The climbing ability of the Tokay gecko has attracted much attention from scientists trying to reproduce the adhesive found on the animal's feet. Many of the impressive properties of the adhesive have been attributed to the micrometer- and nanometer-sized fibers and how they are moved, or articulated, during adhesive placement and separation. Using microfabrication techniques, fibrillar structures were created and tested using a home-built testing apparatus specifically designed to characterize tribological properties over millimeter-sized areas. The results confirm that fiber geometry and fiber articulation are both important for gecko-inspired adhesives.

Tilted and vertical microfabricated polydimethylsiloxane (PDMS) rectangular flaps were created to determine if non-circular structures, similar in shape to the terminal features of the gecko, could be used to create a controllable adhesive. The control of the adhesive was achieved by varying shear length and direction to contact different parts of the flaps. A high adhesion force was achieved when contacting the side face of the flaps, lower adhesion force when contacting the top of the flaps, and almost zero adhesion force when contacting the edge between the top and side faces of the flaps. The addition of tilt resulted in anisotropic behavior, creating both a gripping direction and a releasing direction.

The effect of fiber shape was then explored by using a semicircular cross section. The PDMS fibrillar adhesive was the first single material vertical adhesive without fiber tip modification to exhibit adhesion and friction anisotropy. Using a simple microfabrication process, differences in contact area caused anisotropic behavior when displaced towards and against the flat face of the fiber. The vertical fibers even showed higher anisotropy values than some tilted structures.

As a continuation of the testing on vertical semicircular fibers, tilt was again added to the design. The adhesion forces and anisotropy of the PDMS angled fibers were higher than the previous designs, illustrating the importance of both fiber shape and tilt for anisotropy. The shear forces were also high and the shear pressure reached 88% of the value obtained when testing the two front feet of the gecko. The adhesive was then tested over 10,000 cycles and retained 118% of its initial shear force and 77% of its initial adhesion force. The performance of this adhesive is comparable to other high lifetime gecko-inspired adhesives.

As an alternative to the geometric design of the fibers, the influence of fiber articulation, that is how the fibers are moved, was explored using a common vertical testing procedure and was compared to an angled testing procedure. Both tests were shown to result in similar maximal shear and adhesion forces. The angled testing procedure also showed high adhesion to preload ratios (mu') which have been shown to be important for robot stability when climbing. By choosing the correct approach angle, shear length, and retraction angle, a single testing procedure was shown to achieve a high shear force, a high adhesion force, and a high mu' value, resulting in an optimal articulation.