A bio-dampener
A Bio-dampener is a small device developed by Erin and Magnus Hansen that cloaks the lifesigns of the person wearing it by creating a field around the body that simulates the physiometric conditions within the person's environment, such as a vessel. Magnus used the device to board a Borg cube undetected while conducting research on the Collective in the 2350s. The crew of Voyager then used them to try and get a transwarp coil from a borg ship. The Borg did not seem to of adapted to the technology. (VOY: "Dark Frontier")