title: Elastic modulus of tree frog adhesive toe pads.
abstract: Previous work using an atomic force microscope in nanoindenter mode indicated that the outer, 10- to 15-μm thick, keratinised layer of tree frog toe pads has a modulus of elasticity equivalent to silicone rubber (5-15 MPa) (Scholz et al. 2009), but gave no information on the physical properties of deeper structures. In this study, micro-indentation is used to measure the stiffness of whole toe pads of the tree frog, Litoria caerulea. We show here that tree frog toe pads are amongst the softest of biological structures (effective elastic modulus 4-25 kPa), and that they exhibit a gradient of stiffness, being stiffest on the outside. This stiffness gradient results from the presence of a dense network of capillaries lying beneath the pad epidermis, which probably has a shock absorbing function. Additionally, we compare the physical properties (elastic modulus, work of adhesion, pull-off force) of the toe pads of immature and adult frogs.
authors.affiliation: Centre for Cell Engineering, Institute of Molecular, Cell and Systems Biology, Joseph Black Building, University of Glasgow, Glasgow, Scotland, G12 8QQ, UK. Jon.Barnes@glasgow.ac.uk
authors.forename: W Jon P
authors.initials: WJ
authors.lastname: Barnes
authors.forename: Pablo J Perez
authors.initials: PJ
authors.lastname: Goodwyn
authors.forename: Mohsen
authors.initials: M
authors.lastname: Nokhbatolfoghahai
authors.forename: Stanislav N
authors.initials: SN
authors.lastname: Gorb
mesh_terms: D000268:Adhesiveness; D000367:Age Factors; D000818:Animals; D001001:Anura / Q000502:physiology; D001696:Biomechanical Phenomena; D055119:Elastic Modulus; D004817:Epidermis / Q000502:physiology*; D005552:Forelimb; D006614:Hindlimb; D007368:Interferometry; D011312:Pressure; D014034:Toes / Q000502:physiology*; D014160:Transducers, Pressure