Using chemo-mechanical oscillations to mimic the behavior of protocells in fabricated microcapsules

The complexity of life on Earth was derived from its simplicity: from the earliest protocells through the growth of any organism, individual cells aggregate into simple clumps and then form more complex structures. The earliest cells lacked complicated biochemical machinery; Evolving into multicellular organisms required simple mechanisms to generate chemical signals that caused cells to both move and form colonies.

Replication of this behavior in synthetic systems is necessary to advance areas such as soft robotics. Chemical engineers at the University of Pittsburgh’s Swanson School of Engineering have demonstrated this feat in their latest advance in biomimicry.

The research paper “Lifelike Behavior of Chemically Oscillating Mobile Capsules” was published in the journal matter. The lead author is Oleg E. Shklyaev, a postdoctoral fellow with Anna Balazs, Distinguished Professor of Chemical and Petroleum Engineering and the John A. Swanson Chair of Engineering.

“We used a computer model with red, blue and green capsules. With the addition of appropriate reactants, each capsule initiates one of three interconnected reactions that convert the reactants into products. When the volume of the reactants differs from the products (as is often the case in biocatalytic reactions), the liquid will comprise density gradients that spontaneously generate buoyancy forces. The forces drive the flow of the surrounding solution and propel the submerged capsules,” explains Shklyaev.

“Due to this dynamic behavior, the capsules constantly experience new chemical environments and neighbors. “If the moving capsules are too far apart, the ‘networking’ amounts to an exchange of constant chemical signals, allowing the capsules to ‘know’ about the presence of others,” he continues When capsules are brought sufficiently close together, their chemical ‘communication’ becomes more involved, resulting in the ‘triad’ traversing chemo-mechanical vibrations in space and time.”

Namely, the simple system, which originally involved a time-independent exchange of chemical signals, self-assembles into a colony that exhibits chemo-mechanical oscillations, similar to the oscillations of the chemoattractant cAMP in amoeba colonies or even the periodic beating of a living heart. The system demonstrates lifelike autonomy as the “fuel” for the movement of the capsules is self-generated and the spontaneous movement of the liquid in turn stimulates the communication of the capsules and biomimetic, collective vibrations. With reactants to initiate catalysis, the remaining processes are performed by the system itself.

The specific interconnected reactions acting on the model capsules form a bio-inspired negative feedback loop (the “repressilator”) in which each capsule suppresses chemical production by the next in the loop. The repressilator model was used to successfully simulate and further understand communication (quorum sensing) in bacterial colonies. In the “rest” state, when the capsules are sufficiently far apart, the feedback loop coupled capsules do not exhibit vibrations, but rather produce a 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 biomimetic collective response: an oscillating chemical signal accompanied by the mechanical vibrations of the constituents.

Balazs notes that while their microcapsule system does not involve motifs, it appears to replicate basic biological functions because of the simple rules imposed on the system and the introduction of reactants (nutrients) into the solution. In other words, the seemingly complex chemo-mechanical oscillations can result from simple mechanisms inherent in chemical solutions.

“When designing remote systems and tiny machines, you want the systems to be as autonomous as possible and work without complex programming and hardware,” she said. “We have shown that simple chemical processes, coupled to buoyancy forces that arise naturally in chemical solutions, provide the instructions for particles to form potentially complex systems and movements, just as in the earliest forms of life.”