This study provides an overview of the implementation of a nonlinear microstructural constitutive model in ABAQUS employing a user subroutine at the level of the biomedical engineer. Two different element formulations are employed: a continuum incompressible and a plane stress incompressible. All examples are validated by performing a number of deformations on 2D and 3D square elements and comparing the analytical formulation in a programming language and the user subroutine in ABAQUS. Application models will be presented that provide a deeper look into the impacts of soft tissue deformation, damage, and fracture. Additionally, we investigate the mechanical behavior of skin layers in terms of the nominal stress-strain curve using uniaxial and cyclic loading tests on porcine skin specimens in two forms: dermis integrating epidermis and hypodermis. Experiments were performed on specimens from the belly and breast of the pigs and under both orthogonal orientations with respect to the spine direction. All tests were carried out at room temperature with cyclic loading at a constant strain rate and increasing stretch increments. Finally, data is fitted using microstructural constitutive model.

Despite the alarming incidence rate and potential for debilitating outcomes of sports-related concussion, the underlying mechanisms of injury remain to be expounded. Since as early as 1950, researchers have aimed to characterize head impact biomechanics through in-lab and in-game investigations. The ever-growing body of literature within this area has supported the inherent connection between head kinematics during impact and injury outcomes. Even so, traditional metrics of peak acceleration, time window, and HIC have outlived their potential. More sophisticated analysis techniques are required to advance the understanding of concussive vs subconcussive impacts. The work presented within this thesis was motivated by the exploration of advanced approaches to 1) experimental theory and design of impact reconstructions and 2) characterization of kinematic profiles for model building. These two areas of investigation resulted in the presentation of refined, systematic approaches to head impact analysis that should begin to replace outdated standards and metrics.

<p>Musculoskeletal degradation and musculoskeletal injuries place a substantial burden on the healthcare system. Advancing the understanding and prevention of the injury potential associated with these injuries in various demographics as well as advancing performance optimization requires knowledge of the loading distribution among the various musculoskeletal tissues at the joints. Accurate muscle force estimates are needed for characterizing these distributions due to their influence on the loading of the system. This dissertation discusses</p> <p>the development and validation of a physiologically-driven skeletal muscle force model that is suitable for application on an individualized level. The derivation of the skeletal muscle force model began with dimensional analysis and a selection of critical parameters that define muscle force generation. One of the key parameters included was measured muscle voltage using electromyography sensors. This provided the model with the ability to be easily used</p> <p>in application-based studies. It also incorporated the muscle force-length, force-velocity, and force-frequency curves, providing an even stronger physiological basis to the model. Validation was performed by multiple studies using experimental data from subjects conducting exercises chosen to target specific muscles of interest. Data was collected from a Vicon Vero motion capture system, an instrumented Bertec treadmill, and Delsys Trigno electromyography sensors. The first study analyzed the ankle joint of seventeen subjects using the two Newton-Euler equations of rigid body motion and the skeletal muscle force model. The average percent error across all subjects was 8.2% and ranged from 4.2% to 15.5%. The second study analyzed the sensitivity of two sets of parameters within the model. The first was conducted on a set of observed and fitted constants from the dimensionless pi terms and aimed to identify which, if any, could be excluded from an optimization routine. Results indicated that only two of the nine constant parameters needed to be optimized. The second sensitivity analysis focused on the anatomical kinematic parameters in order to identify the impact that the incorporation of MRI scans for subject-specific anatomical models would have on the accuracy of the model’s output. Results demonstrated sensitivity to the muscle insertion points, suggesting that the use of MRI scans could increase the accuracy of the model. The third study was a case study focused on evaluating the assumption of a constant within the skeletal muscle force model remaining constant over time. Results indicated that the collection of maximum EMG recordings for these studies may not have been controlled to a desirable level and that the inclusion of specialized equipment for maximum EMG recordings would likely validate this assumption. The final study analyzed the</p> <p>knee joint of ten subjects in a similar fashion to that of the ankle joint. The goal was to observe the model’s performance on a more anatomically complex joint. The average percent error across all subjects was 20.6%, approximately two times higher than the ankle joint.</p> <p>However, the majority of the error associated with this study came from the deviation in calculated moments about an axis of much smaller importance and magnitude than the primary flexion/extension axis. When errors were excluded from this axis, the average percent error for all subjects was 8.8%, almost identical to that of the ankle joint application. These findings as a whole indicate that the model has predictive ability and is capable of providing reasonable estimates of both muscle forces and surrounding musculoskeletal tissue loading. Therefore, the model could be used in various biomechanical advancements and applications in injury prevention, performance optimization, tissue engineering, prosthetic design, and more.</p>

