Mountain Scholar :: Browsing by Author "Popat, Ketul, committee member"

Browsing by Author "Popat, Ketul, committee member"

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Open Access
A novel approach for critical bone defect repair
(Colorado State University. Libraries, 2022) Schneiderhan, Adam, author; Prawel, David, advisor; Popat, Ketul, committee member; Séguin, Bernard, committee member
Critical bone defects are defined as defects that will not naturally heal over a patient's lifetime, even with surgical stabilization. When these occur in the long bones of the axial skeleton (secondary to trauma, tumor resection, etc.), limb-sparing surgery can be performed to avoid amputation of the limb. This procedure typically involves the installation of a steel locking plate over the defect, along with an endoprosthesis or allograft to fill the void of resected bone. Much progress has been made in the natural bone regeneration using tissue engineering (TE) scaffolds in place of these grafts. Porous hydroxyapatite (HAP) is a well-established bone TE scaffold biomaterial but lacks sufficient mechanical strength when fabricated at porosities shown to best induce osteogenesis. To remedy this, polymers such as polycaprolactone (PCL) are often mixed with HAP to fabricate scaffolds with increase load-bearing capacity. However, the addition of PCL makes the scaffold less osteogenic and dramatically slows the degradation rate of the scaffold. This translates into reduced new bone volume where the PCL cannot be remodeled as new bone is formed. This project involves a pilot clinical trial of a novel method that augments the gold-standard limb-sparing procedure by implanting a 3D printed endoprosthetic "sleeve" device that attaches to the locking fixation plate and contains and protects the brittle HAp scaffold. The PCL sleeve alleviates the dependency on scaffold strength which enables use of the most osteogenic possible biomaterials at ideal porosities to maximize the rate and density of new bone formation. The purpose of the study is to clinically validate the construct design and surgical procedure. Thus far, pilot limb-sparing surgeries have been performed on 4 client-owned dogs, in which sleeve-scaffold devices were installed in the critical defects caused by the removal of osteosarcomas in distal epiphyseal radii. Recombinant human bone morphogenic protein-2 (rhBMP-2) was added to the scaffolds to further encourage osteogenesis. Mechanical tests were performed on both the sleeves alone and the full construct installed in canine cadaver limbs. Results from this testing demonstrate the sleeve's ability to prevent mechanical failure of the HAp scaffolds. Similarly, no scaffold failure has been observed in clinical trial patients, with some having the device installed for greater than 24 weeks. Additionally, pressureometry and gait analysis confirmed excellent return of limb function in these animals. However, to date, no new bone formation has been observed within the scaffold devices, which has likely been inhibited by anti-cancer treatment. Regardless, results from ex vivo testing and the clinical trial validate the construct design and the viability of our novel method for protecting and maintaining brittle bone tissue engineering scaffolds, while aiding in restoration of normal limb function.
Open Access
A novel design methodology for osseointegrated implants and the effects of heat-treatment on shape setting nitinol foil
(Colorado State University. Libraries, 2020) Morrone, Adam, author; Simske, Steven, advisor; Popat, Ketul, committee member; Kawcak, Christopher, committee member
Nitinol, approximately equiatomic nickel and titanium and a popular shape memory alloy, has been used extensively in modern, implantable medical devices due to its natural biocompatibility, remarkable shape memory properties, and superelasticity. Much of the current literature on processing and handling this material focuses on thin wires, as this is what has historically been of most interest (e.g. for orthopedics, orthodontia, and orthognathics); however, as this technology advances, there are emerging applications of nitinol that require other form factors such as films and foils. In addition, although many manufacturers can produce three-dimensional nitinol structures, much of the information on shaping techniques is still proprietary. In an effort to fill these gaps in the literature and add to the knowledge of nitinol shaping techniques, this study compares the effects of various heat-treatments on the shape-setting of nitinol foil. Foils of two different NiTi compositions (50.2 and 50.8 percent Ni by atomic mole fraction) were rigidly fixed into a cylindrical shape and heat-treated at five different temperatures (400, 450, 500, 550, and 600 degrees C) and for five different durations (5, 10, 15, 20, and 25 minutes). The morphological rebound of these samples was evaluated, and a model was developed to described this shape setting behavior. In addition, the Austenite finishing temperature (Aƒ), and fatigue effects of all samples were evaluated to further quantify the effects of heat-treatment. The results from this materials study were then used in part to develop a novel design methodology for osseointegrated implants. Devices using this methodology have anchors that deploy from the main body to lock the implant in place. The contact points act as "active sacrificial zones" which can experience bone resorption without losing rigidity, while the remainder of the implant body undergoes normal loading conditions. This methodology aims to improve the quality and speed of bone ingrowth.
Open Access
A personal thermophoretic sampler for airborne nanoparticles
(Colorado State University. Libraries, 2010) Thayer, Daniel Lee, author; Marchese, Anthony, advisor; Volckens, John, advisor; Popat, Ketul, committee member; Prieto, Amy, committee member
Engineered nanoparticles are materials with at least one dimension measuring less than 100 nm that are designed on the molecular scale to produce unique or enhanced properties that differ from the bulk material. However, the same properties that make engineered nanoparticles attractive to industry also may present potential health risks to the workers who manufacture them. Very little human exposure data exist for these particles, although they are known enter the body through a number of routes (e.g., respiration, dermal penetrations, and ingestion). Nanoparticles that enter the body can also translocate from one organ to another by virtue of their small size. A cost-effective personal sampler is necessary to evaluate levels of worker exposure to these materials to determine the relative levels of individual risk. Such a sampler must be capable of collecting nanoparticles with high efficiency for subsequent analysis of size, surface chemistry, morphology, and other properties. In addition, the sampler must be able to differentiate between incidental nanoparticles, which are nanoparticles that are naturally present in the environment, and engineered nanoparticles. As detailed in this thesis, a small thermal precipitator was designed to measure breathing-zone concentrations of airborne nanoparticles. The thermal precipitator samples aerosol by producing a 1000 °C cm ' temperature gradient between two aluminum plates (0.1 cm separation distance) using a resistive heater, a thermoelectric cooler, a temperature controller, and two thermistor sensors. The collection efficiency was evaluated for 15, 51, 100, and 240 nm particles at flow rates of 5 and 20 mL/min. Tests were also performed with a zero temperature gradient to determine losses in the device for measurement correction. The homogeneity of particle collection across the collection surface was evaluated using electron microscopy and imaging software. The results indicate that thermal precipitation is a feasible approach for personal monitoring of airborne nanoparticle concentrations in the workplace.
