For many years, silver chloride was the gold standard of photographic printing. The chemical's high pho-tosensitivity made basic imagingpossible because it darkens when exposedto light. Now, the material that moved theworld from sepia tones to classic blackand white may presage the next evolution-ary stage for science and technology. Sound like hyperbole? The goal of many researchers over thepast few years has been to close the gapbetween real life and science fiction: Cre-ate nanoscopic materials that self-assem-ble into fully operating systems such asminuscule robots that sweep a room cleanof allergens or that patrol your blood-stream looking for cancer cells. Asidefrom engineering issues involved in build-ing automatons only a few nanometersacross, there are a few essential design is-sues that are particularly perplexing. Oneof these concerns is communication — toperform any task, the individual machinesin a cloud of nanorobots must be able totalk to each other.
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