Graphene E-Tattoos for Wireless and Mobile Electrodermal Activity Sensing Enabled by Heterogenous Serpentine Ribbons

Electrodermal activity (EDA) is a skin conductance change that indicates psychological or physiological arousal and has been used as a quantitative index of mental stress levels for decades. State-of-the-art EDA devices suffer from short-term wearability when mounted on the palm or low signal fidelity when measured off palm. Our previous innovation of sub-micron-thick, transparent graphene e-tattoos (GET) [http://dx.doi.org/10.1021/acsnano.7b02182, http://dx.doi.org/10.1038/s41699-018-0064-4] is ideal for long-term unobstructive and imperceptible EDA sensing on the palm. The filamentary-serpentine-shaped GET is so thin and so soft that it is imperceptible optically and mechanically. However, there still lacks mechanically robust electrical connection to interface GET with rigid circuit boards [http://dx.doi.org/10.1088/2053-1583/ab4c0f]. To overcome the orders of magnitude mismatch of stiffness between them, we introduce a novel interface design of heterogeneous serpentine ribbons (HSPR), which refer to serpentine GET overlapping with a serpentine gold ribbon. In comparison with heterogeneous straight ribbons (HSTR), a maximum of 50 folds of strain reduction in GET using HSPR have been confirmed. When the HSPR is clamped end-to-end and subjected to uniaxial tension, its resistance only doubles after 42% of applied strain, which is determined as the stretchability. We also provide a guideline to predict the stretchability of HSPR based on FEM. Furthermore, to prevent the damage of the gold ribbon to contact with the rigid circuit board, a reusable conductive soft interlayer is employed and works both as a vertical via and a mechanical buffer layer which could isolate the mechanical scrubbing from the rigid watch. Finally, the combination of HSPR and a conductive soft interlayer between the GET and a rigid EDA watch enables ~15 hrs of continuous and ambulatory EDA monitoring. This method offers a remedy for the long-standing interconnect challenges between ultrathin e-tattoos and rigid electronics, which could significantly enhance the practical use of e-tattoos.