Tree Frog-Inspired Gripping

This research project is funded by the Netherlands Organization for Scientific Research NWO.

Grip between heterogeneous objects can be weak or strong, permanent or reversible, and load dependent or load independent. In some cases, grip needs to be very strong (e.g., aircraft fuselage parts), whereas in other cases it has to be deliberately weak (e.g., screen protection foil for your smartphone). Office tape has to be reversible, whereas a broken vase should be permanently glued back together. A staple can be applied by piercing it through the paper, whereas a surgical gripper should grab soft organs without damaging them.

Grip that is strong, load independent, and reversible at the same time is a great challenge in engineering. Nature, on the other hand, manages quite well: Geckos, for example, can walk on tilted surfaces due to adhesive forces between their toes and the substrate. The stickiness is more than enough to carry the gecko’s weight, but the animal can still easily peel its feet from the surface during walking. Tree frogs can grip on wet, or even flooded, surfaces. Furthermore, their toe pads are soft, thereby eliminating the risk of causing normal stresses to the objects the animal grips upon. With a single foot, tree frogs generate gripping forces strong enough to 100 times carry their weight. Still, tree-frog grip is reversible: The animal detaches from a substrate by peeling its toes off.

The goal of this research project is to gain insight into adhesion on wet and soft surfaces and, inspired by the tree-frog adhesive apparatus, to develop artificial systems that are able to reversibly but firmly grip to objects while minimizing normal stresses.