Break a toe and your body will slowly fuse the bones back together; scrape your knee and your skin will eventually heal itself. The same, however, cannot be said for the man-made materials that we use to build bridges, aircraft and skyscrapers. Once snapped, cracked or bent, materials such as plastic, steel, and concrete are difficult – or impossible – to repair.
But that could soon change. Material scientists are now actively borrowing nature's capacity for regeneration and repair in an attempt to create materials that heal themselves.
Over the years, engineers have tackled the problem using a variety of strategies. But Nancy Sottos, an engineer at the University of Illinois at Urbana-Champaign, has been developing a novel approach that borrows from human physiology. Inspired by human skin, Sottos creates plastics that “bleed” when cut and can heal themselves over and over again. Her work is paving the way for new materials that can respond and react to all sorts of environmental stresses. At a mundane level, this could cut down on costly maintenance and inspections, but perhaps more importantly it could also help prevent catastrophic – and potentially deadly - failures.
When man-made structures fail, they often do so spectacularly—with, say, a bridge collapsing under the weight of rush hour traffic. But such dramatic breakdowns often have much smaller, humbler beginnings. “When a material fails, oftentimes it’s not a big catastrophic event that starts it,” Sottos says. “In most materials, a damage event starts at a very small crack.” The crack grows slowly, unnoticed, until…bam! Bridge collapse. “The goal of a self healing material is to try to prevent that,” Sottos says, “to keep those small cracks from growing.”
In 2001, Sottos made headlines for creating a self-healing plastic that relied on “microcapsules”—tiny, self-contained spheres of liquid resin that were embedded into the material. When a crack formed in the plastic, some of these capsules ruptured, releasing the resin, which filled in the crack and solidified.
The material was a major breakthrough, but it wasn’t perfect. Among the biggest drawbacks was that once the capsules had burst, they were spent - there was no liquid healing agent left inside them to repair another crack in the same area. That's clearly a far cry from how things work in nature - you can cut your arm repeatedly, for instance, and your body will heal the wound every time.
That’s because our skin is criss-crossed by veins, arteries, and capillaries. If you cut yourself, these blood vessels deliver whatever healing compounds are needed directly to the site of the wound, wherever it happens to be. “Biological systems are very good at transporting things around as needed,” Sottos said.
So over the last several years, she has engineered materials that mimic our skin more closely and contain their own microvascular systems full of healing fluids. To create a batch of self-healing plastic, for instance, Sottos begins by constructing a thick polymer sheet that has a dense, three-dimensional web of hollow, internal channels hidden beneath the surface. Then she fills some of these channels with a liquid resin and others with a liquid “curing agent.”
With her new material built, Sottos then carefully tries to destroy it, bending the plastic sheet until a crack forms in its surface. As the crack spreads, it intersects with the fluid-filled veins, releasing both liquids, which flow into the crack. When the fluids mix, the curing agent turns the liquid resin into a hard epoxy, which fills in the crack, repairing the damage.