Browsing by Author "Dasi, Lakshmi Prasad, committee member"

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Open Access
Development of a continuous flow ultrasonic harvesting system for microalgae
(Colorado State University. Libraries, 2014) Hincapié Gómez, Esteban, author; Marchese, Anthony J., advisor; Willson, Bryan D., committee member; Dasi, Lakshmi Prasad, committee member; Peers, Graham, committee member
Microalgae have vast potential as a sustainable source of biofuel. However, numerous technoeconomic analyses have indicated that microalgae harvesting represents a critical bottleneck in the microalgae value chain in terms of energy requirements, capital cost and operating cost. This dissertation presents an approach that uses a combination of acoustophoretic, fluid mechanical, and gravitational forces toward the development of a continuous flow microalgae harvesting system. Ultrasonic Standing Waves have been widely reported in the literature as an approach to manipulate particles in a fluid, a phenomena known as acoustophoresis. These waves exert an acoustic force that agglomerate the cells in the wave nodes or antinodes and the force is directly proportional to the cell acoustic contrast factor. Ultrasonic microalgae harvesting is a promising low cost and low energy approach. However, a better understanding of the acoustic properties of microalgae is essential for the development of this technology. Accordingly, a major component of this work focused on accurately quantifying the acoustic contrast factor of microalgae cells of Nannochloropsis oculata, Nannochloropsis gaditana, Phaeodactylum tricornutum and Chlamydomonas reinhardtii by measuring the average cell density and speed of sound using a vibrating tube densitometer. The results indicate a linear correlation of density and speed of sound as a function of cell concentration. Using this correlation, non-scattering volume average relationships were used to compute density and speed of sound for the average algal cell. The acoustic contrast factor was estimated to be between 0.04 - 0.06 for microalgae cells in their corresponding growth media. Second, particle tracking velocimetry was used to determine the magnitude of the acoustophoretic force. In these studies, in addition to microalgae cells, polyamide seeding particles were used as a surrogate. The results obtained conclude that the maximum acoustophoretic forces are approximately 5 pN for Chlamydomonas reinhardtii cells and the results also show that there is change in the acoustic contrast factor from positive to negative with lipid accumulation. This dissertation also presents a novel device for the acoustic harvesting of microalgae. The design is based on using the acoustophoretic force, acoustic transparent materials and inclined settling (Boycott effect). A filtration efficiency of 70% ± 5% and a concentration factor of 11.6 ± 2.2 were achieved at a flow rate of 25 mL • min-1 and an energy consumption of 3.6 ± 0.9 kWh • m-3. The effects of the applied power, flow rate, inlet cell concentration and inclination were explored. It was found that the filtration efficiency of the device is proportional to the power applied. However, the filtration efficiency experienced a plateau at a 100 W • L-1 of power density applied. The filtration efficiency also increased with increasing inlet cell concentration and was inversely proportional to the throughput of the device as measured flow rate. It was also found that the optimum settling angle for maximum concentration factor occurred at an angle of 50° ± 5°. At these optimum conditions, the device had higher filtration efficiency in comparison to other similar devices reported in the previous literature.
Embargo
Development of bioinspired hyaluronan enhanced synthetic polymers for medical applications
(Colorado State University. Libraries, 2023) Gangwish, Justin Phillip, author; James, Susan P., advisor; Bailey, Travis S., advisor; Dasi, Lakshmi Prasad, committee member; Popat, Ketul, committee member; Monnet, Eric, committee member
The first transcatheter aortic valve replacement (TAVR) was performed in 2002, and over the past two decades has become as prevalent as surgical aortic valve replacements in America. TAVR's have significant potential to provide relief for patients with aortic valve disease, but due to their high cost remain accessible primarily in wealthy nations. Furthermore, TAVR's have limited lifespans and are radiotransparent so they can only be evaluated for function through echocardiography. Thus, an expensive medical implant with limited durability is only replaced after it has begun to fail and cause the patient further stress on their heart. To address these issues this dissertation reviews the research performed in generating a novel heart valve leaflet material that is radiopaque, made primarily out of linear low-density polyethylene (LLDPE), and incorporates the biological polymer hyaluronan (HA). These leaflets could significantly reduce the cost of TAVR's potentially allowing for global adoption of the technology. They are radiopaque and easily imaged using x-ray fluoroscopy allowing changes in leaflet shape or movement to be identified prior to when the echocardiography would have shown a deleterious effect to function. They