Transdermal drug delivery methods, composite computer artwork. Transdermal devices deliver controlled doses of drugs through the skin directly into the bloodstream. At top is a transdermal patch. The drug is stored in the raised reservoir. This type of patch is commonly used for hormone replacement therapy (HRT) and nicotine patches. At centre is an electric patch. An electric current is passed through 2 electrodes on the skin's surface, causing the drug to be pushed through the skin. This process, iontophoresis, is being developed for large ionised drug molecules. At bottom is an ultrasound machine, which uses ultrasonic frequencies to transfer high molecular weight drugs through the skin, a process known as phonophoresis.