Molecular Design of Self-Oscillating Polymer Gels and Their Dynamic Swelling-Deswelling Behaviors

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Abstract
We report a novel "intelligent" gel that undergoes an autonomous swellingdeswelling oscillation without reliance on an alteration in external conditions. The mechanical oscillation in our gel system was produced via oscillating chemical reaction similar to metabolic reaction, called the Belousov-Zhabotinsky (BZ) reaction. We prepared gels composed of crosslinked Nisopropylacrylamide (NIPAAm) networks to which ruthenium tris(2,2'-bipyridine) (Ru(bpy)3), a catalyst for the BZ reaction, was covalently bonded. The BZ reaction occurring within the gel matrix generates periodic redox changes of the catalyst moiety: Ru(bpy)32+ T Ru(bpy)33+. As a result, the gel exhibits a periodical swelling-deswelling change. When the gel size is smaller than the chemical wavelength, the redox change occurs homogeneously in the gel. In this case, the swelling-deswelling oscillation is isotropic and the chemical and mechanical oscillations are synchronized without a phase difference. In the case of a large rectangular gel, the total length of the gel periodically changes with the propagation of chemical waves. However, an "anti-phase mode" between the chemical and mechanical oscillations was observed. This phenomenon was explained by a theoretical model simulation. It has become apparent that a change in the overall gel length is equivalent to that in the last stationary wave at the free end. The dynamic behavior that locally shrunken (or swollen) parts propagate is similar to the peristaltic motion observed in worms.