Dietz and his team built triangular platforms out of DNA, each of which had a rod sticking out from the middle. They stuck these structures to a glass surface and added long DNA arms, which attached to the platforms in a way that allowed them to rotate around the rod.
To create a ratchet effect, the researchers patterned the platforms with bumps that made that rotation more difficult. Only kicks provided by Brownian motion enabled the arms to overcome the bumps and rotate, typically by a half-turn. Without any further intervention, the rotation would go back and forth randomly. So the team also dipped two electrodes into the solution and ran an electrical current in alternating directions. The changing voltage altered the energy landscape experienced by the long DNA arms, and made rotation in one direction more favourable, through a mechanism known as a flashing Brownian ratchet.
This turned the passive devices into actual motors: microscope images demonstrated that under these conditions, each arm — although shaking randomly — kept rotating in the same direction on average. By itself, the nanoscale motor does nothing more than overcome the drag of the surrounding solution. “It’s like when you’re swimming: you’re moving forward and you do a lot of work, and it’s dissipated in water,” Dietz says.
“It’s a remarkable achievement of the team, first of all for them to be able to design a system that folds into such a complex and functional structure with DNA origami and, second, for them to be able to characterize its dynamics so thoroughly,” says David Leigh, a chemist at the University of Manchester, UK.