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


6.5.5 Electrical vs. Mechanical Systems

Because of the ubiquity of electrochemical processes in biological systems, it is natural to assume that electrical power would be the energy of choice to drive nanomedical systems. However, it is clear that mechanically powered nanorobots are competitive because mechanical energy may be transmitted at very high efficiency over nanoscale distances. Fully mechanical motors, pumps, actuators, manipulators, and even computers have been designed.10

The principal disadvantages of electrically-powered nanorobots include:

A. Bioelectric Interactions -- Electrical systems can create stray fields which may activate bioelectric-based molecular recognition systems in biology. While electrical systems allow ready coupling with electrochemical systems in the body, stray fields also may provoke unintended electrokinetic interactions. For example, all galvanic sources have been found to provoke a significant host reaction, including formation of layers of necrotic debris, free neutrophils, granulation tissue and complete fibrous connective tissue encapsulation of long-term implants.590,3512 Microelectrophoretic interactions with possible natural rf oscillations of cells683 could present additional complications. Stray high-frequency vibrations from purely mechanical systems may be less provocative.

B. Electrical Interference -- Electrical nanorobots are susceptible to electrical interference from external sources such as rf or electric fields, EMP pulses, and stray fields from other in vivo electrical devices. (Digital cellular telephones have been reported interfering with implanted cardiac pacemakers, causing the pacemakers to speed up, slow down, or even turn off.3495-3498) Cosmic rays can provoke arc discharges in systems operated at high electrical potential.

C. Thick Insulators -- Very thick insulation is required to prevent electron leakage, especially serious at the smallest sizes where significant quantum mechanical tunneling can occur. Without careful design, the high conductivity of the in vivo medium* can cause sudden power loss, e.g., by "shorting out."

* The purest water (e.g., distilled and deionized) has a specific resistance of 2.5 x 105 ohm-m.390 It is normally difficult to obtain and store water with resistivity exceeding ~104 ohm-m because of the absorption of CO2 and other air gases, and of alkali and other electrolytes leached from glassware; ordinary distilled water in equilibrium with air has resistivity ~1000 ohm-m.390 These values may be compared to 1.59 x 10-8 ohm-m for silver at 293 K ,763 3.5 x 10-5 ohm-m for amorphous carbon at 293 K,1662 0.0893 ohm-m for 1M KCl in water at 298 K and 68.1 ohm-m for 0.001M KCl solution390 (human blood is ~0.15M NaCl), 107-108 ohm-m for synthesized diamond (depending on the growth process used), and >1018 ohm-m dark resistivities in natural diamond.537

D. Thick Wires -- Relatively thick wires are needed to conduct significant power levels without overheating, although future room-temperature superconductors might reduce this disadvantage.

The principal advantages of electrically-powered nanorobots include:

E. Speed of Operation -- Electronic field configurations have a velocity near the speed of light (c ~ 3 x 108 m/sec), much faster than mechanical logic rods which may move as slow as 1 m/sec. Even the classical electron drift speed vd in a bulk metallic conductor at high current density Id may be faster than mechanical rod motions: vd = Id / ne e, where ne = 8.4 x 1028 electrons/m3 for copper and e = 1.6 x 10-19 coul/electron, giving vd ~ 100 m/sec at Id ~ 1010 amps/m2, near the maximum current density. (A more typical Id ~ 106 amps/m2 gives vd ~ 1 cm/sec.) And the mobility of an electron confined to a one-dimensional wire or patterned diamondoid channel can be much higher than in the bulk material, up to ~106 m/sec or ~0.3%c.1097,1098 Thus applications demanding the greatest speeds may require electrical systems.

F. Electromagnetic Coupling -- Detection of photons and electric/magnetic fields is more easily mediated by an electrical transmission device than a mechanical device. For instance, a photocell or rhodopsin antenna absorbs photons to directly create moving charges. This is probably more efficient than using a piezoelectric transducer backwards as a detector, but involves handling higher-energy packets of energy which is inherently riskier.

G. Transmission Attenuation -- Electrical signals at sub-MHz frequencies are only modestly attenuated during passage through human tissue.


Last updated on 18 February 2003