Can't find anything on Google and throughout all my hundreds of hours played I've never come across it.
We have added this to our Known Issues list, but here's some more information:
The Stimpak Diffuser is a Stimpak that restores health to you and your nearby teammates. The recipe for this item will be available in the next update. We're aware that this creates an issue with completing the Tadpole Challenge referencing the Stimpak Diffuser, however you may still complete other world Challenges to earn badges.