The pharmacological term bioavailability refers to a measurement tool that describes the degree and the rate at which an active substance is resorbed by the body and made available to the organism. The bioavailability is indicated in percent, the best possible percentage being 100. This maximum, however, generally, is achieved only by intravenous application of medicines. That makes clear that the bioavailability depends on the substance as well as on the type of application. The bioavailability of nutrients taken in by eating and drinking is influenced by the combination with other ingredients or foods. Coffee, for example, diminishes the bioavailability of many nutrients. In cosmetics, the dermal bioavailability is of great importance. It describes the percentage of active ingredients available to biological use in the skin after local application on skin. Its measurement is relatively difficult; it requires the definition in which skin layers a certain active ingredient is destined to achieve a certain effect.
Bioavailability in cosmetics
The release of active ingredients is of enormous importance for the efficacy of a cosmetic product. Generally, there are two ways of substance intake via the skin: the diffusion through the skin appendages or through the horny layer. The intake through the pores is practically insignificant due to the relatively small number of sebum and sweat glands. The transport through the outer epidermal layer (stratum corneum) is done in a transcellular or intercellular way. The idea that hydrophilic substances are transported transcellularly has proven to be improbable. The present expert opinion is that the intercellular route is the main transport way of lipophilic and hydrophilic substances.
The release and the bioavailability of cosmetic ingredients via the skin are influenced by several factors and substances. The process is a highly complicated interaction between the substance in question and the skin. The first phase, the release of the active substance from the base, includes the diffusion of the dissolved substance to the border vehicle/stratum corneum. In the subsequent penetration into the skin, the substance passes through the barrier. Then it spreads in deeper epidermal layers and the dermis. In general, the release is very rapid. The penetration is regarded as the decisive factor of the rate of resorption. The stratum corneum is a real barrier for the ingredients; one main reason being its low water content. Penetration-promoting agents can improve or boost the bioavailability.
Penetration-promoting agents to boost bioavailability
Certain substances are able to change the structure of the stratum corneum in a reversible way; thereby they enhance penetration and help improve bioavailability.
Skin’s barrier function is decreased while the permeability for cosmetic ingredients increases. Oleic acid is a well-known penetration booster. Combined with propylene glycol, it becomes even more effective.
Branched chain fatty acids, too, have a positive effect, however, not equal to that of oleic acid. Other substances that enhance the penetration and consequently the bioavailability are lecithins and the classic dimethyl sulfoxide (DMSO). The effectiveness of dimethyl sulfoxide could first be proved already in 1965. Scientists noticed a significant increase in the concentration of fluocinolone acetonide and hydrocortisone after DMSO was added to a cream base or an alcoholic solution. Still today, dimethyl sulfoxide is used in many pharmaceutical formulations. The advantage of DMSO with regard to the bioavailability is the rapid effectiveness; however, DMSO is not permitted in cosmetics.
Ethanol is the most frequently used monohydric alcohol, and, with regard to the penetration properties, it is the best tested agent for higher bioavailability. By penetrating the lipid zones of the upper epidermal layers, the substance enhances the solubility of active agents.
Urea is a special substance among the penetration boosters. The body’s own substance, in skin, works as a natural moisturizing factor. Urea, with its enormous water-binding capacity, considerably contributes to the hydration of the keratinized material in the stratum corneum. Even moderate concentrations (approx. 3 percent) suffice to serve as a great penetration booster.
However, the penetration-promoting effect that consequently boosts the bioavailability establishes itself only over a longer period of use.
D-panthenol, the precursor of pantothenic acid, makes skin more permeable and therefore more absorptive to active agents.
Portraits of the most important bioavailability boosters
INCI: Oleic Acid
CAS number: 112-80-1
Solubility: soluble in ethanol and emollients, insoluble in water
Boiling point: 360 degree Celsius
CAS number: 23434-88-0
Solubility: with alcohols, lipids
Melting point: 45°C
CAS number: 57-13-6
Solubility: in water, very good
Melting point: 132.5 to 134.5 degree Celsius (decomposition)
The relevance of bioavailability
A good bioavailability is relevant to an optimum concentration of active agents and the best-possible release of ingredients. The targeted release of active agents and the resulting good bioavailability significantly minimizes the probability of side effects.