The primary characteristic of a ‘‘smart material’’ is that it has the ability to respond to external stimuli in a technically useful and technically controlled way. The words ‘‘technically useful’’ and ‘‘technically controlled’’ are emphasized since all materials respond to external stimuli of some sort or other (as a simple example, all materials respond to temperature by changing their volume), however, to be considered a ‘‘smart material’’ the response must be one that is useful in an engineering application. Thus, any discussion of smart materials must include a consideration of the application of these materials. (Animals and plants could be considered as consisting of a large number of smart materials, however, the scope of this article will be restricted to inorganic and organic materials that are used in a more traditional engineering sense.)
The term smart material often also has a historical context, only being applied to relatively new materials. For example, consider the simple bimetallic strip. Bimetallic strips have been around for centuries and consist of two metals joined so that the difference in the coefﬁcient of thermal expansion causes the strip to bend in response to a change in temperature. This can be used, eg, to open or close a mechanical valve or electrical circuit. The stimuli may either be provided by the natural environment or engineered into a structure that the material is part of. However, bimetallic strips are often not thought of as smart materials because they have been around and used for a long time.
Smart materials are also often characterized by the fact that they transform energy from one mode to another, eg, from electrical energy to mechanical energy. Smart materials are also often incorporated in so-called Smart Structures, which are structures that, as well as being the structural support of a building or vehicle, also have a further function. For example, a