<p>Osteoporosis causes over two million skeletal fractures in the United States every year in people over 50 years of age. Age-related bone loss results from imbalanced bone turnover mainly caused by decreases in sex hormones and skeletal mechanobiology. Estrogen receptor β (ERβ) in osteocytes (Ot) has been proposed to mediate skeletal structural adaptations in response to estrogen and mechanical stimuli. However, direct <em>in vivo</em> studies on Ot-ERβ are lacking, and relevant <em>in vitro</em> studies are mostly made in two-dimensional (2D) culture models, whose cellular environment restricts Ot morphology and biology. To better understand the mechanisms of estrogen-ERs in age-related bone loss, it is important to investigate the role of Ot-ERβ in skeletal turnover in response to sex hormonal and mechanical cues and develop a novel 3D culture model that can reproduce Ot morphology for future <em>in vitro</em> ER studies. The role of Ot-ERβ in bone turnover and skeletal adaptive response to mechanical load were examined in male and female mice at 12wk and 30wk old. Ot-ERβ shows age- and sex-dependent effects on bone morphology. Young male mice with Ot-ERβ deletion (ERβ-dOT) showed increased vertebral cancellous bone, whereas decreased cortical and cancellous vertebral bone mass appeared in adult male ERβ-dOT mice. No difference in bone mass occurred in female mice between genotypes. Ot-ERβ mediates tibial mechanoadaptation in cortical but not cancellous in young and adult male mice but plays an inhibitory role in young female mice during cortical mechanoadaptation. Gonadectomy studies on young adult mice revealed that deletion of Ot-ERβ inhibits the sex hormone withdrawal-induced decreases in bone mass and skeletal strength for male mice but did not play a major role for female mice. Lastly, a novel 3D <em>in vitro</em> culture system was developed using collagen-mineral composites for investigating culture mineralization, osteocyte biology, and osteocyte-osteoblast interaction. Cell viability and cellular differentiation were validated after 3 days and 56 days of culture. Optimal PSC-HA culture conditions were determined based on osteocyte differentiation, gene expression analyses, and tissue mineralization. Overall, this work takes novel steps to demonstrate the <em>in vivo</em> role osteocyte-ERβ plays in skeletal morphology and mechanobiology and develops a novel <em>in vitro</em> 3D culture using PSC-HA composites. These advances will contribute to future mechanistic studies of sex hormone receptors in osteoblasts and osteocytes in age-related bone loss using controlled <em>in vitro</em> environments. </p>

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The femur is the longest limb bone found in humans. Almost all the long limb bones found in terrestrial mammals, including the femur studied herein, have been observed to be loaded in bending and are curved longitudinally. The curvature in these long bones increases the bending stress developed in the bone, potentially reducing the bone’s load carrying capacity, i.e. its mechanical strength. Therefore, bone curvature poses a paradox in terms of the mechanical function of long limb bones. The aim of this study is to investigate and explain the role of longitudinal bone curvature in the design of long bones. In particular, it has been hypothesized that curvature of long bones results in a trade-off between the bone’s mechanical strength and its bending predictability. This thesis employs finite element analysis of human femora to address this issue. Simplified human femora with different curvatures were modeled and analyzed using ANSYS Workbench finite element analysis software. The results obtained are compared between different curvatures including a straight bone. We examined how the bone curvature affects the bending predictability and load carrying capacity of bones. Results were post processed to yield probability density functions (PDFs) for circumferential location of maximum equivalent stress for various bone curvatures to assess the bending predictability of bones. To validate our findings on the geometrically simplified ANSYS Workbench femur models, a digitally reconstructed femur model from a CT scan of a real human femur was employed. For this model we performed finite element analysis in the FEA tool, Strand7, executing multiple simulations for different load cases. The results from the CT scanned femur model and those from the CAD femur model were then compared. We found general agreement in trends but some quantitative differences most likely due to the geometric differences between the digitally reconstructed femur model and the simplified CAD models. As postulated by others, our results support the hypothesis that the bone curvature is a trade-off between the bone strength and its bending predictability. Bone curvature increases bending predictability at the expense of load carrying capacity.

