Self-healing material could replace damaged cartilage in joints

Harvard University researchers have created an extremely stretchy and tough hydrogel that could pave the way to replacing damaged cartilage in human joints.

A hybrid of two common polymers, the new biocompatible material is said to demonstrate exceptional self-healing properties and be able to stretch to 21 times its original length. "Conventional hydrogels are very weak and brittle — imagine a spoon breaking through jelly," said Jeong-Yun Sun, a postdoctoral fellow at the Harvard School of Engineering and Applied Sciences. "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." To create the new material, Sun and his team combined polyacrylamide; known for its use in soft contact lenses, and alginate; a seaweed extract that is frequently used to thicken food. By themselves, polyacrylamide gels and alginate gels are brittle. The new hydrogel, however, has a more complex molecular structure that helps to dissipate stress across a wide area. The research team showed that even with a huge crack or a critically large hole, the hybrid gel can still stretch to 17 times its initial length. It was also capable of maintaining its elasticity and toughness over multiple stretches. "The unusually high stretchability and toughness of this gel, along with recovery, are exciting," noted Sun. "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." Beyond artificial cartilage, the researchers believe the new hydrogel could be used in soft robotics, optics and artificial muscle, as well as a tough protective covering for wounds.