I do a lot of medical device design as part of my work, but also do a lot of medical simulation work for teaching our students and residents. So recently have gotten into 3D printing molds for casting large parts with torturous hollow openings inside (for instance the colon). I’ve been using polyvinyl-alcohol (PVA) as the interior mold core (and guess what, it is laser safe as far as I can tell since it’s basically a simple hydrocarbon) and is a unique plastic in that it dissolves in water but can be used as a thermoplastic. First example of this is making a colon that can be cut into. So I 3D printed the outside mold in PLA (this was a test segment in real live the segments will have coarse screw threads on the ends to allow me to assemble the colon in one chunk). The inner mold I printed in PVA (usually used as a support material), we poured dragon skin around it (a very lifelike silicone) and then placed it in hot water, then tested it with a colonoscope. Worked great, and for about $10 total. (note the “sprue” on the outside of the mold was deliberate as that is the mesentery of the ccolon). The cool thing is I imagine I can use the GF to add surface detail/texture that you could never print (or it would slow stuff down) on smaller items like bones, etc.