In-Phase Synchronization of Chemical and Mechanical Oscillations in Self-Oscillating Gels

A novel gel which undergoes an autonomic and periodical swelling−deswelling oscillation has been prepared by the copolymerization of N-isopropylacrylamide (NIPAAm) with ruthenium tris(2,2‘-bipyridine) (Ru(bpy)3) as a catalyst for the Belousov−Zhabotinsky (BZ) reaction, which is known as an oscillating reaction accompanying a rhythmical change in the redox potential. The swelling−deswelling cycle of a miniature cubic poly(NIPAAm-co-Ru(bpy)3) gel was found to be synchronized with the chemical oscillation without a phase difference. The oscillating profiles of the swelling−deswelling changes as well as the redox changes were studied as a function of the substrate concentrations by using an image-processing method. Both period and amplitude of chemical oscillation varied depending on the substrate concentration. This variation led to a change in the swelling−deswelling oscillation: i.e., the swelling−deswelling amplitude increased with an increase in the period and amplitude of the redox changes. In the synchronization process, a feedback mechanism from mechanical to chemical oscillation was also suggested. The change in gel size with ca. 20% to the initial size was obtained as the maximum value. The swelling−deswelling amplitude of the gel is controllable by changing the initial concentration of substrates or the content of immobilized Ru(bpy)3 catalyst within the gel.