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.

Intervertebral disc (IVD) degeneration is primarily involved in back pain, a morbidity that strongly affects the quality of life of individuals nowadays. Lumbar IVDs undergo stressful mechanical loads while being the largest avascular tissues in our body: Mechanical principles alone cannot unravel the intricate phenomena that occur at the cellular scale which are fundamental for the IVD regeneration. The present work aimed at coupling biomechanical and relevant molecular transport processes for disc cells to provide a mechanobiological finite element framework for a deeper understanding of degenerative processes and the planning of regenerative strategies. Given the importance of fluid flow within the IVD, the influence of poroelastic parameters such as permeabilities and solid-phase stiffness of the IVD subtissues was explored. A continuum porohyperelastic material model was then implemented. The angles of collagen fibers embedded in the annulus fibrosus (AF) were calibrated. The osmotic pressure of the central nucleus pulposus (NP) was also taken into account. In a parallel study of the human vertebral bone, microporomechanics was used together with experimental ultrasonic tests to characterize the stiffness of the solid matrix, and to provide estimates of poroelastic coefficients. Fluid dynamics analyses and microtomographic images were combined to understand the fluid exchanges at the bone-IVD interface. The porohyperelastic model of a lumbar IVD with poroelastic vertebral layers was coupled with a IVD transport model of three solutes - oxygen, lactate and glucose - interrelated to reproduce the glycolytic IVD metabolism. With such coupling it was possible to study the effect of deformations, fluid contents, solid-phase stiffness, permeabilities, pH, cell densities of IVD subtissues and NP osmotic pressure on the solute transport. Moreover, cell death governed by glucose deprivation and lactate accumulation was included to explore the mechanical effect on cell viability. Results showed that the stiffness of the AF had the most remarkable role on the poroelastic behavior of the IVD. The permeability of the thin cartilage endplate and the NP stiffness were also relevant. The porohyperelastic model was shown to reproduce the local AF mechanics, provided the fiber angles were calibrated regionally. Such back-calculation led to absolute values of fibers angles and to a global IVD poromechanical behavior in agreement with experiments in literature. The inclusion of osmotic pressure in the NP also led to stress values under confined compression comparable to those measured in healthy and degenerated NP specimens. For the solid bone matrix, axial and transverse stiffness coefficients found experimentally in the present work agreed with universal mass density-elasticity relationships, and combined with continuum microporomechanics provided poroelastic coefficients for undrained and drained cases. The effective permeability of the vertebral bony endplate calculated with fluid dynamics was highly correlated with the porosity measured in microtomographic images. The coupling of transport and porohyperelastic models revealed a mechanical effect acting under large volume changes and high compliance, favored by healthy rather than degenerated IVD properties. Such effect was attributed to strain-dependent diffusivities and diffusion distances and was shown to be beneficial for IVD cells due to the load-dependent increases of glucose levels. Cell density, NP osmotic pressure and porosity were the most important parameters affecting the coupled mechano-transport of metabolites. This novel study highlights the restoration of both cellular and mechanical factors and has a great potential impact for novel designs of treatments focused on tissue regeneration. It also provides methodological features that could be implemented in clinical image-based tools and improve the multiscale understanding of the human spine mechanobiology.

<p>The extracellular matrix (ECM) plays a significant role in defining the mechanical properties of biological tissues. The proteins, proteoglycans, and glycosaminoglycans that constitute the ECM are arranged into highly organized structures (<i>e.g.</i> fibrils and networks). Cellular behavior is affected by the stiffness of the microenvironment and influenced by the composition and organization of the ECM. Mechanosensing of ECM stiffness by cells occurs at the fibrillar (mesoscale) level between the single molecule (microscale) and the bulk tissue (macroscale) levels. However, the mechanical behavior of ECM proteins at the mesoscale are not well defined. Thus, better understanding of the ECM building blocks responsible for functional tissue assembly is critical in order to recapitulate <i>in vivo</i> conditions. There is a need for the mechanical characterization of the ECM networks formed by proteins synthesized <i>in vivo</i> while in their native configuration. </p> <p>To address this gap, my goals highlighted in this dissertation were to develop appropriate experimental and computational methodologies and investigate the 3D organization and mechanical behavior of ECM networks <i>in situ</i>. The ECM of developing mouse tissues was used as a model system, taking advantage of the low-density networks present at this stage. First, we established a novel decellularization technique that enhanced the visualization of ECM networks in soft embryonic tissues. Based on this technique, we then quantified tissue-dependent strain of immunostained ECM networks <i>in situ</i>. Next, we developed mesoscale and macroscale testing systems to evaluate ECM networks under tension. Our systems were used to investigate tendon mechanics as a function of development, calculating tangent moduli from stress - strain plots. Similarly, we characterized ECM network deformation while uniaxially loading embryonic tissues, since this testing modality is ideal for fibril and network mechanics. Taken together, this information can facilitate the fabrication of physiologically relevant scaffolds for regenerative medicine by establishing mechanical guidelines for microenvironments facilitate functional tissue assembly.</p>

