At Northwestern University, Samuel Stupp is growing bones in a beaker. His advance: specially designed proteins that self-assemble in water to mimic the structure of the proteins that form bones. The designer molecules assemble into tiny fibers a few micrometers long that encourage calcium compounds to form mineral crystals along their lengths.Because self-assembly is faster and simpler than other ways to make bonelike materials, it should be less expensive. Stupp also hopes to create versions of the fibers for engineering pancreatic tissue, which could treat diabetes. Stupp’s group has started experiments with the bone-promoting fibrils in animals, but he estimates that medical use of the molecules in humans is five to 10 years off.