Force spectroscopy on adhesive vesicles

We propose a method to measure weak point forces exerted on giant vesicles adhering to solid surfaces by submicron pinning centers under conditions of thermal equilibrium. The method is based on the analysis of the shape of the contact line between vesicle and substrate using reflection interference contrast microscopy. Force levels below one pN could be measured, considerably below those accessible to available force probes. We illustrate the procedure with a study of the adhesion of giant vesicles adhering to a supported membrane by biotin-streptavidin-biotin linkages.