Liposome Technology for Industrial Purposes

Table 1

Classification of commonly known lipid vesicles according to their structures and/or preparation.

Identification	Definition
Archeosomes	Archeosomes are vesicles consisting of archebacteria lipids which are chemically distinct from eukariotic and prokariotic species. They are less sensitive to oxidative stress, high temperature, and alkaline pH [7, 8].
Cochleates	Cochleates are derived from liposomes which are suspended in an aqueous two-phase polymer solution, allowing the logic partitioning of polar molecule-based structures by phase separation. The liposome containing two-phase polymer solution treated with positively charged molecules such as Ca^2+ or Zn^2+ forms a cochleate precipitate of a particle dimension less than 1 μm [9].
Dendrosomes	Dendrosomes represent a family of novel, nontoxic, neutral, biodegradable, covalent or self-assembled, hyperbranched, dendritic, spheroidal nanoparticles which are easy to prepare, inexpensive, highly stable as well as easy to handle and apply, compared with other existing synthetic vehicles for gene delivery [10].
Dried reconstituted vesicles (DRV)	By this preparation technique, small, “empty” unilamellar vesicles, containing different lipids or mixtures of them, are prepared. After mixing those SUVs with the solubilised drug, dehydration is performed. By addition of water, rehydration leads to the formation of large quantities of rather inhomogeneous, multilamellar vesicles which need further processing [11].
Ethosomes	Ethosomal systems are much more efficient at delivering to the skin, in terms of quantity and depth, than either conventional liposomes or hydroalcoholic solutions. Ethosomal drug permeation through the skin was demonstrated in diffusion cell experiments. Ethosomal systems composed of soy phosphatidylcholine and about 30% of ethanol were shown to contain multilamellar vesicles by electron microscopy [12].
Immunoliposomes	Liposomes modified with antibodies, Fab’s, or peptide structures on the bilayer surface were established for in vitro and in vivo application [13, 14].
Immunosomes	Immunosomes are prepared by the anchorage of glycoprotein molecules to preformed liposomes. Under the electron microscope, immunosomes look like homogenous spherical vesicles (50–60 nm) evenly covered with spikes. Immunosomes have structural and immunogen characteristics closer to those of purified and inactivated viruses than any other form of glycoprotein lipids association [15].
Immune stimulating complex (ISCOM)	ISCOMs are spherical, micellar assemblies of about 40 nm. They are made of the saponin mixture Quil A, cholesterol, and phospholipids. They contain amphiphilic antigens like membrane proteins. ISCOMs already have a built-in adjuvant, Quillaja saponin, which is a structural part of the vehicle [16].
Lipoplexes	Cationic lipid-DNA complexes, named lipoplexes, are efficient carriers for cell transfection but have certain drawbacks due to their toxicity. These toxic effects may result from either cationic lipids or nucleic acids [17, 18].
LUVETs	LUVETs are large unilamellar vesicles prepared by extrusion technique, mainly performed with high-pressure systems [19].
Niosomes	Niosomes are small unilamellar vesicles made from nonionic surfactants also called Novasomes. Their chemical stability is comparable to that of archeosomes [20].
pH-sensitive liposomes	Four basic classes of pH-sensitive liposomes have been described previously. The first class combines polymorphic lipids, such as unsaturated phosphatidylethanolamines, with mild acidic amphiphiles that act as stabilizers at neutral pH. This class of pH-sensitive liposomes has been the most intensively studied. The second class includes liposomes composed of lipid derivatives resulting in increased permeability to encapsulated solutes. A third class of pH-sensitive liposomes utilizes pH-sensitive peptides or reconstituted fusion proteins to destabilize membranes at low pH. The final and most current class of pH-sensitive liposomes uses pH-titratable polymers to destabilize membranes following change of the polymer conformation at low pH [21].
Polymerised liposomes	Polymerized phosphatidyl choline vesicles (35–140 nm) have been synthesized from lipids bearing one or two methacrylate groups per monomer. Compared to nonpolymeric analogues, these vesicles exhibited improved stability and controllable time-release properties [22].
Proliposomes	Proliposomes are defined as dry, free-flowing particles that immediately form a liposomal dispersion on contact with water [23, 24].
Proteosomes	Vesicles of bacterial origin were solubilised, followed by ammonium sulphate precipitation and dialysis against detergent buffer. Proteins and peptides are noncovalently complexed to the membrane, making them highly immunogenic [25].
Reverse-phase evaporation vesicles (REV)	Vesicles are formed by evaporation of oil in water emulsions resulting in large unilamellar liposomes [26].
Stealth liposomes	In the early 1990s, this liposome engineering process culminated with the observation that coating of liposomes with polyethylene glycol (PEG), a synthetic hydrophilic polymer, would improve their stability and lengthens their half-lives in circulation, rendering the use of glycolipids obsolete. PEG coating inhibits protein adsorption and opsonization of liposomes, thereby avoiding or retarding liposome recognition by the reticuloendothelial system (RES). These PEG-coated liposomes are also referred to as sterically stabilized or stealth liposomes. The PEG stabilizing effect results from local surface concentration of highly hydrated groups that sterically inhibit both hydrophobic and electrostatic interactions of a variety of blood components at the liposome surface [27–33].
Temperature-sensitive liposomes	Temperature-sensitive liposomes are considered to be a promising tool to achieve site-specific delivery of drugs. Such liposomes have been prepared using lipids which undergo a gel-to-liquid crystalline phase transition a few degrees above physiological temperature. However, temperature sensitization of liposomes has been attempted using thermosensitive polymers. So far, functional liposomes have been developed according to this strategy whose content release behavior, surface properties, and affinity to cell surface can be controlled in a temperature-dependent manner [34, 35].
Transfersomes	Transfersomes consist of phosphatidylcholine and cholate and are ultradeformable vesicles with enhanced skin-penetrating properties [36].
Virosomes	Virosomes are small unilamellar vesicles containing influenza hemagglutinin, by which they became fusogenic with endocytic membranes. Coincorporation of other membrane antigens induces enhanced immune responses [37].