Browsing by Author "Sampath, Walajabad S., committee member"

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Open Access
316L stainlesss steel modified via plasma electrolytic oxidation for orthopedic implants
(Colorado State University. Libraries, 2022) Michael, James A., II, author; Popat, Ketul C., advisor; Li, Vivan, committee member; Sampath, Walajabad S., committee member
316L stainless steel (SS) is widely used biomaterial for implantable devices and is estimated to the base material for 60% of implantable devices. However, one challenge of the material is the inhomogeneity of the surface morphology which may influence the adhesion process of host cells and bacteria. One method to create a uniform surface of 316L SS is plasma electrolytic oxidation (PEO). PEO creates an oxide layer on the outer surface thus changing the surface topography on the microscale. PEO process on SS functions by anodizing the surface via direct current in electrolyte solution. Preliminary research found that a continuous direct current over a time manufactured undesirable samples, to overcome this challenge the use of pulse timings was utilized during fabrication. This research aimed to answer the questions how do PEO modifications effect cellular adhesion and viability, and how do PEO modifications affect bacteria adhesion and viability. PEO modified 316L SS surfaces were characterized and its effects on the adhesion, morphology, and differentiation of adipocyte derived stem cells, along with the adhesion and morphology of Staphylococcus aureus was investigated.
Open Access
A tabu search evolutionary algorithm for multiobjective optimization: application to a bi-criterion aircraft structural reliability problem
(Colorado State University. Libraries, 2015) Long, Kim Chenming, author; Duff, William S., advisor; Labadie, John W., advisor; Stansloski, Mitchell, committee member; Chong, Edwin K. P., committee member; Sampath, Walajabad S., committee member
Real-world engineering optimization problems often require the consideration of multiple conflicting and noncommensurate objectives, subject to nonconvex constraint regions in a high-dimensional decision space. Further challenges occur for combinatorial multiobjective problems in which the decision variables are not continuous. Traditional multiobjective optimization methods of operations research, such as weighting and epsilon constraint methods, are ill-suited to solving these complex, multiobjective problems. This has given rise to the application of a wide range of metaheuristic optimization algorithms, such as evolutionary, particle swarm, simulated annealing, and ant colony methods, to multiobjective optimization. Several multiobjective evolutionary algorithms have been developed, including the strength Pareto evolutionary algorithm (SPEA) and the non-dominated sorting genetic algorithm (NSGA), for determining the Pareto-optimal set of non-dominated solutions. Although numerous researchers have developed a wide range of multiobjective optimization algorithms, there is a continuing need to construct computationally efficient algorithms with an improved ability to converge to globally non-dominated solutions along the Pareto-optimal front for complex, large-scale, multiobjective engineering optimization problems. This is particularly important when the multiple objective functions and constraints of the real-world system cannot be expressed in explicit mathematical representations. This research presents a novel metaheuristic evolutionary algorithm for complex multiobjective optimization problems, which combines the metaheuristic tabu search algorithm with the evolutionary algorithm (TSEA), as embodied in genetic algorithms. TSEA is successfully applied to bicriteria (i.e., structural reliability and retrofit cost) optimization of the aircraft tail structure fatigue life, which increases its reliability by prolonging fatigue life. A comparison for this application of the proposed algorithm, TSEA, with several state-of-the-art multiobjective optimization algorithms reveals that TSEA outperforms these algorithms by providing retrofit solutions with greater reliability for the same costs (i.e., closer to the Pareto-optimal front) after the algorithms are executed for the same number of generations. This research also demonstrates that TSEA competes with and, in some situations, outperforms state-of-the-art multiobjective optimization algorithms such as NSGA II and SPEA 2 when applied to classic bicriteria test problems in the technical literature and other complex, sizable real-world applications. The successful implementation of TSEA contributes to the safety of aeronautical structures by providing a systematic way to guide aircraft structural retrofitting efforts, as well as a potentially useful algorithm for a wide range of multiobjective optimization problems in engineering and other fields.
