I admit these aren't very good, but it's kinda hard figuring out how to do the effect in the first place. All three are from the top image, with the inside of the mouth starting off as transparent. As this is just my first attempt, I'm gonna try to do something better later on. Let me know your thoughts and if you've done any, let's see them!
Sorry, deleted pictures
I of course cannot take credit for the process as it's far too complicated for me and extremely math-based (fractals and such). I used Josh Sommers'code to do this. If you're interested in making your own, check out his stuff (he's AMAZING and such an inspiration for anything abstract - he also does fantastic texture wrapping)