Anyone (old like me) remember the Visible Man and Visible Woman models? Maybe they weren't as far fetched as you might think... Anyway, the substance is a common food dye: the one that gives Doritos their orange color...and the skin is on a mouse. Then, using light of a certain frequency, you can see through the skin to the underlying structures.  There is a depth limitation of a few millimeters, but still...this opens up avenues for further research. Imagine Doctors could image in...