Fetal Borg drone inside a maturation chamber.
A maturation chamber is a device used by the Borg Collective to accelerate the development of adolescent humanoids until they are ready to service the Collective as a drone. Among those who were placed in such chambers are Annika Hansen (Seven of Nine) and Icheb. (TNG: "Q Who"; VOY: "Collective")
Children assimilated by the Borg are placed in maturation chambers where they will spend the following seventeen cycles. (VOY: "Mortal Coil")
After assimilating the Doctor's mobile emitter and sampling DNA from Ensign Mulcahey in 2375, Seven of Nine's nanoprobes created a modified maturation chamber in one of Voyager's science labs. The growth rate of the drone inside was 25 times faster than normal. The chamber was protected by a force-field. (VOY: "Drone")