S1-3.3 Jan. 5 Similar Elastic Modulus of Setal Keratin for Two Species of Gecko PEATTIE, A M*; MAJIDI, C; CORDER, A B; FULL, R J; Univ. of California, Berkeley firstname.lastname@example.org
Typical bulk adhesives are characterized by soft, tacky materials with elastic moduli well below 1 MPa. Geckos possess subdigital adhesives composed mostly of β-keratin, a relatively stiff material. Biological adhesives like the geckosí have inspired empirical and modeling work that predicts even stiff materials can be effective adhesives if they take on a fibrillar structure. Models suggest that fibrillar adhesives benefit from increased stiffness by allowing higher packing density while avoiding clumping. The structure of β-keratin is highly conserved across birds and reptiles, suggesting that material properties of gecko setae should be constant, but this has yet to be established. Since elastic modulus for β-keratin has only been published for bird and reptile scales, we measured elastic (bending) modulus of setal keratin in two species of gecko. We used a resonance technique to perturb loaded setae and recorded the natural frequency of the setal stalk (eliminating the base and branched tips). Modeling the seta as a cylinder, we inferred the elastic modulus from the known length, radius and natural frequency. We found no significant difference (p = 0.49) in elastic modulus between Gekko gecko (1.57 GPa ± 0.061 SE; n=189) and Ptyodactylus hasselquistii (1.51 GPa ± 0.060 SE; n = 194). We observed no effect of relative humidity (p = 0.93) or temperature (p = 0.23) over the experimental range (15%-65% RH; 20-27°C), and setae did not show a consistent change in stiffness over time after being harvested. If the elastic modulus of setal keratin is conserved across species, it suggests a design constraint that must be compensated for structurally, and possibly explain the remarkable variation in gecko adhesive morphology.