Soft Robots Get A Grip

A high-profile chemistry journal is hardly the place you’d expect to find a paper about robotics. But regular readers of Angewandte Chemie recently got a serving of robot science alongside the journal’s smorgasbord of more traditional chemistry fare.

So what does robotics—a field that conjures up images of whirring electric motors and shiny stainless steel components—have to do with chemistry? Well, robotics, it seems, has a soft side. Chemists at Harvard University have created a so-called soft robot that’s capable of gripping and lifting a raw egg without cracking its delicate shell (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201006464).

The device, which looks like a six-legged starfish, was dreamed up by chemistry professor George M. Whitesides and coworkers Filip Ilievski, Aaron D. Mazzeo, Robert F. Shepherd, and Xin Chen. It is composed of just two kinds of siloxane elastomers, has no hard components, and is powered by air.

The biggest challenge in creating this robot was finding the right material, Whitesides says. The researchers needed an elastomer that could stretch to a large degree without breaking, but they also needed something that was rigid enough to provide support for the structure. They solved the problem by sandwiching a layer of rigid polydimethylsiloxane (PDMS) between two layers of the superstretchy siloxane elastomer known as Ecoflex.

Channels of pneumatic networks, dubbed PneuNets, embedded in the elastomers are the key to getting the device to move. When the researchers push air into the PneuNets between the PDMS and one layer of Ecoflex, the air channels inflate like balloons, expanding the stretchy Ecoflex layer and causing the gripper’s “fingers” to bend. Pumping air between PDMS and the other layer of Ecoflex makes the gripper bend in the other direction, so it can move between convex and concave states.

“You can use these same kinds of structures to make things that walk and crawl and slide in interesting ways,” Whitesides notes.

In the case of the gripper, the soft structure distributes the load of the object it is grabbing over its entire surface, rather than focusing it at a few points. This makes it ideal for handling fragile objects. In addition to the egg, the team also shows that the gripper can lift an anesthetized mouse without breaking any of the creature’s fragile bones.

“This paper is stunning in terms of hierarchy of design,” comments Ralph G. Nuzzo, a chemistry and materials science professor at the University of Illinois, Urbana-Champaign. The robot’s structure of closed cells, thin and thick walls, and stiff and flexible parts all work in concert to perform a task that would be extremely complicated to do with a hard-bodied gripper, he points out.

Cecilia Laschi, a biorobotics professor at the Scuola Superiore Sant’Anna, in Italy, agrees. “One of the major open issues in soft robotics is actuation,” she explains. Among the most highly developed soft robotic actuators, for example, is a pneumatically driven device made of a soft bladder covered in a shell of braided, strong, inextensible fibers. Such actuators, however, can only respond by expanding or contracting when pressurized.

“Unconventional actuators are needed to embed in the compliant materials, without introducing stiffness in the system,” Laschi says. “The solution proposed in this paper presents a nice idea for responding to the need for actuators in soft robotics.” The PneuNet actuators, she adds, can be applied to robots that make different movements by different mechanisms.

“The work presented here is exciting not because of a fundamental scientific advance, but rather because of the insight of the authors in using conventional technologies to produce extremely novel soft and active devices,” adds Jonathan Rossiter, an engineering lecturer at England’s University of Bristol. “There is a sense of organic beauty in these structures and, indeed, the biologically inspired nature of this work results in compact and effective mechanisms which would be difficult to design from scratch.”

Although Whitesides says soft grippers have many possible applications, such as performing surgical tasks or doing the delicate work of disarming explosives, he notes that the Angewandte Chemie article was really intended to be an invitation to chemists.

“The opportunity to think about the materials science of soft-bodied robots is real­ly wide open,” Whitesides says. “There’s a very interesting set of problems that has to do with robots, but there’s a very interesting set of problems that has to be coming from chemistry and chemical materials, which has to do with the soft materials that go into these soft robots.

“There are no standard skeletons, and form is not given by a series of stainless steel rods; it’s given by the polymers or organic materials themselves,” he continues. “Finding materials that fit into this class of structures is just an extremely interesting new class of opportunities for people who like to make elastomers.”

It’s easy to make structures with PneuNets, Whitesides explains, so he hopes designs such as the gripper will provide a test bed for chemists to try out other materials for soft robotics. “The charm of it from our point of view is that it provides a starting point,” he says. “In any new area you need to have a simple system that people can get involved with.” Once folks start tinkering with this simple system, he says, their imaginations will build from there.

Whitesides wonders, for example, how these systems would work with superstretchy membranes or electrically conductive elastomers. “There are just a gazillion things to be done,” he says.

“I think it’s a really interesting way to challenge materials design,” University of Illinois’ Nuzzo says. “Chemistry isn’t all about making and breaking covalent bonds. It’s the characteristics of chemistry that create the property of elasticity.”

“One of the characteristics that I like about this project is that you don’t think of chemistry as being involved in robotics,” Whitesides says. “But chemistry is in a position to play a very important role in this, or it’s in a position to ignore it. My argument is that chemists should look at these kinds of opportunities and see if there isn’t something there they can make a contribution to. It’s good for chemistry. It expands what can be done with the field.” Plus, he says, making robots is “just enormous fun to do.”