(Invited) Anisotropic Self-Oscillating Reaction in Liquid Crystalline Nanosheets Hydrogels
Authors: Miyamoto Nobuyoshih , Shintate Morio , Inadomi Takumi , Hara Yusuke 
Published: February 2020

Soft and wet structures with anisotropy and hierarchy are regarded as key for various intriguing functions and properties of living organisms. Liquid crystal (LC) materials are often utilized to artificially fabricate such sophisticated structures likeliving organisms. For this purpose, lyotropic LC phases of inorganic nanosheets are emerging as new type LC materialsdue to many fascinating characteristics. Actually, inorganic nanosheet LCs have been utilized as unusual anisotropic reaction media,the materials to fabricate tough fibers and inorganic-polymer composites. The most distinctive feature of inorganic nanosheet LCs is that they can effectively barrier the diffusion of molecules. Since the nanosheets are two dimensional objects with huge aspect ratio (the lateral dimension of several micrometer and the thickness of ~1 nm), aligned nanosheets can effectively retard molecular diffusion compared to the materials made with conventional LC molecules. We recently communicated that dye ions anisotropically diffuse into the polymer gel embedded with macroscopically aligned LC nanosheets.1

Meanwhile, combining artificial soft and wet structures with particular chemical reactions that have characteristic features of biological systems is fascinating because such combinations will lead us to fabrication of artificial living organisms or molecular robots. Yoshida et al. reported the hydrogel of poly(N-isopropylacrylamide) (pNIPA) copolymerized with ruthenium trisbipyridine (Ru(bpy)3) units, which is a catalyst of self-oscillating Belouzov-Zhabozinsky reaction (BZ-reaction)2. Not only BZ reaction proceeded in this gel, but also the gel showed self-oscillating motion. After this pioneering work, many kinds of self-oscillating gels with improved properties such as faster and larger oscillation, were reported in view of applications for micro-robotics and micro electro mechanical systems (MEMS). The concentric circlesor rotating spiral waves that appear during BZ reaction is also interesting phenomena as a model of chemical waves in biological systemssuch as embryo, neural networks, and heart muscles. In this context, controlling and modifying the wave patterns in anisotropic, heterogeneous, or patterned media are interested. The BZ-reactions under electric field, in the lamellar phase of surfactant solution and in a porous glass were investigated to control the reaction. However, anisotropic BZ reaction has not been investigated so far.
In this study3, anisotropic chemical wave propagation of self-oscillating Belouzov-Zhabozinsky (BZ) reaction was demonstrated in the poly(N-isopropylacrylamide gel films embedded with macroscopically aligned liquid crystalline inorganic nanosheets. While the average propagation rate of chemical wave was 3.56 mm min-1in the gels without nanosheets, the propagation was retarded in the gels with 1 wt% of nanosheets: v//= 1.89 mm min-1and v⊥= 1.33 mm min-1along the direction parallel and perpendicular to the nanosheet planes, respectively. Thus, the wave propagation is anisotropic with the anisotropy ratio v// / v⊥= 1.42 in these gels and the periodic patterns formed by the BZ-reaction were concentric ellipses, different from circles seen in isotropic gels. Furthermore, the propagation rate and degree of anisotropy were controllable by nanosheet concentration. These phenomena can be explained that the diffusion of molecules inside the gel is effectively hindered along the direction perpendicular to the nanosheet planes due to very large aspect ratio of the aligned nanosheets.The present systems will be applicable for anisotropic self-oscillating soft actuators with one-dimensional motions as well as for ideal model system of BZ reactions.

Acknowledgement

This research was supported by: Research Center for Materials and Energy Devices of Fukuoka Institute of Technology (FIT-ME) (Strategic Research Foundation Grant-Aided Project for Private University from MEXT; #S1511036L); KAKENHI (#24104005, #15K05657, and #17H03209); Canon Foundation; Network Joint Research Center for Materials and Devices (#201507 and #20166009); and Electronics Research Laboratory of Fukuoka Institute of Technology.

References

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