Smart wire is a shape memory alloy (SMA) that changes its length with a useful pulling force when a small current is passed through it
SMA is a smart material which, as its name suggests, has a memory
The most common SMA is an alloy (mixture of metals) of nickel and titanium ‐called nitinol
By means of special heat treatment, a piece of SMA can be made to 'remember' a shape
For example, a length of wire can be made to remember that it should be straight at temperatures above 70¼C
If you bend this wire at normal room temperature into the shape of a paper clip, it stays bent and will continue acting as a paper clip
However, if you place it in a glass of water whose temperature is above 70¼C, it immediately straightens out! When cool, it remains straight until it is bent again
This cycle of bending and then straightening when heated can be continued millions of times
The temperature at which SMA 'remembers' its original form is called the transition temperature and when this point is reached, it changes shape
SMA has a relatively high electrical resistance and can be heated to its transition temperature by passing an electrical current through it
