You could be forgiven for thinking that the toughest, most durable robot would be some sort of ultra-hard, metal alloy skeleton like the Terminator, but sometimes soft and flexible is the way to go.

This week, engineers at the École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland published the results of their efforts to make a really neat, flexible quadcopter drone in the journal Science Robotics.

Like a growing number of microrobots currently in development, these drones also borrow some of their design elements from the lightweight folded architecture of origami. But, the researchers tell Inverse, with one key difference: The design, a new composite that’s flexible enough to absorb damage, but rigid enough to remain aerodynamic, and was inspired by the material properties of insect wings.

Drone Quadcopter Flexible Rigid New Insect Wings EPFL Stefano Mintchev
The EPFL team has designed the drone's wings with a tailored combination of stiff (external) and elastic (internal) layers.

“Most origami structures are made of rigid laminated materials and rigid joints (i.e., inextensible joints made of polyimide or nylon), which leads to limitations,” says Stefano Mintchev, a postdoctoral researcher at the École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland. “Rigid origamis are fragile, prone to tear, and easily fail when overloaded during collisions.”

Zhi Ern Teoh, an unaffailiated Harvard-trained mechanical engineer who has developed origami-inspired microrobots of his own, told Inverse he was impressed with the Lausanne team’s work.

“Because the main wing spar is completely rigid, when it hits an obstacle, the wing can break,” Teoh explains. “What they’ve done with regard to tailoring the ‘threshold force’ — the threshold at which the switch from rigid to soft occurs — I think that’s pretty key for structures that need to withstand impacts.”

A demonstration of the EPFL quadcopter's impact resistant flexibility.

At the core of the Swiss group’s innovation is a stretched-out elastic layer, sort of like a rubber band that desperately wants to return back to form. This layer is then surrounded and adhered to a more rigid, segmented exoskeleton. It’s kind of similar to one of those push-up wooden puppets that droop and collapse when you loosen the string inside them. Upon impact, the drone bends at its segmented joints, with the elastic layer stretching. Then, it springs back into shape ready to continue flying.

Of course, the construction can’t be too flexible either, and finding that delicate balance is where the EPFL team’s materials engineering expertise came in.

“If the threshold force is too low,” Teoh says, “the quadcopter won’t be able to take off, right? Because the moment you turn on full thrust, the thrust is gonna collapse the structure.”

You could imagine a bunch of things happening: the propellers could crash into each other; they could draw the wings in separate directions, basically drawing and quartering the drone. It would be, to be frank, a mess. Finding the right tensile strength, i.e. the threshold at which bending would be desirable, took studying similar features in insect wings, according to Mintchev.

“The challenge of strategically tailoring rigidity and softness in foldable structures is mastered by insects,” says Mintchev. “[Their] evolved origami wings are composed of rigid tiles of cuticle connected through soft resilin joints.”

A flexible gripping claw made with the same "bilaminate" composite of rigid and hard materials as the Swiss group's quadcopter.

Mintchev and the group see other applications for this design too, including more flexible gripping mechanisms, which they have already built prototype of.

Some of the benefits to this design include a grip that would be less likely to break something fragile that it’s attempting to pick up, and a grip that would be incapable of lifting something beyond its capacity (only to risk dropping it, later on).

“The current trend in robotics is to create softer robots,” according to Dario Floreano, the director of the EPFL’s Laboratory of Intelligent Systems and another co-author on the new paper, “that can adapt to a given function and operate safely alongside humans.”

Being gentle, in its own way, is a sign of strength — something that hard-ass Terminator robots should take note of.