Force-controlled equilibria of specific vesicle-substrate adhesion.

We have developed "vertical" magnetic tweezers capable of exerting controlled pico and subpico Newton forces. Using this apparatus, we apply a point-like force to a vesicle that is adhered by means of specific bonds between the vesicle-carrying oligosaccharide sialyl LewisX and the surface-grafted E-selectin. An exponential decrease of the bound vesicle area with the decay rate that is insensitive to the force and the composition of the system is observed. We measure an equilibrium under force that is associated with an increased binding in the center of the contact zone. We also show that the determination of the shape is potentially sufficient to determine the number of formed specific bonds.