(603f) Optimization of the Lipid Vesicle Extrusion Process

Lipid vesicles – hollow spherical assemblies comprising one or more lipid bilayer – offer a platform that encapsulates, protects, and controllably delivers a wide variety of useful compounds across several sectors including pharmaceutical, agricultural, cosmetics, and food. In nearly all cases, it is desirable for lipid vesicles to be unilamellar (i.e., contain a single bilayer) and confined to a specific size range that depends on application. Vesicle extrusion, which consists of passing a lipid vesicle dispersion through a track-etched polycarbonate membrane, is among the more common methods for producing lipid vesicles with desirable characteristics. However, the optimal number of passes necessary to achieve desirable lamellarity and size is not well-developed including whether this information depends on lipid type or membrane pore size. We examined the structural attributes of lipid vesicles extruded through membranes between one and 1000 times using light scattering/transmission (SLS, DLS, transmittance), small angle x-ray scattering (SAXS), and cryo-transmission electron microscopy (cryo-TEM). This assessment was performed for vesicles composed of DOPC extruded through 50-nm, 100-nm, 200-nm, and 400-nm pore size membranes, as well as DOPC, DMPC, and Soy PC extruded through 100-nm pore size membranes. Interpreting structural results using convergence analysis allows us to identify that the optimal number of passes ranges from 10-50 with little trend across either factor considered. Though, the factor ultimately driving the optimal number of passes evolved with membrane pore size: experiments utilizing smaller pore sizes take more passes to achieve desired diameters whereas larger pore size extrusions require more passes to achieve lamellarity convergence. Overall, our results indicate that making 50 extrusion passes in lipid vesicle fabrication is a safe rule of thumb to yield the best possible lipid vesicles for any given PC lipid system.