The shape of things to come

Natasha Loder on how some materials are developing memories of their own

IF you dented your car wouldn't it be great if it simply popped back into perfect shape when you warmed it up a bit?

Soon this might not be such a far-fetched idea. Materials which do exactly that, called shape memory materials (SMMs), are increasingly appearing in cutting edge applications in space, aviation, medicine, robotics and even fashion.

The use of SMMs for biomedical applications is exciting many scientists. One memory material, the alloy Nitinol, has just been approved for use inside the human body by the US's Food and Drug Administration.

"This is a very hot area of technology," explained Dr Tony Anson of Anson Medical, which is developing a series of biomedical products based on Nitinol. Links made of this alloy are already being used for mending broken bones - they are inserted into holes drilled into the bone, where they tighten up and force the bones together on reaching body temperature.

Nitinol is also being developed to make stents, a device used to hold channels - such as arteries - open in the body. Fine wires are woven into a cylindrical shape, which is crushed and put into the body where there is a blockage. When the alloy reaches body temperature it reforms its original shape, and the hollow mesh structure reinforces the walls of the blood vessel. This technique has the potential for treating many types of blockage in the body.

A more gruesome use of SMMs is for creating replacement tooth roots. A rosebud-shaped structure is made out of tiny sheets of SMM and put into the gap in the jaw. On warming, the sharpened edges of the petals splay outwards, embed in the jaw bone, and form a false root. Shape only changes in one direction - so any drop in temperature, say by eating an ice cream, will not make the root drop out.

Two-way SMMs, which are able to change between two set shapes, have been used to make self-regulating window props in greenhouses, and litter bins which open and close at different temperatures. These bins are used in Australia to prevent forest fires, when a lit cigarette is thrown into the bin the heat generated by any fire causes its lid to close. By excluding air the fire is extinguished, the bin cools, and the lid opens again for use.