**6. Substrate Materials and Skin Interfaces**

Due to their flexibility, ease of manufacturing, and low cost, substrates are often made from polymers or fabrics. Elastomers, such as PET, PI, PEN, polyetherimide (PEI), and parylene, are common materials for thin-film-based substrates. Their weak intramolecular forces enable greater elongation and therefore greater stretchability. Substrates such as these offer many opportunities for breakthroughs. For example, Wonryung et al. reported an active, ultra-flexible, multielectrode array that using a 1.2 µm-thick parylene substrate [93]. The 2.6 µm sensor can be used for long-term ECG of dynamically moving hearts due to a 15% strain. Likewise, Shahandashti et al. showed dry stretchable electrodes based on PDMS, which has great biocompatibility, stretchability, and chemical inertness [13]. The substrate showed similar contact impedance to standard wet Ag/AgCl electrodes, though pressure was applied between the electrode and skin using a transparent tape. To improve the adhesion between substrate and skin, Zhang et al. explored a blended film of PEDOT:PSS, waterborne polyurethane, and D-sorbitol prepared by solution processing. These films exhibited low electrode-skin electrical impedances in the frequency range of 1 Hz–10 MHz and adhesion forces above 0.4 N/cm.

#### **7. Wearable Devices**

Long-term, real-time, continuous monitoring is essential in arrhythmia detection. Thus, advances in form factors and device's comfort are critical for better wearable devices for successful cardiovascular monitoring. Hardware platforms, such as watches and smartphones, commonly include rigid sensors for measuring ECG and PPG. However, these devices suffer from motion artifacts, data loss, and low accuracy. Recently, form-factor innovations in lightweight wearable devices have increased user comfort and compliance. Breakthroughs in epidermal electronics, for example, have minimized the bulkiness of wearable devices. Wang et al. reported low-cost electronic sensors based on epidermal electronics that minimized motion and sweat artifacts [94]. The sensors reported up to 45% stretchability, adhering to the skin using only van der Waals forces. The 13-µm thick sensor is tape-free and disposable, allowing for ease of both patient compliance and comfort. The durability of wearable devices is also essential in arrhythmia detection. For example, the conductive gels in the traditional Ag/AgCl electrodes degrade over time, resulting in a signal decrease. In addition, the buildup of moisture, dead skin, and material degradation can impact the durability of wearable devices. Xu et al. attempted to mitigate some of these problems through a washable and screen-printed graphene electrode on textiles [95]. The ECG sensor showed negligible change over nine washing cycles and 2000 bending cycles.
