It's widely accepted today that nanotechnology will soon be able to deliver medicine inside the human body or to do research on cells. But to achieve this goal, you need nano-cargos moving through liquid environments, such as blood. And this is a very difficult challenge because the nano-swimmers have to struggle with blood's viscosity, which has very large effect in a nanoscale environment. But now, two Iranian researchers have found a simple and elegant solution to this problem, based on the principle of non-reciprocal motion and described in "Teaching Nanotech to Swim" by Technology Review. Their nano-swimmer consists of three aligned spheres connected by two rigid rods which can contract and expand. The nano-cargo then advances in the blood like an earthworm inside the soil. Even if these nano-swimmers look promising, nobody knows when they will be able to deliver drugs in our bodies.
Here are some details provided by Technology Review about how the nano-swimmer moves.
To get moving, a nano-swimmer needs a nonreciprocal motion, something in which the movements are never symmetrically reversed -- think 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. 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.
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Here is an image showing from top to bottom how the nano-swimmer moves (Credit: Ali Najafi, Zanjan University, Iran). Here is a link to an animation which shows more precisely the movement of the nano-swimmer. |
These nano-swimmers were also recently described in this short article by Physical Review Focus where I found the title for this post. Here is what it said about the status and the future of this model.
To find out whether the swimmer will really work, however, the theoretical model must be made more "robust," says Golestanian. Teaming up with Armand Ajdari of the Institute of Industrial Physics and Chemistry (ESPCI) in Paris, Golestanian and Najafi hope to account for gravity, internal noise from the engines, and random collisions with individual molecules in a liquid. Then there's the matter of construction materials.
Nanotechnology is still in its infancy, but Golestanian guesses that the swimmer would be powered by molecular motors -- molecules whose bonds expand and contract with energy produced by biochemical reactions or in response to light. Once built, it might someday deliver medicine in the human body or conduct research on cells.
The research work has been published in the Hune 2004 issue of the Physical Review E journal. Here is a link to the abstract of this paper named "Simple swimmer at low Reynolds number: Three linked spheres."
We propose a very simple one-dimensional swimmer consisting of three spheres that are linked by rigid rods whose lengths can change between two values. With a periodic motion in a nonreciprocal fashion, which breaks the time-reversal symmetry as well as the translational symmetry, we show that the model device can swim at low Reynolds number. This model system could be used in constructing molecular-sized machines.
I really hope that these nano-swimmers soon lead to great medical advances.
Sources: Martha Downs, Technology Review, July 21, 2004; Physical Review Focus, June 25, 2004; Physical Review E, Vol. 69, Page 62901, June 16, 2004
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