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"
Either print with a slow direct drive or
On an Ultimaker you'll need an upgrade to a remote direct drive
like e.g. Flex3Drive / Zesty Nimble / DIY
Testprinted with TPU (brand filaflex) at 235°C (Ultimaker Original temp.sensor)