A microfluidic approach is reported for the high-throughput, continuous production of giant unilamellar vesicles (GUVs) using water-in-oil-in-water double emulsion drops as templates. Importantly, these emulsion drops have ultrathin shells; this minimizes the amount of residual solvent that remains trapped within the GUV membrane, overcoming a major limitation of typical microfluidic approaches for GUV fabrication. This approach enables the formation of microdomains, characterized by different lipid compositions and structures within the GUV membranes. This work therefore demonstrates a straightforward and versatile approach to GUV fabrication with precise control over the GUV size, lipid composition and the formation of microdomains within the GUV membrane.

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 dilatational rheology of the poly͑vinylacetate͒ monolayer onto an aqueous subphase with pHϭ2.0 has been studied between 1°C and 25°C. The combination of several techniques, relaxation after a step compression, oscillatory barrier experiments, electrocapillary waves, and surface light scattering ͑SLS͒ by thermal capillary waves, has allowed us to explore a broad frequency range. The relaxation experiments show multiexponential decay curves, whose complexity increases with decreasing the temperature. A regularization technique has been used to obtain the relaxation spectra from the relaxation curves and the dilatational viscoelastic parameters have been calculated from the spectra. The oscillatory barrier experiments confirm the results obtained from the step compression experiments. The dilatational viscosity increases very steeply in the frequency range 0.1-0.001 Hz. The shapes of the relaxation spectra follow the qualitative trends predicted a model recently proposed by Noskov ͓Colloid Polym. Sci. 273, 263 ͑1995͔͒. The temperature dependence of the fundamental relaxation time follows a Williams-Landel-Ferry equation above 14°C. These results correspond to the many-chain dynamics regime. The kilohertz region has been explored by the SLS technique. These results are compatible with the existence of a single Maxwell mode, with a relaxation time that has an Arrhenius-type temperature dependence. In the intermediate-frequency regime ͑10 Hz to 2 kHz͒ a further Maxwell process is found. It might correspond to the dynamics of loops and tails out of the surface plane.

Surface pressure isotherms and ellipsometric measurements of monolayers of two triblock symmetric copolymers, poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide) (PEO−PPO−PEO), at the air−water interface have been carried out. These copolymers are water-soluble, and the difference in hydrophobicity between the blocks is small. This represents a different scenario for brush formation than for most of the hydrophobic−hydrophilic block copolymers reported so far. The surface pressure curves show two different phase transitions. The ellipsometric measurements indicate a thickness transition when the monolayer saturates, which supports the hypothesis for brush formation. The experimental data have been analyzed in terms of the scaling theory of adsorption of polymer brushes. Despite the possibility of diffusion from the interface, the PPO block acts as an efficient anchoring element in the formation of an adsorbed brush, once the adsorption sites at the interface are fully occupied. This is analogous to what has been reported for diblock copolymers with a much larger difference in the hydrophobicity of the blocks.

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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 (PluronicsÒ 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.

Cardiolipin is a cone-shaped lipid predominantly localized in curved membrane sites of bacteria and in the mitochondrial cristae. This specific localization has been argued to be geometry-driven, since the CL’s conical shape relaxes curvature frustration. Although previous evidence suggests a coupling between CL concentration and membrane shape in vivo, no precise experimental data are available for curvature-based CL sorting in vitro. Here, we test this hypothesis in experiments that isolate the effects of membrane curvature in lipid-bilayer nanotubes. CL sorting is observed with increasing tube curvature, reaching a maximum at optimal CL concentrations, a fact compatible with self-associative clustering. Observations are compatible with a model of membrane elasticity including van der Waals entropy, from which a negative intrinsic curvature of −1.1 nm −1 is predicted for CL. The results contribute to understanding the physicochemical interplay between membrane curvature and composition, providing key insights into mitochondrial and bacterial membrane organization and dynamics.

We have studied the surface rheology of mixed solutions of anionic polyelectrolytes and cationic surfactants. Surface elasticity and viscosity exhibit a maximum at a surfactant concentration much smaller than that for solutions of the pure surfactant. Foaming and foam stability were found to depend on these coefficients, although a systematic relation is difficult to establish. Still more surprisingly, the behavior of foams and foam films is very different in these systems.