Mechanical conditioning has been shown to promote tissue formation in a wide variety of tissue engineering efforts. However the underlying mechanisms by which external mechanical stimuli regulate cells and tissues are not known. This is particularly relevant in the area of heart valve tissue engineering owing to the intense hemodynamic environments that surround native valves. Some studies suggest that oscillatory shear stress (OSS) caused by time-varying flow environments, play a critical role in engineered tissue formation derived from bone marrow derived stem cells (BMSCs). There is strong evidence to support this hypothesis in tissue engineering studies of bone. From observing native heart valve dynamics, OSS can be created by means of pulsatility or by cyclic specimen geometry changes. However, quantification of the individual or combined effects of these variables for the maximization of OSS environments in vitro is to date, not known. Accordingly, in this study we examined and quantified the role that i) physiologically relevant scales of pulsatility and ii) changes in geometry as a function of specimen flexure, have in creating OSS conditions for dynamic culture of tissue. A u-shaped custom made bioreactor capable of producing flow stretch and flexure was used. Computational Fluid Dynamic (CFD) simulations were performed through Ansys CFX (Ansys, Pittsburgh, PA) for both steady and pulsatile flow. We have shown that OSS can be maximized by inducing pulsatile flow over straight scaffolds. We believe that OSS promotes BMSCs tissue formation.

Periodontitis is a disease primarily affecting tooth attachment, i.e. concerning oral cavity, however, its connection to a number of systemic diseases is apparent nowadays. Oral microorganisms and their interaction with the immune system of their host play a significant part in the etiology of this multifactorial disease. Rapid development of DNA-based diagnostic methods in the last 15 years dramatically increased the spectrum of identified oral microorganisms and promoted understanding of how particular taxons correlate with periodontal health or disease. Moreover, next generation sequencing methods also bring new possibilities to study the relationship between periodontitis and other diseases such as diabetes mellitus, cardiovascular diseases (CVD), stroke, lung infection or kidney diseases, etc. The presented work focuses on the use of sequencing methods to compare the taxonomic composition of microbiomes within oral cavity and the tissues of aortic valves in patients suffering from CVD. The characterisation of taxonomic composition of microbiome in the analysed tissues was performed using the method of 454 pyrosequencing of variable region IV-V of the bacterial 16S rDNA. The present taxons were determined by comparing the obtained sequences with the Human Oral Microbiome Database. The common...

Mechanical heart valve (MHV) flows are characterized by high shear stress, regions of recirculation, and high levels of turbulent fluctuations. It is well known that these flow conditions are hostile to blood constituents, which could lead to thromboembolism. In the ongoing effort to reduce long-term complications and morbidity, it is imperative that we better understand the flow characteristics of the natural valve as well as that of the mechanical valve. In this study, we overcome many of the limitations imposed by other measurement techniques by employing a powerful, high-speed Time-Resolved Digital Particle Image Velocimetry (TRDPIV) system to map the flow field. We compare the flows downstream from a St. Jude Medical bileaflet MHV, a porcine mitral valve (MV), and a combination of both valves to simulate the technique of chordal preservation. Instantaneous velocity fields and vorticity maps are presented, which provide detailed information about the development of the flow. Time-averaged velocity, vorticity, and turbulent kinetic energy measurements are also discussed. Asynchronous leaflet behavior was observed in all cases involving the mechanical valve. Extensive vortex formation and propagation are present distal to the MHV, which leads to high levels of jet dispersion. The porcine mitral jet exhibits lateral oscillatory behavior, but it does not disperse like the MHV. In the MHV/porcine combination system, the native tissue limits vortex propagation and jet dispersion. The results presented provide insight on the hemodynamic characteristics of natural and MHVs, reveal the detrimental character of asynchronous leaflet opening, document the mechanism of vortex formation and interaction distal to the valve, and illustrate the importance of chordal preservation. These results may improve MHV replacement clinical practice and/or motivate and aid the design of MHVs that better mimic natural mitral flow patterns. / Master of Science

