Non-Cuttable Material Inspired by Mollusk Shells and Grapefruits

Innovation: Academia

Durham University

2020

Image: Brian Jimenez / Unsplash / CC BY SA - Creative Commons Attribution + ShareAlike

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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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 Deformation

When a living system undergoes compression, tension, shear, bending, or twisting, its internal inter-molecular forces can often resist these forces and even change shape temporarily, returning to the original shape when the forces stop. However, if the force is too strong or lasts too long, permanent deformation or structural failure can occur, resulting in death. Therefore, living systems have strategies to resist deformation or help ensure limited deformation. For example, bones have thin crystals and proteinaceous fibers that provide strength and flexibility, protecting them from forces that would otherwise cause deformation on a daily basis.

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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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Proteus from Durham University is a non-cuttable material made of a ceramic spheres within a flexible aluminum structure that interferes with cutting tools.

Benefits

Lightweight
Durable
Non-cuttable

Applications

Security
Protective equipment
Armor

UN Sustainable Development Goals Addressed

Goal 9: Industry Innovation & Infrastructure

The Challenge

Protective equipment must be able to endure potential cutting forces which can damage the material and make it vulnerable to breakage. Once a cut penetrates a material, it makes it easier for the material to collapse, as well as letting dust and dirt enter.

Innovation Details

The non-cuttable material, called Proteus, is a lightweight material made of ceramic spheres within a flexible, cellular aluminum structure. The design was inspired by the tough cellular skin of grapefruit and fracture-resistant mollusk shells. When a cutting tool, like an angle grinder, tries to cut into the material, the ceramic spheres blunt the cutting tool, eventually rendering it effective. Additionally, the force generated between the grinder and the spheres creates a vibration that prevents the grinder from sufficiently penetrating. The ceramic spheres also slowly fragment and fill spaces within the material, hardening the material. This process is increased further as the tool speed increases.

Biological Model

Fruits like the grapefruit and pomelo have excellent damping properties due to the hierarchical organization of their peels. When pomelo fruits fall from the ground, air pockets within the peel collapse like a cushion, absorbing the energy of impact and protecting the peel from damage.

Nacre’s specific composition and construction make it tough and resistant to catastrophic failure that can result from spreading cracks. This means that a greater amount of energy is needed to fracture or break the material. Hard microscale mineral layers in nacre are “glued” together by relatively soft nanoscale organic layers. The arrangement is much like staggered layers of bricks that are held together by mortar in a brick wall.

See Related Content

close up photo of seven pomelo citrus fruits, one with exposed orange flesh

Biological Strategy

Hierarchical Organization of Peel Confers Impact Resistance

Pomelo

The pomelo fruit has excellent damping properties due to the hierarchical organization of its composite peel

Biological Strategy

Weak Interfaces Make Material Tough

Black-lip pearl shell

Layers of weak and stretchy organic material between brittle mineral layers in nacre make the whole composite tough by managing cracks.

References

Journal article

Non-cuttable material created through local resonance and strain rate effects

Scientific Reports

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