A bellow. Usable as damper/spring/hinge. Ends are more massive for predictable behavior and have indents for reversibly locking it into a counterpart with pieces of 3mm filament for mounting.

Design is optimized for air-tightness and has access holes, but it's a bit too stiff to act as pneumatic actuator. It has different (even more nonlinear) spring character with the holes air-tightly covered (air-spring). It is short enough such that compression induced buckling is not an issue.

More details in the *.scad file.

There's also a round version: "Experimental Circular FlexBellow" https://www.thingiverse.com/thing:2949748