Open Access
Characterization of a plasma reactor device for photovoltaic applications
(Colorado State University. Libraries, 2012) Metz, Garrett Eugene, author; Williams, John D., advisor; Mahoney, Leonard, committee member; Sampath, Walajabad S., committee member; Robinson, Steve, committee member
Heated pocket deposition (HPD) sources are used for the rapid manufacture of thin film CdS/CdTe photovoltaic devices. Standard lab devices produced at CSU by the HPD process have achieved efficiencies of 13%. New process methods are required to further improve the quality of the films, increase cell efficiency, and reduce production costs. A plasma-enhanced, close-spacing sublimation (PECSS) technique has recently been developed as a candidate process method. It has been successfully used to eliminate pin holes, to dope CdS with oxygen, and dope CdTe absorption layers; all of which have resulted in higher device efficiencies. In this work we present measurements describing the properties of the PECSS plasma. Specifically the uniformity of the ion current flux to the substrate is presented for nitrogen/oxygen and argon feed gases by means of in situ surface probes fabricated by segmenting a transparent conductive oxide film that is laid over the glass. Plasma properties within the PECSS processing chamber are also presented including plasma density, electron temperature, and plasma potential. Operational characteristics and scaling of PECSS are presented for pressures of 100-300 mTorr and surface areas of 160 - 1700 cm2. A three-dimensional model was developed to calculate plasma production and transport processes, and to gain a greater understanding of the role of energetic primaries versus bulk cold electrons on spatial ionization rates that develop within the PECSS plasma as a function of gas pressure and geometry. Comparisons between the model and experimental measurements are presented and good agreement has been observed when the appropriate spatially varying ionization rates are estimated. This work also presents the development of a diagnostic test bed that will be useful for future work in the development and understanding of the PECSS technique.
Open Access
Effect of destabilized reactions using lithium amide (LiNH2) and doping using titanium based catalyst on the desorption characteristics of lithium aluminium hydride (LiAlH4)
(Colorado State University. Libraries, 2012) Paravasthu, Siddharth, author; James, Susan P., advisor; Sampath, Walajabad S., committee member; Wu, Mingzhong, committee member
In the past few decades there has been a tremendous increase in hydrogen storage research. Numerous materials and material systems have been studied as potential candidates for hydrogen storage, but unfortunately none of those materials demonstrate enough hydrogen releasing capacity under suitable temperature range to be used for hydrogen storage. Research promises to unlock the potential of these materials and ultimately lead to the commercialization of this technology. LiAlH4 is one of those materials that have been exclusively studied as a candidate for hydrogen storage due to its high theoretical hydrogen storage capacity, and its ability to release hydrogen in more than one step at different temperature ranges. Jun Lu and Zhigang Zak Fang studied the effects of titanium based catalyst (TiCl3.1/3AlCl3) and destabilization reactions using LiNH2 on LiAlH4, but did not demonstrate the effects of ball milling on the system. In the present work we have investigated the effects of ball milling, and the effects of destabilization reaction using LiNH2 on the hydrogen release characteristics of LiAlH4 doped with TiCl3. The current market scenario for fuel cell technology and the possibility and consequences of introducing the current system in the market has been briefly discussed. X-ray powder diffraction, thermo-gravimetric analysis and scanning electron microscopy were employed for the characterization of the samples. Both the compounds LiNH2, and TiCl3 worked in effecting the dehydrogenation kinetics of LiAlH4. The duration of ball milling required to affect the dehydrogenation kinetics of LiAlH4 using TiCl3 was optimized. A hydrogen release of 7.3 wt% was observed from the final system i.e. (LiAlH4/LiNH2 doped with 2% TiCl3) at temperatures below 4000C.
