Smart special – Smart materials

Smart Materials Published online on 18 June 2012 by Rebecca Pool Spy drones, raindrop power generators, camouflage clothing and jellyfish robots? Welcome to the wacky world of smart materials. If you thought materials science was mostly about lifeless lumps of…

Smart Materials
Published online on 18 June 2012 by Rebecca Pool

Spy drones, raindrop power generators, camouflage clothing and jellyfish robots? Welcome to the wacky world of smart materials.

If you thought materials science was mostly about lifeless lumps of metal, think again. Thanks to the rapid development of smart materials over the past two decades, the discipline has really got moving, literally.

From colour-changing materials and shape memory textiles to morphing aircraft wings and drug-delivering polymers, what sets these materials apart from your everyday metal, ceramic or polymer, is an ability to respond ‘intelligently’ to the environment. It all kicked off with a quite remarkable snow-ski in the 1990s.

The first smart snow-ski was designed with piezoelectric ceramics; materials that convert mechanical energy to electrical energy and vice versa. Each ski contained a dampening unit, comprising a piezoelectric ceramic card with an electronic control circuit, placed directly where vibrations tend to originate, just in front of the bindings that anchor the skier’s feet.

When the piezoelectric ceramic detected vibrations, electrical signals were sent to the control circuit, which then sent pulses of electrical energy into the piezoelectric material to change its stiffness and damp vibrations. In other words, the ceramic acted as a mechanical actuator, moving in opposition to vibrations and cancelling them out, giving the skier a smoother ride.

Sports equipment manufacturers world- wide were quick to adopt the concept and piezoelectric dampening can now be found in tennis racquets, baseball bats, snowboards and water-skis. But sport is just the beginning.

One rapidly growing application for all kinds of smart materials is covert surveillance. Be it an airborne eye-in-the-sky, an underwater spy-bot or even an indoor fly on the wall, there’s a smart material out there that can make your spy-craft fast and responsive.

Morphing aircraft
The concept really took off around five years ago, when US-based NextGen Aeronautics, part of a US-military-funded project, remotely piloted an unmanned aerial vehicle (UAV) that could change its wing area by up to 40 per cent, thanks to smart materials. Its morphing ‘batwings’ comprised flexible, stretchable polymer skin panels attached to an articulated lattice with piezoelectric actuators in the joints.

These smart skins could slide and fan out to create new wing shapes. If the craft needed to ‘loiter’, its wings would be large but if it then had to fly fast to intercept a target, the wings could transform to narrow, more streamlined structures.

Since this time, the key focus of so-called morphing aircraft research has been to alter the camber of an aircraft wing to boost stability and control. As Juan Gomez, a morphing aircraft researcher at US-based Cornell University, explains, while a myriad of novel ways have been developed to do this, the macro-fibre composite (MFC), pioneered by Nasa, has proved instrumental.

By sandwiching a layer of piezoelectric fibres between copper electrodes and acrylic layers, MFC patches can be fabricated, stuck to an aircraft wing and connected to an on-board power supply to act as simple actuators and better control the craft’s pitch and roll. What’s more, these MFC actuators can be used alongside another type of smart material, the electroactive polymer. These generate very large strains when exposed to high voltages, and have been used to boost deflection, so much so that the wing can actually fold.

Shape memory alloys, materials that deform when heated or cooled, have also been used to morph craft, particularly in much smaller spy-bots used to scout out buildings. Gomez is currently working on a ‘hummingbird’ UAV, which will be able to rapidly flap its wings as well as hover and perch, while researchers from North Carolina State University have unveiled ‘Robo-bat’, a palm-sized bat that “responds to a gust of wind as perfectly as a real bat”. The vehicle comprises shape memory alloy joints as well as smart alloy muscles and wing membranes that flex in response to the heat from an applied electric current.

Clearly, numerous smart material systems are now being presented as the next best way to morph an aircraft. But Gomez predicts this will change.

“Within the next few years I believe more standardisation is going to take place using these types of smart materials and actuation; the systems will be optimised and then commercialised,” he says. “Hopefully we ‘will also know which system can scale up and be used in manned aircraft.”

