Hydrogel-based triboelectric devices for energy-harvesting and wearable sensing applications

Triboelectric nanogenerators (TENGs), which operate based on a combination of contact-electrification and electrostatic induction, have been widely studied as potential mechanical-energy harvesters and self-powered sensors, in which roles they have several advantages. As well as being able to harvest low-frequency mechanical energy – which is the most common type of such energy in the ambient environment – they are easy to make from a wide range of materials; low in cost; and environmentally friendly. This has led to an exponential growth in the popularity of TENGs, especially wearable ones. Hydrogels, meanwhile, are characterized by their high transparency, high stretchability, anti-freezing properties, and self-healing abilities, and have thus emerged as popular materials from which to fabricate wearable TENGs. Accordingly, this paper comprehensively reviews the literature on hydrogel-based TENGs, including work on their fundamental mechanisms; the synthesis and functionalization of the hydrogels used to fabricate them; how those hydrogels are incorporated into them; their most important properties; and their existing applications. As biomechanical sensing and mechanical energy harvesting are two of the most important potential applications for such devices, the challenges and benefits of facilitating such devices’ real-world use in these two areas are also explored. The review concludes that hydrogels are excellent materials for fabricating TENGs for potential application in wearable sensors and energy harvesters. And, as hydrogels and TENGs have already been used successfully, both separately and together, in various areas such as drug and nutrition delivery, the potential applications of hydrogel-based TENGs would appear to be nearly limitless.