In a development that would make the ancient Greeks either very proud or deeply confused, scientists have built a molecular motor that spins when electricity passes through it. The tiny machine, inspired by the Archimedes screw - one of the oldest machines in recorded history - can even reverse direction when the voltage is switched. It's a significant step toward creating working machines at the smallest possible scale, assuming you've ever needed a screw small enough to lose between your couch cushions.
Molecular machines are widely seen as the next frontier in miniaturization. Nature already has working examples inside living cells, where molecular motors use energy to produce movement - because nature, as usual, got there first and didn't even bother to publish. Researchers have spent decades trying to recreate these biological systems artificially, and the field's progress was recognized with the 2016 Nobel Prize in Chemistry, which is the scientific equivalent of getting a gold star for finally catching up to a bacterium.
Scientists first developed electricity-powered molecular motors in 2011, but those earlier designs operated on surfaces and relied on the tip of a scanning tunneling microscope to drive their movement - essentially the molecular equivalent of needing a forklift to turn a doorknob. The new motor takes a different approach, borrowing its design from the Archimedes screw, traditionally used to move water uphill. Instead of water, the new design allows electrons to travel through a spiral-shaped molecule, causing it to rotate as electrical current passes through. Same principle, smaller plumbing.
The motor's central component is a specially designed spiral molecule created by the research group of Prof. Michael Pittelkow at the University of Copenhagen. Measuring only a few millionths of a millimeter, the molecule can attach to gold surfaces through sulfur atoms - because even at the molecular scale, scientists apparently insist on making things unnecessarily fancy.
At the Weizmann Institute of Science in Israel, researchers led by Prof. Oren Tal and Dr. Anil Kumar Singh positioned individual molecules between two gold tips sharpened to atomic dimensions. By carefully changing the separation between these electrodes, the team could stretch or compress the molecular screw. When they applied an electrical voltage, measurements revealed that the molecule began rotating. Even more importantly, they could reverse its direction simply by changing the voltage's polarity - which is the molecular equivalent of putting a car in reverse, except the car is a few millionths of a millimeter long and you can't even see it.
This represents the first demonstration of a molecular motor driven by electrical current, firmly connected to two metal electrodes, and capable of rotating in a controllable direction. That's three firsts in one sentence, which is the scientific equivalent of a hat trick.
The driving force comes directly from electrons traveling through the spiral molecule. As they move, they transfer angular momentum to the molecular structure, producing rotation much like moving air turns a wind turbine. The spiral's shape determines how it responds to this flow, similar to the difference between screws with right- or left-handed threads. Once the applied voltage exceeds a certain threshold, the movement becomes continuous rotation - because even at the molecular scale, things need a good kick to get going.
But the experiments revealed another intriguing effect: the rotating molecule conducted electricity much more efficiently than it did when stationary, raising an important question about what was happening inside the tiny machine. To investigate, Prof. Dr. Fabian Pauly and his PhD students Léon Martin and Yuta Ito at the University of Augsburg used quantum mechanical simulations on the LiCCA supercomputer. Their calculations revealed a close relationship between the molecule's rotation and its ability to carry electrical current. Conductivity changes dramatically depending on the angle of the molecule, increasing by as much as two orders of magnitude - or 100 times - during rotation.
The reason turned out to be particularly surprising. Some of the molecular positions that allow electricity to pass most easily are also positions the molecule naturally avoids when it is not moving. Rotation makes those highly conductive arrangements accessible. "The surprising thing is that it is precisely the positions that the molecule avoids when at rest that conduct electricity best. It is only the rotation that makes these positions accessible," explains Léon Martin. So the molecule is essentially conducting best when it's doing the molecular equivalent of flailing around - which is either inspiring or deeply relatable, depending on your current state of employment.
The researchers then worked with theoretical physicists Per Hedegård of the Niels Bohr Institute in Copenhagen and Jing-Tao Lü of Huazhong University of Science and Technology in Wuhan. Using the simulation results, the scientists developed a simplified theoretical model that successfully recreated both the rotation observed in experiments and the accompanying increase in electrical current. This combination of experimental measurements and theoretical calculations was essential to understanding the mechanism. Although researchers could measure the current passing through the molecule, they needed simulations to determine what was happening at such an extraordinarily small scale - because at that size, you can't just look at it and say, "yep, it's spinning."
"In the experiment, you only see the current. Our calculations allow us to glimpse what is happening inside the nanocontact. It was only through close collaboration between experiment and theory across several countries that the mystery of the current increase could be solved," says Yuta Ito, a PhD student originally from Japan. So it took an international team to figure out what a single molecule was doing - which is either a testament to human cooperation or a sign that we've officially run out of things to do at the macro scale.
The achievement could eventually contribute to the development of molecular electronic components and technologies involving spintronics, which uses properties of electrons beyond their electrical charge. It may also offer possibilities for microfluidic systems, which manipulate extremely small quantities of fluids - because apparently we now need machines to handle amounts of liquid too small to be useful in any practical sense.
Beyond these potential applications, the motor provides scientists with a new way to investigate physical processes at the atomic scale. Its ability to convert electricity into controlled rotation, while allowing that rotation to influence electrical current, introduces a new principle for designing extraordinarily small machines. "We have shown that an electric current can be used to generate a directed rotation and that this rotation, in turn, controls the current. This is a new operating principle for machines at the very limits of miniaturization," explains Pauly.
The work at the University of Augsburg received funding from the German Research Foundation (DFG) through Collaborative Research Center (SFB) 1585 (project number 492723217). The computational research was conducted using the University of Augsburg's LiCCA supercomputer (DFG project number 499211671). Additional support came from the European Research Council (ERC, Horizon 2020, Grant 864008), the Israel Science Foundation (2129/23), the Minerva Foundation, the Villum Foundation, the Novo Nordisk Foundation, the Independent Research Fund Denmark (DFF), the National Key Research and Development Program of China, and the National Natural Science Foundation of China - because apparently even a molecule-sized screw requires a multinational funding consortium.
So there you have it: scientists have shrunk one of humanity's oldest machines to the size of a single molecule, and it works. The ancient Greeks would be impressed. The rest of us are still waiting for a molecular screwdriver.