Integumentary System
Lesson 5: Dermatological Drugs
5-3
5-3. FACTORS AFFECTING SURFACE DRUG ABSORPTION
Many factors affect how the application of a particular drug will be absorbed by the skin surface.
a. Skin Permeability.
(1) Skin is durable because of the dermis, which is composed of connective tissue made up of fibers. Skin is selectively permeable; that is, skin allows only certain substances to enter the pores. The stratum corneum, the first layer of the epidermis, is a dense layer made up of dead, flattened cells that are filled with keratin (an insoluble protein). This layer of dead cells resists substances that are water-soluble or fat-soluble. In other words, the stratum corneum acts as the raincoat of the skin. If the epidermis is removed, the deeper layers of living cells, the dermis, act as a barrier to keep out fat-soluble substances.
(2) Because the skin has an oily secretion, medication applied to the skin surface is absorbed best if such medication is suspended or dissolved in oily media. Drugs combined with inorganic substances such as petroleum are not absorbed as well as drugs combined with synthetic ointment bases that are like sebaceous secretions.
b. Drug Particle Size. The size of the particles in the medication is an important factor in skin absorption. Very little absorption takes place if the particles in the skin medication are large and insoluble; for example, as in zinc oxide ointment. On the other hand, a great deal of absorption takes place when a solution such as oil of wintergreen in olive oil or in a lanolin base is rubbed on the skin.
c. Degree of Skin Hydration.
(1) Medication is absorbed by the skin better if the cornified layer (the top layer of the epidermis) is moist. Ointments soften the skin by wetting it, thus allowing the medication in the ointment to be absorbed into the skin easily.
(2) Another way to get moisture into the skin is to use an occlusive dressing over the skin lesion. An occlusive dressing is a dressing that prevents the loss of moisture from the skin's surface. This type of dressing can be made by placing an airtight plastic film (for example: Saran Wrap® or Handy Wrap®) over the medicated skin. Moisture is kept in the skin allowing the medicine to be absorbed into the skin. If a corticosteroid medication has been used, this medication will reduce skin inflammation faster. The occlusive dressing has kept moisture on the skin as well as prevented the medication from evaporating.
d. Contact Time. Absorption of medication on the skin is increased if the medication is in contact with the skin for longer periods of time. Since all disease organisms are not killed at the same time, there is a gradual decrease in the number of organisms. The longer the medication is on the skin lesions, the more
organisms will be killed.
e. Degree of Friction. Skin medication can be absorbed better as the degree of friction is increased.
f. Skin Temperature. When the temperature of the skin increases, skin medication is absorbed faster. Also, in many cases heat alone is enough to kill disease organisms.
g. Epidermal Damage. The epidermal layer of the skin is the protective layer. Medication applied to an area in which this layer has been damaged means that there is nothing to keep the medication out; therefore, the medication will be absorbed quickly.
