81 Essays on Infinite Lifespans   Robert A. Freitas Jr. What sorts of medical nanorobots could we build, and what would they do, if we could build them? The first simple device that I designed 9 years ago was the respirocyte, an artificial red blood cell [10]. Natural red cells carry oxygen and carbon dioxide through- out the human body. We have about 30 trillion of these cells in all our blood. Half our blood volume is red cells. Each red cell is about 3 microns thick and 8 microns in diameter. Respirocytes are much smaller than red cells, only 1 micron in diameter, about the size of a bacterium. Respirocytes are self- contained nanorobots built of 18 billion precisely arranged structural atoms. Each device has an onboard computer and an onboard powerplant. I show them blue in color, because part of the outermost shell is made of sapphire, a tough ceramic made of aluminum and oxygen atoms that is almost as hard as diamond. These tanks could be safely charged up to 100,000 atmospheres of pressure, but we are conservative and only run them up to 1000 atmospheres. Most importantly, molecular pumps are arranged on the surface to load and unload gases from the pres- surized tanks. Tens of thousands of individual pumps, called molecular sorting rotors, cover a large fraction of the hull sur- face of the respirocyte. As the rotor turns, molecules of oxygen (O₂) or carbon dioxide (CO₂) may drift into their respective binding sites on the rotor surface and be carried into (or out of) the respirocyte interior. There are 12 identical pumping stations laid out around the equator of the respirocyte, with oxygen rotors on the left, carbon dioxide rotors on the right, and water rotors in the middle. Temperature and concentra- tion sensors tell the devices when to release or pickup gases. Each station has special pressure sensors to receive ultrasonic acoustic messages, so doctors can tell the devices to turn on or off, or change their operating parameters, while the nano- robots are inside a patient. The shaded area at left is the O₂