The two most popular types of Fentanyl patches are the reservoir design, and the matrix design. Immediately below is a diagram of the reservoir design:
As you can see, the reservoir patch gets its name from the fact that the fentanyl is contained in a reservoir. In Duragesic fentanyl patches and authorized generics such as the Sandoz fentanyl patch, this reservoir will be filled with a fentanyl gel. And now, the matrix patch:
There is no fentanyl gel in a matrix style patch. Mylan pharmaceuticals makes a matrix patch. A matrix fentanyl patch places the fentanyl in adhesive. There is therefore no gel to link. The matrix design is inherently safer because it cannot leak gel.