Scientists at Harvard University have created a hydrogel that’s tough, biocompatible, self-healing, and can be repeatedly stretched to 21 times its regular length without breaking – all of which are qualities that could make it an ideal replacement for damaged cartilage in humans. Being a hydrogel, it’s composed mostly of water, although it also contains calcium ions, and a mix of two common polymers. While each of those polymers are fairly weak on their own, the results are truly impressive when they’re combined.

The main polymer used in the gel is polyacrylamide, which is also used in soft contact lenses, and in the gel that separates laboratory DNA fragments. The secondary polymer, alginate, is extracted from seaweed and is often used as a thickening agent in foods. The two are blended in an 8:1 ratio.

Within the resulting mixture, the alginate polymer chains bond weakly with one another, capturing the calcium ions as they do so. When the hydrogel is subsequently stretched, the bonds between some of those chains are broken (or “unzipped”), although the chains themselves are left intact. As the bonds are broken, the captured calcium ions are released, causing the gel to expand. Nonetheless, if the hydrogel were simply made from the alginate, it could only stretch about 1.2 times its relaxed length before breaking.

The polyacrylamide chains, however, form into a grid-like matrix that bonds very tightly with the alginate chains. This helps spread out the pulling force when the gel is stretched, causing the alginate chain “unzippings” to be diffused over a wide area, instead of being concentrated in one place and resulting in a crack or tear. Even when a cut was deliberately made in the middle of a sample of the gel, it could still stretch to 17 times its regular length without failing.

Given enough time to recuperate between stretches, the alginate’s ionic bonds are able to re-zip, essentially making the hydrogel good as new. By raising the ambient temperature, that re-zipping process can be hastened.

The scientists have suggested that besides its possible use as a cartilage replacement, the hydrogel could also be utilized in soft robotics, optics, artificial muscle, as a tough protective covering for wounds.

A paper on the research was published yesterday in the journal Nature.

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