The viscosity of water—or its internal resistance to flowing—at small scales becomes sort of syrupy. And as the American Physical Society notes “in that environment, motions that reverse themselves exactly—such as a diver’s scissor kick—will do nothing more than return an object back to its original place.

Only motions that do not look the same in reverse, so-called nonreciprocal motions, such as the rotation of a bacterium’s flagellum, can advance the swimmer in one direction.” Some nanotechnology optimists envision that someday we will be able to insert tiny nanomachines into our bloodstream to hunt down cancer cells or aid organ repair. So do these nanobots need to be designed like bacteria? Not necessarily. Researchers from the Advanced Studies in Basic Sciences in Iran and the Max Planck Institute for Physics in Germany have developed a novel but simple propulsion system consisting of three spheres connected by rods. As the apparatus changes length, it moves. Want to see how the system works? Click here. These machines, say researchers, could be made from molecules that change length in the presence of different proteins.

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