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How the development of smart materials is changing the face of "Technology for life".

Many of the big technological advances of the last fifty years have been due to often unsung engineering advances in materials science.

Some of the most exciting substances to be developed are known as smart materials. These are a class of materials that can alter one or more of their properties when an external stimulus is applied to them. There are a number of different types of smart material

Piezoelectric materials

When piezoelectric materials are squashed they produce a small electric potential. Interestingly, the reverse is also true. When you apply an electric potential to a piezoelectric material it will get bigger. This has made piezoelectric materials very useful for sensors which investigate things like pressure and density.

A good example of a piezoelectric device is the sensor in a car airbag which is used to detect an impact.

Shape memory alloys

These alloys have the amazing ability to 'remember' their original shapes. You can bend a shape memory alloy (SMA) out of shape many times, but by applying heat or an electric current the metal returns to its original shape.

SMAs are already in widespread use. You can find them in things as varied as kettle switches, frames for your glasses and Braille print punches.

Chromogenic smart materials

The chameleons of the smart material world, these materials can change their colour properties when some external stimulus is applied. The most useful external stimulus is usually a small electric current. Materials which change their colour properties when an electric current is applied are called electrochromic.

Chromogenic materials are in use already in various types of display equipment and also in glazing, where they can be used to alter the amount of light passing through a window.

pH sensitive polymers

pH sensitive polymers are materials which respond to the changes in the pH of their surroundings by altering their dimensions. There are two main types of pH sensitive polymer:

Those that increase in volume when exposed to an acid pH
Those which increase in volume when exposed to an alkali pH.

One of the major uses for pH sensitive polymers at the moment is drug delivery.

Smart fluids

These are fluids whose properties can be altered by applying a magnetic (magnetorheological smart fluid) or electric field (electrorheological smart fluid).

So far, the most useful of these materials alter their viscosity in an applied electric or magnetic field. The current model of the Audi TT uses a magnetorheological fluid in its suspension. These fluids have also been used as earthquake shock absorbers in some buildings in Japan.

Electrorheological fluids are currently used in clutches and brakes and it has even been suggested that they be used in bullet-proof vests.

Watch the Nature reinvented video to find out more about the different materials that engineers are working on.