Watch This: Super-Strong New Gel is Also Super-Stretchy

By Ashley P. Taylor | September 7, 2012 2:35 pm

The gel doing an impersonation of a trampoline in the video above is a new synthetic material from Harvard engineers, a substance that stretches to more than 20 times its length and can withstand more force than human cartilage, the resilient tissue that cushions our joints.

This gel starts out as a powder of two different substances, whose molecules link among themselves when mixed with water. Its astounding ability to stretch without tearing comes from the two components’ different cross-linking styles. One component, seaweed extract alginate, forms cross-links using ionic bonds, in which atoms of opposite charges attract. The other component, contact lens ingredient polyacrylamide, forms cross-links using covalent bonds, in which atoms connect by sharing electrons. Ionic bonds are very flexible, while covalent bonds are very strong, and both have their roles to play: When the ball hits the gel, the alginate molecules separate, like two magnets forced apart, absorbing stress. But the gel holds together throughout the bounce thanks to the stronger covalent bonds of the polyacrylamide.

Previous exceptionally stretchy gels were one-time-use materials: they also involved two substances, but both cross-linked using covalent bonds, but one set always broke whenever the stuff was stretched. In contrast, this new gel can recover most of its strength after a period of rest, because the ionic bonds of the alginate reform, like magnets you’ve pulled apart that snap back together when you let them go.

  • Decline

    Interesting. Wonder that the recover time is. That is, in the video, the ball is bounced back but does not fall back down again. If it were, would the gel have recovered enough to be able to stop it again? What is the damping factor? How much internal heating occurs and does that effect the recovery time and ionic bond strength (I am not a chemist).

    Cool stuff!

  • ets_spoon

    Add an adhesive and some sort of compressed gas delivery system and it seems like a cousin to the Spider-Man’s web fluid. Thwiiiiip!

  • Georg

    “”Ionic bonds are very flexible, “”
    Oh, yes indeed!
    That’s what makes sodium chloride (table salt)
    so hard brittle.

  • Grumiester

    That was a very cool demonstration. I have the same question as Decline. Why wasn’t the ball allowed to bounce more than once? I’d rather have seen numerous bounces than just one. Ought to give this to Myth Busters and see what they could do with it.

  • Pat Montana

    @ George… table salt is not a valid comparison since NaCl is a crystalline structure with many shear lines.
    Question in general…is this bio-inert and does it have potential as a cartelidge replacement since they referenced cartelidge in the article?

  • xCory

    What really surprises me is that seaweed alginate and polyacrylamide have both individually been around for quite some time, but this seems to be the first time that these compounds have been mixed. Look at what we get, truly alchemy in action!

  • Stephen W. Anderle

    George, think of underground salt domes. salt creeps and deforms without shattering under pressure,,depends on how tthe pressure is applied. Salt being actually a very strong compound has been used in construction since B.C. It is used today in many different applications.

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