| Research Abstract |
In conventional actuating systems using stimuli-responsive gels, "on-off" switching of external stimuli is essential to instigate the action of the gel. Upon switching, the gels provide only one unique action, either swelling or deswelling. In contrast to these temporary responses to stimuli, there are many physiological phenomena in our body that continue their own native cyclic changes ; examples are heartbeat, brain waves, etc. If such self-oscillation could be achieved for gels, possibilities would emerge for new biomimetic intelligent materials to exhibit rhythmical motion such as molecular pacemaker. self-beating micropump. etc. In this project, we have developed such a self-oscillatlng gel. It spontaneously exhibits cyclic swelling and deswelling under constant conditions, requiring no "on-off" switching of extemal stimuli. Its action is similar to that of a beating heart muscke.
The mechanical oscillation of gel is produced via oscillating chemical reaction. called the Belousov-
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Zhabotinsky (BZ) reaction. The BZ reaction has a cyclic reaction network similar to TCA cycle which is the metabolic reaction in living systems. We have prepared an ionic gel consisting of the cross-linked poly (N-isopropylacrylamide) to which ruthenium tris (2, 2'-bipyridine), a catalyst for the BZ reaction, is covalently bonded. The BZ reaction occurring within the gel matrix generates periodic redox changes of the catalyst moiety. This chemical oscillation is converted into the mechanical oscillation of the polymer network through the change in hydrophilicity of polymer chains. As a result, the gel exhibits a periodical swelling-deswelling change. The oscillating behaviors of the gel have been analyzed in detail.
Further, the micro-actuator to undergo autonomic and periodic motion like living organism has been prepared by micro-fabrication technique. Two photolithographic methods ; (1)using photoinitiator and (2)using the polymer with azidophenyl groups, were employed to control the gel structure. Several shapes of micrometer-sized gels with the same pattern as photomask were synthesized. Optimal synthetic condition for polymerization has been investigated. And also, by using X-ray lithography technique (LIGA), a ciliary motion actuator made of the gel has been demonstrated. As well as these microactuator, self-oscillating polymer chains and nano-gel particles have been prepared as biomimetic nano-machines. Less
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