A positronic device used by Doctor Bashir to take over the functions of damaged brain tissue in Vedek Bareil, keeping him alive through peace negotiations at the cost of his personality.
The positronic implant is an experimental prosthesis for the brain, built on the same principles as an android's positronic matrix. It can carry out the work of destroyed neural tissue, but nobody knows how much of a person survives the substitution.
In the Deep Space Nine episode Life Support, Vedek Bareil is gravely injured in a shuttle accident while helping Kai Winn negotiate a treaty with Cardassia. Bashir revives him, then keeps him working with the drug vasokin, which damages his organs and finally his brain. With Winn pressing for his advice, Bareil asks for the implant, and Bashir reluctantly replaces the failed half of his brain.
Bareil lives to see the talks succeed, but he is changed. When the remaining half of his brain fails, Bashir refuses to replace that as well, telling Kira that what remained would be a machine. Bareil dies with her at his side.