Multi-functional hydrogel electrodes for emerging electronic and robotic applications

Functional hydrogels have emerged as a new class of electronic and robotic materials owing to their unique features. The development of high-performance conductive hydrogels with diverse functionalities has expected to result in important breakthroughs in the fields of electronics and robotics. In this review, the current advances in the fabrication of multi-functional hydrogel electrodes for both electronic and robotic applications are presented. Four different types of hydrogel electrodes are considered depending on their fabrication methods: i) ionically conductive hydrogel, ii) conductive polymer-based hydrogel, iii) conductive nanocomposite hydrogel, and iv) conductive layer-coated hydrogel. Their distinguishing attributes compared to those of other types of soft electrodes, as well as the current challenges for each type of hydrogel electrodes, are discussed. This review also illustrates the recent advances in electronic and robotic applications using hydrogel electrodes. Finally, future directions for capturing these opportunities and overcoming the challenges of conductive hydrogels are discussed.