Following on from the two-part article last year, Ric Williams looks at the subject of anti-ageing, starting with the transport of active ingredients through the skin.
In terms of transport of active ingredients across the skin membrane, the rate of dermal absorption of a substance is proportional to both the concentration of the substance and the surface area over which it is applied.
The wider the contact area and the more concentrated the substance, the greater will be the absorption. However, the chances of materials passing through the skin are slim and many do not pass through the skin at all. The outermost layer is a layer of dead skin and it must pass through the intercellular channels between cells in this layer. The next layers are a series of lipid layers, hence lipophilic ingredients have some chance whereas lipophobic (or hydrophilic) ingredients have great difficulty. Other factors affecting the rate and extent of absorption are:
Structure of skin
The thickness of the skin, especially the stratum corneum, also determines the degree to which substances are absorbed. Thicker skin is a greater barrier to passage of foreign substances. Depending on skin thickness, there can also be variability in absorption of a given substance by different regions of the body. For example, hydrocortisone is absorbed over 50-times greater by genital skin versus the skin of the palms. Hair density, and physical health of skin have an effect. Damage to skin, both through disease or direct environmental influence, can also alter the barrier properties of skin and enhance absorption of substances. Even something as innocuous as the removal of outer layers of skin with cellophane tape can apparently dramatically increase dermal absorption.
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