© 1999 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999

10.3.6 Buoyancy Control and Nanapheresis

Another design issue that may arise when operating in an aqueous medium is buoyancy, which can readily be controlled by loading or unloading ballast. At the extremes, a diamondoid sphere of radius 500 nm and wall thickness 5 nm may range in density from ~100 kg/m³ if filled with vacuum to ~3000 kg/m³ if densely packed with diamondoid machinery. Placed in blood plasma, these spheres would rise at ~0.5 micron/sec or fall at ~1 micron/sec, respectively, relative to the local gravity field, according to Stokes' Law for Sedimentation (Eqn. 3.10), taking r_plasma = 1025 kg/m³ and plasma absolute viscosity as h_plasma = 1.1 x 10^-3 kg/m-sec at 310 K. This range of speeds lies near the low end of cytonatation velocities (0.1-10 microns/sec; Section 9.4.6) and well below the maximum sanguinatation velocity (~10,000 microns/sec; Section 9.4.2.6), which suggests that buoyancy control is unlikely to play a significant role during routine in vivo nanorobot locomotion. By comparison, the small difference in density between individual red blood cells (1100 kg/m³) and blood plasma at 310 K causes them to settle out of suspension at a slightly faster 1-3 microns/sec depending on hematocrit (the volumetric percentage of blood occupied by red cells, typically ~46% in humans; Section 9.4.1.2) and degree of RBC aggregation. Free-floating natural erythrocytes appear unhandicapped by their faster settling rate, so active ballast management for free-floating artificial nanorobots is probably unnecessary in normal operations.

However, once a therapeutic purpose is completed it may be desirable to extract artificial devices from circulation. Active ballast control may be extremely useful during nanorobot exfusion from the blood, particularly in the case of simpler devices which are incapable of removing themselves from the body (Chapter 16). Blood to be cleared may be passed from the patient to a specialized centrifugation apparatus analogous to an apheresis circuit such as a cytapheresis or plasmapheresis system. In nanapheresis, acoustic transmitters command passing nanorobots to establish neutral buoyancy. No other solid blood component can maintain exact neutral buoyancy, hence those other components precipitate outward during the 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 nanorobots but few other solids, is drawn off through a simple filter, removing the devices as residue. Plasma filtrate is then recombined with centrifuged solid components and returned undamaged to the patient's body.

Last updated on 24 February 2003