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

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

10.2.3.3 Organic and Bioelectronic Computers

Many organic materials may be useful in electronic computing. Electrical conduction in synthetic organic molecules is well known^{1761,1906-1909,3125-3128} and already exploited in liquid-crystal displays of laptop computers.¹⁹²⁵ Doped polyacetylene is a conducting plastic with a carbon backbone; undoped, it is an insulator. There are many other known families of conducting polymers such as polyaniline, which includes nitrogen atoms in the backbone, and polythiophene, which includes sulfur.¹⁸³³ Three-dimensional interconnect arrays of organic conducting polymers have been fabricated;¹⁸³⁹ conductivity is higher along a chain than between chains, so sheets of oriented chains can produce relative conductivity anisotropies as high as 100-1000.¹⁹²⁷ An all-plastic transistor has been built.¹⁹³⁵ Organic transistors have been used as the active semiconducting element in thin-film transistors fabricated with organic-material thicknesses ranging from 5-150 nm,¹⁸⁴² and organic microscale transistors and other organic devices may exploit bulk-effect electron transport just like silicon-based semiconductor devices.¹⁹²³ Conjugated polymer poly(1,6-heptadiester) was employed to make an optical correlator with a peak processing rate of 3 x 10¹⁶ operations/sec.¹⁸⁴¹ Spin-transition polymers could serve as thermal sensors or memory devices with the theoretical ability to store one bit in a ~4 nm cube (~2 x 10⁷ bits/micron³ storage density) with ~GHz addressing speeds at room temperature.¹⁸⁴⁰ Other organic polymers possibly useful in logic circuits have been investigated.²⁸⁹⁹

Natural biological materials may also be useful in electronic computing,^{1777,1925,3129} particularly because of the ability of biological materials to self-assemble. Biomolecular electronics is a subfield of molecular electronics that investigates the use of native as well as modified biological molecules (chromophores, proteins, etc.) in place of the organic molecules synthesized in the laboratory.¹⁷⁷⁷ In theory, a three-dimensional DNA or protein scaffolding could be self-assembled, with bacteriorhodopsin, ferritin and magnetite, or related bioelectromagnetic molecules inserted into precise locations within the structure to produce a crystal lattice bioelectronic or biooptoelectronic nanocomputer. DNA already has been used as an atomic scaffold upon which to build silver wires ~100 nm in diameter¹⁹⁶¹ and has been decorated with fullerenes.³⁰²⁴ Self-assembly can generate membrane-based^1931,1932 and biotubule-based¹⁹²⁶ devices, and site-directed mutagenesis is a valuable tool for high-resolution protein device engineering.¹⁹³³ Nucleotide sequence-specific electron transfer between metallic electron donor and acceptor complexes covalently or noncovalently intercalated into strands of B-DNA has been demonstrated.¹⁹³⁴

Perhaps the best-known bioelectronic (and biochemical) computer is the neuron cell and its congeries -- the ganglia, nerve trunks, and brains. In 1997, David Stenger (NRL) and James Hickman (SAIC) were attempting to culture and link together neuronal cells to build a bioelectronic cell-based sensor "computer" for performing complex pattern-recognition tasks.^1966,1967 In early 1999, W.L. Ditto at Georgia Tech announced the creation of a biological computer comprised entirely of leech neurons, that was capable of perfoming simple sums.³²⁴⁵ We can certainly imagine large numbers of cultured neurons arranged in artificial three-dimensional spatial patterns to produce a synthetic bioneural computer, but such a computer would operate at only ~KHz frequencies and would be very energy inefficient since each neuron consumes ~10¹⁰ kT per discharge (Section 4.8.6.2) -- thus offering few advantages.

Last updated on 24 February 2003