Helical-Shaped Self-Oscillating Gels Showing Autonomous and Magnified Mechanical Oscillation

Helical structures exist across multiple length scales, from DNA, collagen fibrils, and bacterial flagella to plant tendrils. They are important in living organisms in a biological context because of their unique properties, such as traveling and sensing. From an engineering point of view, these helical structures can be used in microscale soft robots.<br/>"Self-oscillating" gels have become a distinguished class of smart soft materials because they undergo spontaneous and cyclic swelling-deswelling changes without on-off switching of external stimuli.[1] However, the practical applications of self-oscillating gel are still limited due to a small change in the degree of swelling (deswelling) and non-directional actuation by isotropic deformation.<br/>Herein, to improve the degree of deformation of gels, we report an unprecedented method to convert an isotropic deformation into anisotropic and unidirectional deformation of gel. As a result of introducing a three-dimensional periodic gradient structure by photopolymerization into the gel network, a helical-shaped gel could be synthesized. The helical-shaped gels exhibited amplified uniaxial winding and unwinding during the Belousov–Zhabotinsky (BZ) reaction. In addition, the helical-shaped gels showed a significant change in length according to self-oscillation compared to the conventional rod-shaped gel. When one end of the gel is fixed, the helical-shaped gel repeatedly performs winding and unwinding movements without fatigue with a large amplitude (~14%), even at a constant temperature and without applying external stimuli. When the gel is floated on the water surface, and a BZ reaction occurs spontaneously inside the gel, it reciprocates at a constant speed. This presentation will discuss synthetic strategy, structure analysis, and improved self-oscillation of helical-shaped gels.<br/><br/>Reference<br/>[1] R. Yoshida <i>et al., J. Am. Chem. Soc.,</i> <b>118</b>, 5134 (1996).