Substances are molecules that don’t penetrate or absorb through the skin as easily as one would think. How do cosmetic manufacturers ensure optimal penetration for efficacy in skincare? What is the optimal penetration? How can water, lipophilic molecules, liposomes or nanoparticles penetrate through the stratum corneum and reach deep into the epidermis and below?
Deep skin penetration is defined when a molecule is applied to the skin and penetrates through to the bottom layer of the epidermis to stratum basale. The epidermis is the top layer of the skin and does not have any vasculature for the molecule to be absorbed into the bloodstream. Thus, the molecule does not have an effect on body systems. When a molecule is truly absorbed, this means that the molecule has reached the dermis where there is vasculature.
Lets first discuss the possible routes through the skin’s layers. The stratum corneum is the first line of defense molecules encounter when trying to penetrate our skin.
There are three methods to move through the skin.
The majority of topically applied molecules will permeate the skin via this route. In between the corneocytes are layers of lipids organized in a lamellar structure. This route is through the fluids between each cell.
This route is molecules with hydrophilic and lipophilic characteristics; they are called amphiphilic. This path is the most difficult because it must first cross the lipophilic membrane of each cell.
These are molecules that are known to follow down the hair shaft and into the hair follicle.
Once penetration is obtained, absorbency is the next question. Polar movement is an ionic movement “energy” with the positive and negative attraction of biochemistry in the human body.
Hydrophilic molecules tend to favor polar pathways due to dipole-dipole and hydrogen-bond interactions with other polar molecular entities in the path. Lipophilic molecules tend to favor the non-polar pathway since they are non-polar themselves. Molecules that meet these requirements improve absorbency and distribution creating a localized systemic effect.
Factors that affect skin penetration:
- The concentration of the molecule in the formula.
- The size of the molecule is essential; the smaller it is (<500 Da), the more penetration.
- The solubility of the molecule through the epidermis is a balance of hydrophilic and lipophilic character.
- The charge of the molecule or ion, skin, is negatively charged at physiological pH 4.5; it tends to attract cationic molecules.
- Where the molecule is applied since the epidermis varies in thickness, the skin on your face has a thinner stratum corneum than the palm of your hand; it can also differentiate by race.
- The moisture content of the skin.
- The skin’s health, affected by age, exposure to solvents, poor skincare routine, overall health condition, environmental factors.
- The temperature of the skin, the warmer the skin, the more fluid the lipid barrier, and easier for the molecule to flow through.
Consider the technical concepts of penetration and absorbency when choosing a cosmetic skincare product for both its effects and its safety. Having a highly bioavailable delivery system with inferior ingredients could create the opposite effect in delivering toxicity, which we all do not need. Study the ingredients for safety!