Proc. Natl. Acad. Sci. USAVol. 82, pp. 1874-1878, April 1985Applied Physical Sciences

The molecular electronic device and the biochipcomputer: Present status

(memory/brain/switch)

R. C. HADDON AND A. A. LAMOLAAT&T Bell Laboratories, Murray Hill, NJ 07974

Communicated by C. Patel, November 26, 1984

ABSTRACT The idea that a single molecule might func-tion as a self-contained electronic device has been of interestfor some time. However, a fully integrated version-the bio-chip or the biocomputer, in which both production and assem-bly of molecular electronic components is achieved throughbiotechnology-is a relatively new concept that is currentlyattracting attention both within the scientific community andamong the general public. In the present article we draw to-gether some of the approaches being considered for the con-struction of such devices and delineate the revolutionary na-ture of the current proposals for molecular electronic devices(MEDs) and biochip computers (BCCs). With the silicon semi-conductor industry already in place and in view of the continu-ing successes of the lithographic process it seems appropriateto ask why the highly speculative MED or BCC has engen-dered such interest. In some respects the answer is paradig-matic as much as it is real. It is perhaps best stated as thepromise of the realm of the molecular. Thus it is envisionedthat devices will be constructed by assembly of individual mo-lecular electronic components into arrays, thereby engineeringfrom small upward rather than large downward as do currentlithographic techniques. An important corollary of the con-struction technique is that the functional elements of such anarray would be individual molecules rather than macroscopicensembles. These two aspects of the MED/BCC-assembly ofmolecular arrays and individually accessible functional molec-ular units-are truly revolutionary. Both require scientificbreakthroughs and the necessary principles, quite apart fromthe technology, remain essentially unknown. It is concludedthat the advent of the MED/BCC still lies well before us. Thetwin criteria of utilization of individual molecules as functionalelements and the assembly of such elements remains as elusiveas ever. Biology engineers structures on the molecular scalebut biomolecules do not seem to be imbued with useful elec-tronic properties. Molecular beam epitaxy and thin-film tech-niques produce electronic devices but they "engineer down"and are currently unable to generate individual molecularunits. The potential of the MED/BCC field is matched only bythe obstacles that must be surmounted for its realization.

The idea that a single molecule might function as a self-con-tained electronic device has been of interest for some time(1-4). However, a fully integrated version-the biochip orthe biocomputer, in which both production and assembly ofmolecular electronic components are achieved though bio-technology-is a new concept that is attracting attentionwithin the scientific community (5) and among the generalpublic (6, 7). In the present article we attempt to draw to-gether some of the approaches being considered for the con-struction of such devices and to delineate the revolutionarynature of the current proposals for molecular electronic de-vices (MEDs) and biochip computers (BCCs).

It has been suggested that the MED is the next logical de-velopment in computer logic elements (5). Computer compo-nents and their associated hardware have been shrinking re-lentlessly since the inception of data-processing machines.The features on the current generation of silicon chips are assmall as 1 gtm and single chips may contain as many as 1million bits of random access memory. It is claimed that ad-vances in lithography will soon allow feature sizes of 0.011m and chip capacities in the gigabit range; it is even claimedthat three-dimensional systems with stacked elements can beconstructed-all by extension of conventional chip manu-facturing techniques (5-7). It is therefore important to real-ize that the silicon semiconductor industry presents a mov-ing target and has yet to achieve its full promise.

If feature sizes of 0.01 gm are within the grasp of a knowntechnology it seems appropriate to ask why the highly specu-lative MED or BCC has engendered such interest. In somerespects the answer is paradigmatic as much as it is real. It isperhaps best stated as the promise of the realm of the molec-ular. Thus it is envisioned that devices will be constructed byassembly of individual molecular electronic components intoarrays, thereby engineering from small upward rather thanlarge downward as do current lithographic techniques. Animportant corollary of the construction technique is that itallows the possibility of using microscopic (molecular) prop-erties rather than macroscopic (bulk) properties. These twoaspects of the MED/BCC-utilization of individual mole-cules as functional elements and the assembly of such ele-ments-are truly revolutionary. Both require scientificbreakthroughs and the necessary principles, quite apart fromthe technology, remain essentially unknown.

