Because of its viscoelastic nature, the stress response at any instant of time depends not only on the tissue strain at that time, but also on the history of the deformation. the stress–strain curve shows two distinct paths during the loading and unloading cycle ( Fig. 7.3c). Soft tissues also exhibit considerable hysteresis under a cyclic load, i.e. Alternatively, when the tissue is stretched to a certain stress level and then kept constant, its strain increases with time ( Fig. 7.3b). This phenomenon is called stress relaxation. When a tissue is stretched to a certain strain level and kept constant, the stress gradually decreases with time ( Fig. 7.3a).
#ELASTIC REALITY VHS INTRO SKIN#
Skin tissues show stress relaxation under constant strain and creep under constant stress. It is necessary to consider and specify the strain rate or load rate when conducting tests and reporting the results. With increasing strain rate, the material becomes stiffer and stronger. The mechanical properties are strain-rate-dependent. Soft tissues consist of both solid and fluid, and behave as viscoelastic material. Viscoelasticity describes time-dependent mechanical properties. Basic research in the field of HA has important keys to future medical applications of HA. The newly found cell biological functions of HA may lead to new medical applications of HA in the next generation. Therefore, HA obtains novel functions after its depolymerization ( Fig. Moreover, low molecular weight HA and HA oligosaccharides have cell biological functions that high molecular weight HA does not show ( 1–5). Recently, new HA-binding proteins have been found ( 71–73) however, most of their functions are still unknown. HA preparations can be used as medicines, medical devices or both. HA has important roles through its cell biological functions as well as its physicochemical properties. Recently, signal transductions via HA receptors have been elucidated. However, some of the effects of HA in medical applications are also based on the cell biological functions. Viscoelasticity is the main characteristic of HA. For this reason, haemodynamics and mechanical behaviour have to be studied simultaneously.ĪKIRA ASARI, in Chemistry and Biology of Hyaluronan, 2004 V. Viscoelasticity, in most biological organs, is determined by different causes it is an intrinsic property of the tissue, as in all polymers and elastomers, but may also be affected by the flow of liquid through the porous matrix. The response occurring at high relaxation times (from 1 s to h) can be explored by experiments of creep (a load is applied and kept constant, while deformation is followed) or relaxation (the material is strained and kept under constant strain, while the stress is tracked temporally). The response occurring at short or very short relaxation times (less than 1 s) must be investigated in dynamic tests, by applying an oscillating stimulus at constant frequency or sweeping through a frequency range. Viscoelastic behaviour normally occurs at different time scales (relaxation times) in the same material. This means that the response to a stimulus is delayed, and there is a loss of energy inside the material. Viscoelasticity is the time-dependent anelastic behaviour of materials. Barberis, in Biomaterials for Bone Regeneration, 2014 9.1.2 Viscoelasticity This non-linear phenomenon will be discussed in more detail in Chapter 3, along with viscosity. In fact, the coefficient of normal stresses depends on shear rate, Ψ 1 ( γ ˙ ). However, this is not true for all real viscoelastic liquids. According to Eq 2.5.14, the coefficient of normal stresses is also a material constant. The same is valid for the coefficient of normal stresses, determined in the theory of viscoelasticity as the second moment of a relaxation spectrum. Therefore, a separate chapter of this book is devoted to non-Newtonian flow and these effects are further discussed in Chapter 3.
The theoretical and applied meaning of non-Newtonian behavior of liquids is complex. Non-Newtonian flow is a non-linear effect not described within the framework of the linear theory of viscoelasticity. This effect is known as non-Newtonian behavior of liquids.
However, it is well known (and this is one of the basic effects of rheology) that viscosity of numerous liquids is not constant but depends on conditions of flow such as shear rates, η ( γ ˙ ), or shear stresses, η(σ). This regards Newtonian viscosity according to the definition of Eq 2.1.1. By this definition, it is expected that viscosity is constant, i.e., does not depend on shear rate or stress. Viscosity appears in the linear theory of viscoelasticity as the first moment of a relaxation spectrum, according to Eq 2.5.12. Dr.Avraam I Isayev, in Rheology Concepts, Methods, and Applications (Second Edition), 2012 2.8.1.1 Non-Newtonian viscosity
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