Viscoelasticity, energy storage, and transmission and dissipation by extracellular matrices in vertebrates
The extracellular matrix (ECM) of vertebrates is an important biological mechanotransducer that prevents premature mechanical failure of tissues and stores and transmits energy created by muscular deformation. It also transfers large amounts of excess energy to muscles for dissipation as heat, and in some cases, the ECM itself dissipates energy locally. Beyond these functions, ECMs regulate their size and shape as a result of the changing external loads. Changes in tissue metabolism are transduced into increases or decreases in synthesis and catabolism of the components of ECMs. Viscoelasticity is an important feature of the mechanical behavior of ECMs. This parameter, however, complicates the understanding of ECM behavior since it contains both viscous and elastic contributions in most real-time measurements made on vertebrate tissues. The purpose of this chapter is to examine how time-dependent (viscous) and timeindependent (elastic) mechanical behaviors of an ECMare related to the hierarchical structure of vertebrate tissues and the macromolecular components found in specific tissues. In most ECMs, energy storage is believed to involve elastic stretching of collagen triple helices found in the cross-linked collagen fibrils comprising vertebrate connective tissues, and energy dissipation is believed to involve sliding of such collagen fibrils by each other during tissue deformation. It may be concluded that viscoelasticity differs markedly among different ECMs and is related to ECM hierarchical structure at the molecular and supramolecular levels of any particular vertebrate tissue.