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Artificial hand features 'muscles' made from smart metal wires

25 March 2015

Engineers at Saarland University have taken a leaf out of nature's book by equipping an artificial hand with muscles made from shape-memory wire.

Filomena Simone, an engineer in the research team led by Stefan Seelecke, is working on the prototype of the artificial hand (photo: Oliver Dietze)

The new technology enables the fabrication of flexible and lightweight robot hands for industrial applications and novel prosthetic devices. The muscle fibres are composed of bundles of ultra-fine nickel-titanium alloy wires that are able to tense and flex. The material itself has sensory properties allowing the artificial hand to perform extremely precise movements.

The research group, led by Professor Stefan Seelecke will be showcasing their prototype artificial hand and how it makes use of shape-memory 'metal muscles' at the Hannover Fair from April 13 to April 17. The team, who will be exhibiting at the Saarland Research and Innovation Stand in Hall 2, Stand B 46, are looking for development partners.

"Shape-memory alloy [SMA] wires offer significant advantages over other techniques," says Seelecke. "Tools fabricated with artificial muscles from SMA wire can do without additional equipment, making them light, flexible and highly adaptable. They operate silently and are relatively cheap to produce. And these wires have the highest energy density of all known drive mechanisms, which enables them to perform powerful movements in restricted spaces."

Multiple strands of shape-memory wire connect the finger joints and act as flexor muscles on the front-side of the finger and as extensor muscles on the rear. In order to facilitate rapid movements, the engineers copied the structure of natural human muscles by grouping the very fine wires into bundles to mimic muscle fibres. These bundles of wires are as fine as a thread of cotton, but have the tensile strength of a thick wire.

"The bundle can rapidly contract and relax while exerting a high tensile force," says Filomena Simone, an engineer who is working on the prototype of the artificial hand as part of her doctoral research.

Professor Stefan Seelecke (photo: Oliver Dietze)

"The reason for this behaviour is the rapid cooling that is possible because lots of individual wires present a greater surface area through which heat can be dissipated," she says. "Unlike a single thick wire, a bundle of very fine wires can undergo rapid contractions and extensions equivalent to those observed in human muscles. As a result, we are able to achieve fast and smooth finger movements."

Another effect of using the shape-memory metal wires is that the hand can respond in a natural manner when someone intervenes while a particular movement is being carried out. This means that humans can literally work hand-in-hand with the prototype device.

A semiconductor chip controls the relative motions of the SMA wires allowing precise movements to be carried out. And the system does not need sensors.

"The material from which wires are made has sensor properties," says Seelecke. "The controller unit is able to interpret electric resistance measurement data so that it knows the exact position of the wires at any one time."

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