Researchers at Hokkaido University and Duke University have developed a hydrogel that heals and strengthens itself as it is overloaded and damaged. The proof-of-concept demonstration could lead to improved performance for situations where soft but durable materials are required, such as load-bearing connections and joints within machines, robots and even people.
The research appears online Feb. 26 in the journal Nature Materials.
Hydrogels sound like a fancy term, but they are everywhere in the world all around us, such as soft contact lenses, gummy bears and cartilage. This broad class of materials is defined by web-like matrices of long molecular chains that absorb and hold a lot of water.
It is challenging to create hydrogels that are soft and easily deformed but also difficult to tear apart. Imagine slowly pushing down on a Jell-O mold with your hand; it will bulge out and remain intact for a little while, but eventually, it will split and lose its structural integrity.
In 2003, Jian Ping Gong, a professor of soft and wet matter at Hokkaido University, invented what is called double-network hydrogels. In these materials, an additional harder, more brittle interior skeleton is placed within the hydrogel to provide additional strength and durability.
"The concept is similar to the tires on your car," said Michael Rubinstein , the Aleksandar S. Vesic Distinguished Professor of Mechanical Engineering and Materials Science at Duke. "The rubber is very soft, but by putting a network of connected carbon particles inside of it, one makes tires much harder, stronger and tougher."
One drawback to double-network hydrogels, and hydrogels in general, is that once the internal networks break apart, there's no going back. You can't put a squished Jell-O mold back together by scooping its parts into its original shape.
To address this issue, researchers have been working on schemes to create self-healing hydrogels that work in real-time. To date, however, these effects have worked on timescales much too slow to be useful in practical applications.
In this new paper, Gong, Rubinstein and their colleagues demonstrate a technique to create double-network hydrogels that not only heal themselves but do so much faster than previous examples while also becoming stronger.
The key to their work is incorporating sacrificial segments of internal scaffolding that are quick to break but enable the swift creation of new structural supports. Every end of the broken strands creates radicals that react with nearby bifunctional and multifunctional monomers to form chains and crosslinks to create a new network.
"An ordinary double-network hydrogel loses its support structure when the rigid internal network breaks down," explained Rubinstein. "But when these structures break, they release radicals that react with free-floating building blocks (monomers) to quickly form new networks. Every time a part of a network breaks, it becomes the seed for more reactions so that it reinforces itself and never falls apart."
The result is a hydrogel that quickly and efficiently resists cracking and other forms of damage. Once a crack starts to form—once the insides of the Jell-O mold start trying to spill outside—the material forms new bonds around the site and becomes stronger to stop fracture in its tracks.
This initial proof-of-concept was able to keep pace with cracks forming at a rate of roughly two inches per minute. While this may not sound very fast, it would still prove useful in many applications where slow degradation and wear and tear is of more consequence than quick movements and failures.
It's also just the beginning of this research journey. Rubinstein and his laboratory are continuing to develop a robust computational model of how these internal dynamics work. With this fundamental understanding, they plan to figure out ways to tweak these materials and make the healing process faster and more robust.
"This is only version one, and we're already working toward version 2.0," said Rubinstein.
This work was supported by JSPS KAKENHI (JP22H04968, JP22K21342, JP24H00848), JST FOREST (JPMJFR221X), JST PRESTO ( JPMJPR2098), and the National Science Foundation Center for the Chemistry of Molecularly Optimized Networks (MONET), CHE-2116298.
CITATION: "Rapid self-strengthening in double-network hydrogels triggered by bond scission." Zhi Jian Wang, Wei Li, Xueyu Li, Tasuku Nakajima, Michael Rubinstein & Jian Ping Gong. Nature Materials, 2025. DOI: 10.1038/s41563-025-02137-6