Biological Facets

More than anything else it seems that modern biology hasmade science bold. The directed synthesis of extremelycomplex molecules afforded by modern molecular biologyunder its new heading of "genetic engineering" has alreadybeen demonstrated. It is also argued that, since coded self-assembly properties of macromolecules can lead to struc-tures as complex as the human body, a biochip or a biocom-puter could, in the same way, be assembled from individualmolecular electronic components.

Brain. The line of reasoning given above demands carefulattention for there is little structural analogy between biolog-ical systems, including the brain, and present computers inwhich control of electron circuits provide the operational ba-sis. In the brain, impulses within neurons are transmittedelectrochemically along the axon (or dendrite) by diffusionof sodium and potassium ions across semipermeable mem-branes. At the junctions between neurons (synapses) theelectrochemical action potential stimulates the release of oneof a number of chemical agents that acts as a neurotrans-

Abbreviations: MED, molecular electronic device; BCC, biochipcomputer.

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Applied Physical Sciences: Haddon and Lamola

mitter by migration across the synaptic cleft. The neuro-

transmitter may have an excitatory or inhibitory effect on

the neuron that is the reception point of the signal. Thuswhile it seems clear that the brain utilizes conductive path-ways and a sophisticated switching network, the active ele-ments are ions and molecules rather than electrons andholes. It therefore comes as no surprise that the cycle time ofbrain circuitry is quite slow; the propagation velocities alongthe axons and dendrites are of the order of 25 m/sec. Ittherefore remains a mystery as to how information is proc-

essed and problems formulated and resolved with such greatrapidity. This is usually explained in terms of an extensiveexploitation of parallel processing architecture. Neverthe-less it should be stressed that our current understanding ofthe molecular basis of the higher brain functions such as

memory and reason is essentially nonexistent and thereforeprovides a poor precedent for a "synthetic biocomputer."

Processes of the Brain and the Operation of an ElectronicComputer. The operational similarities between the brainand an electronic computer are superficial at best. Even theanalogy between the electronic switches in a present-daycomputer and the units in neural membranes responsible forthe control of ion flow (gated ion channels) is of a cursory

nature. The opening and closing of the ion channels are sto-chastic processes and the overall membrane conductance re-

flects the sum of the activities of many channels. Choosing,instead, the neuron as the counterpart of the electronicswitch does not improve the analogy because the neuron isfar more complex than an electronic component. It has beensuggested that input/output responses in mature neurons are

modified ("switched") in the course of higher brain functionsuch as learning and memory (9). However, it is conjecturedthat such neuronal modification involves alterations or denovo synthesis of proteins via complicated biochemicalmechanisms that are not yet understood (10).

Active Materials of the Brain and Components of an Elec-tronic Computer. Even if it were possible, there seems littlelikelihood that one would choose the massive and slow ionsand molecules ofthe brain as the active carriers in a BCC. Sowhat is envisaged for this hypothetical device is a piece ofcomputing machinery entirely distinct from anything foundin nature but constructed with the same (or a greater) degreeof miniaturization and based on electronic circuitry usingelectrons (or holes) as carriers.Now, the primary building block in biology is the protein

molecule and it does not seem very likely that this type ofchemical functionality could provide electronic circuitry, al-though an electronic theory of biology was proposed bySzent-Gyorgyi (11) as long ago as 1941. With certain excep-tions, it is now accepted that electron movement is highlyrestricted in biosystems. Some naturally occurring sub-stances such as the biopolymer melanin may be given semi-conducting properties by suitable manipulation in vitro but,in general, polypeptides possess large band gaps, character-istic of electrical insulators.The electron transport that occurs in the mitochondrial

respiratory chains and the electron-hole separation in photo-synthetic reaction centers make use of protein-bound pros-thetic groups with relatively low redox potentials. Many ofthe individual electron transfer steps are slow (hopping con-ductivity), and cannot begin to compare with conduction ve-

locities in semiconductors. Thus there is little evidence forthe general utilization of conduction band electron transportin biology.Molecular Switching and Chemical Codification. Most pro-

posals (5) for a (bio)molecular device have been based on thesupport and control of electron conduction-that is, molecu-lar mimicry of a conventional solid state device. Alternative-ly, binary status could be dependent on a change in molecu-lar state relating to geometry, bonding of a group, or position

of localized electrons or protons. Such processes are under-gone by many molecular entities, including biomolecules andare reasonably well understood. In fact, much of the activityin the development of organic compounds for high-densitymemories is based on materials that undergo such changes ofmolecular state (see below). However it should be recog-nized that these chemical processes are relatively slow be-cause they depend on nuclear motion. With the exception ofproton tunneling events and some excited-state isomeriza-tions, these phenomena do not approach the switchingspeeds of current solid-state devices employing an electronicchange of state. The coding of genetic information within theDNA molecule is the most striking example of chemical cod-ification. However, the information is stored in read-only-memory form (apart from an error-correction capability),and access time is relatively slow.

