General Mechanical Characteristic of Soft Tissues
Jon Trister MD
Soft connective tissues of our body are complex fiber-reinforced composite structures. Their mechanical behavior is strongly influenced by the concentration and structural arrangement of constituents such as collagen and elastin, the hydrated matrix of proteoglycans, and the topographical site and respective function in the organism.
Collagen is a protein which is a major constituent of the extracellular matrix of connective tissue. More than 12 types of collagen have been identified [M. E. Nimni and R. D. Harkness. Molecular structure and functions of collagen. In M. E. Nimni, editor, Collagen, pages 3–35. CRC Press, Boca Raton, FL, 1988.]
The most common collagen is type I, which can be isolated from any tissue. It is the major constituent in blood vessels. The rod-like shape of the collagen molecule comes from three polypeptide chains which are composed in a right- handed triple-helical conformation. Most of the collagen molecule consists of three amino acids; glycine (33%), which enhances the stability of the molecule, proline (15%) and hydroxyproline (15%) [G. N. Ramachandran. Chemistry of collagen. In G. N. Ramachandran, editor, Treatise on Collagen, pages 103–183. Academic Press, New York, 1967.].
The intramolecular crosslinks of collagen gives the connective tissues the strength which varies with age, pathology, etc. (for a correlation between the collagen content in the tissue, % dry weight, and its ultimate tensile strength see Table 1). The function and integrity of organs are maintained by the tension in collagen fibers. They shrink upon heating due to breakdown of the crystalline structure (at 65 C, for example, mammalian collagen shrinks to about one-third of its initial length [Y. C. Fung. Biomechanics. Mechanical Properties of Living Tissues. Springer-Verlag, New York, 2nd edition, 1993.]
Elastin, like collagen, is a protein which is a major constituent of the extracellular matrix of connective tissue. It is present as thin strands in soft tissues such as skin, lung, ligamentum flavum of the spine and ligamentum nuchae (the elastin content of the latter is about 5 times that of collagen).
The long flexible elastin molecules build up a three-dimensional (rubber-like) network, which may be stretched to about 2.5 of the initial length of the unloaded configuration. In contrast to collagen fibers, this network does not exhibit a pronounced hierarchical organization. As for collagen, 33% of the total amino acids of elastin consists of glycine. However, the proline and hydroxyproline contents are much lower than in collagen molecules.
The mechanical behavior of elastin may be explained within the concept of entropic elasticity. As for rubber, the random molecular conformations, and hence the entropy, change with deformation. Elasticity arises through entropic straightening of the chains, i.e. a decrease of entropy, or an increase of internal energy .
Elastin is essentially a linearly elastic material (tested for the ligamentum nuchae of cattle). It displays very small relaxation effects (they are larger for collagen).
[C. A. J. Hoeve and P. J. Flory. The elastic properties of elastin. J. Am. Chem. Soc., 80:6523–6526, 1958;G. A. Holzapfel. Nonlinear Solid Mechanics. A Continuum Approach for Engineering. John Wiley & Sons, Chichester, 2000.]
The mechanical properties of soft tissues depend strongly on the anatomical location, co-morbid conditions, age, gender, physical, hormonal and chemical environmental factors such as temperature, osmotic pressure, pH, and on the strain rate.