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Emulsions are mixtures of liquids that do not normally blend (water and oil): oil-in-water (O/W), in which oil droplets are dispersed in water, or water-in-oil (W/O), in which water droplets are dispersed in oil [20]. The former is more commonly used in topical formulations. Emulsions are milky with coarse dispersion and a droplet size in the range of micrometers. They are thermodynamically but not kinetically stable and thus will eventually phase-separate. Micro-emulsions and nano-emulsions are formulations of water and oil, similar to emulsions, but with the addition of surfactants.

Micro-emulsions are dispersions with a droplet size between 10 and 100 nm [21]. They are clear or translucent and thermodynamically stable. The advantages of micro-emulsions include: the ease and low cost of preparation, the possibility of incorporating both hydrophilic and lipophilic drugs at the same time, the increased drug loading, and the penetration-enhancing ability. The absorption is improved by the use of penetration enhancers in the micro-emulsion’s oil phase, such as oleic acid, or by the use of surfactants. Their clear appearance and ease of application increases their desirability and use in many cosmeceuticals, including moisturizers, sunscreen preparations, tanning products, antiaging products, antiperspirants, deodorants, hair care and coloring products, and perfumes. A common concern related to micro-emulsion use for topical delivery is their potential side effects, mainly skin irritation potential and comedogenic effects. These side effects are generally associated with exposure time and the composition and concentration of components, especially of surfactants, and components of the oil phase.

Table 1. Vesicular lipid-based delivery system

Nano-emulsions are emulsions with droplets smaller than 100 nm, comparable to the size of micro-emulsions despite what the name implies [22]. Nano-emulsions present the advantage of being formed with smaller amounts of surfactants, and thus lower skin irritation potential [23]. The preparation of stable nano-emulsions generally requires expensive, high-energy input methods. Nano-emulsions are kinetically, not thermodynamically, stable [24]. Their instability leads to a more favorable use of other nano-sized delivery systems like nanosomes or solid lipid nanoparticles (SLNs), which will be discussed later. Nano-emulsions are used for transcutaneous delivery of multiple agents, including gamma tocopherol, caffeine, and plasmid DNA [25–27].