Delayed Mechanical Response to Chemical Kinetics in Self-Oscillating Hydrogels Driven by the Belousov-Zhabotinsky Reaction.

Tunde Geher-Herczegh,Tsukuru Masuda,Zuowei Wang,Nandini Vasudevan,Ryo Yoshida,Yoshikatsu Hayashi

doi:10.1021/acs.macromol.1c00402

Tunde Geher-Herczegh, Tsukuru Masuda + Show 4 more

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https://doi.org/10.1021/acs.macromol.1c00402

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Journal: Macromolecules	Publication Date: Jul 2, 2021
Citations: 19	License type: CC BY 4.0

Affiliation: The University of Tokyo, University of Reading

Abstract

We show experimentally that chemical and mechanical self-oscillations in Belousov–Zhabotinsky hydrogels are inherently asynchronous, that is, there is a detectable delay in swelling–deswelling response after a change in the chemical redox state. This phenomenon is observable in many previous experimental studies and potentially has far-reaching implications for the functionality and response time of the material in future applications; however, so far, it has not been quantified or reported systematically. Here, we provide a comprehensive qualitative and quantitative description of the chemical-to-mechanical delay, and we propose to explain it as a consequence of the slow nonequilibrium swelling–deswelling dynamics of the polymer material. Specifically, standard hydrogel pieces are large enough that transport processes, for example, counterion migration and water diffusion, cannot occur instantaneously throughout the entire gel piece, as opposed to previous theoretical considerations. As a result, the volume response of the polymer to a chemical change may be governed by a characteristic response time, which leads to the emergence of delay in mechanical oscillation. This is supported by our theoretical calculations.

Highlights

Smart polymer materials that are nonliving yet exhibit complex “life-like” or biomimetic behaviors have been the focus of intensive research over the past decades, in the quest to broaden our understanding of how living systems function and how life could have emerged.[1−4] One branch of such smart materials is the extensively studied Belousov−Zhabotinsky (BZ) selfoscillating hydrogels, first synthesized in the 1990’s by Yoshida et al.,[5] that are capable of exhibiting a rich variety of physical− chemical and biomimetic behaviors[6−9] and show great promise as potential soft actuators, drug delivery systems, and other applications.[10,11]
We propose that CM delays may arise from the interplay among the diffusion rate-limited gel-solvent mixing process, chemical reaction oscillation, and gel volume fraction change, provided that we assume a nonequilibrium nature in the swelling−deswelling response, as opposed to the already existing theoretical considerations in the literature.[27−29] Previously, BZ gels have been described to have different equilibrium swelling ratios in their fully reduced and oxidized states (see, for instance, the systematic study by Masuda et al for the newer type of poly(NIPAAm-co-NAPMAm-co-Ru(bpy)3NAPMAm) BZ gels23), which results in volume changes when the chemical environment shifts between these redox states
We have shown experimentally that the autonomous chemomechanical self-oscillation in BZ hydrogels has an inherently delayed swelling−deswelling mechanical response with regard to periodic redox changes

Summary

Introduction

Smart polymer materials that are nonliving yet exhibit complex “life-like” or biomimetic behaviors have been the focus of intensive research over the past decades, in the quest to broaden our understanding of how living systems function and how life could have emerged.[1−4] One branch of such smart materials is the extensively studied Belousov−Zhabotinsky (BZ) selfoscillating hydrogels, first synthesized in the 1990’s by Yoshida et al.,[5] that are capable of exhibiting a rich variety of physical− chemical and biomimetic behaviors[6−9] and show great promise as potential soft actuators, drug delivery systems, and other applications.[10,11] They demonstrate spontaneous periodic swelling−deswelling changes known as chemomechanical selfoscillation, reminiscent of the rhythmic beating of cardiac cells, by utilizing the well-known BZ chemical reaction. A phenomenon in excitable nonlinear systems such as the BZ reaction, readily emerges in self-oscillating hydrogels as well, provided that the size of the gel is large enough (several millimeters).[12] By controlling the exact shape and size of gel samples, various two-dimensional patterns have been shown to evolve over time.[13] On the other hand, when samples are cut to small pieces (typically sub-millimeter, smaller than the wavelength of the propagating chemomechanical wave), another type of behavior, isotropic volume oscillation, becomes possible, that is, the gel swells and deswells homogeneously in all spatial directions.[14]

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