The mechanical heart implanted in Jean-Luc Picard after he was stabbed by a Nausicaan as a young ensign, which twice came close to costing him his life.
Picard's artificial heart dates from 2327, when a Nausicaan's blade destroyed his own during a brawl at Starbase Earhart. He is embarrassed by the story and keeps it private.
In Samaritan Snare the implant is found to be faulty and he travels by shuttle with Wesley Crusher to Starbase 515 to have it replaced away from his own crew. The operation goes badly, and he wakes to find that the specialist flown in to save him is Doctor Pulaski.
In Tapestry an energy weapon fired at close range fuses the heart, and the dying Picard is met by Q, who observes that a natural one would have survived. Offered the chance to avoid the stabbing, he learns how much the injury shaped the man he became.