Hydraulic actuators work the way they do because the fluid under pressure performs uniformly: move a piston 50% of the way along its length, and the connected arm, leg, or whatever, will also move 50% of the way it’s built to.

This robot, built by researchers at Cornell University’s College of Engineering, uses a different approach. By using viscous fluid, which behaves unevenly, the team were able to get the actuators in the robotic legs to move more smoothly. The difference is akin to that between a jointed limb on a mammal and the movement of a caterpillar or worm; the clever thing is that by utilising the properties inherent in a viscous fluid, the hope is that robots using these actuators will need less computing power to perform quite complicated movements.