Kinetically Trapped Uniform Nano-Size Unilamellar Vesicles Made of Thermodynamically Stable Multilamellar Vesicular Phospholipid Solutions

Lipid-based carriers (liposomes) are available for delivering therapeutics.  Naturally, micron-size multilamellar vesicles (MLVs) are found in zwitterionic long-chain lipids in aqueous solutions.  The lamellar spacing of these MLVs does not swell upon dilution.  In order to produce uniform, nano-size unilamellar vesicles (ULVs), which have higher loading capacity for hydrophilic drugs, multi-stage extrusion is commonly applied to MLV solutions.  The mass production of ULVs through this method is labor intensive and sometimes membrane fouling and rupture can be problematic.  It has also been investigated that doping short-chain and charged lipids into the zwitterionic long-chain lipids results in uniform nano-sized ULVs at increased temperature (i.e., T > T_M, the melting transition temperature of the long-chain lipid).  The ULVs are found to be thermodynamically stable.  

In this report, we will demonstrate that kinetically trapped ULVs are attainable in a lipid mixture, where MLVs are thermodynamically stable.  The lipid solutions initially form uniform nano-discs, which undergo a slow self-folding process into ULVs, presumably related to the rigidity of the bilayered membrane induced by charged lipids.