Open Access
A study of oxide/CdTe interfaces for CdTe photovoltaics using atomistic modeling
(Colorado State University. Libraries, 2021) Thiyagarajan, Aanand, author; Sampath, W. S., advisor; Weinberger, Christopher, advisor; Martinez Pozzoni, Umberto, committee member; Sites, James R., committee member; Popat, Ketul, committee member
Solar photovoltaics (PV) has undergone a dramatic transformation over the past few decades and is now a widespread electricity generation source. Among currently existing PV technologies, the thin film sector led by cadmium telluride is the most promising. Cadmium Telluride (CdTe) PV has experienced unprecedented growth and is now a major commercial player. However, the field has a few challenges to overcome until it reaches its full potential. The focus of this study is the interface between the CdTe-based absorber and the front window layer. Traditionally, cadmium sulfide has been used as the window layer in such devices. At the Next Generation Photovoltaics (NGPV) center in Colorado State University, superior devices have been demonstrated using magnesium zinc oxide (MgxZn1-xO or MZO) as the window layer. This is attributed to the larger bandgap of MZO causing a pickup in the current and the open circuit voltage. A magnesium to zinc atomic ratio of 23:77 has shown optimal performance characteristics. Alloying CdTe with Se to form cadmium selenium telluride (CdSexTe1-x or CST) has resulted in further improvements. One way to determine the quality of an interface is to study the electronic band alignment at that interface. Existing band alignment models show only limited features and hence there is a need for a more sophisticated approach to investigate complex characteristics. This study uses atomistic modeling based on Density Functional Theory (DFT) to investigate certain structural and electronic properties of the oxide and the oxide/absorber interface. The technique solves for electronic structures of materials based on electron density and predicts the structural properties of materials to a high degree of accuracy. Electronic characteristics are determined using a semi-empirical method known as DFT-1/2. A mathematical formulation called Green's Function (GF) has been incorporated within the model to simulate device structures. The bulk properties of MZO such as lattice constant, band gap, band edges and electron effective mass are established and compared to experiment. Following this, the band alignment at the MZO/CdTe and MZO/CST interfaces is determined, along with band offsets and interface states. The influence of chlorine in the deposition process is also investigated. This work is the first of its kind to study the oxide-CdTe and oxide/CST interfaces using DFT+GF and provides new insights into the electronic characteristics at the interface. Bulk properties of the MZO match experimental reports. Termination chemistry plays a significant role in the band bending and in the presence of defect states at the oxide/absorber interface. Calculations indicate that a Mg/Zn-Te interface is energetically preferred, with experimental reports pointing to the same. Moreover, varying the magnesium composition in the MZO alloy affects the magnitude of the band offsets. The interface band alignment results are close to those seen experimentally. A small amount of chlorine may help alleviate interface defect states by chemical passivation, possibly due to the removal of dangling bonds.
Open Access
Addressing absorber quality in CD1-xMGxTE wide bandgap solar cells for tandem applications
(Colorado State University. Libraries, 2018) Reich, Carey L., author; Sampath, Walajabad S., advisor; Sites, James, committee member; Popat, Ketul, committee member
Tandem photovoltaic (PV) solar cells, which use multiple absorbing layers to convert light into electricity, have the potential to surpass the conversion efficiency limits of PV which uses a single absorber. This has been proven using epitaxially grown III-V semiconductors, but these are expensive and are only commonly used for extra-terrestrial applications. To realize terrestrial, cost effective tandem PV, low cost production of these highly efficient cells is required. Using absorbers which are similar to cost effective, mass produced PV such as CdTe, Si, or CIGS, this is possible. Si and CIGS have appropriate properties for the IR absorbing layer in a tandem cell, but there is no common PV material with the ideal properties for the UV/Visible light absorbing layer, although CdTe is quite close. Even better, CdTe's properties can be altered to those of ideal with the addition of ternary elements such as Mg, Zn, and Mn. Issues still remain however as the quality of solar cells produced using ternary alloys of CdTe is much lower than that of the base material. These quality issues seem to stem from the CdTe bulk passivation process, which involves a thermal treatment in the presence of Cl (commonly CdCl2 is used as a source) to passivate the grain boundaries and catalyze the recrystallization and grain growth process which annihilates detrimental planar crystalline defects in the absorbing material. The work presented in this thesis addresses issues with the absorber quality of solar cells using Cd1-xMgxTe by using concurrent Cl sources with CdCl2, diffusion barriers during CdCl2, and tweaking the absorber material with the addition of quaternary elements or novel layers in the device stack. This work culminated in the production of a 10.6% efficient device, a record for devices using CdMgTe as an absorber, and concludes with paths for future improvements in device performance.
