https://www.cnet.com/news/3d-printed-transparent-skull-lets-scientists-see-a-mouse-brain-work/

A video released by the university shows a sped-up mouse brain scan as seen through the See-Shell. "Changes in brightness correspond to waxing and waning of neural activity. Subtle flashes are periods when the whole brain suddenly becomes active," the school notes.

The researchers digitally scan a mouse's skull and use the data to create a matching transparent piece using a 3D printer. The skull is then surgically replaced with the See-Shell. The mouse studied by the team did not reject the implant, which allowed them to monitor its brain over several months.