It could be the world's tiniest EEG electrode cap, created to measure activity in a brain model the size of a pen dot. Its designers expect the device to lead to better understanding of neural disorders and how potentially dangerous chemicals affect the brain. This engineering feat, led by Johns Hopkins University researchers and detailed today in?Science Advances, expands what researchers can accomplish with organoids, including mini brains--the lab-grown balls of human cells that mimic some of a brain's structure and functionality.