Reconfigurable assemblies of active, autochemotactic gels

Pratyush Dayal; Olga Kuksenok; Anna C. Balazs

doi:10.1073/pnas.1213432110

Edited by Joseph M. DeSimone, The University of North Carolina, Chapel Hill, NC, and approved November 19, 2012 (received for review August 2, 2012)

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 self-recombining 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.

Footnotes

↵¹Present address: Department of Chemical Engineering, Indian Institute of Technology, Gandhinagar 382424, India.
↵²To whom correspondence should be addressed. E-mail: balazs{at}pitt.edu.

Author contributions: P.D., O.K., and A.C.B. designed research; P.D., O.K., and A.C.B. performed research; P.D. analyzed data; and P.D., O.K., and A.C.B. wrote the paper.
The authors declare no conflict of interest.
*By intrinsic frequency of a small portion of the gel (with a given value of u in the outer fluid), we are referring to the frequency of the oscillations that an isolated gel of the same size would exhibit if it were placed in a fluid with the same value of u (23).
^†We assume that BrMA, in the presence of Ru(bpy)₃²⁺, is the only contributing factor to the photochemical production of bromide (Br) ions in the presence of visible light of wavelength ∼450 nm. The presence of oxygen can also contribute towards the production of Br ions (44). Importantly, however, it was also noted in ref. 44 that the concentrations of bromate and BrMA, which are the two major bromine compounds in the BZ reaction and are related to ɛ in our model, do not change appreciably during one chemical oscillation. We, therefore, assume that the value of ɛ is unaffected by light irradiation. In addition, we assume that the rate of change of the concentration of Br ions is negligible, even in the presence of illumination, based on the experimental evidence provided in ref. 44.
^‡A more complicated reaction mechanism for the light sensitivity of the Ru(bpy)₃²⁺-catalyzed BZ reaction was proposed in ref. 45, where the authors accounted for two separate processes: the photochemical production of bromide from bromomalonic acid and the photochemical production of bromous acid from bromate. Importantly, the authors noted that all the essential qualitative features of the dependence on the illumination intensity can indeed be reproduced with the modified Oregonator that considers only the first process (i.e., light induced production of bromide), but they also found more accurate quantitative agreement with their experiments if both processes are taken into account (45). Notably, while the concentration of the bromomalonic acid was assumed to be constant in the reaction scheme in ref. 45, it was latter added as an independent variable in a four-variable (46) and five-variable (47) Oregonator-class models.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1213432110/-/DCSupplemental.