the answer is, one is enough!

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with a twist, you can open the door. This doorknob is made of only one 3D printing part.

compared with traditional doorknobs, the parts are still much simpler:

you can also make a "clamp" in a similar way, which does not require any connecting parts, and the whole thing is printed at once:

these demos are from the Haseo Platner Institute of Software (Hasso Plattner Institute). These structures are printed out of silicone, and the degree of movement of the parts is controlled by the printed structure: the thinner grids are movable parts that can be deformed when the doorknob is twisted, while other thicker parts retain shape. By combining these two structures, you can avoid the trouble of late assembly and directly print out the components that can be active in a specific way at once.

they also made a magic gadget with four legs:

by the way, the researchers also put the software they used to design these 3D printed objects online. If you are interested, you can read here: https://jfrohnhofen.github.io/metamaterial-mechanisms/

Source:

https://hpi.de//en/baudisch/projects/metamaterial-mechanisms.html

https://www.dezeen.com/2016/09/27/metamaterials-mechanisms-design-3d-printed-plastic-door-handle-hasso-plattner-institute-technology/