Background and Objectives: Mitral stenosis with extensive mitral annular calcification (MAC) remains surgically challenging in respect to clinical outcome. Prolonged surgery time with imminent ventricular rupture and systolic anterior motion can be considered as a complex of causal factors. The aim of our alternative hybrid approach was to reduce the risk of annual rupture and paravalvular leaks and to avoid obstruction of the outflow tract. A review of the current literature was also carried out. Materials and Methods: Six female patients (mean age 76 9 years) with severe mitral valve stenosis and severely calcified annulus underwent an open implantation of an Edwards Sapien 3 prosthesis on cardiopulmonary bypass. Our hybrid approach involved resection of the anterior mitral leaflet, placement of anchor sutures and the deployment of a balloon expanded prosthesis under visual control. Concomitant procedures were carried out in three patients. Results: The mean duration of cross-clamping was 95 31 min and cardiopulmonary bypass was 137 60 min. The perioperative TEE showed in three patients an inconspicuous, heart valve-typical gradient on all implanted prostheses and a clinically irrelevant paravalvular leakage occurred in the anterior annulus. In the left ventricular outflow tract, mild to moderately elevated gradients were recorded. No adverse cerebrovascular events and pacemaker implantations were observed. All but one patient survived to discharge. Survival at one year was 83.3%. Conclusions: This “off label” implantation of the Edwards Sapien 3 prosthesis may be considered as a suitable bail-out approach for patients at high-risk for mitral valve surgery or deemed inoperable due to extensive MAC.

"A quantitative understanding of the complex interactions between cells, soluble factors, and the biological and mechanical properties of biomaterials is required to guide cell remodeling towards regeneration of healthy tissue rather than fibrocontractive tissue. The goal of this thesis was to elucidate the interactions between the boundary stiffness of three-dimensional (3D) matrix and soluble factors on valvular interstitial cell (VIC) phenotype with a quantitative approach. The first part of the work presented in this thesis was to characterize the combined effects of boundary stiffness and transforming growth factor-β1 (TGF-β1) on cell-generated forces and collagen accumulation. We first generated a quantitative map of cell-generated tension in response to these factors by culturing VICs within micro-scale fibrin gels between compliant posts (0.15-1.05 nN/nm) in chemically-defined media with TGF-β1 (0-5 ng/mL). The VICs generated 100 to 3000 nN/cell after one week of culture, and multiple regression modeling demonstrated, for the first time, quantitative interaction (synergy) between these factors in a 3D culture system. We then isolated passive and active components of tension within the micro-tissues and found that cells cultured with high levels of stiffness and TGF-β1 expressed myofibroblast markers and generated substantial residual tension in the matrix yet, surprisingly, were not able to generate additional tension in response to membrane depolarization signifying a state of continual maximal contraction. In contrast, negligible residual tension was stored in the low stiffness and TGF-β1 groups indicating a lower potential for shrinkage upon release. We then studied if ECM could be generated under the low tension environment and found that TGF-β1, but not EGF, increased de novo collagen accumulation in both low and high tension environments roughly equally. Combined, these findings suggest that isometric cell force, passive retraction, and collagen production can be tuned by independently altering boundary stiffness and TGF-β1 concentration. In the second part, by using the quantitative information obtained from the first part, we investigated the effects of dynamic changes in stiffness on cell phenotype in a 3D protein matrix, quantitatively. Our novel method utilizing magnetic force to constrain the motion of one of two flexible posts between which VIC-populated micro-tissues were cultured effectively doubled the boundary stiffness and resulted in a significant increase in cell-generated forces. When the magnetic force was removed, the effective boundary stiffness was halved and the tissue tension dropped to 65-87% of the peak value. Surprisingly, following release the cell-generated forces continued to increase for the next two days rather than reducing down to the homeostatic tension level of the control group with identical (but constant) boundary stiffness. The rapid release of tension with the return to baseline boundary stiffness did not result in a decrease in number of cells with α-SMA positive stress fibers or an increase in apoptosis. When samples were entirely released from the boundaries and cultured free floating (where tension is minimal but cannot be measured), the proportion of apoptotic cells in middle region of the micro-tissues increased more than five-fold to 31%. Together, these data indicate that modest temporary changes in boundary stiffness can have lasting effects on myofibroblast activation and persistence in 3D matrices, and that a large decrease in the ability of the cells to generate tension is required to trigger de-differentiation and apoptosis. "

