Health monitoring systems have undergone significant developments in the last decade, mainly thanks to the advent of wearable technology, which contrasts with the bulky, rigid, and obstructive equipment commonly found in clinics and hospitals.[1]
In the last few years, wearable devices were proposed for long-term and continuous electrophysiological monitoring.

This includes both e-textiles or ultrathin adhesive e-skins for the recording of biopotentials (electrocardiography – ECG,[2–5] electromyography – EMG,[6–9] electroencephalography – EEG,[10–14] or electrooculography – EOG),[9,12,15]respiration rate,[16–18]temperature,[19,20]pulseoximetry,[21,22]or human motion.[23,24]The ultimate goal is to enable long-termelectrophysiological monitoring, increasethe patient comfort during such analysis,and to reduce the healthcare burden byenabling remote patient monitoring – anaspect to which more importance hasbeen given after the recent COVID-19pandemic.[25]It also allows patients to bemonitored 24/7 while remaining in theirhomes and keeping their routines, whichhas been proven to speed-up recoverieswhile reducing healthcare costs.[26,27]This has been a major driving force forthe novel field of stretchable and thin-filmelectronics, which intends to develop advanced electronic systems that can adapt to the dynamic morphology of the humanskin. Materials and fabrication techniques for advancement ofthis field have been a major focus of investigation in the pastfew years.[28,29]This includes techniques for implementationof deterministic structures,[30]conductive composites,[31–36]and liquid metals,[37–39]which include soft lithography,[40–44]laser patterning,[45–49]stencil printing,[12,34,44,50]or direct deposition.[16,31,32,46,49]Another requirement is the integration ofskin-interfacing electrodes to collect biopotentials from theepidermis for EMG/ECG/EEG monitoring, which includethe traditional Ag/AgCl electrodes, novel wet hydrogel electrodes[51,52]as well as printed dry electrodes based on carbon,[12]silver[2,12,20,53,54]or polymers such as PEDOT:PSS.[55]Incorporation of thin-film printed electrodes generally is preferable dueto higher user comfort, easier implementation, a more desirable form factor, and the possibility of very low-cost and scalablefabrication. However, compared to gel electrodes, these printeddry electrodes suffer from a lower skin-conformance, a higherelectrode-skin impedance, and thus a lower signal quality.[56]In summary, implementation of a truly wearable, comfortable, thin-film, and low-cost electrophysiological monitoringsystem that provides a medical-grade interfacing quality is stillnot demonstrated. A few implementations of “electronic tattoos”[5,15,21,31,57]are valuable progress, however, these usuallyfall short either in terms of the required robustness for longterm monitoring, or in terms of signal quality that usually getsdegraded over time due to reduced electrode-skin conformance.

A) Depending on its shape and placement in the body, the proposed adhesive patches can be used for detection of multiple electrophysiological signals: brain waves (EEG), eye movement (EOG), neuromuscular activity (EMG), cardiac activity (ECG), and respiration. B) The various layers and components that compose the e-patch. C) Schematic model of the trinary microstructure of the biphasic conductive polymer. Adapted with permission from Ref. [29]. Copyright 2021 American Chemical Society. D) Fully printed flexible adhesive patch on a patterned background, evidencing the transparency of the substrate. E) Rigid analog front end.

A) Skin wrinkles are imprinted on the adhesive interface layer of the patch. B) Cross-section image of a printed electrode conformed to a nonplanar skin replica. C) Images of a patch conformed to the rough surface of an orange peel. D) The dynamic conformal nature of the e-patch: it keeps its conformability to the skin even under deformation. E) Strain–Stress curves obtained from quasi-static tensile testing for electrode and patch stackups showing a rupture strain ≈450% for both cases. F) Young’s modulus for both multi-layer stackups obtained from linear regression. G) Fatigue testing of the 4-layer patch stack for 500 cycles at a strain of 30%. H) Fatigue testing of the 2-layer electrode stack for 500 cycles at a strain of 30%. In plots E–H, one sample per test was used.