Open Access
Alternative heart assistance pump
(Colorado State University. Libraries, 2021) Sharifi, Alireza, author; Bark, David, advisor; James, Susan, advisor; Scansen, Brian, committee member; Popat, Ketul, committee member; Gao, Xinfeng, committee member
On average, the human heart beats around 115,000, and pumps around 2,000 gallons of blood daily. This essential organ may undergo systolic or diastolic dysfunction in which the heart cannot properly contract or relax, respectively. To help hearts pump effectively should these types of failures occur, ventricular assist devices (VAD) are implemented as a temporary or permanent solution. The most common VAD is the left ventricular assist device (LVAD) which supports the left ventricle in pumping the oxygen-rich blood from the heart to the aorta, and ultimately to the rest of the body. Although current VADs are an important treatment for advanced heart failure, generally VADS come with many complications and issues after implantation. These complications include incidents of hemolysis (tearing of the blood cells), thrombosis (clotting of the blood), bleeding (especially in the gastrointestinal tract), and infection at the driveline site. Specifically, the current continuous flow pumps are associated with a much higher incidence of gastrointestinal bleeding, myocardial perfusion, kidney problems, among others, compared with the earlier generation pulsatile pumps. However, the presence of several moving mechanical components made the pulsatile pumps less durable, bulky, and prone to malfunction, ultimately leading to favor toward continuous flow designs. The goal of the present study is to develop a novel heart assist pump, overcoming drawbacks to current commercially available pumps, by improving hemodynamic (blood flow) performance, pulsatility, and eliminating bleeding disorders. Our design will overcome the current pumps which suffer from non-physiological flow, and blood damage. The impact of this work goes beyond heart assist devices and would be applicable to other blood pumps. The fundamental biological and physical principles of designing a blood pump will be reviewed in chapter one. In addition, recent studies on current LVADs and the motivation behind these studies will also be discussed. Then, the idea of using a contractive tubular heart as an alternative pump will be presented in chapter two. To understand the pumping mechanism of the tubular heart, a detailed study on the embryonic heart is presented in this chapter. Subsequently, the effect of flow forces on blood cells will be studied in chapter 3. Moreover, the relation between flow regime and bleeding disorders have been studied in the same chapter. A discussion of our design, including the pump design, testing set up, experimental results will be presented in chapter 4. Finally, the limitations of the present study and future work will be presented in chapter 5.
Open Access
An investigation of the effect of surface released nitric oxide on fibrinogen adsorption
(Colorado State University. Libraries, 2014) Lantvit, Sarah Marie, author; Reynolds, Melissa, advisor; Borch, Thomas, committee member; Fisher, Ellen, committee member; Kennan, Alan, committee member; Popat, Ketul, committee member
The search for improved biomaterials is a continually ongoing effort to prevent the failure of medical devices due to blood clotting. Each group of researchers has their own set of methods to create the ideal material for biological systems. In the pursuit of materials to prevent blood clot formation, these attempts have been focused on alterations in surface properties, pre-adsorption of proteins, and release of drugs. In this work I took a high-throughput approach to the prevention of device failure by investigating a model material system. Starting with a nitric oxide (NO) releasing material, a sample preparation method was developed to ensure that surface properties could be compared to a non-NO releasing control. With this material, the effect of the NO release on fibrinogen adsorption to these surfaces could be isolated. Fibrinogen is instrumental in the formation of blood clots. Determining the effect that NO has on this protein will help determine why NO has been previously found to prevent clotting in blood-contacting systems. Once the model system was developed, further investigation into changes in the fibrinogen resulting from its interaction with the released NO could be undertaken. A full investigation was completed on control non-NO releasing, low NO flux, and high NO flux materials. A qualitative assessment of the fibrinogen adsorption shows that the high NO releasing material exhibits significantly higher fibrinogen adsorption compared to both the control and low NO flux materials. Quantitative assessment of fibrinogen adsorption was attempted through a variety of methods, which indicate that conformational changes are happening upon adsorption of fibrinogen to all materials. To this end, FTIR spectra from the adsorbed fibrinogen and native fibrinogen were compared to elucidate changes in the protein's conformation. Control and low NO flux materials had too little protein to gain insight into these changes. For the high NO flux material, the fibrinogen had a significant decrease in α-helices and an increase in random chains compared to native fibrinogen. To begin understanding the effect that these changes will have on blood clot formation, these materials were further analyzed for platelet adhesion. A comparison of the control, low NO flux, and high NO flux materials with and without fibrinogen adsorbed to the material surface shows that the fibrinogen has a distinct effect on platelet adhesion and aggregation. The high NO flux materials exhibited less aggregation and full activation of platelets when fibrinogen was adsorbed prior to incubation with platelets than if fibrinogen was not present before incubation. Overall, the effect of NO on fibrinogen adsorption can be seen through these measurements. Nitric oxide release causes an increase in fibrinogen adsorption, as well as protein reorganization. Surprisingly, we see that this adsorbed fibrinogen actually improves the viability of platelets. Further study must be done using whole blood and in vivo measurements to fully understand what effect the adsorbed fibrinogen will have on the device. Despite this we can say that the adsorption of fibrinogen onto these NO releasing materials helps to improve the biocompatibility of this biomaterial due to its bulk adsorption and conformational changes.
Open Access
Are lines of arrested growth in bone indicative of seasonal metabolic suppression in bears?
(Colorado State University. Libraries, 2016) Hinrichs, Jason, author; Donahue, Seth, advisor; Norrdin, Robert, committee member; Popat, Ketul, committee member
Large hibernators such as bears have seasonal metabolic suppression, hibernation (Tøien et al. 2012). During hibernation bear's activity is very low; to the point most other animals would exhibit disuse bone resorption. However bears do not exhibit disuse bone resorption during this time (McGeeLawrence et al. 2008). Are lines of arrested growth (LAGs) in bone indicative of seasonal metabolic suppression in bears? Through the use of toluidine blue stain light microscopy slides and backscattered scanning electron microscopy images (SEM), LAGs were counted and correlated with age. LAGs have a strong correlation with age. This is indicative of LAGs formation once per year, during set hibernation cycles. LAGs are metabolic markers, in bears with set hibernation cycles. These metabolic markers could be used to identify the specific time in which there is metabolic suppression, in bears. This identification could be used in the future to track blood serum and other chemical markers in an attempt to understand bear's natural resistance to disuse bone resorption. Bears ability to not exhibit disuse bone resorption could be biomimetically studied, in an attempt to adapt this protection to humans. Since humans experience disuse osteoporosis (extended bed rest and spaceflight) and osteoporosis (older population specifically women).
