Repair kit

The Mire medical kit was a tool used by the Mire to repair themselves if necessary after a battle. After the a battle with the Mire in a Viking village, the Twelfth Doctor used a part of the kit to resurrect Ashildr, who had been killed during the battle. The kit brought her back to life and would never stop repairing her, making her effectively immortal. The Doctor gave Ashildr an extra kit to use for someone she wanted to keep with her. (TV: The Girl Who Died)

By 1651, Ashildr had not given the second chip to anyone, but when Sam Swift's death opened a portal to Delta Leonis, Ashildr put the chip into his head, which healed him and closed the portal. This brought Sam back to life, but, according to the Doctor, it was unclear if the power drain from the portal had used up the chip's power to make him immortal. (TV: The Woman Who Lived)