Cartilage covers the ends of the bones in joints such as our knees, letting them move smoothly against one another without wearing down the bone tissue underneath. It's made up of a porous matrix of collagen fibers, proteoglycan proteins, and elastin protein fibers. That matrix absorbs a viscous liquid known as synovial fluid, which is produced in the joints.
As the joint moves, the interfacing cartilage surfaces gradually release that fluid, providing lubrication. At the same time, the absorbed fluid also helps the cartilage to withstand being irreversibly deformed by compressive forces, thanks to a hydroelastic effect. And while researchers have previously tried to create artificial cartilage, they've typically used soft hydrogels that can't cope with such forces.