Optogenetics provides a unique opportunity to merge materials engineering with neuroscience. Shown are three flexible, wirelessly powered microscale light-emitting devices juxtaposed with needles. Each device activates opsins at different wavelengths and can interface with deep-brain tissue by means of a variety of cellular-scale physiological sensors. Image from the protocol by McCall et al. doi:10.1038/nprot.2013.158. Cover design by Jamel Wooten.