Once, alchemy ruled our understanding of the material world. Part science and part mysticism, its practitioners experimented with alloys, searched for a universal solvent, and hoped for a philosopher's stone—a substance that turned to gold any metal it touched.
These days, materials science takes a more rigorous multi-disciplinary approach to this sort of engineering. Materials scientists design and discover extraordinary variations on the metals, ceramics, and polymers we think we know. And lately, they're taking their cues from how the human body heals itself, transmuting those mechanics into asphalts, concretes, and metals that can mend their own cracks.
Self-healing or "smart" materials may seem as magic as the alchemy of old, but they carry the very real potential to change our roads, buildings, and means of transportation.
With a Touch of Heat, Steel-Wool Asphalt Lives On
As TripNet reported last year, more than one-quarter of major urban roads in the U.S.—Interstates, freeways and other arteries—have pavement in substandard shape. Heavy with potholes and tears, these rough-riding roads incur the average American driver $377 annually—$80 billion nationally—in operating and repair costs. And a recent estimate from the American Society of Civil Engineers shows that combined, the economic losses of decaying roads, bridges, and other infrastructure zaps $129 billion per year from federal coffers.
Clearly, we need to mix better asphalt—stronger asphalt, less costly to repair.
Which is what Erik Schlangen has done. Simply by mixing in strands of steel wool to asphalt's usual combination of pebbles and bitumen, the Dutch civil engineer at Delft University has successfully created a road-ready material that’s practically self-healing. As he puts it, it heals itself “with a little bit of help from the outside.”
In the video of his TED talk, you can watch Dr. Schlangen demonstrate his miracle asphalt onstage: In front of an audience of undergrads, he karate-chops a block of asphalt into two. As he begins to talk about how nice it is to drive on asphalt, he places the two pieces side-by-side in an industrial microwave.
That's the trick: It's induction that heals the asphalt. The microwave heats the steel wool, which in turn melts and mixes the sticky bitumen around it. Take the heat away, and the bitumen cools, mending the asphalt as it goes.
But actually applying what happens in the lab (or at TED) to the real world is a different matter. Where would transit officials obtain gigantic microwaves? Schlangen’s lab has developed a special vehicle that passes induction coils over the road. He estimates that transportation workers would need to run the machine every four years or so to repair small damages and prevent potholes, thereby extending the life of a given road perhaps two-fold. Dutch officials have estimated the technology (which they've funded in part) could save the country €90 million annually.
Here's CNN screenshot of the induction unit rolling over Schlangen's asphalt:
Having originally worked in concrete, Schlangen started on asphalt in 2008 after receiving a grant from the Dutch government. He describes the relative ease of testing asphalt compared to his old material. "It's much harder to test concrete," he says, "since you can't just make a bridge as an experiment and tell people to drive on it. With roads it’s much easier, because if it doesn't work, you just take it off."
Schlangen's self-healing asphalt is still in testing phase; He's now applied it to five strips of road across the Netherlands. "So far it's working quite well," he says.
Bacteria That Fills Concrete's Cracks
From the foundations of our houses to our bridges and roads, concrete is the most common construction material in the world. And there’s no melting or smoldering involved; Simply mix your water, gravel aggregate, and cement and you've got a thick, gray sauce that'll harden pretty fast.
But concrete isn't always as tough as might appear. If a cement bridge cracks, for example, water entering in will break up and stir the concrete's materials, eventually filling in the broken space, to an extent. "In that way it has self-healing properties naturally," Steve Kosmatka, Vice President of Research and Technical Services at the Portland Cement Association, tells me. "Water gets things moving around."
But while water has some Band-Aid like properties, it wears down more than it heals. It can also corrode steel structures that engineers often embed inside concrete for tensile strength and reinforcement. Chlorides from de-icing products can also seriously degrade structures.
Materials scientists have long been keen to develop a concrete that holds better in encounters with water. What they've come up with is a concrete that's partly alive—with spores. Spores!
