A vesicular shuttle: transport of a vesicle within a flexible microtube

Micromanipulation of the external bilayers of nonionic surfactant vesicles (niosomes) and liposomes allows the formation of tethers, which are fluid state lipid/surfactant lamellar nanotubes. The technique allows investigation of some of the factors affecting tether formation and vesicle-tether interactions. In this paper the movement of a vesicle along, or more precisely in, tethers derived from the vesicle, has been studied. When a vesicle is supported by bipolar tethers, stretching the tether on one side of the vesicle, initiates the movement of the vesicle in the opposite direction, at velocities ranging up to 2.5 microm s(-1) thus creating a 'vesicular shuttle'. Movement of the vesicle occurs inside the tether structures, a process akin to the movement of a sphere in a flexible tube with a diameter much less than that of the sphere. The factors involved in the movement of vesicles in the tethers include the radial stretching and subsequently contraction of the tethers and the minimisation of elastic energies stored in the tether membranes. Vesicle velocity is not constant: there is deceleration near the end of the trajectory. While the relevance to the design of novel delivery systems is as yet tenuous, the system allows vesicle-vesicle collisions to be observed when the vesicle is propelled towards a stationary vesicle, and directly observation of the flow properties of vesicles in flexible 'capillaries', a neglected topic.