By Paul E. Burrows Ph.D., guest contributor to Small Times
November 20, 2007 — The Society of Manufacturing Engineers’ Technology Roadmap for Productive Nanosystems (TRPN) workshop, held last month (October 9-10) in Washington, D.C., drew about 100 attendees. The event capped a two-year effort led by the Foresight Institute in collaboration with the Waitt Family Foundation, Battelle Memorial Institute and others.
Although not explicit in the title, both the workshop and the associated roadmap focus on a controversial theme: atomically precise manufacturing (APM). Few areas of technology have generated as much visceral argument as the potential to use nanotechnology to manufacture macroscopic amounts of material with atomic precision. Popularized by K. Eric Drexler in his 1981 book Engines of Creation, the idea that massively parallel arrays of tiny “assemblers” could build practically anything – including more assemblers — led to the now infamous “gray goo” scenario of uncontrolled replication.
The field still suffers from the vision being technologically distant from what we can accomplish today. The Foresight-Battelle roadmap aimed to address this “vision gap,” suggesting incremental steps from what we can do now to the improvements that are required to enable macroscopic APM. So what has changed to justify the considerable expenditure of effort that went into the TRPN?
Presentations at the workshop made a convincing case. The first answer lies in the International Technology Roadmap for Semiconductors (ITRS), which shows that if another 10 – 15 years of Moore’s law progress can be achieved, it leads inevitably to processing at close to atomic resolution in silicon transistors. Atomic-scale manufacturing therefore begins to seem somewhat inevitable. The first two workshop presentations gave a second answer: that APM is steadily being achieved now, at ever greater levels of integration, using both bottom-up and more traditional top-down technologies.
Enzymes have often been cited as extremely complex, but natural, atomically precise machines. Prof. Christian Schafmeister (Temple Univ.) argued that the design of artificial enzymes which fold in a predictable pattern is difficult because of the large number of degrees of freedom in the constituent polypeptide, where there is free rotation around almost every bond. Schafmeister has therefore designed a series of peptide analogs which bond to one another at two points, making a rigid ladder structure. With 14 available building blocks, he has demonstrated predictably folded structures up to 18 units long. He thinks 40 – 50 units is achievable. While these are not yet functional structures, it is possible to imagine them becoming so with further development. For example, tethered chemical groups could be brought into close contact as the ladder folds, causing them to react.
By contrast, John Randall (Zyvex Research) regards self assembly as “powerful but limited” and has adopted a top-down methodology based on the selective depassivation of a hydrogenated single crystal silicon surface using a scanning tunneling microscope. This yields a hydrogen mask which permits etching of silicon (or possibly silicon-germanium) structures with atomic precision. The hydrogen mask appears to be stable against surface diffusion up to 200 – 300°C; a key challenge is to achieve silicon epitaxial deposition within such a temperature range, enabling structures to be built up in addition to being etched away. Randall, however, sees commercial products within five years. These will be simple, low volume but valuable products such as atomic-resolution metrology standards for the semiconductor industry.
Prof. Sir Fraser Stoddard was presented with the 2007 Feynman award in experimental nanotechnology during the workshop and gave a key plenary lecture in which he reviewed his work on rotaxanes and calixaranes — complex organic molecules that have been shown to behave as molecular switches, capable of storing information.
It has always been the case that organic chemists routinely make atomically precise constructs (molecules) but their precursors are brought together by diffusion. The error rate is large and the products are subsequently separated from a complex mixture of reactants and by-products. Can we replace diffusion by the deterministic positioning of atoms? For certain reactions, one molecule at a time, the answer is now yes. Success in scaling such processes to macroscopic volumes remains a long way off but perhaps no longer so far that that a roadmap is not a useful idea. Prof. Stoddard’s molecules and the DNA-based self assembly techniques developed by Rothemund et al. point to a rapid increase in the complexity of molecules that we can assemble in high volume. Simultaneous improvements in top-down manipulation using atomic probe tips is one method of arranging such molecular building blocks into higher order structures.
As pointed out by the introductory speakers, the roadmap must be an evolutionary document. Parallels with the ITRS are inevitable. That roadmap, however, was first assembled in the context of an existing microprocessor industry. In contrast, the TRPN is willfully more speculative. Breakthroughs yet to be invented will no doubt significantly change its path in future years. Drexler himself is now of the opinion that, regardless of feasibility, self replicating assemblers would not be the best way to go about APM and further refinements of the vision are to be expected. The usefulness of a roadmap, however, is that it gives confidence to a researcher in one area that other necessary parts of the technology will be invented. In that sense, this effort may be enabling. There is ample reason to try: The TRPN includes a call for government agencies to fund specific research aimed at APM with applications in clean energy and advanced healthcare and an extensive Appendix includes contributions from experts on how APM could revolutionize a plethora of applications in these and other areas.
The Technology Roadmap for Functional Nanosystems needs to be read, discussed, criticized, and (as many speakers at the workshop pointed out) repeatedly revised. But it should not be ignored. The Foresight Institute expects to make it publicly available soon; watch the organization’s website in the coming weeks.
Paul E. Burrows PhD is Laboratory Fellow at the Pacific Northwest National Laboratory.