Magnetic microrobot swarm moves objects with water

Robots tend to move things physically, using arms or other appendages. But what if robots could move objects without physically touching them? Researchers from the Max Planck Institute for Intelligent Systems, the University of Michigan and Cornell University have developed robotic swarms that can manipulate objects using only water, by inducing a fluidic torque.

Strong viscous interactions exist in microscale systems, which can be used to generate fluid flows that actuate passive objects. In their previous research, the researchers found that this manipulation can be influenced by the number of microrobots, the spin rate of microrobots and the position of the microrobots relative to the object. This latest work, published in Science Advances, has gone one step further, demonstrating that a magnetic robot swarm can assemble, transport and reorganize objects that are many times larger than the microrobots themselves.

“This study is the third in a series of papers where our team explores how microscale robot swarms can coordinate using simple global control signals,” says Kirstin Petersen of Cornell University, “Rather than controlling each robot individually, we broadcast the same signal to the entire group and rely on the robots’ interactions with each other and with their environment to produce different collective behaviours. Here, we showed that those interactions could also be used to manipulate external structures through the fluid flows generated by the swarm”.

The robots are microdisks with diameters of about 300 µm and because they are magnetic, they can be rotated using an externally applied magnetic field. When each individual microrobot spins, it drags the fluid around it, which generates a force in the liquid. While this force is small for an individual robot, combining hundreds of robots together that spin in unison (and/or increasing the spin speed of the robots) creates a much larger flow force in the water – generating a high enough torque to move objects.

“The most exciting result is that the robot collective can use the fluidic torque it generates to manipulate structures much larger than the robots themselves, without physical contact. It suggests that you could add actuation to otherwise passive objects simply by introducing microrobots in the surrounding fluid,” Petersen tells Physics World.

To demonstrate this approach, the researchers positioned the microrobots inside and outside of concentric floating ring structures, and used the number of robots, their positions and spin speeds to act as a form of control for moving objects. They found that the robots could spread out and surround the object, rotating it in the process, or they could crawl around the edges of an object, allowing them to reorganize objects. The ability to change these parameters and obtain different torques provided a tuneable and programmable way of using the microrobot swarms.

The researchers extended the principles to mechanical systems, using the microrobot to turn miniature gear trains (after turning the first gear, the other gears moved by conventional mechanical contact). They also rotated 3D floating objects that were 45,000 times the mass of an individual robot. Here, placing the robots on top of the object generated sufficient torque to rotate it, despite the mass difference.

The team also found that the microrobot swarm could dynamically assemble objects using coordinated fluid flows, in which the robots switched between their rotational function and crawling ability to move objects along a surface. This adaptive behaviour not only allowed the manipulation of objects, but also their reorganization – including expelling, dispersing and aggregating objects – based on the environment and task requirements.

The introduction of these small robots into fluids essentially turns the fluid from a passive medium into a small-scale motor. For applications where there is a risk of structural damage from mechanical manipulation, contactless manipulation could be highly beneficial. For example, this type of mechanism could be useful in microscale manufacturing and biomedical engineering, particularly for miniature device assembly, biological matter transport and targeted manipulation within the human body.

When asked about what’s next for this research, Petersen tells Physics World that “the other authors are focusing specifically on innovating microrobots, whereas my lab is studying the broader question of how collectives coordinate through their shared environment while keeping individual agents simple. We are exploring natural and engineered fluid-coupled swarms across a wide range of size scales”.

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