(w/w)] along with an alkanol component. But the kinetically stable nanosized emulsions can be prepared by using relatively lower surfactant concentrations. For example, a 20% o/w nanosized emulsion may only require a surfactant concentration of 1–5%. The kinetic stability of the nanosized emulsions can be achieved by creating a barrier at the oil–water interface, protecting the emulsion from breakage (Capek 2004). These barriers may be of electrostatic or steric nature and prevent emulsion droplets from direct contact. The most common way to stabilize emulsions is by surfactant adsorbed at the interface between the dispersed oil droplets and continuous aqueous dispersion medium. Surfactant adsorption layers do not only reduce the interfacial tension but can also provide an electrical charge to the emulsion droplets (ionic surfactants) or create the strong steric barrier via bulky molecular groups directed toward the dispersion medium (nonionic surfactants).
Traditionally, lecithins or phospholipids are the emulsifiers of choice to produce o/w nanosized emulsions, because the phospholipid emulsifier molecule structure is more or less similar to the endogenous phospholipids, which build the cells and/tissues. However, additional emulsifiers preferably dissolved in the aqueous phase are usually included in the emulsion composition. A typical example of the aqueous soluble emulsifiers are nonionic surfactants (e.g., Tween 20), which are preferred because they are less irritant than their ionic counterparts. The nonionic block copolymer of polyoxyethylene‐polyoxypropylene (PEO‐PPO), Pluronics F68 (Poloxamer 188) is included to stabilize the emulsion through strong steric repulsion. However, surfactants such as Miranol MHT (lauroamphodiacetate and sodium tridecethsulfate) and Miranol C2M (cocoamphodiacetate) were also used in earlier ophthalmic emulsions (Muchtar and Benita 1994). It should be added that commercially available cyclosporin A‐loaded anionic emulsion (Restasis®) contains only polysorbate 80 and carbomer 1,342 at alkaline pH to stabilize the anionic emulsion. To prepare a cationic emulsion, cationic lipids (stearyl‐and oleyl‐amines) or polysaccharides (chitosan) are added to the formulation. Strikingly, a stable emulsion prepared based on chitosan–lecithin combination was also reported (Ogawa et al. 2003). Conversely, a cationic emulsion based on an association of poloxamer 188 and chitosan without the incorporation of lecithin was prepared and also demonstrated adequate stability (Calvo et al. 1997; Jumaa and Müller 1999). Similarly, a report from our group also indicated the stability of oil droplets through the cation conferring chitosan along with poloxamer 188 as a mixed emulsifier (Tamilvanan et al. 2010). Since the free fatty acid generating phospholipid emulsifier molecule is omitted from the nanosized emulsion system, the stable nanosized emulsion produced from chitosan–poloxamer emulsifier combination would significantly reduce the generation of microclimate acidic pH in the vicinity of oil phase, oil–water interface, and water phase of the emulsion (Tamilvanan et al. 2010). These non‐phospholipid‐based emulsions should therefore pave the way to incorporate the acid‐labile molecules like therapeutic peptides and proteins, and to delineate the scope of applying lyophilization process for the development of a solid or dry emulsion. With the addition of suitable cryo‐ or lyo‐protectant at optimum concentration, the preparation of lyophilized solid dry‐powder form of non‐phospholipid‐based o/w nanosized emulsions is possible in recent years. Figure 2.1 shows the o/w nanosized emulsions in liquid form (before lyophilization) and solid‐dry powder form after the addition of different cryo‐ or lyo‐protectant molecules.
