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.