It’s Skin Deep

The skin may be selected as an administration site for drug actives for a number of reasons. There may be a requirement for treating a localised skin condition (topical delivery – for treating psoriasis) or there may be advantages to using the skin as an administration platform for achieving systemic delivery and avoiding biological hurdles such as first pass metabolism (transdermal delivery – such as nicotine patches and testosterone). Formulating products for either topical or transdermal delivery presents particular challenges to the formulation scientist and requires good understanding of the biology of the skin, as well as an appreciation of the pharmaceutics of liquid and semi-solid dosage forms (creams, ointments and gels, for example).

THE SKIN AS A BARRIER

Skin performs admirably as a barrier between the outside world and the intricate workings of the body. It has to protect the body from physical damage and from invasion from microorganisms and has a vital role to play in controlling transport of water into and out of the body. The structure of the skin is shown in Figure 1.

The outmost layer is the stratum corneum, or horny layer, and is the key protective component of the skin. The stratum corneum is only 10 to 15 cell layers thick (approximately 10-20μm) but is a formidable barrier, consisting of dead, keratinised cells. The space between the keratinocytes is occupied by lipids, consisting of ceramides, fatty acids and cholesterol. The stratum corneum can be compared with a stone wall, with the keratinocytes acting as the stones and the intercellular lipids acting as the mortar.

Beneath the stratum corneum is the epidermis (which can be further subdivided into stratum granulosum, stratum spinosum and stratum basal layers), which is comprised of living cell layers, though no vascular structure. The cells from the stratum granulosum die and replenish keratinocytes to the stratum corneum.