Chemomechanical Self-Oscillatory Microgel Motility in Stratified Chemical Media.

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The design of chemomechanical self-oscillators, which execute oscillations in the presence of constant stimuli lacking periodicity, is a step toward the development of autonomous and interactive soft robotic systems. This work presents a simple design of prolonged chemomechanical oscillatory movement in a microgel system capable of buoyant motility within stratified chemical media containing spatially localized sinking and floating stimuli. Three design elements are developed: a stimuli-responsive membranized calcium alginate microgel, a Percoll density gradient for providing stratified antagonistic chemical media, and transduction of microgel particle size actuation into buoyant motility via membrane-mediated displacement of the Percoll media. The presence of citrate or calcium ions in different layers of the Percoll media gives rise to swelling (buoyancy) or contraction (geotaxis), respectively, which in turn mediate the shuttling of the microgels between the layers to produce prolonged or damped chemomechanical oscillatory trajectories. The concentration-dependence of the oscillatory behavior in the stratified media, the density gap between the Percoll layers, and the kinetic asymmetry of microgel swelling and deswelling are studied. The illustrated modular design allows for the development of chemomechanical self-oscillators responsive to light, pH, or temperature, which will find applications in interactive soft robotics, autonomous microbots, and intelligent materials.