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Given the central role that topical medications play in treating skin disease, nurses must understand the delivery systems of topical preparations to make the most informed decisions for their patients and provide adequate patient education to ensure the best clinical outcomes.
A primary method to treat skin disease is with the use of topically applied medications. To properly use these medications, it is important to understand the subject of percutaneous absorption. The fundamentals of percutaneous absorption, characteristics of the skin and medications that affect absorption, along with the implications for clinical practice will be discussed.
The Stratum Corneum and Percutaneous Absorption Mechanisms
Role of the stratum corneum. Percutaneous absorption refers to the absorption of topical medications through the epidermal barrier into underlying tissues and structures with transfer into the systemic circulation. The outermost layer of the epidermis, the stratum corn, forms the important barrier that regulates the amount and rate of percutaneous absorption. In its normal dry state, it is a very effective barrier. This epidermal permeability barrier in the stratum corneum serves the vital function of preventing a variety of substances from penetrating into the skin.
The formation of this barrier is accomplished through the intercellular lipids along with corneocytes; the primary cell of the epidermis (see Figure 1). The lipids comprise free fatty acids, ceramides, as well as cholesterol and are deposited in the intercellular interstices within the stratum corneum. The intercellular lipids provide the primary barrier to molecular movement across the stratum corneum by allowing diffusion at a rate 1,000-fold less than is allowed by cellular membrane. Corneocytes are cells that have differentiated into structures that contain primarily proteins and only 15% to 30% water. In comparison, other living cells contain approximately 80% to 90% water. The dry corneocytes and hydrophobic intercellular lipids comprise a highly organized and differentiated structure that forms an effective barrier to passage of substances to underlying tissues (Redelmeier & Schaefer, 1999). This structure is often described as mortar, the intercellular lipids, between bricks; represented by corneocytes designed specifically to perform the important protective function of the skin (see Figure 2).
Process of percutaneous absorption. Percutaneous absorption of topically applied medications is accomplished by the process of passive diffusion (Redelmeier & Schaefer, 1999). It requires substances...