chemical principles exemplified
ROBERT C. PLUMB
Biogalvanic Cells Illustrating principles of electrochemistry
Suggested by Professor W. D. Hobey, Worcester Polytechnic Institute Oxygen, hydrogen ions, and other chemical species in a person's body can vary substantially in concentration from one region of the body to another. Can these concentration variations be utilized with suitable implanted electrodes to generate electricity? If they could, heart pacers and biomedical telemetering devices could be substantially improved. Such devices are now powered by primary batteries which must be replaced periodically. (One alternative, a long-lived nuclear power source, is in the 'development stage.) No one has yet devised a galvanic cell consisting solely of two chemically inert electrodes implanted in the body fluids, but researchers have made progress toward this goal. The principles which are involved are just those taught in most classroom introductions to electrochemistry; thus the biogalvanic cell provides an interesting illustration for chemistry teachers. Before constructing a biogalvanic cell, one would need to find two physiologically innocuous electrode reactions. Oxygen is relatively abundant in the body, and examination of a table of half-cell reactions suggests that one of the electrodes could be an inert platinized platinum electrode with the reaction e-
+ 1/40n + H+ = 1/2HnO
Eon
=
1.23 V
The body's system could easily respond to the generation or depletion of chemical species when this electrode operates, providing the power levels were modest. It is not so easy to choose a second electrode. Another platinized platinum electrode in a diierent part of the body with a different concentration of oxygen and/or hydrogen ion would produce a concentration cell, hut apparently this possibility has not been suc-
cessfully explored. Another possibility is to find a biochemical oxidation reaction which could be a source of electrons when coupled with the oxygen reduction reaction. A third possibility, and this has met with some success,' is to use a consumable metal-met.al ion electrode. The electrode which was chosen for study was Zn
=
Zn2+
+ 2e-
Eon
=
-0.76 V
Combining the two electrode reactions, the net cell reaction is Zn
+ 1/20, + 2H+ = Zn2++ H 2 0
AEo
=
1.99 V
The normal human body contains considerable zinc, between 2 and 4 g, serving as an enzyme cofactor in cells. If the zinc ion from the cell reaction is generated slowly enough it could be excreted by the body without toxic effects. Batteries consisting of zinc anodes and platinized platinum cathodes have been tested in animals for periods of up to 128 days without detectable physiological effects. How long could a zinc electrode last and at what rate would the body have to dispose of zinc? This provides an illustration of Faraday's law. The battery which was tested was designed to operate steadily at 0.8 V and 40 #W of power, sufficient to drive the electronic devices of concern. The current drain from the battery was 50 MA. Applying Faraday's law, one finds that only 0.53 g of zinc would be consumed per year. A bio'galvanic battery of this construction, which could last a lifetime, seems possible. This particular biogalvanic cell may not be the ultimate solution to the problem. It is, however, conceivable that reliable, safe, electrode systems will one day be developed which can be implanted and provide a permanent battery powered by the body's biochemical system.
' General Electric Technical Information Report No. 68SD307.
Volume 49, Number 6, June 1972
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