Optical Dynamometry to Study Phase Transitions in Lipid Membranes

The fluidity of the lipid matrix of cell membranes is crucial for the mobility of various inclusions like proteins. When the lipid bilayer undergoes phase transition from fluid-to-gel phase, the shear surface viscosity of the membrane diverges, thus hindering the motion of the membrane inclusions. On the other hand, the membrane bending stiffness drops down, and below the main phase transition, drastically increases with lowering the temperature. A tool to study the membrane properties when the lipid bilayer crosses the phase transition is provided by optical trapping and manipulation of microspheres attached to the membrane. Giant unilamellar vesicles are used, which allow for direct visualization of the membrane response, as model membranes. Following the motion of one or two particles attached to a vesicle, the microscope can provide evidence for the membrane elasticity and state of fluidity. As forces acting on the spheres, one can use gravity, thermal noise, or radiation pressure force.