PERSORPTION* By AMOS E. LIGHT The 14/el/come Research Laboratcries, Tuckahoe 7, N.Y. AS DEFINED IN the dictionary, skin is "the membranous external investment of an animal." Actually it has turned out to be an "invest- ment" of inestimable value for the human animal and the study of ways and means of keeping the skin in good condition has become a very profit- able investment for the drug and cosmetic industries. A human being weighing 70 kg. has approximately 1.83 square meters of skin. This average value is obtained by raising one-tenth of the value of the weight to the two-thirds power (1). The skin weighs about 16 per cent of the body weight. In burns during mass casualties, the area of skin damage is most important in planning for the care of the patient especially in regard to fluid administration. Normally the skin prevents a large loss of moisture except in times of increased body heat during fever or activity at which times the sweat glands are called into action. It may be of value to remember the Rule of Nine where the area of each arm and head is 9 per cent of the total and the surface of each leg and each side of the trunk is 18 per cent of the total (2). The thickness of the skin usually measures between 2.5 mm. and 8 mm. and the different layers also vary in a similar manner (3). It has been recorded that an average strip of skin one inch square contains some 3,000,000 tissue cells, one yard of blood vessels, four yards of nerves with 25 sensory ends, some 400 or more sweat glands, and fi'om 15 to 30 hair follicles with like numbers of sebaceous glands (4). The skin is an organ and uses all of the above structures to protect the body. The production of keratin is considered one of the skin's greatest achievements (5). Usually it is extremely unwise to introduce a new term to describe old events however, after careful perusal of the literature it became apparent that no single word yet used could describe the act of substances passing through various layers of the skin. After considering many combinations of prefixes and suffixes the word "Persorption" evolved to amply describe this action. The cosmetic chemist is probably more interested in absorp- tion, an example of which would be the stratum corneum swelling following * Presented at the September 26, 1956, Meeting, New York Chapter, New York City. 117

118 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS the application of water, whereas the pharmaceutical chemist is concerned with persorption which indicates passage of material into the deeper sections of the dermis and possibly into the lymph or blood systems. In fact the cosmetic chemist may not desire to have the lower layers of the skin affected. Penetration of the stratum corneum by detergents to the area of living cells may be a major factor in the severity of some eczemas. Protective creams have been devised to prevent such penetration (6). Penetration of the skin may be by solid matter causing trauma, or by energy such as heat and x-radiation. With the administration of heat the temperature of the skin rises, the rapidity of this rise depending on the thickness of the tissue it is less rapid when the superficial tissue is thinner allowing deeper penetration and faster dissipation of the heat by the blood (7). The study of the passage of chemicals through the skin, persorption, offers a great challenge to research workers in the field of dermatology. Much information has been gained in the past few years but much more is needed to treat the various skin disorders prevalent today (8). It would be out of place here to present all published data on skin struc- ture and physiology (9) and the literature on percutaneous absorption (10). These subjects have been reviewed before as listed in the bibli- ography. In general it has been found that a substance with water and lipoid solubility has a much better chance of passing through the skin than other types. The rate of penetration is related to the ether-water partition coeflqcient, and the barrier to be passed appears to be an aqueous phase separating two lipoidal layers. There are many exceptions to this so that each individual compound should be tested for persorption. Water itself ' has been studied for its persorption properties, but it has been found that the most important property of this substance is to keep the stratum corneum hydrated with resultant improvement in dry skin (11). The surface layer of epidermis usually contains only 10 per cent of moisture in the keratin as compared with 40 per cent in the deeper layers and 70 per cent below the stratum granulosum (12). The skin as a whole becomes a depot of water for the rest of the body in certain cases of electrolyte imbalance (13). It is possible that a trace of water may go through the membrane at the stratum lucidurn boundary but the eccrine sweat glands and hair follicles are probably the main pathways for any slight passage of moisture through the skin. It is interesting to observe that the diameter of the eccrine sweat gland opening is only about 30 microns or less. This small opening will undoubtedly prevent particles having a large diameter from entering this gland. The much narrower tube spirals down into the skin with only a double layer of cells along its lining this would facilitate persorption (14). The much larger hair follicle has a lining whose cells become more and more undifferentiated as it penetrates into the skin allowing easier passage of material. The sebaceous gland openings into the