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TOPIC 5: NATURAL POWER 1 Two presentations were made in this session, by George Whitesides of Harvard University (a committee member) and Michael Helter of the University of California, San Diego. Their papers are summarized below. BID-BASED NANO SYSTEMS George Whitesides began with the admonition that we should focus not on technologies that are commercially viable but rather on where there is intelligence value. Examples include the following: 1. Obtaining better human intelligence, 2. Detecting and tracking weapons of mass destruction (WMD), 3. Better surveillance, and 4. Exfiltration getting information in and out of a sensor system. On item 1, it will be important to invent a new technology base as opponents become more adept at shielding their activities from overhead assets. One key area could be modifying humans or animals to harvest natural internal chemical gradients to produce biobatteries ("a goat with an electric socket on its sided. Energy harvesting from natural sources could be the best solution for powering sensors or exfiltrating information.. Examples might include harnessing the sodium and potassium gradients across cell membranes; oxygen gradients within the body; or light-harvesting implants based on m~tochondrial chylom~crons, the very efficient photosynthetic organelles in green plants. On item 2, the question is how we look for WMD. One approach might be to develop ubiquitous sensors that are cheap and disposable an example of which might use all-organic electronics. While such materials have poor properties as sem~concluctors, if we move to transistor gate sizes of 50 nm the performance might be adequate. On items 3 and 4, a major issue in surveillance is refueling of the sensor system. A laser might be used remotely to recharge a battery, for example. Insteac! of having a solar cell that converts light to electricity, one court! use the cell to store energy in chemicals or to charge a battery. RetrorefIectors on the sensor system might also be powered! by interrogating laser light. For some applications that require higher power, nuclear materials might be a good solution for some batteries, in spite of the political 17
1 18 Summary of the Power Systems Workshop issues. Another option might be to genetically engineer E. co1ti bacteria to make sensors that detect chemical weapons or even changing light levels. These bacteria would feature nanometer-sized components, be self-replicating, and be cheap once the research has been done. Nanotechnology brings three things to the problem of power sensors: 1. New materials, 2. Better use of existing materials, and 3. New forms of catalysis. On item 1, new battery materials such as carbon, silicon, and titanium offer favorable thermodynamics but terrible kinetics. They form self-passivating layers that slow down the process. This can be solved by raising the temperaturefor example, by using a nuclear material as a fuel source. Nanotechnology may offer a solution to the generic membrane problem. In the past, the kinds of pores one could obtain were determined by how materials precipitate. The field of engineered nanoscale pores is a key R&D area. On item 2, carbon nanotubes look promising as current collectors. These materials were first patented in 1982, but the field has been in stasis for the past 20 years because of the "60 million dollar chicken-or-egg problem." That is, potential users want a manufacturing line to produce large quantities of cheap material, but before building such a line, producers want some assurance that markets will be forthcoming. Whitesides believes that Japanese companies will solve this problem: Carbon nanotubes can provide high electrical conductivity when loaded into a polymer matrix at low volume fractions, and the market for conducting polymers is very large about 10 million pounds per year. Multiwalled nanotubes are so stiff that they cannot be stuffed into a small space, but an oriented forest of tubes can be grown instead. The virtue of the polywall structure is that only the outside wall interacts with the environment; the inside walls do not, so these materials are self-insulating. On item 3, vapor-liquid-solid (VLS) catalysis can be used to make carbon nanotubes from iron nanoparticle melts. The carbon diffuses along the surface of the iron and precipitates as nanotubes on the back side. Silicon tubes form in a more perfect structure than carbon tubes. New forms of catalysis are needed. BID-BASED NANO SYSTEMS: CHALLENGES IN DEVELOPMENT OF BIOMIMETIC NANOMECHANICAL SYSTEMS FOR ENERGY CONVERSION Michael Helter presented an overview of the 2002 NRC report on the National Nanotechnology Initiative, Small Wonders, Endless Frontiers. In his view, the development of nanotechnology thus far has been evolutionary rather than revolutionary. He is concerned about the growing technophobia surrounding nanotechnology, as expressed in recent books and magazine articles. We need better micro- nano manufacturing methods, particularly for making integrated devices. Heller discussed the potential of nanotechnology in composites and advanced materials, microelectronics, computing, and energy conversion. A largely unexplored area is bionanotechnology. Biological systems do things very differently. They use uniquely organized, highly efficient nanoscale structures (one such biological process is photosynthesis). Heller clescribed experiments aimed at developing an efficient nanomachine for converting chemical energy into mechanical motion using kinesin anti dynein nanomotor proteins with ATP synthase. In living organisms, enzymes are the dynamic catalytic nanomachines that run all synthetic, energy conversion, and animation processes. People can make some pieces of these systems, but they cannot bring them together. Heller's message was that chemomechanical nanostructures and their catalytic, dynamic, and mechanistic properties are the business end of living systems and have the most potential for leading to a truly new generation of biomimetic animation and energy transduction nanodevices. 1
Topic 5: Natural Power TOPIC 5 DISCUSSION The discussion for this session began with a series of comments. If one wants to look for wiclely clispersec! "ball stufiF'~hemical agents or explosivesone should consider using insects, e.g., lightning bugs (or something in between E. cold and goats). Power is a major issue. If one wants to use a laser to refuel a sensor system, it would have to be weapons-class in size. On the other hand, natural systems have a low energy density, and this requires equally low-power methods of analysise.g., by biological processing. DARPA has studied engineered bugs, the virtue being that they are small ant! their breeding time is short. In fact, the government has spent enormous amounts on the development of bio-nano. If one uses dragonflies or hummingbirds, their "batteries" must be refueled very often. Humans are "100 watt objects." Bees have also been considerect as detectors for explosives. Biological organisms use some 20 amino acids and DNA uses 4 different bases in its structure. One cannot mimic biological function without using amalgams of comparable complexity that is, one 19 cannot just use nanotubes. One view was that rather than creating synthetic mimics of biological function, it would be better to use biological structures already in place. Pound for pound, insect flight muscles are equivalent in power to a jet engine, but they run more efficiently and on a lower-grade fuel. Whitesides's view is that it is desirable to use the whole organism rather than remove pieces and use those; living organisms often provide enormous amplification of signals. The immune response system is a gold mine of information; by reading the immune system, one can obtain a history of exposures for the organism. One can use cell machinery to make more robust biological molecules, but it is important to remember that while some cells remain stable at 100°C, the indiviclual components may not be stable when taken out of the organism. One should not pick apart the sensor and the power source they should ideally be integrated together. \ 1 c