For many materials, their ability to stretch and expand under compression and tension is a defining characteristic. Cartilage is like this and now researchers at Harvard University have created a hydrogel with the potential to be an artificial cartilage or spinal disk. It also could see use in soft robotics.
Hydrogels are in an interesting class of material which has a great deal of water in them and molecules with loose bonds between each other. Polymers, by comparison, typically have quite strong bonds between their molecules. This particular hydrogel is actually made from two common polymers which, on their own, are not very stretchy. However, when they are combined at the proper ratio, they form a network of bonds that will only pull apart slightly, while the hydrogel is stretched over a distance as much as 21 times its original length. When the hydrogel is allowed to relax, what bonds were broken will actually reform and heal, enabling the material to be stretched again.
The hydrogel also is biocompatible, which is important if it is ever to be used as artificial cartilage or some other implant. It is not limited to that one use though as artificial muscle could benefit from this stretchiness and it could even be utilized as a covering for wounds. Only time and further research will tell us what it is capable of.