The Bioelectricity Security System is a device invented by Linh Garan in order to prevent Earthens from being manipulated by Lunars, and also to keep Lunars from using the gift and protect them from Lunar sickness.
The device attaches to the applicant’s nervous system at the top of the spine. When used in Lunars, it prevents them from manipulating bioelectricity and from seeing hallucinations associated with “Lunar Sickness”. When used in an Earthen, it prevents them from being manipulated by Lunars and the ability to see through Lunar glamour.
In Cinder, it is mentioned that Cinder has a device that prevented her from using her lunar gift, although little other information regarding the device is mentioned. Dr. Erland breaks the device by applying pressure to it under the pretense of "adjusting her spine." This creates pain where the device is in times of strong emotion throughout the book, and it eventually lets Cinder use her lunar gift at the end of the book.
In the book Scarlet, it is revealed that Michelle Benoit also has the device, and is able to resist manipulations from the Lunar Special Operatives who kidnapped her in order to get information from her regarding Selene’s whereabouts. The Lunar Special Operatives do not figure out what is blocking her from their manipulation, so eventually she perishes. It goes unknown throughout the series until Cinder informs everyone after her recovery.
The device also comes up in Winter during the trial of Linh Adri on Luna, presented as evidence against her. Levana asks Adri why she did not choose to capitalize on the device once Garan passed away. Adri says that she had no knowledge of the device. Levana then claims that Adri is lying, although this was untrue. It is unclear whether Levana knew that Adri had no knowledge or not. It is also mentioned that the patent and all other documents regarding the device had been destroyed shortly before the trial. Adri eventually made it back to earth.
The device is later significant regarding the alliance between Earth and Luna. During the meeting, the Australian Governor-General Williams brought up the issues with free travel between Earth and Luna — the possibility of Lunars using their gift to manipulate Earthens. Cinder mentions the device and reveals that although the original paperwork was destroyed, there is a blueprint stored in the android Iko. Linh Adri later negotiated a royalty for the device, bringing her great wealth.