One of the qualities that distinguishes living systems from inanimate matter is the ability to adapt to changes in the environment. Smart materials have the ability to perform both sensing and actuating functions and are, therefore, capable of imitating this rudimentary aspect of life. Poled piezoelectric ceramics, for instance, are capable of acting as both sensor and actuator. External forces are detected through the direct piezoelectric effect, and a response is elicited through the converse piezoelectric effect, in which a voltage of suitable phase, frequency, and amplitude is applied to the same ceramic.

In this special issue, emphasis is placed on actuators, with articles on piezoelectric, electrostrictive, magnetostrictive, and shape memory materials. This is not to say that sensor materials are any less important; it is simply a matter of space. Optical fiber sensors, chemical sensors, thermistors, micromachined semiconductors, and other smart materials deserve special issues of their own.

Smart materials can be conveniently subdivided into passively smart materials that respond to external change without assistance, and actively smart materials that utilize a feedback loop enabling them to both recognize the change and initiate an appropriate response through an actuator circuit.

Zinc oxide varistors are passively smart materials capable of self-protection against high voltage breakdown. When struck by lightning, the ceramic varistor loses most of its electrical resistance, and the current is bypassed to ground. The resistance change is reversible, and acts as a standby protection phenomenon.