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

 

6.3.4.5 Chemoelectric Cells

The human body provides a renewable source of chemoelectric energy that may be tapped by various means. As long ago as 1959, Pinneo and Kesselman595 reported powering an FM transmitter by simply inserting two steel electrodes into the brain of a cat, generating 0.5 microamperes at 40 millivolts. In another experiment by Reynolds596 in 1963, electrodes inserted subcutaneously and abdominally into an anesthetized rat produced 400 millivolts and ~10 microwatts, powering an oscillator circuit for 8 hours with no sign of decline.* Since then, three classes of bioelectric energy have been distinguished: ionic concentration, biogalvanic, and biofuel cells.

* Interestingly, when the rat was finally administered an instantly lethal intracardial injection of Nembutal the voltage required 75 minutes, after death, to fall to zero.

The ionic concentration cell may exploit differences in chemical concentration by placing two similar electrodes in different compartments of the body having different chemical composition, generating a voltage proportional to the logarithm of the concentration ratio. By this means, it has been proposed that power could be drawn from the differences in oxygen and hydrogen ion concentration between arterial and venous blood,590 or the acid-alkali differences between the stomach fluids and surrounding tissue. Similarly, transcytomembrane electrodes could develop potentials of 1-100 mV between cytosol and intercellular fluid. This would allow a 10 pW nanorobot to be powered by a current flow of 10 nanoamperes at 1 mV through a 105 ohm load resistance, although power draws exceeding ~100 pW might prove electrokinetically disruptive to the cell. Also, this source may be less reliable for dying or challenged human tissue.

Biogalvanic energy sources590,3527-3530 exploit the electrochemical potential between metallic electrodes (the fuel) in an electrolyte solution. Two broad classes of galvanic pairs which generate electrical energy have been investigated for medical use: those in which both anode and cathode are consumed and go into solution, and those in which the cathode is inert. In the first group, zinc (anode) and silver chloride (cathode) electrodes have been implanted in human test subjects for up to two years. The Zn goes into solution as Zn++; the AgCl cathode is converted into Ag and the Cl ion goes into solution. This chemoelectric source generates steady currents of ~0.2 picoamperes/micron2 at ~1 volt or ~10 pW/50 microns2 of electrode surface at 70%-90% efficiency, an energy density of 107 watts/m3 assuming 20-nm thick electrodes. Similar output is obtained from galvanic pairs in the second group, of which the best studied is a zinc anode coupled with a palladium or platinum-black cathode; molecular oxygen combines with water at the cathode producing OH and H+ ions. Unfortunately, galvanic sources 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

The biofuel cell590,3526,3527 relies upon redox reactions in which neither anode nor cathode is consumed but merely act as catalysts, potentially a great advantage in nanomedical applications. The biofuel cell that has received the most attention for nanomedicine is the oxyglucose cell, which could rely upon an electrochemical process in which glucose is oxidized at the anode and molecular oxygen is reduced at the cathode according to the following equations:

{Eqn. 6.22}

{Eqn. 6.23}

Decades-old experiments using platinum black electrodes594 produced ~0.6 picoamps per micron2 of electrode surface at ~0.3 volts, with power levels of 0.2 pW/micron2 in vitro (declining over time due to the formation and absorption of gluconic acid at the anode), but only 0.004 pW/micron2 in vivo (when implanted in rat or rabbit test animals) in part due to poisoning of the catalytic anode action by proteins. Complete electrochemical oxidation of glucose has not yet been demonstrated experimentally.1015 Controlled-permeability membranes may eliminate these problems in nanomedical applications, but even using the aforementioned electrodes might allow a continuous power density of 107 watts/m3 for a 10 pW oxyglucose biofuel cell using 50 micron2 of electrodes 20-nm thick. Since gluconic acid can also be oxidized at potentials which oxidize glucose, gluconate may not be the only byproduct, thus appropriate means of disposing of any potentially undesirable byproducts must be included in a practical design. The addition of specific catalysts (e.g., ruthenium) to the electrodes could be helpful.

Ethanol fuel cells, though inefficient, are already in wide commercial use. For example, the Lion alcolmeter fuel cell alcohol sensor2428 employs catalytic platinum electrodes (supported on a ~1 cm3 PVC matrix disk assembly impregnated with acidic electrolyte) to produce electron flow by oxidizing ethanol to acetic acid (producing two free electrons per alcohol molecule) at the anode and reducing atmospheric oxygen at the cathode. Normal alcohol working concentrations are 5900 parts per million typically producing up to ~10 microamps at ~5 millivolts across a 390 ohm load, or ~50 nanowatts (power density at most ~0.1 watt/m3). The Lion fuel cell can also produce energy from all primary and secondary aliphatic alcohols but not from aldehydes, ketones, ether, esters, hydrocarbons or carboxylic acids. Other manufacturers of electrochemical fuel cells for alcohol sensing include PAS Systems, Guth Laboratories, and Intoximeter.2429 Direct methanol fuel cells also were under development in 1998.

