Topology of multivesicular liposomes, a model biliquid foam

Freeze-fracture electron microscopy was used to characterize the microstructure of a novel multivesicular liposome (MVL) currently under investigation as a depot-type drug delivery vehicle. The interior of the MVL includes bilayer-enclosed, water-filled compartments surrounded by an encapsulating outer membrane. The contacts between compartments exhibit tetrahedral coordination analogous to a gas-liquid foam that persists to the bilayer level, and the compartments take on a variety of polyhedral shapes and sizes. Close examination of the distribution of polyhedra shows that it is nearly identical to theoretical predictions and computer simulations of polytetrahedral random close packing. As such, the MVL structure is one of the first experimental visualizations of the polytetrahedral random close packed molecular structure predicted for liquids and metallic glasses.