Does Skincare Really Penetrate the Skin? The Science of Cosmetic Absorption
It has evolved to keep substances out, and the stratum corneum is its main protective barrier. So when we talk about a cosmetic ingredient “penetrating the skin,” what do we actually mean? And does a more penetrating formula necessarily make a better skincare product?
The answer is more nuanced than a simple yes or no.
Absorption and penetration are not exactly the same thing
In cosmetic science, skin absorption and skin penetration describe different processes.
Absorption refers to material moving from the skin surface into the skin. Penetration goes further, describing movement through the skin and potentially towards the systemic circulation. (Cosmetic Formulation: Principles and Practice, 2019, Heather A. E. Benson, Michael S. Roberts, Vânia Rodrigues Leite-Silva and Kenneth A. Walters, eds.)
For everyday skincare, however, the more useful question is often not whether an ingredient reaches the bloodstream. It is:
Where does the ingredient actually go after we apply it?
An ingredient may remain primarily on the surface, enter the stratum corneum, become temporarily retained within the skin, or move further into deeper layers. These are very different outcomes.
The skin barrier is a formidable obstacle
The stratum corneum is the main barrier to substances moving through the skin.
Its structure is highly organised, with corneocytes surrounded by lipid regions. This arrangement makes it difficult for many substances to move through the skin freely.
The properties of the ingredient itself also matter. Molecular size, water and lipid solubility, and the tendency of a substance to partition between the formulation and the skin can all influence how readily it moves into the skin. In general, larger molecules have greater difficulty crossing the barrier. (Dermal Absorption and Toxicity Assessment, 2008, Robert C. Scott, Rebecca H. Guy, John Hadgraft and colleagues, eds.)
This is one reason why two active ingredients at the same concentration can behave very differently after application.
The ingredient is only half of the story
There is another factor that is often overlooked: the formulation itself.
An active ingredient does not reach the skin on its own. It is delivered in a vehicle — perhaps a cream, lotion, gel, serum or another type of formulation.
That vehicle can influence what happens next.
The affinity between the active ingredient and the vehicle can affect how readily the ingredient leaves the formulation and partitions into the skin. If an ingredient is held very strongly by its vehicle, its release into the skin may be reduced. At the same time, some vehicle components can interact with the stratum corneum and change its barrier properties. (Dermal Absorption and Toxicity Assessment, 2008, Robert C. Scott, Rebecca H. Guy, John Hadgraft and colleagues, eds.)
Research discussed in the Handbook of Cosmetic Science and Technology provides a particularly useful example. In an experimental study using lidocaine, the formulation affected the initial absorption and also influenced how much of the substance remained within different layers of the skin. (Handbook of Cosmetic Science and Technology, 2001, André O. Barel, Marc Paye and Howard I. Maibach, eds.)
In other words, the vehicle is not just packaging for the active ingredient. It can influence its behaviour on and in the skin.
What determines whether an ingredient enters the skin?
There is no single factor that determines skin penetration. Several variables work together.
Molecular size
Smaller molecules generally have an easier time moving through the skin barrier than larger ones.
Water and oil solubility
An ingredient needs suitable physicochemical properties to move from the formulation into the skin. Its affinity for the vehicle and its affinity for the skin both matter.
The formulation
Creams, gels, emulsions and other vehicles can create different environments for the same active ingredient. Solvents, oils, emulsifiers and other formulation components can influence its release and distribution.
The condition of the skin
A healthy, intact skin barrier behaves differently from damaged or compromised skin. Disruption of the stratum corneum can increase permeability and therefore change how substances move through the skin. (Dermal Absorption and Toxicity Assessment, 2008, Robert C. Scott, Rebecca H. Guy, John Hadgraft and colleagues, eds.)
Where the product is applied
Skin thickness varies across the body, and this can affect permeability. The characteristics of facial skin, the forearm, the palm and other areas are not identical.
How the product is used
Application time, repeated application and occlusion can also influence what happens after a product is applied.
These factors make skin absorption considerably more complicated than simply asking whether an ingredient “penetrates.”
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Does deeper penetration always mean better skincare?
Not necessarily.
This is an important distinction.
For some cosmetic products, the desired effect is primarily at the skin surface or within the outer layers of the stratum corneum. A moisturizer, for example, does not need to reach deep tissues to provide a useful cosmetic effect.
In other cases, a formulation may be designed to encourage an ingredient to reach particular layers of the skin.
The objective should therefore be appropriate delivery, rather than maximum penetration.
This is particularly important because increasing penetration is not automatically beneficial from a safety perspective. The deeper an ingredient moves, the more important it becomes to understand its exposure and potential biological effects.
That is why penetration studies are used not only to investigate product performance, but also as part of safety assessment for substances applied to the skin. (Dermal Absorption and Toxicity Assessment, 2008, Robert C. Scott, Rebecca H. Guy, John Hadgraft and colleagues, eds.)
How do scientists study skin penetration?
Skin penetration cannot be judged reliably just by looking at a product or by how it feels after application.
Laboratory methods can be used to compare how different substances or formulations behave.
One approach uses sections of excised skin in a diffusion system. The test substance is applied to one side of the skin, and researchers can then examine how much remains in the skin or appears on the other side over time.
Different techniques can also be used to analyse individual layers of the skin.
These methods are particularly useful when comparing different formulations of the same ingredient. They can show, for example, whether changing the vehicle alters how much of an ingredient enters or remains within the skin. (Cosmetic Formulation: Principles and Practice, 2019, Heather A. E. Benson, Michael S. Roberts, Vânia Rodrigues Leite-Silva and Kenneth A. Walters, eds.)
Importantly, laboratory penetration results should not automatically be interpreted as identical to what happens during everyday human use. Factors such as washing, sweating, clothing, application amount and other real-world conditions can change exposure. (Dermal Absorption and Toxicity Assessment, 2008, Robert C. Scott, Rebecca H. Guy, John Hadgraft and colleagues, eds.)
What does this mean for cosmetic formulation?
For a cosmetic formulator, the key lesson is quite simple:
You cannot look at an active ingredient separately from the formulation that delivers it.
A promising ingredient may have very different behaviour depending on its concentration, solubility, vehicle, stability and interaction with the skin.
The objective is therefore not necessarily to make an ingredient penetrate as deeply as possible. The goal is to develop a formulation that provides the appropriate distribution and performance for the intended cosmetic purpose.
At A&T Formulation, this is one of the reasons formulation development goes beyond selecting active ingredients. The active, the vehicle and the skin need to be considered as a complete system.
The key takeaway
So, does skincare really penetrate the skin?
Yes, some cosmetic ingredients can enter the skin — but the extent and location of that penetration depend on many factors.
The ingredient itself matters. So does its molecular structure, the formulation surrounding it, the condition and location of the skin, and the way the product is applied.
Most importantly, deeper penetration does not automatically mean a better cosmetic product.
Good formulation is about understanding where an ingredient needs to act and designing the product accordingly. That is where formulation science turns an interesting ingredient into a well-designed finished skincare product.
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