If graphene had been around when Philip K Dick was writing he would have surely used it as the muscle for his fictive robotic inventions. No other material can match it for its flexibility and responsiveness to electricity.
Researchers in the robotics field are always hoping to discover the perfect material for simulating muscle; ceramics and conductive polymers are just too heavy and unresponsive to finesse the simulation of movement, and produce too little force for the power input. And so, in answer to this problem, a team of Chinese scientists have turned to graphene to develop graphene-based actuators that convert electricity into motion (ACS Nano, DOI: 10.1021/nn3006812).
The new actuators use the graphene as a backing and electrical bridge for the polymer polydiacetylene. Polydiacetylene had been previously shown to deform in response to electrical current, but suffered from the problem of being too brittle for use. The graphene backing enables the polydiacetylene crystals to perform the function of electrical pliability whilst maintaining their integrity.
The graphene paper is made by filtering graphene flakes and fusing them together to make a sheet. The paper is then spin-coated with a solution of the monomers needed to make polydiacetylene, and baked with ultraviolet light to create the polymer. The resulting crystalline layer of polydiacetylene is firmly attached to the graphene layer and is able to flex without breaking.
Wiring the actuator sheet to an electrical source produces movement according to the nature of the input. An alternating current produces an oscillation that matches the current’s frequency, a degree of responsiveness unlikely to occur in other materials and still not fully understood by the research team.
When the scientists apply low levels of direct current, the material generates as much stress as natural muscle can. It can produce 160 megapascals of stress per gram of material, making it stronger than most polymers.
“Muscle is very soft, and responds to a request on demand,” he says. For example, he says, muscles in the fingers contract to avoid dropping a glass before a person is even aware that it is slipping.
The graphene actuator could eventually be used in prosthetics, but for the time being the team is refining their research with work on a bee-sized helicopter-style robot that would use the actuators to spin the blades.