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Sci Tech
Rise of smart materials and structures
CAN A material possess intelligence? The answer depends on what one means by intelligence. One tends to associate intelligence with what a human brain can do: perception, memory, thinking, problem solving, learning, creativity, etc.
A personal digital computer can do some of these tasks, but in terms of more sophisticated aspects of intelligence, it is no match even to a modest human brain. The recognition of these ultimate limits has inspired many scientists to study biological system as a model for developing intelligent or smart systems.
Several smart systems (or structures) are in various stages of development for civil and military applications. Air forces require smart planes that can continuously change the wing's shape to achieve supersonic speed and evade radar screen.
Diabetics need medical systems to sense sugar levels and deliver insulin. Architects are designing smart buildings with self-adjusting windows and self cleaning materials. Smart transducers can talk to fish, to herd them like cattle. How do smart systems accomplish all this? They are made from smart materials, which have the ability to respond to changes in pressure, temperature, moisture, pH, or electric and magnetic fields.
When these materials are incorporated in the structures and subjected to mechanical, electrical, magnetic or thermal loads, they deform or deflect the structure by changing their own physical properties.
Some of the present day smart materials are piezoelectric materials characterised by swift linear shape change in electric field and vice-versa. Electrostrictive and magnetostrictive materials characterised by swift change in materials size in response to either an electric or magnetic field, conversely produce voltage when stretched.
Magnetorheological and electroorheological fluids can transfer reversibly from liquid state to solid state instantly through the application of a magnetic or electric field.
Shape memory alloys, change their shape in response to heat or cold. Pyroelectric materials are materials which produce electricity when heat is supplied to it.
Smart gels are polymeric gels that shrink or swell by a large factor. PH sensitive materials, electrochromic materials, self assembling materials etc. are the other class of smart materials. Shape Memory Alloys (SMA) can be trained to exhibit a two-way shape memory effect, whereby heating SMA results in one memorized shape, while cooling results in s another shape. Nickel-titanium (Ni-Ti) shape memory alloy has been commercially developed.
The largest commercial successes of SMAs are in the field of bioengineering and biomedical applications. Most successful is the use of orthodontic arch wires (in contrast to similar stainless steel wires) will gradually return to their shape exerting a small and nearly constant force on the misaligned teeth.
The result is much less patient discomfort and more efficient and faster tooth movement. Another fast growing field of application is its use as blood clot filter. This involves the use of titanium-nickel wires that are first trained to blood clot trapping coiled configuration prior to the insertion of the cooled straightened wire.
The wire is inserted into the vena cava, where, due to the heat caused by blood flow, reverts to the original blood-clot filtering configuration.
The constriction of vessel restricts blood flow, so that surgeons have to often resort to by-pass and this can be avoided by the smart implant.
Gels based on electroactive polymer metal composites have been recently developed. These polymer gels will change its dimensions when activated by small DC voltage.
Many systems have been developed, based on this smart gel, such as artificial muscle surface wiper, robotic arm mover etc. polyvinylidiene fluride (PVDF) film exhibit piezo electric and pyroelectric properties.
Sensors based on pyroelectric PVDF are developed to detect concealed heat sources and human presence. Using piezo PVDF a touch sensor which can simulate human dermis has been developed.
A concept of smart healing is being evolved by implanting piezo PVDF based sensor in the jaw. This would generate electric field every time the patient bites on it and this electric field would promote healing process. In transportation, PVDF is used for sensing traffic either by registering vibrations or by measuring the heat generated by trucks.
In India, a national programme on smart materials is being undertaken jointly by Defence Research and Development Organisation.
Reji John
DRDO, Cochin
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