Reverse osmotic propulsion

Modern biomedical applications such as targeted drug delivery require a delivery system capable of enhanced transport beyond that of passive Brownian diffusion. In this work an osmotic mechanism for the propulsion of vesicles is proposed. By maintaining a solute gradient inside the vesicle, a seepage flow of the solvent (water) across the membrane is generated which in turn propels the vesicle. We develop a theoretical model for this vesicle-solute system in which the fluid flow through the membrane is described by a model similar to Darcy's law. Using this model, we characterize the motility of the vesicle in relation to the concentration distribution inside the vesicle. We show by explicit calculation that for a weakly permeable membrane the interior fluid flows from the regions of high solute concentration to low—-a reverse osmotic flow. Any osmotic solute is able to propel the vesicle so long as a concentration gradient is present. To maintain such a gradient, we propose to use active Brownian particles (ABPs) with spatially varying activity as the solute. By tuning the swim speed distribution of ABPs confined inside the vesicle, a spherically asymmetric density distribution can emerge and lead to net motion of the vesicle.