Abstract

The ultimate in miniaturization - controlled structuring on the nanometer scale - is becoming possible. Still, speculation should not become expectation. There are many hurdles yet to be surmounted.

The scientific and popular attention paid to the burgeoning field of nanotechnology is well deserved. Controlled structuring on the nanometer scale is the ultimate in miniaturization and theoretically allows us to produce materials with exceptional properties. However, as James Murday of the U.S. Naval Research Laboratory explained in his talk at the Sixth Foresight Conference on Molecular Nanotechnology, nanotechnology doesn't yet exist. Today, we have nanoscience. The first commercialized applications that will make the field into a technology are years, maybe decades away.

Despite the many remarkable scientific advances presented at Foresight, it was difficult to tell whether the meeting was a scientific conference, a Star Trek convention, or a cult gathering. Take the belligerent audience member who accused the Department of Defense of "wasting its time" by funding "incremental research" rather than preparing for the coming of the universal assembler, a device that would make all military infrastructure obsolete by, for instance, assembling a battleship in a beaker in mere days. And then there was Foresight chairman K. Eric Drexler's diatribe that decried the efforts of the established scientific community to discredit his ideas.

Nanoists' dreams are spun from Drexler's works. His popular speculative science book, Engines of Creation, promises revolutionary fantasies, and his technical work, Nanosystems, lends his notions an air of legitimacy. These books suffer from one overriding fault, however: for all his detailed calculations, Drexler supports his arguments primarily with hyper-extended analogy. Due to space constraints, I will only address one of these in detail - the argument that life exemplifies working nanotechnology. But I have similar objections to Drexler's visions in engineering, which involve simply scaling down macroscale mechanical components to molecular dimensions, and basic science, which demand that hero experiments be used repeatedly and in combination to create even the most basic building blocks of a molecular assembler.

Life Does It, So We Can, Too

The molecular assembler is one of Drexler's most popular visions. Swarms of these nanorobots, working from encoded programs, would construct bulk materials or nanoscale devices atom by atom. To create and sustain these swarms, Drexler suggests self-replication. As is quoted on the jacket of Engines of Creation, "Living things are proof that nanotechnology works." Life does it, so we can, too.

But, as with most proofs by analogy, Drexler invokes it only when convenient. For example, he envisions atomically perfect machines, and with good reason - a single misplaced atom will cause one of his proposed bearings or sliding rods to seize. Drexler's devices have zero defect-tolerance. Life, though, does not require system-wide perfection. Lipid bilayers do not mandate single precise arrangements of their component molecules. Cellular organelles are not mutually oriented with atomic precision. Life has extended defect-tolerance so far that inaccuracies crippling to a Drexlerian nanorobot simply aren't relevant in biology.

Life's robustness and flexibility come largely from its architecture of reversibly bound building blocks: amino acids form proteins and nucleotides form DNA and RNA. But Drexler abandons these Lego-like components, using instead highly reactive species, wielded with atomic precision by robotic manipulator arms, to deposit atoms singly or in small groups. The final devices are monolithic, unlike the facile aggregates of separately functional components typical of nature.

Life exists in a remarkably useful medium: fluid. Flow and diffusion mediate materials transport, and the fluid carries away the heat released by chemical reactions. Drexler instead envisions a vacuum environment. It's clean, but requires mechanical conveyer belts to move covalently bound materials. The design is further complicated by the chemical reactions used to transfer molecules from belt to belt; somehow the device itself must dissipate the heat released by each of these transfers.

Drexler may argue that self-replicating nanomachines are feasible because life already does it, but his proposals reject many of the features of life that make its designs so feasible. And while he suggests that careful attention to design will render negligible such problems as atomic mispositioning and heat dissipation, he ignores relevant lessons taught by the model system he touts. So much for careful attention to design.

A More Likely Future

Just as the space program spawned such far-reaching visions of the future as 2001:A Space Odyssey, nanoscience spawned Drexler's vision. The resulting interest and funding has produced much good science that may lead to technological applications. Nanostructured materials are already bridging the gap between the macroscale and the nanoscale, as evinced by the increasing use of carbon nanotube atomic force microscope tips. Both hybrid materials, such as carbon nanotube reinforced polymers, and conventional electronics applications, such as field-emission displays, are on the horizon.

True nanotechnology applications are also possible, particularly nanoscale electronic computing, where carbon nanotubes or the molecular electronics components developed by James Tour of the University of South Carolina would be combined into nanocircuits. In one scheme, a DNA template would arrange tagged molecular transistors into desired patterns. Novel circuit architectures such as neural nets may overcome the problem of nanoscale interconnects and the necessity for fault and defect tolerance in nanoelectronics.

The specifics of Drexler's vision are, of course, implausible, as is all speculation so far removed from current technology. Speculation can be fun and even useful for generating ideas, identifying problems, and producing workarounds. But when speculation becomes expectation, the battleship-in-a-beaker masquerades as a plausible near-term goal. Drexler's nanotechnology is a theoretical flight of fancy. The experiments of nanoscientists stand to produce a technology more grounded in reality.

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