Soft robots take cues from the wild kingdom
Soft robots are built with compliant materials, such as those that give organisms in the wild the flexibility to navigate and adapt to their environments — compliance meaning the ability to yield elastically and deform shape in response to an impediment or force. Take the example of a soft-bodied invertebrate like the octopus — no robot is capable of performing similar motions, including escaping from inside a bottle by squeezing its body through the hole. This lack of compliance in robots, based on their interconnected rigid links, also limits their ability to handle fragile objects.
To provide flexibility, soft robots are fabricated with stretchable polymers and rubbers to achieve motions that conventional robots can’t realize because of their rigid skeletons. Absent a skeleton, a soft robot can conform to the surface of any object — for example, wrapping its structure along the surface of the object to grasp it. The soft structure also enables a soft robot to withstand large stretches and squeezes before it breaks, and to interact with humans –say, in home healthcare applications.
My research group, the FlexiLab at Purdue, has developed a new class of soft robots and actuators that re-create “bioinspired,” high-powered, high-speed motions using stored elastic energy. The robots are fabricated with stretchable polymers similar to those in rubber bands, with internal pneumatic channels that expand upon pressurization. A robot’s elastic energy is stored by stretching its body in one direction or multiple directions during the fabrication process, following nature-inspired principles.
Various animals offer models. For example, taking a cue from the chameleon’s tongue strike, a pre-stressed pneumatic soft robot can expand five times its own length, catch a live beetle, and retrieve the insect in just 120 milliseconds.
Many birds, including three-toed woodpeckers, achieve zero-power perching while asleep by using the elastic energy stored in the stressed tendons in the backs of their legs. The anatomy of these birds has bioinspired us to fabricate robotic grippers that use zero power, and are capable of 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 the contact area, enhancing grasping and facilitating high-speed catching and zero-power holding.
It’s not only animals that exploit elastic energy to attain fast motion using “trap mechanisms.” The Venus flytrap plant uses elastic energy stored in its bistable, curved leaves to rapidly close on prey that are exploring its inner surface. My research team created a soft-robotic Venus flytrap that closes in only 50 milliseconds after a short pressurized stimulus.
These soft robots 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 patterned with anti-slip microspikes, which increase their traction and enable them to perch upside down over prolonged periods of time.
In mirroring nature, these design and fabrication strategies pave the way for a new generation of soft robots capable of harnessing elastic energy to achieve speeds and motions inaccessible for existing robots, completing many automated tasks faster and more accurately.