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Mechanical Characteristics of Soft Tissues
Adil Al-Mayah
1.1 Introduction
General • Source of Mechanical Response in Soft Tissues • General Mechanical Behavior
1.2 Linear Elasticity
Model Description • Elastic Properties of Human Tissues
1.3 Hyperelasticity
Model Description • Hyperelastic Properties of Human Tissues
1.4 Viscoelasticity
Model Description • Viscoelastic Properties of Human Tissues
1.5 Poroelasticity
Model Description • Poroelastic Investigations of Human Tissues
1.6 Isotropy and Homogeneity of Tissues
1.7 Conclusion
1.1 Introduction
1.1.1 General
Soft tissues are defined as the tissues that support and connect body structures. They include skin, muscles, fat, tendons, ligaments, blood vessels, nerves, cartilages, and other tissue matrices. In some cases, they are simply defined as body tissues that exclude hard tissues such as bones, teeth, and nails. As bones, a major component of nonsoft tissues, represent 12%–15% of the human body mass, it can be concluded that most of the human body is composed of soft tissues. Soft tissues are known for high flexibility and soft mechanical properties, differentiating them from mineralized stiff tissues, such as bones (Holzapfel 2001).
1.1.2 Source of Mechanical Response in Soft Tissues
Modeling biological phenomena and material behavior can be performed at the atomic, molecular, microscopic, and macroscopic scales. The mechanical response of tissues can be well addressed at the macroscopic scale of biological modeling and to a limited extent at the microscopic (multicellular) scale. Therefore, these scales will be the focus of this section.
At the cellular level, each cell consists of a cellular membrane, cytoplasm, and nucleus. The membrane and structural cytoplasmic component, known as the cytoskeleton, are the main contributors to the structural performance of cells. The membrane separates the intra- and extracellular environments and plays a role in the interaction between these two environments. The cytoskeleton provides structural integrity of cells. It consists of three types of filaments: (1) actin (8 nm diameter), (2) an intermediate rope-like structure (10 nm diameter), and (3) microtubules (25 nm diameter). Actin filaments are stiffer in extension than microtubules but they rupture at a much lower extension. The intermediate filaments exhibit an intermediate extensional stiffness at lower extensions, but they can sustain much larger extensions than the other two types of filaments while exhibiting a nonlinearly stiffening response. The microtubules are long cylinders that exhibit high bending stiffness compared to other filaments.
At the tissue level, typical tissues consist of three main components: (1) epithelial, (2) stromal, and (3) mesenchymal cells. Epithelium is one of the four basic animal tissues along with connective, muscle, and nervous tissues. It is composed of packed epithelial cells arranged in varying numbers of layers that line the body cavities and surfaces and form glands. Epithelial tissues’ main functions include protection and secretion. The epithelial cells are attached to each other at many locations through adherence junctions, tight junctions, and spot-like adhesion (desmosomes). These epithelial cells rest on a membrane through a keratin-based cytoskeleton and adhesion-based junctions (hemidesmosomes). The thin semipermeable membrane separates the epithelium from the stroma. The stroma is a loose connective tissue that may rest on layers of muscles or bones. It is composed of extracellular matrix (ECM), blood vessels, nerves, and lymphatic vessels. The ECM consists of a scaffolding of fibers, such as collagen and elastin, embedded in a mixture of water and glycoproteins, and is of particular interest in terms of mechanical performance.
Collagen represents the main structural component of hard and soft tissues in animals, and is responsible for the mechanical performance and strength of many elements of the human body, including blood vessels,...
