Nano- to Micro-Scale Communication in Biomimetic, Chemically Active Posts

The rich, collective dynamics exhibited by biological cilia has prompted researchers to probe cooperative behavior in synthetic analogues, i.e., arrays of closely spaced, stimuli-responsive gel posts that are tethered to the surface of a fluid-filled chamber. For these dense arrays, cooperative behavior is facilitated not only by action of external stimuli, but also by the close proximity, which enables physical contact between the neighboring posts. There has, however, been little attention paid to cilia-like arrays in the opposite limit, where the tethered posts lie relatively far apart and do not rely on external fields for their actuation. Such studies are key to designing biomimetic synthetic systems that can spontaneously transmit long-range signals to distant components and thereby achieve greater control over the entire system. We model arrays of well-separated elastic posts in solution that are solely responsive to dissolved chemical reactants. A few of these posts are coated with a catalyst and thus, are chemically active; the remaining non-coated posts are passive. Through theory and simulation, we find that chemical reactions occurring at the immersed, active posts generates a surprising variety of long-range, cooperative interactions in a sparse array, including stable and unstable behavior; damped and highly damped oscillations; and non-reciprocal self-oscillations. Designing self-oscillating chemical systems, which convert constant input into oscillating output, remains a significant challenge. Oscillatory behavior is, however, vital to achieving self-regulation in synthetic materials; oscillating systems are constrained to operate within a certain parameter range and thereby maintain normal operating conditions.<br/>The dynamic versatility and self-regulation found in these systems allow, for example, one non-oscillating active post to initiate synchronized oscillations between two passive ones. Furthermore, one active oscillating post and two passive ones can communicate to synchronize the oscillations among all three. We also find that a single array with two chemically different active posts can simultaneously produce and sustain different modes of motion in the array. Moreover, with different catalyst coatings, one can address and actuate individual posts by selectively adding the appropriate reagent to trigger the specific reaction. For this system, the combination of chemistry, hydrodynamics and fluid-structures causes the array to propagate a distinct message; each post to interpret the message; and the system to respond with a specific mode of organized, collective behavior. This level of autonomous, remote control is relatively rare in synthetic systems, particularly as this system operates without external electronics or power sources. Eliciting the system’s dynamics only requires the addition of chemical reactants.<br/><br/>The behavior of these chemically active posts is driven by solutal buoyancy, which arises as the catalytic reactions at the active posts transform reactants into products. If the volumes occupied by the reaction products are different from those of the initial reactants, then the solution will encompass local gradients, which generate forces that act on the confined fluid, the posts and walls of the chamber. The chemically generated flow in turn performs mechanical work as it actuates and deforms the compliant pillars. The flowing fluid continues to advect the dissolved chemicals to the active posts and through the above cascade of interlocking events sustains this chemo-mechanical coupling.