The skull, made by an Australian firm, is actually fairly transparent. Lots of plastics can be transparent - acrylics, polycarbonates, etc. - but the stringent medical requirements, and also print requirements, place limits on the possible. While the transparency may be incidental, the picture above shows the clear advantage of such transparency: one can see the underlying brain and vasculature. Not only is this a nice feedback to see how things are going macroscopically, it also entices with potential to optically image activity in the brain like never before.