Material Interaction Varies Bone Strength

Biological Strategy

Mammals

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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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Mammals

Class Mammalia (“breast”): Bats, cats, whales, horses, humans

Mammals make up less than 1% of all animals on earth, but they include some of the most well-known species. We know first-hand some of the characteristics that make mammals unique, like having hair, being able to sweat, and producing milk through mammary glands. Another critical shared feature is a set of highly-specialized teeth. Unlike sharks or alligators, for example, whose teeth are generally all the same size and shape, mammals have differently shaped teeth in different areas of the jaws to target specific foods or foraging strategies.

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Surface of bone fibril in mammals alters mechanics of collagen by interfacial interaction between collagen and hydroxyapatite.

The Strategy

Bone is a hard material made up mostly of a substance known as collagen. Collagen in bone self-assembles in a staggered, parallel manner to form fibrils. Between the staggered molecules of collagen, the mineral part of bone fills in. This mineral is known as hydroxyapatite (HAP), and it is a crystalized form of calcium phosphate. The way that HAP orients between collagen molecules alters the way that the bone responds to mechanical tests of its properties. “The HAP crystal platelets nucleate in hole zones, and grow in length along collagen long axis, and in width, along channels influenced by the presence of mineral in the proximity” (Katti et al. 2010). Much of the rest of the space near the surface of bone is occupied by water. Collagen, HAP, and water all interact with one another in varying manners. Researchers struggle to study the mechanical properties of collagen because of the varied responses they get along the surface. The HAP mineral makes the collagen stiffer in some areas and the presence of water (or lack of HAP) makes the collagen less stiff in other places along the surface.

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Bones Heal Themselves

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Last Updated August 23, 2016

References

Journal article

Directional dependence of hydroxyapatite-collagen interactions on mechanics of collagen

Journal of Biomechanics | 09/03/2010 | Dinesh R. Katti, Shashindra M. Pradhan, Kalpana S. Katti

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