Browsing by Subject "biomechanics"
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Item Biomechanics of common carotid arteries from mice heterozygous for mgR, the most common mouse model of Marfan syndrome(2009-05-15) Taucer, Anne IreneMarfan syndrome, affecting approximately one out of every 5,000 people, is characterized by abnormal bone growth, ectopia lentis, and often-fatal aortic dilation and dissection. The root cause is a faulty extracellular matrix protein, fibrillin-1, which associates with elastin in many tissues. Common carotids from wild-type controls and mice heterozygous for the mgR mutation, the most commonly used mouse model of Marfan syndrome, were studied in a biaxial testing device. Mechanical data in the form of pressure-diameter and force-stretch tests in both the active and passive states were collected, as well data on the functional responses to phenylephrine, carbamylcholine chloride, and sodium nitroprusside. Although little significant difference was found between the heterozygous and wild-type groups in general, the in vivo stretch for both groups was significantly different from previously studied mouse vessels. Although the two groups do not exhibit significant differences, this study comprises a control group for future work with mice homozygous for mgR, which do exhibit Marfan-like symptoms. As treatment of Marfan syndrome improves, more Marfan patients will survive and age, increasing the likelihood that they will develop many of the vascular complications affecting the normal population, including hypertension and atherosclerosis. Therefore, it is imperative to gather biomechanical data from the Marfan vasculature so that clinicians may predict the effects of vascular complications in Marfan patients and develop appropriate methods of treatment.Item Characterization of Engineered Tissue by Multimodal Optical Imaging and Biaxial Mechanical Testing(2014-04-28) Bai, YuqiangTo better understand the relationships between mechanical stimuli and cellular responses, we developed a 3D tissue bioreactor coupling to both a biaxial mechanical testing platform and a stage for multimodal microscopy. Fibroblast seeded cruciform fibrin gels were investigated. A multimodal nonlinear optical microscopy-optical coherence microscopy (NLOM-OCM) system was developed to delineate relative spatial distributions of original fibrin, deposited collagen and fibroblasts non-invasively. Serial in-culture mechanical testing platform was also applied to track the evolution of bulk mechanical properties under sterile conditions. Wall stress depends on sample thickness and our multimodal imaging system measured evolving construct thickness as a function of mechanical stretch during biaxial tests. Through one month culture, cell and deposited collagen randomly distributed in non-stretched constructs. While under stretched condition, cell and deposited collagen fibers, which aligned with cell bodies, appeared preferentially parallel with principal stretch. Surprisingly both non-stretched and stretched constructs showed isotropic mechanical properties with increasing stiffness with culture time. In summary, our biaxial bioreactor system integrating both NLOM-OCM and mechanical testing provided complementary microstructural information and mechanical properties and thus may broaden fundamental understanding of soft tissue mechanics and mechanobiology.Item Characterizing strain in the proximal rat tibia during electrical muscle stimulation(Texas A&M University, 2007-09-17) Vyvial, Brent AronHindlimb unloading is a widely used model for studying the effects of microgravity on a skeleton. Hindlimb unloading produces a marked loss in bone due to increased osteoclast activity. Electrical muscle stimulation is being investigated as a simulated resistive exercise countermeasure to attenuate this bone loss. I sought to determine the relationship between strain measured at the antero-medial aspect of the proximal diaphysis of tibia and plantar-flexor torque measured at the ankle during electrical muscle stimulation as an exercise countermeasure for hindlimb unloading in rats. A mathematical relationship between strain and torque was established for the exercise during a 28 day period of hindlimb unloading. The strain generated during the exercise protocol is sufficient to attenuate bone loss caused by hindlimb unloading. Twelve six-month old Sprague-Dawley rats were implanted with uni-axial strain gages in vivo on the antero-medial aspect of the proximal diaphysis of the left tibia. Strain and torque were measured during electrical muscle stimulation for three time points during hindlimb unloading (Day 0 (n=3), Day 7 (n=3), Day 21 (n=3)). Peak strain decreased from 1,100 strain at the beginning of the study to 660 strain after 21 days of hindlimb unloading and muscle stimulation. The peak strain rate measured during muscle stimulation was 10,350 strain/second at the beginning and decreased to 6,670 strain/second after 21 days. The changes in strain are not significant, but the underlying trend in strain values may indicate an increase in bone formation