Harvard scientists develop self-healing rubber

CAMBRIDGE, Mass.—Scientists at Harvard University have developed a self-healing dry rubber, and the world is beating down their door.

"Normally, when we publish a paper, we don't get any public reaction," said Liheng Cai, a postdoctoral fellow at Harvard's John H. Paulson School of Engineering and Applied Sciences and corresponding author of the paper that appeared earlier this year in the journal Advanced Materials.

"But with this paper, we are receiving numerous phone calls from all over the rubber industry," Cai said.

Collaborating with Jinrong Wu, a visiting professor from China's Sichuan University, and David Weitz, a core faculty member of Harvard's Wyss Institute for Biologically Inspired Engineering and Mallinckrodt professor of physics and applied physics at SEAS, Cai created an entirely new hybrid rubber with both covalent and reversible bonds.

Cai, Wu and Weitz developed the concept of randomly branched polymers, which allows two previously unmixable bonds to be mixed homogeneously on a molecular scale, according to the Wyss Institute. The result is a transparent, tough, self-healing rubber, it said.

The concept of self-healing polymers has been around for about 20 years, according to Cai. It began with the work of Jianting Gong at the University of Hokkaido in Japan, who has spent her entire career on research and development in self-healing hydrogels, he said.

A Harvard paper on self-healing hydrogels published in 2012 was based partly on Gong's research, according to Cai. He said he was not aware of any current commercial applications for self-healing hydrogels. However, the inventors say the potential applications include medical implants, tissue engineering, man-made cartilage and stretchable electronics, Cai said.

The concept of self-healing dry rubber is more challenging than self-healing hydrogels, because of permanent, covalent bonds, according to the Wyss Institute.

"While these bonds are incredibly strong, they will never reconnect once broken," it said.

Previous research used reversible hydrogen bonds to connect polymers, but reversible bonds are intrinsically weaker than covalent bonds, according to Cai. He came up with the concept of blending them, but the two types of bonds are like oil and water, he said.

To solve this problem, Cai, Weitz and Wu devised the concept of randomly branched polymers, in which two previously unmixable bonds are roped together to be mixed homogeneously on a molecular scale.

"Typical rubber tends to crack at a certain stress point when force is applied," the Wyss Institute said. "When stretched, hybrid rubber develops so-called crazes throughout the material, a feature similar to cracks but connected by fibrous strands.

"These crazes redistribute the stress, so there is no localized point of stress that can cause catastrophic failure," it said. "When the stress is released, the material snaps back to its original form, and the crazes heal."

Self-healing rubber has obvious potential applications, including tires and medical goods, according to Cai.

"We've received several phone calls from tire makers, and also from a medical glove maker in Malaysia," he said.

But despite the technology's promise, Cai and his colleagues still have much more work to do, he said. They still don't understand why crazes form when the hybrid rubber is stretched, and there also is much more to do on specific applications for the rubber, he said.

Making enough hybrid rubber for commercial purposes is also a major issue to be worked out, according to Cai.

"To make the rubber more useful, we have to scale it up," he said. "We have found a way to make a hybrid rubber that heals itself. Now we have to learn how to optimize its properties."

Harvard's Office of Technology Development has filed a provisional patent application for the technology, as well as actively seeking commercialization opportunities. The patent application process will not be finished until at least sometime next year, according to Cai.

Meanwhile, Cai will depart SEAS in January 2018 to accept a tenure-track position as an assistant professor in the School of Engineering and Applied Science at the University of Virginia in Charlottesville.

The self-healing rubber research was supported by the National Science Foundation, the Harvard Materials Research Science and Engineering Center, and the National Institute of Health/National Heart, Lung and Blood Institute.