SMA is a smart material which, as its name suggests, has a memory
The most common SMA is an alloy (mixture of metals)
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
At room temperature this spring is soft enough to pull out to approximately 50mm by applying a small force
When heated to 70¼C by passing an electric current through it (or by other means such as hot water or a heat gun, etc), the spring contracts to its original length with a useful pulling force (equivalent to lifting a 0.5kg weight)
