Robots with fingernails can grasp thin edges

Robotic hands are getting their nails done.

Researchers have designed a new three-fingered robotic hand whose digits come with a rigid fingernail on soft material. The design gives the robotic hand the ability to peel fruit, open containers with lids and pick up flat objects, the team posted to arXiv.org February 5. The results offer a glimpse at the kinds of real‑world chores and industrial handling tasks the robot could do.

The tips of most conventional robotic hands have a soft pad on a rigid structure, giving the fingertip a squarish shape. The new robot fingertips have a soft material wrapped around the finger “skeleton,” with a rigid structure on top, giving the tip an oval shape, more like our fingers. “A square shape only adapts well to forces coming straight on, but our design can also respond flexibly to twisting or side forces,” says mechanical engineer Dong Ho Kang of the University of Texas at Austin.

Soft fingertips with that square design adapt to objects and grasp them, but that same softness makes them less precise. In humans, rigid fingernails on soft fingertips correct this issue by stiffening the fingertips and focusing pressure.

Taking inspiration from our own nails, the researchers tested robotic hands with three linked, motorized fingers — index and middle fingers and a thumb — with and without nails. The fingers grasped flat, outward-bulging and inward-curved objects, while the objects were being pulled upward. The fingertips with nails demonstrated a stronger grasping force, meaning they had a tighter grip, Kang says. Fingernails were an advantage when grabbing curved objects. Without them, the soft fingers were easily deformed and had a less stable hold on the objects.

Fingertips with nails also excelled at pulling out a single sheet from a stack of papers, opening the lid of a sealed container, picking up thin objects like coins and cards from a surface, and flipping cards. Although they were able to open the lid on some attempts, the soft fingertips without the nails failed at all other tasks and were unable to establish contact with the edges of the objects. Kang says the next step is to extend this work to a full robotic hand.