Abstract

The advantages of transdermal drug delivery over other routes of drug delivery are manifold. Examples of approved transdermal applications are estradiol and testosterone in hormone replacement, clonidine for hypertension, fentanyl and lidocaine for pain management, nicotine for smoking cessation, nitroglycerin for angina pectoris, norelgestro- min/ethinyl estradiol for contraception, and scopolamine for motion sickness. Only a limited number of drugs can be successfully developed into transdermal products because of the skin barrier. To overcome the skin barrier, chemical and physical enhancers are used to facilitate drug transport across the skin. Chemical enhancers are commonly used in transdermal products and are generally effective for therapeutic agents of small molecular size and moderate lipo- philicity. Physical enhancers are usually developed with the ultimate goal to enhance the delivery of macromolecules. Among the physical enhancers, the concept of using electric field to enhance transdermal transport has been around for decades, and this technology has evolved into commercial products for drug delivery. Other technologies such as ultrasound, abrasion, and microneedles are based on the mechanism of disrupting the skin barrier with physical means. A complete review of transdermal delivery technologies is exhaustive and will not be included in this chapter. This chapter will provide a general overview of skin morphology, basic transdermal transport theories, and transdermal drug delivery technologies with the main focus being technologies already used in devices in late- stage clinical trials, recently completed New Drug Application filing, or approved by Food and Drug Administration (FDA). In particular, the mechanisms of enhanced transdermal transport such as iontophoresis, electroporation, sonophoresis, and abrasion with microneedles, heat, and laser will be discussed.