Insects, whether they creep or fly, live in a world of hard knocks. Who has not stepped on a cockroach, then raised her shoe to watch the creature get up and scoot under a door? Bees and wasps, for their part, face a never-ending obstacle course of leaves, stems, and petals—bumblebees crash their wings into obstacles as often as once a second. Now, researchers are learning how these creatures bend but don’t break.
The results do more than explain why cockroaches are so hard to kill. By mimicking the combination of rigid and flexible parts that gives insect exoskeletons and wings their resilience, biomechanicists are making robots tougher. “Bend but not break is a lot of what happens in these insects,” says Harvard University roboticist Robert Wood. “We’re trying the same thing to see if we can have similar robustness in our robots.”
Until recently, most engineers designed for a tough-and-tumble world by making machines stiff and sturdy or agile enough to avoid danger. Modern cars incorporate a third approach: They absorb impacts by crumpling, sacrificing the structure to protect the occupants. “Nature has come up with a tactic that we don’t have,” says David Hu, a mechanical engineer at Georgia Institute of Technology in Atlanta. “Crumple … and then keep on going.”
To see how cockroaches do it, integrative biologist Robert Full at the University of California (UC), Berkeley, and Ph.D. student Kaushik Jayaram coaxed the insects through ever smaller slits or tighter tunnels while filming them with a high-speed video camera. They also lowered weights of up to 100 grams onto different parts of the insects’ bodies and watched how the creatures collapsed.
Full and Jayaram found that when the 9-millimeter-tall Periplaneta americana approaches a slit no more than 3 millimeters high, the roach first inspects the opening with its antennae. Then it jams its head through, follows with its front legs, and begins pulling the rest of its body into the breach. The back legs splay but continue to push. In about 1 second, it emerges on the far side unscathed. That ability to squeeze through a tight spot “goes far beyond any other animals that we have measured, except maybe the octopus,” says Stacey Combes, a biologist at UC Davis. But an octopus—a model for the “soft” robots some designers are pursuing—can’t match the speed of a cockroach or other arthropods. “Not only insects, but crabs, spiders, and scorpions are pretty good at going anywhere and are pretty indestructible,” Full says.
Jayaram and Full’s study, published this week in the Proceedings of the National Academy of Sciences, showed that the cockroach’s secret lies in a hard but still flexible exoskeleton. It consists of hard yet bendable plates—capable of efficiently transmitting energy to its legs—connected by elastic membranes that allow the plates to overlap as the insect compresses. Thanks to spines that give traction when its legs are splayed, a cockroach can scuttle even at maximum scrunch.
At a meeting of the Society for Integrative and Comparative Biology last month in Portland, Oregon, Harvard postdoc Andrew Mountcastle reported that a similar blending of hard and soft parts enables bees and wasps to survive their aerial obstacle courses. Using high-speed video, he found that wasp wings actually buckle during collisions and then snap back into place. He also noticed that the wings have a big patch of an elastic protein called resilin about 65% down the wing. He and Combes hypothesized that the patch serves as a hinge.
To test the idea, Mountcastle developed a way to mount a wasp on a rotational motor and hit the wing over and over. “He showed the wing can pop out many, many times,” Hu says. When Mountcastle splinted the hinge so the wing couldn’t buckle, the wing quickly wore down. He and Combes also found that many insects have a similar hinge, but that bumblebee wings incorporate a different design principle. The veins that support the bee wing are concentrated close to the body, resulting in a flexible wingtip that can bounce off obstacles with less wear and tear. “It’s different means to the same ends,” Mountcastle says.
Both the roach exoskeleton and the insect wings are inspiring robot design. Jayaram has built a 75-millimeter-tall robot, called CRAM, with a roachlike collapsible exoskeleton and legs with “spines” that work both in the uncompressed and compressed positions. It can squeeze to one-half its height and still move 5 to 10 times faster than soft robots, Jayaram says. “What is exciting is that this gives us an order of magnitude reduction in voids where we can deploy robots,” says Robin Murphy of Texas A&M University, College Station, who specializes in robots for disaster search and rescue operations.
Mountcastle has joined forces with Jayaram—now also a Harvard postdoc—and Wood to refit hinged wings to an insect-sized flying robot called Robobee. “Designing [it] was not trivial; they are not simple, linear hinges,” Mountcastle said at the Portland meeting. The group hopes begin testing the new design in the real world by spring.
Hu applauds the insect-inspired designs: “It would be great to see more robots built with potential damage in mind.” As for killing cockroaches—Jarayam suggests slamming that shoe hard and holding it down.