Dermal absorption is the process by which substances applied to the skin are transported into the body, either remaining within the skin layers or entering the bloodstream. It represents one route through which various compounds, including both beneficial and harmful substances, can enter the biological system.
The Skin’s Role in Absorption
The skin serves as a barrier, with its outermost layer, the epidermis, providing the primary defense against external substances. The stratum corneum, the most superficial part of the epidermis, acts as the main protective layer. This layer is composed of flattened, dead skin cells embedded in a lipid matrix, resembling a “brick and mortar” structure. Its composition and organization are responsible for its barrier function, limiting the passage of most materials.
Substances can penetrate the skin through several pathways. The transcellular pathway involves direct movement through the skin cells themselves, crossing the cell membranes. The intercellular pathway, considered the more common route for many substances, involves movement through the lipid-rich spaces between the skin cells. Substances can also be absorbed through skin appendages like hair follicles and sweat glands, though these pathways contribute a minor amount to overall absorption.
What Influences Absorption
The rate and extent of dermal absorption are influenced by a combination of factors related to the substance itself, the characteristics of the skin, and the conditions of exposure. These elements determine how readily a compound can overcome the skin’s barrier.
Substance characteristics play a role in determining absorption. Molecular size is a factor, with smaller molecules penetrating more easily than larger ones. Lipid (oil) solubility is also important; substances that can dissolve in both oil and water absorb well, as they can navigate both the lipid-rich intercellular spaces and the more aqueous cellular environments. The concentration of the substance on the skin surface directly impacts absorption, with higher concentrations leading to greater uptake. The vehicle, or the substance in which the active compound is dissolved or mixed, can also affect absorption by influencing the compound’s solubility and how it interacts with the skin.
Skin characteristics also contribute to absorption variability. Skin hydration can increase permeability, as a well-hydrated stratum corneum becomes more pliable and less resistant to penetration. The integrity of the skin is another factor; damaged or compromised skin, such as cuts or abrasions, allows substances to absorb more readily than intact skin. The thickness of the skin varies across the body, influencing absorption rates; for instance, skin on the palms and soles of the feet is thicker than on areas like the eyelids. Regional differences in skin thickness and appendage density also influence absorption.
Exposure conditions further modify absorption. The duration of contact between the substance and the skin directly correlates with the amount absorbed, with longer exposure times resulting in greater uptake. Temperature also affects absorption; elevated skin temperatures can increase blood flow to the skin and alter the stratum corneum, potentially enhancing the penetration of some compounds.
Real-World Importance
Dermal absorption has implications in various aspects of daily life, ranging from medical treatments to considerations of chemical safety. Understanding this process informs how we interact with many products and environmental agents.
In therapeutic applications, dermal absorption is harnessed through transdermal drug delivery systems, such as medicated patches. These systems allow medications like nicotine for smoking cessation, hormone replacement therapies, or pain relief drugs to be absorbed through the skin and enter the bloodstream. This method offers benefits such as sustained and controlled release of medication, which helps maintain steady drug levels in the body and avoids the “peak and trough” effects seen with some oral medications. It also bypasses the digestive system, which can be advantageous for drugs that might be degraded by stomach acids or metabolized too quickly by the liver.
Dermal absorption is also relevant to chemical exposure and safety. Everyday items like cosmetics, household cleaning products, and pesticides can contain chemicals that may be absorbed through the skin. In occupational settings, workers can be exposed to industrial chemicals through skin contact, which can contribute to their overall exposure. Awareness of dermal absorption helps in developing safety guidelines, such as using protective clothing, to minimize exposure and health risks from these substances.