Tuning the Morphology of Suction Discs to Enable Directional Adhesion for Locomotion in Wet Environments

Reversible adhesion provides robotic systems with unique capabilities, including wall climbing and walking underwater, and yet the control of adhesion continues to pose a challenge. Directional adhesives have begun to address this limitation by providing adhesion when loaded in one direction and releasing easily when loaded in the opposite direction. However, previous work has focused on directional adhesives for dry environments. In this work, we sought to address this need for directional adhesives for use in a wet environment by tuning the morphology of suction discs to achieve anisotropic adhesion. We developed a suction disc that exhibited significant directional preference in attachment and detachment without requiring active control. The suction discs exhibited morphological computation-that is, they were programmed based on their geometry and material properties to detach under specific angles of loading. We investigated two design parameters-disc symmetry and slits within the disc margin-as mechanisms to yield anisotropic adhesion, and through experimental characterizations, we determined that an asymmetric suction disc most consistently provided directional adhesion. We performed a parametric sweep of material stiffness to optimize for directional adhesion and found that the material composition of the suction disc demonstrated the ability to override the effect of body asymmetry on achieving anisotropic adhesion. We modeled the stress distributions within the different suction disc symmetries using finite element analysis, yielding insights into the differences in contact pressures between the variants. We experimentally demonstrated the utility of the suction discs in a simulated walking gait using linear actuators as one potential application of the directional suction disc.