For the real thing, please see portable regenerator.
The portable regeneration unit was actually the mobile emitter used by the USS Voyager's holographic Doctor.
In 2377, the Lokirrim detained Seven of Nine, Harry Kim and The Doctor while the three were on an away mission aboard the Delta Flyer. Seven was forced to download The Doctor's program into her cybernetic matrix to prevent him from being detected by the Lokirrim, who believed he was a photonic insurgent. When Ranek found The Doctor's mobile emitter, Seven identified it as her portable regeneration unit to mask its true purpose. Ranek subsequently asked another Lokirrim to have it analyzed. (VOY: "Body and Soul")