due to the electrical muscle stimulation countermeasure. A mathematical model that relates measured strain to peak eccentric torque during muscle stimulation was created to facilitate estimation of strain for future studies of electrical muscle stimulation during hindlimb unloading.Item Computational modeling of biological cells and soft tissues(2009-05-15) Unnikrishnan, Ginu U.Biological materials are complex hierarchical systems subjected to external stimuli like mechanical forces, chemical potentials and electrical signals. A deeper understanding of the behavior of these materials is required for the response characterization of healthy and diseased conditions. The primary aim of this dissertation is to study the mechanics of biological materials like cells and tissues from a computational perspective and relate its behavior with experimental works, so as to provide a framework for the identification and treatment of pathological conditions like cancer and vascular diseases. The first step towards understanding the behavior of a biological cell is to comprehend its response to external mechanical stimuli. Experimentally derived material properties of cells have found to vary by orders of magnitude even for the same cell type. The primary cause of such disparity is attributed to the stimulation process, and the theoretical models used to interpret the experimental data. The variations in mechanical properties obtained from the experimental and theoretical studies can be overcome only through the development of a sound mathematical framework correlating the derived mechanical property with the cellular structure. Such a formulation accounting for the inhomogeneity of the cytoplasm due to stress fibers and actin cortex is developed in this work using Mori-Tanaka method of homogenization. Mechanical modeling of single cells would be extremely useful in understanding its behavior in an experimental setup. Characterization of in-vivo response of cells requires mathematical modeling of the embedding environment like fibers and fluids, which forms the extra cellular matrix. Studies on fluid-tissue interactions in biomechanics have primarily relied on either an iterative solution of the individual solid or tissue phases or a sequential solution of the entire domain using a coupled algorithm. In this dissertation, a new computational methodology for the analysis of fluid-tissue interaction problem is presented. The modeling procedure is based on a biphasic representation of fluid and tissue domain, consisting of fluid and solid phases. The biphasic-fluid interaction model is also implemented to study the transfer of low-density lipoprotein from the blood to the arterial wall, and also the nutrient transfer in the tissue scaffolds of a bioreactor.Item Integrating Biomechanics, Hemodynamics, and Vascular Adaptation to Relate Mechanisms of Vascular Adaptation to Arterial Pulsatile Pressure in Health and Disease(2014-08-07) Nguyen, Phuc HoangThe inherent complexity of the mammalian systemic arterial system has presented numerous challenges to relating basic vascular biology to clinically-relevant derangements of blood pressures and flows. The field of biomechanics has identified how local changes in pulsatile blood pressures and flows lead to changes in local endothelial shear stress and circumferential wall stress. The field of mechanobiology has identified how local changes in wall circumferential stress and endothelial shear stress lead to changes in arterial radii, wall thicknesses and stiffnesses. The field of pulsatile hemodynamics has identified how changes in local radii, wall thicknesses and stiffnesses lead to changes in the complex distributions of pressures and flows throughout an arterial network. These three fields have primarily been studied in isolation, and yet the properties of a single vessel emerge from the interaction of these three processes. The effect of adaptation of one artery on hemodynamics, stress, and structure of all other vessels in the network makes the arterial system a complex adaptive system that is difficult to study experimentally. This dissertation addresses this unmet need by integrating hemodynamics, vessel mechanics, and vascular adaptation by developing a novel framework with mathematical models at different scales. Allowing arteries simultaneously to adapt to mechanical stresses in a computational model of the human systemic arterial system, the present work illustrated that simple arterial adaptation to wall circumferential and endothelial shear stresses are sufficient to explain nine salient features of the cardiovascular system when traversing away from the aortic root towards the peripheral arteries: decrease in lumen radii, wall thicknesses, vessel compliances, shear stresses, wall stresses and pulsatile flows, and increase in wall stiffnesses, pulse wave velocities, and pulsatile pressures. In addition, it revealed that pulse pressure homeostasis emerges to mechanical perturbations such as reduced ejection fraction, increased peripheral resistance and aortic coarctation. Finally, it illustrated how changes