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Nanoscale graphene actuators power micro-machines

By Douglas Main February 13, 2018
Nanoscale graphene
3D structures with graphene-glass biomorphs and polymer panels. These shapes include (a) tetrahedrons, (b,c) helices of controllable pitch, (d) high-angle folds and clasps, (e) basic origami motifs with bidirectional folding, and (f) boxes. The left column shows the flattened devices before being released, the middle column shows their self-assembled shapes, and the right column shows paper models of each shape.

Origami-inspired methods are perfect for making micro- and nanoscale objects, because they allow researchers to turn extremely thin materials into three-dimensional (3D) shapes. A team of researchers has now created machines that can autonomously move and fold up into a variety of shapes, powered by nanoscale actuators, when subjected to changes in heat and pH.

As described in a study published in a recent issue of Proceedings of the National Academy of Sciences, researchers with the Kavli Institute for Nanoscale Science at Cornell University have created micro-machines with panels only 2 µm thick. They are held together and propelled by actuators made of a bimorph, a two-layered material, consisting of graphene and a 2 nm layer of silicon dioxide glass.

Led by first author Marc Miskin and senior co-authors Paul McEuen and Itai Cohen, the researchers created machines that can fold into shapes such as cubes, tetrahedrons, and helices. One of these tetrahedral machines fits within a 12 µm sphere, which makes it three times smaller than a large neuron, and three times larger than a red blood cell. (The tetrahedron has panels that are shaped like isosceles triangles, with sides that are 15 µm long.)

There is no limit to the shape and variety of devices one could create, says McEuen, a professor of physics at Cornell and the director of the Kavli Institute. In the future, the panels—currently made of “a boring material,” an epoxy called SU8 used for proof-of-principle—“can be anything you want,” McEuen says. The researchers are currently working to incorporate electronics into these tiny machines, he says. This technology could have many uses, for example to one day create tiny biomedical robots. The technique could also be used to create surfaces that change color, reflectivity, and texture, with a wide range of potential applications from chemical sensors to displays.

The research team showed that the machines could be induced to rapidly fold up into their desired shape when heated to 100°C using a 1064-nm laser. When glass heats up it expands but the graphene does not, and this force causes the bimorph actuator to bend in one direction and lift the panels, McEuen explains.

Similarly, pH and electrolytic tinkering can cause conformation changes. For example, a flat sheet will fold up when the balance between hydronium and sodium ions passes a tipping point. Both ions enter the glass under different conditions. But hydronium ions are larger than sodium ions, and the former causes the actuators to swell and bend, moving the machine. By adjusting the relative levels of sodium and pH, they were able to make the shapes reversibly fold or straighten up in acidic and basic conditions.

To fabricate the micro-machines, the researchers first used atomic layer deposition to create a 2 nm-thick sheet of glass on a layer of aluminum. Atop the glass, they used a wet transfer technique to put down the graphene, which was grown on copper foils. Then they spun a thin layer of the epoxy, SU8. The researchers created the desired patterns and shapes using photolithography techniques and finally dissolved the aluminum backing in a bath of dilute hydrochloric acid, setting the little machines free.

The achievement is significant because it is “very hard to take an atomic layer [such as graphene] and bend and curve it,” says David Gracias, a professor at Johns Hopkins University who also works on assembling tiny machines. His group has also made devices using bimorphs, but using a polymer instead of glass. The graphene in their machines was also more than twice as thick.

The study represents a step toward making viable micro-machines, but several challenges need to be overcome, the authors write. For one, the fabrication process will have to improve. The researchers’ current process produced a viable machine about 10–20% of the time. Engineers will also have “to develop specific processing techniques that seamlessly link nanoscale origami to photonic, chemical, and electronic technologies,” they write. Scientists will also have to make actuators that can fold in both directions, come up with mechanisms to produce sequential folds (allowing for a higher degree of movement control), and invent scalable approaches that can be used to manufacture the machines in clean rooms, they add.

Read the abstract in Proceedings of the National Academy of Sciences.