Teaching Nanotech to Swim
When things get really small, they don’t move through fluids with quite the same ease. But a new mathematical model of nanoscale motion could lead to elegant devices that deliver tiny cargoes.
One of the cultural icons of nanotechnology is the image of the surgical team from the 1966 science fiction movie, Fantastic Voyage, motoring along through the bloodstream of scientist Jan Benes. Everyone knows the image is about as closely related to real-life nanotechnology as Jurassic Park is to genomicsstill, its hard to shake. The scene, and others like it have inspired a generation of researchers to pursue technologies that may one day deliver cancer-fighting drugs directly to tumors or clot-busting machinery to the site of a blockage.
To reach those ends, engineers need a way of moving tiny cargos through fluidsa task that turns out to be a bit more challenging when filmmakers yield the territory to the mathematical modelers. One new approach to nano-locomotion solves a longstanding problem in an ingeniously simple way.
When objects have vanishingly small masses, the effects of viscosity become far more important than the effects of inertia. The upshot is that in the nanoworld, there is no such thing as glide. Translated to the nanoscale, the scissors kick that sends a skin diver coasting over a coral reef would produce only a surge forward then back to the same spot. Moving even a micrometer-sized object through water becomes a lot like trying to breaststroke through honey. Move down the scale to the nanometer realm, and the problem is even worse.
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To get moving, a nano-swimmer needs a nonreciprocal motion, something in which the movements are never symmetrically reversedthink crawl stroke instead of scissors kick. In biology (and in movies), the problem is often solved by using a flagellum, or whip-like drive, but the mathematics and the molecular engineering of such a system are daunting. Now, Ali Najafi and Ramin Golestanian of the Institute for Advances Studies in Basic Sciences in Zanjan, Iran, have proposed a solution that requires only the shortening and lengthening of two rigid rods.
Their model consists of three spheres, connected by two rod-like structures. One movement cycle involves first shortening the left arm, then shortening the right arm, next lengthening the left arm, and finally lengthening the right arm. The result is a series of big surges to the right and small surges to the left. (Click here for an animated graphic depicting this scheme.) The closest biological analog is an earthworm pushing its way through the soil. In the case of the nano-swimmer, effective friction is greater when the connecting rods are longer, so the longer segment always provides a partial anchor for the movement.
The work builds on earlier proposals that involved three or more hinged segments. But since the nano-swimmers driving motion is in one dimension, rather than two, the mathematics of this approach are greatly simplified. “This is an old problem,” says Najafi, “but this swimmer is really simple to calculate and should also be simple to manufacture.”
The motion may be mathematically simple and the shapes straightforward to construct, but they would still need to be connected by some kind of active spring-like moleculeand that will take delicate engineering. Then there is the problem of Brownian motion, says Howard Stone, who studies fluid dynamics and microfluidic systems at Harvard University. For truly nanoscale machines to function in a fluid, they would need to overcome not only the stickiness of this highly viscous environment, but also the continuous buffeting of molecules close to their own size. So even if the equations say such a device could travel in a straight line, its progress is likely to look a lot more like a puppy on a crowded playgroundmuch like the path from imagination to reality.
