Nanoprobes are submicroscopic robots, central to the assimilation process used by the Borg. They are delivered by injection tubules, which normally extend from the back of a drone's hand and can penetrate any known alloy, energy field, or biological tissue.
Once in the victim's bloodstream, the nanoprobes replicate and latch onto the subject's blood cells, altering their genetic code, so that they enter into the first stage of becoming a Borg drone. It takes just a few seconds from when the victim is injected to their skin turning gray and mottled, and for their attitude to become passive, ready for the next stage of assimilation when cybernetic implants are sugically added.
As well as organic tissue, nanoprobes are also encoded to utilize any technology that they encounter, adapting alien equipment so the Borg can make use of it. Both the U.S.S. Enterprise NCC-1701-E and the U.S.S. Voyager NCC-74656 have been partially assimilated, but perhaps the most extraordinary example of nanoprobes taking over alien technology happened during a transporter accident on Voyager, when some of Seven of Nin's nanoprobes fused with the Doctor's 29th-century holoemitter. The nanoprobes then used the technology in Voyager's science lab to create 'One,' the equivalent of a 29th-century Borg drone.
Nanoprobes are also used by the Borg to reactivate drones as long as 73 hours after their 'death,' a technique assimilated from Species 149. In 2374, Seven was able to modify nanoprobe technology to bring Neelix back to life, after the Talaxian had been dead for 18 hours following a shuttle accident. This process involved extracting 70 micrograms of nanoprobes from Seven's bloodstream and then matching them to Neelix's physiology. They were then released into Neelix's bloodstream, where they reversed the effects of cellular necrosis while his brain was stimulated with a neuroelectrical isopulse at one second intervals to bring him back to life.
Using the same principles, nanoprobes can be used to slow down the aging process. This makes them an extremely valuable commodity. Three members of the Ferengi believed they had found a way to bring Seven to the Alpha Quadrant, where they planned to sell the 3.6 million nanoprobes contained in her body at six bars of gold latinum per unit.
Nanoprobes can also be used to make a weapon. In 2373, the Doctor managed to access the nanoprobe recoding system so that they could be used against Species 8472. He reprogrammed them to emit the same electrochemical signatures as Species 8472's cells, effectively allowing them to override the species' natural rejection mechanism that had previously made them impervious to assimilation. Using this modified nanoprobe technology, the Borg and the crew of Voyager were able to turn photon torpedoes into bio-molecular warheads that were capable of destroying Species 8472's bioships.