A flexible substrate-free electrochemical tattoo sensor with alcohol transfer printing method for real-time monitoring of sodium ions in sweat

Tattoo sensors, known for the ultrathin and highly flexible nature and excellent adhesion to human skin, have attracted significant attention in recent years. In this study, a flexible, substrate-free electrochemical tattoo sensor is developed based on nickel-dropped EGaIn (Ni-EGaIn), utilizing an alcohol transfer printing method for real-time sodium ion monitoring in human sweat. This novel printing technique is both simple and rapid, offering exceptional precision and accuracy when applied to human skin. The resulting tattoo sensor maintains strong adhesion under bending (0–150°) and twisting (0–360°) conditions. Furthermore, acid-treated multiwall carbon nanotubes (MWCNTs) are introduced to chitosan-prussian blue nanocomposites (ChPBN), which significantly enhances the conductivity and sensitivity (104.23 mV/decade) of the ion-selective electrode (ISE). Experimental results confirm that the MWCNTs/ChPBN-based electrochemical tattoo sensor exhibits impressive recovery rates for sodium ion detection in human sweat samples, while this sensor can also operate effectively across a broad temperature range (−20℃ to 80℃). This work introduces an innovative, efficient method for fabricating flexible, substrate-free tattoo sensors for electrochemical biomarker monitoring in human biofluids.