Nano/micro-engineered metallic flexible electrodes: advancing the era of epidermal electronics

Metals, indispensable since the Bronze Age, remain pivotal in modern technologies due to their exceptional properties and versatility. Beyond traditional machining, advanced nano/micro-machining techniques enable the fabrication of metallic nano/micro structures with high precision in shape, size, and pattern. These structures endow flexible electrodes with outstanding electrical, mechanical, optical, and electrochemical performance, enabling growing applications in flexible optoelectronics, epidermal electronics, energy harvesting, and biochemical sensing. This review provides a comprehensive overview of the fabrication strategies for flexible electrodes made from metal meshes, metal nanowires, and liquid metals. The current advancements, existing challenges, and emerging technologies are systematically discussed. Furthermore, the progression toward ultra-thin, soft epidermal electrodes is explored, with an emphasis on novel in situ and transfer fabrication methods. We examine the underlying mechanisms, performance indicators, and their integration for on-skin applications, including bioelectric sensing, electrical stimulation, and energy harvesting. Finally, we highlight the remaining challenges in performance improvement and industrialization of flexible and epidermal electrodes, along with future opportunities for integrating multimodal systems and leveraging artificial intelligence to enhance their functionalities.