Strong Composite Material Inspired by Nacre

Innovation: Academia

MIT

2020

Image: Jovica Smileski / Unsplash / CC BY SA - Creative Commons Attribution + ShareAlike

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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Modify Material Characteristics

The materials found in living systems are variable, yet often made from the same basic building blocks. For example, all insect exoskeletons consist of a material called chitin. Because material resources are limited, each material within or used by a given living system must frequently serve multiple purposes. Therefore, living systems have strategies to modify materials’ softness, flexibility, and other characteristics. To ensure survival, the benefits of these modifications must outweigh the living system’s energy and material expenditure to generate them. For example, spiders store the liquid components of spider silk in a gland, converting them into silk thread when needed. Some threads have different characteristics, such as elasticity and UV reflectance, than others.

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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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Bio-compatible composite material from MIT is made of thin interlocking mineral layers that increase the strength and toughness.

Benefits

Increased strength
Increased toughness
Sustainable

Applications

Medical implants
Building materials

UN Sustainable Development Goals Addressed

Goal 3: Good Health & Wellbeing
Goal 9: Industry Innovation & Infrastructure

The Challenge

Materials can be incredibly strong at the nano-scale, but when put to test at a larger scale they become more prone to failure. This is partially due to the macroscopic arrangement of the particles. If the inherently strong particles are not arranged in a way that reinforces and emphasizes their strength, the overall material may be weak.

Innovation Details

The composite material is made with sheets of aragonite, similar to nacre. The sheets are formed on a wavy surface of chitosan film. The sheets are interlocked and the spaces between are filled with a silk fibroin. All in all, there are 150 interlocked layers that add up to be the thickness of a penny. Additionally, the material is bio-compatible and has been tested as a medical implant.

Biological Model

Nacre, or mother of pearl, is the iridescent material that forms the inner layer of the shells of some molluscs. It is made of aragonite formed into stacked hexagonal plates. The plates are twisted relative to each other by 5%. Each plate overlaps its top and bottom neighbors by 20% of its depth, leaving the twisted offset corners as overhangs. When nacre is subjected to force, the overhanging lips interlock, making the material tough.

See Related Strategy

close up photograph of purple, blue, pink and green shell

Biological Strategy

Interlocking Increases Material’s Toughness

Mollusks (Snails, Octopuses)

Plates in nacre increase toughness by interlocking

References

Journal article

Tough and Strong: Cross-Lamella Design Imparts Multifunctionality to Biomimetic Nacre

ACS Nano

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