incorporate HA which is found in the interior lumen of blood vessels and has been shown to decrease calcification and thrombosis, both of which have caused polymeric leaflets to fail in previous research. Finally, they can be easily shaped and reinforced allowing for a potentially far greater device lifespan. The leaflets are made by melt pressing in radiopaque powders of either tungsten or bismuth trioxide into sheets of LLDPE followed by treatment with HA to form an interpenetrating polymer network. The material properties of the leaflets were evaluated for their tensile mechanical properties, their hydrophilicity, their radio transparency (or lack thereof), and their hemodynamics. The biocompatibility of the leaflets was evaluated through a cytotoxicity assay, whole blood clotting on the surface of the material, and the ability for platelets to adhere and activate on the material surface. The results demonstrate the material has significant potential to function as a heart valve leaflet in a TAVR. Beyond evaluating this novel material, the process by which HA is incorporated into LLDPE was examined and optimized for commercial scale up. First, the need for solvent distillation and nitrogen blankets during treatment were determined to be unnecessary to produce an HA IPN with LLDPE. Then the rate at which the LLDPE is drawn from the HA solution as well as the vacuum pressure and temperature during this process was found to affect the amount of active HA at the surface of the material. Finally initial evidence was found that shows that HA IPN does not form through the bulk of the material, but rather in the first few microns of the LLDPE. Unrelated to TAVR's a study was performed to enhance the non-woven polypropylene used in surgical masks and N-95 masks against COVID-19 using HA and polyethylene glycol (PEG). HA was used to form a microcomposite on the surface of the non-woven polypropylene while PEG was grafted to the surface with oxygen plasma. The resulting materials were evaluated for their tensile mechanical properties, breathability, chemical composition, hydrophilicity, cytotoxicity, and ability to adsorb COVID-19 spike protein. The results indicate the material has notable potential to make masks more effective at preventing the transfer of COVID-19, however further studies using live SARS-CoV-2 virus beyond the capabilities of this laboratory are necessary before that potential can be fully confirmed.
Open Access
Dynamics and structure of stably stratified turbulence
(Colorado State University. Libraries, 2012) Schaad, Simon, author; Venayagamoorthy, Subhas Karan, advisor; Julien, Pierre Y., committee member; Dasi, Lakshmi Prasad, committee member
The dynamics and turbulent structures of stably stratified turbulence are explored via direct numerical simulations (DNS). The structural features of stratified turbulence and its relationship to the flow dynamics has been the subject of many recent investigations. In strongly stratified turbulent flows, the formation of large-scale quasi-horizontal vortices in layers with strong vertical variability has been observed in laboratory experiments. Enstrophy isosurfaces of strongly stable flows indicate the emergence of randomly distributed 'pancake'-like structures with near horizontal orientation at later times. The strongly stratified simulations are diffusive and dominated by linear internal waves. The results suggest a decoupling between horizontal and vertical dynamics as the vertical dynamics can be described using rapid-distortion theory (RDT) while horizontal dynamics continue to be dominated by non-linear effects not captured by RDT. The integral flux Richardson number for decaying turbulence is the ratio of background potential energy gain to turbulent kinetic energy loss. The traditional flux-based formulation converges upon this ratio only when integrations are performed over an entire event, while the irreversible formulation converges rapidly without error from reversible effects. Mixing efficiency is a property of the flow for energetic flow but becomes a property of the fluid for diffusive flows and subject to Prandtl number effects. RDT models predict the flux Richardson number scales as the inverse Prandtl number at the diffusive limit when the Prandtl number is greater than unity. Mixing efficiency comparisons between DNS and physical grid-tow experiments reveal a large discrepancy for strong stratification, which is attributed in part to the low Reynolds numbers attained in both DNS and grid-tow experiments. Overturns are unstable conditions where heavier fluid resides above lighter fluid. The collapse of these local instabilities produce additional patches of turbulence and mixing making overturns an important mechanism in stratified turbulence. The overturning structures in strongly stratified flow resemble the quasi-horizontal vorticity structures and were found to be correlated with increased horizontal vorticity. The Thorpe scale, a measure of overturning structure height, and the Ozmidov scale equate only at the critical condition where inertial and buoyancy effects are equal (i.e. the turbulent Froude number is unity). The error of inferred dissipation rates from equating the Thorpe and Ozmidov scales was found to be up to two orders of magnitude.
