Microcircuit fibers were cybernetic technology used by the Borg. Microcircuit fibers extensively infiltrated the tissue surrounding Borg implants, during the assimilation process and rewrote the DNA of the host's cells. When a Borg drone was separated from the Collective, the DNA immediately began to return to normal, as did the individual's vital signs. (TNG: "The Best of Both Worlds, Part II" )