Magnetic vesicles
Résumé
The ability of certain lipids to self-assemble in water as bilayers is crucial for the properties of the outer membrane of biological cells and many of their internal structures as well (Golgi apparatus, endosomes, lysosomes, ...). Usually we call a vesicle such a bilayer membrane forming a closed shape with both an inner and an outer aqueous compartments separated by a hydrophobic barrier with a nearly zero permeability to hydrophilic solutes. This vesicular morphology well-known for liposomes prepared from either natural or artificial lipids was also obtained recently for polymersomes, which membrane is made of block copolymers[1]. It is worth noticing that unlike other manmade capsules or natural shells, the membrane of liposomes and polymerosomes is a 2-dimensional fluid. The result is a large flexibility of the shape and a mobility of the molecules within the membrane. The encapsulation of the magnetic nanoparticles of ferrofluids into vesicles was motivated by the idea of controlling their shape by an external magnetic field. Giant magneto-liposomes were first obtained by pre-hydrating a synthetic phospholipid (DOPC) with an aqueous “citrated” ferrofluid at pH7 and subsequent swelling of the bilayers by water[2]. Their diameters ranging from 5 to 20 um, it was possible to use optical microscopy to study their elongation in a homogeneous magnetic field and deduce the bending modulus of the DOPC bilayers Kb=20kBT from those measurements. In following experimental[3] and theoretical[4] works, it was shown that a magnetic field could produce either a prolate or an oblate ellipsoidal deformation of the magnetic liposomes depending on another control parameter important for ionic ferrofluids: the citrate salt concentration. When the concentration of those small ions is small indeed, the spontaneous curvature due to the difference between the Debye lengths on each side of the bilayer becomes sensitive to the presence of the nanoparticles inside and their absence outside. It was shown that the bending energy dominates over the magnetic energy in that case, and thus that an oblate flattening of the vesicle near the magnetic poles is preferred to the prolate elongation observed at high salinity. The citrate salt concentration in the outer medium of the giant magneto-liposomes was also used as an osmotic agent to measure the permeability of water through DOPC bilayers[5]. In other experiments, tubular giant magneto-liposomes were favoured by shearing the bilayers during their swelling by water[6]. The true morphology was rather like dumbbells because the cylindrical portions of membranes were connecting two quasi-spherical vesicles with a high concentration of ferrofluid inside. The deformation of such connected vesicles by an applied magnetic field is much larger than for isolated vesicles because the connected tubules act as reservoirs of lipids adding to the surface area of the spherical parts. Above a threshold field intensity, a pearling instability of the tubules is induced by the increase of the surface tension. Current researches on giant magneto-liposomes deal with their use as directly observable objects (under an optical microscope) to visualize the intermediate steps during the solubilization of lipid bilayers by n-octylglucoside (OG), a non ionic surfactant commonly used in pharmacology. By using the deformation experiment with a magnetic field to probe the membrane elasticity, it was shown that at bulk concentrations below the CMC, the penetration of surfactants in the bilayers causes a 10% relative increase of their surface area and a lowering of Kb down to 5 kBT. [7] At a lower (sub-micronic) scale, large unilamellar vesicles (LUV) were successfully prepared with a high encapsulation yield of magnetic nanoparticles[8] (Fig. 1), which could be used as new magnetic vectors In spite of the lack of experimental example, the deformation by a field of a vesicle with a fluid and magnetic membrane was addressed theoretically[9]. Having managed to confine magnetic nanoparticles in the membrane of polymersomes (Fig. 2) much thicker than lipid bilayers[10], we are now studying their response to a field.for drug targeting or contrast agents in MRI.