Adhesion-based Walking

Starfish are incredibly versatile walking animals, all without the centralized processing of a brain. They can walk over rocks and coral with their bodies bent in various ways and can walk up smooth glass walls or even in inverted orientations!

A starfish walks by using hundreds of hydraulic appendages called tube feet on the underside of its body. They can point in all directions and stick to surfaces via chemical adhesion both in air and when submerged, and the adhesion is strong enough to support its body weight or withstand waves and tidal surges.

This ability could be useful for robots that need to inspect underwater surfaces like pipelines or stick to submerged objects like ship hulls to do tasks!

Walking via an array of soft actuators with soft suction discs

Here, we combined soft pneumatic actuators with soft suction discs to create a robust walking motion. 

The tube foot actuators extend to press the suction disc against the surface and the softness of the actuator allows it to bend so that the disc is flush against the surface. A pressure sensor linked to the inside of the suction disc can detect when the disc is sealed against the surface, which induces the tube foot actuators to contract and pull the body forward. We use active suction to create the adhesion so that the soft tube foot actuators don't need to create large preload forces to stick the suction disc to the surface (soft actuators aren't very good at pressing something into the surface - they tend to buckle instead of pressing).

The array of actuators creates robustness to some environmental obstacles; when one disc can't adhere to the surface because of the obstacle, the other actuators in the array can continue to drive locomotion. The softness of the actuators also can enable it to squeeze through smaller passages without getting stuck.

Ishida M., Sandoval J. A., Lee S., Huen S., Tolley M. T. (2022) "Locomotion via active suction in a sea star-inspired soft robot", IEEE Robotics and Automation Letters/IROS, 7 (4), 10304-10311.  

Embodied intelligence of soft suction discs for walking

During walking, a foot should have high adhesion to stick to the surface at the start of a step and low adhesion at the end of a step so that it can lift up to take the next step.

Here, we created soft, multimaterial suction discs that have different adhesion strengths based on what direction they are pulled. At the start of the step (pulled at a negative angle), the discs have high adhesion and so they stick to the surface and react the force from the body to create forward locomotion. But at the end of the step (pulled at a positive angle), the discs have low adhesion and pop off of the surface!

This embodied intelligence allows us to create suction discs that attach and detach during the walking gait without a specific controlled degree-of-freedom that engage and disengages the suction. 

Sandoval J. A.*, Ishida M.*, Jadhav S., Huen S., Tolley M. T. (2022) "Tuning the morphology of suction discs to enable directional adhesion for locomotion in wet environments", Soft Robotics, 9 (6), pp. 1083-1097.