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Nanoemulsion is an o/w type of emulsion with an average droplet diameter of 50–500 nm. The term “nanoemulsion” is used to define the dispersions of water and oil that are two immiscible liquids to form a thermodynamically stable and isotropically transparent system along with surface molecules involved in interfacial film formation. In addition, it should have an inner core of water or oil as an o/w or w/o emulsion. Nanoemulsion is composed of ingredients that are generally recognized as safe (GRAS) by FDA, approved surfactants for human use. The nanoemulsions consist of water-immiscible oil phase prepared under high shear pressure, or by mechanical extrusion system available throughout the world. Large-scale production of emulsion is easy. The use of nanoemulsion across various routes is favored due to their large surface area; thus, it is used for efficient drug delivery throughout the body [46]. Nanoemulsions are stable and have the ability to dissolve an increased amount of lipophilic drug along with certain vectors that prevent their enzyme degradation and hydrolysis [47, 48]. Reducing the size of droplets to nanoscale results in several fascinating physical properties such as visual transparency and peculiar elastic behavior. They are very promising in the non-material sector, as they are useful for the dispersion of deformable nanoscale droplets from fluid to highly solid and deformation of optical characteristic from opaque to nearly transparent [49].

Preparation of nanoemulsion contains oil and aqueous phase along with drug as well as surfactants/co-surfactant and additives. The physical and chemical characteristics of these components play an important role in formulation stability and their performances. The choice of surfactant must also be taken into account as per the hydrophilic lipophilic balance (HLB) and critical factor. Strong HLB (8-18) surfactants are used in nanoemulsion preparation, while surfactant with low HLB (3 to 6) can be used in w/o nanoemulsion preparation. The right combination of high and low HLB surfactants results in the formation of stable nanoemulsion.

The hybrid nanoemulsion preparation process combines low-energy emulsifying and high-energy emulsifying applications. Due to their drug solubilizing capacity in oil core without premature leakage, they are particularly preferred as the drug delivery system. The interactions between the lipid droplets on administration routes also reveal their targeting properties such as oral drug delivery, parental drug delivery, transdermal drug delivery, anticancer drug delivery, and vaccine drug delivery. Nanoemulsion can be used for both local and systematic targeting effectively, e.g., delivery through skin, lungs, brain, and ligand mediated drug targeting.