Viscoelasticity and Bone Properties PDF

Viscoelasticity Dr/ Ayat Nada Viscoelasticity  Viscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation.  Viscoelasticity is defined as the time-dependent response of a material subjected to a constant load or deformation...

Viscoelasticity Dr/ Ayat Nada Viscoelasticity  Viscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation.  Viscoelasticity is defined as the time-dependent response of a material subjected to a constant load or deformation.  Synthetic polymers, wood, and human tissue, as well as metals at high temperature, display significant viscoelastic effects. In some applications, even a small viscoelastic response can be significant.  The viscoelasticity of blood and other biological fluids is important in understanding their flow behavior and interactions with biological tissues and medical devices.  In medical imaging, viscoelasticity can provide valuable information for diagnosing and monitoring diseases such as cancer, which can alter the mechanical properties of tissues. Therefore, understanding viscoelasticity is crucial for many biomedical applications.  Characterization of Properties of Tissues Tendon  A tendon is a fibrous connective tissue that attaches muscle to bone. Tendons may also attach muscles to structures such as the eyeball. A tendon serves to move the bone or structure. Tendon Tendon Non-Linear Elasticity The structure and composition of tendons allow for their unique mechanical behavior, reflected by a stress-strain curve consisting of three distinct regions :  Toe region: this is where “stretching out” or "un-crimping" of crimped tendon fibrils occurs from mechanically loading the tendon up to 2% strain. This region is responsible for nonlinear stress/strain curve, because the slope of the toe region is not linear.  Linear region: this is the physiological upper limit of tendon strain whereby the collagen fibrils orient themselves in the direction of tensile mechanical load and begin to stretch. The tendon deforms in a linear fashion due to the inter-molecular sliding of collagen triple helices. If strain is less than 4%, the tendon will return to its original length when unloaded, therefore this portion is elastic and reversible and the slope of the curve represents the Young's modulus.  Tendon Tendon Non-Linear Elasticity  Yield and failure region: this is where the tendon stretches beyond its physiological limit and intramolecular cross-links between collagen fibres fail. If micro-failure continues to accumulate, stiffness is reduced and the tendon begins to fail, resulting in irreversible plastic deformation. If the tendon stretches beyond 8-10% of its original length, macroscopic failure soon follows. Tendon Tendon Non-Linear Elasticity(con.)  Since there are many muscles in the body, each tendon differs in its function and therefore its mechanical properties.  For example, the Young’s modulus of the human patellar tendon is 660 ± 266 MPa, whereas the tibialis anterior tendon is about 1200 MPa.  Aging also significantly affects the mechanical properties of tendons: Young’s modulus of human patellar tendons aged 29–50 years is about 660 ± 266 MPa, but is about 504 ± 222 MPa in those aged 64–93 years. Tendon Tendon Viscoelasticity  Tendons also have viscoelastic properties (likely the result of collagenous proteins, water, and the interactions between collagens and proteoglycans), meaning their mechanical behaviour is dependent on the rate of mechanical strain.  In other words, the relationship between stress and strain for a tendon is not constant but depends on the time of displacement or load. Creep  Indicates increasing deformation under constant load.  This is in contrast with the usual elastic material, which does not elongate, no matter how long the load is applied. Tendon Tendon Viscoelasticity(con.) Stress Relaxation  Indicates stress acting upon a tendon will eventually reduce under a constant deformation Hysteresis or energy dissipation  When a viscoelastic material is loaded and unloaded, the unloading curve is different from the loading curve.  The difference between the two curves represents the amount of energy that is lost during loading.  If loading and unloading are repeated several times, different curves can be obtained. Bone BONE  Human bones, are divided into external cortical bone and internal cancellous bone.  Cortical bone is much denser than cancellous bone, harder, stronger and stiffer.  Bone is an organ that is able to change in relation to a number of factors such as: Hormones Vitamins Mechanical influences Bone Bone Non-Linear Elasticity  One of the primary differences between the cortical and cancellous bone is the increased porosity within the trabecular bone. This can be measured in terms of apparent density.  The cancellous bone has an apparent density ranging from 0.1 to 1.0 g/cm3 whereas for the cortical bone it is equal to 1.85 g/cm3.  The apparent density has a profound effect on the compressive stress-strain behavior of the cancellous bone which is quite different from the cortical bone.  The cortical bone is stiff and brittle ,but the cancellous bone is soft and ductile. Cancellous 0.1and 1.0  A comparison is shown between healthy bone (black curve) and more porous, osteoporotic cortical bone (blue curve). Bone Bone Viscoelasticity  Bone is a viscoelastic material, that exhibits both creep and stress relaxation.  Bone has noticeable viscoelasticity, as collagen fiber in bone matrix is viscoelastic.  Viscoelasticity of bone can arise from multiple factors related to structures on multiple length scales. Bone is a composite of the bio-polymer collagen and the bio-ceramic hydroxyapatite. Additionally, the collagen is plied in various directions around the bone.