12. Frog Skin: A Giant Leap for Engineering Applications

Frog skin’s surface structure, color variations, and stickiness are protective features that have been widely explored by researchers for engineering applications. Many frogs have a thin film of water formed on their skin, and it is necessary for them to retain it in order to breathe through their skin (Mackean, Frogs—an introduction, http://​www.​biology-resources.​com/​frog.​html, 2004). Some frogs are adapted to secrete a thick mucous that prevents water from escaping and their skin from drying out (Science Score, Did you know that frogs breathe through their skin? http://​blog.​sciencescore.​com/​fun-facts-for-kids/​did-you-know-that-frogs-breathe-through-their-skin, 2012). Another function of the moist film is the lubrication effect it imparts. Color variation is one of the powerful defensive tactics of frogs that help them blend in with their natural surroundings. Some frogs such as dart frogs often display their bright skin with specific colored patterns to warn predators that they are highly toxic (Tesler, The amazing adaptable frog, http://​www.​exploratorium.​edu/​frogs/​mainstory/​index.​html, 1999). Despite the great variation in types of frogs, all frogs have independently evolved to have adhesive pads on their toes, signifying that the adhesive pad is an optimal evolutionary design for frogs (Barnes, Adhesion in wet environments: frogs. In: Bhushan B (ed) Encyclopedia of nanotechnology. Springer, Netherlands, pp 70–83, 2012). The surface morphology of these toe pads consists of hexagonal disk-like pads with channel-like spaces, as well as fine microstructure pegs (Federle et al., J R Soc Interface 3(10):689–697, 2006). Toe pad adhesion is made possible by van der Waals forces and capillary pressure forces (Lau and Messersmith, Wet performance of biomimetic fibrillar adhesives. In: von Byern J, Grunwald I (eds) Biological adhesive systems. Springer, Vienna, pp 285–294, 2010). Advances in biomimetics have been made in which toe pad replicas have been produced. For example, an early attempt at creating a replica featured polydimethylsiloxane (PDMS), and a later replica featured hierarchical micro- and nano-structures of epithelial cells (Barnes, Adhesion in wet environments: frogs. In: Bhushan B (ed) Encyclopedia of nanotechnology. Springer, Netherlands, pp 70–83, 2012).