Assembly. If the advantages of modem solid-state devicesare to be retained, where conduction velocities approach thespeed of light and switching speeds are of the order of pico-seconds, it seems certain that the MED will be composed ofmolecules that do not occur in nature. There is therefore nobiological precedent to suggest that methods will be found toengineer such systems at the molecular level. It has beenargued that protein could still be used to fabricate aMED/BCC but, rather than using the assembled polypeptideas the active element, it would merely provide an appropri-ate template on which to graft nonbiological MED compo-nents (12).Although it is not clear at this time what molecular struc-

tures might be used to build a molecular electronic (or non-electronic) device and computer, certain consequences ofusing such small active elements are expected. Thermody-namic considerations dictate a minimal error rate in the con-struction of complex molecular structures. Furthermore,side reactions, both thermal and radiation-induced, will leadto defective elements at some constant rate. This rate is sig-nificantly larger for complex organic materials, comparedwith the inorganic materials of present day solid-state de-vices. Living systems solved these problems by devoting asubstantial fraction of available free energy to error-correc-tion, repair, and replacement functions. A large number ofenzyme systems are involved in the detection and repair ofthe cellular DNA (13), and complex energy-intensive pro-cesses are associated with error suppression in the biosyn-thesis of nucleic acids and proteins (14). Enzymes and otherfunctional proteins degrade and are removed and replaced("turned over") at commensurate rates (15). It would addseveral more layers of complexity to the molecular chip toinclude repair functions; thus, redundancy would be em-ployed to circumvent errors and component breakdowns,much like the approach used currently in some advanced sili-con chips.

Dimensional Limitations in Molecular Devices

Thus while the task of assembling the MED is one of Hercu-lean dimensions, what of the other side of the coin-the mo-lecular components themselves? Here again there is littleprecedent, but the quintessential question in this respect re-volves around the effect on electronic properties of the tran-sition from macroscopic bulk materials to microscopic indi-vidual molecules. Can electronic conduction and switchingprocesses in general be expected to remain invariant to suchminiaturization?

Dimensional Electronic Localization in Synthetic Metals. Apartial answer may be supplied to the foregoing questionbased on the work on synthetic metals (16), and the answeris that conduction band electron transport is extremely sen-sitive to dimensional aspects of materials. Recent work onorganic conductors has produced materials with the charac-

Proc. NatL Acad ScL USA 82 (1985) 1875

1876 Applied Physical Sciences: Haddon and Lamola

teristic properties of insulators, semiconductors, semimet-als, metals, and superconductors (16). Some compounds en-compass a number of these characteristics and exhibit well-defined transitions as a function of temperature, pressure, orelectric or magnetic field strength. The primary emphasis inthe area of synthetic metals has been on transport proper-ties, but even here single molecular strands have not beenisolated and studied. The information available from thisfield comes from the study of bulk materials that possesshighly anisotropic electronic properties in which the conduc-tivity is not the same in all directions. Many of these com-pounds are therefore thought to behave as though they werecomposed of an array of single molecular strands or a collec-tion of individual molecular sheets and they are thereforetermed pseudo one- and two-dimensional, respectively.Nevertheless, it is not clear that such materials really doserve as a prototype for a molecular conductor. Although theprimary transport process is along the chain, it seems clearthat medium effects such as the effective dielectric constantwill exert a large influence, particularly in the presence ofdopants. Even assuming the viability of the one- and two-dimensional synthetic conductors as models for a molecularwire, the immediate prospects are not encouraging. The ma-terials with highly one-dimensional transport properties areprey to a host of electronic instabilities, all of which serve todecrease conductivity (metal-insulator transition). The sin-gle current exception to this behavior appears to be the sug-gestion of the existence of nonlinear collective modes suchas sliding charge density waves (17, 18) and solitons (19),which may enhance the transport properties of some materi-als. It is interesting to note that the genesis of the field ofsynthetic metals was provided by the Little model (1964) fora high-temperature organic superconductor that provided amolecular engineering blueprint for a complex polymer (2).Although the proposed structure has never been constructedand the model has never been verified, the idea inspired aburst of interest in the field that has led to the synthesis ofconducting materials with other structures.Dimensional Electronic Localization in Metals. There is as

