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
6.2.3 Chemical Energy Storage
Chemical energy storage offers an ideal combination of high storage density, abundant physiological resources, and superior safety in the event a device is physically compromised. As shown in Table 6.1 (values computed from stoichiometric reaction formulas), chemical energy storage density (fuel only*) ranges from 108 joules/m3 to 1011 joules/m3, as compared to 106-109 joules/m3 for mechanical media and 106-108 joules/m3 for electric/magnetic storage (Section 6.2.4). Electrochemical batteries typically achieve 107-1010 joules/m3 (e.g. 5-micron thick rechargeable lithium microbatteries have energy density ~2 x 109 joules/m3,588 and tin-lithium material can reach ~8 x 109 joules/m3.715) As of 1998, prototype ~4000 nm3 Cu/Ag nanobatteries had demonstrated ~7 x 107 joules/m3 or ~2 x 104 watts/m3 power density.589
* Not all fuels require oxidizer, e.g. ATP. For fuels that require oxidizer, taking oxidizer volume into account (e.g. O2 at 1000 atm) reduces overall energy storage density to ~7% of fuel-only density for diamond, ~25% for most organics, and ~65% for hydrogen. Also, note that Eavail = Estorage - Eactivation, where Eavail is available energy and Eactivation is the net unrecoverable activation energy required to release the stored chemical energy.
Table 6.1 offers many interesting revelations:
(1) Assuming "safe" ~1,000 atm storage, a pressurized hydrogen/oxygen mixture has the greatest energy per unit mass (excluding the mass of the containment vessel) but has almost the lowest energy per unit volume, hence is too inefficient for free-swimming medical nanorobots where interior volume is a scarce commodity.
(2) The classic explosive, nitroglycerine (included solely for comparison), ranks poorly on either measure of energy density.
(3) ATP, sometimes proposed as an alternate energy source for in cyto operations,261,1259 has the lowest energy density on the list, though its use cannot be excluded in the earliest-generation medical nanodevices for metering out small energy packets of known magnitude.
(4) Lipids and fats generally store more energy per unit volume than carbohydrates, proteins, or even hydrogen.
(5) Cholesterol, twice as plentiful as serum glucose on a gm/cm3 basis (Appendix B), is in theory the most favored biochemical energy storage molecule with nearly double the storage density of glucose.
(6) Diamond has the highest known oxidative chemical storage density, possibly surpassed only by fullerene materials, probably because it also has the highest atomic number density per unit volume.
(7) Hydrogen stored in solid form (H2 solidifies at ~57,000 atm at room temperature, making 600 kg/m3 crystals),568 yields an extraordinarily high chemical energy density, second only to diamond.
Combining the values for molecular energy density given in Table 6.1 with the bloodstream concentrations given in Appendix B reveals that the chemical energy content of blood plasma (~3 liters in adult males) includes ~3 x 108 joules/m3 for all lipids (~8 x 107 joules/m3 from total cholesterol alone), ~2 x 107 joules/m3 for serum glucose, ~1 x 106 joules/m3 for each of the common amino acids, and even ~4 x 106 joules/m3 for urea. Cytoplasmic chemical energy content is of similar magnitude, with the addition of ~6 x 104 joules/m3 for ATP. (See Table 6.4 for related data.)
The question naturally arises whether some biocompatible artificial energy molecule could be added to the human bloodstream to provide a supplementary chemical energy source for a working in vivo nanorobot population. To this end, an injection of ~0.7 cm3 of diamond colloid (the most energy-dense chemical fuel known -- it has been used as rocket fuel) encapsulated in trillions of suitable submicron-scale biocompatible carrier devices provides an energy resource equal in size to the entire serum glucose supply, a negligible ~0.01% addition to whole blood volume. Ten trillion 0.1-micron3 passive carriers would have a mean separation of ~10 microns in the blood.
Last updated on 18 February 2003