Integrated experimental and numerical comparison of different approaches for planar biaxial testing of a hyperelastic material

Planar biaxial testing has been applied to a variety of materials to obtain relevant information for mechanical characterization and constitutive modeling in presence of complex stress states. Despite its diffusion, there is currently no standardized testing procedure or a unique specimen design of common use. Consequently, comparison of results obtained with different configurations is not always straightforward and several types of optimized shapes have been proposed. The purpose of the present work is to develop a procedure for comprehensive comparison of results of biaxial tests carried out on the same soft hyperelastic material, using different types of gripping methods and specimen shapes (i.e., cruciform and square). Five configurations were investigated experimentally using a biaxial test rig designed and built by the authors, using digital imaging techniques to track the displacements of markers apposed in selected positions on the surfaces. Then, material parameters for a suitable hyperelastic law were determined for each configuration examined, employing an inverse method which combines numerical simulations with the finite element method (FEM) and optimization algorithms. Finally, efficiency of examined biaxial configurations was assessed comparing stress reductions factor, degree and uniformity of biaxial deformation, and operative strain ranges

Computational modeling of electroactive hydrogels for cartilage-tissue repair using electrical stimulation

The self-repair capability of articular cartilage is limited due to the lack of vascularization and low turnover of its extracellular matrix. In quest of therapeutic options, electrical stimulation has been proposed for improving tissue engineering approaches for the repair of articular cartilage. The use of electrical stimulation for the repair of cartilage tissue requires detailed preliminary analysis. In this thesis, computational models have been studied that can be used for optimizing the experimental protocols for cartilage–tissue repair using electrical stimulation.Die Fähigkeit des Gelenkknorpels, sich selbst zu regenerieren, ist aufgrund der fehlenden Vaskularisierung und des niedrigen Umsatzes seiner extrazellulären Matrix begrenzt. Ein alternativer Therapieansatz zu klassischen operativen Eingriffen ist die elektrische Stimulation des Knorpelgewebes im Gelenk. Diese neuartige Methode bedarf ausführlicher Voranalyse. In dieser Arbeit wurden Computermodelle untersucht, die zur Optimierung der experimentellen Protokolle für die Knorpelgewebereparatur mittels elektrischer Stimulation verwendet werden können

Multiscale Mechanical Characterization of Soft Matter

Ph.DDOCTOR OF PHILOSOPH

SIMULATION-BASED DESIGN AND MATERIAL MODELING FOR ENT IMPLANTS

Ph.DDOCTOR OF PHILOSOPH

Cell Migration within 3D Microenvironments: an Integrative Perspective from the Membrane to the Nucleus