The following analysis was prompted by R. Merkle's suggestion that an oxyglucose biofuel cell using nanoscale membranes might achieve significantly higher power densities than indicated by past experiments (Fig. 6.5). Consider a proton (H+) exchange nanomembrane consisting of a 1 nm thick diamondoid sheet containing a number of very narrow pores each lined with atoms of oxygen, fluorine or nitrogen, creating negatively charged channels with high proton affinity. A channel narrow enough to admit only protons but nothing else (~0.1 nm, excluding even helium atoms) and surrounded by a ~3 atom thick support wall makes a ~1 nm3 pore structure ~1 nm wide.

The power generated by each pore due to proton flow is Ppore = q 'N Vp, taking q = 1.6 x 10-19 joule/proton-volt, 'N is the proton flow rate in protons/sec, and Vp is the electrical potential through which the charges fall. Experimental observations suggest Vp may reach 0.75 volt at zero load, but averages 0.3-0.6 volts at moderate loads;594 for this analysis, Vp will be taken as ~0.5 volt.1017 As for 'N, the nicotinic acetylcholine receptor channel (Section 3.3.3) has a 0.65-2.2 nm inside diameter and allows 2.5 x 107 Na+ ions/sec to pass while the channel is open, comparable to the transport rate of artificial transmembrane peptide nanotube ion channels.1177 Taking this flow rate as representative for 'N, then Ppore ~ 2 pW/pore.

Redox fuel cells using transition metal catalysts typically achieve ncat ~ 10 catalytic events/sec per catalytic atom.1017 Oxidation of each glucose molecule produces 24 protons, so the required catalytic rate is 'N/24 ~ 106 glucose molecules/sec per pore which in turn requires 'N / (24 ncat) ~ 105 metal catalyst atoms per pore. Taking Pt atoms as representative, 105 Pt catalyst atoms have a volume of ~1500 nm3. Including the 1 nm3 pore structure and doubling the volume to allow for catalyst dispersal, fluid access and structural overhead gives a 3000 nm3 single-pore catalytic unit generating 2 pW, or a power density of ~7 x 1011 watts/m3. (Assumed catalyst atom usage is ~1017 atoms/watt; in 1999, the best Pt-catalyzed proton exchange membranes required ~1018 atoms/watt.3264) The current flow of 4 picoamps through each (0.1 nm)2 pore gives a current density of 4 x 108 amps/m2, well below the ~1010 amp/m2 maximum current density in bulk aluminum (Section 6.4.3.1). Since ~106 glucose molecules/sec are consumed per pore and each glucose molecule represents 4765 zJ of free energy, then total energy available is ~4.8 pW/pore. Thus it appears the device may be 2 pW / 4.8 pW ~ 40% efficient, though this figure is highly voltage-dependent.

J. Soreff notes that the oxyglucose biofuel cell can in theory operate near the thermodynamic limit but may require the design of a complex sequence of catalytic reactions, not unlike microbial metabolism.2427 The glucose/gluconic acid source may have lower efficiency due to incomplete oxidation but could be useful in early systems because it requires optimization of catalytic sites for just one reaction. On the other hand, the production of protons and electrons from complete oxidation of glucose potentially encompasses a lengthy biochemical chain which possibly may be implemented using sets of immobilized enzymes in the manner of mitochondrial respiration. (Mitochondrial power density is 105-106 watts/m3.)781,786

There are limits to efficiency set by mismatches between redox potentials at various steps in a respiratory chain. Given a single anodic chamber with a common electric potential and pH, concentrations of intermediates must be unequal to compensate for the redox potential mismatches at various steps. If these concentrations are too unequal, then the lowest concentration in the chain becomes the rate-limiting intermediate for the entire process. Throughput may be optimized by careful choice of intermediates, couplings between reactions, and catalyst design. To oxidize glucose, in biology, glycolysis requires ~18 enzymes, the TCA cycle ~9 enzymes, and the pentose phosphate pathway ~9 enzymes.526 Soreff suggests that a slightly different strategy is to have separate oxidation/proton chambers for each stage in the catalytic pipeline, allowing intermediates to diffuse through semipermeable membranes or to be transported via molecular sorting rotors. This transfers the problem of matching redox potentials into the electrical domain where it is more easily resolved.

Oxyhydrogen fuel cells are another possibility, though probably at much lower power density because hydrogen must first be produced from the glucose fuel. This occurs naturally in the energy-generating hydrogenosomes of the trichomonads (which use the enzymes pyruvate:ferredoxin oxidoreductase and hydrogenase) and in the laboratory using related enzymes (from bacteria that live near hot underwater vents) to convert glucose into hydrogen gas and water. In a modern 350 K oxyhydrogen fuel cell, O2 and H2 are fed into adjacent chambers separated by a proton exchange membrane. This membrane allows only H+ ions to flow to the O2 side, making water as the sole waste product and establishing a net negative charge on the H2 anode and a net positive charge on the O2 cathode, an electrical potential of ~0.6 volts under load. This chemoelectric process is at least ~50% efficient but typically achieves only 104-106 watts/m3 partly due to hydrogen's low volumetric energy density and partly due to the crude membranes currently in commercial use. Bacterium-based biofuel cells have also been investigated.2427,3531

 

Last updated on 18 February 2003