Open Access
Effect of interpass temperature on the structure and properties of multipass weldments in high performance nickel alloys
(Colorado State University. Libraries, 2011) Petro, John S., author; Smith, Frederick W., advisor; Sampath, Walajabad S., committee member; James, Susan P., committee member; DuChateau, Paul C., committee member; Klarstrom, Dwaine, committee member
Nickel alloys comprise an important group of engineering materials which are used primarily for their exceptional resistance to corrosion and their ability to maintain good mechanical strength over a wide temperature range, (both low and high) in demanding industrial applications. Welding is a primary fabrication process for these alloys. It has been a generally accepted practice to maintain a maximum interpass temperature of 200°F or lower when multipass welding many nickel alloys to prevent defects such as cracking or loss of corrosion resistance. This practice has been based on recommendations by many of the nickel alloy producers. A low maximum interpass temperature can increase the welding time which increases fabrication costs. According to the author's industry contacts and based upon the author's industrial experience as well as the author's examination of the literature, there has been little or no systematic research on the effect of interpass temperature for multipass welding of nickel alloys. In fact, the same is true for the establishment of the basic robotic welding parameters using the new generation of digital power supplies for these alloys. This dissertation presents research on the effect of interpass temperature on two nickel alloys; HASTELLOY® C-2000® and HASTELLOY® B-3®. Welding parameters were also developed for these alloys and also for HAYNES® 230® alloy using Gas Metal Arc Welding, GMAW, as a single process for both the root and fill weld passes. Weldments were made at 5 different interpass temperatures, 100°F - 500°F, in 100°F increments, for these alloys in thicknesses of 0.25 inch and 0.5 inch. Transverse weld specimens were then tested according to AWS B4.0:2007 using tensile, bend, and hardness tests. Transverse weld specimens were corrosion tested according to ASTM G28A for the HASTELLOY C-2000 alloy and the HASTELLOY B-3 alloy was subjected to 20% HCl at 149°C for 96 hours in an autoclave. The specimens were also examined using optical light microscopy for intergranular corrosion attack, weld fusion, cracking, and heat affected zone (HAZ) microstructure effects. (HASTELLOY, HAYNES, C-2000, B-3, and 230 are registered trademarks of Haynes International, Inc.) No significant loss of tensile strength was found at any of the higher interpass temperatures. All ultimate tensile strengths for both alloys were above the ASME Boiler and Pressure Vessel Code Section IX minimum. All samples passed 2T transverse face bend tests. Some lack of fusion was observed at the root of some samples at random interpass temperatures. No noticeable change in the HAZ microstructure or cracking was observed at the highest interpass temperature for both the HASTELLOY C-2000 and the HASTELLOY B-3 alloys. No significant corrosion attack was found along the weld, face or root sides, for both alloys at the higher interpass temperature of 500°F. It was concluded that a higher interpass temperature could be specified for these alloys without any appreciable loss of strength, weld soundness, loss of corrosion resistance, or detrimental effect to microstructure. It was also shown that the GMAW process could be used as a sole welding process but more development is needed to decrease process variability in the root pass and to develop a complete welding procedure specification.
Open Access
Improvement in dye sensitized solar cell efficiency through functionalization of redox mediators and passivation of the photoanode using a home-built atomic layer deposition system
(Colorado State University. Libraries, 2017) Thomas, Joshua D., author; Prieto, Amy L., advisor; Fisher, Ellen R., committee member; Menoni, Carmen S., committee member; Sampath, Walajabad S., committee member
The efficiency of dye sensitized solar cells (DSSCs) is driven based on the contributions of a vast array of kinetic and thermodynamic processes which must all function in sync with one another. The redox mediator factors into a majority of these processes and thus its proper function is vital to adequate function of the DSSC as a whole. The function of the redox mediator can be altered in two ways: changing the identity of the redox couple used and modifying one of the components which the redox couple is interacting with. Herein, both methods have been performed to optimize the properties and processes involved in efficient DSSC function. Several cobalt bipyridine coordination complex type mediators have been synthesized and differentiated through the modification of the ligand structure. The purpose of the modification was to generate complexes with more positive redox potentials to increase the open circuit voltage of the cells. Subsequently, attempts were made to further modify the ethyl ester substituted ligands which yielded the highest redox potential in order to provide higher stability for the resulting mediator. While the outcome of the synthesis was unsuccessful at this point, promising results have been shown. Further, an apparatus was constructed in order to cheaply perform atomic layer deposition of aluminum oxide on the surface of the mesoporous titanium dioxide photoanodes for DSSCs. Atomic layer deposition has been shown to reduce the rate of recombination with the oxidized mediator. In this case, improvement in the open circuit voltage of the cell was shown. However, the overall improved performance of the DSSCs shown in the literature could not be replicated. It is hoped that more high resolution analytical techniques could be used to elucidate the deficiencies still present in the use of this technique.
