الفهرس 
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Abstract
The skin is a complex organ covering the whole surface of the body, and is continuous with the mucous membranes lining the body’s orifices [1].It is the heaviest organ of the human body, on average accounting for 10% of the body mass and covering nearly 2m2 of the body surface area .It defines the boundary between the body and its surroundings [2]. The primary function of the skin is to provide a barrier between the body and the external environment. This barrier protects against the permeation of ultraviolet radiation (UVR), chemicals, allergens and microorganisms, and the loss of moisture and body nutrients. In addition, the skin has a role in homeostasis, regulating body temperature and blood pressure. The skin also functions as an important sensory organ in touch with the environment, sensing stimulation in the form of temperature, pressure, and pain .While the skin provides an ideal site for administration of therapeutic compounds for local and systemic effects, it presents a formidable barrier to the permeation of most compounds [3].
The most common routes of drug delivery are the oral and parenteral routes with the majority of small molecule drugs conventionally delivered orally. The oral route has the advantage of pre-determined doses, portability and patient self-administration. For these reasons, the oral route remains the most convenient means of delivering medications. However, most therapeutic peptides or proteins are not delivered by the oral route, due to rapid degradation in the stomach and size-limited transport across the epithelium. The primary mode of administering macromolecules is therefore via injection which is not without limitations, such as the invasive nature of injections eliciting pain and lower acceptance by patients, in addition to the requirement for administration by a trained administrator .Rationally, the conventional routes of medication delivery have many inherent limitations which could potentially be overcome by advanced drug delivery methodologies such as transdermal drug delivery (TDD) [4].
The administration of chemical agents to the skin surface has long been practiced, whether for healing, protective or cosmetic reasons. Historically, the skin was thought to be totally impervious to exogenous chemicals. Thus, topical drug therapy typically involved the localized administration of medicinal formulations to the skin, generally when the skin surface was breached by disease or infection and a route of drug absorption into the deeper.