Classification of Liposomes

Liposomes are effective carriers for drug delivery, including small and large molecules (proteins, peptides, and DNA). Generally, we can classify liposomes based on the preparation method, the number, and size of vesicle bilayers, etc.

Classification based on Structure

According to the size and number of bilayer membranes (lamellarity) forming vesicles, liposomes can be divided into the following categories:

• Small unilamellar vesicles (SUV): 20-100 nm.
• Large unilamellar vesicles (LUV): >100 nm.
• Giant unilamellar vesicles (GULV): >1000 nm.
• Oligolamellar vesicles (OLV): 100-1000 nm.
• Multilamellar large vesicles (MLV): >500 nm.
• Multivesicular vesicles: >1000 nm.

Fig.1 Schematic representation of different sizes and lamellar structures of liposomes.

Classification based on Composition and Application

Liposomes can be divided into several different types according to their composition and application, including:

• Conventional liposomes

Conventional liposomes are the first generation of liposomes. They are lipid bilayer molecules surrounding the aqueous chamber and are the basis of all subsequent liposomes.

• Immunoliposomes

Immunoliposomes are vesicles specially designed for active targeting of the drug substances inside the body.

• Long circulating liposomes

The surface modification or PEG modification of liposomes is called PEGylation of liposomes, and the modified liposomes are called long circulating liposomes or stealth liposomes. Compared with conventional liposomes, PEG liposomes can avoid phagocytosis and circulate for a long time in systemic circulation.

• Cationic liposomes

Cationic liposomes can be prepared by adding cationic phospholipid into bilayer membrane. This allows high rates of DNA incorporation, and for this reason, such liposomes may be more suitable for gene and antisense therapy.

• Stimuli-responsive

Liposomes can be easily functionalized through the introduction of functional materials, such as stimulus-response materials. Their structure, configuration, and other properties can be changed under certain in vivo or in vitro stimulation, such as the change of heat, light, magnetism, and pH value.

Fig.2 Classification of liposomes based on composition and application. (Le, 2019)

Classification based on Preparation Method

In addition to the above two classification methods, liposomes will also be classified according to their preparation methods. This classification depends on using the organic solvents, obtaining different lamellarity of liposomes, changing the size and applications of liposomes.

REV SUVs/OLVs/MLVs: made by reverse-phase evaporation method

SPLV: stable plurilamellar vesicle

DRV: made by dehydrated rehydrated method

VET: vesicles prepared by extrusion technique

FATMLVs: frozen and thawed MLVs.

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For Research Use Only. Not For Clinical Use