This board answers many questions I have been asked about auto-injectors and how they work. I fired them, took them apart and did experiments on them in order to figure out how they worked and answer questions about them.
FDA cleared and hopefully soon to be marketed, Vaccject is a combined syringe-cartridge product with an integrated retracting needle. It could be an inexpensive alternative to auto-injectors and a safer and faster alternative to syringes and vials/ampules, for EMS systems and hospitals. It might also provide an option for patients in nearly half the world where epinephrine auto-injectors are unavailable.
The EpiPen needle shown here fired slightly off-center. This seems fairly common. It may be due to how it compresses the rubber sterility barrier that protects the needle (shown in the bottom image, where the white needle shield is partially cut away).
Mylan's Generic vs Brand Epi Auto-Injector, the only difference is the label-the devices and medication are identical.
This clear-shelled Auvi-Q reveals the intelligent design inside. The larger spring and attached pin (not visible) pierces the canister, releasing the argon gas that propels the needle (replaced by plastic in this model) and epinephrine. When fully injected, release of the gas from the upper compartment allows the smaller spring to retract the needle, all in less than 2 seconds.
In their Sept 8, 2016 letter to Congress, Mylan wrote: "This sterile injectable drug-in-device includes 26 separate parts". For the EpiPen Jr, I count 19 including the drug and the label; 22 if I include the case and the clip between two cases. The EpiPen would have one fewer part, missing the little black circular piece that goes over the needle hub, effectively shortening the needle.