Negative Chemotaxis of Urease-Functionalized Polymeric Microcapsules.

Enzyme-driven micromotors provide a promising platform for mimicking biological chemotaxis and for developing responsive materials. We recently developed a method to make cell-sized asymmetric urease-functionalized poly(lactic-co-glycolic acid) (PLGA) microcapsules that are highly motile in the presence of urea. Here, we investigate the chemotactic behavior of these microcapsules in urea gradients in a chemotaxis chamber. We demonstrate that urease-functionalized microcapsules exhibit negative chemotaxis, migrating down urea gradients over a wide range of gradient magnitudes. The chemotactic flux increases with increasing urea gradients but saturates at high gradients, whereas increasing the average substrate concentration in a fixed gradient suppresses directional migration. We find that the direction of migration is independent of the orientation of the asymmetric particles. Buffered environments suppress chemotaxis, supporting a mechanism of motion that involves local pH changes or ionic self-diffusiophoresis. A transport model of substrate consumption demonstrates that chemotactic behavior correlates with the relative concentration difference across the particle, whereas particle speed scales with the absolute concentration/corresponding osmotic pressure difference. These results establish negative chemotaxis as a robust feature of urease-functionalized polymeric microcapsules and provide mechanistic insight into how enzymatic activity, substrate gradients, and ionic environments govern directional motion.