Vascular tissue engineering (VTE) with the use of polymeric scaffolds offers the potential to generate small-diameter (<6 mm) arteries. In this thesis, a degradable polar hydrophobic ionic (D-PHI) polyurethane porous scaffold was synthesized with the objective of demonstrating its potential application for VTE. D-PHI scaffold synthesis was optimized, maximizing isocyanate and methacrylate monomer conversion. Through the incorporation of a lysine-based crosslinker, scaffold mechanical properties and swelling were manipulated. Furthermore, D-PHI scaffolds demonstrated the ability to support the growth and adhesion of A10 vascular smooth muscle cells (VSMCs) during two weeks of culture. This study also investigated the effect of a double porogen approach on D-PHI scaffold properties, demonstrating an increase in the total scaffold porosity and pore interconnectivity. Specifically, it was found that the use of 10 wt% polyethylene glycol and 65 wt% sodium bicarbonate porogens resulted in a porous (79±3%) D-PHI scaffold with the mechanical properties (elastic modulus=0.16±0.03 MPa, elongation-at-yield=31±5%, and tensile strength=0.04±0.01 MPa) required to withstand the physiologically-relevant cyclic mechanical strain (CMS) that is experienced by VSMCs in vivo. Furthermore, the effects of uniaxial CMS (10% strain, 1 Hz, 4 weeks) on human coronary artery smooth muscle cells (hCASMCs), which were cultured in a porous D-PHI scaffold, were studied using a customized bioreactor. Four weeks of CMS was shown to yield greater DNA mass, more cell area coverage, a better distribution of cells within the scaffold, the maintenance of contractile protein expression and the improvement of tensile mechanical properties. The in vitro and in vivo degradation as well as the in vivo biocompatibility of D-PHI scaffolds were also investigated. Following their subcutaneous implantation in rats (100 days), porous D-PHI scaffolds demonstrated more cell/tissue infiltration within their pores and degraded in a controlled manner and at a faster rate when compared to in vitro studies (120 days), retaining the mechanical integrity required during neo-tissue formation. This thesis provides significant insight into the role of the D-PHI scaffold in combination with physiologically-relevant CMS in modulating VSMC proliferation and phenotype. The findings of this work can be used to tailor vascular tissue regeneration by regulating VSMC function in a directed manner.

Vascular tissue engineering (VTE) with the use of polymeric scaffolds offers the potential to generate small-diameter (<6 mm) arteries. In this thesis, a degradable polar hydrophobic ionic (D-PHI) polyurethane porous scaffold was synthesized with the objective of demonstrating its potential application for VTE. D-PHI scaffold synthesis was optimized, maximizing isocyanate and methacrylate monomer conversion. Through the incorporation of a lysine-based crosslinker, scaffold mechanical properties and swelling were manipulated. Furthermore, D-PHI scaffolds demonstrated the ability to support the growth and adhesion of A10 vascular smooth muscle cells (VSMCs) during two weeks of culture. This study also investigated the effect of a double porogen approach on D-PHI scaffold properties, demonstrating an increase in the total scaffold porosity and pore interconnectivity. Specifically, it was found that the use of 10 wt% polyethylene glycol and 65 wt% sodium bicarbonate porogens resulted in a porous (79±3%) D-PHI scaffold with the mechanical properties (elastic modulus=0.16±0.03 MPa, elongation-at-yield=31±5%, and tensile strength=0.04±0.01 MPa) required to withstand the physiologically-relevant cyclic mechanical strain (CMS) that is experienced by VSMCs in vivo. Furthermore, the effects of uniaxial CMS (10% strain, 1 Hz, 4 weeks) on human coronary artery smooth muscle cells (hCASMCs), which were cultured in a porous D-PHI scaffold, were studied using a customized bioreactor. Four weeks of CMS was shown to yield greater DNA mass, more cell area coverage, a better distribution of cells within the scaffold, the maintenance of contractile protein expression and the improvement of tensile mechanical properties. The in vitro and in vivo degradation as well as the in vivo biocompatibility of D-PHI scaffolds were also investigated. Following their subcutaneous implantation in rats (100 days), porous D-PHI scaffolds demonstrated more cell/tissue infiltration within their pores and degraded in a controlled manner and at a faster rate when compared to in vitro studies (120 days), retaining the mechanical integrity required during neo-tissue formation. This thesis provides significant insight into the role of the D-PHI scaffold in combination with physiologically-relevant CMS in modulating VSMC proliferation and phenotype. The findings of this work can be used to tailor vascular tissue regeneration by regulating VSMC function in a directed manner.