Transdermal patch is an adhesive pad placed on the skin to deliver medication into the bloodstream. For thousands of years, people have applied substances to the surface of the skin to treat a variety of conditions. However, few such substances, called topical medications, fully penetrate the skin and enter the bloodstream in significant amounts. As a result, most topical medications affect only the part of the body where they are applied. Scientists have long been interested in developing a method to effectively deliver drugs into the bloodstream through the skin without an injection. Such drugs can then have systemic (whole body) effects.
The first transdermal patch sold commercially, called Transderm Scop, was introduced in 1981. This patch is used to administer a drug for the prevention of nausea and vomiting associated with motion sickness. Drug safety agencies in many countries—including the Australian Therapeutic Goods Administration, Canada’s Therapeutic Products Directorate, the United Kingdom’s Medicines and Healthcare Products Regulatory Agency, and the United States Food and Drug Administration (FDA)—have since approved several drugs administered through transdermal patches. These include drugs for angina (heart pain) and other types of pain. Transdermal patches are a popular way to deliver a continuous dose of nicotine, the addictive ingredient in cigarettes, for people who are trying to quit smoking. Other patches supply estrogen or testosterone for hormone replacement therapy. Patches for hypertension (high blood pressure) and contraception (prevention of conception or pregnancy) are also available in many countries.
Transdermal patches have several advantages compared to drugs that are swallowed or taken through an injection. Transdermal patches are easy to use and help ensure that patients follow a physician’s instructions for drug use. Unlike injections, the patches provide a continuous, steady dose of a drug into the bloodstream. Administering a medication through the skin also avoids any breakdown of the drug in the stomach, intestines, and liver. Up to 90 percent of the effective dose of a swallowed drug can be lost through the digestive system.
There are two main types of transdermal patches, passive-delivery patches and active-delivery patches. Most transdermal patches use passive-delivery. In some passive-delivery patches, the drugs are mixed with adhesive used to stick the patch to the skin. In others, the drug is kept in a liquid reservoir or semi-solid layer resting above or below the adhesive layer. In some types, the drug is included in a semi-solid material called a hydrogel. The drug in the patch then diffuses (spreads out) directly through the skin into the bloodstream. In some patches, chemicals are applied to the skin to enhance the absorption of the drug. Passive-delivery patches can be single-layered or multi-layered to control the release of the drug over time.
Active-delivery patches use an electric current, ultrasound (high-frequency sound waves), or other energy source to improve the delivery of a drug across the skin. In active patches, the drug is usually incorporated in a semisolid substance. Power supply devices are either built into the patch or are connected to the patch when needed. In iontophoretic patches, an electric current assists the delivery of ionic drugs (drugs that have molecules with a positive or a negative charge) through the skin. Another type of active patch, called a sonophoretic patch, also called a phonophoretic patch, utilizes a brief skin treatment with ultrasound. The ultrasound creates cavitations (vacuum spaces) in the skin that allow the drug to pass through into the bloodstream.
See also Drug (Entrance into the body).
