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Next-generation flexible electronics require highly stretchable and transparent electrodes. Few electronic conductors are both transparent and stretchable, and even fewer can be cyclically stretched to a large strain without causing fatigue. Fatigue, which is often an issue of strained materials causing failure at low strain levels of cyclic loading, is detrimental to materials under repeated loads in practical applications. Here we show that optimizing topology and/or tuning adhesion of metal nanomeshes can significantly improve stretchability and eliminate strain fatigue. The ligaments in an Au nanomesh on a slippery substrate can locally shift to relax stress upon stretching and return to the original configuration when stress is removed. The Au nanomesh keeps a low sheet resistance and high transparency, comparable to those of strain-free indium tin oxide films, when the nanomesh is stretched to a strain of 300%, or shows no fatigue after 50,000 stretches to a strain up to 150%. Moreover, the Au nanomesh is biocompatible and penetrable to biomacromolecules in fluid. The superstretchable transparent conductors are highly desirable for stretchable photoelectronics, electronic skins, and implantable electronics.fatigue-free | adhesion | biocompatibility | topology | stretchability F lexible transparent electrodes are crucial to the emerging fields of flexible solar cells (1, 2), flexible electronics (3-5), electronic skins (e-skins) (6), and implantable electronics (7,8). Among the several modes of flexibility, including bending, folding, twisting, and stretching, stretching generates the largest strain and therefore is the most demanding (9). What is even more challenging is to make transparent electrodes fatigue-free under cyclic stretches. Fatigue often happens during strain cycling, even if the strain level is relatively low. It determines the real loading that can be applied to a material in practical applications. However, metallic materials often exhibit high cycle fatigue (10), and fatigue has been a deadly disease for metals.Several types of transparent conductors, including graphene sheets, carbon nanotube (CNT) films, metal nanowire (NW) networks, composites based on Ag NWs, metal meshes, and ultrathin metal films have been found to be stretchable (1,3,6,(11)(12)(13)(14)(15)(16)(17)(18). However, sheet resistance (R sh ) of existing stretchable transparent electrodes often sharply increases when highly stretched, or repeatedly stretched to relatively small strains for thousands of cycles. Graphene can be stretched one time to 30%, or cyclically stretched to 6% for a few times (11). Metal meshes made of straight lines and ultrathin metal films are also stretchable, but typically they cannot be stretched to more than 100% (16, 17). The Bao group has shown that CNT network film with a serpentine morphology can be stretched one time to 170% before failure, or repeatedly stretched to 25% for 12,500 cycles with a modest increase of resistance (6). Here we show that optimizing topology of a Au nanomesh can ...

Noncovalent adhesion has long been developed for numerous applications, including pressure-sensitive adhesives, wound closure, and drug delivery. Recent advances highlight an urgent need: a general principle to guide the development of instant, tough, noncovalent adhesion. Here, we show that noncovalent adhesion can be both instant and tough by separately selecting two types of noncovalent bonds for distinct functions: tougheners and interlinks. We demonstrate the principle using a hydrogel with a covalent polymer network and noncovalent tougheners, adhering another material through noncovalent interlinks. The adhesion is instant if the interlinks form fast. When an external force separates the adhesion, the covalent polymer network transmits the force through the bulk of the hydrogel to the front of the separation. The adhesion is tough if the interlinks are strong enough for many tougheners to unzip. Our best result achieves adhesion energy above 750 J/m2 within seconds. The adhesion detaches in response to a cue, such as a change in pH or temperature. We identify several topologies of noncovalent adhesion and demonstrate them in the form of tape, powder, brush, solution, and interpolymer complex. The abundant diversity of noncovalent bonds offers enormous design space to create instant, tough, noncovalent adhesion for engineering and medicine.

Manipulating charges is fundamental to numerous systems, and this ability is achieved through materials of diverse characteristics. Electrets are dielectrics that trap charges or dipoles. Applications include electrophotography, microphones, air filters, and energy harvesters. To trap charges or dipoles for a long time, electrets are commonly made of hard dielectrics. Stretchable dielectrics are short–lived electrets. The two properties, longevity and stretchability, conflict; existing electrets struggle to attain both. This work describes an approach to developing stretchable electrets. Nanoparticles of a hard electret are immobilized in a matrix of dielectric elastomer. The composite divides the labor of two functions: the particles trap charges with longevity, and the matrix enables stretchability. The design considerably broadens the choice of materials to enable stretchable electrets. Silica nanoparticles in the polydimethylsiloxane elastomer achieve a charge density ∼ 4 × 10–5 C m–2 and a lifetime beyond 60 days. Long–lived, stretchable electrets open extensive opportunities.