Abstract: Nanoemulsions are kinetically stable emulsions often mystified as microemulsions, which are thermodynamically stable emulsions. The present review focuses on various components of nanoemulsions, design of colloidally stable nanoemulsions, classification of nanoemulsion and recent advances in manufacturing processes of these systems. Low energy as well as high energy techniques for preparation of nanoemulsions, which are suitable for industrial scale up, is discussed. Apart from the potential for improving the oral absorption of poorly permeable bioactives, they are well suited for skin care products and topical delivery, by virtue of their good sensorial and biophysical powers. Nanoemulsion has been reported to be equally efficacious in parenteral and nasal delivery. Nanoemulsions have recently been of key interest worldwide in target specific delivery of DNA/siRNA making them a significant delivery system for not only therapeutically active agents but also for gene delivery.

 
Introduction: A number of novel drug delivery systems offering various advantages are being proposed and commercialized recently to improve bioavailability of poorly soluble drugs. Almost 60% molecules in drug development programmes are poorly water soluble with consequent poor bioavailability1. Oral delivery of such drugs always remains a formidable challenge to formulation scientists. Besides, intravenous delivery also becomes difficult as drug has to be either solubilized or formulated as stable colloidal delivery system that does not result in drug precipitation in blood vessels.
Various approaches have been tried to improve solubility and/or bioavailability of such drugs. The most frequently reported ones include nanosuspensions2, polymeric nanoparticles3, and cyclodextrin inclusion complexes4 for oral administration. Besides these, a few novel lipid based nanoparticulate systems have been explored for effective oral, parenteral and topical delivery5.
However, one of the most preferable delivery systems for effective drug delivery of lipophilic drugs has been emulsions. Emulsions possessing droplet size in the nanometric scale (20–200 nm) are often referred to in the literature as miniemulsions, nanoemulsions, ultrafine emulsions, submicron emulsions and appear transparent or translucent to the naked eye. However, there is a significant difference between nanoemulsions and microemulsions, since the later unlike former is thermodynamically stable system. Both o/w and w/o nanoemulsions have been reported, however, the former has been evaluated to greater extent.
Nanoemulsions of o/w type are physically stable, biocompatible, biodegradable and most importantly, easy to produce on a large scale using proven technology6. Nanoemulsions are expected to penetrate deep into tissues through fine capillaries and bypass the fenestration present in the epithelial lining in liver due to its small size. Nanoemulsions have been exploited in targeted delivery for bioactives as well as gene delivery7.