Super stretchy gel doesn’t crack under pressure, self-heals too

Imagine a substance made of about 90% water that is resilient, compatible with living tissue and can stretch to more than 20 times its own length without breaking.

A team of American and Korean researchers have managed to make just such a material by combining two unremarkable gels — creating a supergel with strengths derived from both.

Though it’s meant to be a proof of concept, gels that are similarly tough and stretchy could one day be used to make replacements for cartilage and other biological tissues, which must be flexible enough to work inside a living body while also strong enough to bear the forces exerted on it.

The gels being studied consist of chains of linked polymers that float in water, their primary ingredient. The result is a hydrogel with a unique set of characteristics. The gels have a range of uses, including tissue engineering and drug delivery. The tougher variety of these gels is used to make contact lenses.

But the particular hydrogel made by the American and Korean team has a stretchiness that puts it in a class by itself. It was described this week in the journal Nature.

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

Many gels are brittle -- they break easily. Engineers have gotten around that problem by combining different types of gels whose structures work together to effectively dissipate energy for maximum toughness.

Most of these combinations consist of two types of gels — a relatively strong and stiff one with a dense network of chemically-bonded polymers and another one whose polymer network is more loosely packed. That way, when a crack propagates through a stiff gel and breaks its chemical bonds along the way, the looseness of the second gel helps dissipate the energy and reduce the breakage.

But some breakage is inevitable, and it’s hard to repair. The result is a gel that’s weakened and becomes fatigued quickly after repeated compressions.

Suo’s team solved this problem by including in the mix a second gel whose chains of polymers were linked by calcium ions. These ionic bonds are weaker than covalent bonds. But unlike covalent bonds, they can re-form very easily. So when a crack seems to be imminent, the calcium ions break off and “unzip” the polymer chains, dissipating some of the energy. By taking that hit, the covalent bonds in the dense gel can remain intact.

Later, when the stress subsides, the calcium ions can return back to their initial positions, “re-zipping” the ionic bonds back together.

This new hydrogel has been stretched up to 21 times its original length. (For the sake of comparison, a typical rubber band can stretch about six times its resting length.) It can withstand mechanical stresses of up to 9,000 joules per square meter of fracture energy — nine times better than cartilage, and about as good as natural rubber.

This type of gel could be a boon for patients suffering a range of conditions, Kenneth Shull, a materials scientist at Northwestern University, wrote in a commentary on the paper. Perhaps artificial kneecaps and better scaffolds for growing organs might be around the corner.

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