Mechanics Challenges and Considerations for Sub-Micron Thin E-Tattoos

Compared with conventional wearable devices, skin-conformable ultrathin and ultrasoft e-tattoos offer unique advantages such as low contact impedance comparable with gel electrodes, effective electrical, thermal, optical and/or mass transfer across the skin-tattoo interface, minimal relative motion against the skin during human movement, mechanical imperceptibility, and unobstructive to human touch sensation. We have therefore built a theoretical framework to predict the skin-conformability of e-tattoos based on the tattoo stiffness and thickness, skin roughness, and skin-tattoo interfacial adhesion.[1, 2] The framework was used to guide the thickness design of our graphene e-tattoos (GET).[3, 4] While sub-micron-thin e-tattoos can achieve unprecedented skin conformability, their connection to rigid printed circuit boards (PCB) for data acquisition (DAQ), storage and/or wireless transmission suffer from orders of mismatch in thickness and stiffness. The significant strain concentration at such interfaces makes them notoriously vulnerable and has prevented the mobile and ambulatory use of skin-conformable e-tattoos. We herein propose a simple yet effective solution named “heterogenous serpentine ribbons (HSPR)”,[5] which refers to the strategic overlap between a sub-micron-thin serpentine ribbon and a thicker metallic serpentine ribbon. Compared with heterogeneous strain ribbons (HSTR), HSPR offers a fifty-fold strain reduction and correspondingly, fifty-fold enhancement of stretchability at such interfaces. To demonstrate the capability of HSPR, we have fabricated 300-nm-thin serpentine-shaped GET to form HSPR with 750-nm-thin gold-on-polyimide (Au/PI) serpentine ribbons. While graphene is in contact with the skin, Au is facing up to form the electrical contact with a rigid EDA watch through a soft conductive elastomer. The HSPR construct for GET allows long-term, continuous, unobstructive and ambulatory EDA sensing on human palm during our daily activities as well as during sleep. We believe that the HSPR could be a generic solution to robustly connect any sub-micron-thin e-tattoos to rigid circuits, which could significantly enhance the reliability and usability of ultrathin, skin-conformable e-tattoos.<br/><br/>1 Wang, L., and Lu, N.: ‘Conformability of a Thin Elastic Membrane Laminated on a Soft Substrate With Slightly Wavy Surface’, Journal of Applied Mechanics, 2016, 83, (4), pp. 041007<br/>2 Wang, L., Qiao, S., Kabiri Ameri, S., Jeong, H., and Lu, N.: ‘A Thin Elastic Membrane Conformed to a Soft and Rough Substrate Subjected to Stretching/Compression’, Journal of Applied Mechanics, 2017, 84, (11), pp. 111003<br/>3 Ameri, S.K., Ho, R., Jang, H.W., Tao, L., Wang, Y.H., Wang, L., Schnyer, D.M., Akinwande, D., and Lu, N.: ‘Graphene Electronic Tattoo Sensors’, Acs Nano, 2017, 11, (8), pp. 7634-7641<br/>4 Ameri, S.K., Kim, M., Kuang, I.A., Perera, W.K., Alshiekh, M., Jeong, H., Topcu, U., Akinwande, D., and Lu, N.: ‘Imperceptible Electrooculography Graphene Sensor System for Human-Robot Interface ’, npj 2D Materials and Applications, 2018, 2, pp. 19<br/>5 Jang, H., Sel, K., Kim, E., Kim, S., Yang, X., Kang, S., Ha, K.-H., Wang, R., Rao, Y., Jafari, R., and Lu, N.: ‘Graphene e-tattoos for unobstructive ambulatory electrodermal activity sensing on the palm enabled by heterogeneous serpentine ribbons’, Nat Commun, 2022, accepted