Novel biomaterials provide a spectrum of possibilities. They can be engineered in different forms to understand how they would perform as different bioprosthetic conduits. Bacterial cellulose membranes may be suitable candidates as prosthetic valve leaflets in valve replacement surgeries due to their functional properties (hemodynamics, resistant to thrombosis). Biomaterials used for most bioprosthetic heart valves are cut, trimmed and sutured. A major challenge for the bi-leaflet configuration is that the cutting and suturing of biopolymers fabricated as sheets into a cylindrical form increases failure risk due to greater number of suture points and irregular coaptation. The objective was to culture the bacterial cellulose membrane as a continuous cylindrical construct and evaluate its mechanical properties. Various design features of the fabrication process such as culturing media and the hollow carrier-mandrel characteristics were evaluated. A comparative study of how bacterial cellulose grows on different hollow carrier membranes was conducted and thin smooth surface silicone tubes fabricated in the lab were found to be most suitable. A bioreactor for culturing cylindrical bacterial cellulose tubes on the outer surface of the hollow carrier was designed and fabricated. The mechanical properties of the fabricated tubes, specifically, their tensile strength, flexure, suture retention and tear resistance were characterized. Mechanical characterization studies showed the cylindrical bacterial cellulose tubes to be anisotropic, with preferential properties in the longitudinal (axial) direction of the tube. Preliminary results show that cylindrical bacterial cellulose tubes can be a promising candidate for use in prosthetic valve conduits.

Currently, mitral valve repair techniques have shown substandard mid-term and long term results. In order to improve the efficacy of these repair techniques, detailed knowledge of normal mitral valve function and the alterations to the valvular and subvalvular apparatus which occur under pathological conditions is required. Furthermore, current techniques may be optimized through a better understanding of the function and mechanics of the mitral valve after a particular repair. The experiments which comprise this study were designed using an in vitro approach since this technique has the clear advantage of isolating and independently controlling specific parameters that are of importance to valvular mechanics and function. The experiments were conducted in the Georgia Tech Left Heart Simulator using native porcine and human mitral valves. The first set of experiments measured the chordal force distribution and anterior leaflet strain of the mitral valve in its normal geometrical configuration. Subsequent experiments measure mitral regurgitation volume and chordal force distribution in conditions associated with ventricular dilation. The last set of experiments simulated two commonly used mitral repair techniques. For the Alfieri stitch experiments, the effects of mitral flow rate, transmitral pressure, and mitral annular area on valve stenosis, mitral regurgitation and Alfieri stitch force were evaluated. For annuloplasty, the effect of annular saddle curvature on anterior leaflet strain was quantified. In Conclusion, the normal geometry of the native mitral valve optimized its function and mechanics. Under pathological conditions associated with ventricular dilation, significant alterations to mitral valve function and mechanics were present. Although the studied repair techniques may have significantly restored valve function, severe alterations to the mechanics of the valve still persisted.

Bileaflet mechanical heart valves (BMHVs), though a life-saving tool in treating heart valve disease, are often associated with serious complications, including a high risk of hemolysis, platelet activation, and thromboembolism. One likely cause of this hyper-coagulative state is the nonphysiologic levels of stress experienced by the erythrocytes and platelets flowing through the BMHVs. Research has shown that the combination of shear stress magnitude and exposure time found in the highly transient leakage jet emanating from the b-datum gap during valve closure is sufficient to cause hemolysis and platelet activation. Regions of flow stasis in the valve vicinity may also allow activated platelets to aggregate and form thrombus. This thesis addresses the hypothesis that passive flow control may have the potential to reduce flow-induced thrombogenicity by altering the fluid mechanics of bileaflet mechanical heart valves. To test this hypothesis, a steady model of the regurgitant b-datum line jet was developed and studied. This model served as a test bed for various vortex generator array designs. The fluid mechanics of the b-datum line jet model was investigated with flow visualization and particle image velocimetry. In vitro tests with whole human blood were performed with and without the vortex generators in order to determine how the presence of the passive flow control affected the propensity of the blood to form thrombus. An effort was then made to correlate the fluid mechanics of the jet model with the procoagulant potential results from the blood experiments. The effect of the vortex generators on the fluid mechanics of the valve under physiologic pulsatile conditions was also investigated via flow visualization in the Georgia Tech Left Heart Simulator. By studying a steady model of the regurgitant b-datum line jet, it was found, using an in vitro system with whole human blood, that the presence of vortex generators significantly decreased the blood's propensity for thrombus formation. The potential of applying passive flow control to cardiovascular hardware in order to mitigate the injurious effects of shear-induced platelet activation is thus demonstrated. The investigation into the effect of vortex generators on the fluid mechanics of the b-datum line jet showed that the jet oscillated aperiodically and that the effect of the applied flow control was played out at both the scale of the chamber (large-scale) and on the scale of the vortex generator fins (small-scale). On the large scale, the presence of vortex generators appeared to decrease the magnitude or frequency of jet oscillation, thereby stabilizing the jet. After removing the effect of the large-scale oscillations via phase averaging, the effect of the vortex generators on the small scale was examined. On the small scale, the jet without flow control was found to have higher levels of velocity RMS, particularly on the jet periphery, and higher levels of Reynolds shear stress. It is proposed that the vortex generators effect this change by generating vorticity in the plane of the jet. This vorticity is theorized to stabilize the jet, delaying roll-up of the jet shear layer which occurs via the Kelvin-Helmholtz instability. The method by which the vortex generators acted on the fluid mechanics of the steady jet system to decrease the blood's procoagulant potential was investigated via flow visualization and DPIV. The results from these studies implicate two possible mechanisms by which the vortex generators may act. First, the peak turbulent shear stresses in the jet were reduced by 10-20% with the application of vortex generators. Even if only a few platelets were activated in each passage through the valve, the cumulative effect of this difference in peak stresses after many passes would be greatly magnified. Thus, this reduction in turbulent shear stresses may be sufficient to explain the change seen in the blood's procoagulant potential with the application of passive flow control. It is suspected, though, that the second mechanism is dominant. The flow fields revealed that the presence of the vortex generators delayed or prevented the roll-up of the Kelvin-Helmholtz instability in the b-datum jet's shear layers into discrete vortices. By doing so, it is thought that opportunities for the interaction of activated and unactivated platelets entrained in these vortices were prevented, thereby inhibiting further propagation of the coagulation cascade. Even if the rate at which platelets were activated was similar for cases with and without flow control, it seems that the flow fields experienced by the platelets subsequent to activation can determine the level of procoagulant potential. Under the steady conditions observed in this experiment, the jet influenced by vortex generators was thus shown to induce significantly lower levels of procoagulant potential.