How it works: concrete manufacturers pour into their mix a batch of bacterial spores, which are sheltered inside tiny, water-permeable capsules. They build as planned, and the concrete sets. After construction, the spores remain inert, like hibernating cubs, until a crack forms and water finally seeps in. As soon as the water permeates the bacterial capsules, the spores, loving water as living things do, germinate and move toward the source of the water. As they do, these bacteria naturally produce calcite, which acts as a kind of bio-cement, filling in the cracks in the concrete as the bacteria move.
Researchers from all over the globe—the University of Illinois, the University of Ghent, Cardiff University, and even Schlangen at Delft—are working with bacteria-based self-healing concrete. But other approaches are also out there: Some engineers are looking at ways of using fibers that shrink in response to water infiltration, effectively squeezing the concrete together. Still others are attempting to embed in concrete a kind of vascular network that would send a glue-like healing compound to any crack in a structure.
A concrete that heals itself would extend the life of buildings and infrastructure, and potentially translate into huge savings. According to estimates from the University of Ghent, the E.U. could save €120 million in annual maintenance cost, were it to use self-healing concrete in bridges, tunnels and earth-retaining walls alone.
And that's to say nothing of the environmental impact a better concrete might have. The production of cement, the primary ingredient in concrete, contributes 5 percent of global carbon dioxide emissions. A longer-lasting concrete could mean less cement overall. "Any time a structure lasts longer, that’s an improvement to a society as a whole," Kosmatkas says. "If you have a road that needs to be replaced every 15 years versus one that lasts 30 years with no maintenance, there’s a carbon savings right there."
Self-healing concrete could become the eco-friendly building material of choice, but it remains early to say for sure; It's still being tested. And Kosmatka predicts it'll probably be an expensive option for builders, so will likely get the most use in projects that need a lot of long-term durability. "Not for sidewalks or roads right in front of houses," he says. "But for a bridge you want to get a hundred years out of."
A Metal That Cracks So Far It's Together Again
Last year, MIT scientists uncovered a mechanism that, in principle, can close cracks in alloys under stress.
Materials scientist and engineer Michael Demkowicz and colleagues were examining breakages in the grain boundaries of nickel. Grain boundaries are the divisions between the tiny grains, or crystals, that make up the structure of a metal or alloy. The team was experimenting with applying different levels of tension to a partly broken piece of nickel—that is, exerting a force that you'd think would have pulled the metal apart even further. But they found that under certain conditions, the tension actually caused the crack to close, fusing the edges of the boundary back together.
"It was totally accidental," Demkowicz says. "It was not something we expected to see." You can watch the self-mending happen in this seven-second video, courtesy of MIT. Notice what happens to the rectangular shape (a crack) near the bottom of the video, on the right.
That rectangular crack* interacts with neighboring grain boundaries, which change the way the crack behaves. When enough tension is applied, the grain boundaries themselves can move, which generates additional forces on the crack. That's key for the crack to mend, Demkowicz says: When the additional forces tend to close the crack rather then opening it, self-healing in metals can occur.
The discovery is still recent, and is far from practical applications. But in principle, says Demkowicz, the self-healing mechanism could be applied to metals in order to repair small issues before they spread further.
"The question now is, how do we design a material that heals itself when you want it to?" he says. "We don't really know yet. But if this really gets promoted in the engineering world, you'd have enormous possibilities. You could make lighter metals, since they wouldn't degrade as much. Imagine improving the efficiency of cars, planes, subways, the earthquake resistance of buildings. That's all very new."
The discovery also reflects a surprising fact that Demkowicz stresses: There's a lot we don't really understand about metals, even the simplest ones. Inside a tiny piece of iron or nickel is an intricate architecture, he explains—like bricks make buildings make a neighborhood make a city, atoms make crystal grains make aggregates make a shard of metal. "The behavior of the material is really a collective property of all the levels of its structure," he says, "including defects and the grain arrangement, and the boundaries between them." And it's precisely how these many layers interact that we don't fully understand.
Call it a bit of magic left in materials science—some alchemy left to perform.
*Correction: An earlier edition of this post incorrectly referred to the crack as a "disclination." It is not.