Open Access
Influence of polymeric nanowire topography on the differentiation of adipose-derived stem cells
(Colorado State University. Libraries, 2014) Trujillo, Nathan Anthony, author; Popat, Ketul, advisor; Williams, John, committee member; Dasi, Lakshmi Prasad, committee member; Kipper, Matt, committee member
Considering the many advances in tissue engineering, there are still significant challenges associated with restructuring, repairing, or replacing damaged tissue in the body. Recently biodegradable synthetic scaffolds have shown to be a promising alternative. However, based on the application, it is essential that the scaffold possess specific surface properties that promote cell-scaffold interactions and aid in extracellular matrix deposition. In this work, we present a novel solvent-free, template-synthesis technique for creating substrate-bound nanowire scaffolds from polycaprolactone, a biocompatible and biodegradable polymer. Nanowire surfaces were also fabricated from polycaprolactone that included 1 wt% hydroxyapatite nanoparticles for osteogenic studies. The fundamental concept behind successful synthetic tissue-engineered scaffolds is to promote progenitor cell migration, adhesion, proliferation, and induce differentiation, extracellular matrix synthesis, and finally integration with host tissue. Recently, lipoaspirate tissue has been identified as a viable alternative source for mesenchymal stem cells because it contains a supportive stroma that can easily be isolated. Adipose derived stem cells can differentiate into a variety of mesodermal lineages including the osteogenic, chondrogenic, and adipogenic phenotypes. The results indicated that during the growth period i.e., initial 7 days of culture, the nanowire surfaces supported adhesion, proliferation, and viability of the cells in addition to morphological changes. Osteogenic, chondrogenic, and adipogenic differentiation potential of adipose derive stem cells was evaluated with and without differentiation supplements to determine the influence of nanowire architecture on mechanotrasnduction. It was determined that nanowire topography stimulated the expression of osteogenic marker proteins, osteocalcin and osteopontin, as well as mineralization and alkaline phosphatase activity.
Open Access
Tailoring solid-liquid interactions to control droplet wetting and dynamics
(Colorado State University. Libraries, 2019) Vahabi, Hamed, author; Kota, Arun K., advisor; Dasi, Lakshmi Prasad, committee member; Tavener, Simon, committee member; Bandhauer, Todd M., committee member
Recent advances in micro/nano-scale fabrication techniques and synthesis of novel chemicals with a variety of functionalities have opened up new avenues in tailoring solid-liquid interactions. In this work, by systematically tuning the wettability and slipperiness of solid surfaces, we developed a multitude of novel surfaces and strategies. First, we developed metamorphic superomniphobic surfaces that display wetting transition in response to heat. Second, we systematically studied the dynamics of droplets of various liquids during coalescence-induced jumping on textured super-repellent surfaces. Third, we developed a simple and passive strategy consisting of superomniphobic surfaces with a protruding macrotexture to demonstrate coalescence-induced jumping with significantly higher energy conversion efficiency, compared to state-of-the-art surfaces. Fourth, we developed a simple "grafting to" technique to fabricate a novel non-textured hydrophilic surface that is counterintuitively slippery with unprecedented potential to enhance the heat transfer coefficient in dropwise condensation. Fifth, we developed a novel triboelectric-based droplet manipulation technique on smooth hydrophobic slippery surfaces that is very simple without any complex fabrication of manipulation platform or expensive actuation system. Overall, the novel surfaces and strategies developed in this work have significant implications for phase-change heat transfer, liquid transportation, anti-fouling, self-cleaning, drag reduction, corrosion control, and manipulation of liquid droplets.
