Disposable Pen Injector Pen Injector is a type of reusable liquid medicine injection device that contain liquid medicine solution with a syringe. The Liquid medicine medication is placed in the pen in a cartridge, eliminating the need to extract liquid medicine each time. Simply remove the pen cap, adjust the dial to the desired dose, and then perform the liquid medicine injection. Currently, NEST Pen Injector are available in two categories: Reusable Pen Injector and Disposable Pen Injector.