Open Access
CdTe alloys and their application for increasing solar cell performance
(Colorado State University. Libraries, 2016) Swanson, Drew E., author; Sampath, W. S., advisor; Sites, James R., committee member; Williams, John D., committee member; Popat, Ketul, committee member
Cadmium Telluride (CdTe) thin film solar is the largest manufactured solar cell technology in the United States and is responsible for one of the lowest costs of utility scale solar electricity at a purchase agreement of $0.0387/kWh. However, this cost could be further reduced by increasing the cell efficiency. To bridge the gap between the high efficiency technology and low cost manufacturing, a research and development tool and process was built and tested. This fully automated single vacuum PV manufacturing tool utilizes multiple inline close space sublimation (CSS) sources with automated substrate control. This maintains the proven scalability of the CSS technology and CSS source design but with the added versatility of independent substrate motion. This combination of a scalable deposition technology with increased cell fabrication flexibility has allowed for high efficiency cells to be manufactured and studied. The record efficiency of CdTe solar cells is lower than fundamental limitations due to a significant deficit in voltage. It has been modeled that there are two potential methods of decreasing this voltage deficiency. The first method is the incorporation of a high band gap film at the back contact to induce a conduction-band barrier that can reduce recombination by reflecting electrons from the back surface. The addition of a Cd1-xMgxTe (CMT) layer at the back of a CdTe solar cell should induce this desired offset and reflect both photoelectrons and forward-current electrons away from the rear surface. Higher collection of photoelectrons will increase the cells current and the reduction of forward current will increase the cells voltage. To have the optimal effect, CdTe must have reasonable carrier lifetimes and be fully depleted. To achieve this experimentally, CdTe layers have been grown sufficiently thin to help produce a fully depleted cell. A variety of measurements including performance curves, transmission electron microscopy, x-ray photoelectron spectroscopy, and energy-dispersive x-ray spectroscopy were performed to characterize these cells. Voltage improvements on the order of 50 mV are presented at a thin (1 μm) CdTe absorber condition. However an overall reduction in fill factor (FF) is seen, with a strong reduction in FF as the magnesium incorporation is increased. Detailed material characterization shows the formation of oxides at the back of CdMgTe during the passivation process. A CdTe capping layer is added to reduce oxidation and help maintain the uniformity of the CdMgTe layer. A tellurium back contact is also added in place of a carbon paint back contact, reducing the impact of the valance band offset (VBO) from the CMT. With the addition of the capping layer and tellurium back contact a consistent 50 mV increase is seen with improved FF. However this voltage increase is well below modeled Voc increases of 150 mV. CMT double hetero-structures are manufactured and analyzed to estimate the interface recombination at the CdTe/CMT interface. The CdTe/CMT interface is approximated at 2*105 cm s-1 and modeling is referenced predicting significant reduction in performance based on this interface quality. To improve interface quality by removing the need for a vacuum break, the deposition hardware is incorporated into the primary deposition system. Second, CdTe has a somewhat higher band gap than optimal for single-junction terrestrial solar-cell power generation. A reduction in the band gap could therefore result in an overall improvement in performance. To reduce the band gap, selenium was alloyed with CdTe using a novel co-sublimation extension of the close-space-sublimation process. Co-sublimated layers of CdSeTe with various selenium concentrations were characterized for optical absorption and atomic concentrations, as well as to track changes in their morphology and crystallinity. The lower band-gap CdSeTe films were then incorporated into the front of CdTe cells. This two-layer band-gap structure demonstrated higher current collection and increased quantum efficiency at longer wavelengths. Material characterization shows the diffusion of selenium through the CdTe during passivation resulting in improved in lifetime and a reduced voltage deficit at lower band gaps.
Open Access
Characterization of chromatin remodeling in mesenchymal stem cells on the application of oxidative stress
(Colorado State University. Libraries, 2022) Kabi, Neda, author; Ghosh, Soham, advisor; Popat, Ketul, committee member; Goodrich, Laurie, committee member; Johnstone, Brian, committee member
Chromatin is a highly dynamic entity of the eukaryotic cell nucleus. Contrary to previous belief that chromatin maintains a well-defined permanent architecture in the interphase nucleus, new evidences are emerging with a support of the notion that chromatin can locally and globally rearrange itself to adapt with the cellular microenvironmental changes. Such microenvironmental changes can be related to biophysical such as change in the stiffness of extracellular matrix or the force applied on the cell as well as biochemical such as change in the oxidative stress, osmolarity or the pH. It is not well understood how the chromatin architecture changes under such environmental changes and what is the functional significance of such change. Characterization and quantification of chromatin remodeling is therefore a first step to understand the chromatin dynamics for elucidating complex subnuclear behavior under the influence of single or multiple environmental changes. Towards that end, in this work, human bone marrow derived mesenchymal stem cells were used to characterize such chromatin level changes under the changing oxidative stress on the cells. Oxidative stress was applied using hydrogen peroxide treatment. After validation of the application of oxidative stress, a series of experiments and subsequent analysis was performed to understand the hallmarks of chromatin remodeling at high spatiotemporal resolution. Specific chromatin remodeling pattern was observed in the heterochromatin, euchromatin and the interchromatin regions. Finally, a key component of chromatin remodeling complex called ARID1A was identified which is critical for the chromatin remodeling process.
