Designing Dual-functionalized Gels for Self-reconfiguration and Autonomous Motion.

Olga Kuksenok,Anna C Balazs

doi:10.1038/srep09569

Abstract

Human motion is enabled by the concerted expansion and contraction of interconnected muscles that are powered by inherent biochemical reactions. One of the challenges in the field of biomimicry is eliciting this form of motion from purely synthetic materials, which typically do not generate internalized reactions to drive mechanical action. Moreover, for practical applications, this bio-inspired motion must be readily controllable. Herein, we develop a computational model to design a new class of polymer gels where structural reconfigurations and internalized reactions are intimately linked to produce autonomous motion, which can be directed with light. These gels contain both spirobenzopyran (SP) chromophores and the ruthenium catalysts that drive the oscillatory Belousov-Zhabotinsky (BZ) reaction. Importantly, both the SP moieties and the BZ reaction are photosensitive. When these dual-functionalized gels are exposed to non-uniform illumination, the localized contraction of the gel (due to the SP moieties) in the presence of traveling chemical waves (due to the BZ reaction) leads to new forms of spontaneous, self-sustained movement, which cannot be achieved by either of the mono-functionalized networks.

Highlights

Human motion is enabled by the concerted expansion and contraction of interconnected muscles that are powered by inherent biochemical reactions
The remarkable feature of the BZ gels[8,9] is that they are the only known polymer networks that undergo periodic swelling and deswelling in the absence of external stimuli and an imposed flow. (Other remarkable examples of oscillating gels include pH-responsive systems based on the bromate–sulfite (BS) reaction that undergo oscillations in a continuously-stirred tank reactor[10] or gel-based oscillators that occur due to a chemo-mechanical feedback mechanisms in multi-component systems such as hydrogel-enzyme oscillators[11,12,13] or self-regulating, homeostatic materials14.) When Ru(bpy)[3] is grafted to a poly(NIPAAm) gel and the material is immersed in a solution of BZ reagents, the ensuing chemical reaction provides the fuel for this BZ gel’s rhythmic mechanical oscillations
The gel swells as the Ru catalyst is oxidized and collapses when Ru is reduced by the oscillating reaction. These mechanical oscillations are accompanied by traveling chemicals that propagate throughout the sample

Summary

Methods

We modified our gel lattice spring model, or gLSM20–22, which was originally developed to capture the dynamic behavior of BZ gels[8,23,24,25,26]. This can be captured by a modified version[20,21] of the two-variable Oregonator model[35,36] that explicitly accounts for the polymer volume fraction, w20,21. The chemical reactions within the SP-BZ gels occur simultaneously with the following three dynamic processes: 1) the movement of the grafted oxidized catalyst, v, and chromophores, cSP, with the polymer at the velocity v(p), 2) the transport of the activator for the BZ reaction, u, along with the solvent at the velocity v(s) 5 2w/(12w)v(p), and 3) the diffusion of the dissolved activator u throughout the polymer network with a diffusion flux given by[21] j(u) 5 2(12w)=(u(12w)[21]). The dynamics of the SP-BZ gels can be described by the following set of equations: LcSP Lt

Lu Lt

Additional information