Cyborg left hands

During mass-scale cybernetic development during the mid-to-late 2010s, military-grade cyborgs' left hands were usually stored with various amounts of information, usually combat-related data, and normally stored on a holographic drive.

Content and development
Prior to the fall of the Patriots in 2014, 31 individuals, including Raiden, ended up undergoing cybernetic surgeries, which resulted in their left forearms being made into memory drives. Raiden himself would go on to aid in taking out the Patriots, while the other thirty would join the PMCs World Marshal and Desperado. The content was stored on a 3D state drive, which could render images and data from various cyborgs into 3D holograms via light impacting it in a specific way. This resulted in a much faster processing speed and an increased storage capacity than regular solid-state-drives, to the extent that the standard 3D holographic storage, a 2-inch square cube of lithium niobate crystal can store 4-terabytes of data, and was highly resilient to damage due to 3D state drives uploading its bit data via a optical diffractor spread across the medium rather than localizing it in one spot. The drive itself, however, will sometimes end up wiped prior to a cyborg's self-destruct protocol being initiated in order to ensure that the enemy couldn't retrieve their data, in case the left arm survived both the resulting explosion itself and the shockwave, as the cyborgs themselves often contained classified data that suppliers wanted to keep classified, and damage will sometimes result in the data contained inside to be corrupted, requiring a quarantine and retrieval upwards of several weeks.

The short list of information that could be stored in the cyborgs left hands included energy consumption, damage level, intracranial and blood pressure, blood sugar, other vitals, GPS history, speed and acceleration data, radio and optical readings, recorded computations from the internal expert battle-support system, Time-stamped readings of distortion and accelerations gauges whenever the body was impacted, stresses applied to the CNT muscle fibers in all four limbs, as well as any localized breakdowns, Power consumption rates, remaining battery charges type and amount of weaponry used, Codec logs and wavelengths, voice transcripts, encryption methods, and more. The cyborgs IDs were also located inside the arm, which also allowed them to act as effectively ID cards for entry and exit from secure locations. Their arms could also be uploaded as videologs for the purposes of downloading the data into Unmanned Gears, and their left arms also possessed a USB cable for this purpose.

The arm's storage memory data was directly connected to the cranial nerves, which was required for the cyborg to record everything that they saw, heard, and the like. As such, when removed, the cyborg's hippocampus was stimulated, resulting in unpleasant effects such as memory loss and intense flashbacks. Accessibility was largely the reason why the left arm was chosen to be a memory drive, as the arm could be removed voluntarily or can be procured easily in the event that it was forcibly removed by an enemy.

Usage
In 2014 through 2018, Raiden, thanks to his cyborg surgeries courtesy of the Patriots, had his left arm turned into a memory storage data, which contained several classified information. Eventually, during the Ambush at Africa, Raiden's left arm was forcibly removed by Samuel Rodrigues during their duel, with it being corrupted as a result. After the arm was retrieved, it was then hooked up to Maverick's database and underwent file retrieval while Raiden himself underwent emergency surgery, where he suffered the side effects of his left arms' forced removal.

The German cyberneticist Dr. Wilhelm Voigt intended to aid in creating a cyborg database via various left arms from cyborgs to aid in future development of cyborgs in terms of modifications, advances, and analyzing data for every conceiveable instance in cyborg-related combat, as cyborg warfare was still in its infancy. He had managed to collect at least 56 left hands and presumably upload their data by the time Raiden was dispatched to quell the Abkhazian Coup in 2018. In addition, in exchange for his supplying Raiden with his new cyborg body, he also requested that Raiden retrieve the left hands of any cyborgs that he encountered during his mission. He eventually gained left arms formerly belonging to 30 cyborg commanders from Raiden, and noted that they contained data relating to the Patriots, due to their earlier being experimented on by the Patriots just as Raiden himself had been.

At least one cyborg soldier stationed at a lab in Mexico had recorded Sundowner and Steven Armstrong's meeting with the scientist and his staff regarding the Sears Program, of which Maverick later uncovered when trying to investigate the facility.

Behind the scenes
Left arms were a collectible item in Metal Gear Rising: Revengeance. The storage capacity of the left arms were first hinted at in the viral marketing site "Make it Right" for Metal Gear Rising: Revengeance, which depicted file retrievals from Raiden's severed left arm as well as his surgery after being wounded in a mission. In-game, the player can retrieve left arms from cyborg commanders. See cyborg commander for more details.

In the weapons descriptions for the unique weapons (the weapons the Winds of Destruction had previously used), it was mentioned that they were created from analyzing combat data from the respective fallen Winds, although it is not made clear whether they were analyzed via their left arms.