It can be difficult to fully recover from knee injuries or other damage to your joints, if just because there hasn't been an artificial replacement for cartilage that can withstand as much punishment as the real thing. That may not be an issue in the long run, though: scientists have developed a Kevlar-based hydrogel that behaves like natural cartilage. It mixes a network of Kevlar nanofibers with polyvinyl alcohol to absorb water at rest (like real cartilage does in idle moments) and become extremely resistant to abuse, but releases it under stress -- say, a workout at the gym.