Conventional stimuli-responsive hydrogels provide one unique action, either swelling or deswelling in response to external stimuli such as a change in solvent composition, 1,2 pH, 1-3 temperature, 4-6 etc. Here we report a novel polymeric gel which autonomously swells and deswells periodically in a closed homogeneous solution without any external stimuli, similar to autonomic phenomena in life such as heartbeat. The mechanical oscillation is achieved by inducing the Belousov-Zhabotinsky (BZ) reaction 7 within the gel. We prepare a copolymer gel of N-isopropylacrylamide (IPAAm) in which ruthenium(II) tris-(2,2′-bipyridine) (Ru(bpy) 3 2+ ), a catalyst for the BZ reaction, is covalently bonded to the polymer chain. The poly(IPAAmco-Ru(bpy) 3 ) gel swells and deswells at the oxidized and reduced states of Ru(bpy) 3 , respectively. The BZ reaction in the gel generates periodic redox changes of Ru(bpy) 3 , and the chemical oscillation induces mechanical oscillation of the polymer network.Many polymeric gels undergo abrupt volume change in response to external stimuli, and they have been utilized to design intelligent materials which imitate stimuli-responding functions in living systems, such as actuators 8,9 and pulsatile drug release devices, 10-12 etc. In contrast to the unique gel action of either swelling or deswelling toward a stable equilibrium state, many physiological systems maintain rhythmical oscillations in a nonequilibrium state, as represented by the autonomic heartbeat, brain waves, periodic hormone secretion, etc. If such self-oscillation without any external stimuli is achieved for gels, a new concept will be created for functional materials that work under dynamic oscillating states similar to life systems, such as self-walking actuators, new pacemakers and timers, drug delivery systems synchronized with human circadian rhythms, etc. For example, a strategy for an oscillatory drug release system utilizing a gel membrane coupled with an enzyme reaction is studied numerically by Siegel et al. 13 In a previous report, 14 we achieved periodic swelling-deswelling changes of gels soaked in an autonomous pH-oscillating solution in a continuously-stirred tank reactor. However, gels that provide volume oscillation without external control in a closed solution have not been developed yet. In gel systems, the only oscillatory phenomenon known both theoretically and experimentally is sol/gel transition. [15][16][17] The BZ reaction is well-known for exhibiting temporal and spatial oscillating phenomena with periodic redox changes of the catalysts in a closed solution, and its significance has been recognized in understanding some aspects of life phenomena in transmission of information, pattern recognition and selforganization, etc. 18-21 We attempt to convert the chemical oscillation of the BZ reaction to the mechanical changes of gels and generate an autonomic swelling-deswelling oscillation under nonoscillatory outer conditions. Whereas hydrogels have been used as the BZ reaction medium in order to suppress ...

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.

Self-oscillation of polymer chains in an aqueous solution has been achieved. The ruthenium catalyst for the Belousov-Zhabotinsky reaction was polymerized by using N-isopropylacrylamide and dissolved into the solution containing the BZ substrates. Periodical soluble-insoluble changes of the polymer chain were spontaneously induced by the BZ reaction. The conformational oscillations of the polymer were measured as the optical transmittance changes of the solution. This is the first report that rhythmical and reversible soluble-insoluble changes of polymer chains are realized under constant and homogeneous conditions. The transducing system from chemical energy of the BZ reaction to optical information has been constructed.

We report on a beating polymer gel that exhibits periodical volume changes (swelling and deswelling) in a closed solution without external stimuli, like autonomous heartbeat. The mechanical oscillation is driven by the chemical energy of the oscillatory Belousov-Zhabotinsky (BZ) reaction. The gel is a copolymer gel of N-isopropylacrylamide (NIPAAm) in which ruthenium tris(2,2(')-bipyridine) [Ru(bpy)(3)], known as a catalyst of the BZ reaction, is covalently bonded to the polymer chain. The poly[NIPAAm-co-Ru(bpy)(3)] gel provides an open system where the BZ reaction proceeds, when immersed in an aqueous solution containing the reactants of the BZ reaction (with the exception of a catalyst). The chemical oscillation in the BZ reaction generates the periodical changes of the charge of Ru(bpy)(3) in the gel network between reduced [Ru(II)] and oxidized [Ru(III)] states. The gel swells at the oxidized state because the hydrophilicity of the polymer chains increases, while at the reduced state the gel deswells. Thus, the chemical energy is transduced into the mechanical energy to drive the polymer gel oscillation with a period of about 5 min, depending on the composition of the surrounding solution. The oscillation mode of the gel depends on its size scaled by the wavelength of the BZ pattern. Sufficiently small bead-like gels demonstrate isotropic beating. A large rectangular gel shows mechanical oscillation with a peristaltic motion coupled with the propagating chemical waves. The dynamic behavior of the chemical and mechanical oscillations have been analyzed with a model simulation. (c) 1999 American Institute of Physics.

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We succeed in measuring the oscillating force of cylindrical poly(N-isopropylacrylamide) gel having a side group of ruthenium tris(2,2′-bipyridine) (Ru(bpy)3) accompanied with the periodic redox reaction of Ru(bpy)3 2+/3+ , the Belousov-Zhabotinsky (BZ) reaction. The measurements are made for three gels with different diameters, 0.65, 1.17, and 1.88 mm, at 15 °C. It is found that the amplitude of oscillatory tensile stress of the reacting cylindrical gel with a diameter of 0.65 mm is much higher than the stress theoretically expected from the Donnan osmotic pressure. The amplitude is explained by an oscillatory change of the interaction parameter, χ, which might be induced by the hydration and dehydration of the chain due to the oscillatory charge density on the polymer chain. The oscillation behavior of the BZ reaction is found to be strongly dependent on the diameter of the gel. The staying period in the oxidative state and the period of the oscillation increase with a decrease in the diameter of the cylindrical gel.