Open Access
Characterization of osseointegrative phosphatidylserine and cholesterol orthopaedic implant coatings
(Colorado State University. Libraries, 2013) Rodgers, William Paul, author; James, Susan, advisor; Popat, Ketul, committee member; Ehrhart, Nicole, committee member; De Long, Susan, committee member
Total joint arthroplasties/replacements are one of the most successful surgeries available today for improving patients’ quality of life. By 2030 in the US, demand for primary total hip and knee arthroplasties are expected to grow by 174% and 673% respectively to a combined total of over 4 million procedures performed annually, driven largely by an ageing population and an increased occurrence of obesity. Current patient options for load-bearing bone integrating implants have significant shortcomings. Nearly a third of patients require a revision surgery before the implant is 15 years old, and those who have revision surgeries are at an increased risk of requiring additional reoperations. A recent implant technology that has shown to be effective at improving bone to implant integration is the use of phosphatidylserine (DOPS) coatings. These coatings are challenging to analyze and measure due to their highly dynamic, soft, rough, thick, and optically diffractive properties. Previous work had difficulty investigating pertinent parameters for these coating’s development due in large part to a lack of available analytical techniques and a dearth of understanding of the micro- and nano-structural configuration of the coatings. This work addresses the lack of techniques available for use with DOPS coatings through the development of original methods of measurement, including the use of scanning white light interferometry and nanoindentation. These techniques were then applied for the characterization of DOPS coatings and the study of effects from several factors: 1. the influence of adding calcium and cholesterol to the coatings, 2. the effect of composition and roughness on aqueous contact angles, and 3. the impact of ageing and storage environment on the coatings. This project lays a foundation for the continued development and improvement of DOPS coatings, which have the promise of significantly improving current patient options for bone integrating implants. Using these newly developed and highly repeatable quantitative analysis methods, this study sheds light on the microstructural configuration of the DOPS coatings and elucidates previously unexplained phenomena of the coatings. Cholesterol was found to supersaturate in the coatings at high concentration and phase separate into an anhydrous crystalline form, while lower concentrations were found to significantly harden the coatings. Morphological and microstructural changes were detected in the coatings over the course of as little as two weeks that were dependent on the storage environment. The results and understanding gained pave the path for focused future research effort. Additionally, the methods and techniques developed for the analysis of DOPS coatings have a broader application for the measurement and analysis of other problematic biological materials and surfaces.
Open Access
Characterization of tellurium back contact layer for CdTe thin film devices
(Colorado State University. Libraries, 2018) Moffett, Christina, author; Sampath, W. S., advisor; Sites, James, committee member; Popat, Ketul, committee member
Cadmium Telluride (CdTe) thin film photovoltaic technology has shown favorable progress due to inexpensive and efficient processing techniques. However, efficiencies have yet to reach the overall projected CdTe device efficiency, with the back contact being a main source of CdTe performance limitations. Tellurium (Te) applied as a back contact has led to significant increases in fill factor and an overall progress in device efficiency. Devices deposited with Te show significant improvement in uniformity, even without intentional Cu doping, when compared to devices without Te. In current - density measurements, Te shows stability even at low temperatures, which is indicative of a low barrier developed at the CdTe/Te interface. X-ray and ultra-violet photoelectron spectroscopy were carried out to examine the valence band offset at the CdTe/Te back contact interface. The valence band offset was shown to be highly dependent on the Te thickness and was largely affected by oxidation and contamination at the surface. Capacitance measurements were carried out to study the effect Te has on the absorber depletion width. Data indicate a decreased depletion width with Te applied at the back of thin film CdTe devices, which agrees with increased device performance. Te thickness was varied in all studies to understand the effect of application thickness on device performance and material characteristics. With a thicker Te layer leading to overall improvement in device performance and favorable device characteristics.
Embargo
Coexistence of weather radars and communication systems: model to identify interfering sources and mitigation solutions
(Colorado State University. Libraries, 2023) Vaccarono, Mattia, author; Chandrasekaran, Chandra V., advisor; Cheney, Margaret, committee member; Jayasumana, Anura, committee member; Popat, Ketul, committee member
Electromagnetic spectrum is a finite resource. Weather radars are one of the many sources which use electromagnetic waves. The availability of spectrum bands that can be assigned to a specific user is limited. Consequently, the electromagnetic spectrum is shared by different application in the same frequency band. This is the specific case of C-band weather radars, which operate in the 5.6GHz band, sharing the same frequencies with Radio Local Area Networks, Wireless Local Area Networks and HiperLAN systems. These telecommunication systems are continuously increasing in rural areas as broadband Internet access points. The coexistence of C-band weather radar and such systems is nowadays a primary topic in the weather radar community. The amount of interference received by weather radars are affecting the data quality, especially for polarimetric observations. Electromagnetic interference may also appear at higher frequencies, such as the X-band located around 9.3GHz. These frequencies are used by weather radars for hydrological purposes. The dense radar network deployed in Dallas Fort Worth area and the mobile radar managed by Arpa Piemonte operate at X-band and they receive interfering signals. These signals have been detected during a field measurement campaign using both the mobile weather radar and a vector signal analyzer able to perform real time analysis. A technique to identify the likely interfering sources is discussed, which can be used by the National Regulatory Authorities or Regional Agencies, such as the Physics and Industrial Risk Department of Arpa Piemonte, Italy, in charge of the telecommunication authorization processes. The model may be applied to a telecommunication tower transmitting at the same frequency of a given radar and in case of likely interference, mitigation strategies could be set during the tower installation, i.e. changing the antenna direction or tilt. Over the years, many RFI removal and mitigation tools have been discussed in the literature, but only few are currently implemented on operational weather radars. This work, instead, aims to implement mitigation solutions that can be implemented by National Weather Services. The electromagnetic interference may be removed at different levels: from the received signals to the processed radar products, such as reflectivity maps that are shown to general public. In order to make possible the interference removal also to those National Weather Services, or radar management services, which are not able to act on the radar signal processor to implement deeper mitigation tools, a RFI mitigation solution based on image processing is shown. This method does not require to access the radar signal processor, but it does not mitigate the effect of interference overlapped with weather echoes. Then, based on the interfering signals features, a mitigation solution has been developed. The interfering signals are removed before received signals are processed to obtain radar moments. The proposed method has been tested with good performances in clear air echoes at both C and X-bands. A study case has been selected to evaluate its performances during precipitation events. The proposed mitigation solution is applied to the received signals to remove interfering signals and to reconstruct the residual information. The radar reflectivity is computed and it is compared to the operational radar Z product. A Swiss C-band radar is selected as reference to validate the mitigation solution. The interfering signals are properly removed and the missing data in the received radar pulses are computed by smoothing from adjacent range gates and pulses. Actually, removing only the interfering signals the proposed solution is able to preserve the meteorological echoes which lead to a better estimate of the reflectivity values, especially in case of weak echoes (i.e. light rain or drizzle). The Interference to Signal Ratio (ISR) is considered the metric to quantitatively evaluate the mitigation performance as ISR difference between processed and received signals. The proposed mitigation solution can achieve up to 20dB suppression.
