As researchers learn more about the brain, it has become clear that responsive neurostimulation is becoming increasingly effective at probing neural circuit function and treating neuropsychiatric disorders, such as epilepsy and Parkinson’s disease. But current approaches to designing a fully implantable and biocompatible device able to make such interventions have major limitations: their resolution isn’t high enough and most require large, bulky components that make implantation difficult with risk of complications.