Cold Sleep

Cold Sleep (コールドスリープ) is a method of preserving a living subject, freezing them into an inactive state in such a way that they can be revived at a later stage if the proper steps are taken. Not widely recognized as a medical technique, only Academy City's advanced technology and methods have been shown to be capable of successfully accomplishing cold sleep.

Specific Examples
In the case of Kihara Noukan, who was mortally wounded beyond the capability of Kihara Yuiitsu to save him in time and subsequently placed into cold sleep until a means could be found to save him, a flash-freezing machine at a nearby research facility was used for the freezing process. He was eventually revived and treated by Heaven Canceller, though the exact methods used in the process of unfreezing and saving him are not known.

Cold Sleep Murder Case
The Cold Sleep Murder Case (コールドスリープ殺人事件) was a crime perpetrated by the serial killer Innai Chigiri, known as Loophole for his exploitation of flaws in the social and legal systems, which involved the use of cold sleep.

In this incident, the culprit abducted a 14 year old girl, Asajimo Saemi, stripped her naked and used cold sleep methods to freeze her in a block of ice, leaving her in a state where she was technically dead but not definitely so. Loophole also left a large vertical wound on her back, such that she would start haemorrhaging if anyone tried to unfreeze her normally and die from the blood loss. He deliberately used the outdated saline catalyst method, which would harm her body and leave her unable to be revived if she wasn't unfrozen and revived within a certain time frame.

Other
In a file on the hypothetical impact of a large-scale blackout on Academy City and the steps needed to be taken for such an event, Kihara Yuiitsu noted the effects on subjects in cold storage, with Yuiitsu having a strict policy of installing secondary freezing systems using chemical coolants rather than electricity. With regards to cold sleep devices, she recommended giving them a thermos-like structure and sealing them in with chemical coolants like liquid nitrogen or helium. She also noted that care needed to be taken in handling them, as exposing them to room temperature while sealed would cause an explosion due to expansion from vaporization, with the thermos-like structure requiring a plug to allow vaporized gases to escape.

Partial freezing technology, similar yet different from proper cold sleep, is used in the HsF-00 and HsB-07 to freeze the bodies of the pilots at -70 degrees Celsius and reinforce them to endure the high inertial Gs, with life support machines carrying out organ functions and the brain's decision-making ability being the only thing left running, with brain signals being read through various methods to fly the aircraft.

Background
At some point in the past, the Beginning Child, the first esper produced by Academy City (over half a century before the present) and the foundation of the Power Curriculum Program, was secretly frozen and preserved, being officially recorded as having gone missing. A year before the start of the series, during Kamijou Touma's third year of middle school, she was awakened by L.S.S.'s Matsuo Ryuusuke using Mitsuari Ayu's Mental Stinger, having previously failed to rouse her using stimulation from artificial specimens he had created as a Biohacker.

In the early days of cold sleep research, a saline-based catalyst was used in the process, however this was superseded by other substances, the leading one being a synthetic preservative derived from the structure of egg white.

During her captivity when she was younger, Kihara Enshuu figured out and wrote down a proof of the foundational ideas behind a cold sleep device.

In a file she wrote on a hypothetical disaster scenario involving a large-scale blackout in Academy City, Kihara Yuiitsu included recommendations on the design of cold sleep devices.

Toaru Kagaku no Railgun: Cold Game
In the Cold Sleep Murder Case, the serial killer Innai Chigiri, known as Loophole, abducted Asajimo Saemi and froze her in a block of ice, leaving her in a state where she was technically dead but not definitely so, while also leaving a wound which would kill her if she was unfrozen normally and using an outdated method which left a limited time frame for her to be revived. The frozen girl was left below a ice skating rink in School District 15 where she was discovered by customers the following morning, and subsequently transferred to cold storage in a university hospital in School District 2 which her twin sister Asajimo Mebuki attended.

In his trial after being apprehended, Loophole and his attorney Tsujinaka Kyouka used the precarious and legally uncertain state of the frozen Asajimo Saemi in order to delay the proceedings and buy time to carry out a scheme to destroy or render the evidence against him unusable,     but Loophole's plan to save himself, which even involved sacrificing his attorney so the thwarted scheme could be passed off as solely her doing,  ultimately failed when Asajimo Saemi was successfully revived, due to Misaka Mikoto and Asajimo Mebuki's combined efforts.

World Rejecter Arc
After Kihara Noukan was mortally wounded by Kamisato Kakeru on December 3rd, unable to save him with what she currently had, Kihara Yuiitsu took him to a nearby freezing facility and used a flash-freezing machine to place him into cold sleep until a time when he could be saved.

Coronzon Arc
Around the time of the evacuation of Academy City, Heaven Canceller successfully revived and healed the frozen Kihara Noukan.