I'm looking at this copy of Technology Review It has a robot that has gone through many iterations of face to be appealing to humans yet the face is made of plastic with mechanical expression changes
They emphasize that robots must give social feedback to be truly useful thus robots with expressive
faces speak to the supposed "most" of human communication that is nonverbal
I think that just laser drawing different expressions on the robot would create an even more rapid way to prototype the interactions with humans (basically they have these robots communicate with humans then move the appearance structure towards likeability plus utility) as a way to research then find higher amplitudes of appreciation of nuanced communication
I know its obvious
Yet they haven't done it yet n MIT has been doing robot faces since the 90s or earlier
If you just had to mention it I think there are likely UV compounds that like LCD change transmissivity with UV thus use a laser internal to the head thus creating halftonable skinlike appearance Perhaps a tripolarized light emitter just like color LEDS could do this as well with ordinary nonelectric liquid crystal
also with a UV laser the face could have expression plus turn slightly authentic (fluorescent) pink or have iris dynamics or makeup all of which are known to affect human perception of other humans
a vector laser behind the face drawing expressions seems easier than a UV responsive raster fabric behind the face