Recent Progress in Natural Biopolymers Conductive Hydrogels for Flexible Wearable Sensors and Energy Devices: Materials, Structures, and Performance

Natural biopolymer-based conductive hydrogels, which combine inherent renewable, nontoxic features, biocompatibility and biodegradability of biopolymers, and excellent flexibility and conductivity of conductive hydrogels, exhibit great potential in applications of wearable and stretchable sensing devices. Compared to traditional flexible substrates deriving from petro-materials-derived polymers, conductive hydrogels consisting of continuous cross-linked polymer networks and a large amount of water exhibit more fantastic combination of stretchability and conductivity because their polymer networks endow the hydrogels with mechanical flexibility and the water offers them a consecutive ionic transport property. Different from petro-materials-derived polymers, biopolymers that are extracted from bioresource with intrinsic biocompatibility and biodegradability are commonly considered as appropriate candidates for constructing wearable devices. For example, biopolymers such as cellulose, chitosan, and silk fibroin are usually chosen as promising candidates to construct conductive hydrogels, endowing the hydrogels with enhanced mechanical properties and remarkable biocompatibility. This review summarizes the recent progress of natural biopolymer-based conductive hydrogels that are utilized for electrical sensing devices with a series of typical biopolymers including cellulose, chitosan, silk fibroin, and gelatin. The chemical structures and physicochemical properties of the four typical biopolymers are demonstrated, and their applications in diverse conductive hydrogel sensors are discussed in detail. Finally, the remaining challenges and expectations are discussed.