Smart materials are synthetic substances engineered to mimic certain behaviors of living things. Scientists and engineers typically design a smart material to respond in a specific way to a stimulus (something that produces a reaction), such as electricity, heat, or magnetism. The development of smart materials is related to a broader field of study called biomimetics, which involves designing and building structures and machines that mimic the functions of living things. The term biomimetics comes from two Greek words meaning life imitating.
The simplest types of smart materials are sometimes referred to as responsive materials or passively smart materials. They respond directly to a particular stimulus in a consistent way. Some of these materials are employed as actuators (devices that produce movement when given a signal) because of their ability to change shape in response to a stimulus. Others serve as sensors, devices that measure temperature, pressure, or other properties.
More complex smart materials include a controller that processes signals, as well as at least one sensor and one actuator. These materials can be thought of as actively smart materials. They can sense a signal or condition and “decide” how to respond.
The most sophisticated smart materials, sometimes called smart systems, contain networks of interconnected sensors, actuators, and controllers. Such materials can have many advanced capabilities, including diagnosing problems within their own structures and repairing themselves. These materials may use sophisticated computers as controllers. A single smart material may contain a variety of different types of sensors and actuators.
Types of responsive materials.
There are several classes of responsive materials. The properties of each of these classes of materials may make them suitable to serve as actuators, sensors, or both.
Piezoelectric and electrostrictive materials are ceramics or plastics that change shape quickly in response to electrical charges (see Piezoelectricity ). These materials can constrict (become smaller) or expand when a voltage is applied to them. This ability to change shape enables these materials to serve as actuators. In addition, when an external force compresses or stretches them, they generate an electric charge. Thus, materials of this class can also work as sensors. Magnetostrictive materials constrict and expand when exposed to a magnetic field, a region in which a magnetic force can be detected (see Magnetism ). In addition, when an external force compresses or stretches magnetorestrictive materials, they generate a magnetic field. Thus, these materials can serve as actuators or sensors.
Shape memory alloys (SMA’s) are metals that can take on and “remember” different shapes. An SMA that is bent out of shape at a low temperature will return to its original undeformed shape when heat is applied to it. However, this change in shape occurs slowly. SMA’s typically serve as actuators.
Some optical fibers, transparent fibers of glass or plastic that carry information as patterns of light, can be combined with other smart materials to create smart composites. Changes in the wavelength of light that occur as the light passes through the fiber can be used to measure various properties, such as the temperature or pressure surrounding the composite. Thus, these fibers can be used as sensors.
An active fluid thickens almost instantly when a certain type of field, either magnetic or electric, is applied to it. The fluid thins out again when the field is turned off. An electric field is an influence that an electrically charged object creates in the region around it. Fluids affected by electric fields are called electrorheological fluids. Those affected by magnetic fields are called magnetorheological fluids. When such fluids are contained in closed cavities within construction beams, they can actively control vibration in a structure.
Uses of smart materials.
SMA’s have applications in numerous medical devices, ranging from blood clot filters for use in arteries to prosthetic hip joints. These alloys are also used to make tubelike mesh devices called stents, which are inserted into weakened or narrowed blood vessels to strengthen or widen them. The stent is cooled to reduce its diameter so that a surgeon can more easily insert it. After insertion, the stent warms to body temperature and expands to the size needed to repair the blood vessel.
Some civil engineering structures, such as large commercial buildings, dams, and bridges, feature smart materials for sensing and recording changes in structural conditions. Parts of the structure can be adjusted to improve function or to prevent damage using information provided by embedded sensors. Such applications of smart materials can reduce maintenance costs, increase safety, and extend the useful life of structures.
Smart materials can be incorporated into many different types of machinery and mechanisms to enhance performance. Many early applications of smart materials were in aerospace and automotive equipment. Such equipment includes vibration-absorbing supports, failure-prediction devices, clutches, and shock absorbers.
Smart skis contain piezoelectric devices that sense the condition of the snow beneath the skier and the sharpness of the turns on the slope. Signals from the devices stiffen or relax the skis to improve the skier’s performance. Smart materials have other applications in sports equipment, such as dampening vibration in baseball bats, golf clubs, and tennis racquets.
Scientists and engineers expect the economic and technological importance of smart materials to grow dramatically in the future. Sophisticated smart materials with such characteristics as self-repair, self-learning, and self-diagnosis will be applied to innovative products in many industries.
See also Materials (Smart materials) .