yet no firm experimental evidence for room temperatureelectronic localization in two-dimensional (thin films) orone-dimensional (narrow wires) made from highly conduct-ing metals. For high-resistance wires at low temperatures,however, the case for localization effects is strong (20-24).The difficulties in fabrication have hindered attempts to lo-cate a critical diameter for localization in highly conductingwires, but it seems certain that if an electric current is toflow in a wire below a certain cross-sectional area the elec-trons will require sufficient energy to hop between localizedstates (20-25).Dimensional Restrictions in Photonics. There are severe

difficulties in connecting or addressing a nonelectronic mo-lecular device. If states of components are to be sensed viaspectroscopic signatures certain limitations obtain. The lim-iting spatial resolution (minimum size of individually ad-dressable region) is of the order of the wavelength of theelectromagnetic radiation employed. For reasons of specific-ity and to avoid excessive radiation damage, the near-ultra-violet region of the spectrum (-250 nm) would probably bethe lower limit. Thus the spatial resolution would be of theorder of 0.25 Am, which is far greater than the dimensions ofmost organic molecules.Dimensional Limitations of Packing Density. As the dimen-

sions of individual electronic components decrease and thecomponent packing density increases there is a concomitantincrease in the complexity of the problems in addressing in-dividual components, connecting components, and prevent-ing unwanted interactions among components (cross talk).Cross talk occurs when the elements of a chip are sufficient-ly close for electrons to tunnel between the components. If

the device is electronic in nature, the distance between com-ponents rather than their dimensions, may set the limit ofminiaturization.

Current Approaches

It is now appropriate to consider the approaches to theMED/BCC currently under investigation. We format thediscussion along the lines of a hierarchy or protocol for thedevelopment of a (biological) MED. Beginning with the mostbasic structures we will progress to the more complex, thusthe discussion starts with wires, moves on to componentssuch as switches and rectifiers, and concludes with the com-plete MED/BCC.

Molecular Wire. Proposals for a molecular wire have beenin the scientific literature for some time; the model of Little(2) for an excitonic superconductor made use of this idea in1964. Most of the proposals for a molecular wire, such asthose advanced by Carter (26, 27) are based on conductingpolymers such as polyacetylene and polysulfurnitride. Cur-rent methods of preparation make the isolation of individualmolecular strands exceedingly difficult. Furthermore itseems likely that these materials would not be sufficientlygood conductors to be used in the pristine state. Generationof carriers through oxidation or reduction of the polymerchain might improve the situation, although the presence ofcounter ions could lead to decreased conductivity as a resultof Coulomb localization. At least in principle a number ofthese questions are open to experimental test. Althoughthere are many uncertainties associated with the idea of amolecular wire, this probably represents the logical first stepin the construction of a MED/BCC. If a molecular wirecould be synthesized, isolated, and successfully tested, thiswould represent a genuine beginning to the era of the MED.

Molecular Switch. The development of a molecular switchis also basic to the implementation of the MED. Such a featwould be comparable with the discovery of the transistor,which heralded the introduction of solid-state electronics.

Prerequisites for an operational switch. Two basic ap-proaches have been advocated and these involve geometri-cal and electronic bistability for the binary change of state ina molecule. A number of molecular systems exist in two (ormore) stable states and a subset of these are capable of un-dergoing transitions between the various possibilities. Thesemolecules therefore possess the basic requirement of aswitch: bistability. For an operational molecular switch,however, there are three other essential ingredients. First,the switching process must be controllable-it must be pos-sible to unambiguously set the state of the switch. Second,the state of the switch must be readable-it must be possibleto unambiguously sense the state of the switch. Third, thetwo preceding functions must be executable at the molecularlevel with full addressability-it must be possible to selec-tively switch or address an individual molecule. Althoughmany materials exhibit bistability and thus may be classifiedas switches, the attempt to develop an operational molecularswitch is an extremely daunting challenge.

