(159i) Surface Energy and Adhesion of Phospholipid Bilayers

The phospholipid bilayer is a major component of the cell membrane where it serves as a boundary to protect the interior of cells and to regulate molecular transport. Within the bilayer, the bilayer surface energy (γ_b) and critical interleaflet bilayer adhesion force (F_cr) (the force per length that maintains the structure of two lipid leaflets adhering together) are fundamental properties for understanding cell and vesicle mechanics. In this work, a modified droplet interface bilayer (DIB) technique has been utilized to measure γ_b and F_cr. In this method, bilayers are created when pendent and sessile drops covered with phospholipid monolayers are brought together. Unlike previous DIB methods where conjoined droplets are observed in a quiescent state, the present procedure actively tugs on the droplets, causing them to deform and for the bilayer to progressively unzip. For the γ_b determination, an energy conservation analysis is undertaken, which yields bilayer surface energies that are in agreement with previous methods. For the F_cr determination, the approach is to unzip the resulting bilayer formed at the interface of the drops step by step by slowly pulling the drops apart. With this sequential separation of aqueous pendent and sessile drops, the F_cr is determined through a force balance at the edge of the bilayer interface up until complete bilayer leaflet detachment. Both the surface tension and force measurements are reproducible and independent of droplet size.