An overview of conductive composite hydrogels for flexible electronic devices

Conductive hydrogels (CHs) have shown great potential in smart wearable devices and energy storage due to their unique advantages, such as the mechanical properties and physiological characteristics similar to human skins and tissues (stretchability, low modulus, flexibility, biocompatibility, etc.), the function and structure design with diversity, and the transformation of external stimuli (force, deformation, temperature, etc.) into electronic signals. Currently, a large number of CHs have been developed and designed for specific applications. In this review, we first focus on the recent progress on the synthesis strategies of all kinds of CHs. Their categories involve conductive nanomaterial-based, free ionic-based, and conductive polymer-based CHs. Whereafter, the promising applications of CHs in flexible electronic devices are discussed in detail. The relevant applications include wearable strain sensors, bioelectronics, epidermal patch electrodes, temperature sensors, triboelectric nanogenerators (TENG), energy storage devices, and gas sensors. Finally, we also discuss the future research direction of CHs and the possible challenges in the field.