"The second ambiguity involves the term 'practical.' If there is any use, to anyone, of any scientific advance, I regard it as practical, although others might feel differently. A key near-term goal of our laboratory is the assembly of stick polyhedra into periodic arrays, in order to attack the key problem of macromolecular crystallography, the crystallization problem. We hope to assemble our polyhedra by means of sticky-ended cohesion so that they will diffract x-rays and so that crystallographers can determine their structures and the structures of suitably oriented molecular guests contained within the polyhedra; we expect to be able to form these arrays within five years. It should then be possible to make substantial advances in determining the structures of biological molecules, an advance that will have potential biomedical impacts. To me, this is a practical benefit."

Clifford P. Kubiak and Jason I. Henderson in the department of chemistry at Purdue University in West Lafayette, Ind., add this joint response:

"How far we are from realizing practical benefits from nanotechnology really depends on which aspect of nanotechnology is being considered. Some nanotechnologists envision gears, camshafts and motors engineered on the nanometer (billionth of a meter) scale. Others think of integrated circuits whose smallest features are on the scale of tens of nanometers. Still others see chemical 'self-assembly' as a means of building up larger functional structures or devices from molecular building blocks 0.5 to five nanometers in size. There is a common goal shared by all these researchers, however: to make devices that are smaller than anything now available.

"In general, there are two distinct approaches to constructing very small things: (1) to etch, chisel or sculpt small features into an existing structure or (2) to build up tiny structures from even smaller ones. Research in nanotechnology can be classified according to which approach is employed. People in the first group are using techniques such as scanning tunneling microscopy, atomic-force microscopy, electron beam lithography and other forms of lithography to define very small features, down to the atomic scale (0.1 nanometer). Miniature gears, tiny sensors and smaller integrated circuits are but a few of the objects being fabricated via this approach. The other group is building up larger structures by manipulation of molecular components, a process called chemical self-assembly. Single-electron transistors, highly ordered arrays of nanoscale metal or semiconductor clusters and microcontact stamps that can transfer submicron-scale patterns are a few of the recent breakthroughs resulting from the second approach.

"The question of how far in the future the real, practical benefits lie has been considered in the most detail by the Semiconductor Industry Association. The semiconductor industry has the largest business interest in the tremendous improvements in computer processor speeds and information storage densities that will result from nanoscale devices. The smallest feature of an Intel Pentium processor is currently on the order of 350 nanometers (0.35 micron). The industry's 'National Technology Roadmap for Semiconductors' sets a 70-nanometer minimum feature size as a goal to be realized by the year 2010. The image of a roadmap provides a nice conceptual model for nanotechnology research and development. Proved, existing technologies are the high-volume superhighways. But the next-generation technologies might be found on what are in 1996 only unimproved roads, footpaths or unblazed trails! It is widely believed that the 70-nanometer goal will not be achieved by incremental improvements of present-day lithographic processes.

"Paradigm shifts may be necessary that totally revolutionize the most fundamental architectures of logic and memory devices. A recent article in Scientific American ('Blue-Laser CD Technology,' by Robert L. Gunshor and Arto V. Nurmikko, July 1996) described blue-laser CD technology. That technology would replace the existing 820-nanometer-wavelength laser technology with 460-nanometer-wavelength blue lasers, greatly increasing the amount of data that could fit on a compact disk. All nine of Beethoven's symphonies could be played from a single audio CD, instead of just one symphony. Clearly, this technology is analogous to a paved road, and it may soon see widespread application. A radical paradigm shift to, say, single-electron transistors that take advantage of nonlinear 'staircase' current-voltage responses (which are only possible in nanostructured materials) may take many years to move from being a vague footpath to something resembling a road.

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