Changing mechanical properties of lipid vesicle bilayers investigated by linear viscoelastic measurements

The complex viscosity of lipid vesicle dispersions as a function of frequency was monitored as the vesicles aged. Of the two relaxation processes that occur in fresh vesicle dispersions, the first (i.e., longest) relaxation time did not shift, while the second (i.e., shortest) relaxation time proved to increase dramatically in time. The first relaxation process pertains to an entropic relaxation process in which redistribution occurs of a shear-disturbed distribution of hard spheres. The second relaxation time pertains to vesicle deformation where the surface shear modulus μ of the bilayer plays a dominant role. We will show that μ decreases several orders of magnitude as the vesicles age. With increasing age, a third relaxation process became measurable. It is inferred that this pertains to vesicle deformation where the surface dilatational modulus κ plays a dominant role. We found some boundary values for the surface shear modulus, surface dilatational modulus, and the curvature modulus for fully aged bilayers. The process responsible for the changed bilayer mechanical properties is the peroxidation of the lipids. This process causes unsaturated lipids to break and modify, imposing many changes on the constitution of the bilayer.