83Essays on Infinite LifespansRobert 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,
