Tattoo electrodes in bioelectronics: a pathway to next-generation wearable systems.

Tattoo-based electronics have emerged as a transformative platform for next-generation wearable bioelectronics. Unlike conventional wearable devices, which rely on substrates, tattoo electrodes are directly formed or transferred onto the skin or internal organs, ensuring superior comfort, breathability, and long-term usability. This intimate interface minimizes motion-induced artifacts and enables reliable biosignal acquisition across diverse physiological and anatomical regions. However, the absence of a supporting substrate imposes unique challenges in fabrication and material design. The fabrication processes must be tailored to accommodate direct skin application, and the selection of functional materials is more constrained. Materials must not only be biocompatible and flexible but also capable of maintaining performance under the dynamic conditions of the human body. This review presents a comprehensive overview of tattoo electrode technology, beginning with fabrication strategies, including direct and indirect patterning methods. We then discuss a range of materials, such as metallic networks, carbon-based materials, polymers, and materials recently being studied. Finally, we explore the diverse applications of tattoo electrodes in strain and electrophysiological sensing, temperature and humidity detection, biochemical monitoring, and energy harvesting and storage. Through this review, we aim to highlight the potential and future directions of tattoo-based electronic systems.