Open Access
Density functional theory and Green's function approach to investigate cadmium telluride based thin film photovoltaics
(Colorado State University. Libraries, 2020) Nicholson, Anthony P., author; Sampath, Walajabad S., advisor; Weinberger, Chris, advisor; Popat, Ketul, committee member; Sites, James, committee member; Martinez, Umberto, committee member
In recent years, cadmium telluride (CdTe) based thin film photovoltaics (PV) have exhibited remarkable improvements in overall efficiency and device performance. As the most notable thin film PV technology, CdTe PV is developed and manufactured in the U.S. as the leading cost-competitive option for electricity generation in comparison to other PV technologies such as silicon and CIGS PV. However, CdTe PV faces major challenges that limit its achievable performance during the solar energy conversion process. It has become increasingly evident that to improve PV efficiency, an understanding of surfaces and interfaces is necessary. Therefore, high-fidelity quantum-based atomistic simulations will be used to calculate energy band alignment of CdTe thin film surfaces and interfaces to resolve the issues found in such problematic areas and advance PV efficiency. Ab initio simulation models implement density functional theory (DFT) coupled with Green's function (GF) for investigating the electronic and structural properties of thin film surfaces and interfaces within CdTe PV device configurations. Comprehensive studies on spatially-dependent energy band alignments with respect to plane orientation, terminated surfaces, carrier concentrations and elemental composition were computationally evaluated to determine their possible effect on CdTe solar cell device performance. A total of 14 unique CdTe-based surfaces and 3 different CdTe/Te interfaces were simulated to determine their effect on energy band alignment. A number of key insights were gained that include: 1) the band bending directions dictated by the termination layer based on surface theory; 2) the role of surface reconstruction in flattening the CdTe surface energy band alignments while neutralizing surface states due to the fulfillment of the electron counting rule; 3) the formation of a cusp energy potential feature along the CdTe{111} plane oriented energy band alignments as observed by external literature studies within the CdTe/Te interface. Results to date indicate that the DFT+GF atomistic modeling approach to constructing energy band alignments matches closer to experiments than conventional band alignment methods. State-of-the-art DFT+GF calculations on CdTe-based thin film surfaces and interfaces provide a methodology for studying quantum mechanical effects in thin film PV devices such as high-efficiency single junction CdTe solar cells and tandem solar cells.
Open Access
Design of durable de-icing, superhydrophobic, superoleophobic coatings
(Colorado State University. Libraries, 2015) Beemer, Darryl Lewis, author; Kota, Arun K., advisor; Bailey, Travis, committee member; Popat, Ketul, committee member
This work looks at the issue of ice accretion on surfaces and efforts to reduce this ice accretion and the subsequent ice adhesion strength in order to make ice removal easier and more cost effective for wider implementation. Ice accretion on various surfaces is a major economic and safety issue for a variety of industries, including air travel, power production and transmission, maritime shipping, and more. While efforts have been taken to diminish ice accretion and subsequent ice adhesion strength, existing technology is limited in its ability to prevent ice accretion in a wide range of conditions and to then have a low ice adhesion strength once ice has accumulated on a surface. With the background of icing and solid mechanics of ice removal in mind, materials were developed to exhibit a low ice adhesion strength while maintaining the durability characteristic of a non-sacrificial coating. After development and testing, it was found that the developed materials exhibited an adhesion strength lower than any currently available technology, with extended durability under both ice removal and mechanical abrasion conditions. As a secondary effort, an initial exploration into the development of durable superomniphobic surfaces was performed in order to reduce and/or prevent adhesion of water-based paint and a low surface-tension fluid to various surfaces. Development of a variety of surface types (spray coated layered and mixed surfaces, etched stainless steel surfaces, and more) was performed, with initial results providing an encouraging path forward for future development of this durable coating work.
