A couple of years back, I had an Idea - I don't recall whether I shared here, but since there's no prospect of my pursuing it, I'll go ahead and describe it.
The idea, see, is that you'd engineer strains of bacteria that (a) could live just beneath the surface of human skin, and (b) were bioluminescent, in a variety of colors.
Now put little colonies of them in itty-bitty cages so they'll stay where they're put.
Glow-in-the-dark tattoo ink!
And now... someone's gone and put bacteria in cages.
Though I'd assumed the cages would look more like buckyballs and less like serving dishes for monkey brains.