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.

The Belousov−Zhabotinsky (BZ) reaction in a gel system was studied as a function of initial concentration of substrates (malonic acid, sodium bromate, and nitric acid), through which the understanding of the reaction mechanism has been attempted. We prepared gels composed of cross-linked N-isopropylacrylamide networks to which ruthenium catalyst was covalently bonded. To compare the period and waveform of the BZ oscillations in the solution with those in the gel, an image processing method was developed and applied to the gel system. We monitored the oscillating change in the mole fraction of oxidized ruthenium catalyst. The period dependence on the initial substrate concentrations showed that there is a marked difference between the solution and gel systems. In particular, malonic acid and NaBrO3 affect the period over their low concentration ranges. A marked effect of malonic acid concentration appeared in the waveform in the gel but not in the solution. By analyzing these results with the Field−Körös−Noyes (FKN) mechanism, diffusion limitation of substrate and product was found to play an important role in the oxidizing and reducing rates of the ruthenium catalyst within the gel system.