Robotic jellyfish
Instead of using traditional rigid materials,’scientists are beginning to use new materials that are electro active, which means if electricity is applied to them they move. A normal robotic limb would have actuators and gears to give it movement, but research projects at the University of West England (UWE) have been inspired by how organic muscles make a human limb operate. Peter Walters, a researcher at the Centre for Fine Print Research at UWE, has created a soft robotic tentacle, incorporating shape memory alloy (SMA) micro-actuators, enabling them to exhibit lifelike movement when stimulated by the application of electric current. “The tentacle can move in a lifelike way by combining the 3D-printed elastomer with the SMA micro-helix muscle, so when you apply voltage it contracts just like a real muscle.”

UWE are not the only University to utilise SMA. Earlier this year, researchers from accross the pond at Virginia Tech and the University of Texas, Dallas, unveiled ‘Robojelly’, an unmanned seawater-powered submarine drone – or robotic jellyfish – for undercover surveillance or underwater search and rescue operations.

According to researcher Yonas Tadesse from US-based Virginia Tech, the jellyfish has a simple swimming action in which circular muscles on the inside of its body contract, pulling the body inwards to eject water and propel it forward. He and his team replicated this action in a silicone jellyfish using shape memory alloy actuators.

The actuator comprises a nickel-titanium shape memory alloy wire, wrapped in carbon nanotube sheets coated with a platinum catalyst. The wire is then actuated with a chemical fuel source, in this case, water.

The actual silicone underwater vehicle comprises eight ‘muscles’, each containing a shape memory alloy actuator. Water is injected at the top and flows through channels to each muscle to activate the SMA and get the bot swimming.

It’s still early days for Robojelly. While a real jellyfish deforms by 42 per cent when on the move, the water-powered robot flexes by 13.5 per cent. As Tadesse explains, matching the natural deformation is crucial to replicating the body dynamics and creating an artificial version that is as fast and agile as the real thing.

“We’re researching new ways to deliver the fuel into each segment so that each one can be controlled individually,” he adds. “This should allow the robot to be controlled and moved in different directions.”

Mean machines
Beyond spy drones, ‘clean’ power generation is fast becoming a key industry for smart materials. In April last year, researchers at UK-based University of Bolton revealed a piezoelectric device that can generate energy from raindrops and wind.

Their piezoelectric device comprises a strip of piezoelectric film wedged between long, flexible polymer strips. They first experimented with releasing water drops onto the strip to generate a voltage and then placed it in a custom-built wind tunnel, measuring the voltage generated by various wind speeds.

According to lead researcher Professor Elias Siores, his team showed that light impact from raindrops and only moderate wind speeds could generate enough energy for low-power electronic devices, such as wireless sensors. His team has gone on to develop inexpensive, hybrid piezoelectric and organic photovoltaic fibres – organic solar cells are deposited on piezoelectric fibres – that produce an electrical voltage when, again, vibrated by wind or raindrops. Importantly, the photovoltaic cell generates constant direct current.

“Being an organic photovoltaic, the efficiency reaches [only] 3.5 per cent, and the life expectancy is currently being evaluated,” says Siores. “However, the application horizon is vast. Such hybrid structures can harness the sun, wind, rain, waves and tides as well as performing well in space applications.”

His team is now considering different polymer materials for the fibre, including nylon, and is also looking to add small doses of carbon nanotubes to enhance the piezoelectric effect. As Siores emphasises, his team focuses on developing fibres made from inexpensive raw materials and using simple processing routes.

But it’s not just Siores who’s vying for a piece of the ever-growing renewable energy market. A US research team, headed up by General Motors, has successfully developed the much-coveted shape memory alloy engine, which can harvest waste heat and convert it into useful mechanical work.

The team’s prototype uses a shape memory wire that has been looped around three pulleys that form the corners of a triangle. One segment of wire between two pulleys is heated, causing it to contract, while a second is cooled, causing it to stretch out. By contracting on the warm side and expanding on the cooler side, the wire pulls itself around the loop, spinning the pulleys. And, of course, attach a pulley to a generator and you generate power.

As Jan Aase from GM points out, this is actually the first SMA heat engine that can operate through convective heat transfer with air; previous systems operated in water. So how have they done it?

Fossil-free energy
Working with colleagues from Dynalloy, HRL Labs and Michigan University, the researchers tailored the original composition of the nickel-titanium alloy wire to respond to smaller temperature variations. They also strengthened the wire loop by laser-welding the ends together, rather than using fasteners or nuts. As a result, the researchers say the loop can now turn the pulleys in a heat engine for several years, rather than the meagre few hours demonstrated in the past.

According to the Aase, a 10g strand of wire can be used to generate 2W, enough to power a night light, and his team first hopes to recycle heat from a car’s exhaust system to power, say, air-conditioning or the radio.

