Tiny building blocks: A small vial contains a billion viruses, each with a slightly different genetic modification. These can be screened to determine which of them will bind to specific inorganic materials, such as those used in rechargeable batteries. Credit: Porter Gifford

Angela Belcher leans in to watch as a machine presses down slowly on the plunger of a syringe, injecting a billion harmless viruses into a clear liquid. Instead of diffusing into the solution as they escape the needle, the viruses cling together, forming a wispy white fiber that's several centimeters long and about as strong as a strand of nylon. A gradu­ate student, Chung-Yi Chiang, fishes it out with a pair of tweezers. Then he holds it up to an ultra­violet light, and the fiber begins to glow bright red.

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