Metallic alloys and polymers that can change their shapes at will... that's the future

Published: Sunday 15 March 1998

The shape of things to come?<s (Credit: Science Photo Library)they are smart, can change shapes automatically and are all set to play a crucial role in numerous aspects of day-to-day life in the coming millennium. Meet the Shape Memory Alloys (smas). These are metals and plastic derivatives that are currently emerging from infancy in British research facilities. Researchers are not only developing new metallic alloys and plastic materials that are "smarter" than ever, but are also identifying a rapidly growing range of uses for them that promises to bring major practical benefits, particularly in the industrial, domestic and medical environments (Spectrum , No 261).

First discovered almost three decades ago, smas can have a predetermined shape "implanted" in their structure. These smas can then be deformed completely into a different shape to serve a particular purpose.They will only revert to their original "memorised" shape when triggered by a speci-fic stimulus, such as temperature or a chemical.

Considerable worldwide effort is currently focusing on expanding the smas' abilities and their potential applications. Anson Medical, a pioneering firm based in Oxford, uk, is building on technology to create sma actuators for use in controlling life-science experiments in unmanned space stations, an environment that demands extreme levels of reliability. This company also was the first to produce a sma device to help repair fractured bones in the human body. The device applies its memory effect (or, changes back to its implanted shape) to drasti-cally reduce the healing time required broken bones to mend, using a continuous and controlled force to bring the broken parts together smoothly and accurately.

Further industrial and medical applications for these smas are being sought by researchers using a variety of metallic and polymer-based materials, in which a set geometric form can be established that will remain as latent memory. Among the metallic alloys that are proving to be the "smartest" are copper/zinc/aluminium, copper/iron/ zinc and nickel/titanium, the last of which is particularly useful since it can be triggered at different temperatures to as many as four different memorised shapes. This alloy is a tenacious metal, difficult to a machine to high degrees of accuracy required and is resistant to being welded or fused to other components. However, its highly regular crystalline geometry, appearing much like a snowflake under high magnification, can easily be prompted to change. This is due to its lattice-like crystalline structure comprising cubes of atoms that take on a more rhomboidal shape when triggered by a predetermined temperature, causing the alloy to change its shape.

This internal geometry can be instilled easily. The alloy component is held in a jig in the shape that the sma is to "remember" and is heated to between 400 and 600 o c until it is thermally soaked. An immediate quench in cold fluid locks in the memory, after which these smas is deformed to the new shape it requires foe installation.

Anson Medical is currently developing new and faster methods of processing smas. These involve very fine nickel and titanium particles that are pressed into a mould in the shape to be remembered and then heat-treated to install the memory. This expedites the production process by removing reliance on slow machining or fabrication of the alloy component to the correct shape.

The smas find innumerable medical and industrial applications. Simple, reliable and long-lasting sma devices are ideal for use as linear and rotary actuators in a wide range of equipment.

In home and at the workplace, smas are arriving in thermostats, computer equipment, mobile telephone antenna supports, hot water anti-scalding and fire-damping doors and louvers.

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