My name is Wyatt Felt, I'm a Junior in Mechanical Engineering at BYU.
When I saw the Harvard Soft Robot wiggling around I was impressed at the simplicity of the actuation. I liked how the legs re-extended with their own elasticity. I wondered whether the idea could be scaled up for skeletal systems.
I decided to make these uber-cheap, pull-type actuators out of twisty balloons. While there are still some questions of stress relaxation and control, these actuators work at least on a small scale. In theory, they can have a much greater stroke length than either air muscles or pneumatic cylinders.
It was fun to see the tech teachers at Robogames interested in building them for their classes. If you already have an Arduino and power supply the whole actuator system can be done for under $50.
You'll need (what I used)
Arduino or other micro-controller Arduino Uno
NPN Transistors (2x per actuator, 1x per pump) TIP29C (with 1A current limit)
Micro-solenoid valves (two per leg) AJK-F0504 though I could've used The Jameco one
Small pump AJK-B2703 though Ponoko has one
Leg material Flexible plastic (PETG) or an antagonizing spring (another balloon)
Frame 5.6mm Acrylic though you can use thinner
flexible tubing, with barbed tees and connectors Home Depot (see pictures)
High current 7v power supply or batteries
Small breadboard or Prototyping shield
Hot melt glue
Thin gauge wire
Heat shrink or electrical tape