Engineering 101

Bioinspired Soft Robots Harness Stored Elastic Energy

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Robotics researchers look for inspiration not only from the humans with whom they’re designed to interact, but also at other species. Amphibians, with their sticky tongues and stored elastic energy, can catch insects located up to one-and-a-half body lengths away within a tenth of a second. And some toads can catch up to 1000 insects a day.

Purdue FlexiLab researchers took a look at amphibians, birds, and the Venus flytrap to design a new class of entirely soft robots and actuators capable of re-creating bioinspired high-powered and high-speed motions using stored elastic energy. These robots are fabricated using stretchable polymers similar to rubber bands, with internal pneumatic channels that expand upon pressurization.

The elastic energy of these robots is stored by stretching their body in one or multiple directions during the fabrication process following nature-inspired principles. Similar to the chameleon’s tongue strike, a pre-stressed pneumatic soft robot is capable of expanding five times its own length, catching a live fly beetle and retrieving it in just 120 milliseconds.

“We believed that if we could fabricate robots capable of performing such large-amplitude motions at high speed like chameleons, then many automated tasks could be completed more accurately and in a much faster way,” Martinez said. “Conventional robots are usually built using hard and heavy components that slow down their motion due to inertia. We wanted to overcome that challenge.”

Many birds, like the three-toed woodpecker, achieve zero-power perching using the elastic energy stored in the stressed tendons at the back of their legs, allowing them to not fall off a perch when asleep. The anatomy of these birds has served as an example to enable the fabrication of robotic grippers capable of zero power holding up to 100 times their weight and perching upside down from angles of up to 116 degrees.

The conformability of the soft arms of these grippers to the gripped object maximizes contact area, enhancing grasping and facilitating high-speed catching and zero-power holding.

Some plants also know how to exploit elastic energy to achieve high-speed motion using “trap mechanisms.” The Venus flytrap uses the elastic energy stored in its bistable, curved leaves to rapidly close on prey exploring their inner surface.

Inspired by the trap mechanism of the Venus flytrap and studying how lizards catch insects, the Purdue team created a soft robotic Venus flytrap, which closes in only 50 milliseconds after receiving a short pressurized stimulus.

Martinez said these new pre-stressed soft robots have several significant advantages over existing soft robotic systems. First, they excel at gripping, holding and manipulating a large variety of objects at high speed. They can use the elastic energy stored in their pre-stressed elastomeric layer to hold objects up to 100 times their weight without consuming any external power.

Their soft skin can be easily patterned with anti-slip microspikes, which significantly increases their traction and enables them to perch upside down over prolonged periods of time and facilitates the capture of live prey.

“We envision that the design and fabrication strategies proposed here will pave the way toward a new generation of entirely soft robots capable of harnessing elastic energy to achieve speeds and motions currently inaccessible for existing robots,” Martinez said.

Source: Purdue University

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