Reconfigurable assemblies of active, autochemotactic gels

Abstract

Using computational modeling, we show that self-oscillating Belousov-Zhabotinsky (BZ) gels can both emit and sense a chemical signal and thus drive neighboring gel pieces to spontaneously self-aggregate, so that the system exhibits autochemotaxis. To the best of our knowledge, this is the closest system to the ultimate selfrecombining material, which can be divided into separated parts and the parts move autonomously to assemble into a structure resembling the original, uncut sample. We also show that the gels' coordinated motion can be controlled by light, allowing us to achieve selective self-aggregation and control over the shape of the gel aggregates. By exposing the BZ gels to specific patterns of light and dark, we design a BZ gel "train" that leads the movement of its "cargo." Our findings pave the way for creating reconfigurable materials from self-propelled elements, which autonomously communicate with neighboring units and thereby actively participate in constructing the final structure.self-oscillating gels | autochemotactic gels | autochemotactic self-organization S pecies ranging from single-cell organisms to social insects can undergo autochemotaxis, where the entities move toward a chemo-attractant that they themselves emit. This mode of signaling allows the organisms to form large-scale structures, with amoebas (1) and Escherichia coli (2) self-organizing into extensive multicellular clusters and termites constructing macroscopic mounds (3). Notably there are few equivalents of such autochemotactic-driven assembly in the synthetic world. Although researchers have devised a range of nano-and microscopic selfpropelled particles (4), hardly any exhibit autochemotaxis (5) that leads to the formation of extended structures (6). Although recent theoretical models provide insight into the autochemotaxis of a self-propelled walker (7) and active Brownian particles (8), these studies provide few guidelines for synthesizing specific materials that display autochemotactic self-organization. The latter materials would open new routes for dynamic, reconfigurable self-assembly, where self-propelled elements communicate with neighboring units and thereby actively participate in constructing the final structure. Herein, we use computational modeling to show that millimeter-sized polymer gels can display such self-sustained, autochemotatic behavior. In particular, we demonstrate that gels undergoing the self-oscillating Belousov-Zhabotinsky (BZ) reaction (9) not only respond to a chemical signal from the surrounding solution, but also emit this signal and thus, multiple neighboring gel pieces can spontaneously self-aggregate into macroscopic objects. These findings indicate that BZ gels can undergo a form of "selfrecombining": if a BZ gel is cut into distinct pieces and the pieces are moved relatively far apart, then their autochemotactic behavior drives the parts to move autonomously and recombine into a structure resembling the original, uncut sample (Fig. 1). We also show that the gels' coordinated motion can ...

Chemo-responsive, self-oscillating gels that undergo biomimetic communication

“…The following reaction schemes apply to a cross-linked polymer network of poly(N-isopropylacrylamide) (NIPAAm) in which the catalyst ruthenium tris(2,2 0 -bipyridine) (Ru(bpy) 3 ) is anchored onto the chains. 4,5,8,[37][38][39][40] This polymer network is swollen in an aqueous solution of sodium bromate (NaBrO 3 ), nitric acid (HNO 3 ), and malonic acid (MA); MA acts as an organic substrate for the BZ reaction.…”

“…In these studies, we use the computational model outlined in Section 4.1; the model, however, was recently modified 3 to prevent the freely moving pieces from colliding and penetrating into each other. In particular, we introduce an inter-gel interaction, 3 which is based on the repulsive part of the Lennard-Jones potential, between nodes on different gels that lie within a critical distance of each other. By introducing these excluded volume interactions between mesoscopic pieces, we can readily capture the late-time behavior of the system, where the gels self-aggregate into larger structures with the individual samples oscillating in close proximity to each other.…”

“…We note that while researchers have developed a range of nano-and microscopic self-propelled particles, 28 few of these exhibit auto-chemotaxis, 29 and even fewer 30 use this mechanism to self-organize into large-scale structures. Research on synthetic auto-chemotaxing particles is still in its infancy, but recent advances 2,3 point to promising efforts in the creation of mechanically compliant materials that appear to communicate and effectively collaborate. Separate advances in automated responses for self-healing polymeric structures 31,32 and actuating structures 33 have achieved impressive macroscopic motion with nonfunctional polymers, which may eventually be augmented by use of functional, autochemotactic materials.…”

“…4 In that pioneering study, the ruthenium tris(2,2 0 -bipyridine) complex, Ru(bpy) 3 , which is known to be a catalyst for the BZ reaction, was incorporated as a pendent group on cross-linked polymer chains of NIPAAm. The polymer network with the attached metal-ion catalyst was swollen in an aqueous solution of the BZ reagents: NaBrO 3 , HNO 3 , and malonic acid. The ensuing BZ reaction generated a wave of swelling and de-swelling that propagated along the length of the BZ gel.…”

“…4,18 Inclusion of the Ru(bpy) 3 complex affects the degree of swelling of the gel to an extent that depends on the electric charge of the metal ion. Oxidation of the catalyst has a hydrating effect, and the gel swells.…”