Nitinol, an alloy of nickel and titanium developed at the Naval Ordnance Laboratory in the 1950’s, has been widely used in medical devices because of its extreme shape memory property. The majority of coronary stents, for example, are nickel-titanium because the material allows them to pop open once delivered through a very narrow catheter. Researchers at University Saarland in Germany have harnessed this feature of nitinol to power a prosthetic hand in an entirely new way.

The prosthesis has bundles of Nitinol wires, each about the width of human hair, running through its interior. There are no motors, pulleys, or many other mechanical components found in existing prostheses. The wires have a predefined shape that they like to be in, but when electricity is run through them, the wires loosen and the arm moves. Stopping the flow of current returns the metal wires to their original position. This technology allows the power system to be small and fit into tight spaces, letting designers create new prosthetic designs that are not limited by traditional mechanics.

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

source:University Saarland