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
Development of LC-MS and degradation techniques for the analysis of fungal-derived chitin
(Colorado State University. Libraries, 2020) Allison, Christopher L., author; Reynolds, Melissa M., advisor; Farmer, Delphine, committee member; Bailey, Travis, committee member; Popat, Ketul, committee member
The research contained within this dissertation began with the following question: Can liquid chromatography-mass spectrometry (LC-MS) be used as a screening method for fungal infections? The ensuing projects investigated various aspects of that question, taking a ground-up approach that started with the analysis of the simplest constituents of the biomarkers used, chitin and chitosan. The complexity of the systems investigated was gradually increased, culminating in the extraction and detection of these biomarkers from pertinent fungal cells. Chitin constitutes 10-30% of the mass of filamentous fungi. While not found endogenously, it is the second most abundant naturally-occurring polysaccharide, next to cellulose. Chitin is composed of >50% N-acetylglucosamine (GlcNAc) and D-glucosamine (GlcN). In nature and in numerous applications, chitin can be deacetylated to produce chitosan. Chitosan is the deacetylated (>50% GlcN) counterpart to chitin and is also found in some species of fungi. This dissertation began with the development of electrospray ionization mass spectrometry (ESI-MS) methods to analyze GlcN and GlcNAc, as well as oligomers composed of both residues. The optimization of methods to analyze the components of chitin served as the foundation on which to advance the applicability of these methods. Following method optimization, the ability of mass spectrometry to analyze polymeric chitosan was explored. Detecting polymeric chitosan was determined to be infeasible using ESI-MS; hence, the focus of subsequent studies was turned to the use of degradation studies to generate low molecular weight chemical fingerprints that could be correlated to the presence of chitin and chitosan. The first experiments performed to study the degradation of chitosan evaluated the impact nitrosating conditions have on the structure of chitosan. Both mass spectrometry and spectroscopic methods were used to track the formation of a degradation product, 2,5-anhydro-D-mannose (2,5-AM), to demonstrate that nitrous acid-based conditions induce degradation in polymeric chitosan. Following these experiments, degradation studies were expanded to include a wider range of starting materials. Chitosan polymer was used again, in addition to two chitin polymers with varied degrees of deacetylation. In addition to examining the effect of nitrosating conditions on these polymers, degradation methods were expanded to include hydrochloric acid (HCl), hydrogen peroxide (H2O2), and enzymatic degradation agents (lysozyme, lipase, and hemicellulase). The susceptibility of each polymer to degradation protocols was assessed by ESI-MS or LC-MS analysis of degradation products generated. Results from these studies indicated that HCl, H2O2, HNO2, and lysozyme generate distinct products from chitin and chitosan polymers. Identification of unique chemical fingerprints produced from chitin and its derivatives provided the necessary information to apply these studies to pertinent fungal cells. The final experiments in this dissertation apply cleanup, cell lysis, degradation methods, and LC-MS to identify GlcN produced from Aspergillus niger fungi. Cumulatively, the following research contains a thorough overview of degradation methods for chitin and its derivates, along with the characterization of low molecular weight fingerprints that each protocol generates. ESI-MS or LC-MS methods were used to identify low molecular weight products formed during degradation. Finally, both degradation and LC-MS methods were applied to Aspergillus niger to validate that representative fungal species can be detected using the proposed techniques.
Open Access
Efficacy of locally delivered parathyroid hormone for treatment of critical size bone defects
(Colorado State University. Libraries, 2018) Wojda, Samantha J., author; Donahue, Seth, advisor; Yaszemski, Michael, committee member; Popat, Ketul, committee member; Reynolds, Melissa, committee member
Large segmental defects in bone (e.g., due to trauma or tumor resection) commonly have complications or fail to heal properly, resulting in delayed or non-union. Around 2.2 million orthopaedic procedures utilize autografts or allografts each year to repair large defects; however, neither is without disadvantages. Disability due to orthopaedic injury has a significant impact on both the patient and the healthcare system. Quality of life for these patients can be severely impacted as healing time may exceed 9 months and multiple treatment attempts may be required if the first is unsuccessful. Research into bone graft substitutes, like Infuse® and OP-1® (Bone Morphogenetic Protein and a collagen sponge), has become prominent. PTH is another bioactive molecule that may promote bone regeneration and provide an alternative to autograft and BMP use for treatment of large segmental defects and non-unions. Daily injections of PTH are well known to have an anabolic effect on bone and are presently FDA approved for use as an osteoporosis treatment that results in increases in both bone mineral density and bone volume. Off label PTH 1-84 treatment also resulted in the healing of a non-union fracture that was unresponsive to BMP. Current FDA approval is for daily injections of PTH (intermittent administration), as continuously elevated PTH often has a catabolic effect on bone. However, post-menopausal women with mild primary hyperparathyroidism (PTH levels are not as severely elevated) demonstrate trabecular bone preservation. Low levels of continuous PTH have also been shown to increase bone formation rate and marrow vascularity in mice. Thus, there is some evidence to suggest that low dose continuous PTH could be beneficial as an anabolic therapy in bone and may enhance bone regeneration. Continuously released, locally delivered PTH has been shown to improve healing/formation around dental implants in dogs and drill defects in sheep. However, dose response to local continuously delivered PTH is still unknown. Whether or not the benefits of PTH treatment observed in these models translate to critical size defect models is also unknown. The contribution of the research described in this dissertation increases understanding of the effects of locally delivered PTH on osteoblasts as well as its potential to enhance bone regeneration in a critical size long bone defect. This contribution is significant because presently the effects of low dose continuous PTH are not well understood. Continued development of the approaches described herein could lead to improved therapies for treatment of non-union and critical size defects in bone. Bone regeneration through locally delivered parathyroid hormone has the potential to improve functional restoration, even beyond that of allografts and without the drawbacks of current treatments, which would improve the quality of life for patients.
