Optogenetics has been hailed as a breakthrough in biomedical science—it promises to use light to precisely control cells in the brain to manipulate behavior, model disease processes, or even someday to deliver treatments.

But so far, optogenetic studies have been hampered by physical constraints. The technology requires expensive, bulky lasers for light sources, and a fiber-optic cable attached to an animal—an encumbrance that makes it difficult to study how manipulating cells affects an animal’s normal behavior.