Constitutive modeling and testing of biological soft tissue.

Biological soft tissue is a complex engineering material whose mechanical response is intimately connected to its heterogeneous, multilayered, biphasic structure. Tissues such as skin and heart are composed of load bearing fibrous networks of collagen and elastin; ground substance containing proteins, sugars, and water; cells such as fibroblasts; and other tissue specific components like vasculature and hair follicles. The presence and orientation of these components result in a constitutive response that is nonlinear, anisotropic, and viscoelastic. Constitutive models have been proposed to model the mechanical behavior of these tissues but most are phenomenological such that the material parameters do not reflect the histology of the tissue. A sound microstructurally based model has the capability to accurately capture the constitutive response of tissue, satisfy basic continuum mechanical requirements by virtue of its physical basis, and yield clinically useful metrics of the mechanical response that relate to the underlying tissue histology. A new microstructural model has been developed that has as its basis an orthotropic network of nonlinear elastic fibers within an incompressible medium. The resulting constitutive response predicted by this model is nonlinear, hyperelastic, and orthotropic. The parameters within the model reflect the orientation and alignment of the underlying load bearing collagen network and thus reflect the histology of the tissue. This model has been used to fit data from uniaxial tension and load controlled biaxial tension experiments. Additionally, the orthotropy and nonlinearity demonstrated in newly acquired data from uniaxial compression tests on canine myocardium have been successfully captured using this model. Most mechanical tests on soft tissue do not result in homogeneous deformations, due to either complex boundary and loading conditions or tissue heterogeneity within the sample. For application of the new model to these problems, the model has been incorporated into commercial finite element code. The resulting computational model has been used to demonstrate the effects of spatially varying fiber orientation on the response of tissue to various common experimental protocols.Ph.D.Applied SciencesBiomedical engineeringMechanical engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/124834/2/3016806.pd