The adhesive performance of tree frogs on various substrates

Secure and gentle grip of delicate biological tissues

Tree frogs have very soft adhesive toe pads, with which they can adhere to various, even wet surfaces. This makes them an appropriate model for the development of novel, biomimetic surgical tools. On the way to a better understanding of the fundamental mechanisms providing the frog’s remarkable sticking capabilities, a combination of experimental (force measurements, kinematics) and numerical (FEA, CFD) analyses of the tree frog’s toe pad is required.

Project content

The properties of the substrate are a crucial factor influencing the adhesive performance of the frog. For the species analysed in this project, H. cinerea and L. caerulea, the effect of changes in substrate roughness, moisture level and surface energy on toe pad functionality is not known. Thus, using a rotating platform, adhesion and friction forces are to determine. The following subtopics are available:

•    The effects of substrate roughness

The epithelial cell layer of the tree frog’s toe pad shows a remarkable structured pattern of hexagonal cells (d ≈ 10 μm; Ernst, 1973), each of them covered by an even finer pattern of cellular protrusions, so called nanopillars (d ≈ 350 nm). In a parametric study the interaction between this multi-hierarchical toe pad structure and substrates with different levels of roughness will be tested. It is assumed, that the toe pad design is optimised for a range of roughness found in nature. Furthermore, dependencies on species and age/size will be investigated.

•    The effects of substrate moisture

In between the hexagonally shaped cells found on the tree frog’s toe pads, channels filled with a watery mucus can be observed (Green, 1979). It already was assumed early, that this mucus supports the mechanism of wet adhesion, which provides the frog its remarkable sticking abilities (Nachtigall, 1974). In a parametric study the influence of increasing levels of substrate wetness and substrate coverage with varying viscosity is in the focus of research. Furthermore, dependencies on species and age/size will be investigated.

•    The effects of substrate surface energy

In nature, tree frogs face substrates with highly diverse properties. An important substrate characteristic is the surface energy, which directly influences capillary effects of the mucus found on the tree frog’s toe pads and thereby the mechanism of wet adhesion. In a parametric study the influence of different levels of substrate surface energy is analysed. Furthermore, dependencies on species and age/size will be investigated.

These subjects are available during the whole of 2015-2016. They are best suited for BSc students, but also can be extended into a MSc project.

The following requirements should be fulfilled by the applicant:

-    Basic knowledge in biophysics

-    Basic understanding of adhesion

-    Good knowledge in MS Office; experience with MATLAB will be helpful

-    Basic knowledge in statistics will be helpful

Literature

Ernst, V.V. (1973): The digital pads of the tree frog, hyla cincerea. I. the epidermis. Tissue and Cell, 5, 1, pp. 83–96.

Green, D.M. (1979): Treefrog toe pads: comparative surface morphology using scanning electron microscopy. Canadian Journal of Zoology, 57, pp. 2033–2046.

Hanna, G. and Barnes, W.J.P. (1991): Adhesion and detachment of the toe pads of tree frogs. Journal of Experimental Biology, 155, 1, pp. 103–125.

Nachtigall, W. (1974): Biological Mechanisms of Attachment: The Comparative Morphology and Bionengineering of Organs for Linkage, Suction, and Adhesion. 1st edn. Springer/Verlag, Berlin, Heidelberg, New York.