Fig. 9. Young's modulus (E) of materials including approximate values of
bulk ß-keratin and effective modulus (Eeff) of
natural setal arrays (Geisler et al.,
2005). A value of E
100 kPa (measured at 1 Hz) is the
upper limit of the Dahlquist criterion for tack, which is based on empirical
observations of pressure sensitive adhesives [PSAs
(Dahlquist, 1969;
Pocius, 2002)]. A cantilever
beam model [equation 5.3 (Sitti and
Fearing, 2003)] predicts a value of Eeff near
100 kPa, as observed for natural setae and PSAs. It is notable that geckos
have evolved Eeff close to the limit of tack. This value
of Eeff may be tuned to allow strong and rapid adhesion,
yet prevent spontaneous or inappropriate attachment.