Soft Shield Distributes Force

a brown and orange river cooter turtle stands on rocks in shallow water

Biological Strategy

Turtles

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Image: Bruce Dupree, ACES / Flickr / Public Domain - No restrictions

Prevent Fracture/Rupture

High force impact or stress can cause materials that comprise living systems to separate into two or more pieces (called fracturing) or to break or burst suddenly (called rupturing). For example, a scallop prevents structural failure from fracture because its shell is comprised of two materials of varying stiffness. When a crack moves from the scallop’s stiff material to the less stiff one, the latter reduces the force at the tip of the crack, thereby stopping it from spreading farther.

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Manage Compression

When a living system is under compression, there is a force pushing on it, like a chair with a person sitting on it. When evenly applied to all sides of a living system, compression results in decreased volume. When applied on two sides, it results in deformation, such as when pushing on two sides of a balloon. This deformation can be temporary or permanent. Because living systems must retain their most efficient form, they must ensure that any deformation is temporary. Managing compression also provides an opportunity to lessen the effects of other forces. Living systems have strategies to help prevent compression or recover from it, while maintaining function. For example, African elephant adults weigh from 4,700 to 6,048 kilograms. Because they must hold all of that weight on their four feet, the tissues of their feet have features that enable compression to absorb and distribute forces.

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Manage Impact

An impact is a high force or mechanical shock that happens over a short period of time, such as a hammer hitting a nail rather than a hand pushing slowly against a wall. Because of their speed and force, impacts don’t allow materials to slowly adjust to the force, which can lead to cracks, ruptures, and complete breakage. Therefore, living systems have strategies that can absorb, dissipate, or otherwise survive that force without the need to add large amounts of material. For example, the Toco toucan’s large beak is very lightweight, yet can withstand impacts because it’s made of a composite material with rigid foam inside and layers of a hard, fibrous material outside.

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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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Turtle shells use a two-layer structure to prevent penetration injury by dispersing force.

Introduction

The turtle is a fascinating organism known for its unique shell, which serves as both protection and support. This is especially critical as turtles inhabit diverse environments, from terrestrial to aquatic. The remarkable feature we’ll explore here is the turtle shell’s bumper-buffer mechanism, a natural design strategy that significantly enhances its resilience to surface indentations and provides valuable insights for .

The Strategy

The turtle shell comprises a bi-layer skin coating over a hard, brittle bony core. This bi-layer skin consists of a keratin layer (similar to human nails) on top, a collagen layer beneath it (similar to collagen in our cartilage and skin), and the underlying bony core. The remarkable aspect of this design is how it deals with surface indentations. When subjected to an indentation force, the skin stresses reach the yielding strength of keratin in the indentation region, acting like a “bumper.” This localized plasticity blunts the penetration effect and reduces stress concentrations.

Furthermore, the presence of the softer intermediate collagen layer between the keratin and the bulk bone acts as a “buffering” interface. It distributes indentation stresses over a greater region, significantly diminishing stress on the bone. The combined effect of the bumper and buffering mechanisms confines the indentation damage to the bi-layer skin region, protecting the underlying brittle bony core.

The Potential

The turtle shell’s bumper-buffer mechanism provides valuable inspiration for various design, engineering, and material science applications. Here are some ways in which humans could adopt or be inspired by this strategy for better design:

Protective Materials: Engineers can replicate this bi-layer design in protective materials for various applications. By combining a hard outer layer with a soft, energy-absorbing inner layer, we can create materials that are more resilient to surface damage.
Safety Gear: Sports equipment like helmets and pads could benefit from this strategy. It could reduce the risk of injury by better distributing and absorbing impact forces, especially in high-impact sports.
Structural Integrity: Applying the bumper-buffer concept in architecture and construction materials could improve the resilience of buildings and structures, making them better equipped to withstand external forces and natural disasters.
Automotive Design: In the automotive industry, this design could be used to enhance vehicle safety by creating car exteriors that are more resistant to dents and damage.
Aerospace: For spacecraft and aircraft, this concept could be employed to protect critical components from micrometeoroid impacts in outer space.

In summary, the turtle shell’s bumper-buffer mechanism demonstrates a remarkable strategy for mitigating surface damage. By emulating this natural design in various human-made materials and structures, we can enhance the resilience and safety of numerous applications, ultimately contributing to better engineering and sustainability, in alignment with the principles of biomimicry.

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Last Updated July 23, 2019

References

Journal article

Surface protection in bio-shields via a functional soft skin layer: Lessons from the turtle shell

J Mech Behav Biomed Mater | 01/01/2017 | Shelef Y, Bar-On B

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