A specific dose of medication delivered through the skin

A transdermal patch or transdermal system (TDS) is a medicated adhesive patch that is placed on the skin to deliver a specific dose of drug through the skin and into the bloodstream.
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The fact that transdermal patches can provide a controlled release of medication from 24 hours up to seven days gives it a major advantage over other types of drug delivery, such as oral or other types of topical administration (for example, gels or creams).

The delivery of a drug via a transdermal route of application offers the best of oral and intravenous administration. Avoidance of the first-pass effect leads to better bioavailability and may result in fewer side effects. Consistent and controlled drug delivery can minimize or avoid periods of under- and overdosing. Transdermal patches deliver drugs over a longer period of time, thus allowing for reduced dosing frequency. A transdermal patch is administered by the patients themselves and administration is stopped by removal of the patch. Contrary to intravenous drips or infusion pumps, patient activity is not restricted. These numerous benefits lead to significantly improved patient compliance and as a consequence transdermal patches can offer a critical edge to new and established drugs in competitive markets.
A typical transdermal system is composed of an adhesive matrix sandwiched between a backing foil and a release liner. In the simplest case, the matrix consists of the active pharmaceutical ingredient (API) and one type of adhesive, with polyacrylate, silicone or polyisobutylene adhesives being the most commonly-used patch adhesives.

The release liner, which is removed before application, is usually a siliconized or fluoro-siliconized polyester film. The backing foil that stays on the patch, and forms the outer layer of the transdermal system, can be made from a wide range of materials. It can come in many colors and can also be printed on. There are two types of backing foils, those that greatly restrict vapor transmission (occlusive backing foils) and those that permit vapor transmission (non-occlusive). Both types of foils demonstrate advantages and disadvantages, ranging from wearing comfort to influence on patch performance.
These fundamentals form the basis for developing transdermal delivery systems for specific APIs. Molecular size and charge are crucial characteristics of an API when considering its suitability for transdermal delivery; to pass through the outer skin layers the molecule should be small, have an appropriate solubility and lipophilicity, and should be available as a free base. Solubility and stability of the API in the adhesive polymer and performance of a prototype transdermal patch with this API in in vitro skin permeation experiments will then determine the choice of adhesive and the need for additional excipients.