Elastic fibers: The near ideal linear springs of the extracellular matrix

This chapter attempts to paint a picture of elastin as the ultimate simple object of mechanics, the ideal linear spring. Tropoelastin, the monomer of the elastic fiber, comes very close to this idealization. This is a consequence of the large number of hydrophobic regions surrounded by water, which maintain a certain disorder. When stretched, water molecules reorganize themselves as the tropoelastin backbone unfolds, both reducing entropy and generating a recoil force. Following secretion, tropoelastin aggregates into elastic fibers, which still behave as a linear spring, but they are stiffer with a reduced range of linearity. Fibers and their networks also dissipate energy, which makes them only “near” ideal. Elastin’s design allows it to support the vasculature for at least 1 billion stretch cycles with near ideal performance. Evolution has achieved this by strong cross-linking of the ideal molecular spring providing stability and robustness at the expense of some compromise in ideality. Although biology favors order, disorder is a fundamental characteristic of elastin at both molecular and fiber scales. Hence, in contrast to structure-function relations, elastin’s elasticity can be said to derive from a highly conserved disorder-function relationship.