Shape-Memory Alloy Could Enhance Prostheses Performance

Originally Published MDDI September 2002

R&D DIGEST

Nitinol, an unusual alloy that lends shape-memory characteristics to vascular stents and other small-scale products, may someday prove useful in medical devices of much larger size.

Following studies of biomedical applications of nitinol, researchers at the University of Florida (Gainesville) recently began to explore the potential use of the alloy in the construction of prosthetic limbs. So far, the researchers have succeeded in building a nitinol device capable of moving the equivalent of more than 100 lb. The device was fabricated by graduate student Jose Santiago-Anadon, under the direction of University of Florida mechanical engineering professor Carl Crane, PhD.

According to the researchers, the apparatus currently has little function beyond that of a weight-lifting machine. They believe, however, that their work may eventually lead to a nitinol artificial muscle that can duplicate the strength and motion of the real thing. They speculate that such an implanted actuator could perform the work of a tendon or major muscle in a next-generation prosthesis.

Says Crane, "Basically, [the apparatus is] almost the size of a tendon or large muscle. It requires a lot of electricity, but it does not require the kind of bulky motors or hydraulic pumps that drive similar devices." Because nitinol responds to heat, inducing the desired effect requires considerable energy levels. The researchers explain that these energy requirements are one reason development efforts have focused on smaller products that contain comparatively little nitinol. They add, however, that where space or weight are more critical factors than energy, large nitinol devices could be quite useful.

The machine developed by the researchers uses 104 threadlike nitinol wires to displace the equivalent of 135 lb, as measured by the wires' action against a metal block attached to mechanical and hydraulic springs. When power is applied, the wires constrict in response to the generated heat. The device can lift the metal block more than 1 in. Once the power is turned off, the hydraulic springs slowly return the wires to their original size.

Although maximum performance of the machine requires some 1200 W, the wires are quite light and take up less space than a hydraulic system or electric motor that would be required to perform a similar function. Santiago-Anadon believes that the device is a good candidate for use in prosthetic limbs "because of the properties of the actuator itself—light, noiseless, and small." He says, "The major issue to overcome is the power consumption (finding a portable power source big enough). Another big obstacle is the control aspect of the device. Controlling shape-memory devices is not as easy as it looks since they are very susceptible to ambient temperature and their performance will vary."

The researcher explains that, "for the application of prosthetic limbs, this aspect gets more complicated since several joints must be controlled at the same time." Nevertheless, Santiago-Anadon believes that development of a prototype prosthetic arm based on the shape-memory alloy could be possible within five years.

Use in prosthetic limbs is not the only potential application for large-scale nitinol devices. For example, nitinol already is used in satellite components, such as cylinders placed around bolts that elongate under temperature change. The change in shape breaks the bolts, opening a box and releasing a structure such as an antenna. Other nitinol devices that could replace much-larger motors or pumps also could be useful in space applications, says Santiago-Anadon. He adds that unobstructed heat from the sun serves as an excellent energy source to cause the shape-memory effect.

Copyright ©2002 Medical Device & Diagnostic Industry