Bones Heal Themselves

black and white x ray of a broken human clavicle

Biological Strategy

Vertebrates (Mammals, Fish, Birds, Reptiles)

Sam Stier

Image: Harlie Raethel / Unsplash / Free non-commercial use

Optimize Shape/Materials

Resources are limited and the simple act of retaining them requires resources, especially energy. Living systems must constantly balance the value of resources obtained with the costs of resources expended; failure to do so can result in death or prevent reproduction. Living systems therefore optimize, rather than maximize, resource use. Optimizing shape ultimately optimizes materials and energy. An example of such optimization can be seen in the dolphin’s body shape. It’s streamlined to reduce drag in the water due to an optimal ratio of length to diameter, as well as features on its surface that lie flat, reducing turbulence.

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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.

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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 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.

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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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Vertebrates (Mammals, Fish, Birds, Reptiles)

Subphylum Vertebrata (“jointed”): Mammals, fish, birds, reptiles

Vertebrates are a subgroup of chordates, which all have a flexible, rod-shaped structure that supports the body called a notochord. Non-vertebrate chordates include tunicates, hagfish, and lancelets. In vertebrates the notochord ultimately becomes part of the spine, usually encased in bony joints. All chordates also have a dorsal hollow nerve cord that forms the nervous system and pharyngeal slits that open outside the body during development (and persist to form gills in aquatic animals). Lastly, chordates have a tail at the back of the body––it’s just that sometimes you need an x-ray to see it.

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Bones maintain skeletal homeostasis by balancing the activities of osteoblasts and osteoclasts.

Introduction

One stark difference between the things humans make and the things the rest of nature makes is that when something we make breaks, it doesn’t fix itself. Break a bone, though, and it begins to heal almost immediately. This instant repair is the work of a diverse team of cell types, who appear like royal artisans called to reshape and rebuild overnight.

A science illustration of the bone healing process.

The Strategy

Within a couple hours of a bone breaking, a blood clot containing special immune cells known as phagocytes (“cells that eat” in Greek) forms around the break. The phagocytes clear out bone fragments and any unwanted microbes that are trespassing at the site. Next, a soft callus develops, made of collagen––the same material that keeps your skin supple. Cells called osteocytes (simply meaning “bone cells”) seem to coordinate the behavior of two other kinds of cells, the osteoblasts (“bone builders”) and osteoclasts (“bone removers”).

Long filaments emanating from the cell body of the osteocyte mechanically sense changes to the bone’s structure, and signal osteoblasts to begin construction. The osteoblasts work in groups to deposit calcium-and-phosphate crystals into the soft collagen callus, hardening the substance into new bone tissue. Finally, osteoclast cells perform the clean-up, removing any unwanted bone around the fracture site until the repaired bone assumes a shape similar to its appearance before injury.

Image: Shahfa84 / Wikipedia / CC BY - Creative Commons Attribution alone

An osteocyte bone cell showing extensive filaments (canaliculi), which mechanically sense changes to bone stress, and signal osteoblasts to begin construction.

"The idea of self-healing has inspired humans to completely reimagine the things we make and the way we make them."

The Potential

The idea of self-healing has inspired humans to completely reimagine the things we make and the way we make them. Imagine, for example, bridges that could fix developing microfractures before they became larger cracks and eventually sacrificed the bridge’s integrity. Materials scientists have developed models of self-healing concrete that do just this, filling the matrix with tiny repair kits ready to release their contents to fill developing cracks. Bridges built this way would be safer, last longer, and could reduce the vast amounts of material people use for infrastructure.

Concrete robots may seem impractical initially, but think of advanced ceramics and the applications they have had. There could also be self-healing metallic alloys, perhaps incorporating solder particles that could be activated with targeted application of light, heat, or electrical current. Additionally, self-healing materials wouldn’t need to be used for the entirety of a machine, but could be used in smaller pieces at strategically significant points in the structure.

And what about the idea of embedded, living repair teams in our constructions? Microscopic organisms could one day lead not only to self-healing structures, but self-improving ones, applying nature’s deep responsiveness to laws of physics and geometry we still have yet to learn ourselves.

Related Innovations

Innovation: Industry

Generative Design Software Inspired by Slime Mold and Human Bones

Autodesk

Project Dreamcatcher from AutoDesk Research is a generative design system that uses nature-based algorithms to create optimized design solutions in real time.

Innovation: Industry

Self-Healing Concrete Inspired by Bones

Hendrik Marius Jonkers

Self-repairing concrete contains bacteria that produce limestone to fill any cracks that occur.

Last Updated March 1, 2024

References

“Bone remodeling is the result of the coordinated activity of osteoblasts, which form new matrix, and osteoclasts, which resorb bone…Bone remodeling is a temporally and spatially regulated process that results in the coordinated resorption and formation of skeletal tissue (1). Bone remodeling is carried out in basic multicellular units in which osteoclasts resorb bone and osteoblasts form newly synthesized matrix in a coordinated process that takes about 4 months…The number and function of osteoclasts and osteoblasts are regulated by extracellular and intracellular signals acting in a coordinated fashion to maintain skeletal homeostasis.” (Canalis 2008:1)

Journal article

Notch Signaling in Osteoblasts

Science Signaling | 24/06/2008 | E. Canalis

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