PEATTIE, A.M.*; DELANNOY, S.M.; RUSSELL, A.P.; FULL, R.J.; Univ. of California, Berkeley; Univ. of Calgary; Univ. of Calgary; Univ. of California, Berkeley: Deformation of isolated gecko setal arrays: bending or buckling? 1. Kinematics.

The adhesive material of the gecko foot consists of arrays of millions of microscopic, branched hairs (setae). To attach effectively to rough surfaces, adhesive material must be compliant. Setae are composed of a stiff keratinous material. Compliance appears to result from the geometry of the hairs. One model treats setae as fibrils oriented perpendicular to the surface that buckle during attachment (Jagota, et al. 2004). An alternative model suggests that setae are positioned at an angle to the substrate and bend like cantilevers as they are loaded (Sitti & Fearing, 2003). We isolated arrays of 8 to 200 Tokay gecko setae (100µ long) and attached them to a probe. We made the first direct observations and shear force measurements of gecko setae attaching in an environmental scanning electron microscope (ESEM). The shear force measured inside the evacuated chamber of the ESEM was similar to that measured in ambient conditions (100 µN/seta). We observed that setal stalks bend in unison, rather than buckle randomly, under the compressive load that is required for them to engage and subsequently generate force. While maximal engagement required low attachment angles (<30°), shear force was generated at higher angles of attachment as long as setae were correctly oriented. Setae have a resting curvature, but straighten as they engage. Bending was localized to the base of each seta, which is narrower than the rest of the stalk. A direct measurement of effective compliance is necessary to demonstrate that these kinematics result in a reduction in compliance consistent with effective adhesion. Supported by UCB IB Summer Grant, NSF NIRT 0304730 and DARPA RiSE M000219.