As a new class of high-strength hydrogels, we designed a tetra-PEG gel by combining two symmetrical tetrahedron-like macromonomers of the same size. Because the nanostructural unit of the gel network was defined by the length of the tetrahedral PEG arm, the gel had a homogeneous structure and resultant high mechanical strength comparable to that of native articular cartilage. Furthermore, since the gel was formed by mixing two biocompatible macromonomer solutions, the gelation reaction itself and the resultant gel were also biocompatible. The breaking strength had local maxima at the overlap concentration of the macromonomers (C*) and at 2C*. Dynamic light scattering measurement indicated the near absence of inhomogeneities in the network at C*. Thus, we successfully designed and fabricated a high-strength hydrogel by controlling the homogeneity of network structure for the first time, which will lead to multiplied effects, i.e., contributing to the understanding of ideal networks, providing a universal strategy for designing high-strength gels, and opening up the biomedical application of hydrogels.

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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 ...

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