Open Access
Interaction of adipose-derived stem cells with titania nanotube surfaces
(Colorado State University. Libraries, 2018) Cowden, Kari Miller, author; Popat, Ketul C., advisor; Park, Juyeon, committee member; Sampath, Walajabad S., committee member
The need for joint replacement will continue to grow and increase significantly in the coming decades due to the aging population. Unfortunately, many joint implants experience failure after 10-15 years requiring revision surgery. With the growing need for more implants and the high cost of medical expenses for orthopedic surgery, it is imperative that implants are effective and have long term success. Since joint implant materials come into direct contact with bone it is vital that they mimic the structure of bone to improve osseointegration, or the direct structural and functional connection between living bone and the implant surface. Improving the osseointegration of the implant can increase the stability of the implant, thus, reducing micro motions that cause loosening and lead to implant failure. Current joint implants have microscale coatings and texturing, however, bone is composed of both micro and nano components. In order to mimic the nanostructure of bone, different nanotopograhies are currently being studied. These nanostructures have been shown to improve cellular response in terms of adhesion, proliferation, and osteogenic differentiation. However, the optimal size of nanosurfaces to promote cell adhesion, proliferation, and differentiation is still disputed. Titania nanotubes (NT) have been shown to improve cellular response in vitro and improve integration in in vivo animal studies. This thesis investigates the surface characteristics of titania NT and the effect of nanotube size on adhesion, proliferation, and differentiation of adipose-derived stem cells (ADSC) in vitro. The results presented in this thesis indicate that ADSC differentiated and performed better on NT surfaces than Ti surfaces. Additionally, the size of titania NT altered the proliferation and osteogenic differentiation of ADSC. Further studies should be directed toward in vivo animal studies to confirm that implants with NT surfaces enhance osseointegration and further define their potential to improve implant stability.
Open Access
Luminescence measurements inform a strategy for unlocking the full potential of CdTe-based photovoltaics
(Colorado State University. Libraries, 2023) Jundt, Pascal M., author; Sites, James R., advisor; Sampath, Walajabad S., committee member; Yost, Dylan C., committee member; Gelfand, Martin P., committee member; Kuciauskas, Darius, committee member
Cadmium telluride (CdTe) photovoltaics are characterized by simplicity and speed of fabrication with low usage of materials, all of which translate into low cost. These significant advantages have earned CdTe the second-highest adoption rate of all photovoltaic technologies. However, conversion efficiencies, while functional, are significantly lower than the theoretical limit for this material. This discrepancy is almost entirely a discrepancy in voltage, and the so-called "voltage deficit" of CdTe has stubbornly persisted for decades. While many strategies are being pursued to attempt to reduce the voltage deficit, this issue is fundamentally one of excessive nonradiative recombination due to defects within the absorber material, as will be demonstrated in this dissertation. Recombination is evaluated primarily by luminescence measurements, and as such this class of measurements is particularly relevant to the challenges faced by CdTe research today. The rate of recombination is parameterized by the carrier lifetime, and time-resolved photoluminescence (TRPL) is the most common method of determining this parameter in CdTe. Historically, accurate determination of bulk lifetime was as simple as extracting the time constant of the slowest component of a TRPL decay. However, significant gains in material passivation and doping over the last few years have both decreased the relative influence of trap-assisted recombination and increased the influence of p-n junction fields on TRPL measurements. Consequently, when measurements are performed on complete cells, extracting the tail time constant from a TRPL decay no longer necessarily gives an accurate representation of the bulk material lifetime, and the result is distorted by field effect contributions. This fact is not necessarily well-known by the CdTe community, and extraction of the tail time constant is still the most common way to report lifetimes, even in measurements on complete state-of-the-art cells. This dissertation demonstrates the skewing effects of junction fields, and identifies under which conditions they manifest and how. To probe field effects, external electrical bias was incorporated during TRPL measurements, which allows fairly precise manipulation of fields. Biased TRPL measurements were performed on a variety of samples, and a model was developed to substantiate and better explain the results. It was found that the same characteristics which enable good performance (high lifetime, doping, and mobility) are the same which add complexity to TRPL interpretation. It was also found that field effects can be effectively suppressed by significant forward bias, leading to far more accurate determination of bulk lifetime. TRPL and external radiative efficiency (ERE) luminescence measurement results have indicated very low rates of nonradiative recombination and associated very high lifetime for some CdTe-based materials deposited at Colorado State University, particularly the cadmium selenium telluride (CdSeTe) alloy. While these attributes should allow voltages approaching 1 V and efficiencies on the order of 25%, when incorporated into "traditional" cell architectures these materials typically achieve middling performance at best, and often no performance at all. To unlock the great potential indicated by luminescence measurements, a different cell architecture is proposed which aims to accommodate these materials and take advantage of their characteristics. In an n-i-p configuration, an intrinsic absorber material is sandwiched between two carrier-selective contacts, at least one of which must be transparent. This design eliminates the requirement that the absorber be doped, which penalizes lifetime. Based on the findings of modeling reported here, undoped CdSeTe appears to be an ideal intrinsic layer material. The currently-utilized SnO2:F/MgZnO front contact appears to be excellent as the n-type electron-selective layer. The one missing component is the p-type hole-selective layer; modeling in this dissertation describes in detail what attributes are required of this material. Most important is band alignment with CdSeTe, which should produce a valence band offset as close to zero as possible, and a conduction band offset which forms a sufficiently high electron barrier. Sufficient p-type doping is also quite important. Based on these criteria, ZnTe was identified as a suitable candidate material, and several cells were fabricated with this architecture. While preliminary cells achieved relatively poor performance compared with traditional designs, J-V curves were surprisingly well-behaved, and the almost immediate achievement of functioning cells using an entirely new approach is promising. Luminescence characterization of these structures identified several areas for improvement, namely the use of a p-type dopant other than copper and the replacement of ZnTe with another material with similar band structure but more compatible lattice constant.
