A fiber-optic cable is a bundle of thousands of light-carrying glass threads. A start-up called Illumina in San Diego plans to put a test tube on the tip of each.

Born in the Tufts laboratory of chemist David Walt, the scheme uses hydrofluoric acid to etch a dimple at the end of each fiber. The teensy wells-each holds just a billionth of a microliter-can be filled with reagent-bearing beads or cells. Expose the bundle end to a patient's blood or to a test chemical that induces a light-generating reaction and each test tube sends in a report via its fiber. Illumina is betting that the lab-on-a-tip will speed diagnostic tests, chemical sensing and genome experiments.