Biological membranes spatially deform in response to environmental stresses. It is not understood how adhesion to a support, such as a solid surface or a cytoskeletal network, influences this remodeling. Supported lipid bilayer (SLB) is a model membrane system consisting of a bilayer adsorbed to a solid surface. Using SLB, we quantitatively investigate membrane transformations from planar to tubular to spherical morphologies.

Spherical shells (vesicles) exposed to a solid surface adsorb and rupture to form planar bilayer. We investigated this process using fluorescence microscopy. Low lipid concentrations made it possible to resolve previously inaccessible features of vesicle adsorption and bilayer formation. In particular, these experiments revealed that, after a period of constant accumulation, vesicle adsorption accelerates as bilayer patches appear. As the patches spread and coalesce into a continuous SLB, unruptured vesicles desorb. Based on these observations, we hypothesize that the increased rate of adsorption and subsequent loss of vesicles can be attributed to the emergence and disappearance of bilayer edges. These results pertain to zwitterionic lipids and glass surfaces in particular, but we also demonstrate SLB formation using charged lipids and sapphire surfaces.

We tested this hypothesis using an SLB with excess bilayer edge. Such an SLB can be made based on the large mismatch in thermal expansivity between bilayer and glass, which causes the SLB to form holes upon cooling. Vesicles adsorb to the glass surface exposed by the holes at a faster rate and a higher density than on bare glass.

We further explore the transformation from planar to tubular morphologies. Small increases in temperature (∼1°C) expand the SLB more than its support, but the difference is small enough to relax via bilayer creep. Larger increases (∼5°C) result in an irreversible transformation in which semi-flexible filaments (worms) emerge from the SLB.

Individual worms are tubules of <1 microm diameter, but often reach hundreds of microns in length before spontaneously retracting into discs. At high ionic strength, the sub-resolution worms adhere to the SLB, enabling measurement of their radius to within +/-5 nm using fluorescence microscopy. Finally, we measure the retraction rate and comment on retraction disk morphology.