Phospholipid bilayers are viscoelastic

Lipid bilayers provide the structural framework for cellular membranes, and their character as two-dimensional fluids enables the mobility of membrane macromolecules. Though the existence of membrane fluidity is well established, the nature of this fluidity remains poorly characterized. Three-dimensional fluids as diverse as chocolates and cytoskeletal networks show a rich variety of Newtonian and non-Newtonian dynamics that have been illuminated by contemporary rheological techniques. Applying particletracking microrheology to freestanding phospholipid bilayers, we find that the membranes are not simply viscous but rather exhibit viscoelasticity, with an elastic modulus that dominates the response above a characteristic frequency that diverges at the fluid–gel (L α − L β ) phase-transition temperature. These findings fundamentally alter our picture of the nature of lipid bilayers and the mechanics of membrane environments.