How Can We Get There?
The wondrous capabilities described here are, for the most part, theoretical. How can they be made real? How can we build a general-purpose, programmable manufacturing system using highly reactive, positionally controlled tools that could inexpensively manufacture most diamondoid structures?
The magnitude of this challenge should not be underestimated. Present proposals for an assembler able to fabricate diamondoid structures involve hundreds of millions or billions of atoms-with no atom out of place. Even a simple robot arm, which might be composed of only a few million atoms, would have to be accompanied by other components. The robotic arms would work in a vacuum, for instance, dictating the need for a shell around the arm to maintain that vacuum. Other ancillary gadgets that will be needed include acoustic receivers, computers, pressure-actuated ratchets, and binding sites. If each operation, such as hydrogen abstraction or carbene deposition, typically handles one or a few atoms, then the error rate must be fewer than one in a billion.
Although such perfection is theoretically attainable, today’s technology is not up to the task. A chemical synthesis process that chemists view as very good converts 99 percent of the reactants to the desired product. Yet that 99 percent yield represents an error rate of one in 100, which is ten million times less perfect than we desire for a mature nanotechnology. The synthesis of proteins from amino acids by ribosomes has an error rate of perhaps one in 10,000. DNA, by relying on extensive error detection and correction along with built-in redundancy (the molecule has two complementary strands), achieves an error rate of roughly one base in a billion when replicating itself.
No existing technology can approach this level of performance. One technique that can position individual atoms, for example, is the scanning probe microscope (SPM), in which a sharp tip is brought down to the surface of a sample so that a signal is generated that lets us map out the surface being probed, like a blind person tapping with a cane to sense the path ahead. Some SPMs literally push on the surface and note how hard the surface pushes back. Others connect the surface and probe to a voltage source, and measure the current flow when the probe gets close to the surface. A host of other probe-surface interactions can be measured, and are used to make different types of SPMs.
The SPM can not only map a surface but can change it-depositing individual atoms and molecules in a desired pattern, for example. In a well-publicized case, scientists arranged 35 xenon atoms on a nickel surface to form the letters identifying their employer: IBM. But this SPM manipulation required cooling to 4 degrees above absolute zero-not exactly ideal conditions for large-scale manufacturing. More recently, IBM scientists have precisely arranged molecules at room temperature on a copper surface. However, SPMs have error rates high enough that they must use relatively sophisticated error detection and correction methods. And while these systems can move around a few atoms or molecules, they can’t manufacture large amounts of precisely structured diamond of the kind that might be used to build a car or a plane.
Finally, today’s SPMs are much too slow. In nature, ribosomes take tens of milliseconds to add a single amino acid to a growing protein. But if an assembler is to manufacture a copy of itself in about a day, and if this takes a few hundred million operations, then each operation must take place in a fraction of a millisecond. An SPM, by contrast, takes hours to arrange a few atoms or molecules. Rather than attempting to solve all these problems in a single giant leap, we might approach them more incrementally-developing a series of intermediate systems. One approach, for example, would be to eliminate the requirement that the assembler be made from diamondoid structures. Diamondoid is attractive, as we’ve seen, because of its strength, stiffness, and electrical properties. But an intermediate system need only be able to make a more advanced system, and perhaps products that are impressive in comparison with today’s products. It doesn’t have to be diamondoid itself.
This suggests what might be called building blockbased nanotechnology. Rather than building diamond, we’ll build some other material from relatively large molecular units consisting of tens, hundreds, or even thousands of atoms. Such large building blocks reduce the number of assembly steps, so fewer unit operations are required, and they need not be as reliable. Soluble building blocks that stick only to other building blocks, not to the solvent or low concentrations of contaminants, eliminate the need for working in a vacuum.
In selecting such building blocks, we have many choices: any of the many molecules that chemists have synthesized, or could reasonably synthesize, with the desired properties. Each molecular building block should have at least three sites where it can link to other building blocks. Units with two bonding sites suggest the polymers ubiquitous in biological systems, such as DNA, RNA, and proteins. Building blocks that have three bonding sites make the design of stiff three-dimensional structures much easier.
Such building blocks could be linked to each other using any one of a variety of well-understood chemical reactions. A particularly attractive possibility is the Diels-Alder reaction, in which a diene (a hydrocarbon with carbon-carbon double bond) can be made to react with a specific molecule.