Nanorobots working on an embryo, computer artwork. Nanotechnology is an area of science concerned with producing mechanical entities whose size is measured in nanometres (billionths of a metre). It is hoped that robots of this size will have medical applications; they could be used at a cellular level to fight illnesses. It is also possible that they could be applied to embryology: to perform operations in the womb or to administer gene therapy. A human embryo is defined as being less than eight weeks old. At this stage it develops primitive organ systems and the beginnings of limbs.