Nature Vol. 248 April 5 1974, p475 ( News and Views )
Semiconductors in the human body?
from our Solid State Physics Correspondent
It was realised about 20 years ago that the production of energy from oxygen in the cell mitochondrion was possibly the result of direct electronic transport through haemoproteins rather than the interaction of mobile ions through an aqueous mediurn (see, for example, Szent-Gyorgyi, Discuss. Faraday Soc., 27, 111; 1959; Carden and Eley, ibid., 115). It was thus necessary to think about a sort of wet solid-state Physics which could embrace reasonably high rates of transport of free electrons or holes through biological solids, especially through certain haemoproteins involved in respiration. Szent-Gyorgyi proposed a theory that a suitable conduction band could be produced by orbital overlaps in amino acid chains and one could predict from this theory that proteins might achieve conductivities in the semiconductor range. The theory was conceptually very attractive but was not widely taken up, largely because it was not at the time susceptible of proof, nor had any investigation ever revealed a way of making analogous but simpler organic materials conduct to the level required.
Now, at least, one biological material has been shown to have a strikingly large conductivity when correctly excited. McGinness, Corry and Proctor, of the University of Texas Cancer Center, Houston, report in Science (183, 853; 1974) that melanins can be made to 'switch' from a poorly conducting to a highly conducting state at fairly low electric fields (say from 101 n cm to 10' n ent at a field of 3 x 101 V cm-1). This remarkable phenomenon occurs both in melanin made synthetically from tyrosine and in that extracted from a human melanoma. The large conduction is not destructive in any way and is reversible;. According to some tests, conduction seems to be electronic rather than ionic. Also tests of a few other likely biological materials in the same form (a compressed solid pellet inside a quartz tube, mildly hydrated and of length ranging from 0. 1 to 10 mm) suggest that the effect is confined to the melanins and the authors note a similarity in the I-Y characteristics of the sample to those of some amorphous inorganic semiconductors which undergo 'threshold switching'. But apart from the major difference in the electric field at which the threshold effect occurs (of the order of 1000 V cm-1 for melanin and 10' V crn-1 for chalcogenide glasses), the current theory of the inorganic switching phenomenon rests on filamentary conduction, leading to a controlled degree of segregation of the constituents of the glass (for example, segregation of pure tellariurn from Ge-Te alloys) and possibly strong injection at the electrodes under the high local fields (Bosell and Thomas, Phil. Mag., 27, 665-81; 1973).
Neither of these effects seems even likely in the system described, especially since the switching becomes unstable at thicknesses of chalcogenide greater than a few micrometres. Thus, the suggestion of McGuiness et al. that melanin in the human body can be a cause rather than a by-product of disease and that its mode of action can he related to this 'electronic device' action is probably premature, especially considering the preliminary nature of the experiments. A revival of discussion on in vivo electronic effects in some biological materials associated with oxidation-reduction is, however, welcome if only because science has perhaps moved far enough since the 1950s that it can now devise adequate tests for the basic theories of transport in wet solids. Also a new approach to the treatment of melanotic diseases may well be stimulated by this particular revival of an intellectually stimulating discussion.
keywords: conductive organic metal polymers polymer metals conductors semiconductors conductors.