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Conductive hydrogels have emerged as excellent candidate materials for designing and fabricating flexible wearable sensors. However, there are still critical challenges in preparing high-performance, strongly adhesive, and biocompatible hydrogels through robust and green strategies. A central issue is the lack of an efficient, environmentally friendly and adaptable catalytic system for hydrogel construction. Herein, a series of natural polyphenol/Fe3+ nanodots (PPL/Fe3+ NDs) has been developed as nanoreactors to catalyze the self-gelation of vinyl monomers. The NDs could provide confined spaces for electron transfer, achieving an exceptionally high catalytic efficiency. Remarkably, they were effective in promoting the polymerization of vinyl monomers to form hydrogels even at the Fe content as low as 0.13 μg mL−1. The resulting hydrogels exhibited outstanding mechanical properties, excellent adhesive capacity, stable ultraviolet (UV)-shielding ability, and feasible electrical conductivity. These advantages allow the hydrogel as a flexible wearable sensor to monitor various human motions. It is believed that these natural polyphenolic NDs could open up new possibilities for the advancement of conductive hydrogels and contribute to the design and implementation of human–machine interaction devices. [ABSTRACT FROM AUTHOR]