Voltage-gated ion channels found in membranes of certaincells are of macromolecular dimensions, of the order of 5nm 2, and fulfill some of the requirements of an operationalswitch. However, as was mentional above, switching is notunambiguously controllable and individual channels are notaddressed.Nonelectronic molecular switch. As noted above many

molecules are capable of existing as isomers and particularlyin the case of organic systems some of these may be inducedto undergo conformational changes that carry one form intoanother. A number of agents may be effective in inhibiting orfacilitating this change such as redox systems, ligation, light,and an applied electric or magnetic field. Of course the light-

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induced reactions form the basis for a number of photochro-mics (28) and the systems developed by Heller (29) were al-most brought to commercial practice by the Plessey Corpo-ration for use as holographic memories (30). Such opticalmemories operate with high bit densities, but because of thelimitations on read-out sensitivity the system operates farfrom the wavelength-limited bit size, which in turn is farfrom molecular resolution.There has been much interest in the idea of using systems

that employ a hydrogen atom as the mobile group as a resultof the very fast switching times arising from the proton tun-neling mechanism. A variety of compounds have been sug-gested in the scientific literature and some of these havebeen examined by spectroscopic techniques and their poten-tial energy surfaces characterized with respect to proton mo-tion. In this area detailed proposals of molecular architecturehave been made by Carter (31) and the concept of "smartmolecules" was enunciated by Haddon and Stillinger (32).The actual implementation of such concepts is far behind theideas which have been advanced. Aviram et al. (33) havestudied a class of hemiquinones and considered the effect ofan electric field on proton switching. Rosetti et al. (34, 35)investigated proton motion in the ground and excited statesof hydroxyphenalenones. Such studies have produced inter-esting new information in chemical dynamics but have yet topoint the way for the development of a practical molecularswitch. Although the speed of proton tunneling makes thehydrogen atom an attractive group in conformational switch-ing processes, the leakage back through the barrier, withconsequent loss of integrity of information stored, remains a

problem. Similarly, the questions of single molecule switch-ing, addressing, and interrogation remain to be resolved.Photochemical hole-burning techniques circumvent some ofthese problems and allow information storage in the frequen-cy domain (36); currently such work requires cryogenic tem-peratures.

Electronic molecular switch. The only candidates for truemolecular electronic switches seem to be based on the ideasof Carter (26, 27, 31) and of Aviram and Ratner (37). Thelatter have proposed a molecular diode or rectifier that in-corporates electron-rich (donor) and electron-poor (accep-tor) components within the same molecule and, theoretical-ly, for some range of applied voltage, is expected to allowcurrent flow in only one direction. All of these proposals formolecular electronic switches still await experimental real-ization and verification.

Molecular Biology. Biochip concept. The proposals forMEDs have been extant for some time-it is the concept ofthe BCC that has caught the imagination of scientists and thepublic at large. Again, it is the success of modern biologythat has lent impetus to the idea of a computer based on mo-

lecular electronic devices (MEDC). Within conventionalchemistry there seems at present no obvious way to assem-ble a structure of the inherent complexity of a MEDC. Largeand intricate molecular systems are routinely constructed inbiology and it has been proposed that genetic engineeringshould be brought to bear on the MEDC, thus producing theBCC and circumventing the need for the conventional chem-istry approach.

Genetic engineering. Two groups have championed thetechniques of recombinant DNA and genetic engineering inthe fabrication and self-assembly of the BCC. Ulmer (12) andMcAlear and Wehrung (38) have stressed the fact that all ofthe genetic information required to produce an organism isencoded within the DNA although the preexisting cytologi-cal structure is necessary for interpretation of this informa-tion. This, together with the ability of many biopolymers toundergo self-assembly, led to the suggestion of a BCC syn-thesized by genetic engineering techniques. Quite apart fromthe viability of the necessary components (discussed earlier)

it is extremely difficult to delineate the potentialities of thismethod of fabrication. The chief problem is that it is not yetknown which molecules are to be incorporated as the elec-tronically active constituents of the BCC. It seems clear (asdiscussed above) that for electronic circuitry the moleculeswill be quite distinct from anything that occurs in nature.Will genetic engineering techniques be capable of dealingwith such compounds? When constructed will they retainthe ability of biopolymers to self-assemble to a controllablestructure?At present it is possible to direct bacteria to produce

copies of a protein possessing any desired sequence of aminoacids (providing the protein is not toxic to the microbe).Thus the current status of biotechnology only allows theconstruction of proteins, which are unlikely candidates forthe active electronic elements in a MED. However, proteinmay be able to serve as a matrix for the organization of elec-tronic components composed of other classes of com-pounds. Conceivably it may be possible to fabricate molecu-lar logic devices that are not based on conduction band elec-tron transport but depend on molecular processes such asisomerization. Compounds of this type may be inherentlymore compatible with the current techniques of biotechnolo-gy than the MED components.At the moment, however, whether it be for use as a pas-

