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Elastic modulus of tree frog adhesive toe pads.

W Jon P Barnes¹, Pablo J Perez Goodwyn, Mohsen Nokhbatolfoghahai

¹Centre 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

Journal of Comparative Physiology. A, Neuroethology, Sensory, Neural, and Behavioral Physiology

|June 14, 2011

Summary

Tree frog toe pads are remarkably soft biological structures, with stiffness increasing towards the outer surface. This gradient is likely due to a capillary network, aiding in shock absorption.

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Area of Science:

Biomechanics
Amphibian biology
Materials science

Background:

Previous studies measured the elastic modulus of the outer keratinized layer of tree frog toe pads (5-15 MPa).
Limited information existed on the physical properties of deeper toe pad structures.

Purpose of the Study:

To measure the stiffness of whole tree frog toe pads (Litoria caerulea) using micro-indentation.
To investigate the stiffness gradient within the toe pads.
To compare physical properties between immature and adult frogs.

Main Methods:

Micro-indentation was employed to assess the mechanical properties of whole toe pads.
Elastic modulus, work of adhesion, and pull-off force were measured.

Main Results:

Tree frog toe pads exhibit an effective elastic modulus of 4-25 kPa, classifying them as very soft biological materials.
A stiffness gradient was observed, with the outermost layer being the stiffest.
A dense capillary network beneath the epidermis likely contributes to the observed stiffness gradient and shock absorption.

Conclusions:

Tree frog toe pads possess a unique stiffness gradient, crucial for their adhesive and shock-absorbing functions.
The findings provide insights into the biomechanics of amphibian adhesion.
Further research can explore the role of the capillary network in toe pad mechanics.