Much of the matter we typically deal with in our everyday lives is a solid. The idea of "solid" is typically said to be something that retains is shape. This is an idealization — it really means "it doesn't change its shape while I'm watching it."  As we know, a solid can change its shape if you push on it hard enough (by deforming or breaking). Some things that appear solid will change their shape slowly under the gentle pull of gravity. We consider a spring a solid even though we can make it deform (and see it do so). So perhaps what we really mean is that a solid is something that retains its shape if left alone and returns to its shape if we deform it (without breaking). That doesn't really work either since you can bend a solid wire and it stays in its bent shape. And what is it with butter, anyway? Deformable and elastic solids are an essential part of all biological organisms so perhaps we shouldn't lean too hard on trying to come up with a rigorous definition.

A uniform solid is one where every part of it is the same as every other part. This is a model statement, since all matter is composed of atoms and at the atomic level no matter is uniform since there are spaces between the atoms. When we say something is a "uniform solid" we mean that we are going to be examining it on a scale large enough that we can ignore atomic discontinuities.

Some uniform solids consist of atoms or ions in a regular pattern, such as diamonds composed only of carbon atoms in a regular lattice, or a salt crystal composed of sodium and chloride ions in a regular array. However, these are relatively uncommon and so for the most part we will consider solids to made up of molecules. Solids can be formed from a single compound where all the molecules are the same, or multiple compounds, with a mixture of molecules.  A multi-component solid is sometimes harder to characterize as it can be a non-uniform mixture or conglomerate.

Biological solids can be quite complex. They are often not uniform, but instead are composites, made of particles embedded in layers composed of different materials.  Your skin is a good example.  It is composed of a thick layer of connective tissue (dermis) underlain by a  membrane and overlaid by a surface (epidermal) layer. The dermis is actually a three dimensional network of collagen fibers that are embedded in a protein-polysaccharide matrix. The stretchiness of your skin is provided by additional elastin fibers which are distributed throughout the dermis.*

In spite of the complexities of biological materials, they still have all the properties of other natural or engineered materials.  These properties include density, bulk modulus, Young’s modulus, shear modulus and breaking strain.  These properties give us some idea of how a material will respond when subjected to some kind of force.  These properties can be measured by some basic methods, whether the materials are simple or complex.  We  describe these properties as well as what is known for some of the more interesting biological materials on the following pages:

* S. A. Wainwright, Mechanical Design in Organisms, (Princeton U. Press, 1982) p. 132

Karen Carleton and Joe Redish 10/20/11

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Last Modified: March 5, 2019