Ultrathin, Breathable and Self-Powered Electronic Tattoos with Graphene and Silk-Reinforced Cellulose Nanofibers for On-Skin Electronics

Electronic skins and electronic tattoos (E-tattoos) represent notable advancements in wearable technology, offering benefits in healthcare monitoring, energy harvesting, human-machine interfaces, and artificial intelligence prostheses. These devices emulate human skin functionalities, enabling real-time, noninvasive detection of environmental stimuli like temperature, pressure, and vibrations. E-tattoos, particularly, are ultrathin, lightweight, and skin-adhesive, allowing seamless integration with human skin, making them valuable in clinical diagnostics, therapeutic applications, and real-time monitoring of physiological signals. Despite advancements, challenges remain in integrating these technologies with human skin while maintaining high sensitivity in sensing and minimal invasiveness.<br/><br/>To fabricate E-tattoos, synthetic polymers like poly(dimethyl siloxane), polyimide, and poly(ethylene terephthalate) are commonly used. However, these polymers lack biocompatibility and biodegradability. Conductive materials are often blended into these polymers to maintain skin adhesion and signal monitoring capabilities. Yet, the insulating nature of most polymers reduces overall conductivity, necessitating thick electrodes that compromise breathability, leading to sweat accumulation and potential skin issues. Addressing these limitations requires resilient, adaptable E-tattoos with multifunctionalities. Hybrid and nanostructured biomaterial composites can be viable solutions.<br/><br/>The present study reports an imperceptibly operable, skin-compatible, multifunctional E-tattoo based on a composite of silk sericin (SS), cellulose nanofiber (CNF), and graphene. CNFs provide mechanical stability, flexibility, biocompatibility, and porosity, while SS offers strong adhesion and skin compatibility. The hybrid biomaterial-based tattoo is ultrathin, lightweight, highly porous, and displays exceptional adherence to skin, mechanical robustness, and skin compatibility. Graphene provides favorable electrical and thermal properties, resulting in high electrical conductivity of ~0.16 S/cm, stable under various mechanical deformations. The E-tattoo also features high water-vapor transmission and no skin irritation. It facilitates temperature sensing and can function as a skin heat patch which can be activated by electrical and optical signals. The temperature can increase by more than 20 °C at a bias voltage of 3 V as well as under light-emitting diode (LED) illumination with a power density of ~34.5 mW/cm². Due to the tribopositive nature of cellulose, the E-tattoo generates a high open-circuit voltage (<i>V_oc</i>) of ~320 ± 20 V and a power density of ~7.2 mW/cm², sufficient to glow multi-LEDs, demonstrating its potential in self-powered sensory systems for Morse code transmission and real-time monitoring of body movements. Overall, the study presents a novel E-tattoo platform with potential applications in healthcare and secure communications, demonstrating significant advancements in wearable bioelectronics.