Oil‐in‐water emulsion compositions based on α‐tocopherol (or α‐tocopherol derivative) as the disperse phase has been described in a patent granted to Dumex (Sonne 2015). Interestingly, the emulsifying agent used to make tocol‐based emulsions are restricted to vitamin E TPGS (D‐alpha‐tocopheryl polyethylene glycol 1000 succinate) taking into consideration of toxicological issues. According to a patent by Nakajima et al. (2003), functional emulsions for use in food, APIs, and cosmetics were reported. These emulsions were stabilized with various span products such as Span 80, Span 40, etc. There is a group of emulsions that were not prepared by using the traditional anionic, cationic, and nonionic surfactant molecules. These emulsions do contain the oil or oil combination and therefore they can be termed as “surfactant‐free emulsions.” Another group of colloidal dispersions whose final appearance is white similar to the traditional emulsions but these dispersions do not contain both surfactants and oil or oil combination. Taking the physical appearance (white color) into consideration, these colloidal dispersions get the term “surfactant‐ and oil‐free emulsions.” Both of these two emulsions (surfactant‐free and surfactant‐ and oil‐free) are briefly discussed in Chapter 7.
Figure 2.1. Freeze‐dried emulsions using different cryo‐ or lyo‐protectants and reconstitution of freeze‐dried powder into nanosized emulsion.
2.2.3. Importance of Charge‐Stabilized Nanosized Emulsions
At present, emulsions stabilized by positively charged, cationic surfactants are most often used as colloidal API carriers (Tamilvanan 2004). Kim et al. (2005) used an emulsion of squalene in water stabilized by the cationic surfactant 1,2‐dioleoyl‐sn‐glycero‐3‐trimethylammoniumpropane (DOTAP), which facilitated gene transfer in biological fluid even in the presence of 90% serum in the dispersion medium. The emulsion droplets play the role of mucosal gene carriers and can form stable complexes with DNAs. Here, the DNA was incorporated at the end of emulsion preparation by the de novo method. Compared with liposomal carriers, cationic emulsions demonstrated a 200‐fold increase in transfectional efficacy in both lungs and tissues (Kim et al. 2003, 2005). The nature of oil selected as the dispersed phase is another important factor that can affect the applicability of such emulsions for transfection. Three different oils were used for the disperse phase: soybean oil, linseed oil, and squalene (Kim et al. 2003). The transfection activities of the nanosized emulsion carriers in the presence of serum followed the order squalene > soybean oil > linseed oil, and the squalene emulsions were also most stable. From these data, the authors concluded that stability of a carrier system is a necessary requirement to form stable complexes with DNA, and this stability determines the in vivo transfection.
It is known from the literatures that the interaction between cationic liposomes and polyanionic macromolecules like DNA is dependent on ± ratio, and at the ratio of maximum transfection there is a major aggregation leading to destabilization of formulation or desorption of DNA from the formulation (Liu et al. 1997). Furthermore, Simberg et al. (2003) suggest that an understanding of the interplay between lipoplex composition, its interaction with serum, hemodynamics, and target tissue properties (susceptibility to transfection) could explain the biodistribution and efficient in vivo transfection following intravenous administration of cationic lipid‐DNA complexes (lipoplexes) into mouse. However, it is interesting to see what could happen when the cationic nanosized emulsion is applied to in vitro cell culture models in the presence of serum. The serum stability of emulsion/DNA complex was reported (Yi et al. 2000). Further studies are, however, necessary to be carried out to understand clearly the origin of the serum stability of this emulsion. In addition, the transfection efficiency of this emulsion was not affected by time up to 2 h post‐emulsion/DNA complex formation. This means that the o/w cationic nanosized emulsion allows the experimenter to have a wider time window to work within during transfection study.
The o/w nanosized emulsions stabilized by both cationic and anionic lipidic emulsifiers were investigated in order to compare the degree of binding and uptake by specific cells that over‐expressed tumor receptors (Goldstein et al. 2007a). Immunoemulsions were prepared by conjugating an antibody to the surfactant molecule via a hydrophobic linker and then the antibody‐conjugated surfactant was used to make the emulsion by the de novo method. The anionic stabilized emulsions showed decreased stability leading to phase separation after 20 days of storage. The reduced stability of anionic immunoemulsion could be attributed to the rapid decrease of