Fluid Microstructure and Transdermal Delivery

by The Cosmetic Chemist Staff

All of the cosmetic or drug products we put on our skin have a characteristic fluid microstructure and rheology. These physical properties play a pivotal role in determining how the product will interact with the skin. This is especially important for vehicles that need to deliver an active ingredient(s). A major challenge for formulators is to develop a product that can traverse the barrier of the stratum corneum and bring the active ingredient to the targeted vascular components of the deep dermis.

Recent attention has turned to the use of microemulsions to carry out such a task. Microemulsions are thermodynamically stable oil-in-water or water-in-oil emulsions in which small droplets are dispersed in a continuous phase. They are are stabilized by surfactants, and as indicated by the name, are of a scale much smaller than normal emulsions. In fact, the use of the term microemulsion is quite confusing, since it implies a particle size on the micron level scale, when in fact sizes are normally in the nanometer range. The adoption of this terminology is due to historical reasons and does not correlate with the actual size of the particles. Equally confusing, the term nanoemulsion refers to small particles—which can be larger than microemulsions—that are thermodynamically unstable.1

photograph of fluid

The concept of microemulsions is not new. In fact, there has been considerable interest in this area for the last several decades. A recent report details the status of microemulsions and their use as transdermal delivery agents.2 Included in this work is a review of the methods to monitor transdermal delivery as well as a qualitative description of how the composition of the microemulsions ultimately determines transdermal behavior. It should be noted that microemulsions can be irritating to the skin, possibly caused by the high levels of surfactants used to stabilize them. It is intriguing to ponder if such high levels of surfactant may actually weaken the stratum corneum barrier, through dissolution of lipids, thereby facilitating transdermal drug delivery. In any event, this continues to be an active and exciting field of research in the realm of skin care and drug delivery.

1. D.J. McClements, Nanoemulsions versus microemulsions: terminology, differences, and similarities. Soft Matter, 8, 1719-1729 (2012).
2. R. Liuzzia, A. Carciati, S. Guido, and S. Caserta, Transport efficiency in transdermal drug delivery: What is the role of fluid microstructure? Colloids Surf. B, 139, 294-305 (2016).