Along with things like flying cars, miniscule robots thinner than a human hair or smaller than a cell are one of the technologies that screams, “We’re in the future.” Unlike flying cars, tiny robots are already feasible. But despite the popularity of the idea of a swarm of tiny robots carrying out vital tasks in applications like healthcare or monitoring, producing large quantities of these tiny bots has always been tricky.
Robots and robotic components at these miniscule measurements are usually intended to work collectively or in a high volume, which means having to build them one at a time isn’t feasible. However, producing them in large quantities is difficult because the equipment needed to do so is complex and expensive.
Enter the engineer’s favorite material, graphene, and a team of researchers from MIT. Using a method the team developed for controlling the natural fracturing process of brittle, atomically-thin materials, they have demonstrated a way to mass-produce tiny robots the size of cells. Their technique directs the facture lines in a brittle material to create tiny pockets with a uniform, predictable size and shape.
By embedding electronic circuits and other materials inside these pockets, the researchers created what they call “syncells” (short for synthetic cells), microscopic devices that can collect, record and output data. These robots could eventually be used to monitor conditions inside an oil or gas pipeline, such as pressure or temperature, or to test for disease or medication levels while floating through a human’s bloodstream.
Controlling Fracturing to Produce Tiny Robots
The team’s process is called “autoperforation.”
The first step of the process involves layers of two-dimensional carbon—graphene—used to form the syncells’ outer structure. One layer of graphene is laid down on a surface, followed by the deposition of tiny dots of a polymer material that contains the electronics for the devices, using a sophisticated laboratory version of an inkjet printer. A second layer of graphene is laid on top, forming a graphene shell around the electronic components.
When hearing about graphene, most people are aware of the material’s most well-known properties: being ultrathin, but extremely strong. But even knowing graphene is a strong material, knowing it’s thin often leads to thinking that graphene is also “floppy.”
However, graphene is actually a brittle material, explained Michael Strano, professor of chemical engineering at MIT. But the team determined that instead of considering that brittleness to be a problem, the team figured out how they could use it to their advantage.
“We discovered that you can use the brittleness,” said Strano. “It's counterintuitive. Before this work, if you told me you could fracture a material to control its shape at the nanoscale, I would have been incredulous.”
But that’s exactly what the MIT team’s autoperforation process does: control the fracturing process to produce pieces of a brittle material that have a uniform size and shape, rather than random shards and slivers like the way window glass shatters.
“What we discovered is that you can impose a strain field to cause the fracture to be guided, and you can use that for controlled fabrication,” Strano explained.
Breaking Things Down
“Imagine a tablecloth falling slowly down onto the surface of a circular table,” said Albert Liu, a graduate student on the research team. “One can easily visualize the developing circular strain toward the table edges, and which is analogous to what happens when a flat sheet of graphene folds around these printed polymer pillars.”
What Liu is describing illustrates the key aspect of the autoperforation process. The top layer of graphene that is laid over the array of polymer dots forms round pillar shapes. The edges of each polymer dot, where the graphene layer drapes down, forms lines with high strain.
This leads to the fractures being concentrated along those boundaries, Strano said. “And then something pretty amazing happens: The graphene will completely fracture, but the fracture will be guided around the periphery of the pillar.”
The result? A tiny, neat, circular bit of graphene that looks as clean-cut as being created by a microscopic hole punch.
Like a micro-sized pita bread, the top and bottom layers of graphene adhere at the edges of the disk around the polymer pillars, sealing the polymer inside. “The advantage here is that this is essentially a single step,” Strano explained. In comparison, traditional processes require multiple complex clean-room steps to produce microscopic robotic devices.
This process isn’t limited to graphene, either; the team has tested additional materials, such as molybdenum disulfide and hexagonal boronitride work equally well.
What Do You Do with Robots the Size of Human Cells?
These tiny bots measure between 10 micrometers across, up to around 10 times that size—similar to the size of a human red blood cell. “They start to look and behave like a living biological cell. In fact, under a microscope, you could probably convince most people that it is a cell,” Strano said.
The current work on autofabrication follows from previous research by Strano and his students, which focused on developing syncells capable of using surface sensors to gather information about the chemistry or other properties of their environment and storing that information for later retrieval. As an example, a swarm of these particles could be injected into one end of a pipeline, and retrieved at the other end, where the sensor data could be used to understand conditions inside the line. However, these sensor syncells were assembled individually, limiting their prospective use; in contrast, while the new version of syncells don’t have as many capabilities at the moment, the team demonstrated that they would be easy to mass produce.
And, while it’s great to imagine all the potential uses for these syncells, from industrial monitoring to healthcare, it’s the innovative way the devices themselves can be produced that is the highlight of this research.
“This general procedure of using controlled fracture as a production method can be extended across many length scales,” Liu said. “[It could potentially be used with] essentially any 2-D materials of choice, in principle allowing future researchers to tailor these atomically thin surfaces into any desired shape or form for applications in other disciplines.”
He added that it’s “one of the only ways available right now to produce stand-alone integrated microelectronics on a large scale” which are capable of functioning as independent, free-floating devices. By changing the type of electronics inside, devices could be given different capabilities, such as for movement, detection of various chemicals or memory storage, to name a few.
Cell-sized robotic devices such as the syncells could be suitable for a host of applications.
For one demonstration, the team “wrote” the letters M, I, and T into a memory array inside a syncell, which stored the information as varying levels of electrical conductivity. An electrical probe could then “read” this information, which showed how the material can function as electronic memory that allows information to be written, read and erased. The syncell also doesn’t need power to retain the data, which allowed stored information to be collected at a later time. According to Strano, another experiment demonstrated the particles to be stable over a period of months, even when they were floating around in water, which is a harsh solvent for electronics.
“I think it opens up a whole new toolkit for micro- and nanofabrication,” Strano added.
How could microscopic robots be used? Check out our story on How Nanotechnology Will Help Us Explore Other Planets.