processable,Omnidirectionally Stretchable,and High‐Pressure‐Sensitive Piezoresistive Device

The development of omnidirectionally stretchable pressure sensors with high performance without stretching‐induced interference has been hampered by many challenges. Herein, an omnidirectionally stretchable piezoresistive pressure‐sensing device is demonstrated by combining an omniaxially stretchable substrate with a 3D micropattern array and solution‐printing of electrode and piezoresistive materials. A unique substrate structural design and materials mean that devices that are highly sensitive are rendered, with a stable out‐of‐plane pressure response to both static (sensitivity of 0.5 kPa^?1 and limit of detection of 28 Pa) and dynamic pressures and the minimized in‐plane stretching responsiveness (a small strain gauge factor of 0.17), achieved through efficient strain absorption of the electrode and sensing materials. The device can detect human‐body tremors, as well as measure the relative elastic properties of human skin. The omnidirectionally stretchable pressure sensor with a high pressure sensitivity and minimal stretch‐responsiveness yields great potential to skin‐attachable wearable electronics, human–machine interfaces, and soft robotics applications.     

0.2 λ0 Thick Adaptive Retroreflector Made of Spin‐Locked Metasurface

The metasurface concept is employed to planarize retroflectors by stacking two metasurfaces with separation that is two orders larger than the wavelength. Here, a retroreflective metasurface using subwavelength‐thick reconfigurable C‐shaped resonators (RCRs) is reported, which reduces the overall thickness from the previous record of 590 λ₀ down to only 0.2 λ₀. The geometry of RCRs could be in situ controlled to realize equal amplitude and phase modulation onto transverse magnetic (TM)‐polarized and transverse electric (TE)‐polarized incidences. With the phase gradient being engineered, an in‐plane momentum could be imparted to the incident wave, guaranteeing the spin state of the retro‐reflected wave identical to that of the incident light. Such spin‐locked metasurface is natively adaptive toward different incident angles to realize retroreflection by mechanically altering the geometry of RCRs. As a proof of concept, an ultrathin retroreflective metasurface is validated at 15 GHz, under various illumination angles at 10°, 12°, 15°, and 20°. Such adaptive spin‐locked metasurface could find promising applications in spin‐based optical devices, communication systems, remote sensing, RCS enhancement, and so on.     

Electrically Controllable Molecularization of Terahertz Meta‐Atoms

Active control of metamaterial properties is critical for advanced terahertz (THz) applications. However, the tunability of THz properties, such as the resonance frequency and phase of the wave, remains challenging. Here, a new device design is provided for extensively tuning the resonance properties of THz metamaterials. Unlike previous approaches, the design is intended to control the electrical interconnections between the metallic unit structures of metamaterials. This strategy is referred to as the molecularization of the meta‐atoms and is accomplished by placing graphene bridges between the metallic unit structures whose conductivity is modulated by an electrolyte gating. Because of the scalable nature of the molecularization, the resonance frequency of the terahertz metamaterials can be tuned as a function of the number of meta‐atoms constituting a unit metamolecule. At the same time, the voltage‐controlled molecularization allows delicate control over the phase shift of the transmitted THz, without changing the high transmission of the materials significantly.     

Stretchable Thin‐Film Electrodes for Flexible Electronics with High Deformability and Stretchability

Flexible and stretchable electronics represent today's cutting‐edge electronic technologies. As the most‐fundamental component of electronics, the thin‐film electrode remains the research frontier due to its key role in the successful development of flexible and stretchable electronic devices. Stretchability, however, is generally more challenging to achieve than flexibility. Stretchable electronic devices demand, above all else, that the thin‐film electrodes have the capacity to absorb a large level of strain (>>1%) without obvious changes in their electrical performance. This article reviews the progress in strategies for obtaining highly stretchable thin‐film electrodes. Applications of stretchable thin‐film electrodes fabricated via these strategies are described. Some perspectives and challenges in this field are also put forward.     

Crumpling and Unfolding of Montmorillonite Hybrid Nanocoatings as Stretchable Flame‐Retardant Skin

Flame‐retardant coatings are widely used in a variety of personnel or product protection, and many applications would benefit from film stretchability if suitable materials are available. It is challenging to develop flame‐retardant coatings that are stretchable, eco‐friendly, and capable of being integrated on mechanically dynamic devices. Here, a concept is reported that uses pretextured montmorillonite (MMT) hybrid nanocoatings that can undergo programed unfolding to mimic the stretchability of elastomeric materials. These textured MMT coatings can be transferred onto an elastomeric substrate to achieve an MMT/elastomer bilayer device with high stretchability (225% areal strain) and effective flame retardancy. The bilayer composite is utilized as flame‐retardant skin for a soft robotic gripper, and it is demonstrated that the actuated response can manipulate and rescue irregularly shaped objects from a fire scene. Furthermore, by depositing the conformal MMT nanocoatings on nitrile gloves, the firefree gloves can endure direct flame contact without ignition.     

Solution‐Processed Lead Iodide for Ultrafast All‐Optical Switching of Terahertz Photonic Devices

Solution‐processed lead iodide (PbI₂) governs the charge transport characteristics in the hybrid metal halide perovskites. Besides being a precursor in enhancing the performance of perovskite solar cells, PbI₂ alone offers remarkable optical and ultrasensitive photoresponsive properties that remain largely unexplored. Here, the photophysics and the ultrafast carrier dynamics of the solution processed PbI₂ thin film is probed experimentally. A PbI₂ integrated metamaterial photonic device with switchable picosecond time response at extremely low photoexcitation fluences is demonstrated. Further, findings show strongly confined terahertz field induced tailoring of sensitivity and switching time of the metamaterial resonances for different thicknesses of PbI₂ thin film. The approach has two far reaching consequences: the first lead‐iodide‐based ultrafast photonic device and resonantly confined electromagnetic field tailored transient nonequilibrium dynamics of PbI₂ which could also be applied to a broad range of semiconductors for designing on‐chip, ultrafast, all‐optical switchable photonic devices.