Periodic minimal surface structures in bicontinuous lipid–water phases and nanoparticles

The accumulated evidence for the existence of periodic minimal surface (PMS) bilayer structure of the bicontinuous cubic phases in lipid–water systems is summarized. There are three fundamental PMS: the G, D, and P types. These three and no others have been observed in lipid–water systems. Due to the so-called Bonnet relation, the PMS structures are isometric conjugates to one another at coexistence, which determines the cubic axis ratio and water content at equilibrium. A number of lipid systems exhibiting coexisting cubic phases have been examined in relation to fulfilling the Bonnet relation. In all cases with known unit cell dimensions, the phases are Bonnet-related. This is regarded as strong evidence for their PMS structure. The phase transitions in relation to swelling and water content also fulfill the Bonnet relations. Aside from bulk phases with PMS structure, it has also been shown possible to produce nanoparticles with internal PMS structure. New developments enable the manufacturing of such particles with uniform size and internal structure, of interest in practical applications such as in vivo delivery of drugs.