Actuator Design

Balloons of the type used for twisting animals and other fun things for kids seemed the clear choice of material for the inner bladder. The 350 type balloons (3 inches wide, 50 inches long) were chosen over the 260 type (2 inches wide, 60 inches long) most commonly used for twisting balloon shapes. The 350 balloons felt more durable than the 260s; the latex may have greater wall thickness. These balloons are easily available and suited the purpose well. 

The selection of the reinforcing material, however, took considerable trial and error. PVC tubing of various sizes and types, and even latex reinforced work gloves were tried. The standard PVC tubing split easily, while the braid reinforced type was too hard to bend. The gloves did not provide enough structure. In the end, what worked best was a ribbed plastic hose that came from an inexpensive foot pump. This is the blue hose shown in the picture below.

Different materials tried for the actuator

The hose has a slight natural bend to it, similar to a human finger when held at rest. Stress relief holes are used on each side of the hose between each rib to reduce the chance of the hose splitting. Slices between each rib along the upper side of the curve, extending to the stress relief holes, allow the finger to bend when the balloon is inflated.

Ribbed hose with cuts and stress relief holes

The tip of the finger is blocked so the balloon cannot expand in that direction. So when the balloon is inflated with pressurized air, the only way the balloon can expand is by bending the finger.