La migración celular es fundamental para la vida y el desarrollo. Desafortunadamente, la movilidad celular también está asociada con algunas de las principales causas de morbilidad y mortalidad, incluidos los trastornos inmunitarios, esqueléticos y cardiovasculares, así como la metástasis del cáncer. Las células dependen en su capacidad para percibir y responder a estímulos externos en muchos procesos fisiológicos y patológicos (p. ej., desarrollo embrionario, angiogénesis, reparación de tejidos y progresión tumoral). El objetivo global de esta tesis doctoral fue investigar la respuesta migratoria de células individuales a señales bioquímicas y biofísicas. En particular, el enfoque de esta investigación se centró en los mecanismos que permiten a las células percibir e internalizar señales bioquímicas y biofísicas y la influencia de estos estímulos en la respuesta migratoria de las células individuales.El primer estudio tuvo como objetivo establecer una metodología para facilitar la integración de estudios teóricos con datos experimentales. Al minimizar la intervención del usuario, el sistema propuesto basado en técnicas de optimización Bayesiana gestionó de manera eficiente la calibración de los modelos in silico, que de otro modo sería tediosa y propensa a errores. Posteriormente, se construyó un modelo in silico para investigar cómo los estímulos bioquímicos y biofísicos influyen en el movimiento celular en tres dimensiones. Este modelo computacional integró algunos de los principales actores que permiten a las células percibir y responder a señales externas, que pueden actuar a diferentes escalas e interactuar entre sí. Los resultados mostraron, por un lado, que las células cambian su comportamiento migratorio en función de la pendiente de los gradientes químicos y la concentración absoluta de factores químicos (por ejemplo, factores de crecimiento) a su alrededor. Por otro lado, estos resultados revelaron que la respuesta migratoria de las células a la rigidez y densidad de la matriz depende de su fenotipo. En general, la tesis destaca la dependencia de la migración celular tridimensional al fenotipo de las células (es decir, el tamaño de su núcleo, la deformabilidad del mismo) y las propiedades del microambiente circundante (por ejemplo, el perfil químico, la rigidez de la matriz, el confinamiento).Cell migration is fundamental for life and development. Unfortunately, cell motility is also associated with some of the leading causes of morbidity and mortality, including immune, skeletal, and cardiovascular disorders as well as cancer metastasis. Cells rely on their ability to perceive and respond to external stimuli in many physiological and pathological processes (e.g., embryonic development, angiogenesis, tissue repair, and tumor progression). The global objective of this doctoral thesis was to investigate the migratory response of individual cells to biochemical and biophysical cues. In particular, the focus of this research was on the mechanisms enabling cells to perceive and internalize biochemical and biophysical cues and the influence of these stimuli on the migratory response of individual cells. The first study aimed at establishing a methodology to facilitate the integration of theoretical studies with experimental data. By minimizing user intervention, the proposed framework based on Bayesian optimization techniques efficiently handled the otherwise tedious and error-prone calibration of in silico models. Afterward, an in silico model was built to investigate how biochemical and biophysical stimuli influence three-dimensional cell motion. This computational model integrated some of the main actors enabling cells to probe and respond to external cues, which may act at different scales and interact with each other. The results showed, on the one hand, that cells change their migratory behavior based on the slope of chemical gradients and the absolute concentration of chemical factors (e.g., growth factors) around them. On the other hand, these results revealed that cells’ migratory response to matrix stiffness and density depends on their phenotype. Overall, this thesis highlights the dependence of three-dimensional cell migration on both cells’ phenotype (i.e., nucleus size, deformability) and the properties of the surrounding microenvironment (e.g., chemical profile, matrix rigidity, confinement).<br /

The Role of Endothelial Mechanosensing in Capillary Development and Organization.

Ischemic injury is a leading cause of morbidity and mortality with the most common causes being heart attack, stroke, and peripheral artery disease. Therapies attempt to improve healing, in part, by promoting angiogenesis in these ischemic sites. Angiogenic invasion and maturation into a new capillary network may be affected by the altered microstructure and the mechanical properties of the ischemic tissue, in particular, the extracellular matrix (ECM). It is known that endothelial cells (EC) are mechanosensitive and reorient in response to both shear and normal stresses in vessels. Further, they generate traction forces and displacements in 2D culture to coordinate motion. However, the question of whether EC use cell-generated ECM forces to communicate in 3D culture to direct capillary organization and anastomosis is currently unresolved. Hydrogels formed from natural extracellular matrix (ECM) proteins readily support the formation of vasculature in vitro. The ECM is a highly ordered meshwork of various macromolecules. This anisotropic microstructure produces non-linear viscoelastic mechanical properties which confound attempts towards modeling the mechanical environment around cells. To overcome these issues, we developed a biosynthetic hydrogel consisting of polyethylene glycol diacrylamide conjugated to macromolecular type-I collagen. Through acrylamide-based cross-links, these materials allow for independent control of physical properties and bulk ligand concentration. Photoencapsulation of EC and fibroblasts within this hydrogel material and their subsequent co-culture led to the formation of capillary vessel-like networks with well-defined hollow lumens. Patterned hydrogel constructs were produced to assess angiogenic invasion independently of other stages of EC organization. ECM displacements were observed over time and mechanical modeling was used to compute cell-generated stresses and strains. We found that regions of strain exceeding 9% and stress exceeding 1,500 pico-Newtons per square micron co-localized with regions of capillary invasion (r=0.44). Thus, capillaries were found to generate stresses which propagated though the ECM. Through these studies, we developed an engineered ECM which enabled the magnitudes of cell-generated stresses during a complex 3D morphogenetic process to be quantified for the first time. These findings could yield a better understanding of the physical principles guiding capillary morphogenesis and provide new strategies for treating ischemic disease.PHDBiomedical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/111562/1/rahsingh_1.pd

