Researchers at MIT CSAIL developed a 3d printing process named "printable hydraulics" where you can 3d print with soft and hard materials at the same time. This creates articulated objects which can move when pressure is applied.
They demonstrated it by 3d printing an insectoid robot that moves when motor and battery is added. Other robotic accessories such as hydraulic grippers can also be 3d printed.
Hydraulic hexapod in action with other flexible parts demonstrated:
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