GAINESVILLE, Fla. --- To keep blood vessels from clogging, surgeons sometimes implant compressed stents that expand to the right size and shape when warmed by body heat.
The tiny stents get their so-called "shape memory" from an unusual alloy called nitinol, which exhibits one shape when cool, but forms another when heated. This property has made the alloy useful in an increasing number of small-scale medical and consumer products that depend on motion, yet do not have enough space for motors or pumps.
Intrigued by the alloy's biomedical potential, University of Florida researchers have recently begun investigating it for the much larger application of 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 merely a weight-lifting machine now, the hope is the research will one day lead to a nitinol "muscle" that can mimic the strength and motion of the real thing – doing the work 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."
Nitinol was discovered by U.S. Navy researchers in the early 1960s. The name combines the abbreviations for nickel and titanium with the acronym for the Naval Ordnance Laboratory, where the first research on the alloy was done.
For years, nitinol was used only in niches. 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 flips 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 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.
Prostheses, however, are not the only potential application for such large-scale nitinol devices. Nitinol already is used in outer space, for example in cylinders placed around bolts that elongate under temperature change, breaking the bolt to open a box and release a structure such as an antenna on a satellite. Because space is at such a premium in orbital vehicles, any other nitinol devices that replace motors or pumps could be useful, Santiago-Anadon said. The unobstructed heat from the sun in space, meanwhile, serves as an excellent energy source to cause the shape memory effect.
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