The skin of Sigma's hand is ABT.
Artificial Biological Tissue, often shortened to ABT, is a type of artificial skin. It's incredibly realistic and detailed, even mimicking the pores in human skin. It's able to feel touch, heat, and other such things. It's usually worn by GAULEMs and applied over their metal skeleton, but has been used for human prosthetics as well.
Nine Hours, Nine Persons, Nine Doors[]
Although unconfirmed, it is implied Light Field has an ABT precursor. His left arm and left hands are seen in different positions, such as a clenched fist and open palm, possibly implying that he also has cybernetic nerves.
Virtue's Last Reward[]
In the secret file GAULEM obtained from the GAULEM Bay, it is revealed that Zero Sr. created ABT in preparation of the AB Game. It is unknown if Zero Sr. is the sole creator, or if there were any other contributors to the development of ABT (such as Hephaestus Systems).
Luna END[]
After Sigma, Alice and Luna investigated the GAULEM Bay, G-OLM suddenly activated and started to talk with them. While conversing, G-OLM informed the three about ABT and that the tissue appeared very realistic on GAULEMs. He even went as far to say that the nine of the participants wouldn't be able to tell if a GAULEM was among them, and suddenly was deactivated.
It was later revealed to Sigma and Phi in the Director's Office that Luna was a GAULEM, and that she looked human due to ABT. The two still allied with Luna in the AB Game, and reached nine BP. As the two were escaping the facility with Quark, Sigma decided to chase Luna to the B. Garden, leaving Phi and Quark to escape without him.
Luna's ABT falling off.
When he reached the B. Garden, Sigma talked with Luna about the events that happened at the facility. During their discussion, Luna revealed that she had disobeyed orders from Zero III and that she would be deactivated because of it. She thanked Sigma for trusting her, and started to cry. The tubocurarine she was injected with earlier caused the ABT to fall off of her face, and she was deactivated by Zero III.
Sigma END[]
Sigma's ABT after being cut.
While Sigma is trying to stop Alice from killing herself because of Radical-6, Sigma cuts his left hand with the scalpel he took from Alice. After taking Alice back to the infirmary, Sigma and Luna head to the AB Rooms for the second round of the AB Game. While inside the room with Luna, she realizes that Sigma has something white on his hand. Sigma is shocked to see that it is coming from the cut on his hand.
After everyone votes ally, Luna decides to speak with Sigma in the B. Garden. During their conversation, Luna asks Sigma if he is a robot because he knew about the garden before he ever visited it. Luna then explains that the white liquid is ABT and it is similar to robot blood. She tells him that the ADAM can determine if he is a robot. Soon after, Phi interrupts them and tells Sigma and Luna that she found Bomb 1.
Sigma's cybernetic arms contain ABT.
After Dio admits to setting the bombs and activating them, Sigma decides to go to the Infirmary to have Luna scan him. The ADAM's results show that Sigma is not a robot, but he has cybernetic arms. This leaves Sigma confused because he doesn't remember having cybernetic arms at any point in his life. Just before the bombs explode, Luna tells Sigma her GAULEM ID.
Zero Time Dilemma[]
Sean bleeding white blood.
It's implied Sean has either ABT or a precursor since he also bleeds white liquid.