The 50 Best Inventions of 2009

Tens of thousands of amputees in the developing world wear an inexpensive prosthetic called the Jaipur Foot. But poor patients who lose a knee joint have few options: a titanium replacement can cost $10,000, and crude models don't work very well. Now a team of Stanford engineering students has designed a knee that's not only dirt cheap — just $20 — but also mimics the natural joint's movements. Developed with the Jaipur Foot group, the JaipurKnee is made of self-lubricating, oil-filled nylon and is both flexible and stable, even on irregular terrain. The device is being tested in India; more than 300 people have been fitted so far.

The JaipurKnee comprises five pieces of plastic and four nuts and bolts. It requires no special tools and takes just a few hours to manufacture