The design of many robots has been inspired by living creatures, from the humanoid machines that have appeared in science fiction for decades to the mechanical cockroaches that scurry around some research labs. There has even been a robotic tuna used to explore the ocean. But our reliance on the mechanical has left a very large area of the animal kingdom left out: soft bodied creatures with neither skeletons nor shells. In a paper that will be released by PNAS, researchers describe a soft-bodied robot that can crawl around lab, powered by compressed air.

The limits in robot design have been very practical. We don't yet have something that will mimic muscles well, which leaves our creations articulating their joints with things like gears and engines, which require a fairly rigid support structure. But the creators of this new robot were inspired by squid, which perform impressive feats of flexibility using a soft body that's supported by the ocean's buoyancy.

They figured they could skip the buoyancy requirement by using a tough elastomer that can stand up to the force of gravity while still retaining enough flexibility to move around. In practical terms, their work required two elastomers, one that was able to stretch when put under an appropriate force, another that would flex, but not stretch. A chamber that had these elastomers on opposite surfaces would flex in a specific way when the chamber was pressurized, with stretching on only one side causing it to curve in the opposite direction. A series of theses chambers linked together could create the sort of "muscle" that would propel their creation.

To create an actual robot, they fabricated a "tetrapod," with four limbs spreading out from a central body in a tall X shape. Each of the limbs could curve down when pressure was applied, and the "body" at the center of the X could either be held rigid or allowed to flex.

Before you get images of a giant, rubbery X shambling down the street, we'll point out that the robot here was only about 15cm (six inches) long. On the plus side, that allowed it to move using only about a half-atmosphere of pressure—about 10 percent of what you'd find in a typical bike tire. The pressurized air was supplied externally through a set of flexible tubes, but there doesn't seem to be a reason that a small pump couldn't be carried along, though it might take much longer for it to move.

The simplicity of the system also allowed it to be programmed with a remarkably low-tech approach: the movements were set up in a spreadsheet that was imported by the control software and used to control the valves that set the robot's pressure.

The authors were able to get it to move in two different manners. The first involved an undulation, where the two hindlimbs would curve and draw forward and then relax, pushing the robot ahead after the body and forelimbs also curled up. A separate algorithm flexed the legs one at a time to allow the robot to crawl. The authors were even able to get it to slide itself under a barrier with a low clearance, during which its entire body flattened out—not something you'd find easy to do with a traditional robot.

Right now, the response to pressure is limited by the fact that these materials aren't tough enough to keep the robot from popping under high pressure. That's something that can be changed but, even now, the soft, flexible robot is a challenge for traditional control systems. The authors were able to figure out how to send the appropriate commands empirically, but they say that the response to pressure is nonlinear, making it a poor fit for control software designed to handle simple joints and gears. Before we try building more complex soft-bodied machines, we're either going to have to master nonlinear control systems, or get a neural network to do the empirical learning for us.

PNAS, 2011. DOI: 10.1073/pnas.1116564108 (About DOIs).