E-Tattoos: Toward Functional but Imperceptible Interfacing with Human Skin.

The human body continuously emits physiological and psychological information from head to toe. Wearable electronics capable of noninvasively and accurately digitizing this information without compromising user comfort or mobility have the potential to revolutionize telemedicine, mobile health, and both human-machine or human-metaverse interactions. However, state-of-the-art wearable electronics face limitations regarding wearability and functionality due to the mechanical incompatibility between conventional rigid, planar electronics and soft, curvy human skin surfaces. E-Tattoos, a unique type of wearable electronics, are defined by their ultrathin and skin-soft characteristics, which enable noninvasive and comfortable lamination on human skin surfaces without causing obstruction or even mechanical perception. This review article offers an exhaustive exploration of e-tattoos, accounting for their materials, structures, manufacturing processes, properties, functionalities, applications, and remaining challenges. We begin by summarizing the properties of human skin and their effects on signal transmission across the e-tattoo-skin interface. Following this is a discussion of the materials, structural designs, manufacturing, and skin attachment processes of e-tattoos. We classify e-tattoo functionalities into electrical, mechanical, optical, thermal, and chemical sensing, as well as wound healing and other treatments. After discussing energy harvesting and storage capabilities, we outline strategies for the system integration of wireless e-tattoos. In the end, we offer personal perspectives on the remaining challenges and future opportunities in the field.

Towards Simultaneous Noninvasive Arterial and Venous Oxygenation Monitoring with Wearable E-Tattoo.

While noninvasive arterial blood oxygenation is easily estimated using peripheral pulse oximeters, noninvasive venous blood oxygenation monitoring is still a critical unmet need. Critical conditions that lead to inefficient extraction of oxygen from the blood, such as sepsis or shock, can only be detected by analyzing the oxygen content of the venous blood. In this work, we introduce a soft wearable e-tattoo sensor that simultaneously measures the arterial and venous pulses from the wrist. First, we prove that the origin of the signal is venous pulsatility. We hypothesize that a significant obstacle for simultaneous SaO2 and SvO2 extraction is the close proximity of the artery and vein, thus leading to crosstalk. We characterize this crosstalk with simulation, in vitro, and in vivo experiments. Finally, we offer a potential solution for minimizing the crosstalk through spatial filtering.Clinical Relevance- This lays foundational work for a novel method of noninvasively and simultaneously measuring arterial and venous blood oxygenation to improve clinical diagnoses of sepsis, shock, and metabolic abnormalities.