A surgeon upgraded his patients as part of Operation Exodus. After they kept protesting in pain, he developed an inhibitor to stop them caring about it.
The surgeon was a descendant of Mondasian crew members aboard a colony ship. He was affiliated with Operation Exodus, upgrading his patients so that they could survive the harsh conditions of Floor 1056, and so that they could eventually climb to the upper floors (notably Floor 0507 and Floor 0000) and take command of the ship.
After Bill Potts was shot on Floor 0000, she was brought to the surgeon's Conversion Theatre by a group of Patients where he fitted her with a metal heart.
Some years later, he sent Mr Razor to collect Bill and deceptively bring her to the Conversion Theatre. The surgeon showed Bill a new prototype head-piece which would inhibit her emotions. Against her will, he proceeded to convert her into a Cyberman. (TV: World Enough and Time)
Sometime later he was presumably converted himself, though as he was the one performing the operations, it can be assumed he was one of the last to be so.