µm-Thick and Water-Taping Protein Electronic Tattoos for Multifunctional On-Skin Electronics.

Seamless integration between on-skin electronics and the skin is crucial for advanced personalized diagnostics, therapeutics, and human-machine interfaces. The challenge lies in creating an ideal interface that directly connects thin-film electronics with soft skin, ensuring both free skin respiration and stable device performance. Here, an electronic tattoo (e-tattoo) based on the ever-thinnest protein hydrogel that conforms to the skin's minutiae without air gaps is reported. The combination of sub-µm-thick silk film and carbon nanotube nanosheet (CNT-NS) enables the implementation of µm-thick e-tattoos. The extremely thin and strong skin-adhesion of silk allow conformal integration with skin contours, while mechanical and electrical properties of CNT-NS enable stable on-skin electronic operation. The e-tattoo exhibits high breathability, mechanical strength, and Ohmic electrical conductivity, supporting various biomedical applications. Strong light absorption of the CNT-NS enables photothermal patch applications, and the seamless skin-interface allows electrocardiogram detection with higher signal-to-noise than that of commercial gel electrodes. Additionally, the e-tattoo is applied as a triboelectric nanogenerator operated by bare skin touch, functioning as a self-powered Morse code transmitter. By sandwiching an electrolyte-infused silk film between two e-tattoos, a supercapacitor is implemented, demonstrating stable performance even under highly iterative charge/discharge cycles and mechanical deformation.