Bioavailability: How does care get into the skin?

Dabs of serums and creams on sand colored background.

Our skin is a protective wall against germs and everything that harms us. At the same time, we try to support and care for it with various care products. But how are all the highly effective serums, creams and waters supposed to leave visible traces in the best sense if the skin doesn't let anything through ...? The answer is: it's not that simple, because an intact skin barrier does not distinguish whether the intruders are unpleasant contemporaries such as bacteria or a support squad of vitamins. The bioavailability of active ingredients is important in this context. 

What is bioavailability?

Bioavailability is a measured quantity. It describes what proportion of an applied active ingredient is actually available later in the body. It is expressed as a percentage. For example, with a day cream you apply Retinol onto your facial skin. But only a certain proportion of it, by no means all of it, passes through the horny layer into the deeper skin layers, where the retinol can develop its effect. 

The bioavailability indicates what percentage of the originally applied retinol actually reaches the skin: 

If the bioavailability of an active ingredient is 100 percent, everything that has been applied at the top reaches the lower layers of the skin; at 0 percent, correspondingly, nothing at all. In reality, the value is always in between, in other words: a little of the active ingredient always passes through, but all of it never reaches the addressed skin cells. 

Penetration enhancers: the active ingredient smugglers

In active ingredient cosmetics, particularly high bioavailability is desirable because the effectiveness of a skin care product stands and falls with it. So how do manufacturers ensure that the coveted moisture, the valuable lipids and, last but not least, all the good antioxidants do not stick to the surface of the skin? The large chemical boxes of the cosmetics industry also have a gallant solution to this problem, and it is called: penetration enhancers, alternatively also called permeation enhancers. 

This refers to substances that are able to temporarily change the structure of the stratum corneum. They have penetration-enhancing properties and thus increase bioavailability. In other words, penetration enhancers reduce the barrier function of the skin, while at the same time increasing the absorption capacity for cosmetic ingredients. 

There are various substances that have penetration-enhancing properties. These include for example 

  • Urea (Urea),
  • Fruit acids, 
  • oils rich in fatty acids and 
  • Alcohols. 

How exactly they transport the active ingredients into the skin is determined by numerous factors. A very complex interplay takes place between each substance and our skin until the active ingredient has finally overcome the skin barrier. 

Liposomes in active cosmetics 

Liposomes play a remarkable special role here. This is because they not only make the skin barrier more permeable to active ingredients, but also help to repair it at the same time. What initially sounds contradictory is made possible by their special shape: liposomes are tiny, hollow spheres whose shell consists of double-walled phospholipids. 

Phospholipids consist of a water-friendly (hydrophilic) head and a fat-loving (lipophilic) tail - this is how they form the double lipid layer typical of liposomes in aqueous solutions. Liposomes are thus the advanced version of the simpler micelle. An important difference is that the shell is not made of surfactants but of the body's own phospholipids. There are even liposomes that are coated by several such double lipid layers - then we are talking about multilamellar liposomes. 

Schematic illustration of a liposome and a micelle
Due to their special, double-walled structure, liposomes are able to hold water-soluble active ingredients in the core and transport them into deeper skin layers. This distinguishes them from micelles: although they also combine water- and fat-loving properties, they have a simpler structure (and are fat-loving in the core).

Advantages and disadvantages of liposomes

Due to their membrane-like structure and skin-like composition, liposomes easily latch onto the skin's barrier layers. There they are a highly welcome repair helper: because the phospholipids supply the skin with essential fatty acids and promote the formation of ceramide I. Both are important for a strong skin barrier. 

So far, so caring, but liposomes can do even more: As mentioned at the beginning, they are inherently hollow. This hollow space can be loaded with water-soluble active ingredients. Active ingredients transported by liposomes spread very well in the skin, are stored like depots and often show a higher efficacy. This in turn allows lower concentrations, which is associated with better tolerability. 

But liposomes also have disadvantages: they tend to clump together and do not like oxygen, heat or a low pH value. So they are relatively unstable by nature. This is the reason why you may well find a small amount of alcohol in liposomal creams. This serves to stabilize the liposomes. In addition, lecithin should be quite high in the list of ingredients, because only then is there a sufficient amount of liposomes in the product. 

Penetration enhancers make the skin more permeable - in both directions

Regardless of which substance is used as a penetration enhancer, the following applies: The increase in barrier permeability affects both directions. This means that increased transepidermal water loss (TEWL) may occur in the short term. That is why it is important that the respective formulation also contains moisturizing active ingredients that prevent the skin from drying out. 

However, one thing must not be forgotten: In principle, penetration enhancers also open the door for ingredients that one would not actually want under the skin. Denatured alcohol (ethanol), for example, also transports phthalates through the skin barrier. PEGs (polyethylene glycol), which are considered critical, also make the skin barrier more permeable. 

With active cosmetics, it is therefore of great importance that the entire formulation is of high quality. Because once the skin barrier is open, the good guys and the bad guys sneak through. 

More effect through cleaning

By the way, besides penetration enhancers, there are other strategies to increase bioavailability: Mild but careful cleansing prepares oily skin areas in particular to better absorb serums and creams. A light degreasing can therefore definitely be in the interest of the skin. And another tip: where the skin feels rough, horny skin flakes stand out. This is also a barrier that opposes serums and emulsions. Regular exfoliation, which gently removes the dead flakes, is an equally simple and effective "effect booster" for your skin care.