The idea of microscale robots such as single-atom transistors is not brand new, but previous designs were not able to move or change in response to environmental stimulus, which limited their applications. Researchers from Cornell University have combined nanorobotics with single-atom thick graphene and the Japanese art of origami to building something very new: imagine a computer more powerful than the spacecraft Voyager that fits within a single cell because it’s capable of folding itself into different shapes.
The Cornell team devised a type of robot exoskeleton that can quickly change its shape as it senses chemical or thermal changes in the environment. When the robots carry electronic, photonic, and chemical “payloads,” these human cell-sized machines can be adapted for work, particularly in biomedical applications. They represent an opportunity to create a “Fantastic Voyage” type device that contains a collection of microelectronics elements -- voltage sensors, pH sensors, memory chips and communication devices -- using standard silicon-based electronics and folded into a cell-sized package that can be introduced into a living body to do work.
|A range of small shapes made by graphene origami. The smallest shape, the tetrahedron, is only three times the size of a red blood cell, while the largest cube is half a hair's width. Photo credit: Marc Miskin|
To develop the tiny robot, researchers drew on origami, according to Marc Miskin, a Kavli Institute postdoctoral fellow in Nanoscale Science at Cornell who worked with co-researchers Kyle J. Dorsey, Baris Bircan, Yimo Han, David A. Muller, Paul L. McEuen, and Itai Cohen.
“One of the fascinating things that drew us to origami-inspired robotics is that it’s scale invariant: if you design a robot at one size scale, and then shrink down every dimension -- the length, the width and the thickness -- the miniaturized version will work in the same way,” Miskin told Design News. “To nanotechnologists, this is an extremely powerful idea. Scalability is one of the central design principles that drove the microelectronics revolution, although it has been difficult to achieve the same thing with machines. We deliberately picked origami inspired fabrication as a platform because it allows large-scale concepts to easily be translated to the nanoscale.”
Working with materials only one atom thick requires entirely new tools and “recipes” to create folding, moving parts in the right places. For the purpose of their research, the Cornell team created a “biomorph” from graphene and glass. Because of the different properties of the two materials, parts of the biomorph can bend in different ways when exposed to stimuli such as temperature or electricity. Controlling the stimuli means the biomorph can be manipulated to fold in a variety of ways into complex geometric shapes. The inherent strength of graphene – a material comprised of carbon atoms one atom thick – means the tiny machines can be made to carry “payloads” of significant weight. Glass was chosen to create an actuator that would provide as much force as possible when bending, according to Miskin.
“We wanted a material that is as stiff as possible to provide a large force, but also flexible enough to bend to micron radii of curvature without breaking,” he told Design News. “It turns out that if you plot the strength and stiffness of every material known and then ask which ones are optimal for actuators with graphene, glass gets singled out as the optimal choice. It’s somewhat surprising since we tend to think of glass as brittle and breakable, but because our devices are so thin, they can easily bend to cell-sized radii of curvature without sustaining any damage. If you couple that with the fact that glass is extremely stiff, you wind up with a fantastic motor for cell-sized robots.”
Because of the stiffness of the two materials, the result is optimal strength and extremely high force output. The team demonstrated that the devices were able to lift pads two micrometers thick, although they estimate, based on what is known about graphene, that the pads could be 100 times thicker. To put this in perspective, an appropriately designed graphene-glass actuator could lift a strand of human hair, though the machine lifting it would be only 21 atoms thick. The researchers also believe the biomorphs could work indefinitely for an unlimited number of folds and bends.
“We designed everything to be elastic, and so far there is no evidence of fracture, or breaking in the glass or graphene,” noted Miskin. “In principle, the lifetime of the machines is unlimited. We're working right now on testing how true that is, but currently the limiting factor is us; We tend to break the machines by accident in the lab before they fail from fatigue.”
The team used atomic layer deposition to create their biomorphs, a process that allowed them to grow films with extremely precise thicknesses. This kind of control is critical to building machines at the cellular scale: for a bimorph, the two thicknesses of the layers have to be precisely tuned or no bending takes place. Compellingly, the nanorobots were built using standard semiconductor tools, chemicals and processes, which means that an appropriately equipped cleanroom could produce them for a variety of applications.
“Currently, we're exploring how they can be used to sense pH in the environment around them, detect DNA, pump their surrounding fluids, harvest energy from their environments, and integrate with electronics,” Miskin told Design News. “It’s already a broad scope, but I think it’s probably just the tip of the iceberg. Biology hints at truly incredible possibilities for the distant future: every human on Earth is living proof of the power and potential of cell-sized machines.”
The team’s research has been presented in an article, “Graphene-based Bimorphs for Micron-sized, Autonomous Origami Machines,” and published in the journal Proceedings of the National Academy of Sciences.