A Windmill at the Nanoscale

Going with the flow is how we’ve harnessed energy for millennia. Water wheels, windmills, and turbines have, for many a century, been turned by the elements to spin our axles, do our grinding, and produce our electricity. But that’s all happened at the human-can-climb-it scale. Now, researchers from the Cees Dekker lab at the Delft University of Technology in the Netherlands have created a flow driven rotor at the nanoscale. And it’s made out of DNA.
 
Cells, of course, derive their own power. Bacteria, for instance, have flagellar motors that power them. So it was not so outlandish that the Delft researchers—a good seven years ago—had the idea to create something similar, at a similar scale.
 
The researchers in the Dekker group came up with a host of different design ideas. “They were very impressive, and very complicated,” said Xin Shi, a postdoctoral student in the university’s bionanoscience department and author of the paper “Sustained Unidirectional Rotation of a Self-organized DNA Rotor on a Nanopore,” which appeared in August in Nature Physics. “Some of the experiments worked in theory but didn’t work in the lab. They had too many parts or were too big of a structure. I wanted to do it in a simpler way.”

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The centerpiece of the nanomotor Shi devised is a 450-nanometer-long bar of DNA. The bundle of six helixes has a 50-nanometer nub sticking perpendicularly out of its side, which is essentially the axle and the anchor. This little nub is dropped into a nanopore in a silicon-nitride membrane. Once docked, the DNA bar twists on its own, becoming a naturally angled turbine blade. Later, Shi and his team added a “leash”—also of DNA—which is attached to this protrusion, allowing it to more easily be dragged across the membrane and into a pore.
 
This nano feat was no small feat. “At nano scale, everything is wobbly,” said Shi. The assembly of the nanopored membrane took place in a clean room usually used to make quantum computers. But that twist in the DNA beam was a kind of lucky coincidence.
 
 “In the beginning the bar was not part of the design,” said Shi. “It was designed as a load and wasn’t meant to be rotating.”
 
But rotate it did. When a voltage was applied to the membrane, it started to spin at 20 revolutions per second. The DNA rotor also spun when there was a salt gradient across the membrane (without any applied voltage).

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Now that he’s proved that DNA can be used to build a such a simple rotor, Shi and his collaborators at Delft as well as the Hendrik Dietz lab at the Technical University of Munich, and the Ramin Golestanian’s group at the Max Planck Institute for Dynamics and Self-Organization in Göttingen, are looking at making it more sophisticated. A gear box, for instance, could be added to switch the direction that the rotor turns. They’re also working with the ‪Aleksei Aksimentiev lab at University of Illinois at Urbana-Champaign to expand their molecular simulations so that they can better test designs before trying to assemble them.
 
The minuscule machine is by far the smallest fluid powered motor the world has seen. But Shi isn’t shooting for a Guinness world record. He’s hoping to power the microscopic automata of the future. A flow-driven rotor may soon have a place powering tiny robots programmed to swim through arteries for targeted drug delivery and pinpointed surgery.
 
 “To clear blood clots and kill cancer cells—it all starts with nanoscale engines,” said Shi. “If you want a robot at micro scale, you need nanoscale motors.”
 
Michael Abrams is a science and technology writer in Westfield, N.J.