Progress in the flexible and stretchable epidermal electrodes for wearable ECG monitoring

As the primary skin-contact interface in wearable electrocardiograph (ECG) devices, epidermal electrodes play a pivotal role in determining both signal quality and biocompatibility. With continuous advancements in materials science and structural engineering, next-generation flexible and stretchable bioelectrodes have emerged, enabling long-term ECG monitoring and offering superior signal-to-noise ratios compared to conventional clinical electrodes. Their performance in ensuring reliable signal acquisition and user comfort is primarily governed by key interfacial mechanical and electrical properties, including mechanical compliance (i.e., flexibility and stretchability), interfacial adhesion (i.e., conformability and adhesion strength), and electrical characteristics (i.e., contact impedance). In recent years, significant progress has been made in enhancing the signal acquisition capabilities of flexible and stretchable bioelectrodes by optimizing these critical interfacial attributes. This review highlights the latest advances in conformable epidermal electrodes, encompassing traditional wet electrodes, flexible dry electrodes, novel dry electrodes based on organic electrochemical transistors, and integrated wearable systems. We systematically examine strategies for improving skin-electrode interface performance in ECG monitoring. Finally, we discuss ongoing challenges and future directions to advance epidermal electrode technologies for next-generation wearable healthcare applications.