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If a robot is sent to disable a roadside bomb—or delicately handle an egg while cooking you an omelet—it needs to be able to sense when objects are slipping out of its grasp. Yet to date it’s been difficult or impossible for most robotic and prosthetic hands to accurately sense the vibrations and shear forces that occur, for example, when a finger is sliding along a tabletop or when an object begins to fall.

Now, engineers from the University of Washington and UCLA have developed a flexible sensor “skin” that can be stretched over any part of a robot’s body or prosthetic to accurately convey information about shear forces and vibration that are critical to successfully grasping and manipulating objects.

The bio-inspired robot sensor skin, described in Sensors and Actuators, mimics the way a human finger experiences tension and compression as it slides along a surface or distinguishes among different textures. It measures this tactile information with similar precision and sensitivity as human skin, and could vastly improve the ability of robots to perform everything from surgical and industrial procedures to cleaning a kitchen.

According to phys.org, traditionally, tactile sensor designs have focused on sensing individual modalities: normal forces, shear forces or vibration exclusively. However, dexterous manipulation is a dynamic process that requires a multimodal approach. The fact that our latest skin prototype incorporates all three modalities creates many new possibilities for machine learning-based approaches for advancing robot capabilities,” said co-author and robotics collaborator Veronica Santos, a UCLA associate professor of mechanical and aerospace engineering.

The new stretchable electronic skin, which was manufactured at the UW’s Washington Nanofabrication Facility, is made from the same silicone rubber used in swimming goggles. The rubber is embedded with tiny serpentine channels—roughly half the width of a human hair—filled with electrically conductive liquid metal that won’t crack or fatigue when the skin is stretched, as solid wires would do.

When the skin is placed around a robot finger or end effector, these microfluidic channels are strategically placed on either side of where a human fingernail would be.

As you slide your finger across a surface, one side of your nailbed bulges out while the other side becomes taut under tension. The same thing happens with the robot or prosthetic finger—the microfluidic channels on one side of the nailbed compress while the ones on the other side stretch out.

The research team has demonstrated that the sensor skin has a high level of precision and sensitivity for light touch applications—opening a door, interacting with a phone, shaking hands, picking up packages, handling objects, among others. Recent experiments have shown that the skin can detect tiny vibrations at 800 times per second, better than human fingers.