A team of researchers at Cornell University has created a new class of magnetically controlled microscopic robots (microbots) that operate at the visible-light diffraction limit. Termed diffractive robots, these microbots can interact with waves of visible light and still move independently, so that they can maneuver to specific locations to take images and measure forces at the scale of some of the body’s smallest structures.
A diffractive robot. Image credit: Smart et al., doi: 10.1126/science.adr2177.
Diffractive robotics connects, for the first time, untethered robots with imaging techniques that depend on visible light diffraction — the bending of a light wave when it passes through an opening or around something.
The imaging technique requires an opening of a size comparable to the light’s wavelength.
For the optics to work, robots must be on that scale, and for the robots to reach targets to image, they have to be able to move on their own.
Controlled by magnets making a pinching motion, the robots can inch-worm forward on a solid surface. They can also ‘swim’ through fluids using the same motion.
The combination of maneuverability, flexibility and sub-diffractive optical technology create a significant advance in the field of robotics.
“A walking robot that’s small enough to interact with and shape light effectively takes a microscope’s lens and puts it directly into the microworld,” said Cornell University’s Professor Paul McEuen.
“It can perform up-close imaging in ways that a regular microscope never could.”
“These robots are between 2 and 5 microns. They’re tiny. And we can get them to do whatever we want by controlling the magnetic fields driving their motions.”
“I’m really excited by this convergence of microrobotics and microoptics,” said Cornell University’s Dr. Francesco Monticone.
“The miniaturization of robotics has finally reached a point where these actuating mechanical systems can interact with and actively shape light at the scale of just a few wavelengths — a million times smaller than a meter.”
To magnetically drive robots at this scale, the team patterned the bots with hundreds of nanometer-scale magnets that have an equal volume of material but two different shapes — long and thin, or short and stubby.
“The long, thin ones need a larger magnetic field to flip them from pointing one way to pointing the other, while the short, stubby ones need a smaller field,” said Cornell University’s Professor Itai Cohen.
“That means you can apply a big magnetic field to get them all aligned, but if you apply a smaller magnetic field, you only flip the short, stubby ones.”
To create the robots, the authors combined this principle with very thin films.
“One of the main optical engineering challenges was figuring out the most suitable approach for three tasks — tuning light, focusing, and super-resolution imaging — for this specific platform, because “different approaches have different performance trade-offs depending on how the microrobot can move and change shape,” Dr. Monticone said.
“There’s a benefit to being able to mechanically move the diffracting elements in order to enhance imaging,” Professor Cohen said.
The robot itself can be used as a diffraction grading, or a diffractive lens can be added. In this way, the robots can act as a local extension of the microscope lens looking down from above.
The robots measure forces by using the same magnet-driven pinching motion that enables them to walk to push against structures.
“These robots are very compliant springs. So as something pushes against them, the robot can squeeze,” Professor Cohen said.
“That changes the diffraction pattern, and we can measure that quite nicely.”
Force-measurement and optical abilities can be applied in basic research, as in explorations of the structure of DNA; or they might be deployed in a clinical setting.
“Looking to the future, I can imagine swarms of diffractive microbots performing super-resolution microscopy and other sensing tasks while walking across the surface of a sample,” Professor Monticone said.
“I think we are really just scratching the surface of what is possible with this new paradigm marrying robotic and optical engineering at the microscale.”
The study was published in the journal Science.
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Conrad L. Smart et al. 2024. Magnetically programmed diffractive robotics. Science 386 (6725): 1031-1037; doi: 10.1126/science.adr2177
