Strong adhesion of giant vesicles on surfaces: Dynamics and permeability

Evanescent wave induced fluorescence microscopy combined with phase contrast microscopy is applied to study the strong electrostatic adhesion of giant unilamellar vesicles (GUV) on flat surfaces. This technique gives the shape of the adsorbed vesicles and an insight into the dynamics of spreading. In addition, it is used to investigate the possible induction of membrane permeability by adhesion. We show that the vesicle shape on the surface is a spherical cap and that a three-regime process with characteristic times tau(1), tau(2) and tau(3) rules the dynamics of spreading. tau(1) is the time elapsed between the moment when the vesicle reaches the surface and the instant when the vesicle actually adheres. tau(1) stands between 10 s and one minute and is determined by the drainage of the liquid film between vesicle and substrate. tau(2) and tau(3) are the characteristic times of the biexponential law that fits the curve of the radius of the contact area as a function of time. tau(2) is about 0.05 s and corresponds to a regime of vesicle adhesion at constant volume. During tau(2), the negatively charged lipids of the outer monolayer of the membrane bilayer flow toward the surface while the excess area of the membrane is resorbed; the vesicle bilayer is then under tension. The regime ts corresponds to a regime of water permeation through the membrane. We find that 0.5 s < tau(3) < 5 s according to the experimental conditions. On average, vesicles lose 7% of their inner content through a process of pore formation along the contact line. The role of parameters such as vesicle size, medium viscosity, lipid composition, and surface charge on the adhesion phenomenon is investigated and analyzed.