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A polyconvex continuum-level proteoglycan Cauchy stress function was developed based on the continuum electromechanical Poisson-Boltzmann unit cell model for proteoglycan interactions. The resulting proteoglycan model was combined with a novel collagen fibril model and a ground substance matrix material to create a polyconvex constitutive finite element model of articular cartilage. The true collagen fibril modulus , and the ground substance matrix shear modulus , were varied to obtain the best fit to experimental tension, confined compression, and unconfined compression data for native explants and explants cultured in insulin-like growth factor-1 (IGF-1) and transforming growth factor-β1 (TGF-β1). Results indicate that culture in IGF-1 results in a weakening of the COL fibers compared to native explants, and culture in TGF-β1 results in a strengthening of the COL fibers compared to native explants. These results elucidate the biomechanical changes in collagen fibril modulus, and ground matrix shear modulus following in vitro culture with IGF-1 and TGF-β1. Understanding the constitutive effects of growth factor stimulated culture may have applications in AC repair and tissue engineering.

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To successfully develop a functional tissue-engineered vascular patch, recapitulating the hierarchical structure of vessel is critical to mimic mechanical properties. Here, we use a cell sheet engineering strategy with micropatterning technique to control structural organization of bovine aortic vascular smooth muscle cell (VSMC) sheets. Actin filament staining and image analysis showed clear cellular alignment of VSMC sheets cultured on patterned substrates. Viability of harvested VSMC sheets was confirmed by Live/Dead® cell viability assay after 24 and 48 hours of transfer. VSMC sheets stacked to generate bilayer VSMC patches exhibited strong inter-layer bonding as shown by lap shear test. Uniaxial tensile testing of monolayer VSMC sheets and bilayer VSMC patches displayed nonlinear, anisotropic stress-stretch response similar to the biomechanical characteristic of a native arterial wall. Collagen content and structure were characterized to determine the effects of patterning and stacking on extracellular matrix of VSMC sheets. Using finite-element modeling to simulate uniaxial tensile testing of bilayer VSMC patches, we found the stress-stretch response of bilayer patterned VSMC patches under uniaxial tension to be predicted using an anisotropic hyperelastic constitutive model. Thus, our cell sheet harvesting system combined with biomechanical modeling is a promising approach to generate building blocks for tissue-engineered vascular patches with structure and mechanical behavior mimicking native tissue.

abstract: An orthotropic elasto-plastic damage material model (OEPDMM) suitable for impact simulations has been developed through a joint research project funded by the Federal Aviation Administration (FAA) and the National Aeronautics and Space Administration (NASA). Development of the model includes derivation of the theoretical details, implementation of the theory into LS-DYNA®, a commercially available nonlinear transient dynamic finite element code, as material model MAT 213, and verification and validation of the model. The material model is comprised of three major components: deformation, damage, and failure. The deformation sub-model is used to capture both linear and nonlinear deformations through a classical plasticity formulation. The damage sub-model is used to account for the reduction of elastic stiffness of the material as the degree of plastic strain is increased. Finally, the failure sub-model is used to predict the onset of loss of load carrying capacity in the material. OEPDMM is driven completely by tabulated experimental data obtained through physically meaningful material characterization tests, through high fidelity virtual tests, or both. The tabulated data includes stress-strain curves at different temperatures and strain rates to drive the deformation sub-model, damage parameter-total strain curves to drive the damage sub-model, and the failure sub-model can be driven by the data required for different failure theories implemented in the computer code. The work presented herein focuses on the experiments used to obtain the data necessary to drive as well as validate the material model, development and implementation of the damage model, verification of the deformation and damage models through single element (SE) and multi-element (ME) finite element simulations, development and implementation of experimental procedure for modeling delamination, and finally validation of the material model through low speed impact simulations and high speed impact simulations. / Dissertation/Thesis / Doctoral Dissertation Civil, Environmental and Sustainable Engineering 2019

Indiana University-Purdue University Indianapolis (IUPUI) / Advances in peripheral nerve electrode technology have outpaced the advances in chronic implantation reliability of the electrodes. An observable trend is the increased deposition of fibrotic encapsulation tissue around the electrode to shift its position away from the implantation site and subsequently reducing performance. A finite element model (FEM) is developed in conjunction with tensile testing and digital image correlation of strain to understand the relationship between cuff electrode attachment and the strain environment of the nerve. A laminar and bulk nerve model are both developed with material properties found in literature and geometry found from performing histology. The introduction of a cuff electrode to an axially stretched nerve indicates a significant behavior deviation from the expected response of the axial strain environment. When implemented in ex-vivo tensile testing, results indicate that the reduction of strain is statistically significant but becomes much more apparent when paired with a digital image correlation system to compare predicted and measured effects. A robust FEM is developed and tested to emphasize the effect that the boundary conditions and attachment methodology significantly effects the strain environment. By coupling digital image correlation with FEM, predictive models can be made to the strain environment to better design around the long term chronic health of the implant.

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