The complexity of life on Earth stemmed from simplicity: from the earliest protocells to the growth of any organism, individual cells aggregate into basic clusters and then form more complex structures. Early cells lacked complicated biochemical machinery; to evolve into multicellular organisms, simple mechanisms were needed to produce chemical signals that prompted cells to move and form colonies.
Replicating this behavior in synthetic systems is necessary to advance fields such as soft robotics. Chemical engineering researchers from the Swanson School of Engineering at the University of Pittsburgh have established this feat in their latest breakthrough in biomimicry.
The research, “Realistic Behavior of Chemically Oscillated Moving Capsules,” was published in the journal Question. The lead author is Oleg E. Shklyaev, postdoctoral associate with Anna Balazs, Emeritus Professor of Chemical and Petroleum Engineering and holder of the John A. Swanson Chair of Engineering.
“We used a computer model involving red, blue and green capsules. With the addition of appropriate reagents, each capsule triggers one of three interconnected reactions that convert reactants into products. If the volume of the reactants is different from that of the products (as is frequently the case in biocatalytic reactions), the fluid will encompass density gradients, which spontaneously generate buoyant forces. The forces drive the flow of the surrounding solution and propel the submerged capsules,” says Shklyaev.
“Because of this dynamic behavior, the capsules are always experiencing new chemical environments and new neighbors. If the moving capsules are too far apart, ‘networking’ amounts to a constant exchange of chemical elements. chemical signalsallowing the capsules to ‘know’ of each other’s presence,” he continues. “If, however, the flow brings the three different types of capsules sufficiently close together, their chemical ‘communications’ become more involved, leading the ‘triad’ to undergo spatial and temporal chemomechanical oscillations.”
Namely, the simple system which initially featured a time-independent exchange of chemical signals self-organizes into a colony that displays chemomechanical oscillations, similar to the oscillations of chemoattractant cAMP in amoeba colonies or even the periodic beating of a heart living. The system exhibits realistic autonomy as the “fuel” for capsule movement is self-generated, and in turn, the spontaneous fluid movement triggers the capsules’ biomimetic and collective communications and oscillations. With reactants to initiate catalysis, the rest of the processes are accomplished by the system itself.
The interdependent specific reactions acting on the model capsules form a bio-inspired whole negative feedback loop (the “repressilator”), where each capsule suppresses chemical production by the next in the loop. The repressilator model has been used to successfully simulate and better understand communication (quorum sensing) in bacterial colonies. In the “dormant” state, when the capsules are far enough apart, the capsules coupled via the feedback loop do not exhibit oscillations, but instead produce constant chemical output and translational motion through the fluid . Eventually, the moving capsules come into contact with new neighbors and form a colony that exhibits a collective biomimetic response: an oscillatory chemical signal accompanied by the mechanical oscillations of the constituent parts.
Balazs notes that although their microcapsule system does not encompass any pattern, it appears to mimic fundamental biological functions due to the simple rules imposed on the system and the introduction of reagents (nutrients) into the solution. In other words, seemingly complex chemo-mechanical oscillations can result from simple mechanisms that occur inherently in chemical solutions.
“When developing remote systems and tiny machines, you want the systems to be as autonomous as possible, working without the need for complex programming and hardware,” she said. “We showed that simple chemical processes coupled with buoyant forces, which occur naturally in chemical solutions, give particles instructions to form complex systems and movements, potentially just as with early life forms.”
Oleg E. Shklyaev et al, Realistic behavior of chemically oscillating moving capsules, Question (2022). DOI: 10.1016/j.matt.2022.06.063
University of Pittsburgh
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