Breathable and imperceptible on-skin electronic tattoos with a hybrid of silk and cellulose and highly conductive electrodes for monitoring skin hydration

Skin, the largest organ protecting the body, acts as a pathway connecting the inside and outside of the body, allowing us to examine health conditions. Therefore, on-skin electronics are attractive for monitoring biosignals in daily life for point-on-care healthcare. However, integrating highly conductive electrodes while maintaining all the properties suitable for on-skin devices, such as flexibility, imperceptibility, breathability, and biocompatibility, is still challenging. Here, we present breathable and imperceptible electronic tattoos (E-tattoos), on which highly conductive gold (Au) electrodes are integrated. The E-tattoo, which a hybrid of two nanostructured biomaterials, ultrathin silk film and cellulose nanofiber mat, possesses all favorable properties for on-skin electronics. Due to the inherent strong adhesion of silk to Au, patterned Au electrodes, with a high conductivity (2.84 × 10⁷ S/m) comparable to that of pure Au (4.01 × 10⁷ S/m), can be integrated on the E-tattoo. High water-vapor transmission and low leakage current through E-tattoos provide skin-compatibility (nonirritating response). With these advantages, the E-tattoo is applied to monitor skin hydration. On-skin impedance measurements reveal dependency on skin hydration, and impedances measured with E-tattoos show better signal stability than those measured for Au nanomesh patches. This study presents a new on-skin electronic platform for monitoring skin conditions.