Smart materials

While smart materials are modern materials, modern materials are not necessarily smart.

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To be classed as a ‘smart material’ they need to exhibit a physical change in response to some external stimuli.

In other words, they change when you do something to them, and when you remove what is causing that change they return to their original form.

Shape-memory alloys (SMA) are metal alloys that can remember their shape when heated. These alloys have been utilised on spectacle frames that spring back to shape if they are squashed.

Nickel titanium (nitinol) is a type of SMA, and it contracts when heated, whereas most metals expand. When braces are made from nitinol, they heat up in the mouth and ‘pull’ on the teeth, so they move with the nitinol.

Thermochromic pigments change colour when their temperature changes. The term ‘thermo’ relates to heat, and chroma means colour - so thermochromic pigments change colour when they are heated up. These pigments can be mixed with paint or polymers to give the materials the same colour-changing properties as the pigment. An example of this technology is seen on colour-changing mugs or bath items for children.

A pair of hands showing a blue/green thermochromic pigment against a black backdrop.

Body temperature can cause a reaction in thermochromic pigments

Photochromic pigments work in a similar way but 'photo' refers to light - so these pigments change their properties when exposed to ultraviolet (UV) light. A well-known example would be glasses where the lenses are clear when worn inside a building, but become more like sunglasses when exposed to bright sunlight outside. The same technology has been used in windows to prevent rooms from getting too hot in warm weather.

Quantum-tunnelling composite (QTC) is an insulating rubber containing tiny particles of metal. When squashed, the metal particles meet and allow the flow of electrical current. As a result, QTC is an insulator when resting and a conductor when pressure is applied. It is often used in outdoor applications where water might otherwise damage tiny microswitches. It has been used in clothing to control smartphones and portable music players, in power tools to give variable speed controls and in touch-sensitive pads.

Self-healing materials have the ability to repair themselves, which can extend the lifespan of the products that use them. These include polymers that can heal knife cuts in themselves, metals that resist corrosion and concrete that can heal when cracked.

Ferrofluids can be formed by a magnetic field and are being used in hydraulic suspension pistons, with the strength of the magnetic field allowing the suspension to be hard or soft depending on what is necessary. They also have friction reducing properties allowing magnetic objects to glide across the surface.

Polymorph is a polymer that becomes malleable when heated to about 62°C. When it cools down it becomes hard enough to drill and cut. This makes it perfect for modelling as it can be reheated and formed again. It is also excellent for creating ergonomic handles.

A large heap of fine white thermoplastic granules on a white background.
Polymorph granules

Modern and smart materials are constantly being engineered, so it’s good to try to keep up to date with the latest developments. They can often be incorporated into new consumer products without people noticing.