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.