Mechanical behavior of a supported lipid bilayer under external shear forces

Shear forces from a pressure-driven bulk flow in a microfluidic channel can be used to induce and control the motion of a supported lipid bilayer (SLB) formed on the walls of the channel. We here present a theoretical model that relates the experimentally observed drift velocities of an egg yolk phosphatidylcholine (egg PC) SLB to the hydrodynamic drag force from the bulk flow, the intermonolayer friction coefficient, b, of the bilayer, and the friction coefficient, bls, between the lower leaflet of the bilayer and the supporting substrate. The drift velocity and diffusivity of the lipids in the SLB were obtained by photobleaching a delimited area of fluorescently labeled lipids and subsequently monitoring the recovery and convective motion of the bleached spot. A striking observation was that the drift velocity of the lipids was observed to be nearly 6 orders of magnitude smaller than the bulk velocity at the center of the channel. This predicts a value for bls that is at least 25 times as high as predicted by the traditional model with the SLB and the support spaced by a homogeneous 1 nm thick film of water. In addition, the intermonolayer friction coefficient was estimated to 2x10(7) Pa s/m, a value that increased after addition of glycerol to the bulk solution. This increase was accompanied by an equal decrease in the lipid diffusivity, with both observations indicating an increased viscous drag within the bilayer when glycerol was added to the bulk solution.