Fibers Reinforce Hydrostatic Structures

close up photograph of the back of a yellow flower

Biological Strategy

Common to all organisms

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Image: Mark Freeth / Flickr / CC BY - Creative Commons Attribution alone

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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Animals

Kingdom Animalia (“having breath or soul”): Mammals, reptiles, sponges, corals, insects, birds

This kingdom captures a staggering variety of life—from the smallest coral polyp to the largest elephant. Although animal body shapes and life histories vary, there are a few defining characteristics. Animals are heterotrophs, meaning they gain energy from eating other organisms, and have developed from single-celled ancestors to highly diversified and coordinated multicellular forms. Unlike plant and fungi cells that have rigid cell walls, animal cells are bound by more flexible combinations of proteins. Most animals have cells organized into tissues and are able to move within their environment.

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Hydrostatic structures found in sunflowers and many other organisms serve various functions but almost always use helical fibers as reinforcement.

Introduction

In the natural world, many organisms rely on water-filled structures to provide support and maintain their shape. These structures, called hydrostatic skeletons (or hydroskeletons), are found in a wide range of plants and animals. Unlike more rigid skeletons made of bone or wood, hydrostatic skeletons are soft and flexible, yet they can still provide the strength and stiffness an organism needs. They are particularly common in fungi, young or herbaceous plants such as sunflowers, and in a variety of invertebrates, including worms, sea anemones, and squid. A key feature of these hydroskeletons is their arrangement of reinforcing fibers, which offers mechanical advantages that allow for flexibility and resistance to internal pressure.

The Strategy

At the heart of the hydrostatic skeleton’s function is the combination of internal pressure and helical fiber reinforcement. In many organisms, such as worms and squids, muscles contract against the fluid-filled cavity, generating pressure that helps the body maintain its shape. The surrounding fibers are arranged in a helical pattern, wrapping around the pressurized core like the fibers of a woven basket or a coiled spring. This helical structure is not unique to a single species or group; it appears in various life forms across plants and animals.

In plants, the young stems of herbaceous species like sunflowers have internal water pressure that allows them to remain upright without woody tissue. Helically arranged fibers in their stems reinforce the plant, preventing the stem from buckling or collapsing under pressure. This arrangement is also put to prominent use in the body walls of various soft-bodied animals, such as flatworms, roundworms, and mollusks.

The common thread in all these systems is the arrangement of fibers in a helical pattern. This configuration offers both flexibility and strength, as it allows the structure to resist deformation when pressure is applied. The variation in materials—collagen in animals, in plants—demonstrates how different organisms achieve similar mechanical outcomes using the resources available to them.

Image: Louisa Howard, Charles Daghlian, Dartmouth Electron Microscope Facility /

Scanning electron microscope image of x-section through Helianthus annus [sunflower] vascular bundle, showing the vascular cambium between the phloem and xylem.

Image: Louisa Howard, Charles Daghlian, Dartmouth Electron Microscope Facility /

Scanning electron microscope image of longitudinal section of Helianthus annus [sunflower] stem showing area of vascular bundles.

The Potential

The helical fiber arrangement found in natural hydrostatic skeletons offers promising insights for human engineering and design. For example, soft robotics could benefit from mimicking these structures to create flexible, pressure-resistant bodies that can adapt to changing environments. Medical devices, such as stents or soft prosthetics, could use helical fiber reinforcement to maintain shape and function without relying on rigid structures, providing comfort and durability.

In architecture and structural engineering, applying the principles of helical reinforcement could lead to more flexible and resilient materials that withstand internal and external pressures. Sustainable design strategies could draw inspiration from the way these natural systems use available materials, avoiding reliance on toxic substances or energy-intensive processes. By studying the ubiquitous yet elegant design of hydrostatic skeletons, humans can create innovative, sustainable technologies that harmonize with nature’s principles.

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Last Updated September 10, 2024

References

“With few exceptions, nature uses the second arrangement of fibers for her internally pressurized, water-filled cylinders. These structures (often termed ‘hydrostatic skeletons’ or ‘hydroskeletons’ as well as ‘hydrostats’) have helical reinforcing fibers. And this particular arrangement is no rare or once-evolved thing. It occurs in the stems of young herbaceous (nonwoody) plants such as sunflowers; it provides a wrapping for flatworms (platyhelminths and nemerteans), roundworms (nematodes), and segmented worms (annelids); it stiffens the body wall of sea anemones; it determines the response to muscle contraction of the outer mantle of squids; and it’s a major functional component of shark skin. The material of the fibers varies widely, the functions of these hydroskeletons are even more diverse, but the wrapping is almost always helical.” (Vogel 2003: 409)

Book

Comparative Biomechanics: Life's Physical World, Second Edition

Princeton University Press | 17/06/2013 | Steven Vogel

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