Wearable and flexible electrodes in nanogenerators for energy harvesting, tactile sensors, and electronic textiles: novel materials, recent advances, and future perspectives

There are numerous drivers in the context of sustainable energy production from ambient mechanical energy sources, such as body motions, due to the increasing world demand for alternative energy. Recent progress has been made in the energy harvesting technologies based on piezoelectric nanogenerators (PENGs) and triboelectric nanogenerators (TENGs) to convert such ambient biomechanical energy into electricity. The PENGs and TENGs technologies have been successfully utilized to provide sufficient energy for low-power electronic devices, such as biomedical sensors for health monitoring. However, the successful implementations of such technologies, including their electrodes as the critical component of the nanogenerators, require unique properties such as flexibility, wearability, and stretchability. As a result, this review summarizes recent progress on PENGs and TENGs technologies and applications with a focus on new electrode materials that could provide flexibility, wearability, and stretchability capabilities to these types of nanogenerators. This review shed light on the role of wearable electrodes in different applications such as devices with smart tactile sensing mechanisms and electronic textiles. This review also outlines the future prospect and potential direction toward the advancement of such technologies and their performance.