Manufacturers promise flawless skin if we use cosmetics that they claim penetrate the skin and improve cell functioning. And yes, of course they do penetrate – but to what depth? The skin’s outermost layer, specially designed to act as a barrier, is formed of nearly impermeable tissue. So, how can cosmetics penetrate the skin?
No cosmetic active ingredient has yet been invented that crosses the epidermal barrier and penetrates deep into the skin. In fact, a substance that appears to penetrate the dermis and hypodermis is most likely absorbed by the blood vessels. In that case it would be a drug, not a cosmetic active ingredient, because it affects metabolism. With nicotine patches applied to the skin, for instance, tiny nicotine molecules travel via the skin layers until they reach blood vessels. Does nicotine act on the skin on its way to the blood? The answer is no.
Cosmetic manufacturers know that, unlike what happens with skin patches, their active ingredients remain on the skin surface (the epidermis) and yet are effective.
How deep do active ingredients go?
The outer horny layer of the epidermis, the stratum corneum, is the most impermeable. If it weren’t, we wouldn’t be able to have a soapy bath or apply a moisturizer: the water, cream and soap would accumulate under the skin in blisters and would endanger our health. The barrier function is therefore essential to maintaining the balance of the epidermis and the body.
The stratum corneum retains the water we drink by preventing it from evaporating. It also filters everything that enters the skin. Molecules that care for the skin have to pass this barrier in order to penetrate the epidermis. But there the journey ends, in that layer measuring 0.5 mm on average (1.5 mm maximum in the palms of the hands).
Not even the tiniest nanocosmetics can penetrate beyond the epidermis. A study by the University of Bath (UK) confirms that nanoparticles do not penetrate any deeper than the molecules commonly used in cosmetic formulations.
In, not through, the skin
Active cosmetic ingredients penetrate the epidermis in three ways. The smallest molecules and lipophilic molecules (which dissolve in oil) pass between spaces in the stratum corneumformed by fatty acids that separate cells from each other. Some soften the bonds linking cells in order to improve penetration.
Larger molecules penetrate the epidermis through the hair follicles. Travelling through the small ducts where hair grows, they reach the point where the epidermis meets the dermis. Other substances penetrate the skin through sweat gland pores.
The largest molecules (greater than 10 microns) do not cross the skin barrier. As one example, collagen – a protein that makes the skin elastic – cannot penetrate the epidermis. Thus, no collagen-based cream functions in this way: the collagen has to be injected directly into the deeper skin layers.
Traditional and new vehicles
The question that obsesses the cosmetics industry is how to improve the penetration capacity of cosmetic ingredients. The means by which an active ingredient travels into the epidermis is called the vehicle and the most traditional and popular vehicles are fats (oils), water and alcohol.
Overcoming the limitations of traditional ones requires new vehicles to be designed. Today’s cosmetics industry has managed to develop vehicles that deliver active molecules without wasting a single one. These vehicles do not penetrate any deeper but they do ensure that all (or almost all) of the active ingredient is available to influence cell functioning. This is done by protecting and delivering the active ingredient in capsules.
These tiny cylinders, made from a combination of water and oil, keep the active ingredient in optimum conditions until it reaches its destination. Depending on their exact chemical composition, these capsules are known by various names, such as liposomes,microemulsions (or nanoemulsions if transporting nanoparticles) and micelles.
These tiny capsules deliver, directly to epidermal cells, antioxidant ingredients (like vitamins E and C derivatives) and other molecules such as hyaluronic acid and polyphenols (e.g., resveratrol).
Some of these transport systems are so effective that they have been called smart capsules: they use receptors to identify the skin cells where the active ingredient is to be delivered.