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.
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. (VOY: "Sacred Ground")
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. (VOY: "Dark Frontier")