Self-healing polymers are a relatively new class of “smart” materials that have the ability to repair damage autonomously, much the way biological systems can heal cuts and bruises. The applications for these self-healing polymers are numerous, and include automobile coatings, luggage, cell phones, and factory machinery. Not only can they repair micro-damage without any human intervention or even diagnosis, they can prevent small damage from becoming major damage and extend the life span of the coating, device, or machine.

Traditionally, however, these polymer gel-based materials needed to be soft and wet (water serves as a non-toxic clue in the self-healing process) which is good for rainy outdoor environment but limits them in dry conditions such as interiors or machinery that can’t get wet. Researchers from Osaka University in Japan recently developed a new self-healing polymer solution that works in mostly dry conditions and with harder materials. Only a small amount of water vapor is needed to facilitate self-healing in the dried film state. It does this by combining physical and chemical self-healing mechanisms rather than using one or the other.

The research was published in the November 10 issue of Chem.

Senior author Akira Harada, a supramolecular polymer chemist at Osaka University, and co-author Yoshinori Takashima, an associate professor at Osaka University, told Design News that prior to their research, self-healing polymers have been categorized into two types: physical approach and chemical approach. The physical mechanism often relies on polyrotaxane, a macromolecule composed of ring molecules threaded by an axle polymer, with bulky stoppers at both ends. The ring molecules are able to move along the axle.

The physical approach includes microcapsules or vascular networks filled with healing agents embedded in the materials and a stress relaxation mechanism in the materials based on polyrotaxane. The material is embedded with microcapsules or pathways filled with healing agents, or they use a material like polyrotaxane, which changes shape in response to damage.

Chemical self-healing materials use reversible bonds ranging from reversible chemical reactions to intermolecular interactions such as hydrogen bonding. Rather than rely on one method or another, the work of Osaka University researchers advocates a new design principle of self-healing materials through a combination of physical and chemical self-healing mechanisms.

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“We utilized polyrotaxane as a backbone structure, imparting materials with physical self-healing ability,” Professor Harada told Design News. “Our material design is polyrotaxane networks cross-linked by reversible interactions: in this case, boronate-diol interactions. The polyrotaxane structure enables stress-relaxation in recovering shallow dents. In addition, the reversible nature of the bonds enables chemical self-healing from a deep cut. The combination of physical and chemical self-healing enables multifunctional self-healing materials that exhibit rapid and efficient self-healing even in a dried, hard state, compared to the material having either physical or chemical self-healing mechanism.”

The result, according to the researchers, was a rigid material that was able to repair 99 percent of a cut on the surface even in semi-dry conditions and with harder materials. The combined approach allowed the materials to recover up to 80 percent of their strength within 10 minutes (The researchers estimated that without the combined approach, the materials would have repaired only 30 percent of their strength after an hour).

Professor Harada noted that the maximum size of microcracks that can self-heal will depend on the thickness of the coating.

“Theoretically, our materials will show the healing property regardless of the depth, unless the microcrack reaches to the ground surface,” he told Design News. “This is because our self-healing mechanism is similar to the zipper mechanism. On the basis of this report, microcracks with around five micrometer depths are healed autonomously.”

As for the amount of water required for the self-healing to initiate, the researchers noted that their materials do not need to reach a maximum of a swelling degree, unlike materials used in the past.

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“It’s difficult to answer clearly the percentage of water required, but our materials show the healing property simply under high humidity conditions with no need to apply water to the surface,” said Professor Harada.

Going forward, the researchers hope to develop a hard material that can self-heal under ambient conditions without any external cues to facilitate self-healing. Polymeric materials that are both tough and self-healable could open up a new frontier in materials science.

“Our discovery process will continue to pursue the development of ideal healing materials,” said Professor Harada. “In addition to the combination of physical and chemical self-healing techniques, structure and orientational controls as a third function should be added to the materials to exceed biological materials and open a new paradigm in materials science.”

Tracey Schelmetic graduated from Fairfield University in Fairfield, Conn. and began her long career as a technology and science writer and editor at Appleton & Lange, the now-defunct medical publishing arm of Simon & Schuster. Later, as the editorial director of telecom trade journal Customer Interaction Solutions (today Customer magazine) she became a well-recognized voice in the contact center industry. Today, she is a freelance writer specializing in manufacturing and technology, telecommunications, and enterprise software.