sive matrix or active element, it is not known how to specifyan amino acid sequence that will yield the three-dimensionalconformation necessary for specific function (15, 38, 39). In-deed, it is not currently possible to derive a detailed three-dimensional structure of natural proteins from their aminoacid sequence or the function of a protein from its three-dimensional structure. In fact, the relationships between thefunctions and structures of natural proteins, where both areknown, are not fully understood in spite of three decades ofactive investigation.McAlear and Wehrung (40-42) and Hanker and Giammara

(43) have developed a technique that uses conventional lith-ography to expose a pattern on the synthetic protein polyly-sine that can be subsequently developed with silver to give aconducting network. This process has been described as afirst step toward the construction of a very small device thatin turn is construed to be an intermediate stage in the devel-opment of a true MED. Within the frame of reference adopt-ed in this article, however, this system does not qualify as aMED. That is, it is engineered down rather than up, and ituses macroscopic bulk properties rather than molecular phe-nomena. The first application of proteinaceous material togenerate a pattern occurred some time ago with the use ofdichromated gelatin (44).

Neurological prostheses. In addition to the connotation ofa logic device produced by using modern biotechnology, theterm "biochip" is also being used to describe implantablesolid-state devices used as neurological (or physiological)prostheses. The latter concept stems from the work of Brind-ley and Lewin (45) on the production ofphosphenes (percep-tion of flashes of light by the blind) in patients by electricalstimulation of the visual cortex. Recent work by Dobelle etal. (46) made use of an 8 x 8 Teflon matrix of electrodes thatallowed the patient to recognize simple patterns. In an ap-proach suggested by McAlear and Wehrung it is envisagedthat connections of devices to the brain will be made by thegrowth of embryonic nerve cells between a protein-coveredmatrix of electrodes and the visual cortex. Thus, in this hy-pothetical device the molecular engineering is once again leftto biology.Brain architecture and algorithms. So far our discussion

has centered around the possible use of materials and assem-bly technology derived from modern molecular biology toproduce computing devices of the future generation. But thisis not the only way biology may influence computer science

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and technology. As the architecture and circuitry of nervoussystems become better known, novel concepts should sur-face and influence the computer field. For example, studiesof the circuitry involved in motor control and its interfacingwith sensory systems in animals from mollusks to humans(47) may eventually have an impact on robotics.Knowledge of neuronal network architecture in the central

nervous system and of the properties of human memoryhave led theorists to ideas for organizing present-day solid-state components to produce logic and memory devices thatwould function in novel ways. In an exciting synthesis ofconcepts from neurobiology and solid-state physics, Hop-field (48) has shown, by means of computer simulations, thata content-addressable memory emerges naturally as a collec-tive property of systems containing large numbers of appro-priately coupled switches. His computer model, which usesan algorithm that can be described as asynchronous parallelprocessing, correctly yields an entire memory from any sub-set of sufficient size. Additional emergent collective proper-ties may provide capacity for categorization, error correc-tion, and time sequence retention. The collective propertiesare quite insensitive to the failure of individual components.

In a similar vein, Barker (49) has devised computer modelsof arrays of electronic components at very high densitieswhere "cross talk" occurs. In simulations he finds thatthese arrays spontaneously undergo cooperative transitionsamong a set of states.Concluding Statement

It is clear that the advent of the MED/BCC still lies wellbefore us. In the short term a demonstration of feasibilitywould be most worthwhile; the construction of a molecularcomponent that could function as a wire or switch would domuch to add credibility to the field.The twin criteria of utilization of individual molecules as

functional elements and the assembly of such elements re-main as elusive as ever. Biology engineers structures on themolecular scale but biomolecules do not seem to be imbuedwith useful electronic properties. Molecular beam epitaxyand thin-film techniques produce electronic devices but they"engineer down" and are currently unable to generate indi-vidual molecular units. The potential of the MED/BCC fieldis matched only by the obstacles that must be surmountedfor its realization.

It is a pleasure to acknowledge the contributions of E. A. Chan-dross, G. J. Dolan, A. Gelperin, J. J. Hopfield, and F. H. Stillinger.

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Proc. NatL Acad Sci. USA 82 (1985)