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Fuel cells created by the team at the University of Texas at Dallas's NanoTech Institute could rival natural human muscles in energy efficiency and power.
High-energy-density fuels such as hydrogen, methanol, or formic acid can power two types of artificial muscles. Researchers from the University of Texas at Dallas (UTD) NanoTech Institute (Richardson, TX) have built fuel cells that could replace the standard batteries that tie otherwise-powerful robots to stationary power sources.
Among the varied applications for the fuel cells could be use in artificial limbs, powered by food-derived fuels. The researchers even suggest the devices could eventually be used for artificial hearts.
According to the research team, converting chemical energy from a fuel source to mechanical energy is a natural process—that's how real body muscles extract energy. Thus far, however, chemically powered artificial muscles have typically been too large and too weak for artificial limb applications.
By harnessing carbon nanotubes, the UTD team believes it has addressed the size limitations and made powerful muscles that can be used in various applications, including prostheses.
The first design uses a catalyst-containing carbon nanotube as a fuel-cell electrode. The nanotube also acts as a supercapacitor to store the electrical energy and as a muscle electrode to transform the electrical energy into mechanical energy. Fuel-powered charge injection in a carbon nanotube electrode produces the dimensional changes needed for actuation. It is triggered by a combination of quantum mechanical and electrostatic effects on the nanotube.
The second and more-powerful design harnesses chemical energy in the fuel as it is converted to heat by a catalytic reaction of a mixture of fuel and oxygen in the air. The resulting temperature increase in this shorted fuel-cell muscle causes contraction of a shape-memory metal muscle wire that supports this catalyst. Subsequent cooling completes the cycle by causing expansion of the muscle.
“The shorted fuel-cell muscles are especially easy to deploy in robotic devices, since they comprise commercially available shape-memory wires that are coated with a nanoparticle catalyst,” says Ray Baughman, a professor of chemistry, director of UTD's NanoTech Institute, and head researcher.
Baughman estimates that the fuel cells hold more than 30 times higher energy density than the most advanced batteries, without needing refueling. That means long operational lifetimes. According to a paper published in the journal Science, the power density is about 68 W/kg–1 (similar to natural skeletal muscle power, which is typically 50 W/kg–1).
The study also explains that refueling requires negligible time compared with recharging batteries. “Since the muscles are not used at the same time, temporarily inactive muscles of the first muscle type can be used as ordinary fuel cells and then as supercapacitors to provide for the electrical needs of prosthetic limbs.”
Using this principle, Baughman says, the fuel-cell muscles can be combined to obtain the best of both designs. And to create food-fueled prosthetics, the team says it could replace the metal catalyst with tethered enzymes.
With all these ideas flying around, Baughman cautions that there are still some kinks to work out. “The major challenges,” he says, “have been in attaching the catalyst to the shape-memory wire to provide long muscle lifetimes and in controlling muscle actuation rate and stroke. Students and scientists will be working on optimizing and deploying our artificial muscles.”
One of the team members, Von Howard Ebron, explains that there are numerous steps planned for the next year or so. “Right now we are trying other possible materials and systems for artificial muscles that involve the same mechanisms.”
The team is also continuing research to improve the properties of the muscles. For example, it is looking for more-efficient ways to coat the shape-memory muscle wires with platinum catalyst. “We are also looking into making simple devices that show how to control fuel delivery to the muscle to be able to control the movement,” Ebron says.
Ebron estimates that the fuel cells will be ready for finished devices within the next five years. “I believe that industrial partners will be needed to achieve this goal. Our role is to conduct research to determine the science behind these devices and make things easier for products to come along.”
The UTD team has several patents pending for the artificial muscles. The multinational team has even built a demonstrational arm-wrestling machine for humans to challenge the artificial muscles.
The Defense Advanced Research Projects Agency (DARPA), the Robert A. Welch Foundation, and the Strategic Partnership for Research in Nanotechnology funded the research.