“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.