On-scalp printing of personalized electroencephalography e-tattoos

Electroencephalography (EEG) is crucial for diagnosing neurological disorders and facilitating brain-computer interfaces. Traditional EEG setups with wet gels and cumbersome cables are labor intensive, uncomfortable, and degrade over time. Dry, skin-conformable e-tattoos offer a comfortable and user-friendly alternative but struggle with hairy scalps. To tackle this problem, we introduce non-contact digital printing of e-tattoos directly on the hairy scalp. Biocompatible inks are crafted for low-impedance electrodes and high-conductivity interconnects. The fabrication system includes a custom sensor layout design algorithm based on individual head scans and a 5-axis robot controlling a microjet printhead for safe and precise on-scalp ink delivery. After printing, these inks rapidly self-dry into conductive films perfectly conformed to scalp skin, with enhanced wearability, longevity, and skin adhesion compared with gel electrodes and transferred e-tattoos. Motion imagery and error-related potentials are effectively measured by these printed e-tattoos. This innovation heralds a new era in on-body manufacturing of personalized e-tattoos.