Swelling is ubiquitous for conventional hydrogels but is not favorable for many situations, especially underwater applications. In this study, an antiswelling and mechanically robust polyacrylic acid (PAAc)/gelatin composite hydrogel is reported with a rapid gelation process (10 1 s) under mild conditions via the synergy of MXene-activated initiation and zirconium ion (Zr 4+ )-induced cross-linking, without the requirement of external energy input. The MXene is found efficient to activate the chain initiation, while the Zr 4+ is prone indispensable for facilitating the cross-linking of formed polymer chains. The resulting hydrogel exhibits integration of exceptional anti-swelling properties and high mechanical performance at room temperature, thanks to the dense hydrogen bonds between PAAc and gelatin chains that enable an upper critical solution temperature above room temperature. Also, desirable electrical conductivity emerges in the hydrogel due to the simultaneous contribution of MXene and Zr 4+ , allowing stable electrical signal output of the gel upon deformation underwater. As a demonstration, an underwater communicator by harnessing the gel as a sensing module is assembled, which is capable of wirelessly delivering messages to the decoder on the ground via Morse codes. This study provides an exemplary way for the rapid gelation of tough and anti-swelling hydrogels for durable underwater applications.

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