When we work out, the strain breaks fibers in our muscles. Our body repairs those fibers with a steady supply of amino acids that it knits into proteins, ultimately building us stronger muscles. Scientists have now made a muscle-like hydrogel that works the same way, strengthening itself when mechanically stressed. The work, they say, could lead to longer-lasting tires or soft robots made of flexible plastic that can repair themselves and even grow.
The stretchy hydrogel material comes from a team led by Hokkaido University’s Jian Ping Gong and Tasuku Nakajima and takes advantage of polymer mechanochemistry, in which mechanical force initiates a chemical reaction. The soft yet tough hydrogel is made up of 85% water and two intertwined, cross-linked polymer networks. One of the networks—made of poly(2-acrylamido-2-methylpropanesulfonic acid) sodium salt—is taut. The other network—made of polyacrylamide—is slack.
When pulled, the slack polymer stretches out, which prevents the hydrogel from ripping, while the taut polymer breaks, forming carbon radicals at the broken ends of the polymer chains. These radicals quickly react with monomer interspersed throughout the material to rebuild the polymer network so that when the hydrogel rebounds to its relaxed state, it’s stronger than it was originally. The researchers demonstrate this by showing that the material can lift heavier weights each time it’s stretched. (Science 2019, DOI: 10.1126/science.aau9533).
Creating a molecular architecture that contains a enough radicals to make a macroscopic change in the hydrogel’s properties is a technical feat and represents “a significant step forward in the area of smart responsive polymers,” says Eindhoven University of Technology’s Rint P. Sijbesma, an expert in smart materials. This work, he says, contrasts with earlier efforts in which researchers tried to make similar systems but failed because the polymer architecture they used didn’t generate enough radicals to make a noticeable difference in strength.
Even so, there’s plenty of room for improvement in the system, Nakajima points out. For example, he says, the researchers need to figure out a way to continuously supply the system with monomer. In the current system, the monomer gets depleted after a five or six stretches, and the hydrogel becomes stiff and fragile. The system is also sensitive to oxygen, a common problem in radical polymerizations.
“Conceptually the advance is inspiring,” says Jeffrey S. Moore, a polymer mechanochemistry expert at the University of Illinois at Urbana-Champaign. “While chemists are comfortable performing chemistry in the controlled environment of a flask, this work pushes us to think about the challenges of chemistry in the wild, robust enough to work under demanding conditions.”