In part 1 of our look at different colloidal drug carrier systems used by pharmaceutical researchers, we looked at nanosuspensions and liposomes. In part 2, we looked at mixed micelles and colloidal liquid crystalline structures.
Today in part 3, we look at microemulsions and nanoemulsions.
- Microemulsions
Microemulsions are clear, thermodynamically stable (a state in which a chemical system is not consuming or releasing heat energy) isotropic mixtures of water, oil (or possible different oilefins and hydrocarbons), and surfactant. A cosurfactant may also be added to the mix.
There are three primary kinds of microemulsions: direct, reversed and bicontinuous. Direct microemulsions are comprised of oil dispersed in water (o/w). Reversed microemulsions are comprised of water dispersed in oil (w/o). Bicontinous microemulsions are comprised of oil and water existing as a continuous phase.
Microemulsions have droplets that are generally between 10-100 nm, and because of their large interfacial areas, relative to micellar solutions, they have greater solubilizing capacity for hydrophilic and lipophilic drugs (e.g. Sandimmun Optoral™, Neoral™, etc.). However, because of their significant concentration of surfactant, microemulsions are typically limited to dermal and peroral applications.
- Nanoemulsions
Nanoemulsions, which are sometimes called sub-micron emulsions (SME), are tiny lipid droplets with mean diameters of typically between 50 – 1000 nm, and an average droplet size of 100 – 500 nm. They are comprised of surfactants (usually 10-20% oil with .5-2% emulsifying agent like egg or soybean lecithin) that have been deemed Generally Recognized as Safe (GRAS) by the Food and Drug Administration (FDA).
Nanoemulsions are created using high-pressure homogenization, and because of their lipophilic interior (capacity for dissolving in lipids), they are often used to transport lipophilic compounds. Nanoemulsions also have significant bioactive effects, and are capable of transporting lipids into the skin. Also of particular interest and important to pharmaceutical researchers is the fact that nanoemulsions do not cream, and thus can be used in sprayable products.
Stay Tuned for Part 4
In part 4 of our overview of colloidal drug carrier systems, we will look at nanocapsules and polymer nanoparticles.
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