These artificial muscles could find use in prosthetic limbs, exoskeletons that enhance human strength, airplane wings that mimic those of birds, or humanoid robots that march in the front lines of battles, said researcher Ray Baughman, director of the University of Texas at Dallas NanoTech Institute.
Future artificial muscles could even incorporate enzymes to power them "using just the sugars in your body as fuel, perhaps eventually for an artificial heart. But that's very futuristic, not anywhere in the near term," Baughman said.
Scientists have experimented on artificial muscles for decades, which often work by converting electricity into mechanical energy. The problem is that batteries do not store very high amounts of energy for the space they occupy, nor do they deliver energy very quickly, Baughman said.
The artificial muscles Baughman developed, along with Nobel laureate Alan MacDiarmid and their colleagues in Texas and Korea, use fuel instead of batteries. "Methanol, for instance, has 30 times the energy storage density of a battery," Baughman explained.
Currently the most powerful type of muscle the researchers created has the nickel-titanium "shape memory" alloy muscle coated in platinum particles only 3 to 6 nanometers or billionths of a meter wide. The nanoparticles behave as catalysts that react fuels such as hydrogen, methanol or formic acid with the oxygen in the air.
The resulting heat contracts the alloy, while cooling expands it. This artificial muscle generates more than 100 times more force of natural muscle of the same diameter, "and we haven't optimized it at all. It's clear we can dramatically improve it," Baughman said. "But what we have now is deployable. I believe the shape memory alloy muscle can be implemented in less than three years."
The other artificial muscle the researchers developed uses strips of carbon nanotubes laminated with a mixture of platinum-coated carbon and proton conductive plastic. The result is a muscle capable of converting the chemical energy in hydrogen fuel into electricity, which it can then use for mechanical energy, store or dispense for other purposes.
"That means in a robot, while some muscles do mechanical work, you could use other muscles to generate electricity or to store excess electrical energy," Baughman said. Currently this nanotube-based muscle suffers irreversible deformation, which the researchers hope can be overcome in longer nanotubes.
"One of the key areas for muscle research is to develop humanoid robots -- look at Asimo from Honda -- that have the same dexterity, human-like features etc of real humans. To achieve this aim we need to develop actuators that are very small and lightweight. Motors are too difficult to make really small. Pneumatics and hydraulic systems need pumps and fluid reservoirs that take up too much space," said materials scientist Geoff Spinks at the University of Wollongong in Australia. "Enter Baughman and his team -- now they show that we don't need batteries!"
Future applications might also include space exploration, "where multi-functional planetary landers are being built smaller and smaller and with more and more functionality," Spinks said. To be practical, however, all these muscles need to be faster than so far shown, but "Baughman and his colleagues hint that they can make their fuel cell muscles go faster," Spinks added.
Patent applications for the artificial muscles are pending. Baughman and his colleagues report their findings in the March 17 issue of the journal Science.