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Evaluating the functionality of an engineered material lies in the proper characterization of its material and functional properties. In the treatment of musculoskeletal disorders, engineered bone or fat tissue must behave as an adequate replacement else failure of the material could result in discomfort and further surgical procedures. A significant material characteristic that reflects tissue development is the mechanical properties (i.e. shear strength and viscosity). Shear strength and viscosity provide an indication of how efficient the material is in dissipating energy. Energy dissipation occurs naturally in many tissues including fat and can prevent damage to deeper tissues. Many of the techniques for determining a material’s shear modulus result in the destruction of the construct. However, few methods exist that can assess this property by evaluating a noninvasive cross-section of the construct. As a result a need exists for the development of a nondestructive way to assess the biomechanical properties of engineered materials both before and after they have been implanted. In an effort to improve the quality of constructs being produced, a recently developed magnetic resonance imaging (MRI) technique termed magnetic resonance elastography (MRE) was applied to evaluate the development of adipogenic (fat) and osteogenic (bone) tissue constructs derived from mesenchymal stem cells. MRE is a technique in which motion from a mechanical actuator is synchronized to a phase contrast imaging pulse sequence and used to measure the generated displacement. The captured displacement is displayed in shear wave images from which the properties of shear stiffness can be derived. For differentiation of the bone marrow-derived mesenchymal stems cells, the use of differentiation media kits was applied. Change in stiffness was observed over the four weeks of in vitro growth. Constructs initially measured at approximately 3 kPa developed into 22 kPa osteogenic and 1 kPa adipogenic tissues. Following four weeks in vitro growth, constructs were implanted in athymic mice and assessed with an MRE system custom built for animal imaging. The following thesis demonstrates the application of MRE to evaluate the mechanical properties of engineered constructs through in vitro growth and in vivo regeneration in an animal model.
Advisor: Shadi F. Othman