Subdermal bioprobe

A subdermal bioprobe was a medical device that could be inserted under the skin of a subject to record data, which could then be analyzed with a physiometric program. It could also incorporate a homing beacon, allowing a transporter lock to be established on the individual.

During their studies of the Borg in the 2350s, Magnus and Erin Hansen used subdermal probes to keep track of specific Borg drones. 

In 2373, The Doctor injected Kathryn Janeway with a subdermal bioprobe so that he could monitor her biochemistry as she underwent a Nechisti Order purification ritual in an effort to save Kes, who had been injured by a Nechisti shrine. The Nechani bio-scanners detected the bioprobe, but they had no objections to Janeway using the device. 

In 2375, Kathryn Janeway and Tuvok injected a subdermal probe into a Borg drone in the Unicomplex so that they could determine the modulation of a Borg force field when the drone passed through. This allowed Janeway to adapt her bio-dampener to pass through the force field as well, though it only worked once.