Nanotechnology and Enabling Technologies
Improving the ability to control matter has long been a major aim of technology. Progress in science suggests the feasibility of achieving thorough control of the molecular structure of matter via controlled molecular assembly. The consequences of such an ability will be enormous, in areas as diverse as medicine, computation, manufacturing, and the environment. We can gain some understanding of these future possibilities by observing examples from nature.
Molecular assembly in nature operates by means of molecular machines: machines which use individual atoms and molecules as working parts. Enzymes are molecular machines that make, break, and rearrange chemical bonds at a rate of up to a million per second. Muscle fibers work like molecular-scale linear motors. DNA serves as a digital storage medium, directing ribosomes in manufacturing proteins. Nanotechnology will likewise use programmable molecular machines to build complex structures atom by atom.
The term nanotechnology has sometimes been used to refer to any technology giving some control at a substantially sub-micron scale. Here the term is used more narrowly to describe a technology giving more general control of the structure of matter on a nanometer scale: a broad ability to control the arrangement of atoms.
An approach to atomic-scale control was outlined in 1959 by Richard Feynman. He suggested that large machines would be used to build smaller machines, which would build yet smaller machines, working toward molecular dimensions. Ths sequence has been termed a "top-down" strategy.
Modern microtechnology uses a different top-down approach--that of photolithography, thin films, and selective etching--to make microelectronic systems. These techniques are now being used to make mechanical structures such as gears and motors. Currently, such devices are tens of microns in diameter, many orders of magnitude larger than nanometer-scale.
Chemists, in contrast, have started at the atomic level, building molecules precisely. Progress in molecular biology and synthetic chemistry has been steady and impressive: the operation of many naturally-occuring molecular machines has been elucidated, and some have been synthesized.
This work, led by K. Eric Drexler, is to propose a "bottom-up" approach to artificial molecular machines, ultimately including devices such as molecular assemblers, assembler-based replicators, and mechanical nanocomputers. The "bottom-up" style builds toward larger and more complex systems by starting at the molecular level and maintaining precise control of molecular structure. One approach would involve designing new protein-based devices which self-assemble to form molecular machines like those found in nature. Natural molecular devices show that such machines could include parts acting as struts, cables, fasteners, motors, bearings, and numerical control systems.
Non-molecular machines may also be of use in bottom-up strategies. The scanning tunneling microscope (STM), announced by Binning and Rohrer in 1982 and for which the Nobel Prize in Physics was awarded in 1986, and the atomic force microscope (ATM) announced by Binning and Quate in 1986 are both capable of positioning a sharp tip with atomic accuracy. These tips might be equipped with molecular tools capable of carrying out specific reactions.
However, performing only one molecular reaction at a time will be impractical for making large amounts of a product. It appears that until assemblers capable of replication can be built, the parallelism of chemical synthesis and self assembly will be needed; groups of molecules can self assemble quickly due to their thermal motions, which enable them to "explore" their environments and find (and bind to) complementary molecules.
Given their key role in natural molecular machines, proteins are obvious candidates for early work in self assembling artificial molecular systems. Recent work by DeGrado shows the feasibility of designing protein chains that fold predictably into solid molecular objects. Progress is also being made in artificial enzymes and other relatively small molecules that perform functions like those of natural proteins; the 1987 Nobel Prize in Chemistry went to Cram and Lehn for such work. Several bottom-up strategies using self assembly seem feasible.
Bottom-up strategies are being pursued in academic and industrial labs because molecular engineering has short-term rewards. Improved enzymes, sensors, and detectors are obvious applications. More ambitious is work in molecular electronics, aimed at constructing circuitry in which individual molecular parts serve as wires and transistors.
But before reaching this stage, the bottom-up approach should yield gradually more complex molecular machinery, perhaps starting with folding polymer systems able to help synthesize copies of themselves. These could lead to crude "proto-assemblers" (which might or might not incorporate an AFM or STM positioning mechanism) able to manipulate a limited set of molecular tools. With these devices, building an actual assembler with broad capabilities should be within reach.
General-purpose assemblers will use chemical reactions combined with precise location control to produce materials not accessible to conventional synthetic chemistry. Products made by assembler could include many mechanical parts on a molecular scale: drive shafts, cams, roller bearings, levers, even electrostatic motors. Describing potential products of nanotechnology involves the discipline of "exploratory engineering": the study of what can be built with tools that are foreseeable, but not yet available. Exploratory engineering aims not to predict what will be built, but to make a sound case that certain kinds of systems can be built. This favors simple, easily-analyzed systems.
Molecular electronic computers built with nanotechnology seem feasible, but are difficult to design. In contrast, molecular mechanical computers--molecular Babbage machines--can be designed and analyzed more easily. Design calculations indicate that a mainframe computer built with this technology could have a volume as small as a cubic micron. Systems incorporating nanocomputers, assemblers, and suitable programs and auxiliary devices should be able to build copies of themselves as cheaply as bacteria do. Thus, assemblers and their products can eventually become inexpensive.
With replication, assemblers will be able to work in parallel to produce objects of macroscopic size. Special purpose devices could be designed to tackle such enormous tasks as removing excess carbon dioxide from the atmosphere or to produce goods for human consumption. Perhaps most intriguing are the possibilities for radically improved medical care, combining the advantages of molecular action of drugs with those of surgical control.
Almost any technology can be abused, and nanotechnology will be no exception. A technology for inexpensive mass production could be used to produce weapons, and worse scenarios (e.g. programmable "germs" for germ warfare) have been discussed. Nanotechnology will let us control the structure of matter--but who will control nanotechnology? The chief danger isn't a great accident, but a great abuse of power. In a competitive world, nanotechnology will surely be developed. If democratic institutions are to guide its use, it must be developed by groups within their political reach. To keep it from being developed under a shroud of military secrecy, it seems wise to emphasize its value in medicine, the economy, and in restoring the environment. Nanotechnology must be developed openly to serve the general welfare.
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