Cyborg

A Cyborg (サイボーグ), short for "cybernetic organism", is an individual who has been mechanically modified in order to achieve enhanced performance or compensate for compromised or lost body parts. They primarily belong to the Science side and presumably only Academy City has enough technology to properly produce them.

Principles
A cyborg is an individual who has had their body partially or wholly replaced with mechanical parts. The exact definition and extent varies, from artificial organs to full body mechanization, but in Academy City, a cyborg is specifically defined as someone who had the necessary parts all contained within their body - for example, optical fiber nerves might make someone a cyborg, but heavy metal claws would not.

Cyborg technology is used for both medical reasons, such as performing the functions of damaged or lost body parts,   and for combat purposes, such as enhancing one's abilities and weaponry,  with different variants for the technology designed for specific purposes.

While cyborgs are primarily restricted to the Science Side and Academy City, there are a few individuals elsewhere who are referred to as cyborgs but were made through non-scientific means, such as Salome of the Kamisato Faction.

There are a number of projects and experiments which have their roots in cyborg research, including the Total Human Cyborg Replacement Project, which aside from testing technology also had the hidden objective of observing the creation of a soul, and Anatomy Mechatronics, intended to make perfect models of individuals' bodies (such as the Level 5s) using technology.

DRAGON Arc
Shiokishi referenced to cyborgs and their downsides (mainly the need for maintenance every few years and surgery for dealing with the artificial organs), when talking about his choice of a Powered Suit.

Freshmen Arc
Two prominent members of the Freshmen, responsible for the events which occurred on November 5th, were cyborgs - Silvercross Alpha, whose enhancements supported his use of Powered Suits, and Kuroyoru Umidori, whose robotic arms were meant to support her ability.

Agitate Halation Arc
Rensa, a cyborg developed to counter the Level 5s if they rebelled against Academy City, played a prominent role in the events surrounding the Agitate Halation Project.

Salome Arc
Salome, Kamisato Kakeru's non-blood sister and a member of the Kamisato Kakeru, was noted to have abandoned her original body for a stronger one using a chemical cyborg treatment, in order to drastically raise her abilities in conjunction with her sacrificial magic, Internal Offering and External Offering.

Dream Ranker Arc
The distribution of Indian Poker in September and early October was a result of Kuriba Ryouko trying to find a solution to the problem posed by her mechanical doppelganger, created as a result of the cyborg experiment she participated in. At the same time, she was also working on the cybernetics needed to save her mother. After being accidentally shot during the events at the time of the doppelganger's termination, its parts were used to save her life.

Toaru Kagaku no Dark Matter
Kihara Souji was noted to have started his career in medical science and the development of cyborg technology, specifically mechanical prostheses. During the events surrounding Yuzuriha Ringo and SCHOOL, he replaced at least one DA agent's lost arms with his prosthetic limbs, and after his death, Kuroyoru Umidori took his data and existing prostheses for her own use.

Toaru Jihanki no Fanfare
As a result of injuries from a past experiment, Kihara Nayuta has medical-purpose cybernetics making up over 70% of her body.

Toaru Majutsu no Index: Shokuhou Misaki Figurine
During Shokuhou Misaki's encounter with the Anatomy Mechatronics Misaki, it was noted that the project had branched off from cyborg research.

Souyaku Toaru Majutsu no Index SS
While facing Hikarizawa Megumi, who was connected to a life support system made up of large medical devices, Kuroyoru Umidori compared them to artificial organs too big to fit inside her and the methodology to Shiokishi's previous attempt in the form of a Powered Suit, remarking that the system was two generations out of date compared to the more compact technology making up a cyborg.

Known Cyborgs

 * Kihara Gensei, the man obssessed with Level 6 and creator of Ability Body Crystal
 * Kihara Nayuta, member of Judgment.
 * Mugino Shizuri, member of ITEM
 * Kuroyoru Umidori, member of Freshmen.
 * Silvercross Alpha, member of Freshmen.
 * Rensa, anti-Level 5s rebellion unit. (#28 and #29)
 * Kuriba Ryouko (former) (split in two, later recombined)
 * Salome, member of Kamisato Faction

Trivia

 * In the afterword of , Kamachi Kazuma pointed out that it was Narita Ryohgo (of Bacanno! and Durarara!! fame) who introduced the concept of cyborgs into the universe and thus decided to use the concept as a tool in the main series.
 * Kamachi also noted that in the volume, there were two types of cyborgs; one of them being a cyborg that altered her outer silhouette rearranging the inside of her body in order to increase the proficiency of her ability and the other one a "powered suit" that altered the outside of the body and optimized the inner workings of his mind.
 * While the Freshmen cyborgs were modified for military purpose and meant for battle, the first cyborg created by Narita was modified for medical purposes, with her enhancement not meant to boost her combat ability.