Using microcantilevers to study the interactions of lipid bilayers with solid surfaces.

We report the use of free-standing microcantilever beams, which have been used as an ultrasensitive method for measuring the surface free energy changes on a substrate induced by the adsorption of thin films, to probe the interactions between a solid surface and a phospholipid bilayer. We relate the observed deflection of a cantilever to the changes in the surface free energy of the solid surface which supports the phospholipid bilayer. We observe that the deflection is influenced by electrostatic and intermolecular interactions of the bilayer with the substrate. Increasing the surface charge density in the supported lipid bilayer (SLB), by increasing the ratio of cationic to zwitterionic lipids in bilayer, resulted in an increase in cantilever deflection. The surface free energy changes due to lipid transfer between anionic unilamellar vesicles and a cationic supported bilayer were also observed using microcantilevers. Finally, the adsorption free energy of a mixed lipid and cholesterol bilayer was measured demonstrating a detectable decrease in affinity between the phospholipid bilayer and the solid surface as a result of the addition of cholesterol. Our results reveal a new technique to probe the adsorption free energy of a SLB system as a function of the interactions governing the structure of supported lipid membranes.