The next step is to build a small-scale demonstrator, that generates kW, rather than MW, for distributed generation, rivalling the likes of home solar panels and wind turbines.

If heat engines can be retrofitted into existing appliances, the fossil-free energy generating applications are endless; think factory boilers, home radiators, fridges, vehicles and more. “We’re confident the cost per unit, and cost per unit output, will be competitive with other forms of renewable energy,” says Aase. “We hope to be in production in much less than ten years.”

Clearly smart materials are destined for an ever-growing range of high-tech applications, but could medicine be the final frontier? Researchers have already developed smart, enzyme-sensitive polymers that can be loaded with drugs and injected into the body to deliver the medication to a specific site. These polymers swell and release the drug in response to specific biological signals.

Low-power irradiation
However, late last year, researchers from University of California, San Diego, unveiled a new smart polymer that actually ‘disintegrates’ in response to harmless levels of low-power, near-infrared irradiation. Such materials could, for example, be filled with anti-cancer drugs, injected into tumours, with the medicine then released in response to low-power irradiation.

To build the new material the researchers re-designed an existing polymer at the molecular level, by adding a more light-sensitive cleavable side chain to its long backbone. They found that within minutes of exposing the new polymer to near-infrared light, it had disassembled into small, fragments that seemed be non-toxic to surrounding tissue.

“Introducing the new triggering group drastically increases the sensitivity of the material to NIR light,” explains research leader Professor Adah Almutairi. “The material is well-tolerated in cells before and after irradiation and we think there is great potential in human patients, allowing previously inaccessible target sites to be reached for both treatment and diagnosis.”

The research team is now integrating near infrared light cleavable groups directly into the polymer’s actual backbone, so the polymer will break down even quicker, and as Almutairi says is “actively pursuing commercialisation”.

“This is, to the best of our knowledge, the first example of a polymeric material capable of disassembly into small molecules in response to harmless levels of irradiation,” she adds.

So, in a little more than 20 years, smart materials have made it from the ski slopes into the skies, and now into the human body. Where next?

Talk to any materials researcher, and he or she will tell you the next cutting-edge application is military protection, energy, medicine or even space. Indeed, right now, Nasa is testing a new shape memory foam under microgravity, in a bid to develop a lightweight actuator for use in future spacecraft. Perhaps instead of asking where next, we should be wondering where will it end?

Design: Shape Shifters
While most materials will expand when they’re heated, get more malleable when they’re warmed or even become better electrical conductors, what makes a material smart is that these changes take place by design. The smart material can respond to a particular stimulus, such as heat, light or an applied voltage, that would leave most other materials unchanged. And importantly the magnitude of this response is large.

Shape memory alloys are a clear example. These first registered with the wider public after they had been used in spectacle frames. The frames were made from a heat-responsive titanium alloy called Titanflex, so if the frames got bent, warming them up would restore the original shape.

How? Shape memory alloys have a different atomic structure at low and high temperatures. At lower temperatures, they take on a ‘Martensite’ structure, which is relatively soft and easy to shape. Meanwhile, at a higher temperature, the atoms re-arrange, transforming the material into ‘Austenite’, which is harder and much more difficult to deform.

So if you take a piece of straight, shape memory wire, at lower temperatures it will be easy to deform and bend into new shapes, much like any ordinary wire. However, heat it up to transform it into the harder Austenite, and its will atoms rearrange turning it back into its original, straight shape. Keep the wire above this ‘transition’ temperature, and no matter how much you deform it, it will spring back to its straight shape as soon as you release the force you’re applying.

Optical: changing shades
Until the development of photochromic glass by Corning Glass Works in the 1960s, spectacle wearers wanting to benefit from both dark and clear lenses were forced to carry around separate indoor and outdoor glasses or, if possible, switch to contact lenses.

But while photochromic lenses have proved to be a great boon in freeing up pocket or handbag space for many, those also requiring polarising lenses in their sunglasses have, until now, been out of luck.

In fact, it was thought for many years that the engineering of a suitable material exhibiting both variable polarisation and photochromism was simply not possible.

This situation, however, is set to change with the launch of Transitions Vantage, a cutting-edge product that unites’these two properties in a lens for the’first time.

The first wave of photochromic glasses was made by embedding silver halide molecules into regular glass lenses. But when plastic lenses were introduced in the early 1970s the inorganic silver halide molecules were no longer suitable due to their incompatibility with the organic plastic.