in sensitivity of arterial adaptation to pulsatile wall stress can lead to manifestations of disease states such as increased pulse wave velocity and isolated systolic hypertension. The governing principles leading to the emergence of complex, adaptive behavior in the systemic arterial system have thus been identified.Item Response of the Femur to Exercise During Recovery Between Two Bouts of Hindlimb Unloading in Adult Male Rats(2012-10-19) Gonzalez, EstelaMechanical unloading with microgravity exposure during spaceflight induces bone loss in weight bearing bones. Combined with loss of bone due to aging, this disuse bone loss puts astronauts at increased risk of fracture upon returning to 1G conditions. It is important to study countermeasures, such as exercise, to mitigate or prevent this bone loss. This study utilized the hindlimb unloaded (HU) rat model to characterize the effects of resistance exercise on recovery dynamics in-between two bouts of HU. In the larger project adult male Sprague-Dawley rats, six months of age, were divided into the following groups: baseline (sacrificed at 6 months of age); aging cage controls (did not undergo any treatment, sacrificed at 7, 8, 9, 10, and 12 months of age); 1HU7 (one month of HU at 6 months of age followed by three months of ambulatory recovery); 2HU10 (one month of HU at 6 months of age, ambulatory recovery for two months, one month HU at 9 months of age, and final two month ambulatory recovery); 1HU10 (one month HU at 9 months of age and two month ambulatory recovery); and 2HU+Ex (One month HU at 6 months of age, two month resistance exercise recovery, and a 2nd bout of HU at 9 months of age). This thesis focused on the 2HU+Ex data, while utilizing data from other groups for comparisons. The data in this thesis includes ex vivo densitometric and biomechanical properties at the femoral neck (FN), femur midshaft diaphysis (FD), and distal femur metaphysis (DFM). All compartments of BMC increased following exercise recovery above AC at the FN and DFM. Ambulatory recovery values revealed incomplete recovery in total and cortical BMC at the DFM and full recovery in other parameters. DFM and FD vBMD data indicated there were further benefits of exercise during recovery. Geometric data revealed periosteal apposition at the DFM and FN following exercise recovery. FD mechanical properties did not produce benefits of exercise. However, FN maximum force increased above all other groups after exercise recovery. Elastic modulus of the DFM showed benefits of exercise recovery in the response to the 2nd HU.Item The feeding biomechanics of juvenile red snapper (Lutjanus campechanus) from the northwestern Gulf of Mexico(2009-05-15) Case, Janelle ElaineJuvenile red snapper are attracted to structure and settle onto low profile reefs, which serve as nursery grounds. Little is known about their life history during this time. However, recent studies from a shell bank in the NW Gulf of Mexico have shown higher growth rates for juveniles located on mud habitats adjacent to low profile reefs, perhaps due to varied prey availability and abundance. To further investigate the habitat needs of juvenile red snapper, individuals were collected from a low profile shell ridge (on-ridge) and adjacent mud areas (off-ridge) on Freeport Rocks, TX, and divided into three size classes (?3.9 cm SL, 4.0-5.9 cm SL, ?6 cm SL). Feeding morphology and kinematics were characterized and compared among size classes and between the two habitats. A dynamic jaw lever model was used to make predictions about feeding mechanics, and kinematic profiles obtained from high-speed videos of prey capture events validated the model?s predictive ability. Model output suggested an ontogenetic shift in feeding morphology from a juvenile feeding mode (more suction) to an adult feeding mode (more biting). Stomach contents revealed a concomitant shift in prey composition that coincided with the ontogenetic shift in feeding mode. The model also predicted that on-ridge juveniles would have faster jaw closing velocities compared to off-ridge juveniles, which had slower, stronger jaws. Analysis of prey capture events indicated that on-ridge juveniles demonstrated greater velocities and larger displacements of the jaws than off-ridge juveniles. Shape analysis was used to further investigate habitat effects on morphology. Off-ridge juveniles differed from on-ridge in possessing a deeper head and body. Results from model simulations, kinematic profiles, personal observations, and shape analysis all complement the conclusion that on-ridge juveniles exhibited more suction feeding behavior, whereas off-ridge juveniles used more biting behavior. Stomach contents demonstrated an early switch to piscivory in off-ridge juveniles compared to on-ridge juveniles, which may account for higher off-ridge growth rates. Habitat disparity, perhaps available prey composition, generated variations in juvenile feeding mechanics and consequently feeding behavior. This disparity may ultimately affect the growth rates and recruitment success of juvenile red snapper from different habitats.