A liposome is a spherical-shaped vesicle that is composed of one or more phospholipid bilayers, which closely resembles the structure of cell membranes. The ability of liposomes to encapsulate hydrophilic or lipophilic drugs have allowed these vesicles to become useful drug delivery systems.
Liposome - Image Credit: CiplaMEDVideo Membrane phospholipids
Cell membranes are typically composed of a phospholipid bilayer. This bilayer contains a hydrophilic, or “water loving” head group, and a lipophilic, or “fat loving” tail, which is composed of a long hydrocarbon chain that repels by water. Phospholipids are therefore classified as amphipathic molecules as a result of the presence of both hydrophilic and hydrophobic components.
When the phospholipid bilayer cellular membrane is exposed to water, the head group is attracted to water and forms a surface facing the water. At the same time, the lipophilic tails are repelled by water and subsequently form a surface that opposes the water. Within a single cell, one layer of the head groups face the exterior of the cell, whereas another layer of polar head groups face the internal cellular environment. The hydrocarbon tails that are attached to both layers of polar head groups face each other, thereby forming the bilayer structure of a cell membrane.
VIDEO The composition of liposomes
Liposomes can be composed of naturally-derived phospholipids with mixed lipid chains, such as egg phosphatidylethanolamine, or of pure surfactant components like dioleoylphosphatidylethanolamine (DOP). It is common for liposomes to also contain a core aqueous solution that is trapped by one or more bilayers. The phospholipid bilayers of a liposome can originate from natural sources, which are biologically inert, immunogenic and exhibit a lower inherent toxicity.
Classification of liposomes
The name liposome is derived from the two Greek words 'lipos,' meaning fat, and 'soma,' meaning body. Liposomes can vary in size from 0.025 micrometers (µm) up to 2.5 µm vesicles. In order to determine the half-life of liposomes, researchers must consider both the size and number of bilayers present within the liposome, as both of these properties play a role in determining the drug encapsulation volume of liposomes.
Liposomes can be classified as multilamellar vesicles or unilamellar vesicles, which can be further classified as large unilamellar vesicles (LUV) or small unilamellar vesicles (SUV). Unilamellar vesicles contain a single phospholipid bilayer sphere that encloses the aqueous solution, whereas multilamellar liposomes will exhibit multiple phospholipid bilayers.
Akbarzadeh, A., Rezaei-Sadabady, R.,
et al. (2013). Liposome: classification, preparation, and applications. Nanoscale Research Letters 8(1). DOI: 10.1186/1556-276X-8-102. Li, M., Du, C., Guo, N., Teng, Y., Meng, X., Sun, H., Li, S., Yu, P., & Galons, H. (2019). Composition design and medical application of liposomes.
European Journal of Medicinal Chemistry 164; 640-653. DOI: 10.1016/j.ejmech.2019.01.007. Further Reading