This is the second part of a series on breakthroughs being made in the quest for smart materials. The author is project manager at CANEUS Bioastronautics Montreal. She is also a materials and biomechanics researcher, adjunct professor in mechanical engineering and surgery at McGill University, and a science writer and columnist. She runs a science and mathematics outreach program for Montreal-area schools called Back to Basics.

A small piece of the self-healing material developed by Daniel Therriault, assistant professor of mechanical engineering at L'École Polytechnique in Montreal. The innovative design contains microchannels similar to blood vessels that allow healing fluid to flow throughout the material, ready to arrest any incoming cracks in their tracks.

Along the southern shore of the St. Lawrence river looms an aluminium-tinselled edifice that headquarters the Canadian Space Agency. Here, Darius Nikanpour and his young team of engineers have been quietly working on a new generation of smart materials for spacecraft.

The Columbia Shuttle tragedy in 2004 drove home the need for structural materials that would resist not only ground-shaking rocket launches, but also be able to withstand the potentially fatal high-speed impact of debris in space.

The potential for space debris to cause another Columbia-like tragedy has spurred the space community to seek novel, sturdier materials from which to build spaceships and satellites. And, like his counterparts at NASA, Darius was intrigued by the prospect of materials that, like human skin, would heal on its own when wounded.

Cooking material

To understand the principle of self-healing materials, let us start by making mayonnaise, the old-fashioned way:

Pour vinegar into an electric blender and set it to swirl at low speed. Gradually pour oil into the mixture and watch it break into small droplets. As you increase the speed of the rotor blade, the oil droplets will get smaller and smaller. Then, gradually pour in an egg yolk and watch the solution turn cloudy and finally turn into a thick white emulsion.

If you looked at the mixture under the microscope, you would see tiny microdroplets dispersed in a liquid. The egg acts as a soap, in that it keeps oil droplets separated from each other, without allowing them to clump together.

At the University of Illinois at Urbana-Champaign, Scott White and his team used a similar principle to make self-healing materials. They dispersed small glue-filled microspheres inside a liquid resin. The liquid resin was then allowed to harden into a tough transparent plastic, but with oily glue-filled bubbles trapped inside the material.

Then, they pocked the material with a sharp instrument to create a dent. The material initially buckled in, as though taken aback, only to patch the stab wound and spring back into shape.

How did that happen?

Quite simply, the blade creates a fissure that ruptures the nearest microsphere, thus spilling the glue into the crack and healing it. Mechanical tests on these healed materials showed that they were able to recover up to 80 per cent of their original strength after healing.

Even as this work garnered international attention and was featured in prestigious scientific journals, White's Canadian PhD student Daniel Therriault was contemplating something even better.

"Here was the problem", explained Therriault, now assistant professor of mechanical engineering at L'École Polytechnique in Montreal. "Once a microcapsule opens and spills its glue, there is no glue left to heal subsequent cracks. What do we do when the same area of a material is struck twice?"

The solution, according to him, was to turn the microdroplets into microchannels. Like blood that continuously flows through our veins, Daniel's innovative new material contained microchannels that allow healing fluid to flow throughout the material, ready to arrest any incoming cracks in their tracks.

To make this new material, Daniel extruded a mixture of ordinary Vaseline with a crystalline substance through a syringe. He then carefully "drew" a three-dimensional network of fine Vaseline lines inside a mould. Next, he poured liquid epoxy over the Vaseline channels and allowed it to harden to a plastic. Then, under vacuum, he gently sucked out the Vaseline and … presto! A plastic material with built-in microchannels. The microchannels were then filled with the same glue to trigger crack healing when necessary.

Clearly impressed by the work of White and Therriault, the Canadian Space Agency has embarked on a project to test the viability of these self-healing materials for spacecraft.

However, Daniel admits that the rate of healing remains a problem. As it stands, it takes about 20 minutes for these materials to self-repair, far too slowly to counter the high-speed impact of debris and other rogue objects in space. Clearly, something to improve upon in the near future.

Of Wood and Cow Hoofs

Professor Caroline Baillie, a materials engineer at Queens' University, has spent years studying the properties of natural materials like wood and flax — even cow hoofs — hoping to decode nature's blueprint, especially its uncanny ability to cobble together sophisticated structures using simple starting materials. Likewise, she urges her colleagues not to overlook simple solutions in their quest for smart materials.

"Take wood, for instance", she explains. "Wood has, what you might call, a self-healing mechanism."

Wood is a "hygroscopic" material, in that it has a high affinity for water. If you are building a boat out of wood, the wood absorbs enough moisture to swell and seal the joints. "So you have a simple way to build a leak-proof boat", she adds.

Furthermore, research work carried out by Baillie showed that the intricate microstructure of wood is such that added moisture renders the wood tougher and less sensitive to defects.

While Baillie is fully aware that moisture is often a liability in industrial materials, she nevertheless encourages her colleagues to heed nature's lessons and consider the role of water in wood when building sturdier, perhaps self-healing, structures.

Towards Unobtainium?

Meanwhile, White and Therriault are moving beyond self-healing materials to so-called autonomous materials — materials that can adapt to their environment by changing their properties.

And they are hoping that microspheres and circulating fluid trapped inside materials can not only self-heal, but also trigger changes in their mechanical, electric and optical properties. Yet another small step towards the ultimate smart material.

In the next column, we will travel down another possible path to unobtainium with shape-memory materials. These are materials that take on a desired shape when heated, then cooled, yet are able to snap back to the original form when re-heated again. How do these materials "remember" their original shape? We will find out in Unobtainium, Part Three.

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