We developed “self-oscillating” gels that undergo spontaneous cyclic swelling－deswelling changes without any on －off switching of external stimuli, as with heart muscle. The self-oscillating gels were designed by utilizing the Belousov －Zhabotinsky (BZ) reaction, an oscillating reaction, as a chemical model of the TCA cycle. We have systematically studied these polymer gels since they were first reported in 1996. Potential applications of the self-oscillating polymers and gels include several kinds of functional material systems, such as biomimetic actuators, mass transport systems and functional fluids. For example, it was demonstrated that an object was autonomously transported in the tubular self-oscillating gel by the peristaltic pumping motion similar to an intestine. Further, self-oscillating polymer brush surface was prepared and the dynamic behavior was evaluated. Besides, autonomous viscosity oscillation was realized via metallo-supramolecular terpyridine chemistry, etc. Self-oscillation between unimer/micelle or unimer/vesicle structures was also realized for a synthetic block copolymer. Here our recent progress on the self-oscillating polymer gels is summarized.

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Uniaxial elongation or compression has often been used to characterize the finite deformation behavior of polymer gels. We demonstrate that biaxial stretching experiments under varying the strains independently in the two directions provide stress-strain data in a wide range of strain revealing the essential features of nonlinear elasticity of polymer gels. We introduce the analysis of the biaxial stretching data for the three types of gel: Highly swollen hydrogels with a water content of 98%, Tetra-PEG gels with nearly uniform network structure, and slide-ring gels whose cross-links are movable along network strands.

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Polymer network in slide-ring gels consists of cyclic molecules and their dimers threaded with polymer chains end-capped. Polymer chains do not bind each other covalently, but topologically connected via the figure-of-eight shaped cross-links. Therefore, polymer chains in slide-ring gels can slide through the cross-links. As the result of chain sliding, slide-ring gels exhibit unique mechanical properties. In addition, a peculiar non-linear pressure-responsive permeability of solvents was found recently. Slide-ring gel is synthesized by cross-linking of a necklace-like mechanically-interlocked polymer, so-called polyrotaxane. For the first decade of slide-ring gels, the network structures, mechanical properties, and various functionalization have been studied intensively by using a single-species polyrotaxane. However, we have recently developed the synthesis of polyrotaxanes to diversify the chemical structures of slide-ring gels, with different backbone polymers, cyclic components, and the ratios of these two components. Here we outline a peculiar viscoelastic property in slide-ring gels found recently, and then introduce several molecular designs on polyrotaxanes for controlling the mechanical properties of slide-ring gels.

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It is well known that polymeric gels based on dynamic covalent chemistry can exhibit unique properties in response to external stimuli. Here we show fascinating properties of dynamic covalent polymer gels, in which reversible carbon－carbon bonds in equilibrium between dissociation and association at room temperature in air are embedded. The dynamic covalent polymer gels have the ability to reorganize their polymer networks autonomously at ambient temperature, and therefore, can show self-healing also due to the small bond dissociation energy. In addition, the tolerance of the blue-colored carbon radicals, which are formed from the cleavage of the carbon－carbon reversible bonds, toward oxygen enables the gels to show repeatable stress-visualization, particularly in response to the stress induced by coagulation of swelling solvents.

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Ionic liquids are ambient temperature molten salts that have attracted considerable attention, because of their unique property such as negligible volatility and thermal stability. In this review, progress, challenges, and opportunities of new polymer gels swollen by ionic liquids, named “ion gels”, are demonstrated. To form polymer networks in ionic liquids, we focus on the self-assembly of amphiphilic block copolymers in ionic liquids, based on micro-phase separation. The obtained ion gels exhibit high ionic conductivity, comparable with neat ionic liquids, together with soft solid consistency. We show here an example of the application as polymer electrolyte of ionic polymer actuators, combined with composite carbon electrodes. Moreover, stimuli-responsive polymers in ionic liquids are proposed. Solubility change of the responsive polymers in ionic liquids can be induced by temperature change and light irradiation. Block copolymers containing stimuliresponsive segments can realize thermo- and photo-reversible ion-gel formation induced by stimuli-sensitive self-assembly changes. Finally, we demonstrate photo-healable soft materials that can be repaired in response to an optical trigger from damaged states.

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