A transdermal patch is normally composed of an external protective layer, an intermediate zone containing the active ingredient and an adhesive that ensures the contact of the preparation with the skin. The external protective layer has the function of supporting and protecting the device and, normally, it is impermeable to drugs and water.
The various types of patches differ in shape, size, release time and characteristics of the intermediate area. The most common is the matrix patch, in which the active ingredient is dispersed in a polymeric substrate that can also act as an adhesive. These patches allow the drug to passively pass to the skin, through a single-layer membrane or a multilayer system that regulates its release.
Instead, tank patches are formed from a sort of pocket that contains the active ingredient in liquid or suspended form. One example is the transdermal patches for menopause, which release the drug at predetermined times thanks to a semipermeable membrane. This type of patch is bulkier than matrix patches and should never be cut.
In any case, however, the release of the active ingredient varies according to the excipients - which make the drug available and absorbable transdermally - and the size of the patch itself.
