I’m attempting to make some very flexible lips for my models. I thought of using spline IK, but it has the draw back that I can’t curl them. Some models I’m making have muzzles and I want them be able be as flexible as one might see a Horse’s mouth. You know how they can curl their lips all way up and such. I haven’t found a way to do that as of yet and shape keys don’t seem to work to well with a spline when they get stretched.
You need to rig it with stretch to constraints following the topology or a circular type of mouth concentric rings. Here is a simple way. https://www.youtube.com/watch?v=Map_ro-xUwg
You can look at how it was rigged for the pitchipoy face which is now rigify in Blender.
Those should get you going.
If you want to get more advanced, take a look at Juan Pablo’s work with Blenrig.
You can always use a combination of bones or bendy bones and shape keys using drivers. Also you can use lattice movements like [Virgilio Vasconcelos] uses to move the lips like this (one of my favorites). 2.79 version not 2.8 (I haven’t tried it in 2.8 yet).
004-Lattice-complete.blend (2.8 MB)
I do remember this method. I’ll try it out to see if works well.