Tough hydrogel stretches to 21 times its length, recoils, and heals itself

Sep 05, 2012
Tough hydrogel stretches to 21 times its length, recoils, and heals itself
The researchers pinned both ends of the new gel in clamps and stretched it to 21 times its initial length before it broke. Credit: Jeong-Yun Sun

A team of experts in mechanics, materials science, and tissue engineering at Harvard have created an extremely stretchy and tough gel that may pave the way to replacing damaged cartilage in human joints.

Called a , because its main ingredient is water, the new material is a hybrid of two weak gels that combine to create something much stronger. Not only can this new stretch to 21 times its original length, but it is also exceptionally tough, self-healing, and biocompatible—a valuable collection of attributes that opens up new opportunities in medicine and .

The material, its properties, and a simple method of synthesis are described in the September 6 issue of Nature.

"Conventional hydrogels are very weak and brittle—imagine a spoon breaking through jelly," explains lead author Jeong-Yun Sun, a postdoctoral fellow at the Harvard School of Engineering and Applied Sciences (SEAS). "But because they are water-based and biocompatible, people would like to use them for some very challenging applications like artificial cartilage or spinal disks. For a gel to work in those settings, it has to be able to stretch and expand under compression and tension without breaking."

Tough hydrogel stretches to 21 times its length, recoils, and heals itself
The researchers used a razor blade to cut a 2-cm notch across the gel. In the image above (left), the gel has been stretched very slightly so that the notch is visible. This damaged gel was still able to stretch to 17 times its initial length without breaking. Credit: Jeong-Yun Sun

Sun and his coauthors were led by three faculty members: Zhigang Suo, Allen E. and Marilyn M. Puckett Professor of Mechanics and Materials at SEAS and a Kavli Scholar at the Kavli Institute for Bionano Science and Technology at Harvard; Joost J. Vlassak, Gordon McKay Professor of and an Area Dean at SEAS; and David J. Mooney, Robert P. Pinkas Family Professor of at SEAS and a Core Faculty Member at the Wyss Institute for Biologically Inspired Engineering at Harvard.

To create the tough new hydrogel, they combined two common polymers. The primary component is polyacrylamide, known for its use in soft contact lenses and as the electrophoresis gel that separates in biology labs; the second component is alginate, a seaweed extract that is frequently used to thicken food.

Separately, these gels are both quite weak—alginate, for instance, can stretch to only 1.2 times its length before it breaks. Combined in an 8:1 ratio, however, the two polymers form a complex network of crosslinked chains that reinforce one another. The chemical structure of this network allows the molecules to pull apart very slightly over a large area instead of allowing the gel to crack.

The alginate portion of the gel consists of polymer chains that form weak ionic bonds with one another, capturing calcium ions (added to the water) in the process. When the gel is stretched, some of these bonds between chains break—or "unzip," as the researchers put it—releasing the calcium. As a result, the gel expands slightly, but the polymer chains themselves remain intact. Meanwhile, the polyacrylamide chains form a grid-like structure that bonds covalently (very tightly) with the alginate chains.

By themselves, polyacrylamide gels (a) and alginate gels (b) are brittle. The new hydrogel (c), however, has a more complex molecular structure that helps to dissipate stress across a wide area. The red circles represent calcium ions, and the blue triangles and green squares represent covalent crosslinks between chains. Credit: Jeong-Yun Sun and Widusha R. K. Illeperuma

Therefore, if the gel acquires a tiny crack as it stretches, the polyacrylamide grid helps to spread the pulling force over a large area, tugging on the alginate's ionic bonds and unzipping them here and there. The research team showed that even with a huge crack, a critically large hole, the hybrid gel can still stretch to 17 times its initial length.

Importantly, the new hydrogel is capable of maintaining its elasticity and toughness over multiple stretches. Provided the gel has some time to relax between stretches, the ionic bonds between the alginate and the calcium can "re-zip," and the researchers have shown that this process can be accelerated by raising the ambient temperature.

The group's combined expertise in mechanics, , and bioengineering enabled the group to apply two concepts from mechanics—crack bridging and energy dissipation—to a new problem.

"The unusually high stretchability and toughness of this gel, along with recovery, are exciting," says Suo. "Now that we've demonstrated that this is possible, we can use it as a model system for studying the mechanics of hydrogels further, and explore various applications."

"It's very promising," Suo adds.

Beyond artificial cartilage, the researchers suggest that the new hydrogel could be used in soft robotics, optics, artificial muscle, as a tough protective covering for wounds, or "any other place where we need hydrogels of high stretchability and high toughness."

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User comments : 11

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ValeriaT
5 / 5 (2) Sep 05, 2012
To watch a short video demonstrating this exceptionally slimy slime, click here (.mp4, 15MB) or here (streamed YT video). The video shows a metal ball dropping onto a very thin sheet of hydrogel without rupturing it.
sirchick
1 / 5 (1) Sep 05, 2012
Wondering what this could be used for :)
atomsk
2.3 / 5 (3) Sep 06, 2012
Wondering what this could be used for :)


How about reading the text?
Beyond artificial cartilage, the researchers suggest that the new hydrogel could be used in soft robotics, optics, artificial muscle, as a tough protective covering for wounds, or "any other place where we need hydrogels of high stretchability and high toughness."
antialias_physorg
not rated yet Sep 06, 2012
This seems like a really cool material. Would be interesting to test whether both types of gel in the material 'heal' or whether it's just one that reknits (i.e. whether a spot that has healed will be weaker in the future or whether it will reaquire its original stability). The article, if I read it right, seems to say that only the alginate gel will rezip?
SatanLover
not rated yet Sep 06, 2012
inorganic skin?
Deadbolt
not rated yet Sep 06, 2012
Couldn't graphene or carbon nanotubes eventually be incorporated to make it even more awesome?
ValeriaT
1 / 5 (1) Sep 06, 2012
The problem is, graphene or carbon nanotubes have no functional groups which would enable the mutual binding of carbon atoms with hydrogels, hydrophilic group the less. But inside of strong acids the carbon atoms are getting partially ionized and they do change in sorta "acidogel", which is indeed stable just under these extreme conditions. http://www.youtub...5If2VU3Q
Tausch
1 / 5 (1) Sep 08, 2012
Possible uses...
These are the future 'bumpers' for cars.
Absolutely useless for the automobile industry.
You can not having 'self healing' bumpers.
Unless longevity - the products' life span - is preprogrammed.
rsklyar
1 / 5 (1) Sep 10, 2012
Just it was some plagiaristic attempt by the professors at MIT and Harvard University in this area- https://www.usw.c...activity
antialias_physorg
not rated yet Sep 10, 2012
Couldn't graphene or carbon nanotubes eventually be incorporated to make it even more awesome?

Graphene and carbon nanotubes ma ynot be optimal, as they represent nanoparticles (and graphene can splinter under load/bending/shear).
The effect of nanoparticles in the body isn't well understood, yet. But since we have no bodily functions desiged to deal with their removal it's best not to plug them in when it can be avoided.
SatanLover
1 / 5 (1) Sep 11, 2012
Just it was some plagiaristic attempt by the professors at MIT and Harvard University in this area- https://www.usw.c...activity


I wouldnt be surpirsed.