Micro-Mechanical Approaches for the Hierarchical Modeling of Soft Biological Tissues
Johns Hopkins University
Load-bearing soft tissues are abundant throughout the human body, including such diverse examples as skin, cornea, tendons, and blood vessels. The mechanical characterization of these tissues is important for applications such as tissue engineering, disease state pathology, and medical device/patient interface modeling. The mechanical properties of soft tissues arise from the underlying collagen microstructure, which varies across the body depending on tissue function. Such specialized microstructures are thought to arise in part from the ability of soft tissues to self-adapt to the mechanical environment by a process known as growth and remodeling. Growth and remodeling is a normal part of tissue development and maintenance; however, it is suspected that an imbalance in the process may contribute to disease states such as osteoarthritis and glaucoma. Though growth and remodeling are well documented, the mechanisms driving the process are not well understood. This work develops a hierarchical, structure-based modeling approach for planar collagenous tissues based on the underlying collagen microstructure. The approach was applied both to characterize human skin mechanics for prosthetic/residual limb interface modeling, and to simulate potential fiber-level mechanisms of the growth and remodeling process. The nonlinear, anisotropic properties of human skin tissues were measured using full-field inflation testing, and a novel analytical method was developed to fit constitutive model parameters to the inflation test data while accounting for bending stresses. Two different anisotropic constitutive models were considered: a fully integrated distributed fiber model, and a more computationally efficient generalized structure tensor model. Finite element analysis was used to show that only the fully integrated distributed fiber model could reproduce the experimentally measured anisotropy of skin tissue. To investigate potential mechanisms of growth and remodeling, the fully integrated model was extended to incorporate a micro-mechanical description of the collagen fibers. This enabled the prescription of fiber-level evolution equations for strain-protected enzymatic degradation and constant collagen deposition as potential mechanisms of the growth and remodeling process. The degradation model was calibrated to fibril-level experiments and used to predict tissue-level experiments for model validation. Strain homeostasis was achieved when the degradation model was paired with constant collagen deposition, supporting these two mechanisms as potential mechanisms of the growth and remodeling process.