Biologically inspired soft robots make a lot of sense in a lot of different scenarios, but as with any class of technology, they have their limitations. Among other things, these compliant structures can struggle to move at the same speed as their more rigid counterparts. To help speed up swimming, a team of researchers at North Carolina State University turned to another bit inspiration from nature: the manta ray.

Looking toward the fish makes sense. These cousins of sharks effectively fly through water courtesy of massive, wing-like pectoral fins. Ultimately, however, the researchers named them after a different animal altogether. The “butterfly bots” are so deemed for their resemblance to the arms of humans performing the butterfly stroke [see: the above image].

Two versions of the robot were built. The first was designed specifically for speed, with the ability to move 3.74 body lengths per second — a big increase over soft robots that struggle to move a single body length in that time. A second, designed for more control, moves around 1.7 body lengths a second. In the first case, the design makes turning an issue. With the second, the team added a second drivetrain to turn the robot by flapping a single wing at a time.