In order to understand the relationship between feeding behavior and the evolution of mammalian skull form, it is essential to evaluate the impact of bite force over large regions of skull. There are about 1,100 bat species worldwide, which represent about 20% of all classified mammal species. Hence, a study in the evolution of bat skull form may provide general understanding of the overall evolution of skull form in mammals. These biomechanical studies are generally performed by first building solid Finite Element (FE) models of skull from micro CT scans. This process of building FE models from micro CT scans is both tedious and time consuming. Therefore a new approach is developed in this research project to build these FE models quickly and efficiently. I have used SolidWorks to build a parameterized, three dimensional surface CAD model of a skull of the short-tailed fruit bat, Carollia perspicillata, by using coordinate data from an STL model of the species. The overall shape of this model closely resembled that of solid model of C. perspiciallata constructed from micro CT scans. Finite element analyses of the solid and surface models yielded comparable results in terms of magnitude and distribution of von Mises stress and mechanical advantage. Using this parametric surface model, the FE plate or shell element models of different bat species were generated by varying two parameters, palate length and palate width. Parametric analyses were performed on these FE plate models of skulls and response surfaces of performance criteria: von Mises stress, strain energy and mechanical advantage were generated by varying the input parameters. After generating response surfaces, species of bats from the morphologically diverse family of New World leaf-nosed bats (Family Phyllostomidae) were overlain on these response surfaces to determine which portions of the performance design space (palate length X width) are and are not occupied. These plots serve as a foundation for understanding the affect of different performance criteria on the evolution of bat skull form.

This study proposed a finite element procedure for protein normal mode analysis (NMA). The finite element model adopted the protein solvent-excluded surface to generate a homogeneous and isotropic volume. A simplified triangular approximation of coarse molecular surface was generated from the original surface model by using the Gaussian-based blurring technique. Similar to the widely adopted elastic network model, the finite element model holds a major advantage over standard all-atom normal mode analysis: the computationally expensive process of energy minimization that may distort the initial protein structure has been eliminated. This modification significantly increases the efficiency of normal mode analysis. In addition, the finite element model successfully brings out the capability of normal mode analysis in low-frequency/high collectivity molecular motion by capturing protein shape properties. Fair results from six protein models in this study have fortified the capability of the finite element model in protein normal mode analysis.

<p>  </p> <p>Traumatic Brain Injuries (TBI) are a major cause of disability and death in the United States. One of the greatest consequences of the disease is the resulting long-term damage, especially in milder injury cases where the damage is initially subclinical and thus lacking acutely observable manifestations that over time can compound significantly. Among these chronic issues, Alzheimer’s Disease (AD) is one of the most serious. While multiple studies demonstrate an increased likelihood of developing neurodegenerative diseases in response to TBI, the underlying mechanisms remain undefined and no current treatment options are available. Multiple hypotheses have been postulated based on various animal and clinical models, which have contributed a great deal to our current knowledge base and implicated several targets of interest in this pathway (i.g. oxidative stress, inflammation, disruptions in proteostasis). While extremely valuable, these <em>in vivo</em> procedures and analyses are physiologically and ethically complex: there is currently no model capable of separating and visualizing TBI-induced sub-cellular damage in the moments (seconds) immediately following injury, and the subsequent associated long-term changes (AD). In addition, no mechanistic study has been performed to link mechanical-trauma independently with neurodegeneration initiation via protein aggregation. It is clear that additional investigative tools are needed to rectify these intricate issues, and while <em>in vitro </em>methodologies generally offer the type of resolution required, no such model replicates these phenomena. Therefore, we introduce the “TBI-on-a-chip” <em>in vitro </em>concussive model, with a series of concomitant targeted-experiments to address this urgent, currently unmet need. This dissertation work describes the development of our cellular trauma model, featuring a multi-disciplinary approach that provides investigatory opportunities into cellular mechanics, molecular biology, functional alterations (electrophysiology), and morphology, in both primary and secondary injury. Utilizing this model, we directly observe evidence of impact-induced electrical/functional and biochemical consequences, in addition to isolating oxidative stress as a key, contributing component. Taken together, these collective efforts suggest that oxidative stress may be a viable target for both acute and chronic potential therapeutic interventions.</p>