Quantitative modeling and control of nascent sprout geometry in in vitro Angiogenesis

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2012.Cataloged from PDF version of thesis.Includes bibliographical references (p. 121-127).Nascent blood vessel growth in angiogenesis is a complex process involving cellular response to biochemical growth factors, degradation of the surrounding matrix, and coordinated migration of multiple endothelial cells up a growth factor gradient. Mechanistic understanding and quantitative modeling of the dominant dynamics involved in nascent vessel growth will enable new strategies for regulating vessel growth rate and geometry, and will have implications in controlling growth of complete vascular networks in many research areas, ranging from cancer treatment and wound healing to tissue engineering. In this thesis, we investigate the dynamics of nascent vessel growth in 3D microfluidic assays, formulate a quantitative process model based on our experimental characterization, and formulate a feedback approach to regulate growth. We begin by developing a new microfluidic assay consisting of a collagen gel scaffold with features to reduce assay-to-assay variability and increase experimental throughput. This high throughput assay reveals that there is an inverse relationship between nascent vessel elongation rate and diameter under diverse biochemical conditions. This finding is supported by immuno-fluorescent staining and biochemical inhibition studies, which give insight into the dominant mechanisms determining nascent vessel diameter. Based on our experimental characterization, we formulate a simple quantitative reaction-diffusion model that relates vessel diameter to elongation rate, and supports our understanding of the relevant dynamics. We conclude by formulating a model-based optimization approach for planning the optimal trajectory of elongation rate vs. time needed to obtain desired sprout geometry, and illustrate in simulation that model predictive feedback control can be used to correct for noise in the response of elongation rate to growth factor inputs.by Levi Benjamin Wood.Ph.D

Development and Modeling of a Polymer Construct for Perfusion Imaging and Tissue Engineering.

The physical and computational modeling of distributed fluid flow to vascular beds remains a challenging issue. The computational resources required, and the complexity of capillary networks makes modeling infeasible. The resolution limits of manufacturing techniques make physical models difficult to fabricate and manipulate under experimental conditions. As such, an in vitro polymer construct was developed with structural properties of small arteries and the bulk flow characteristics of capillary beds. Rapid prototyping and scaffolding techniques were used to fabricate vascular trees amendable to scaffold compartments. Several scaffold architectures were evaluated to achieve target fluid flow characteristics for implementation in a dynamic contrast-enhanced computed tomography (DCE-CT) imaging phantom and endothelial cell bioreactor, respectively. Experimental flow measurements were compared to measurements from computational simulations. In addition, the flow-induced shear stress across the construct was modeled to identify the optimal settings within the bioreactor. In addition, the cytocompatibility of the polymer construct was optimized. Vascular trees were reliably fabricated to achieve arteriole-like flow. Rapid prototyped polycaprolactone (PCL) scaffolds produced distinct differential flow ranges, marked by a decrease in flow rate across the network. The construct served as a viable dynamic flow phantom capable of generating signals typical of organs imaged with DCE-CT. Furthermore, simulations of the construct as a bioreactor provided guidance on the boundary conditions required for stimulatory shear stress within the scaffolds. Under static conditions, endothelial cells were cultured on PCL scaffolds modified with extra-cellular matrix mimicking biological and chemical agents. All surface modifications exhibited similar cell proliferation and function. However, the Arg-Gly-Asp (RGD) surface-modified constructs exhibited an optimal spatial distribution for future endothelial cell bioreactor investigations. This work demonstrates a method for modeling and physically simulating a bifurcating vascular tree adjoined to scaffold compartments with tunable flow, for application to perfusion imaging and in vitro tissue engineering (tissue and tumors).PHDBiomedical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/107136/1/auresa_1.pd