SKIN PENETRATION 617 Cronin and Stoughton (37), using the erythema reaction produced by vasodilators such as ethyl nicotinate and histamine, demonstrated that the forehead, presternal area, and back showed a greater response than the limbs, and the arm was more reactive than the leg. They concluded that the presence of more follicles in the forehead indicated increased penetration through sebaceous glands. A similar conclusion was reached by Feldmann and Maibach (38), in studies of hydrocortisone. Measur- able absorption occurred through all regions of human skin except the heel. Absorption seemed greater in areas where follicles are large or more numerous, such as the forehead and scalp, and decreased where the stratum corneum is thicker, such as the foot. Tregear (39), on the other hand, from studies of rapidly absorbed tributyl phosphate, concluded that hair follicles do not increase penetration. Smith et al. (36) observed a difference between the times taken for local anesthesia to develop on scrotal and abdominal skin after the topical application of lidocaine. With in vitro experiments they also found differences in the penetrability of skin between these two areas. Blood Flow If blood flow through the dermal vessels increases, the rate of clear- ance of materials should also increase as the concentration decreases. This is particularly true of gas permeation (40). Whether the rate of passage through the barrier layer is altered is not quite so clear, although it seems possible that this may occur as the more rapid removal of ma- terial that has penetrated must alter the perfusion gradient across this area. Clinically, erythematous skin is usually diseased, and this may alter the rate of absorption, at least partially due to increased vascular flow (21). SDecies Variation Ituman and animal skins display wide differences in physical char- acteristics such as the number of appendageal openings per unit area and the thickness of the stratum comeurn, thus affecting the penetration pathways and the penetration resistance of skin (41). In spite of limita- tions, animals must be used in studies of percutaneous absorption be- cause biologically dangerous substances, including radioactive com- pounds, are under many restrictions as to the application to human sub- jects. The skin of rabbits, rats, and mice lacks sweat glands and abounds in hair and hair-follicles, in contrast to that of man. The relationship be- tween species is not consistent for different substances, but the average

618 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS permeability order is rabbit, rat, guinea pig, and man in descending sequence (16). The largest deviation from the average is that human skin is very much less permeable to ions than rabbit or pig skin. While penetrability through rabbit skin is rapid, its structure of epidermis or appendages does not appear to differ significantly from that of other animals which are more resistant to penetration (40, 42, 43). PHYSICO-CHEMICAL FACTORS IN SKIN PENETRATION Hydration Hydration of the stratum corneum is possibly the most important factor in skin penetration, increasing the rate of passage of all substances which penetrate the skin. Hydration results from water diffusing from underlying epidermal layers or from perspiration that accumulates after application of an occlusive vehicle or covering on the surface. Under occlusive conditions, the stratum corneum is changed from a tissue which normally contains very little water (5-15%) to one which may contain as much as 50% water. The importance of hydration can be found in those investigations employing occlusive plastic film in steroid therapy. Here, the preven- tion of water loss from the stratum corneum and the subsequent in- creased water concentration in this skin layer enhances the penetration of the steroid (44-48). McKenzie and Stoughton (45) have shown that penetration of corticosteroids may be increased 100-fold by occluding the site of application, and thus hydrating the stratum corncure. Wurster and Kramer (49) measured the rate of penetration of esters of salicyclic acid through skin with dry and hydrated stratum corneum. They found that when the tissue was hydrated the rate of penetration of the most water-soluble ester increased more than that of the other esters studied. Working with aspirin in a temperature-humidity chamber, Fritsch and Stoughton (13, 50) showed the dual importance of these fac- tors on the penetration of excised skin. Full hydration of the keratin, accomplished by layering water over acetylsalicyclic acid on the epidermal surface, dramatically increased the penetration when compared to con- ditions of lower humidity at the same temperature. The mechanism of transport of a drug through hydrated stratum corncure may be quite different from that through normal stratum corneum. The low diffusion constant and high activation energy ob- tained for water and polar alcohol, as well as the selective diffusion ex- hibited by molecules of varying polar character, suggest that extensive