Nanomedicine, Volume I: Basic Capabilities
© 1999 Robert A. Freitas Jr. All Rights Reserved.
Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999
6.5.4 Selection of Principal Power Source
In selecting the ultimate source of power for a given application, the appropriate energy acquisition strategy must first be decided. Two overall strategies are readily distinguished: (1) nonrefuelable and (2) refuelable.
A nonrefuelable strategy implies that all nanorobot power is drawn from internal energy storage. Such a nanorobot simply stops when its internal energy stores are depleted. The relevant parameter is onboard energy storage density, which should be maximized. From Section 6.2, chemical energy storage produces the highest energy density, up to ~1011 joules/m3. (Nuclear storage has a much higher theoretical energy density, but in medically "safe" fissile systems the rate of energy withdrawal from the store cannot be precisely regulated using known technology, thus limiting the effective energy density of this potential source.) A 1 micron3 store of chemical energy at a conservative 1010 joules/m3 powers a 10 pW nanorobot for ~103 sec, which may be sufficient in some applications.
A refuelable strategy implies that onboard energy stores serve mainly as buffers, which are refilled periodically or continuously from external chemical, sonic, electrical, or tethered sources. A relevant parameter is now the rate of energy transfer, or environmental power density, which should be maximized consistent with safety. Again, at least two strategies can be distinguished: (2A) in vivo sources and (2B) ex vivo sources. In vivo power sources may include free-flowing chemical fuels (e.g., bloodstream glucose circulating once every 60 seconds represents ~2 x 105 watts/m3), encapsulated chemical fuels, or implanted dedicated energy organs. Ex vivo power sources may include acoustic (<~2 x 104 watts/m3), electromagnetic (<~2 x 103 watts/m3), or tethered sources. For tethered sources, a safe properly configured 1-meter tether 1 micron in diameter transfers hydraulic energy at ~105 watts/m3 (at 1000 atm), fiberoptic photonic energy at ~106 watts/m3, electrical energy at ~107 watts/m3 (silver wire, ~107 amp/m2 at ~1 volt), chemical energy at ~108 watts/m3 (at 1000 atm), or mechanical energy up to ~1010 watts/m3.
However, this is only the first step of the analysis because the initial energy, once received, must be transduced into other useful forms which may be used to drive onboard systems. Most nanorobot designs will involve energy transduction chains of various lengths, so the efficiency of the entire chain is also a relevant parameter driving the choice of initial power source. Consider, for example, a nanorobot requiring electrical power for some internal function. While the maximum available electrical power density in the environment may be comparatively low, the advantage of a much higher available chemical power density may be offset by a relatively low direct chemoelectric transduction efficiency (Section 18.104.22.168). On the other hand, the relative efficiency of the chemoelectrical pathway may be improved by using a glucose engine to generate mechanical motions, which motions may then be converted to electrical energy via highly efficient mechanoelectrical transduction (Section 6.3.2).
Last updated on 18 February 2003