Oscillatory Swelling Behavior of Hydrogels Incorporated with Ion-Exchange Nanoparticles

α-Zirconium phosphate (α-ZrP) ion-exchange nanoparticles, acting as a proton generator, were incorporated into pH-sensitive hydrogels, poly(acrylic acid) (PAA). The hydrogels exhibited an oscillating volume change behavior in a high pH aqueous solution with influx of Na+ ions. The observed oscillatory swelling behavior was explained by an equilibrium mechanism between reactions and diffusions. This pH-sensitive hydrogels could be utilized for controlled drug delivery that periodically releases enclosed materials without external stimuli.