Self-oscillation of polymer gels coupled with the Belousov-Zhabotinsky reaction

Conventional stimuli-responsive hydrogels which undergo volume phase transition provide one unique action either swelling or deswelling in response to external stimuli such as a change in temperature, pH, etc. In contrast to these temporary stimuli-responding behaviors, we have developed a novel gel that spontaneously exhibits cyclic swelling-deswelling changes in a closed solution under constant conditions requiring no switching of external stimuli. The mechanical oscillation is driven by the energy of the BZ reaction within the gel. We have prepared a copolymer gel of N-isopropylacrylamide, in which Ru(bpy)(3) is covalently bonded to the polymer chain. The gel swells at the oxidized state of Ru(bpy)(3)(3+) because the degree of hydrophilicity of polymer chains increases, while at the reduced state of RU(bpy)(3)(2+), the gel deswells. The BZ reaction induced in the gel generates periodic redox changes of Ru(bpy)(3) sites on polymer chains, and the chemical oscillation induces mechanical swelling-deswelling oscillation of the gel. The gel with bead-like shape demonstrates isotropic beating with interval homogeneous redox changes. For the rectangular gel, mechanical oscillation like peristaltic motion is observed with propagation of chemical wave. The dynamic behavior of the chemical and mechanical oscillations has been analyzed with theoretical simulation.