Medi-drones were autonomous drones capable of administering the correct dosage of a drug. They were among the medical amenities available in a Battle TARDIS.
When Heleyna took control of Vassarian's Battle TARDIS on behalf of the Dalek Time Strategist, Kalan wounded her. Bleeding, Heleyna left a trail to the medical bay and healed herself. Knowing that the War Doctor would follow, she hacked into a medi-drone to delay him. (AUDIO: Eye of Harmony)