Properties of natural materials - Orthopaedic Biomaterials in Research and Practice - page 103

but not contributing to structural integrity or mechanical properties.
Even an element with such a key physiological role as iron represents
a contribution of 6 g to the weight of a 70 kg person or less than 0.01%
of the total. Disregarding its role as an oxygen ligand, iron dictates the
structure of the porphyrins and hemoglobin but is not even able to influ-
ence markedly the shape and mechanical properties of the red blood cell.
Sickle cell anemia, which is associated with reversible shape changes
of the red blood cell, is more directly related to changes in the internal
structure of the hemoglobin molecule than to its atomic composition.
Again, at the molecular level, a similar sparsity of structural ele-
ments persists. The most common structural molecule is collagen. It
constitutes 25% of the organic content of the human body, and despite
the presence of perhaps 10 related structural types, more than 90% of
it consists of types I and II. Several other proteins have structural roles,
and a number of other molecules promote tissue component adhesion
and water retention. The inorganic structural composition is even more
limited, with a single mineral, calcium hydroxyapatite, making up
almost all of it.
Thus, the great variety of structure and mechanical properties present
in the human body arise in a traditional engineering fashion: by com-
bining simple elements into complex assemblages designed to provide
the desired performance. Whatever one's view concerning the role of
creational or evolutionary processes, one must wonder at the richness of
results obtained with such poverty of resources.

Collagen It is not necessary to recite the structure of collagen in detail here. It is

sufficient to remember that both type I and type II consist of three indi-
vidual molecular chains assembled into a right-hand helical spiral fibril.
Each molecule contains perhaps 1000 amino acids, made up of one-
third glycine, one-third proline and hydroxyproline, and one-third other
amino acids. The periodicity and the ability to assemble extracellularly
into a spiral are apparently a consequence of the presence of a regu-
lar repeating triad (glycine plus proline or hydroxyproline plus another
amino acid residue) along the covalently bonded backbone of each chain.
These fibrils are stabilized by internal intermolecular van der Waals
bonds with occasional covalent cross-links. The resulting structures are
300 nm in length and 1.5 nm in diameter with a screw pitch (distance
between “turns”) of 0.27 nm. The fibrils, in turn, are assembled into
larger bundles or fibers with a longitudinal quarter stagger that results in
the familiar 64-68 nm banded structure that is the hallmark of collagen
when seen in the electron microscope.
Thus, a collagen fibril may be considered as a long coiled spring, like
the one formerly used to close screen doors, and a fiber as an assemblage
of springs connected together by a great number of less stiff spring-like
cross-links. This model clearly predicts the stress-strain behavior of
a single fiber, as seen in Figure 5.1. Small stresses tend to uncoil the
springs and to rotate the cross-links, whereas larger stresses begin to
stretch the covalent backbone and to break and remake the interfibril
cross-links. Thus, the curve has two relatively distinct regions, with the