Purity and percutaneous absorption of raw materials
Because of its horny layer the skin is a barrier which is difficult to penetrate. Nevertheless, numerous substances with certain physico-chemical properties are able to overcome this diffusion barrier. This applies not only to drug substances but also a large number of excipients used as vehicles for drug substances or for cosmetic products. To some extent it is not the excipient itself which is absorbable but by-products formed during synthesis which are not sufficiently removed during raffination of the substances.
The raffination of starting materials for the manufacture of dermatological products is a question of technology and cost and varies greatly depending on the quality required by the raw material manufacturer and the degree of acceptance by the cosmetic producer.
This problem can be illustrated taking the universally used polymers as an example. Numerous types of polymers are used in cosmetic products for nails, hair and for skin care. Depending on the type of synthesis and the quality of their raffination, polymers contain monomeric residues (e.g. acrylamides, styrenes), some of which are readily absorbed through the skin and have thus already attracted the attention of the American health authority.
Contrasting with the provisions of the pharmaceutical legislation, global cosmetic regulations place much less stringent requirements on the toxicological testing and quality control of raw materials and products. This is because of the legal classification of cosmetic products as intended for use on healthy skin and not for application to diseased, non-intact skin. Although the regulatory agencies require the cosmetic industry to perform tolerance testing in humans and a toxicological assessment before marketing cosmetic products, considerable differences in the quality of planning, implementing and documenting clinical trials and assessments are also apparent in this area. Moreover, there is little guarantee of the reproducible quality of intended raw materials, since the exchange of raw materials is not monitored and is completely at the discretion of the cosmetic manufacturer.
Studies have been performed to determine the absorbability of preservatives (e.g. parabens) and alpha-hydroxy acids. It was found that appreciable amounts of these substances not only have good penetration ability but can also pass through the epidermis and thus become systemically available.
The penetration of chemical artefacts, raw material constituents, preservatives or fragrances etc. is frequently triggered by the solvents or substances inherently capably of readily penetrating easily through the skin which are present in the formulation and act as carriers. Alcohols and chemical UV filter substances are good penetration enhancers and can have the unfavourable property of promoting the ingress of these substances.