Lipid Vesicles and Other Polymolecular Aggregates—From Basic Studies of Polar Lipids to Innovative Applications
Lipid vesicles (liposomes) are a unique and fascinating type of polymolecular aggregates, obtained from bilayer-forming amphiphiles—or mixtures of amphiphiles—in an aqueous medium. Unilamellar vesicles consist of one single self-closed bilayer membrane, constituted by the amphiphiles and an internal volume which is trapped by this bilayer, whereby the vesicle often is spherical with a typical desired average diameter of either about 100 nm or tens of micrometers. Functionalization of the external vesicle surface, basically achievable at will, and the possibilities of entrapping hydrophilic molecules inside the vesicles or/and embedding hydrophobic compounds within the membrane, resulted in various applications in different fields. This review highlights a few of the basic studies on the phase behavior of polar lipids, on some of the concepts for the controlled formation of lipid vesicles as dispersed lamellar phase, on some of the properties of vesicles, and on the challenges of efficiently loading them with hydrophilic or hydrophobic compounds for use as delivery systems, as nutraceuticals, for bioassays, or as cell-like compartments. Many of the large number of basic studies have laid a solid ground for various applications of polymolecular aggregates of amphiphilic lipids, including, for example, cubosomes, bicelles or—recently most successfully—nucleic acids-containing lipid nanoparticles. All this highlights the continued importance of fundamental studies. The life-saving application of mRNA lipid nanoparticle COVID-19 vaccines is in part based on year-long fundamental studies on the formation and properties of lipid vesicles. It is a fascinating example, which illustrates the importance of considering (i) details of the chemical structure of the different molecules involved, as well as (ii) physical, (iii) engineering, (iv) biological, (v) pharmacological, and (vii) economic aspects. Moreover, the strong demand for interdisciplinary collaboration in the field of lipid vesicles and related aggregates is also an excellent and convincing example for teaching students in the field of complex molecular systems.