Matthew Borgatti has built an air-powered, 3D-printed robot tentacle that waves in a friendly fashion and lends a helping hand. It is in no way erotic. Nuh-uh, not at all.
So, with a very nice looking tentacle in hand, it was time to start experimenting with robotic air control. I believe I've found a system that works in a pretty simple and straightforward way. It still needs some work when it comes to the programming end, but I think the mechanics are well sorted. The idea is to pulse air into the tentacle using a solenoid valve, and have a constant bleed on the line so that flex will entirely be controlled by how long the valve stays on. It's sort of a low frequency PWM. I'd like to get this working using a visual interface in Processing but, given how little I program, progress has been slow. I've got a thread on Adafruit with what I've come up with. In the meanwhile, you might like to check a rough video of the trefoil inflating.