Previously I have made artificial muscles using a 3D printer by printing molds and casting silicone in them. See here:http://www.instructables.com/id/3d-Print-An-Artifi...
That is a fairly involved method and does not produce the same precision as directly 3d printing.
This is a more direct method where the muscles themselves are directly 3D printed using a flexible filament called Ninjaflex. The printing pattern of a 3D filament printer leaves many microscopic holes that will not hold air pressure. So, after printing, the muscles are dip-coated in an flexible elastomeric glue to seal the holes. This allows them to hold air pressure of 22 PSI or higher.
I used a Makerbot Replicator 2 to make these muscles, but other filament printers that can print Ninjaflex can be used.
This is still at the early stage of experimentation, but it shows great promise for producing muscles that can replace servos and gear-motors in robots. This will make them considerably less expensive and when used in soft robots, make them more human friendly to touch and be around.
A Stereo-lithography printer using flexible resins, would make it possible to produce smaller and more precise types of muscles and robot skin that a filament printer cannot. More intricate air channels and unusual shapes that do not need supports could also be printed. A flower robot like this could be printed assembled in one piece.
Step 1 pic shows the flower opening when all six muscles are pressurized at 22 PSI.