**2. The human skin**

The potential of using the intact skin as the port of drug administration to the human body has been recognized for several decades. However, the skin is a very difficult barrier to the ingress of materials allowing only small quantities of a drug to penetrate over a period of time. In order to design a drug delivery system, one must first understand the skin anatomy and its implication of drug-of choice and method of delivery.

The human skin is the largest organ in our body with surface area of 1.8-2.0 m2 . It is composed of three main layers; the epidermis, dermis and hypodermis (subcutaneous layer) (Fig. 1). The skin is a well energized organ that protects the organism against environmental factors and regulates heat and water loss from the body.

(http://www.spring8.or.jp/en/news\_publications/press\_release/2011/110406/. Downloaded April 26, 2014).

**Figure 1.** Structure of the skin (http://www.naturalrussia.com/natural/skin/structure.html. Downloaded on April 26,

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Interlamellar regions in the stratum corneum, including linker regions, contain less ordered lipids and more flexible hydrophobic chains. This is the reason for the nonplanar spaces between crystalline lipid lamellae and their adjacent cells' outer membrane. Fluid lipids in skin barrier are crucially important for transepidermal diffusion of the lipidic and amphiphilic molecules, occupying those spaces for the insertion and migration through intercellular lipid

**Figure 2.** The stratum corneum

2014)

**3.1. The intercellular lipid route**