Embargo
Engineered mRNA therapeutic encoding beta-catenin increased bone formation in a murine tibial fracture model
(Colorado State University. Libraries, 2023) Nelson, Anna Laura, author; Ehrhart, Nicole, advisor; Bahney, Chelsea, advisor; Huard, Johnny, committee member; Popat, Ketul, committee member; Prawel, David, committee member
Fractures continue to be a global economic burden and impaired fracture healing cases, like delayed and non-union, occurring in about 14% of all tibial shaft fractures. Current treatments to aid in fracture healing involve surgical interventions and osteoanabolic, bone-morphogenetic protein-2 (BMP-2), yet is challenged supraphysiological doses and adverse side effects. Given the limited treatment options available, there remains a clinical need to develop injectable therapeutics to accelerate fracture healing in impaired fracture healing cases. Mechanistic data reveals β-catenin as a molecular driver in endochondral ossification. The central hypothesis for this dissertation is a stabilized, non-destructive β-catenin mRNA delivered locally in the fracture callus can accelerate fracture healing in a murine tibia fracture healing model. Using mRNA therapeutically continues to be challenged with stability and immunogenicity of the mRNA. To circumvent these limitations, delivery carriers have been employed to maximize gene stability, minimize off-target effects, and reduce immunogenicity. Recent advancements in liposomal technologies have led to the development of lipid nanoparticles (LNPs), leading to successful clinical translation of several novel and highly effective therapies, like SARS-CoV-2 vaccine. Alternative delivery carriers have emerged involving use of mineral coated microparticles (MCMs) as a biomimetic and biocompatible system to deliver liposomes at the site of a fracture in a controlled manner. Here, we explore mRNA delivery carriers for fracture healing applications, including manufactured cationic liposomes, MCMs, LNPs and a combination of these carriers. Manufactured liposome, Lipofectamine™, was found to be prolong transfection when tested in a murine fracture model in vivo as compared to TransIT Transfection Reagent. Using Lipofectamine™ to deliver mRNA, chemically-doped MCMs enhanced transfection and stimulated bone in vitro when delivered in chondrocytes. When testing these platforms in a murine tibia fracture model, chemically-doped MCM did not promote bone expression through testing RNA in the fracture callus for bone-related genes and through histomorphometry of the fracture callus 2 weeks post-fracture. The chemically doped MCM was found to prolong transfection of reporter gene, firefly luciferase mRNA, in vivo when compared to other treatment groups including the liposome and mRNA complex (lipoplex) alone. Ionizable-based LNPs are positively charged at a low pH and net neutral at physiological pH. Two FDA-approved ionizable phospholipids, MC3 and SM-102, were used to generate ionizable LNPs. First, MC3 LNP was tested for transfection capacity when combined with MCMs. While chemically-doped MCMs when combined with firefly luciferase mRNA encapsulated MC3 LNPs showed improved transfection in vitro, no improvements in transfection efficacy were found in vivo. Next, MC3 and SM-102 LNPs were then complexed with reporter gene, firefly luciferase mRNA to test transfection potential, immunogenicity, fracture interference and biodistribution in vitro and in a murine fracture healing model. SM-102 LNPs showed enhanced transfection efficacy in vitro, prolonged transfection in vivo, minimal fracture interference in vivo and showed no localized inflammatory response in the murine fracture callus. Ex-vivo IVIS images of main organs revealed no biodistributive effects when delivering SM-102 complexed with mRNA locally to the site of the fracture callus. Capitalizing on prior mechanistic data showing β-catenin's critical role in chondrocyte to osteoblast transdifferentiation, a non-destructive β-catenin, β-cateninGOF, mRNA transcript was generated using nucleoside modification, N1-methyl-pseudouridine, and cap analog, CleanCap. When testing the generated β-cateninGOF mRNA encapsulated in SM-102 LNPs in vitro for bioactivity, downstream canonical Wnt genes were significantly upregulated. When testing SM-102-β-cateninGOF mRNA therapeutic in murine tibia fracture model, more bone and less cartilage composition compared to PBS control was determined when analyzing histomorphometry at 25 and 45 μg concentrations at 2 weeks post-fracture. To further confirm SM-102-β-cateninGOF mRNA therapy's capabilities to promote bone in vivo, μCT was performed revealing significantly more bone volume over total volume with 45 μg dose as compared to PBS control. Taken together, we generated a novel mRNA based therapeutic encoding a non-destructive β-catenin mRNA and optimized ionizable LNP, SM-102, to maximize transfection efficacy with a localized delivery. This SM-102-β-cateninGOF mRNA therapeutic may accelerate fracture healing in a murine tibia fracture healing model.
Open Access
Evaluation of nitric oxide releasing polymers for wound healing applications
(Colorado State University. Libraries, 2015) Wold, Kathryn A., author; Reynolds, Melissa, advisor; Henry, Charles, committee member; Kipper, Matt, committee member; Popat, Ketul, committee member; Williams, John, committee member
Chronic, non-healing wounds afflict millions of Americans and represent a costly burden to the healthcare industry. In addition, the overuse and misuse of antibiotics has triggered the widespread emergence of drug-resistant bacteria, making the treatment of infected wounds more challenging. As a result, improved methods for wound care incorporating antibiotic-alternative bactericidal agents are in high demand. Recent wound care advances have focused on the development of dressings incorporating physical structures and biological components which mimic those encountered in a natural wound environment. Nitric oxide (NO), an endogenously produced molecule upregulated to promote cellular function and bactericidal activity during wound healing, has been harnessed in material systems and studied for wound healing potential. This work describes the characterization, bactericidal activity, cell functionality and processing of two NO-releasing polymer systems, one water-soluble and another water-insoluble. The results of this work demonstrate the capability of these polymeric NO-releasing materials to promote high log reductions of planktonic bacteria. Additionally, polymer dosages that promote cell survival and induce cytotoxicity in eukaryotic cells have been determined and nano-scale polymer fibers that maintain NO release properties have been processed. These results represent qualities beneficial towards the development of enhanced materials for the treatment of chronic infected wounds.