The development of a technique to measure the adhesion forces of receptor-ligand pairs under biocompatible conditions is the main focus of this thesis. The established reflection-interference contrast microscopy technique is upgraded to allow for measurements of the absolute distances between adhering membranes and solid surfaces. Giant vesicles are used to study the receptor-mediated adhesion to solid supported membranes. Introducing a soft polymer film between substrate and membrane makes it possible to attach receptors to surfaces without a considerable loss of their functionality. Absolute unbinding forces can be determined by the controlled detachment of the adhered giant vesicles from the substrate using magnetic tweezers. Moreover, the generic unbinding is shown to be a first order phase transition from an detailed analysis of the membrane fluctuations.
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