Fast swelling/deswelling and dispersing/flocculating oscillations of autonomously oscillating hydrogel microspheres (microgels) at periods of single-order seconds were accomplished. The design of the oscillating microgels is based on the incorporation of neutral and hydrophilic acrylamide monomers into conventional microgels to suppress the irreversible aggregation of the microgels during oscillation at high substrate concentrations and high temperatures in the Belousov–Zhabotinsky (BZ) reaction. In contrast to conventional microgels, the aforementioned microgels can carry out the BZ reaction under optimized conditions without irreversible aggregation at higher temperature, resulting in short oscillation periods (∼7 s) that were obtained from optical transmittance measurements. Furthermore, a macrogel composed of organized microgels was prepared, which also showed a remarkably fast swelling/deswelling oscillation (oscillation period: ∼23 s). This new type of oscillating microgel could be applied to advanced materials, e.g. to autonomously oscillating micropumps that imitate the human heartbeat.

The Belousov–Zhabotinsky (BZ) reaction shows temporal or spatiotemporal structures such as redox oscillation of the catalyst, [ruthenium­(II)­tris­(2,2′-bipyridine)]­[PF6]2 ([Ru­(bpy)3]­[PF6]2). In this study, autonomously oscillating hydrogel microspheres (microgels) were investigated, which show swelling/deswelling oscillation induced by the redox oscillation of the BZ reaction inside the gel. Despite the periodically and autonomously induced oscillation that does not require an external stimulus, it has not been possible to perform any manipulation of the oscillatory behavior over time. The results of the present study show that it is possible to reversibly switch the microgel oscillations from an “on” active state of the BZ reaction to an “off” inactive state by changing the temperature in combination with thermoresponsive microgels. To realize on-demand switching, the construction of double-shell structures is crucial; the thermoresponsive first shell allows the microgels to modulate the diffusion of the substrates or intermediates in the BZ reaction, while the second shell maintains colloidal stability under high temperatures and high ion concentrations. The functionalized double-shell microgels were prepared via multistep seeded precipitation polymerization. The oscillatory switching behavior of the BZ reaction was observed directly and evaluated by ultraviolet–visible (UV–vis) spectroscopy. The central concept of this study, i.e., “on–off switching” can be expected to benefit the development of advanced bioinspired materials.

The swelling/deswelling oscillation of autonomously oscillating hydrogel microspheres (microgels) at a frequency close to that of the human heartbeat was accomplished. In this study, poly(oligoethylene glycol) methacrylate (pOEG-MA)-based microgels that contain a tris(2,2′-bipyridyl)ruthenium catalyst were used to create oscillatory chemical systems via the Belousov−Zhabotinsky (BZ) reaction. The oscillating pOEGMA microgels can undergo the BZ reaction without irreversible aggregation at elevated temperatures or high substrate concentrations. By optimizing the chemical composition of the microgels and the BZ reaction conditions, a short swelling/deswelling oscillation period of ∼1.3 s was achieved in the dispersed state. Furthermore, in the assembled state, the microgels exhibited fast swelling/deswelling on the order of seconds (oscillation period: ∼6.2 s). The central concept of this study, that is, "high-frequency oscillation", can be expected to benefit the development of advanced bioinspired actuators that imitate the human heartbeat.

論文要旨：高分子ヒドロゲル微粒子は温度や pH などの外部刺激に対して応答し，その物理化学的性質が 可逆的に変化する『刺激応答性』を有する。また，水溶液中で膨潤し，ブラウン運動をしており，分散／凝 集といったコロイド的性質も併せ持つ。筆者らは，これまで「高分子ゲル微粒子の次元制御とマイクロ空間 場における機能制御」というコンセプトのもと研究を続けており，その中には，非平衡系の中での自己組織 化を活用した機能制御を含んでいる。特に，温度応答性の高分子である poly(N-isopropyl acrylamide) (pNIPAm)から成るゲル微粒子は，その希薄分散液の乾燥後，間隔を空けて配列し，単層の薄膜が自発的 に形成される。筆者らはこの現象に注目し，その乾燥過程を追跡する事で，ゲル微粒子の自発的な構造形成 過程を明らかにしてきた。更に， 『液滴の乾燥』といった極めてシンプルな系とは異なり，この pNIPAm ゲ ル微粒子と化学振動反応(ベローゾフ・ジャボチンスキー(BZ)反応)のカップリングを試み，微粒子に 時間周期構造の付与を行う事で，周期的に体積や粒子間相互作用を変化させる新奇ゲル微粒子(自律駆動ゲ ル微粒子)の開発に成功している。本稿では，筆者らが検討している『ゲル微粒子の自己組織化』研究につ いて，上述した自律駆動ゲル微粒子の話題を中心に紹介し，その発展について述べる。 松井 秀介 信州大学大学院総合工学系研究科 生命機能・ファイバー工学専攻 後期 博士課程 1 年 〒 386-8567 長野県上田市常田 3-15-1 Shusuke MATSUI