Nanodevice

Nanodevices (ナノデバイス) or Nanomachines are devices whose components are at or at or close to the scale of a nanometer (10−9 meters). They are microscopic objects that are used in a variety of ways in Academy City.

Known applications
However, it is also used for other things such as sending a command to the transmitter that can shut down the organs of its host, as what happened with Sunazara Chimitsu. It can also act like bugs that can be implanted on a person's body or clothes. As Kumokawa Maria demonstrates, it can be destroyed through heat.
 * Transmitter: The first use of the term "nanodevice" in the Toaru Majutus no Index novels. It uses the furigana that spells out Nanodevice, but its kanji can be translated as Transmitter (発信機). It is used by Academy City to prevent against the abduction of students by possible agents or spies when they leave Academy City premises. As it's name suggests it most likely transmits a signal which indicates where the student is.
 * Mimosa: The Mimosa can be referred to as nanodevices despite not having circuitry or power. They can be more accurately described as nano-sized reflective alloy.
 * UNDER_LINE: Only about a 70 nanometers across, the miniscule machines that make up UNDER_LINE creates a network spread all over Academy City, allowing him to survey any place he chooses.
 * Type: Mosquito: Baba Yoshio's robotic mosquito can insert nanodevices into a human, incapacitating them. It probably uses a Mosquito Needle, and moreover, seems to use a somewhat similar nanotechnology that incapacitated Sunazara Chimitsu. Yoshio notes that the nanodevices are expensive.
 * Medical purposes: Mugino Shizuri teases to Hamazura Shiage that she may have been revived through nanodevices. It can be assumed that there is technology which uses nanodevices for medical purposes.