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Memory Metals (Public Radio Commentary)

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(For radio stations: Bill's public radio work can be licensed via PRX).

I have in my mouth the most marvelous thing: It isn't edible at all, in fact it's kind of boring looking. It's the archwire that threads through my braces. This slender wire, which supplies the force that moves my teeth about one millimeter a month, is truly a high tech marvel.

It withstands long-term assault in the body's harshest environment. The human mouth contains acids of all types - from food, from the digestive system. Biting and chewing is like subjecting the wire to a jackhammer. And toothpaste and brushing is highly abrasive to the wire. Yet, all that isn't what makes it special.

The wire in my mouth is made from a novel metal that has a memory. It can be bent, then when heated will return to its original shape. It began life in 1958 when William Buehler, a metallurgist at the U.S. Naval Ordinance Laboratory, searched for a metal for missile nose cones.

He wanted one that could withstand the forces and heat of reentry. While testing various alloys, a mixture of metals, he noticed that an alloy made from the elements nickel and titanium behaved differently from others. He demonstrated its new property at a lab meeting in 1961.

His lab assistant pulled out the a strip of the alloy folded like an accordion. Then a lab scientist, who was also a pipe smoker, used his lighter to heat the strip. To everyone's amazement it stretched out to its original form.

Buehler named this memory material Nitinol - N-I-T-I-N-O-L. N-I for nickel, T-I for Titanium, and N-O-L for his work place the Naval Ordinance Laboratory.

It works because the atoms in the metal rearrange into a new phase. Usually changing phase is something dramatic: An ice cube melts changing phase from a solid to a liquid, or we put water to boil on the stove and it changes phase from a liquid to a vapor. Less well known are that such changes occur within in a solid: Solid-to-solid phase changes involve rearrangement of the position of the atoms. Nitinol remembers its shape because above a certain temperature it returns to a rigid arrangement of the atoms.

There are thousands of uses of a shape memory alloy like Nitinol. For example, in a greenhouse, when the temperature inside gets too high, hinges made from Nitinol remember their original shape, and snap open.

In the case of my teeth the Nitinol wire is pliable at room temperature. This gives the orthodontist the flexibility to maneuver the wire into place. Then at body temperature, the wire snaps back to its original shape applying a gentle force to my teeth.

Perhaps the most interesting use is a shirt woven from nylon fibers with Nitinol wires interspersed. When balled up for packing, it can be unwrinkled with a blast from a hair drier. And when the wearer's body temperature rises, the shirt's sleeves automatically get shorter. It only costs, of course, a modest four thousand dollars.

Copyright 2003 William S. Hammack Enterprises