Actuator is a complementary element to the sensor. It is an active device which performs some action according to external stimulus, usually given by the sensor if some threshold value is reached. A typical example of an actuator is any type of motor or heater. Shape memory materials are actuators as well since they can execute some mechanical action - open a valve/switch. A smart actuator is an actuator which acts as a sensor as well and performs an action according to its own measurement. For example, a smart shape memory actuator will act when the external temperature changes.