Viscoelastic Behavior of Allografts and Scaffolds Composed of Extracellular Matrices

Collagen fibers form the basic structural components of the extracellular matrix (ECM) of vertebrates that serve to: store elastic energy during muscular deformation, transmit stored energy into joint movement, and transfer excess energy from the joint back to the attached muscles for dissipation [1]. They also act as mechanotransducers by transferring stress borne by the musculoskeleton to the attached cells in order to regulate tissue metabolism, either upor down, as a result of changes in mechanical loading [2]. The viscoelastic behavior of ECM is very important in understanding the tissue reaction to implanted allografts and tissue engineered materials. It is known that excessive stress at the implant-tissue interface leads to intimal hyperplasia with vascular grafts as well as fat tissue necrosis associated with the pressure produced with breast implants and fat necrosis at the femoral catheter site after use of pressure to achieve closure [3]. For these reasons collagen fiber mechanical behavior in ECMs is intimately related to tissue reactions at the implant-tissue interface. However, analysis of the basic mechanisms behind this behavior is complicated by the viscoelasticity of collagenous tissues, the presence of other components besides collagen, and mechanotransduction processes. The viscoelasticity of collagenous tissues leads to non-linear behavior; however, recent studies have identified methods to analyze this behavior [2,46]. In this paper, we will review the relationship between ECM molecular and hierarchical structures, mechanical properties and mechanotransduction to better understand events that occur at the implant-host tissue interface.