A bio-dampener
A bio-dampener was a small device developed by Erin and Magnus Hansen in 2353 for use in remaining undetected aboard Borg vessels. The device operated by creating a field around one's body which simulated the physiometric conditions of the Borg ship, essentially acting as a form of camouflage. In order to function properly, the dampener had to be precisely tailored to its user's physiology.
In 2376, the crew of the USS Voyager replicated their own bio-dampeners, for use in "Operation Fort Knox," their attempt to steal a transwarp coil from a Borg sphere. Later, Captain Janeway and Lieutenant Commander Tuvok used them to infiltrate the Borg Unicomplex, where Seven of Nine was being held captive by the Borg Queen. While on board, Janeway and Tuvok encountered a force field preventing them from reaching Seven's location. To overcome this obstacle, they utilized a subdermal probe, implanted in a Borg drone, to discern the field modulation of the force field when the drone passed through it. Tuvok then adjusted Captain Janeway's bio-dampener to compensate, allowing her to pass through the field, but, in doing so, her dampener was damaged and ceased functioning. (VOY: "Dark Frontier")