Epidermic Tattoo Sensors Interface—An In-Depth Look at the Electrochemical Features, Biosignal Transduction Mechanism and Breathability Properties

Temporary tattoo sensors are emerging as flexible, soft, and ultra-thin (~ 1 μm) organic skin contact electronic devices. Tattoo electrodes establish an intimate and stable interface with the skin, and transduce signals with high quality without the need for a conductive gel, like in the case of the medical standard Ag/AgCl electrodes.¹<br/>Here we report a detailed and comprehensive description of the tattoo-skin interface. We detail the transduction mechanism, the factors at the basis of such high performances, and the tattoos intrinsic breathability.<br/>By recording the tattoo-skin electrochemical impedance spectroscopy (EIS) over time, we inspected the physical characteristics and electrochemical properties. By modeling the EIS equivalent circuits of skin with tattoos and gelled Ag/AgCl, we revealed that tattoo electrodes establish a dry interface. Conversely to wet electrodes, polymeric tattoo electrodes show to transduce signals through a capacitive mechanism in which the upper skin layer acts as the dielectric. The proposed capacitive coupling explains why the tattoo EIS values at frequencies &lt;10² Hz are not a limiting factor in the recording ability, despite they are higher than in other medical electrodes. Moreover, we showed that tattoo electrodes preserve a dry interface with the skin over several days of wearing, with no sign of skin occlusion.² These findings indirectly demonstrate that the perspiration evaporates through the electrodes, revealing tattoos as breathable devices and pushing us towards further investigations. Breathability in wearable devices is a pivotal quality to ensure physiological trans epidermal water loss (TEWL) and reduce skin inflammation and discomfort.³ In this context, intrinsically breathable films represent an ideal material for e-skin device fabrication and to close the loop of this study, we reported on the tattoos breathability features. The internal structure of tattoo sensors plays a crucial role in the gas-liquid exchange capabilities. Therefore, SEM and AFM micrographs showed us the presence of cavities within the tattoo structure. This evidence is further confirmed by the liquid water permeance experiments that revealed tattoos function as compressible porous membranes and allow for liquid water passage. Complementarily, we experimentally calculated a water vapor transmission rate (WVTR = 70 g m^-2 h^-1) remarkably higher than the physiological TEWL (=4-8 g m^-2 h^-1).⁴ This result confirms the sweat freely passes through and evaporates from the tattoo sensor.<br/>Polymeric tattoo sensors transduce biosignals through a capacitive coupling with the skin, which is stable over time, thanks to their intrinsic breathability. These properties are optimal for prolonged and high-quality biosignal recordings, and their assessment is essential to design the next generation of wearable e-skin devices.<br/><b>References:</b><br/>1. Ferrari, L. M., Ismailov, U., Badier, J.-M., Greco, F. & Ismailova, E. Conducting polymer tattoo electrodes in clinical electro- and magneto-encephalography. <i>Npj Flex. Electron.</i> <b>4</b>, 4 (2020).<br/>2. Ferrari, L. M., Ismailov, U., Greco, F. & Ismailova, E. Capacitive Coupling of Conducting Polymer Tattoo Electrodes with the Skin. <i>Adv. Mater. Interfaces</i> <b>8</b>, 2100352 (2021).<br/>3. Miyamoto, A. <i>et al.</i> Inflammation-free, gas-permeable, lightweight, stretchable on-skin electronics with nanomeshes. <i>Nat. Nanotechnol.</i> <b>12</b>, 907–913 (2017).<br/>4. Galliani, M., Greco, F., Ismailova, E. & Ferrari, L. M. Tattoo electrodes are intrinsically breathable. <i>Sensors & Diagnostics</i>.