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February 1st, 2013

Crocodile scale formation—more materials science than biology

Published on February 1st, 2013 | By: pwray@ceramics.org

Crocodile skin sections (left) indicate that cracks correspond to epidermal bulges that reach the stiff underlying tissues. Immunohistochemistry (right) indicates increased cell proliferation (green) within the skin grooves corresponding to cracks. Abbreviations: primary (pc) and secondary (sc) cracks (ep, epidermis, de, dermis, bo, bone tissue). Credit: Nicolas Di-Poi; Milinkovitch Group, UG.

Today is the last day of ACerS’ back-to-back conferences in Florida—last week’s Electronic Materials and Applications and this week’s 37th International Conference on Advanced Ceramics and Composites. One thing that struck me about the programming at both conferences is the interdisciplinary aspects that our field of ceramic engineering/science brings together.

Biomimetic or bioinspired synthesis is one good example. Is it biology or materials science? Can the secrets of nature be tapped and applied to engineer materials?

Just as we leave the “Gator Nation,” an interesting article in Science reveals that sometimes it is the biologists who look into the world of materials science to gain understanding. In this case, the problem was to understand the mechanism by which scales in the region of the crocodile’s head form.

One might think that they are the result of genetic coding, but, in fact, they form by fracturing of skin. The interesting question is whether there are crossover lessons for materials synthesis.

Lead author, Michel Milinkovitch, professor at the University of Geneva, Switzerland, studies evolutionary developmental genetics and is interested in the intersection between evolution, biology, physics, and mathematics. The biology of reptiles, snakes, and lizards provide some important clues as to how things like skin differentiation occur.

For example, typical keratinized skin “appendages”—e.g., feathers, hairs, or scales—develop in the embryo from genetically controlled cell clusters called “primordia.” While studying the scale patterns of snakes, Milinkovitch noticed that the scales were organized with very predictable shapes and with left side-right-side symmetry. Crocodile head scales, in contrast, were irregularly shaped and without any symmetrical patterning.

Starting with mathematics, the team conducted a sophisticated, three-dimensional statistical analysis of head scales. They found their geometries are characteristic of “qualities and signatures associated with cracking, a phenomenom that is well known in physics, but so far absent in biology,” Milinkovitch says in a sciencemag.org podcast. Instead, the scale patterns are similar to those seen when an adherent layer shrinks but the substrate does not, for example, when mud dries on a pavement, or when a glaze cracks on a ceramic.

Next, the group studied crocodile embryos and looked for primordial regions that would develop into scales, which would be evidence of a genetically controlled process, however, none were found on the heads. Instead, they found that the skin covering the crocodiles’ heads and jaws is especially thick and rigid. As the embryonic skeleton (substrate) grows rapidly, mechanical stress fields build and eventually the skin cracks, and “what you observe is that grooves progressively appear on the face and jaws, propagate, and interconnect to form a continuous network across the developing skin.” (Interestingly, they also found that the scales that form everywhere else on the crocodile are primordial.)

In the podcast, Milinkovitch suggests that this work could help understand the role of mechanical tension in skin development and healing. From a materials processing perspective, I am intrigued by the idea of manipulating the substrate to control the processes occurring on it. Could the crocodile scale-forming mechanism lead to novel, low-temperature processing ideas, maybe even at the nanoscale? I don’t know, but my guess is yes. For example, earlier this week Mufit Akinc from Iowa State University gave a talk at ICACC on biomimetic and bioinspired, bottom-up approaches to designing nanostructured materials. I was not there, but it sounds like crocodiles could have some interesting insights on how to control bottom up processes.

The paper is “Crocodile head scales are not developmental units but emerge from physical cracking
Michel C. Milinkovitch1, Liana Manukyan, Adrien Debry, Nicolas Di-Poï, Samuel Martin, Daljit Singh, Dominique Lambert, and Matthias Zwicker; Science, 4 January 2013 (doi:10.1126/science.1226265).

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