Researchers probe ‘shape memory’ alloy for better prostheses

Researchers at the University of Florida are looking at whether the shape memory alloy nitinol can be used to aid movement in prosthetic limbs.

University of Florida researchers are looking at whether nitinol, an alloy that exhibits one shape when cool, but forms another when heated, can be used in prosthetic limbs.

Under the direction of mechanical engineering Professor Carl Crane, UF master’s student Jose Santiago-Anadon built a nitinol device that can move the equivalent of more than 100 pounds. While the apparatus is a weight-lifting machine now, the hope is that the research will one day lead to a nitinol ‘muscle’, which can mimic the strength and motion of a tendon or other major muscle in a next-generation prosthesis.

‘Basically, it’s almost the size of a tendon or other large muscle,’ Crane said. ‘It requires a lot of electricity, but it does not require the kind of bulky motors or hydraulic pumps that drive similar devices.’

For many years nitinol, an alloy of nickel and titanium, was used only in niche products. But the development of medical applications has spurred the creation of other nitinol commercial products, including showerheads that automatically shut off the flow of water before it becomes hot enough to scald a person.

The shape memory effect occurs in response to heat, which can be generated through electricity or any other energy source. Inducing the effect requires considerable energy, one reason the development has focused on small products with comparatively little nitinol, say Santiago-Anadon and Crane. Where energy is not a factor but space or weight is, however, large nitinol devices could be quite useful, they say.

To demonstrate that, Santiago-Anadon built a machine that uses 104 nitinol wires to displace the equivalent of 135 pounds – a weight measured by the wires’ action against a metal block attached to mechanical and hydraulic springs.

When Santiago-Anadon turns on the power switch, the thread-like wires constrict due to the heat generated by the electricity, lifting the block more than an inch. When he turns the power off, the hydraulic springs slowly return the wires to their original size.

When lifting the maximum weight, the machine requires some 1,200 watts, enough to power four computers. However, the wires are much lighter and potentially more compact than a hydraulic system or electric motor needed to do the same amount of work.

That, according to the researchers, makes it a good candidate for prosthetic limbs, which would be hampered by such conventional machines. Santiago-Anadon predicted that researchers could develop a prototype arm prosthesis using shape memory alloys in about five years.