Helical Wound Fibres Increase Toughness

Biological Strategy

Plants

Eleanor Banwell

Image: Daniel Toh / Shutterstock / Some rights reserved

Physically Assemble Structure

Living systems use physical materials to create structures to serve as protection, insulation, and other purposes. These structures can be internal (within or attached to the system itself), such as cell membranes, shells, and fur. They can also be external (detached), such as nests, burrows, cocoons, or webs. Because physical materials are limited and the energy required to gather and create new structures is costly, living systems must use both conservatively. Therefore, they optimize the structures’ size, weight, and density. For example, weaver birds use two types of vegetation to create their nests: strong, a few stiff fibers and numerous thin fibers. Combined, they make a strong, yet flexible, nest. An example of an internal structure is a bird’s bone. The bone is comprised of a mineral matrix assembled to create strong cross-supports and a tubular outer surface filled with air to minimize weight.

Search this Function

Manage Shear

The effect of shear stress on a living system is parallel internal surfaces sliding past each other. Slippage occurs in parallel with the force. Think about holding two wooden boards on top of each other and sliding one to the right and the other to the left. This may be easy until you add glue, which increases their shear strength and makes them harder or impossible to slide. Shear can occur in solids, liquids, and gases. Living systems must increase their shear strength to overcome these types of forces. For example, darkling beetles lock their wings together in flight to prevent lateral movement by using many small hairs on each wing. These hairs interlock to provide shear strength, just as two hair brushes put together would be difficult to slide past each other.

Search this Function

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.

Search this Function

Manage Tension

When a living system is under tension, it means there is a force pulling on it, like a person pulling on a rope tied to a horse. When applied to a living system, unless the system is completely rigid, the result is that it gets stretched. If stretching exceeds the strength of the living system’s material, it can damage it. Living systems manage tension using materials that are flexible and stretchable enough to survive most tension that occurs in their environment. The ocean’s intertidal zone offers a good example. The waves and incoming and outgoing tides put tension on soft-bodied organisms. Mussels resist tension with flexible threads that hold them onto rocks; in contrast, large algae have stretchy fronds.

Search this Function

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.

Search this Function

Plants

Phylum Plantae (“plants”): Angiosperms, gymnosperms, green algae, and more

Plants have evolved by using special structures within their cells to harness energy directly from sunlight. There are currently over 350,000 known species of plants which include angiosperms (flowering trees and plants), gymnosperms (conifers, Gingkos, and others), ferns, hornworts, liverworts, mosses, and green algae. While most get energy through the process of photosynthesis, some are partially carnivores, feeding on the bodies of insects, and others are plant parasites, feeding entirely off of other plants. Plants reproduce through fruits, seeds, spores, and even asexually. They evolved around 500 million years ago and can now be found on every continent worldwide.

Search this Living System

Cellulose fibers in plant stems increase toughness by winding around tubes at an angle

Materials that are strong and stiff while not being brittle are very useful. However, resistance to fracture is linked to flexibility, meaning these two properties are in conflict with each other: stiff and hard materials are prone to cracking, while those that don’t crack tend to be weaker and prone to bending. Composite materials achieve improved performance, usually by embedding a strong and stiff material in a matrix of a softer and more flexible one to combine the best properties of both.

Wood is a trusted composite that humans have used for building for millennia. Wood is a composite of fibers embedded in a matrix, but its performance is better than models suggest it should be based on its composition alone: the arrangement of the fibers matters. In trees, hollow tubes (which double as transport vessels) are reinforced with spiral-wrapped bundles of cellulose fibers. Wood is hierarchical and bundles of vessels form concentric layers that contribute to strength and stiffness. Because the fibers are helically wound, they do not easily snap or pull out of the matrix—the two main ways in which fiber composites commonly fail. Instead, fibers must unwind as they are pulled out and broken, increasing the energy required to fracture wood.

Bamboo is a woody grass with material properties comparable with steel, which it is often used in place of. Like wood, bamboo is a cellulose composite with helically-wound fibers wrapped around hollow vessel bundles. However, the structure of vessels in bamboo is more complex. In bamboo, vessels are wrapped in a series of concentric layers of cellulose fibers that alternate between thick layers of fibers wound in one direction (at an angle of about 45° to the vertical) and thin layers wound in the opposite direction (at an angle of around 5° to the vertical). When a plant bends, one side of the stem, and of each vascular bundle, is compressed, while the opposite side stretches. When the cellulose fibers are compressed they can bulge and buckle, leading to failure of the material. The inclusion of thin, oppositely-coiled cellulose layers restrain the main load-bearing thick layers, preventing them from bulging and giving bamboo its impressive performance.

When they do break, both wood and bamboo absorb a lot of the energy and broken stems often remain attached, meaning failure is safer than in other materials.