transdermal patch

A transdermal patch is a sticky bandage-like device that you press onto the skin, but instead of just covering a wound it slowly leaks a drug through the skin and into the bloodstream over hours or days. Think of it as a tiny reservoir taped to your arm that drip-feeds medicine without a single needle or swallowed pill.

Mechanically, a patch is built in layers: a backing film that keeps the drug from evaporating, a drug-holding layer (either a polymer matrix that the drug is dissolved in, or a liquid reservoir capped by a rate-controlling membrane), an adhesive that glues it to the skin, and a peel-off liner you remove before use. The drug diffuses out of this layer, crosses the outermost skin barrier (the stratum corneum), and is picked up by capillaries underneath. Because the skin itself often controls the slowest step, many patches deliver an almost steady rate for a fixed wear time, then are replaced.

Only a narrow set of drugs suit this route: they must be potent (a low daily dose), reasonably small, and lipophilic enough to cross skin. Familiar examples include nicotine, fentanyl, nitroglycerin, estradiol, and scopolamine patches. The big advantages are steady blood levels, avoidance of first-pass liver metabolism, and easy removal if a problem arises; the limits are slow onset, possible skin irritation at the application site, and the fact that most drug molecules simply cannot permeate skin fast enough.