Here is a soft walking quadruped robot that doesn’t need any electronics. It is powered by pressurized air. Engineers at the University of California San Diego are behind this 4-legged robot. It uses a light-weight, low-cost system of pneumatic circuits made up of tubes and soft valves.
This robot has 3 valves acting as inverters. As the researchers explain:
The legs are angled downward at 45 degrees and composed of three parallel, connected pneumatic cylindrical chambers with bellows. When a chamber is pressurized, the limb bends in the opposite direction. As a result, the three chambers of each limb provide multi-axis bending required for walking.