These active materials may change colour, shape, opacity or temperature, some even move in response to light, heat, electricity, magnetic fields, chemicals or biological agents. Materials that can change energy into other forms of energy – such as turning light or pressure into electricity – are also classed as active materials.

The ‘cleverness’ of the material itself can be harnessed and applied to create something that reacts, bringing a design to life. Part of the beauty of these materials is the idea of getting something for nothing – you can use the material itself to generate its own power, or use a material that automatically responds to its environment without people having to intervene.

In future these smart materials will be combined so that one stimulus triggers multiple reactions. For instance, a soft helmet composed of several layers of smart materials is not out of the question – perhaps it will contain a piezoelectric material that generates an electrical charge on impact, stimulating an electro-rheological material to harden so that the force of impact is distributed over the entire shell of the helmet, which then softens again to minimise the force transferred to the head.

Although smart materials are typically expensive, some of them can be used economically to replace several parts (a shape memory alloy instead of a motor and linkage assembly, for instance), or for applications where such a reduction in weight results in ongoing cost savings.

The attributes of these materials are leading to some very exciting applications.

Blowing hot and cold

Phase-change materials (PCMs) can release heat when they change from a liquid to a solid, and vice versa. The translucent liquid-filled heat packs sold in camping stores are a common example.

The pack contains liquid sodium acetate and a little metal disk that, when pressed, turns the liquid into a cloudy crystallised semi-solid and gives out heat. These heat packs can be re-used many times – when you place them in boiling water, the crystals are removed and the material returns to a liquid.

PCMs can store thermal energy and then release it when required. They can be used for all sorts of things, including recovering waste heat, the heating and cooling of foods, the transporting of temperature-sensitive materials, the regulation of temperature in buildings and vehicles, and for performance clothing. In the latter example, some apparel manufacturers have developed clothing that keeps you warm or cool – for instance, cooling vests for fire-fighters or athletes, and warming vests for mountaineers.

Magnetocaloric materials such as gadolinium-silicon-germanium alloys change temperature in a magnetic field and can be used for refrigeration, thereby having the potential to reduce greenhouse gases.

Camfridge in the UK is in the process of developing commercial refrigeration and air-conditioning products that will reduce energy consumption without the use of polluting gases. The Ames National Laboratory (US Department of Energy) is also working on magnetocaloric materials.

Shape shifters

Rheological materials are able to change their physical shape and viscosity (from solid to liquid, for instance) with the application of either electricity (electro-rheological materials) or a magnetic field (magneto-rheological materials). The viscosity of these materials can be controlled, and they respond really quickly.

Current applications include shock absorbers and suspension systems for cars, seismic dampeners for buildings, artificial limbs, and some curious art pieces. Ferrofluids are particularly captivating to watch when variable magnetic fields are applied to the liquid.

Hydrogels are polymers that can retain water and simulate biological tissue. Stanford University’s Bio-X interdisciplinary research program (amongst others) has worked on developing a bio-compatible artificial cornea for use in eye surgery.

More comfortable, longer-lasting contact lenses are now made with hydrogels. Smart hydrogels, however, can rapidly change their shape by swelling or releasing water in response to chemical, biological or electrical stimuli.

Smart hydrogels can be used for variable-focus lenses and artificial muscles. The beauty of these materials is that they can be designed to change based on a variety of environmental conditions – they can detect and respond to changes in chemical or biological compositions of air or water, for instance.

Shape-memory alloys are particularly fascinating because of their ability to rapidly recover their initial shape after being deformed. Imagine a coat hanger that uncurls and becomes a straight piece of wire when a current is applied, or a scrunched-up piece of wire that, when heated, folds up to become a coat hanger. And this can be done thousands (if not millions) of times.

The question is, of course, why stop at the coat hanger? Why not the shirt? Apply heat and the collar is instantly straight.

There are two main types of shape-memory alloys: ferromagnetic shape-memory alloys and thermal-responsive shape-memory alloys. Thermal-responsive alloys return to their original shape when heat is applied (either directly or via a current that resistively heats the alloy).

By varying the composition of the alloy, the temperature at which the material responds can be finely tuned – set to change at body temperature or at the boiling point of water, for instance. The alloy, typically in the form of a wire, is not particularly strong when compared to a steel wire (about 100th of the tensile strength), but its elasticity is far superior.

Whilst it is a relatively expensive material, its ability to recover, and therefore move, makes it great as a replacement for motors and actuators – because there are fewer moving parts, it is much lighter. Consequently, the aircraft industry has used them to adjust wing flaps.

The phenomenon of shape-memory alloys was discovered in 1932 but not really utilised until thirty years later when the Naval Ordinance Laboratory developed nickel-titanium alloys now commercially available as Nitinol (incidentally, Nitinol is an acronym; Nickel Titanium Naval Ordinance Laboratory) and Tinel.

The most interesting thing about these materials is the applications for which they are used. The medical industry has pioneered their use – particularly for delicate surgical procedures (keyhole surgery, arterial clips, catheter guiding wires and stents).

Orthodontic use is also becoming more common, and they are being used in penile implants for erectile dysfunction, and for bras that can change shape to provide different types of support.

Shape-memory polymers act in a similar way to shape-memory alloys, but the response time is slower and they are not as strong. However, they have a lot of potential in textiles, where they can be used to create wrinkle-free garments, or fabrics that breathe (like the gills of a fish) in response to changes in temperature.

The advent of biodegradable plastics adds a new dimension to these shape-memory polymers – surgical stitches that tighten at body temperature, and then, after the patient has recovered, dissolve to leave healed tissue.

The shape-memory polymer suture is particularly useful for keyhole surgery as it can be used to provide the right degree of tension to hold the tissue together – not so much that it causes necrosis and not so little that it doesn’t hold the wound together.

Colour changers

Thermo-chromatic materials change colour when warmed up. They can be used to change colour at different temperatures and have been used for all sorts of applications, from liquid crystal displays (LCDs), which sandwich a material that polarises when a current is applied, to the Hypercolor T-shirts of the late 80s, to the hot spot on frying pans.

Thermo-chromatic dye can be blended in with many thermoplastics and then moulded conventionally, or applied as a coating.

Electro-chromic pigments perform in much the same way as thermo-chromatic ones, except the stimulus is an electrical current. Smart, or switchable, glass can change its opacity – from transparent through gradients of translucency to opaque – with the application of a current.

Electricity generators

A variety of active materials can generate electricity, such as the photovoltaic materials used for solar cells. Photovoltaic materials use thin coatings, or films, of light-absorbing materials that absorb photons of light and release electrons.

In turn, an array of photovoltaic cells provides sufficient charge to draw current from. Photovoltaic materials have been around for over 160 years – the effect was first noticed by Becquerel in 1839 – but it took a long time for photovoltaics to be commercially available.

Research into photovoltaic materials is resulting in cheaper and more efficient solar cells, and work is underway to produce completely transparent solar panels on windows using carbon nanotubes (the nanotubes are too small to see when sandwiched between two panes of glass).

Piezoelectric materials generate charge when pressure is applied to them. A common example of their application is gas or cigarette lighters, where pressing a button causes a piezoelectric crystal to generate a spark that lights the gas.

Conversely, if a current is applied to them they can generate movement. Piezoelectric materials only generate small movements, which makes them ideal as highly accurate actuators (high-precision stepper motors) and flat speakers. 

We would like to acknowledge the assistance of materials expert Jonathon Allen with this story.
 

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