The notion that robots must be rigid metallic automatons made mobile by wheels, tracks or even legs has constrained the imagination of their designers. The weight of all those rods, gears and motors quickly adds up, and complex mechanical and electrical control systems are needed for robots to handle delicate objects or navigate across different types of terrain.

A team of researchers, including Harvard University chemist and materials scientist George Whitesides and Robert Shepherd, a postdoctoral fellow at Whitesides's lab, has eschewed this vertebrate-inspired approach in favor of a softer touch. Modeling their work on vastly more flexible, invertebrate squid, starfish and worms, Whitesides and his colleagues, earlier this week, reported online in the Proceedings of the National Academy of Sciences USA that a combination of elastic polymers and pneumatic pumps has supplied the parts list for a simple robot capable of complex motion.

How complex? Their five-centimeter-thick quadruped was able to crawl and undulate its way through a space just two centimeters high. (The researchers actually executed limbolike moves to navigate their bot underneath a glass plate elevated two centimeters above the ground.) The robot, which looks like a pair of Ys joined at the stem, was made using soft lithography in two layers. Soft lithography is an approach to fabricating objects that uses a patterned elastomer as the stamp, mold or mask, as opposed to the more rigid materials used in photolithography.

The most significant breakthrough demonstrated by this flexible robot is that soft materials can provide a solution to natural movement without the need for complex mechanical components and assembly. It also demonstrates the value of considering simple animals when looking for inspiration for robots and machines, the researchers say.

The shape-shifting robot's upper, flexible layer comes embedded with a system of pneumatic channels through which air could pass. The lower one was made of a much more rigid polymer. The researchers placed the actuating layer onto the strain limiting/sealing layer  with a thin coating of silicone adhesive. Air pumped into different valves in the upper layer caused them to inflate and bend the robot into different positions. For example, the robot could lift any one of its four legs off the ground and leave the other three legs planted to provide stability, depending on which channels were inflated.

The researchers are now exploring a variety of methods to design and make such robots autonomous. Onboard condensed-air cylinders and micro compressors are one route. "We will probably need to scale up the size of the robots a bit to support their load," says Whitesides, who is a member of Scientific American's board of advisors . "Additionally, our current tethered, soft robots can be coupled with hard robot systems to transport them to a location and support the load of the offboard cylinders and compressors."

In many applications, tethers are not a disadvantage, and in others, they are desirable or even required, the researchers say. "Remember, most robots—for example, those used in manufacturing—are fixed in place," Whitesides says, adding that autonomous movement is required for only certain tasks.

The researchers acknowledge that simple, inexpensive robots will probably not replace their more costly counterparts, but they could still have multiple uses. Robot-assisted mine rescues offer one possibility. In these, bots carrying cameras trek down narrow-diameter pipes hundreds of meters underground to search for survivors. Such robots are currently made mostly of metal and often become trapped in boreholes when cave-in aftershocks cause the ground to shift.

A potential disadvantage to these Gumbybots is that softer and more pliable material may rupture when moved across rough or sharp surfaces. Still, the researchers say that with the right mix of toughness and flexibility, they can develop robots that are cheaper to produce, lighter, able to be made big or small and much simpler to operate than their hard-metal brethren.

Advances in materials—polymers, in particular—will impact the development of soft robots by enabling them to operate in a higher pressure range, the researchers say. "We would also like elastomers that are tough, in the sense of being resistant to damage by cutting or puncture," Whitesides adds. "The area of soft robotics will provide many interesting problems for polymer scientists and materials scientists to work on."

Advances in artificial muscles would likewise assist in making these pliable robots more compact and provide more reproducible movement. "It would also allow us to mimic some of the very intricate designs to arms, tentacles or other structures directly," Whitesides says.