Self-Repairing Composites

A Material Capable of Healing Itself Just as a Biological Organism Does


Material Is Self-Healing


February 15, 2001

WASHINGTON (UPI) - Researchers have developed a new material they say is capable of healing itself in much the same fashion as a biological organism. The discovery could one day lead to longer-lasting and structurally self-repairing planes, bridges, buildings and prosthetic devices, they will report today in the journal Nature.

Researchers at the University of Illinois have created a composite material containing densely packed capsules. Each is filled with a chemical agent that automatically heals the surrounding material when released.

"We were essentially trying to mimic what the human body does quite naturally," said Scott White, an Illinois chemical engineer and lead author of the paper.

"The body self-assembles, puts itself together in prescribed architecture, and the process is driven naturally. In one small sense, we've achieved that."

Composites are solid materials made of substances with different physical characteristics. Ideally, such a blend of ingredients would each original characteristic, such as rigidity or malleability, while contributing desirable properties to the whole.

Nowadays composites are ubiquitous, appearing in airplanes, sporting equipment, microelectronics and even some types of dinnerware. Yet despite their unique properties, composites often crack if exposed to enough heat or pressure.

"Once a material like fiberglass cracks, the integrity of its structure is compromised," White told United Press International. "But in this material, repairs begin as soon as cracks appear."

Tests show the repaired material capable of regaining about 75 percent of its original strength, he added.

White noted Boeing and Motorola are interested in working with the composite. Circuit boards in electronic devices often crack, interfering with electrical processes. And aircraft made from the composite would theoretically last longer and boost safety.

The capsules can be thought of as tiny microballoons about 100 microns, or 100 millionths of a meter, in diameter, said Richard Wood, a chemical engineer at the University of Delaware. Wood reviewed the paper for Nature.

The capsules are filled with polymer monomers, a basic building block from which the composite material itself is made. When the composite degrades, cracks in the material rupture the tiny balloons, releasing the healing polymers. They then mix with special catalysts sprinkled throughout the material before binding to and repairing the fractured areas.

The researchers said the main design obstacle was finding the right size and thickness for the microballoons. The task was to develop a shell thick enough to withstand heat and agitation during manufacturing but thin enough for a crack to rupture it - the most difficult part of the work, said White.

Self-repairing composites are a grail for material engineers. Since the 1950s the need for light, highly rigid materials to make airplanes, space craft, weapons, sporting equipment and electronics have driven researchers to find newer and better combinations of materials. Civil engineers, for example, want self-repairing composites to build a myriad of structures from bridge parts to roof coatings.

Experts said the discovery was exciting but likely would not appear in everyday products for some time.

"For one thing we have to wait and see if it is economically feasible to produce the material on a mass scale," said Philippe Geubelle, another Illinois researcher on the project. "That is one thing we haven't talked much about."

Other experts questioned whether the microballoons would make the material heavier, which could offset its healing qualities especially in aerospace and biomedical applications.

But White told UPI the microballoons weighed less than the composite itself, making the material lighter than normal.

White's team worked on the project for six years, starting in 1995 with a seed grant from the Army Corp of Engineers. The University of Illinois and the U.S. Air Force, which did not offer comment for this story, provide funding.

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