Influence of Substrate Modulus on Gecko Adhesion

Sheet 1: Whole Animal Gecko TrialsAll trials were done in an environmental chamber, set to 25 C and 35 %$RH. The specimens had thirty minutes to acclimate to the conditions, then were tested vertically on two sets of substrates (cellulose acetate and PDMS) using a force-sensing apparatus, similar to the one used by Niewiarowski et al., 2008. This force-sensing apparatus was measured the force that the specimens were capable of withstanding (for safety of the gecko, a maximum of 20 N was set). Before starting an individual experiment, the gecko had a small strip of medical tape carefully placed around the mouth, avoiding the nostrils, and was fitted with two harnesses around their pelvis, which were then attached to a force sensor positioned vertically on a motorized track. The gecko was placed on the substrate and encouraged to take a few steps, ensuring natural adhesion to the substrate. After each trial the substrate was cleaned.Within each substrate set, gecko trials were performed randomly on the different modulus substrates, with a total of three trials per substrate. Geckos were tested a maximum of three times before having at least a 24 hour rest period. The trials with the highest force for each gecko on each substrates are presented here.Sheet 2: Isolated Lamella AdhesionSkin sheds from 7 naturally molting Tokay geckos were collected and stored in a -20 C freezer. Single lamella were isolated from gecko sheds and adhered to small glass plates with cyanoacrylate glue such that the skin was anchored to the glass, but the setal strip was free to flex. Shear adhesion trials were done with ethanol/water cleaned glass and solvent extracted 1000 micron PDMS substrates.The substrate was brought in contact with the lamella in the normal force direction by motor and loaded to a force of around 5 mN. The lamella was then loaded in the shear direction with a velocity of 5 microns/s such that the substrate would move towards the direction of the tip of the toe, with respect to the lamella. The trial was ended manually when the shear force dramatically dropped, reached a plateau for at least a minute, or the lamella reached the end of the substrate, and the maximum force was taken to be the shear adhesion force.