Microbots

Microbots are tiny nanobots constructed by Hiro Hamada. Though they appear small, the Microbots can join together with each other to form various structures and constructs. Despite being used for a good purpose, Robert Callaghan steals and starts controlling them, using them as a dangerous weapon.

Description
Inspired by Hiro's previous invention, the Megabot, these small-scale machines are designed connect together through electromagnetism to form various shapes and perform tasks cooperatively. Thanks to their size, they are incredibly versatile; they can adapt to any situation and evade damage as well as supply reinforced strength when together.

They are all controlled by a single neurotransmitter, and whoever wears it can command the Microbots to respond by a thought. All Microbots are designed to operate at high efficiency, even if the controller does not command the Microbots to do so.

However, a critical weakness that the Microbots contain is their quantity; Microbots cannot self-replicate and must be manufactured into large amounts to be able to perform heavy tasks or complex, solid formations. Without enough of their own for support, Microbots are vulnerable to attacks and each can quickly fall due to the fragility of their miniature size.

Role in the film
The Microbots were created by Hiro for the San Fransokyo Tech exhibition with the intention of using them to help improve society's efficiency. However, the first generation of Microbots were stolen and used by Robert Callaghan to fake his death. Afterwards, he integrated the transmitter into a mask used for his identity as Yokai and began mass producing the Microbots to recollect the missing pieces of the Silent Sparrow portal. Once the portal was rebuilt, Yokai intended to use it to destroy Krei Tech as his revenge against Alistair Krei for causing the loss of his daughter Abigail. However, one Microbot that was kept by Hiro was reactivated due to Yokai's current use of the Microbots and tried to reunite with its kind. Hiro and Baymax follow it and discover Yokai's Microbot mass production. Because of the duo's discovery of his plan, Yokai commands the Microbots to attack them, but the two are able to escape.

Despite the intrusion, Yokai continues to use the Microbots in his construction, excavating lost parts of the portal out at sea. While this is going on, Hiro and Baymax track him down again with the lone Microbot Hiro kept, which immediately joins with the other Microbots. However, Yokai spots them and their friends Honey Lemon, Go Go Tomago, Wasabi, and Fred, and goes on a rampage after them using the Microbots until the team is able to evade them. When Yokai retreats to the island the portal was originally constructed on, the team (now upgraded into Big Hero 6) trace him there where Yokai attacks them. Knowing that the mask holds the transmitter, each member tries to remove the mask which would rob him of his control over the Microbots. Hiro manages to accidentally remove the villain's mask, temporarily deactivating the Microbots until Yokai escapes back with the mask. The day after, Yokai intercepts Krei's latest ceremony and causes a swarm of Microbots to capture Krei and raise the rebuilt portal to destroy everything Krei has built. Big Hero 6 arrives on the scene, however Yokai uses the Microbots to incapacitate each member with the Microbot's overwhelming pressure bringing each near the verge of death. Fortunately, Hiro instructs his team to send the Microbots into the active portal in hopes of subduing Callaghan's power. The plan is successful, as each Microbot formation destroyed by a member causes a rapid decline in the Microbot numbers while they are sucked into the portal until all of the Microbots disappear. Yokai's mask is destroyed by Hiro and Baymax to ensure that Yokai cannot use the Microbots anymore.

Trivia

 * The Microbots' black color and transforming nature are reminiscent of the Symbiotes from the Spider-Man series. This not surprising as Big Hero 6 was based on a comic published by Marvel Comics.
 * Hiro Hamada uses these as a weapon in Disney INFINITY 2.0.
 * During the film's development, early incarnations of the microbots were more chaotic and organic, and instead acted as a special suit for Yokai. Eventually, the microbots would have lost the need for Yokai and try to assume their own identity. However, this idea was scrapped due to the demand for the Microbots' technology to be from existing science instead of comic science.
 * The same technology that was used to animate Elsa's snowflakes were used to help animate the microbots.
 * An early idea for the Microbots was to make them fluid-like and sentient as if they were their own entity. After this idea was dropped, the crew looked at videos of fire ants and other insects building structures atop each other in order to give movement for the Microbots.
 * The concept of the Microbots was inspired by a real-life drone swarm developed by the University of Pennsylvania the filmmakers found on YouTube.