Scales Minimize Abrasive Damage

close up photograph of orange and brown snake on a dark grey rock

Biological Strategy

Kenyan sand boa

Mary Hoff

Image: reptiles4all / Shutterstock / Some rights reserved

Protect From Dirt/solids

When dirt and other small solids adhere to living systems, they can slow them down, create blockages, reduce their ability to carry out vital functions, or cause surface wear and tear. Due to electrostatic forces, it’s easy for dirt and other solids to adhere to surfaces, so living systems must overcome those forces. An earthworm, for example, uses a small electric current to keep soil particles from adhering to its body as it moves through the soil. This enables it to move more efficiently by reducing drag.

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Manage Mechanical Wear

A living system is subject to mechanical wear when two parts rub against each other or when the living system comes in contact with abrasive components in its environment, such as sand or coral. Some abrasive components are a constant force, such as finger joints moving, while others occur infrequently, such as a sand storm moving across a desert. Living systems protect from mechanical wear using strategies appropriate to the level and frequency of the source, such as having abrasion-resistant surfaces, replaceable parts, or lubricants. For example, human joints like shoulders and knees move against each other all day, every day. To protect from mechanical wear, a lubricant reduces friction between the cartilage and the joint.

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Prevent Fatigue

When materials that comprise living systems are subjected to repeated loads, they can weaken over time. This is called fatigue and it can lead to cracks and eventually failure. However, fatigue doesn’t occur just anywhere on a material; the cracks usually start in areas subjected to elevated local stresses. Therefore, living systems often employ shapes, materials, and/or smooth transitions to decrease the potential for fatigue. An example of using shape is found in brown algae, an intertidal seaweed. Rather than being bent by the constant flow forces of waves and currents, its shape causes it to be pulled. This reduces stress by roughly 800 times compared to another species that bends with the flow, ultimately reducing structural fatigue.

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Reptiles

Class Reptilia (“creeping”): Lizards, crocodiles, turtles, snakes

Reptiles retain some of the key characteristics that first enabled vertebrates to live permanently on land. They have dry skin covered in scales made of keratin that help prevent water loss. (Amphibians still need to stick close to water to keep their skin moist.) Many reptiles reproduce by laying eggs that are watertight and have a yolk, so eggs can develop on land while staying moist and nourished. Reptiles are also ectotherms, meaning they don’t produce their own body heat. This means they burn through energy much more slowly than “warm blooded” creatures of the same size. It also means that they need to keep warm to keep active, which is why you might see them spending a seemingly inordinate amount of time simply basking in the sun.

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Scales of the Kenyan sand boa minimize abrasive damage by exhibiting a gradient in hardness and elasticity.

Introduction

Life is literally rough for the Kenyan sand boa (Eryx colubrinus, sometimes called Gongylophis colubrinus), which lives in arid, sandy places in northern and eastern Africa. It not only slithers along sandpaper-like surfaces to get from one place to another, it also coats itself with coarse grains of sand that provide camouflage as it waits for a meal to walk, hop, or crawl by. As a result, the scaly outer skin that covers its body is exposed to huge amounts of abrasion. How does the sand boa keep from getting literally all worn out by the rough substrate in which it covers itself, while still staying flexible enough to move smoothly across, and stay in close contact with, each other and uneven surfaces? The answer lies in the nature of the skin itself, which uses a combination of materials to give different desirable qualities to different parts of the scale.

Image: David Bygott / Flickr / CC BY NC ND - Creative Commons Attribution + Noncommercial + NoDerivatives

The scales on the bottom surface of a Kenyan sand boa constantly rub against sand and other rough surfaces, creating a need for abrasion resistance.

The Strategy

A snake’s belly skin has an outer scale layer, which touches the sand and other scales. It also has an inner scale layer, which underlies the outer scale layer and does not directly come in contact with the outside world.

The outer scale layer consists of six main layers. The layers on the side that contacts the outer world (the beta layer) contain beta-keratin, a molecule that also makes up bird beaks and claws, and is relatively hard and inflexible. The layers on the side that faces the inner scale (the alpha layer) layer contain alpha-keratin, which is found in structures such as human fingernails and hair, and is softer and more flexible.

Both hardness and flexibility vary across the cross-section, with the outer surface being harder and less flexible and the inner surface being softer and more flexible. For the snake’s skin, this means the ability to be extra scratch-resistant but also have the flexibility needed to subtly adjust the shape of the skin to a rough surface and so distribute stress and reduce the likelihood of cracking. The result is a structure that is tough enough to stand up to sand, yet flexible enough to allow movement and avoid cracking.

The result is a structure that is tough enough to stand up to sand, yet flexible enough to allow movement and avoid cracking.

The Potential

The strategy of combining hardness and flexibility in a layered configuration that provides strength where needed but flexibility too could be useful for designing durable but pliable medical devices, tires, fabrics, and more. Substances and configurations that minimize destruction due to friction can inform the design ofmovable parts in vehicles and other machinery, textiles, and cutting edges such as those on knives and blenders.In addition the Kenyan sand boa’s “best of both worlds” strategy offers a broader lesson: When designing to optimize between two competing needs, it’s wise to remember the best solution may not be a single optimum, but different optima customized to the location and function within the larger system.

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References

“[T]he outer scale layers are harder, and have a higher effective elastic modulus than the inner scale layers. The results obtained provide strong evidence about the presence of a gradient in the material properties of the snake integument. The possible functional significance of this gradient is discussed here as a feature minimizing damage to the integument during sliding locomotion on an abrasive surface, such as sand.” (Klein et al. 2010:659)

“A system which has to endure high amounts of stress under pressure is more effective against abrasion wear, if it has a gradient in structure and properties, since such design leads to more uniform stress distribution and thus to the minimization of the probability of local stress concentration.” (Klein et al. 2010:666)

Journal article

Material properties of the skin of the Kenyan sand boa Gongylophis colubrinus (Squamata, Boidae)

J. Comparative Physiology A | July 10, 2010 | Marie-Christin G. Klein, Julia K. Deuschle, Stanislav N. Gorb

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Reference

“In sliding contacts, local stress concentrations on the surface lead to material fatigue and failure. The surface is more effective against abrasion wear, if it has a gradient in material structure and properties, because it leads to a more uniform stress distribution, and thus to the minimization of local stress concentrations. A hard, inflexible surface material easily forms cracks under pressure, whereas a soft flexible system will be easily worn off under shear stress. As we have previously assumed, the gradient in material properties from a stiff surface to a soft depth will improve wear resistance in snake skin by combining the advantages of stiffness and flexibility.” (Klein & Gorb, 2012:3141)

Journal article

Epidermis architecture and material properties of the skin of four snake species

J. Royal Society Interface | August 15, 2012 | Marie-Christin G. Klein and Stanislav N. Gorb

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