2, Purdue University, West Lafayette, Indiana, United States
3, Birck Nanotechnology Center, West Lafayette, Indiana, United States
4, Thin-Film Device Laboratory & Center for Emergent Matter Science (CEMS), Wako, , Japan
Accurate, imperceptible and long-term monitoring of vital biopotential signals promises to revolutionize healthcare industry by shifting from costly and uncomfortable hospital visits to in-home usage. Currently available wearable electronics are typically rigid with non-conformal skin contact resulting in poor data quality, necessitating the integration of such bioelectronics  directly onto the skin . Increasing the conformity of the artificial electronic skin to the soft, irregular and stretchable human skin typically results in improved signal quality and user comfort .
We report on the fabrication of self-adhesive and conformable to highly irregular three-dimensional soft surfaces, sub-300 nm thin dry electrodes that produce biopotential (sEMG and sECG) recordings of excellent quality (SNR). The electrodes are based on thermally evaporated thin film (100 nm) of Au, sandwiched between two layers (100 nm each) of CVD-deposited biocompatible parylene (parylene/Au/parylene). They are fabricated on glass substrates, with fluorinated polymer (85 nm) and poly(vinyl alcohol) (PVA, 5 µm) sacrificial layers used for delamination and ease of handling. Parylene is etched away at the skin-interface side, allowing for direct Au contact with the skin. Following delamination, electrodes are placed on pre-stretched human skin and sprayed with H2O to remove PVA, forming a skin/Au/parylene structure. The skin is then dried and relaxed, with the ultra-thin film conforming to the skin groves via wan der Waals forces , without any additional adhesives.
These simple-to-fabricate and use, ultra-thin sensors show single-day electrical and mechanical stability of up to ten hours. Their bending stiffness was calculated to be comparable to stratum corneum, the uppermost layer of human skin, at ~0.33 pNm2, which is over two orders of magnitude lower than the bending stiffness of a 3.0 µm thin sensor. Compared with the thicker sensor, its impedance also decreased by almost two orders of magnitude. Laminated on a pre-stretched elastomer, the sensor forms wrinkles with a period of 17 µm and amplitude of 4 µm, agreeing with theoretical calculations.
In contrast to wet adhesive Ag/AgCl electrode, with skin vibrations of up to ~15 µm, the sensor demonstrates motion artifact-less sEMG monitoring. Additional impedance and sEMG measurements reveal that the decrease of impedance, as well as the motion artifact-less operation, is likely due to improved skin adhesion of the sub-300 nm thin sensor.
With compatible fabrication to our previously demonstrated sub-300 nm thin electronics , this demonstrates a path for integration of skin-laminated systems consisting of sensors and electronics.
 M. Irimia-Vladu, et al., Adv. Fun. Mat. 20, 4069-4076 (2010)
 T. Yokota, et al., Science Adv. 2, e1501856 (2016)
 D.H Kim, et al., Nature Mat. 9, 511-517 (2010)
 M. Fernandez, et al., Biomed Inst. Tech. 34, 125 (2000)
 R. Nawrocki, et al., Adv. Ele. Mat. 2, 4 (2016)