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A soft robotic gripper holds a flask of orange juice. 

Sukho Song

Gecko-inspired gripper could help robots climb walls

If you want a robot to pick up a coffee cup, a cherry tomato, or a bag of packaged food, you’ll have to deal with a lot of programming and quite possibly some cleanup in aisle 3. A child can handle all kinds of shapes and materials, and apply the right combination of force and delicacy when lifting them, but getting machines to reproduce these seemingly simple skills hasn’t been easy. Now, a team of researchers has developed a new way to pick up objects that solves many of these problems. Modeled on geckos’ toes, it could also help robots climb irregularly shaped walls.

Engineers have long sought inspiration from nature, and one of nature’s most inspiring specimens is the gecko, that fearlessly climbing lizard with the power to stick to walls and ceilings. These feats rely on Van der Waals forces. At any moment, atoms tend to be slightly positively charged on one side and negatively charged on the other, as a result of the random behavior of their electrons. When two atoms come close to each other, Van der Waals forces can generate an attraction. A gecko’s toe pads are covered with tiny, hairlike fibers that maximize its contact with surfaces, amplifying the Van der Waals effect.

Scientists have created materials that exploit synthetic arrays of microfibers to replicate geckos’ stickiness, but there’s a trade-off. Making geckolike materials stick well takes pressure, which requires a rigid backing like a paddle. But such a backing in turn prevents these materials from adhering to curved surfaces. The new research avoids this tradeoff, offering both flexibility and grip, by arraying microfibers on a thin, stretchy membrane to create what the researchers call fibrillar adhesives on a membrane (FAM), and providing a new type of backing for it. 

In the soft robotic gripper, the FAM covers the wide opening of a shallow funnel made of soft rubber, 18 millimeters across, with the narrow opening connected to an air pump. After the FAM touches a flat or curved object, air is pumped out of the funnel, flattening the shallow cone against the object, whatever its contours.

In tests, researchers found that with a contact area of only 2.5 square centimeters (about the size of a dime), their gripper could lift objects weighing more than 300 grams (close to the weight of a can of soda), they report today in the Proceedings of the National Academy of Sciences. It could also pick up a coffee cup from the convex outside, the concave inside, or a point on the handle; lift a cherry tomato without harming it; or grab a plastic bag of packaged food. Inflating the gripper releases the objects.

“This is a very nice contribution to a field which in its early days was just focused on the nanoscale fibrils rather than the backing,” says Kimberly Turner, a mechanical engineer at the University of California, Santa Barbara (UCSB), who was not involved in the research. “The design of the backing is key to making these adhesives function properly for most applications, and this is a very exciting development.”

The technology has several potential applications, says Metin Sitti, an author of the study and a mechanical engineer at the Max Planck Institute for Intelligent Systems in Stuttgart, Germany, and Carnegie Mellon University in Pittsburgh, Pennsylvania. In factories, such grippers could help assemble delicate electronics or move around objects with complex shapes such as custom car parts. They could also be used in biomedicine for picking up organs. And they would enable robots to climb everything from airplanes to nuclear plants, for inspection and maintenance.

Still, the gripper must meet several requirements before it can be used widely, says Elliot Hawkes, a mechanical engineer at UCSB and Stanford University in Palo Alto, California, who has also worked on geckolike grippers. It must be durable (able to lift and release things hundreds of thousands of times), scalable (able to lift things heavier than a kilogram), and worth the cost relative to other types of grippers such as clamps or suction cups.

Sitti sees no obstacle to making bigger grippers, tens of centimeters wide, and using many of them in concert to grab and lift heavy objects. The team still needs to test the gripper’s durability, however. When a climbing robot fails, cleanup means more than mopping up tomato juice; it means sweeping up the remains of an expensive robot.