The skin is evidently the largest organ of the body. Amongst other functions, it prevents transepidermal water loss (TEWL). As good as the skin is at preventing dehydration through TEWL, it is even better at keeping things out. Things like water too. This sounds a little strange but imagine you absorb water everytime you take a bath or got rained on, think about the myriad of substances that get on the skin by accident or on purpose, what about bacteria and other grimy things that touch us from time to time. Our skin is highly resilient and resistant to external forces.
With this said, the skin does allow some compounds through based on:
- Size of the compound
- Integrity of the skin
- Solubility in oil or water – lipophilicity
- Transport molecules
The stratum corneum, comprised of several layers allow penetration of compounds through three routes (assuming that skin integrity is not compromised) :
- The shunt route.
- Intercellular route
- Intracellular route
Most compounds penetrate the skin through the stratum corneum cornoceocytes i. e. the Intracellular (intra – through the cell) pathway. Some compounds can move better between cells (Intercellular) especially when they are water loving. The shunt route is the follicular path, with the sebaceous gland playing an active part. Jojoba oil is a perfect example of a liquid wax, that penetrates the skin through the shunt route. It also has great similarity with sebum.
The problem with penetration enhancers is that they can evoke structural changes within the skin and lead to immune responses like inflammation and irritation. This may be beat cause of the lack of specificity in action, causing/allowing just about any molecule that is small and lipophilic enough to cross the barrier, harmful or not e. g. Preservatives. That is not to say that they do not have benefits. For example, they can assist out precious cosmeceuticals in reaching deeper levels of the skin where they can exert their effects.
In the next post in this series, I’ll list some common penetration enhancers and maybe how they operate. Till then, can you try to identify penetration enhancers by looking at the ingredients list of your cosmetic products?
Let me know!