Fatigue-free,superstretchable, transparent,and biocompatible metal electrodes

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.Flexible 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, 11–18). 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 significantly improve the stretchability, revealing an R_sh of ∼28 Ω/□ and a transmittance (T) ∼90% when stretched to 300%. Moreover, by tuning the adhesion between the Au nanomesh and the underlying substrate, the conductor exhibits high fatigue resistance: The resistance does not increase and the morphology has little change after 50,000 cycles of stretching to a large strain of 150%. We ascribe the fatigue-free nature to two reasons. First, the ligaments in the Au nanomesh on a slippery substrate can locally shift and reorient to relax stress. Second, the Au nanoserpentines are well interconnected, and the nodes play an important role for the metal nanomesh to return to the original shape after stress is removed. The Au nanomesh is also biocompatible, and penetrable to body fluid, allowing biomacromolecules to pass through freely. The large stretchability, high fatigue resistance, and good biocompatibility of the transparent electrode are highly desired for stretchable photoelectronics, e-skins, and implantable electrodes in medical devices.