Science Scope

Gel stretches to 21 times its length, could replace cartilage

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This hydrogel, the toughest, stretchiest one ever, may be 90% water, but it could someday be used to replace kneecaps or spinal discs.

Wow.

This gel is made of 90% water, and yet it can stretch to 21 times its resting length without breaking. Even a rubber band only stretches to six times its resting length.

Since it's compatible with the living tissue, it could someday be used in the body, such as to replace cartilage (such as in kneecaps), spinal discs or other tissues. Similar such hydrogels are currently used to make contacts.

But this hydrogel is the stretchiest, toughest one ever.

“It’s the toughest hydrogel ever reported, we believe,” said Zhigang Suo, a mechanical engineer at Harvard University and senior author of the paper, which was published in Nature. “So far, nobody has challenged this claim.”

What's it made of?

Gels are usually brittle. In order to create a stretchy gel, engineers have to combine gels whose structures dissipate energy together. Usually, the combination is a strong, stiff gel that has densely packed polymers and another one with a loosely packed polymer network.

The way these work together is that if the stiff gel cracks, meaning its chemical bonds get broken, then the looseness in the second gel reduces the breakage.

Even gels made this way get broken repeatedly, causing the gel to get weaker.

To solve the problem of fatigue, Suo and his colleagues used, for their second gel, one with polymers linked by calcium ions; ionic bonds can re-form easily.

If it seems like a crack is about to form, the calcium ions "unzip," to dissipate energy, and that allows the covalent bonds from the other gel to remain intact.

As the Los Angeles Times puts it, "Later, when the stress subsides, the calcium ions can return back to their initial positions, 're-zipping' the ionic bonds back together."

This new self-healing hydrogel can take up to nine times more mechanical stress than cartilage, making it about as good as natural rubber. And it maintains its elasticity and toughness even after being stretched multiple times. The only thing it needs is some time between stretches for the calcium to re-zip.

It's so self-healing, in fact, that the researchers showed that if they cut a two-inch crack in the gel, it can still stretch to 17 times its starting length.

And don't even try ripping it apart with your bare hands. You can't.

Check out its Herculean strength in the video below.

via: Nature, Nature News, LA Times, Physorg

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Laura Shin

Features Editor

Laura Shin has been published in The New York Times, The Wall Street Journal and The Los Angeles Times, and is currently a contributor at Forbes. Previously, she worked at Newsweek, the New York Times, Wall Street Journal and LearnVest. She holds degrees from Stanford University and Columbia University's Graduate School of Journalism. Follow her on Twitter. Disclosure