Repair kit

Mire repair kits, also referred to as immortality charges, were battlefield medical kits used by the Mire, and stored in their battle helmets. These chips linked biologically with their patients, and repaired them continually.

Following a Mire incursion on Earth in the 9th century, the Twelfth Doctor modified two repair kits for human use, and linked one of those kits to a near-dead Viking girl named Ashildr, resulting in her resurrection and subsequent immortality. The Doctor also left the second modified kit with her, indicating that she could use it on anybody she decided she didn't want to live without. (TV: The Girl Who Died)

Though the immortality charge kept Ashildr healthy, and prevented her from ageing, it did not augment her capacity for memory, and she struggled to retain more than an ordinary human lifespan's worth of recollections. She kept the extra repair kit on her person, as a necklace; in 1651, she told the Doctor she had never found anybody worthy of the kit. It was eventually used on highwayman Sam Swift by Ashildr, to close a portal generated by the Eyes of Hades and prevent a Leonian invasion. Due to the power drain from the portal, the Doctor was uncertain as to whether or not Sam Swift would become immortal, or to what extent his lifespan would be altered by the influence of the kit. (TV: The Woman Who Lived)

Because of the immortality charge, Ashildr remained youthful for billions of years, and was still alive and well during the last 5 minutes of the universe, outliving even other immortal lifeforms. (TV: Hell Bent)