Other designs described in the Science paper, include self-assembling boxes with polymer-gold bilayer lids that can open or shut as needed (see illustration). Such microcavities might serve to deliver drugs more precisely, to carry out nano- and picoliter chemistry tests or to help scientists study cells in an application the authors call "cell clinics." Similarly, the microactuators could be used to block the flow of liquids through, say, a lab-on-a-chip. Jager and his colleagues are also developing with cell biologists an instrument dubbed a "cell tapper." This device would, as its name suggests, tap on an individual cell, thereby making it possible to study the forces the cell exerts under different circumstances.

Dancing Tin

Far more preliminary, but promising investigations from researchers at the Sandia National Laboratories appear in the same issue of Science. Andreas Schmid, Norm Bartelt and Robert Hwang report on the discovery of tin dancing on copper, a phenomenon that resembles the way camphor particles will shimmy across the surface of water. Their hope is that by finding ways to harness and choreograph the tin crystals¿ movements, they can force alloys into assuming useful nanoshapes and create super-efficient nanomotors.

Source: EDWIN JAGER et al. NANO-BOX opens and closes when the conjugated polymers from which it is fashioned bend the flaps together. Such microcavities could serve to deliver drugs more precisely and in studing and compartmentalizing cells within a lab-on-a-chip.

Schmid¿s team made the discovery by watching how tin and copper morph into bronze in real time using both scanning tunneling microscopy (STM) to obtain a topographical map of the material¿s surface and low-energy electron microscopy (LEEM) to watch the movements of the tin on that surface. They noticed that when tin is deposited onto copper, it clumps up into tiny two-dimensional crystalline islands within seconds. And these islands surf over the copper¿s surface, swapping tin atoms for copper atoms as they go. When the islands become bronze crystals, they eject the copper atoms they picked up. After a few moments, bronze clumps cover the surface and the tin islands dissolve.

The researchers provide an explanation for the dancing tin islands: tin already embedded within the copper repels the traveling tin islands, driving them on to find another surface spot. Only when they find such a spot do they make an atomic trade of tin for copper. The islands are so efficient in their search for a non-competitive spot that they will even force themselves into a corner of the copper surface to avoid any backtracking. Schmid explains that this "completely unanticipated cooperative process" takes place because the islands "lower the surface free energy by moving toward unalloyed regions of the surface."

In an accompanying essay, Flemming Besenbacher of the University of Aarhus and Jens Noskov of the Technical University of Denmark note that the discovery "can be viewed as a direct observation of a nanomotor" because the tin islands translate chemical energy into forward motion. This sort of naturally-occuring motor is surely efficient¿and it appears to be remarkably powerful as well: Whereas a car delivers some 0.1 horsepower for each kilogram of its weight, the tin islands offer a power-to-weight ratio closer to 0.3 hp/kg. In conclusion, Besenbacher and Noskov note, "the challenge is to devise nanomotors whose motion can be controlled externally (so that they can be used to move things around at will) and that can be refueled."

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