Adhesion of Latex Spheres to Giant Phospholipid Vesicles: Statics and Dynamics

(Received 24 February 1997, revised 23 June 1997, accepted 15 July 1997)

Abstract
We studied the sequence of phenomena which occur when a solid microsphere is brought in contact with an isolated giant lipid vesicle. We used Latex beads, a few microns in diameter, which were manipulated individually by means of a long-working-distance optical trap. The evolution of the bead/vesicle system was characterized in time, from ~ 1 ms to ~ 100 s. In this time range, we identified different steps, namely adhesion, ingestion, expulsion and recapture. In the adhesion step the sphere moves quickly in direction to the vesicle interior and the surface of the particle becomes wetted by lipids. We propose a simple model, based on the counter-balance between adhesion and stretching of the lipid lamella, which explains the experimental equilibrium configuration. The bead/vesicle configuration after the adhesion step pertains to partial or complete wetting, depending on the initial vesicle state. Partial wetting can be followed by a second step, which we named "particle ingestion", and which leads to complete (or nearly complete) wetting of the particle surface. Ingestion is characterized by a further penetration of the particle across the vesicle contour, in concomitance with a decrease of the vesicle size. The phenomenon is attributed to the occurrence of a dynamically stabilized pore across the membrane, which allows part of the water initially inside the vesicle to flow out. Ingestion can be followed by a back and forth movement (expulsion and re-capture) of the particle. In the ultimate configuration, the solid surface is totally wetted by lipids, however with a finite contact angle between the membrane and the solid surface.