A close-up view of the Doodle Pro panel, after being removed from the frame. The honeycomb pattern that is usually visible is actually part of the same piece of clear plastic that the visible surfaces of the panel are made from. It's only about 1 mm thick, filled with a viscous white fluid, and a few black magnetic particles, which can be pulled towards either the front or back side of the display. Obviously this wouldn't work unless the fluid were remarkably opaque.
This is part of the Evil Mad Scientist Analog PlotBot project, which features a DIY E-Paper display! Read more about this project here.