Liposomes

Liposomes (also known as phospholipid bilayer vesicles) are spherical lipid bilayer membranes that represent models of cell membranes. Liposomes are composed of two main components: phospholipids and cholesterol. Phospholipids are amphipathic molecules composed of a glycerol backbone, two fatty acid tails, and a polar head group. Liposomes are viewed as aqueous cores with a hydrophobic membrane cap. These vesicles display a unique property to encapsulate both hydrophilic and hydrophobic drugs simultaneously. Depending on the size, surface charge, and lipid blend, drug is released from vesicles via three primary mechanisms such as cell fusion, endocytosis, and diffusion. Due to biocompatibility and biodegradability along with minimal toxicity, several clinically approved liposomal formulations have been developed for diagnostic imaging and biosensing [22].

Breakdown of liposome preparation involves four main steps: (1) drying of lipids from organic solvents, (2) redispersion of the lipids in aqueous media (containing drug/dye), (3) mechanical dispersion of liposomes to obtain the desired size, and (4) purification. Several techniques are applied for mechanical dispersion of liposomes such as sonication, membrane extrusion, French pressure cell extrusion, freeze thawing, and microemulsification.

As discussed earlier, liposomes are carriers of both hydrophilic and lipophilic drugs/ dyes, which render these systems ideal candidates for theranostic application. Liposomes of certain size ranges may be engulfed by the macrophage phagocytosis process rendering these vesicles ideal carriers for targeting parasitic and infectious diseases. For imaging purposes, nanobubbles ofcontrast or diagnostic agent can be entrapped into the liposomal core or embedded into the bilayer membrane [23]. A schematic representation of liposomal modification in drug delivery and imaging is shown in Fig. 10.2.

Schematic representation of nanosystems functionalized for theranostic properties

Figure 10.2 Schematic representation of nanosystems functionalized for theranostic properties: (A) liposomal system, (B) solid biodegradable polymer nanoparticles, and (C) dendrimer [32].

 
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