The following nanomedicine FAQ and their answers have been compiled by Robert A. Freitas Jr.
7. How would the nanorobots be retrieved from the body?Some nanodevices will be able to exfuse themselves from the body via the usual human excretory channels; others will be designed to allow ready exfusion by medical personnel using apheresis-like processes (commonly called nanapheresis) or active scavenger systems. It is very design dependent. In the case of the respirocytes, the removal procedure is fairly simple:
"Once a therapeutic purpose is completed, it may be desirable to extract artificial devices from circulation. Onboard water ballast control is extremely useful during respirocyte exfusion from the blood. Blood to be cleared may be passed from the patient to a specialized centrifugation apparatus where acoustic transmitters command respirocytes to establish neutral buoyancy. No other solid blood component can maintain exact neutral buoyancy, hence those other components precipitate outward during gentle centrifugation and are drawn off and added back to filtered plasma on the other side of the apparatus. Meanwhile, after a period of centrifugation, the plasma, containing mostly suspended respirocytes but few other solids, is drawn off through a 1-micron filter, removing the respirocytes. Filtered plasma is recombined with centrifuged solid components and returned undamaged to the patient's body. The rate of separation is further enhanced either by commanding respirocytes to empty all tanks, lowering net density to 66% of blood plasma density, or by commanding respirocytes to blow a 5-micron O2 gas bubble to which the device may adhere via surface tension, allowing it to rise at 45 mm/hour under normal gravitational acceleration."
(Quoted from Robert A. Freitas Jr., "Exploratory Design in Medical Nanotechnology: A Mechanical Artificial Red Cell," Artificial Cells, Volume 26, 1998, pp. 411-430. This paper is apparently the first detailed design study of a specific medical nanodevice (of the general type proposed by Drexler in Nanosystems) that has been published. See earlier description in: Robert A. Freitas Jr., "Respirocytes: High Performance Artificial Nanotechnology Red Blood Cells," Nanotechnology Magazine, Volume 2, October 1996, pp. 1, 8-13.)
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