83 Essays on Infinite Lifespans   Robert A. Freitas Jr. Another medical nanorobot I designed more recently is the microbivore – an artificial white cell [12–15]. One  main  task  of  natural  white  cells  is  to  absorb  and digest microbial invaders in the bloodstream. This is called phagocytosis.  Microbivore  nanorobots  would  also  perform phagocytosis, but would operate much faster, more reliably, and under human control. Like the respirocyte, the micro- bivore  is  much  smaller  than  a  red  blood  cell  but  is  more complex than the respirocyte, having about 30 times more atoms involved in its construction. The microbivore device is a flattened sphere with the ends cut off. It measures over 3 microns in diameter along its major axis and 2 microns in diameter along its minor axis. This size helps to ensure that the nanorobot can safely pass through even the narrowest of human capillaries and other tight spots in the spleen (e.g., the interendothelial splenofenestral slits [16])  and  elsewhere  in  the  human  body.  The  microbivore has a mouth with an irising door, called the ingestion port, into which microbes are fed to be digested. The microbivore also has a rear end, or exhaust port. This is where the com- pletely digested remains of the pathogen are expelled from the device. The rear door opens between the main body of the microbivore and a tail-cone structure. Inside the micro- bivore, there are two concentric cylinders. The bacterium is minced  into  little  pieces  in  the  morcellation  chamber,  the smaller inner cylinder, and then the remains are pushed into the digestion chamber, the larger outer cylinder. In a prepro- grammed sequence engineered digestive enzymes are added, then removed, using an array of sorting rotors. In just 30 sec- onds these enzymes reduce the microbe’s remains to simple chemicals like amino acids, free fatty acids, and simple sugars, which are then expelled harmlessly from the device. A human neutrophil, the most common type of leukocyte or white cell, can also capture and engulf a microbe in a minute or less,