*2.2. Materials for Flexible ECG Devices*

Many ECG electrodes currently used in clinics are made up of three parts: (1) a conductive metal, traditionally Ag/AgCl electrodes; (2) a conductive gel; and (3) an adhesive patch. Conductive gels reduce the impedance from the electrode to the skin. However, they dry up over time, which causes signal quality to deteriorate during long-term monitoring. Therefore, many materials have been explored as alternatives to traditional Ag/AgCl that are conformal to the skin. Although they generally have higher impedances than wet electrodes, flexible dry electrodes are quickly gaining in popularity. Electrodes can be made from any conductive materials. Since metals have high Young's moduli, ultra-thin metal films can be arranged in serpentine or fractal geometries to provide flexibility or stretchability, as shown in Figure 2a [28–31]. Chlaihawi et al. reported an electrode that was screen printed with Ag flake ink [32]. The electrode with the largest area reported a 0.95 correlation coefficient with traditional wet Ag/AgCl electrodes. In addition, the study showed the feasibility of using the high-throughput process of screen printing for the development of flexible dry electrodes.

**Figure 2**. Electrocardiography. (**a**) Photo of a skin-conformal electrode. (**b**) Microneedle **Figure 2.** Electrocardiography. (**a**) Photo of a skin-conformal electrode. (**b**) Microneedle array-based ECG. Illustration of (**i**) traditional Ag/AgCl electrode and (**ii**) microneedle array electrode. (**iii**) Photo of a microneedle array electrode. (**c**) Photo of a stretchable hybrid-electronics device. (**d**) Photo of a soft strain-isolated bioelectric device. (**e**) Foil micrograph of a flexible ECG patch.

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The stretchability of these thin-film metals can be increased by introducing conductive polymers. Polyethylene terephthalate (PET) and polydimethylsiloxane (PDMS) are commonly chosen as polymers due to their biocompatibility, wide availability, and low Young's modulus. To make these polymers conductive, materials such as activated carbon or metal micro/nanoparticles are added to form networks of conductivity. For example, Jung et al. showed a carbon nanotube (CNT)/PDMS composite-based dry electrode to combat motion and sweat artifacts [33]. The performance of the electrode was able to be tuned by adjusting the CNT concentration. This electrode showed no signal degradation over a seven-day period of continuous monitoring, providing similar motion-artifact reduction as wet electrodes and more motion-artifact reduction than other dry electrodes. Zhang et al. showed an electrode that had stretchability up to 500% by combining Ag nanowires (NWs) with polymers [34]. This sensor improved upon the CNT/PDMS structure, which is subject to weak connection of conductive materials when stretched. This polymer/Ag NW sensor also increased the durability over 1000 cycles and exhibited good fatigue resistance.

In recent years, the conductive polymer poly (3,4-ethylene dioxthiophene): polystyrene trans acid (PEDOT:PSS) has been the most common polymer for textile-based electrodes due to its high sensitivity to biological molecules and high response time [35]. Wang et al. showed that PEDOT:PSS could be used to achieve even higher flexibility and lower skin impedance by combining it with a flexible cellulose/polyvinyl alcohol substrate [20]. This process provides new ideas for low-cost manufacturing of environmentally friendly ECG devices.

In addition to material advances, recent studies have decreased the impedance of surface electrodes by changing the form factor. For example, semi-invasive strategies, such as microneedle-based approaches, have been demonstrated to reduce motion artifacts [36,37]. Satti et al. reported a microneedle array electrode (MNE), as shown in Figure 2b, that showed no mechanical failure under compression forces of 16 N and showed that while the signal quality of wet Ag and AgCl electrodes decreased after 3 days and 1 week, respectively, the MNE showed no signs of signal-quality deterioration [36].

Despite the many developments in electrode technology, there is still a lack of allin-one integration. To address these sensor-only systems, innovations in packaging have become important as well. For example, the general impedance challenges of sweat buildup on electrodes have been addressed by integrating hydrophilic poly(urethane-acrylate) into Ag electrodes to increase conductivity during sweating and increasing the breathability of the substrate, as shown in Figure 2c,d [37–41]. All-in-one systems featuring wireless charging, wireless data communication, and onboard data analysis have been developed, as in Figure 2e [42–45].
