Materials That Sense and Repond: An Introduction to Smart Materials

INTRODUCTION Each of us reacts to the world around and within us by sensing and actuating. When our hand is in contact with a hot object, we sense the heat, our brain sends a command, and our arm muscles actuate our hand away from the object. Similarly, because of internal sensing, we will tend to favor the burned hand until it has healed. As technology progresses, it becomes reasonable to ask, “Can we design analogous mechanisms that can intelligently interact with their environment, and structures that assess their own health?” Such smart structures could have a tremendous impact in advancing many fields including medicine, microelectronics, and robotics, among others. Often, simple devices made from a single sensing or actuating material are used in certain applications. However, systems that involve both sensing and actuating materials can be used to build more sophisticated applications. Such systems are referred to as smart structures, which incorporate sensors and actuators with processing/control units connecting them. To get an idea of how smart structures can be implemented, it is necessary to understand the fundamental components of these structures: sensor and actuator materials. For centuries, materials have been known to react to the surrounding environment producing some form of response. For instance, in 1824, Rochelle salt was discovered to become electrically polarized by the application of heat. That was the first discovery of the effect known as pyroelectricity. Since that time, numerous additional materials have been discovered having the inherent capability to convert one form of energy into another. Sensors are materials that respond to a physical stimulus, such as a change in temperature, pressure, or illumination, and transmit a resulting signal for monitoring or operating a control. Actuators are materials that respond to a stimulus in the form of a mechanical property change such as a dimensional or a viscosity change. Table 1 lists the sensor/actuator material classes that will be discussed in this MaterialEASE along with their associated energy stimulus and response forms.