A novel type of ultrathin cationic hydrogel coatings are provided that have high, quick and reversible swelling/shrinkage transitions and surface wettability in response to pH changes. The poly(4-vinyl pyridine) (PVP) hydrogel films are produced by selective cross-linking of PVP copolymers in layer-by-layer (LbL) films assembled by spin-assisted method. These multilayer hydrogels exhibit drastic and reversible 10-fold swelling when the pH is changed from neutral to acidic. The swelling amplitude of these coatings is controlled by varying cross-link densities within the films. The pH-responsive hydrogels open new prospects for developing “intelligent” materials with sharp stimuli-triggered responses for applications in sensing, transport regulation, and self-cleaning coatings.
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