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

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

6.4.4 Dedicated Energy Organs

Delivery of energy to in vivo nanorobots via tethers necessarily involves a hierarchical distribution network of some kind, ranging from maximum span (e.g., maximally horizontal hierarchy: all units directly connected to the external source) to minimum span (e.g., maximally vertical hierarchy: all units connected in series). Many intermediate network topologies are readily imaginable. Networks typically are most efficient at some optimum mix of spans and levels. To support this optimum mix, dedicated energy storage organs positioned at network nodes can add stability and longevity to the system and provide load buffering. Energy organs may act as distribution or routing devices, simply passing power down the line, or they may act as output nodes which are periodically encountered or revisited by in vivo nanorobots to recharge or refuel depleted onboard energy stores.

For example, we may envision a permanent macroscopic implanted device with a transdermal connector port. Through this port, the user recharges the internal energy organ by plugging in a source of compressed gas, by connecting through the port to a battery or a household electrical wall outlet, or by attaching an antenna system to facilitate the receipt of acoustic, mechanical, inductive, rf, infrared, or optical power signals. Chemical tethers could also resupply these energy organs: Assuming Poiseuille flow (Eqn. 9.25), ~40 mm³/sec of stoichiometric oxyglucose fuel supply at 1000 atm pressure differential flowing (separately) through a double-tether ~1 meter in length and ~60 microns in diameter supplies the entire 100-watt human basal power requirement assuming 50% chemical energy conversion efficiency at the receiving end, with the glucose supplied in a saturated 70% aqueous solution at 310 K. Chemical tether power density ~ 4 x 10¹⁰ watts/m³.

Inside the body, components of the implant may protrude into tissues or the bloodstream to supply power to internal nanorobots. These protrusions could act in broadcast mode, emitting acoustic or electrical power intracorporeally. Circumaortic current-carrying loops could energize bloodborne nanorobots as they passed through the vascular solenoid, although energy transfer efficiency may be low (Section 6.4.2) and possible biological effects of these electric fields on cellular systems should be studied further. Energy organ protrusions could release manufactured artificial energy-rich molecules (Section 6.2.3) or microscale fuel tankers containing energetic compounds directly into the bloodstream. Protrusions might act as "energy teats" to which nanorobots could dock and refuel or recharge, although this will require navigational beacons, docking mechanisms, transfer conduits, etc., thus increasing system complexity. Energy organ protrusions could also serve as catalytic or conversion nodes, transducing one form of energy into another, or as manufacturing nodes, converting less energetic chemicals into more energetic chemicals through the application of externally supplied energy. In all these cases, biocompatibility is a critical issue that must be fully addressed³²³⁴ (Chapter 15).

Dedicated energy organs may range in size from hundreds of microns up to several centimeters, depending on the position in the distribution hierarchy and the task to be performed, and may themselves be recharged from other larger energy organs positioned higher up in the hierarchy, or directly from external sources. Given the maximum conventional energy storage density of ~10¹¹ joules/m³ (Section 6.2), the human basal energy requirement of ~2000 Kcal for one day could be stored in a (>4.4 cm)³ cube. A special case is the nuclear powered energy organ containing a similar volume of Gd¹⁴⁸ radionuclide, which could provide the same basal requirement for ~centuries (Section 6.3.7.1). Although a nuclear-powered organ is unlikely to be implemented in this manner, from Eqn. 6.27 a sphere of Gd¹⁴⁸ emitting ~100 watts with a 75-year half-life and measuring 3.41 cm in diameter with a 5-micron Pt shield glows at 1326 K (e_r for Pt at 1326 K is 0.156; Gd melting point ~1585 K, Pt melting point ~2042 K); this is approximately the decomposition temperature of diamond (into graphite) and well above the combustion point for diamond in air (Section 6.5.3), so Pt-coated sapphire (sapphire melting point ~2310 K¹⁶⁰²) may provide a more stable first wall for the radionuclide energy organ. Carnot thermal efficiency for a heat engine using this source could reach, at most, ~76%.

Last updated on 18 February 2003