For the past 50 years, manufacturers have considered carbon fiber a dream material: Though individual fibers are thinner than a strand of human hair, they can be twisted together and fused with a matrix material to form a lightweight composite that is stronger than steel, twice as stiff and a good conductor of heat. And, unlike metals, the material doesn't crack over time. It's been used in a wide range of applications, including air and spacecraft, cars, buildings, medical devices and sports equipment.