Device Mimics Human Muscle Size, Strength

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A new artificial muscle powered by hydrogen is equivalent in size and power to human muscle.
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Hydrogen could soon power a new economy, but in the meantime the lightest element is already powering new artificial muscles.

According to new research published in the journal Smart Materials and Structures, scientists have created a strong and silent hydrogen-powered artificial muscle, modeled on human skeletal muscle.

"People are trying to use metal hydrides as hydrogen storage devices," said Kwamg Kim, a professor at the University of Nevada, Reno who, along with Alexandra Vanderhoff, authored the journal article. "What we did was use metal hydrides to create a new artificial muscle."

Metal hydrides are extremely porous metals that lock onto and hold hydrogen gas at lower pressures and temperatures than is possible with cold and highly compressed pure hydrogen gas. The UNR scientists used a metal hydride made from nickel, copper, calcium and manganese to hold the hydrogen.

Once the metal hydride is full of the gas, it has to be released -- providing the muscle's power source.

"Heat it up and hydrogen comes out, cool it down and the hydrogen goes back into the metal hydride," explained Kim.

Kim uses a thermoelectric device, which takes electrical energy and converts it into heat, to control the release of the gas. As the device heats up and more hydrogen is released, the gas expands, pushing outwards and moving whatever object is attached to the muscle.

During recent lab tests the two-ounce artificial muscle, about six inches long and less than half an inch thick, lifted over 60 pounds in less than two minutes. The artificial muscle also contracted between 15 and 30 percent of its total length, which is about the same as most human skeletal muscle, which contracts between 15 and 20 percent of its length.

Kim is working to both speed up and slow down the muscle. Shrinking the thickness of the metal hydride from about one centimeter down to a couple millimeters, something Kim is working on, would speed up muscle movement. Making the metal hydride thicker would slow the muscle's movement. 

"It's not an electronics problem," said Kim. "It's a how-fast-can-I-put-heat-on-it-and-take-heat-away-from-it problem."

Kim's new actuator isn't the only artificial muscle out there, said Xiaobo Tan, a professor at Michigan State University who also studies artificial muscles.

"What is novel about this new research is the combination of the metal hydride material as a gas compressor using a thermoelectric material," said Tan.

Using a metal hydride as the gas compressor gives the muscle almost as much power as the pneumatic muscles (which use pressurized gas) used in heavy machinery for decades, but without the large and loud gas chambers and air compressors. The new artificial muscle operates smoothly and silently, and with no moving valves or pumps that can fail, like existing pneumatic pumps. 

The new muscle is also much smaller than traditional pneumatic muscles, small enough to replace a human arm one day. Kim is even experimenting with using multiple actuators in a single system, in an effort to more closely mimic how human biceps and triceps work together to raise and lower the arm.

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