The concept of membrane fluidity usually refers to a high molecular mobility inside the lipid bilayer which enables lateral diffusion of embedded proteins. Fluids have the ability to flow under an applied shear stress whereas solids resist shear deformations. Biological membranes require both properties for their function: high lateral fluidity and structural rigidity. Consequently, an adequate account must include, in addition to viscosity, the possibility for a nonzero shear modulus. This knowledge is still lacking as measurements of membrane shear properties have remained incomplete so far. In the present contribution we report a surface shear rheology study of different lipid monolayers that model distinct biologically relevant situations. The results evidence a large variety of mechanical behavior under lateral shear flow.

The transbilayer diffusion of unlabeled ceramides with different acyl chains (C₆-Cer, C₁₀-Cer, and C₁₆-Cer) was investigated in giant unilamellar vesicles (GUVs) and in human erythrocytes. Incorporation of a very small percentage of ceramides (∼0.1% of total lipids) to the external leaflet of egg phosphatidylcholine GUVs suffices to trigger a shape change from prolate to pear shape vesicle. By observing the reversibility of this shape change the transmembrane diffusion of lipids was inferred. We found a half-time for unlabeled ceramide flip-flop below 1 min at 37 °C. The rapid diffusion of ceramides in a phosphatidylcholine bilayer was confirmed by flip-flop experiments with a spin-labeled ceramide analogue incorporated into large unilamellar vesicles. Shape change experiments were also carried out with human erythrocytes to determine the trans-membrane diffusion of unlabeled ceramides into a biological membrane. Addition of exogenous ceramides to the external leaflet of human erythrocytes did not trigger echinocyte formation immediately as one would anticipate from an asymmetrical accumulation of new amphiphiles in the outer leaflet but only after ∼15 min of incubation at 20 °C in the presence of an excess of ceramide. We interpret these data as being indicative of a rapid ceramide equilibration between both erythrocyte leaflets as indicated also by electron spin resonance spectroscopy with a spin-labeled ceramide. The late appearance of echinocytes could reveal a progressive trapping of a fraction of the ceramide molecules in the outer erythrocytes leaflet. Thus, we cannot exclude the trapping of ceramides into plasma membrane domains.

We report an experimental study on the mechanical and permeability properties of giant polymersomes made of diblock (PBD–PEO) and triblock (PEO–PPO–PEO) copolymers. These polymer amphiphiles bear the architecture and macromolecular dimensions adequate for assembling stable flat bilayers with a different hydrophobicity. In the highly hydrophobic case (PBD–PEO) an extremely compact membrane is formed, resulting in rigid polymersomes which represent a permeability barrier against solute transport across. In the case of water soluble PEO–PPO–PEO triblock copolymers, the bilayer structure is less stable in favour of the micellar state; therefore giant vesicles can be solely formed at large PPO contents. These cases (Pluronics® L121 and its mixtures with P85 and P105) are characterised by a much lower chain entangling than highly hydrophobic membranes, their polymersomes being softer than those based on PBD–PEO. Pluronic-based polymersomes are also found to be highly permeable to hydrophilic solutes, even remaining undamaged in the case of an extreme osmotic shock. This high permeability together with their high flexibility endows Pluronics polymersomes smart core/shell properties ideal to catch large biomolecules inside and able to resist under osmotic and mechanical stresses.