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Engineering Stretchable Gel Tougher Than Human Cartilage
A team of engineers at Harvard University has created a strong, stretchable hydrogel that might one day be used as a replacement material for damaged cartilage and spinal discs.
Credit: Jeong-Yun Sun

This Behind the Scenes article was provided to LiveScience in partnership with the National Science Foundation.

Outside of hospital cafeterias, where JELL-O® reigns as the unpopular dessert du jour, hydrogels (water-based gels) are gaining respect in the medical community at large. With funding from the National Science Foundation, a team of engineers at Harvard University, led by Zhigang Suo, Ph.D., has created a new hydrogel that might one day be used as a replacement material for damaged cartilage and spinal discs.

The new hydrogel‘s advantages are its stretchability and toughness. It can be stretched to more than 20 times its initial length. At the same time, it is incredibly tough in a mechanical sense, meaning it can absorb a relatively great amount of energy from sudden blows and shocks before breaking. In fact, the material is nearly 10 times tougher than human cartilage. What truly sets the hydrogel apart, however, is the fact that it can “self-heal,” or return to its original shape, which helps it to maintain the same toughness over multiple stretches.

A tough (but brittle) start

While the main ingredient in a hydrogel is water, it’s held together by polymers (chains of molecules). In 2003, Jian Ping Gong, a materials scientist at the University of Hokkaido in Japan, pioneered the idea of incorporating two types of polymers: long and short-chained, into one hydrogel. The idea was that when force was applied to the gel, the long-chained polymer network would remain intact and provide stretchability, while the short-chained polymer network would provide toughness.

But there was a problem with this combination of polymers: once the short-chain network finally broke, the gel was permanently damaged, and there wasn’t much that could be done to repair it. But without that short-chain network, the gel became brittle.

“If a brittle gel gets a tiny crack in it, the gel will rupture, even with a very small load,” says Jeong-Yun Sun, a postdoctoral fellow in Suo’s laboratory, and first author of the research report.

Self-healing seaweed extract adds renewable toughness

The new hydrogel created by the Harvard team replaced the problematic short-chain network with alginate, a polymer extracted from seaweed. Alginate is most frequently used as a thickening agent in food and beverages, but can be found in everything from paper to textiles to wound dressings.

The researchers chose alginate because of its unique chemical structure. It is held together by weak bonds that break and reform easily, a feature that allows it to self-heal after being stretched slightly. Alone, however, alginate can’t withstand a big stretch without completely tearing.

More than the sum of its parts

The investigators think that when combined with the long-chain scaffolding, which helps to spread the stretch across a larger volume of the gel, the alginate’s self-healing toughness makes the new compound superior to gels that contain only one or the other.

Importantly, both alginate and the long-chain polymer used in Suo’s hydrogel are biocompatible, or safe to use in the human body. Likewise, the new hydrogel containing these materials is biocompatible, according to the results of preliminary tests. Therefore, it may be possible to incorporate the new hydrogen into implantable medical devices that must be tough but flexible.

The hydrogel is also relatively inexpensive and easy to produce, which means that scientists and engineers around the world can (and already have) start using it for other applications besides just the hydrogels.

While Suo speculates that this work may have implications for various products like running shoes and bicycle helmets, his team’s next step is to take a closer look at how each of the components contribute to the toughness and flexibility of the gel.

“We are working on changing the ingredients of the gel to explore what new behaviors emerge,” says Suo. “We want to see if we can further improve its properties.”

The research report, Highly stretchable and tough hydrogels, was published in the journal Nature on September 6, 2012.

Editor's Note: The researchers depicted in Behind the Scenes articles have been supported by the National Science Foundation, the federal agency charged with funding basic research and education across all fields of science and engineering. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation. See the Behind the Scenes Archive.