Eventually, researchers were able to find suitable alternatives; these are typically carbon ring compounds such as oxazines, naphthopyrans and indeno-naphthopyrans. In their normal colourless state these molecules have a complex three-dimensional structure. When a specific area of the molecule is exposed to UV light it breaks apart and flattens out. This changes the way it absorbs light and thus changes its colour.

As with all matter, the molecules inside the material are in constant motion’and so the broken bonds eventually come back into contact with one another allowing them to rejoin like magnets. This allows the material to revert back to its normal colourless state.

Polarising effects are also due to the manner in which a material absorbs electromagnetic radiation. Visible light is made up of electromagnetic waves of differing frequencies vibrating around the direction of travel in all directions.

Typically polarising filters selectively absorb waves vibrating in a single plane. This can be advantageous in a number of situations. For example, when looking across the surface of a lake a large degree of horizontally vibrating light is reflected into the eye producing a blinding glare.

This glare can be absorbed by a lens with a polarising filter, leaving the vertically vibrating light to pass through, allowing the wearer to see beneath the surface of the water.

Early polarisers were largely confined to the laboratory for use in experiments investigating the properties of electromagnetic radiation. They consisted of a series of fine parallel wires arranged with a separation distance less than the radiation’s wavelength. Waves with an electric field aligned parallel to the polariser cause electrons to move along the wires. This results in the wave being reflected directly back. Waves with an electric field aligned perpendicular to the wires are unaffected.

In the late 1920s the Polaroid Corporation was able to reproduce this effect in an inexpensive synthetic polarising sheet using an array of microscopic herapathite crystals. By stretching the sheet during manufacturing they discovered the crystals can be aligned in one particular direction causing the material to act in a manner similar to the wire grid.

The challenge for the team at Transitions was to combine a similar polarisation effect which would change on exposure to UV light, a task which took them ten years of painstaking research and development.

Derek Hoare, director of New Project Management and Technical Services and Vantage project leader, explains: “Polarised lenses are basically sun lenses, they will always be dark. By definition they have to block at least 50 per cent of the light, the light that is oriented in one direction.

“Vantage is the first product ever that combines clear, tinting and polarising in a single lens. We have a lens that is clear indoors and when you bring it outdoors it will get dark but will also become polarised. This is very revolutionary.”

With the company currently filing for more than 80 patents, it’s understandable that Hoare is remaining tight-lipped as to exactly how the team has been able to achieve the variable polarisation effect. However, he is willing to offer a broad-brush explanation.

“We have been able to find a way to put the molecules into the lens in such a way that as the molecules darken they are also aligning in a manner that allows them to polarise light. It filters out light in the horizontal direction and allows light in the vertical direction through.

“It’s a very difficult thing to do because what you are basically trying to do is change the material on a molecular level. The technology hasn’t been there up to now and as far as we are aware people haven’t even been working on this because it is such a novel concept.”

Transitions Vantage lenses were launched in the US on 1 May with launches in the UK and Europe scheduled for later in the year.

Mimicking squid: smart clothing goes psychedelic
Take a look at recent research from UK scientists and you might be excused for thinking they were experimenting with hallucinogenic drugs at the same time. Bristol University researchers have created a smart material-based muscle that can mimic the colour-changing abilities of a squid, and could be integrated to ‘smart clothing’ for camouflage.

As researcher Jonathan Rossiter explains, the artificial muscles are based on chromatophore cells found in many marine creatures. These cells in colour-changing squid have a central sac containing granules of pigment, surrounded by a series of muscles that contract in response to signals from the squid’s brain. This forces the sac to expand, generating an optical effect that makes the squid look like its changing colour.

To simulate the cells, he and colleague Andrew Conn used an electroactive polymer, known as a dielectric elastomer, which stretches rapidly when a voltage is applied. Taking a circular sliver of the elastomer, they smothered each side with black carbon electrode grease, creating a black spot-shaped actuator.

When the researchers applied a voltage, the elastomer stretched in the same way as a squid’s chromatophore, producing the same colour-changing effect. They went onto fabricate a network of artificial chromatophores and experimented with different pigments, simulating lighter and darker colour changes.

“Our artificial chromatophores are both scalable and adaptable and can be made into an artificial compliant skin which can stretch and deform, yet still operate effectively,” explains Rossiter. “This means they can be used… at the physical interface with humans, such as smart clothing.”