Researchers from Cornell have developed tiny graphene-enhanced robot exoskeletons that can rapidly change shape upon sensing chemical or thermal changes in its environment. And, they claim, these microscale machines – equipped with electronic, photonic and chemical payloads – could become a powerful platform for robotics at the size scale of biological microorganisms.
“We are trying to build what you might call an ‘exoskeleton’ for electronics,” said the team. “Right now, you can make little computer chips that do a lot of information-processing … but they don’t know how to move or cause something to bend”.
The machines move using a motor called a bimorph. A bimorph is an assembly of two materials – in this case, graphene and glass – that bends when driven by a stimulus like heat, a chemical reaction or an applied voltage. The shape change happens because, in the case of heat, two materials with different thermal responses expand by different amounts over the same temperature change.
As a redult, the bimorph bends to relieve some of this strain, allowing one layer to stretch out longer than the other. By adding rigid flat panels that cannot be bent by bimorphs, the researchers localize bending to take place only in specific places, creating folds. With this concept, they are able to make a variety of folding structures ranging from tetrahedra (triangular pyramids) to cubes.
In the case of graphene and glass, the bimorphs also respond to chemical stimuli by driving large ions into the glass, causing it to expand. Typically this chemical activity only occurs on very outer edge of glass when submerged in water or some other ionic fluid. Since their bimorph is only a few nanometers thick, the glass is basically all outer edge and very reactive. “It’s a neat trick,” the researchers said, “because it’s something you can do only with these nanoscale systems.”
The bimorph is built using atomic layer deposition – chemically “painting” atomically thin layers of silicon dioxide onto aluminum over a cover slip – then wet-transferring a single atomic layer of graphene on top of the stack. The result is the thinnest bimorph ever made.
One of their machines was described as being “three times larger than a red blood cell and three times smaller than a large neuron” when folded. Folding scaffolds of this size have been built before, but this group’s version has one clear advantage. “Our devices are compatible with semiconductor manufacturing,” they said. “That’s what’s making this compatible with our future vision for robotics at this scale."
Graphene’s relative strength helps it handle the types of loads necessary for electronics applications. “If you want to build this electronics exoskeleton,” the team said, “you need it to be able to produce enough force to carry the electronics. Ours does that.”
For now, these tiny machines have no commercial application in electronics, biological sensing or anything else. But the research pushes the science of nanoscale robots forward, the team explained. “Right now, there are no ‘muscles’ for small-scale machines...so we’re building the small-scale muscles.”