The idea of turning the stickiness of a material on and off is not an intuitive concept, yet this is exactly how geckos run up walls at speeds greater than 1 m/s and cling to ceilings made of glass. Transitioning this capability to spacecraft would constitute a major breakthrough because it would provide improved capabilities for multiple mission types.
JPL’s adhesive mimics the method and performance of a gecko’s adhesive system. Geckos adhere to surfaces using arrays of hierarchical hairs with features at the millimeter (mm), micrometer (μm), and nanometer (nm) scales that generate enough van der Waals forces to support the animal’s weight. The directional bias of these hairs provides a means of turning the adhesion on and off through an applied shear load, a behavior also seen in JPL’s two-stage, mm-μm synthetic structures.
The traditional method of analyzing grasp relies on the use of a contact model to develop a Grasp Map (also known as the Grasp Matrix). Using this model of the contacts, the resistance to external wrenches on the object being grasped can be analyzed. If a grasp can resist arbitrary wrenches, it is said to have force-closure or is fixtured. There are three commonly used contact models: frictionless point contact, point contact with friction, and soft-finger. This innovation takes advantage of a recently reported model, Frictional Adhesion, which accounts for adhesion. Using this model accurately accounts for how a gripper with gecko-like adhesives is able to grip non-traditional surfaces like flat plates and blankets without complex grasp planning or control.
In practice, the applied shear load is generated through a slight sliding motion. Once activated in such a manner, a pad will resist both normal and shear forces aligned roughly to the loading direction. By arranging these pads in counterbalanced pairs, triads, or quads, omnidirectional grip can be achieved. This architecture has been shown at proof-of-concept level at JPL. The pads release with zero detachment force when the applied shear load is removed through the reversal of the slight sliding motion that was used to engage it in the first place. Thus, the clamp would have a zero power on and zero power off state.
To date, the pads have been fabricated and demonstrated at ambient conditions. However, several space-qualified materials have been identified including NuSil CV-1142 and GE Silicones 566, both of which meet NASA SP-R-0022A, a standard for use of polymers in orbit. While some IP already exists in academia for fibrillar adhesives, JPL’s current instantiation is different, and has better performance. Additionally, the configuration of opposing sets of pads to create omnidirectional anchors is new, and the use of these adhesives in orbit is novel. Typically, the adhesives require the gravitational vector to retain the on-off property, but this gripper configuration presented here allows the pads to be useful in orbit.
This work was done by Aaron Parness of Caltech for NASA’s Jet Propulsion Laboratory.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
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Refer to NPO-48587.