‘Self-Healing’ Material Seen In Movies Is Real Possibility

The 1991 film "Terminator 2: Judgment Day" introduced the world to a robot assassin that could instantly repair itself from shotgun blasts and molten lava bursts.


More than a decade later, researchers aren’t far off in their quest to design self-healing materials. Instead of developing killing machines, scientists are pursuing self-healing plastics, metals, and even cement. The hope is to build lighter airplanes that use less fuel, to construct buildings and bridges with stronger, more flexible steel, and to pave a road with cement that heals itself at the first sign of a crack. Researchers believe this technology could save industries and governments billions of dollars if they can harness it in an inexpensive way.

"We’re moving towards a time when the physical objects around us will be made of active materials that can change shape, heal themselves, and have built-in computation," said Christine Peterson, president of Foresight Institute, a nonprofit think tank. "Right now most objects are stupid objects with no ability to respond to us. One day we’ll be able to signal them to change in some way we desire."

The research is being done mainly in the field of nanotechnology, the study of objects at the molecular level. Molecules are independent of one another but are drawn together by things like electrical pulses. The idea is that by changing the properties of individual molecules, researchers can change the overall properties of the structure the molecules form.

To date, the farthest advances in nanotechnology have been made in developing tiny sensors that can detect movement or a change in temperature, among other things. But recently more scientists have begun to study the mechanical aspect of nanotechnology with a view to making stronger products that cost less to maintain.

Most of the research is government funded while some is done at the university level. Individual industries have shied away from the research because commercial applications are more likely to be achieved in five years than a year or two.

"Most industries want the solution yesterday," said Surendra Shah, director of the Center for Advanced Cement-based Materials at Northwestern University.

Much of this research is being done at the National Aeronautics and Space Administration. One of the goals is to have self-healing structures in outer space.

"What we’re after is a material that will heal itself if you penetrate it," said Mia Siochi, assistant head for advanced materials and processing research at the NASA Langley Research Center. "In space, if a meteorite hits a structure and goes through it, we want the structure to be able to close the hole itself. We’ve made materials that show a lot of promise."

Spending Days on the Golf Course

NASA scientists pursuing this research are spending much of their time on the golf course. Instead of playing 18 holes, they are studying the outer-coating of golf balls — a plastic called surlyn. Surlyn is found in everything from bowling pins to dog-chew toys because of its ability to take a beating. Surlyn, Ms. Siochi said, has the ability to heal itself at the nano-level when damaged. For reasons still being studied, surlyn’s molecules unite once they are separated.

NASA doesn’t believe surlyn is the answer for its program because it likely won’t be able to handle the ravages of outer space — including massive radiation, Ms. Siochi said. Still, researchers are studying it because they believe any self-healing material they develop will have many of the same properties as surlyn.

Much of the research at NASA and other labs involves creating individual spheres less than a millimeter across, but larger than molecules. The spheres, which aren’t visible to the naked eye, are embedded into a certain object — say a basketball. Each sphere contains materials that essentially have the same properties as the molecules of the basketball. On the outside of each sphere is a coating of a powder-like substance.

If the surface of the basketball is punctured, the molecules and spheres that make up the surface of the basketball are also punctured. Once an individual sphere is cut, the materials inside are released. Those materials react with the sphere’s powder coating and fill the hole created by the puncture. This would all happen in a matter of seconds.

"It’s pretty wild stuff and yet it’s not talked about that much," Ms. Peterson said.

Cutting Through Water

Draper Laboratories is also studying ways to make molecules instantly come back together once they are separated. One area of research is in nanovelcro, said Amy Duwel, group leader for Draper’s Micro-Electro-Mechanical Systems division. Draper is an independent nonprofit lab that was spun off from the Massachusetts Institute of Technology in the 1970s but often partners with the university on projects.

"The concept of how it works is no different than saying you can’t cut through water," Ms. Duwel said. "When you slide a knife through water you are separating it, but as soon as the knife is removed the water comes back together like nothing happened."

Nanovelcro acts much like regular velcro. The nanovelcro is added to the surface of molecules to make them stick together and, when momentarily separated, to make them stick together again.

The National Science Foundation and the Federal Highway Administration recently took part in a conference to discuss ways to engineer stronger cement and steel. These groups, among others, see a world of promise with nanotechnology and the nation’s infrastructure. Much like the sphere research, some of the research here involves having a layer of chemicals inserted at a nanolevel underneath the surface of say, Portland cement, which is used to pave roads. Once the surface or outer level cracks, the chemicals underneath would act to spur the molecules to bind together more tightly — thereby limiting the crack to a small area. If harnessed, this technology could make the pothole a thing of the past.

Research is also being done to change the properties of the cement at the molecular level. Scientists could add or take away properties in hopes of designing more durable concrete with better traction, said Mikhail Roco, chairman of the National Science & Technology Council’s subcommittee on nanoscale science, engineering, and technology.

Researchers are also testing ways to add new properties to metals. This includes making metals impervious to temperature changes which now lead to expansion or contraction and even improving a metal’s ability to act as a semiconductor, Mr. Roco said.

The U.S. military is conducting much of its own research into self-healing materials and engineering molecules of certain objects to have improved properties. One area of study involves the gecko — a tropical lizard that is able to walk on virtually any horizontal or vertical surface, and even stand upside down on a ceiling.

It was long thought that geckos are able to do this because of some biochemical property. But researchers have found the hairs on the tips of a gecko’s foot are small enough to fill the cracks between individual molecules. With millions of hairs on each foot, the gecko essentially plugs itself into the gaps between the molecules.

The military would like to design boots that allow its soldiers to do the same thing. Think of an army of spider-men.

Write to Tom Becker at [email protected]

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