Читать книгу Introduction to Nanoscience and Nanotechnology - Chris Binns - Страница 32

Human skin, with a typical surface area of 1.5 m², is an impenetrable barrier to the environment preventing the entry of any foreign matter. The structure, shown in Figure 2.8, is divided into three layers called the epidermis, the dermis and the subcutaneous layer with the surface layer of the epidermis composed of a layer of dead cells about 10 μm thick known as the stratum corneum. The dermis is a layer of living cells permeated with blood vessels near its base containing the oil and sweat glands and the hair follicles. The epidermis is also composed of living cells apart from the stratum corneum and contains the specialist melanin producing cells (melanocytes) that determine skin color.

Zinc oxide (ZnO) and titanium dioxide (TiO₂) nanoparticles in the size range 20–30 nm are widely used in cosmetic products such as sunscreens and there has been some concern that penetration to the bottom of the dermis could allow such particles to enter the blood circulation. To date, however there is no evidence that this can occur, indeed, studies of 18 nm ZnO nanoparticles [12] show that they do not penetrate the stratum corneum. Although particles can enter the hair follicles at the hair root, this part of the channel is also covered with a dead layer and prevents the particles reaching live layers. There has been some interest in transdermal applications of drugs, which is possible using microemulsions [13] though this is not relevant to nanoparticles.

Figure 2.8 Structure of skin. Human skin consists of three basic layers labeled the epidermis the dermis and the subcutaneous layer. The dermis is composed of living cells and contains, hair follicles, oil, and sweat glands. The epidermis is also composed of living cells apart from the top 10 μm, which is a layer of dead cells known as the stratum corneum.