Aortic Valve Analysis and Area Prediction using Bayesian Modeling Miheer S. Ghotikar ABSTRACT Aortic valve stenosis affects approximately 5 out of every 10,000 people in the United States. [3] This disorder causes decrease in the aortic valve opening area increasing resistance to blood flow. Detection of early stages of valve malfunction is an important area of research to enable new treatments and develop strategies in order to delay degenerative progression. Analysis of relationship between valve properties and hemodynamic factors is critical to develop and validate these strategies. Porcine aortic valves are anatomically analogous to human aortic valves. Fixation agents modify the valves in such a manner to mimic increased leaflet stiffness due to early degeneration. In this study, porcine valves treated with glutaraldehyde, a cross-linking agent and ethanol, a dehydrating agent were used to alter leaflet material properties. The hydraulic performance of ethanol and glutaraldehyde treated valves was compared to fresh valves using a programmable pulse duplicator that could simulate physiological conditions. Hydraulic conditions in the pulse duplicator were modified by varying mean flow rate and mean arterial pressure. Pressure drops across the aortic valve, flow rate and back pressure (mean arterial pressure) values were recorded at successive instants of time. Corresponding values of pressure gradient were measured, while aortic valve opening area was obtained from photographic data. Effects of glutaradehyde cross-linking and ethanol dehydration on the aortic valve area for different hydraulic conditions that emulated hemodynamic physiological conditions were analyzed and it was observed that glutaradehyde and ethanol fixation causes changes in aortic valve opening and closing patterns. Next, relations between material properties, experimental conditions, and hydraulic measures of valve performance were studied using a Bayesian model approach. The primary hypothesis tested in this study was that a Bayesian network could be used to predict dynamic changes in the aortic valve area given the hemodynamic conditions. A Bayesian network encodes probabilistic relationships among variables of interest, also representing causal relationships between temporal antecedents and outcomes. A Learning Bayesian Network was constructed; direct acyclic graphs were drawn in GeNIe 2.0ʾ using an information theory dependency algorithm. Mutual Information was calculated between every set of parameters. Conditional probability tables and cut-sets were obtained from the data with the use of Matlabʾ. A Bayesian model was built for predicting dynamic values of opening and closing area for fresh, ethanol fixed and glutaradehyde fixed aortic valves for a set of hemodynamic conditions. Separate models were made for opening and closing cycles. The models predicted aortic valve area for fresh, ethanol fixed and glutaraldehyde fixed valves. As per the results obtained from the model, it can be concluded that the Bayesian network works successfully with the performance of porcine valves in a pulse duplicator. Further work would include building the Bayesian network with additional parameters and patient data for predicting aortic valve area of patients with progressive stenosis. The important feature would be to predict valve degenration based on valve opening or closing pattern.

Thesis (M.S.)--Mississippi State University. Department of Agricultural and Biological Engineering. / Title from title screen. Includes bibliographical references.