Forcing the issue: testing gecko-inspired adhesives.

Materials are traditionally tested either by imposing controlled displacements and measuring the corresponding forces, or by imposing controlled forces. The first of these approaches is more common because it is straightforward to control the displacements of a stiff apparatus and, if the material suddenly fails, little energy is released. However, when testing gecko-inspired adhesives, an applied force paradigm is closer to how the adhesives are loaded in practice. Moreover, we demonstrate that the controlled displacement paradigm can lead to artefacts in the assumed behaviour unless the imposed loading trajectory precisely matches the deflections that would occur in applications. We present the design of a controlled-force system and protocol for testing directional gecko-inspired adhesives and show that results obtained with it are in some cases substantially different from those with controlled-displacement testing. An advantage of the controlled-force testing approach is that it allows accurate generation of adhesive limit curves without prior knowledge of the expected behaviour of the material or the loading details associated with practical applications.

Spatially variant microstructured adhesive with one-way friction.

Surface microstructures in nature enable diverse and intriguing properties, from the iridescence of butterfly wings to the hydrophobicity of lotus leaves to the controllable adhesion of gecko toes. Many artificial analogues exist; however, there is a key characteristic of the natural materials that is largely absent from the synthetic versions-spatial variation. Here we show that exploiting spatial variation in the design of one class of synthetic microstructure, gecko-inspired adhesives, enables one-way friction, an intriguing property of natural gecko adhesive. When loaded along a surface in the preferred direction, our adhesive material supports forces 100 times larger than when loaded in the reverse direction, representing an asymmetry significantly larger than demonstrated in spatially uniform adhesives. Our study suggests that spatial variation has the potential to advance artificial microstructures, helping to close the gap between synthetic and natural materials.

A robotic device using gecko-inspired adhesives can grasp and manipulate large objects in microgravity.

Grasping and manipulating uncooperative objects in space is an emerging challenge for robotic systems. Many traditional robotic grasping techniques used on Earth are infeasible in space. Vacuum grippers require an atmosphere, sticky attachments fail in the harsh environment of space, and handlike opposed grippers are not suited for large, smooth space debris. We present a robotic gripper that can gently grasp, manipulate, and release both flat and curved uncooperative objects as large as a meter in diameter while in microgravity. This is enabled by (i) space-qualified gecko-inspired dry adhesives that are selectively turned on and off by the application of shear forces, (ii) a load-sharing system that scales small patches of these adhesives to large areas, and (iii) a nonlinear passive wrist that is stiff during manipulation yet compliant when overloaded. We also introduce and experimentally verify a model for determining the force and moment limits of such an adhesive system. Tests in microgravity show that robotic grippers based on dry adhesion are a viable option for eliminating space debris in low Earth orbit and for enhancing missions in space.