Absorption is also influenced by other skin properties that vary at different cutaneous anatomical sites. For instance, the absorption diminishes greatly as one moves from the palpebral skin to the plantar surfaces .
Age influences skin absorption. Various biological activities are lower in the skin of the aged individual. Great variation is also noted for the premature infant and neonate, who have greater cutaneous permeability . There are no experimental data confirming the validity of friction on transcutaneous absorption . Alterations of the barrier induce modifications of TEWL . In addition, the horny layer may be defined as a biosensor; alterations of external humidity regulate proteolysis of filaggrin, synthesis of lipids, DNA, and proteins within keratinocytes, which can lead also to inflammatory phenomena .
The cutaneous bioavailability of most commercial dermatological formulations is low (within 1-5% of applied dose) .
The active substances of topical formulations are generally absorbed in small quantities; only a reduced fraction passes from the vehicle into the stratum corneum. The greater part remains on the surface of the skin, subject to loss in several ways such as by sweating, chemical degradation, and removal.
f) Pressure waves by intense laser radiation (Transdermal drug delivery)
The photomechanical compression obtained by laser (Q-switched ruby laser) has been tested for modulating the permeation of the stratum corneum . The material (polystyrene) including the solution with the active principle (e.g., δ-amino-levulinic acid) absorbs the laser radiation, while the solution increases the propagation in the stratum corneum. The penetration route is apparently extracellular, as it is for sonophoresis and ionophoresis. The effect is only temporary and the barrier function is restored
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