Stretching Polymer and Lipid Membranes

Abstract

Polymer and lipid membranes can be used as model membranes to mimic real cells or in bio-compatible devices. Both materials self-assemble into vesicles and are similar in their ability to fuse to hydrophilic substrates and form patches with distinct heart or circular shapes. They differ in their composition and size, with polymers showing reduced fluidity and hydration repulsion forces behaviour at patch edges.
Stretching experiments are widely used to study mechanosensory signalling and physical process in cells and model membranes. A custom device to study lipid and polymer membrane stretching in uniaxial and biaxial directions was designed. This device was used in conjunction with fluorescence and atomic force microscopy to study membranes under strain.
Lipids under biaxial stretch open roundish pores and the membrane slides at the edges to accommodate stretch. Conversely, polymers under mechanical stretch open pores which are strongly affected by the friction coefficient between the membrane and substrate, and the tension diffusion in the membrane. This causes them to form roundish to dendritic structures under different temperatures and stretching rates. The immobile fraction which is left behind during stretching reorganises into micelles creating scars after the strain has been released.
Under uniaxial strain, continuous lipid bilayers open oval pores in contrast to circular pores in simulation prediction and pore theory. The membranes show stretching rate dependence with the distance between pores smaller at faster rates, in agreement with tension dissipation in the membrane.