Biomimetic Self‐Oscillating Polymer Gels

Abstract

There have been many studies on stimuli-responsive polymer gels that exhibit a reversible swelling-deswelling change in response to environmental changes such as solvent composition, temperature, and pH change. In contrast, we have developed a novel "self-oscillating" polymer gel that exhibits autonomous mechanical oscillation without an external control in a completely closed solution ( Fig. 11.1). For the design of the gel, the Belousov-Zhabotinsky (BZ) reaction, which is wellknown for exhibiting temporal and spatiotemporal oscillating phenomena [1, 2], was focused. We attempted to convert the chemical oscillation of the BZ reaction into a mechanical change in gels and generate an autonomous swelling-deswelling oscillation under non-oscillatory outer conditions. A copolymer gel consisting of N-isopropylacrylamide (NIPAAm) and ruthenium tris(2,2 -bipyridine) (Ru(bpy) 3 ), acting as a catalyst for the BZ reaction, was prepared ( Fig. 11.2a). When the poly(NIPAAm-co-Ru(bpy) 3 ) gel is immersed in the catalyst-free BZ solution, the reaction occurs in the gel by the catalytic function of the polymerized Ru(bpy) 3 (Fig. 11.2c). The redox changes of the polymerized catalyst moiety (Ru(bpy) 3 2+ Ru(bpy) 3 3+ ) change the volume phase transition temperature as well as the swelling ratio of the gel because the hydrophilicity of the polymer chains increases at the oxidized Ru(III) state and decreases at the reduced Ru(II) state (Fig. 11.2b). As a result, the gel exhibits an autonomous swelling-deswelling oscillation with the redox oscillation in the closed solution under constant condition (Fig. 11.2d).