Sol-gel transition programmed self-propulsion of chitosan hydrogel

Self-propulsion of a calcium alginate surfer

A droplet of sodium alginate dripped into calcium chloride solution results in plate or boat shaped hydrogels. Both exhibit several minute-long self-propelled motion on the liquid surface without any extra fuel added, offering a new method to making active materials. By changing the initial concentrations, we are able to tune the transient dynamic activities from translational to rotational or stop-and-run motion. Dynamics are governed by osmotic pressure induced Marangoni effect, depending on the density difference and initial concentrations.

Recursively positive and negative chemotaxis coupling with reaction kinetics in self-organized inanimate motion

Reconstructing recursive chemotaxis in inanimate self-propelled objects is inevitable in the development of recursively and autonomously artificial mass transport systems. However, the fabrication of inanimately recursive chemotaxis has been extremely challenging because of the difficulty in introducing competitive positive and negative feedback into an inanimate self-propelled object. Herein, a coumarin derivative (coumarin, 4-methylcoumarin (4-MC), or 6-methylcoumarin (6-MC))-based disk floated on water as a self-propelled object exhibited characteristic features of motion; these features include continuous motion, repetition between positive and negative chemotaxis to the Na₃PO₄ powder as a base stimulus, and oscillatory motion above the Na₃PO₄ powder depending on the Na₃PO₄ density of the powder and the functional group of coumarin derivatives. The mechanism of the characteristic features of motion to the base stimulus is discussed in relation to the surface tension of the coumarin derivatives as the driving force of motion and the reaction rate of the hydrolysis between coumarin derivatives and OH^- obtained from Na₃PO₄. This study suggests a novel avenue for developing a recursive chemotactic system coupled with reaction kinetics in self-organized motion.