New intelligent polymer gels: a self-oscillating gel with pacemaking and actuating functions

Abstract

We report an “intelligent” gel that undergoes autonomous swelling-deswelling oscillations without reliance on alterations in external conditions; this aspect reminds us of the heartbeat. The mechanical oscillation in our gel system was produced via an oscillating chemical reaction similar to a metabolic reaction, called the Belousov-Zhabotinsky (BZ) reaction. We have succeeded in synthesizing an ionic gel consisting of the cross-linkedN-isopropylacrylamide (NIPAAm) chain to which ruthenium tris(2,2′-bipyridine) [Ru(bpy)3] was covalently bonded. This Ru(bpy)3 complex acts as a catalyst for the BZ reaction; thus, the BZ reaction occurring within the gel matrix generates periodic redox changes of the catalyst moiety: Ru(bpy)3 2+⇆Ru(bpy)3 3+. This chemical oscillation is converted into the mechanical oscillation of the polymer network. As a result, the gel exhibits a periodic swelling-deswelling change. When the gel size is smaller than the chemical wavelength, the redox change occurs homogeneously in the gel. In this case, the volume change is isotropic and the gel beats as a whole, like a heart muscle cell. In the case of a large rectangular gel, the total length of the gel periodically changes with the propagation of chemical waves. The dynamic behavior that locally shrunken (or swollen) parts propagate is similar to the peristaltic motion observed in worms. This is the first study to achieve spontaneous volume oscillation of gels without external stimuli. Self-oscillation may be useful in a number of important applications to biomaterials, such as pulse generators or chemical pacemakers, self-walking actuators or micropumps with peristaltic motion, and devices for signal transmission.