Open Access
The impact of shear rate and reverse flow on cardiac morphogenesis and gene expression in the embryonic zebrafish heart
(Colorado State University. Libraries, 2015) Zeller, Molly J., author; Garrity, Deborah M., advisor; Mykles, Donald, committee member; Bedinger, Patricia, committee member; Dasi, Lakshmi Prasad, committee member
Missteps in formation of the embryonic heart can have drastic consequences, making cardiac malformations a common human birth defect. During development, biomechanical factors including shear stress and reverse flow impact cardiogenesis. Shear stress is an epigenetic biomechanical force acting upon endothelial cells. Normally, a short period of reverse flow occurs prior to atrioventricular valve formation during ventricle systole and atrial diastole. The goal of our research is to investigate how altered biomechanical forces acting on endocardial cells lead to genetic responses by the heart. The mammalian zinc finger transcription factor Krüppel-like factor 2 (KLF2) responds to shear stress signals. Here, we explore the zebrafish KLF genes: klf2a, klf2b, and klf4. Whole embryo RT-PCR indicates that the three genes are expressed throughout early development, with cardiac expression in all genes present by 48 hours post fertilization. To evaluate how changes in biomechanical environments trigger altered gene expression in endocardial cells, we used comparative qPCR to quantify klf2a, klf2b, and klf4 expression levels in embryonic hearts with altered shear stress or reverse flow. Knockdown of the hematopoiesis gene gata2 was found to decrease blood viscosity, thereby decreasing both shear stress and reverse flow. Knockdown of contractility gene filaminCb was found to decrease shear stress but significantly increase reverse flow. Using high-speed imaging we quantified these forces and correlated changes in klf2a, klf2b, and klf4 expression. klf2a expression levels decreased in response to changes in both blood viscosity and cardiac contractility. klf2b and klf4 expression levels did not significantly change with these changes in biomechanical stresses. Our investigations considered the impact of blood viscosity versus cardiac contractility on KLF expression and determined that klf2a is a flow response gene. This data confirms previous studies that klf2a is in fact a flow response gene and shows that klf2b and klf4 are not responsive to changes in blood viscosity or cardiac contractility. Future studies will use transcriptomic approaches to identify genes regulated by the KLF family in response to shear stress and reverse flow cues.
Open Access
Tissue electrophoresis for generation of porcine acellular dermal matrices
(Colorado State University. Libraries, 2013) Duran, Celso, Jr., author; Orton, Christopher, advisor; Dasi, Lakshmi Prasad, committee member; James, Susan P., committee member
Background: Acellular dermal matrices have several applications including treatment of burns, reconstructive surgery, and treatment of chronic ulcers. Xenogeneic acellular dermal matrices have the advantage of increased availability compared to matrices derived from human cadavers (i.e. allogeneic dermal matrices), however they have a higher potential for generating an inflammatory response in the recipient. One approach to creating an acellular dermal matrix is through chemical and detergent-based processes collectively known as decellularization. Concerns regarding the completeness of soluble protein and antigen removal associated with current detergent-based decellularization treatments have been raised. The aim of this study was to compare the efficacy of a standard detergent-based decellularization and a novel electrophoresis-based method at removing soluble protein and protein antigens. Hypothesis: I hypothesized that tissue electrophoresis would enhance the removal of soluble proteins and protein antigens from porcine dermis compared to a standard detergent-based decellularization protocol. Methods: Skin was harvested from 6 pig cadavers. A portion of skin from each pig was assigned to four treatment groups: 1. Epidermis removal without sodium dodecyl sulfate (SDS) (positive or untreated control) 2. Epidermis removal with 0.5% SDS (epidermis removal control) 3. Epidermis removal with 0.5% SDS and standard 0.5% SDS decellularization treatment with a 6 h passive diffusion washout period 4. Epidermis removal with 0.5% SDS and Tissue Electrophoresis-based decellularization (0.5% SDS, 2% agarose gel, and 0.5 Amp) for 6 h The completeness of soluble protein and antigen removal was evaluated by SDS-PAGE and immunoblot analysis, respectively. Rabbit anti-porcine and human IgG serums were the primary antibodies for immunoblot analysis. Results: Tissue electrophoresis decellularization increased removal of soluble proteins from porcine dermis when compared to standard passive detergent-based decellularization, based on SDS-PAGE analysis. Antigen removal, based on immunoblot analysis, was increased compared to untreated dermis, but was not significantly different between standard detergent-based and tissue electrophoresis-based decellularization treatments. Conclusion: Tissue electrophoresis enhances removal of soluble proteins from porcine dermis compared to standard detergent-based decellularization. This enhanced removal of soluble proteins may translate into reduced inflammatory response to xenogeneic acellular dermal matrices implanted into humans. Optimization of electrophoretic parameters may further increase the efficiency of tissue electrophoresis as a decellularization method.
