What's so great about starfish?
You probably don't have too many thoughts one way or the other about starfish. They're pretty common benign sea creatures. It's rare they terrorize an eastern seaboard and they seldom get on the news. Yet, they've got something utterly stunning that can guide us to making better robots: no blood.
That's right, the humble starfish has no blood coursing through its body. It does have a circulatory system, but it's swishing around pure sea water.
It uses this water vascular system to power anywhere from 1000-100,000 tube feet, which are responsible for most of the creature's locomotion. Water can be blasted out of these feet to quickly burrow into lose sand. The pressure can be reversed by engorging structures above these feet called ampullae, which can clamp a starfish so firmly to a clam's shell that it can pry the bivalve open through brute force. This selective pressure differential also allows it to climb up sheer surfaces like the glass front of an aquarium.
What does this mean for a soft robot? If soft robots eventually end up taking over complicated tasks, chances are they'll be pretty complex themselves. In a traditional electrically controlled robot this would mean more wires in the system relaying signals place to place. In a pneumatic soft robot, however, this means more lines carrying air. To keep air friction down to a minimum and not lose power along the way, air lines have to take up a certain amount of space, which could end up with huge bundles of air lines making complicated molds, cores, and increasing assembly time for each robot made. However, it might be possible to take cues from nature and simplify the structure by adding valves right into the robot.
The starfish uses sequences of low power valves to direct water through its arms and into its tube feet, controlling the complex and powerful structure through a series of small low energy changes. We're hoping to capture the essence of these low power hydraulic valves to get robots that have grippy arms that will always grip as long as there's power in the system, or can oscillate power between a set of chambers without needing an external power source controlling the chambers' timing.
This paper by R. Skyler McCurley and William M. Kier examines some of the intimate structures underlying the starfish's amazing feet. Although dense and somewhat technical, it's a valuable resource for anyone thinking about soft engineering.