Self-Oscillating Gel

Abstract

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 ...

Chemo-responsive, self-oscillating gels that undergo biomimetic communication

“…4,5,8,[37][38][39][40] This polymer network is swollen in an aqueous solution of sodium bromate (NaBrO 3 ), nitric acid (HNO 3 ), and malonic acid (MA); MA acts as an organic substrate for the BZ reaction. The kinetics of the BZ reaction involves two dozen reactive species and tens of chemical reactions.…”

“…Hence, in a Petri dish, an isolated BZ gel sample is seen to pulsate like a beating heart. 4 The BZ gels 4,5,8,9 are truly unique: to date, there are no other gels that oscillate in the absence of external stimuli. Notably, there are various examples of polymer gels that can be driven to pulsate through a periodic change in the local environment, which is accomplished through the use of a continuously stirred tank reactor or controlled variations in external cues.…”

“…It was only in 1996 that Yoshida et al fabricated the first BZ reaction to take place in a responsive gel. 4 In that pioneering study, the ruthenium tris(2,2 0 -bipyridine) complex, Ru(bpy) 3 , which is known to be a catalyst for the BZ reaction, was incorporated as a pendent group on cross-linked polymer chains of NIPAAm. The polymer network with the attached metal-ion catalyst was swollen in an aqueous solution of the BZ reagents: NaBrO 3 , HNO 3 , and malonic acid.…”

“…4,18 Inclusion of the Ru(bpy) 3 complex affects the degree of swelling of the gel to an extent that depends on the electric charge of the metal ion. Oxidation of the catalyst has a hydrating effect, and the gel swells.…”

Self-oscillating gel as novel biomimetic materials

“…Consequently, periodic redox changes induced by the BZ reaction produce periodic swelling-deswelling changes in the gel (Fugure 1). Since being first reported in 1996 as a "self-oscillating gel" (Yoshida et al, 1996;1997), the author have been systematically studying the self-oscillating polymer and gel as well as their applications to biomimetic or smart materials (Figure 2). In fact, as applications to autonomic biomimetic actuators, ciliary motion actuators (Tabata et al, 2002(Tabata et al, , 2003 and selfwalking gels (Maeda et al 2007), etc.…”