Open Access
Titania nanotube arrays: interfaces for implantable devices
(Colorado State University. Libraries, 2012) Smith, Barbara Symie, author; Popat, Ketul, advisor; Gonzalez-Juarrero, Mercedes, committee member; Prasad, Ashok, committee member; Dasi, Lakshmi Prasad, committee member; Dow, Steven, committee member
For the 8-10% of Americans (20-25 million people) that have implanted biomedical devices, biomaterial failure and the need for revision surgery are critical concerns. The major causes for failure in implantable biomedical devices promoting a need for re- implantation and revision surgery include thrombosis, post-operative infection, immune driven fibrosis and biomechanical failure. The successful integration of long-term implantable devices is highly dependent on the early events of tissue/biomaterial interaction, promoting either implant rejection or a wound healing response (extracellular matrix production and vasculature). Favorable interactions between the implant surface and the respective tissue are critical for the long-term success of any implantable device. Recent studies have shown that material surfaces which mimic the natural physiological hierarchy of in vivo tissue may provide a possible solution for enhancing biomaterial integration, thus preventing infection and biomaterial rejection. Titania nanotube arrays, fabricated using a simple anodization technique, provide a template capable of promoting altered cellular functionality at a hierarchy similar to that of natural tissue. This work focuses on the fabrication of immobilized, vertically oriented and highly uniform titania nanotube arrays to determine how this specific nano-architecture affects skin cell functionality, hemocompatibility, thrombogenicity and the immune response. The results in this work identify enhanced dermal matrix production, altered hemocompatibility, reduced thrombogenicity and a deterred immune response on titania nanotube arrays. This evidences promising implications with respect to the use of titania nanotube arrays as beneficial interfaces for the successful implantation of biomedical devices.
Open Access
Turbulence parameterizations for numerical simulations of stably stratified environmental flows
(Colorado State University. Libraries, 2011) Elliott, Zachary, author; Venayagamoorthy, Subhas Karan, advisor; Julien, Pierre Y., committee member; Dasi, Lakshmi Prasad, committee member
Almost all environmental and geophysical flows such as lakes, reservoirs, estuaries, and the atmosphere are turbulent and are also often characterized by stable density stratification. The presence of buoyancy forces due to stratification has a substantial effect on the flow development and turbulent mixing processes, influencing the distribution of pollutants and suspended matter in these flows. Mathematical and computer models can be used to simulate and produce numerical solutions to these flows, providing results that would otherwise not be feasibly attainable in a laboratory setting and that can be used for engineering prediction, design, and analysis purposes. Turbulence models use computational procedures to close the system of mean flow equations and account for the effects of turbulence and stratification through the specification of parameters that characterize the behavior of the flow. In this research, an attempt is made to assess and improve turbulence parameterizations for stably stratified environmental flows. An important parameter describing the transfer of momentum and scalar fluxes in stratified turbulent flows is the turbulent Prandtl number Prt. Specifically, four different formulations of the turbulent Prandtl number Prt are evaluated for stably stratified flows. All four formulations of Prt are strictly functions of the gradient Richardson number Ri, a parameter that provides a measure of the strength of the stratification. A zero-equation turbulence model for the turbulent viscosity νt in a one-dimensional turbulent channel flow is considered to assess the behavior of the different formulations of Prt. Both uni-directional and oscillatory flows are considered to simulate conditions representative of practical flow problems, such as atmospheric boundary layer flows and tidally-driven estuarine flows, to quantify the behavior of each of the four formulations of Prt. It is discussed as to which of the models of Prt allow for a higher rate of turbulent mixing and which models significantly inhibit turbulent mixing in the presence of buoyancy forces resulting from fixed continuous stratification as well as fixed two-layer stratification. The basis underlying the formulation of each model in conjunction with the simulation results are used to highlight the importance of choosing an appropriate parameterization of Prt, given a model for νt in stably stratified flows. Other more complete and dynamic models rely on additional parameters that allow stratified turbulent flow to be modeled as a function of local turbulence quantities rather than mean global properties of the flow. This research also focuses on implementing and testing proposed changes that explicitly account for buoyancy effects in two-equation Reynolds-averaged Navier-Stokes (RANS) turbulence models. Direct numerical simulation (DNS) data of stably stratified homogeneous turbulence are used to study the parameters in two-equation RANS turbulence models such as the buoyancy parameter Cε3 and the turbulent Prandtl number Prt in the k-ε model. Both the gradient Richardson number Ri and the turbulent Froude number Frk are used as correlating parameters to characterize stratification in the k-ε model. It is shown that it may be more appropriate to use Frk as the parameter of choice for the stratification parameter in the k-ε model since it is based on the local properties of the turbulence as opposed to Ri, which is a mean property of the flow. The proposed modifications and alterations to Cε3 and Prt as functions of Ri and Frk are implemented in a one-dimensional water column model called General Ocean Turbulence Model (GOTM) and used to simulate stably stratified channel flows. The results from numerical simulations using the modified versions of the k-ε model are compared to stably stratified channel flow DNS data to assess their efficacy.