These artificial robot muscles are made by laminating thin layers of Oogoo which is a silicone rubber that can be cast in any thickness. For info on Oogoo see here: https://www.instructables.com/id/How-To-Make-Your-Own-Sugru-Substitute/
An artificial muscle segment is cast in two layers with a thin piece of plastic between the layers to allow for air expansion. This creates a simple air bladder that can glued together in many segments with spacers to create curving or straight robot muscles.
The step one pic shows a 3d illustration of a six segment silicone robot muscle. Ideally, an artificial robot muscle like this could be directly printed in one piece. I have not found a 3d printer service that can print silicone or rubber at an affordable price. So, for now, I can only do the illustration and must use hand techniques to create the muscles. I am looking into the 3d printing of plastic molds that could be used to cast the Oogoo robot muscles in one step.
123D was used to create the illustration.