Prognosis for Self-Healing Materials Is a Longer Life, Less Maintenance

Autonomic system of microspheres contains recovery agents to repair cracks and increase strength.

By mimicking the natural response of living tissue to injury, cross-departmental researchers at the University of Illinois at Urbana-Champaign have developed a polymeric material that heals itself when damaged. Cracks can be precursors to structural failure, and the ability to treat weakened regions will result in longer-lasting materials used in a variety of applications from microelectronics to aerospace.

Twenty million tons of composite material are used annually in the defense, engineering and electronics industries. While obsolescence is a fact of life that developers plan around, use of self-repairing components could help extend the life of many objects such as aircraft wings, road signs, artificial joints or circuit boards.

Polymer composites—advanced materials used in everyday products—consist of two components: a reinforcing fiber such as carbon or glass and a liquid molding resin such as epoxy, unsaturated polyester, vinyl ester or urethane. After the fiber is placed in a mold, the resin is injected, resulting in the composite that is ready to accept the self-healing components.

Philippe Geubelle, associate professor of aeronautical and astronautical engineering at the university, is one of the project’s lead team members. “It’s a fascinating project because many things have to come together for this to work,” he says.

Autonomic healing is accomplished by dispersing a series of microspheres that contain a healing agent and a catalyst, known as the Grubbs’ catalyst, in the polymeric composite. When a crack propagates in the material, stress causes the closest sphere to break open and release the healing agent, dicyclopentadiene (DCPD). When the DCPD comes in contact with the catalyst, a chemical reaction begins that allows the agent to polymerize and heal the breach.

Although the concept is a simple one, Geubelle explains that several steps must succeed for the overall process to work. Manufacturing must go smoothly, the microspheres must fracture at the appropriate time, and the chemistry must be correct, Geubelle says.

“As far as the manufacturing of the microcapsule, we had to learn how to do this,” Geubelle notes. “There’s a whole industry devoted to microencapsulation. These microcapsules are used by the perfume industry where they, for example, are put in magazines as a sample of a perfume’s fragrance. When you scratch it, the scent is emitted. … We had to apply this technology to our system.”

University of Illinois Associate Professor of Theoretical and Applied Mechanics Nancy Sottos notes that the capsules must be tough enough to make it through the manufacturing process. “They’re like tiny eggshells, and you have to mix them and pour them into the mold. They have to be fairly resilient,” she offers. However, when a crack occurs, the spheres also must break open. Achieving this delicate balance depends on the mechanics of designing the wall thickness and the adhesive characteristics of the materials. The spheres must stick firmly to the rest of the wall material so cracks will go through the orbs rather than around them.

The team has done extensive mechanical analysis to determine how to achieve the right wall thickness. A property of the material called fracture toughness must measure slightly less than the fracture toughness of the polymer. “If that condition holds, you can get the spheres to crack open. If you make them resilient enough, you can get them to withstand the manufacturing process,” Sottos explains.

Developing an accurate method to assess the efficiency of the system was a challenge. “How do you figure out how well the material is healing itself?” Sottos asks. “You could break it and observe how it goes back together, but it’s a little trickier than that. You have to be careful when establishing controls to which you will compare the system. You have to have a reliable test, but it may not reflect how the material fails in general. You also need a repeatable test.”

One of the checks the scientists used to determine healing efficiency was a standard fracture test, Geubelle notes. A tapered double-cantilever beam specimen was used to perform testing. First, a pre-crack was created in the samples by tapping a razor blade into a molded starter notch, and a load was applied to the pre-crack. Researchers determined the initial fracture toughness by the critical load needed to propagate the crack.

After structural failure, the load was removed and the crack was allowed to heal at room temperature for 48 hours. Next, fracture tests were repeated to quantify the amount of healing. According to Geubelle, these tests demonstrated that the material regained up to 75 percent of its original strength.

Initial tests have been successful, and applications for the system are abundant. “We see the first applications for the lower-end composites such as everyday construction materials,” Sottos says. “For example, decking materials on the back of your house would be a great application where you could fill cracks before water gets in and splits things further. There are composites and plastic materials used routinely in construction and in lower level structures such as bumpers and signposts. These are good target applications where you don’t want cracks in the material, but they would be a good place to improve on the technology since they aren’t critical load-bearing structures.”

Geubelle foresees other possible uses as well. “We see applications with anything that employs polymeric material from microelectronics to larger scale devices such as aircraft,” he reveals. “In the aircraft industry, composites are used more and more because of their properties. They’re very light and very stiff at the same time. We envision putting these kinds of microspheres in composites that make part of a wing, for example, and extend the life of the structure.”

The National Aeronautics and Space Administration (NASA) is also considering the technology for use in long space missions and to maintain computers and electronic systems. NASA is developing a new type of system that extends self-repairing ability to a spacecraft’s internal wiring to protect crucial electronics.

Each application has its own specific properties and ideal environmental conditions, Geubelle says. The chemical reaction between the microspheres and the catalyst is affected by differences in temperature. “We might be able to heal a crack in one system and not be able to heal it in another,” he allows. “That’s why we still need to do a lot of research on the application side.”

For example, the catalyst and healing agent degrade at high temperatures, and the chemical reaction slows at extremely low temperatures. If scientists can overcome the problems caused by temperature extremes, they envision using the system for cryogenic applications. Tanks that are used to keep contents at very low temperatures could be designed to repair themselves. These self-repairing materials also may be able to withstand space travel better than traditional materials.

Another challenge is that once a microcapsule has been broken and its content released on a fractured surface, the next time a crack forms, there is no healing agent left in that area to perform its function. Because of this hurdle, researchers also are looking at different versions of the system. One concept is to use a series of microchannels in place of the spheres. These channels could be turned into a two- or three-dimensional network through the material. “Here it is a bit more complex,” he adds. “The advantage is that the healing process can continue to occur as needed. Every time the agent is used, there is still more to use.”

Future objectives of the research include extending the system to fiber-reinforced composites, Sottos says. “All the work we’ve done so far is with polymers.” She also offers that the work with the spheres is on the micron level, and the team’s goal is to take their work to the nanoscale.

Team members anticipate that the field of self-healing will someday evolve beyond the current method to a procedure that uses biomimetic healing abilities. This concept would incorporate a circulatory system that continuously transports the necessary chemicals and building blocks of healing to the damaged site.