Open Access
Metaheuristic approach to solving U-shaped assembly line balancing problems using a rule-base coded genetic algorithm
(Colorado State University. Libraries, 2015) Martinez-Contreras, Ulises, author; Duff, William S., advisor; Troxell, Wade O., committee member; Labaide, John W., committee member; Sampath, Walajabad S., committee member
The need to achieve line balancing for a U-shaped production line to minimize production time and cost is a problem frequently encountered in industry. This research presents an efficient and quick algorithm to solve the U-shape line-balancing problem. Heuristic rules used to solve a straight line-balancing problem (LBP) were modified and adapted so they could be applied in a U-shape line-balancing problem model. By themselves, the heuristic rules, which were adapted from straight-line systems, can produce good solutions for the U-shape LBP, however, there is nothing that guarantees that this will be the case. One way to achieve improved solutions using heuristic rules can be accomplished by using a number of rules simultaneously to break ties during the task assignment process. In addition to the use of heuristic and simultaneous heuristic rules, basic genetic operations were used to further improve the performance of the assignment process and thus obtain better solutions. Two genetic algorithms are introduced in this research: a direct-coded and an indirect-coded model. The newly introduced algorithms were compared with well-known problems from literature and their performance as compared to other heuristic approaches showed that they perform well. The indirect-coded genetic algorithm uses the adapted heuristic rules from the LBP as genes to find the solutions to the problem. In the direct-coded algorithm, each gene represents an operation in the LBP and the position of the gene in the chromosome represents the order in which an operation, or task, will be assigned to a workstation. The indirect-coded genetic algorithm introduces sixteen heuristic rules adapted from the straight LBP for use in a U-shape LBP. Each heuristic rule was represented inside the chromosome as a gene. The rules were implemented in a way that precedence is preserved and at the same time, facilitate the use of genetic operations. Comparing the algorithm’s results with known results from literature, it obtained better solutions in 26% of the cases; it obtained an equivalent solution in 62% of the cases (not better, not worse); and a worse solution the remaining 12%. The direct-coded genetic algorithm introduces a new way to construct an ordered arrangement of the task assignation without violating any precedence. This method consists of creating a diagram that is isomorphic to the original precedence diagram to facilitate the construction of the chromosome. Also, crossover and mutation operations are conducted in a way that precedence relations are not violated. The direct-coded genetic algorithm was tested with the same set of problems as the indirect-coded algorithm. It obtained better solutions than the known solutions from literature in 22% of the cases; 72% of the problems had an equivalent solution; and 6% of the time it generated a solution less successful than the solution from literature. Something that had not been used in other genetic algorithm studies is a response surface methodology to optimize the levels for the parameters that are involved in the response model. The response surface methodology is used to find the best values for the parameters (% of children, % of mutations, number of genes, number of chromosomes) to produce good solutions for problems of different sizes (large, medium, small). This allows for the best solution to be obtained in a minimum amount of time, thus saving computational effort. Even though both algorithms produce good solutions, the direct-coded genetic algorithm option requires less computational effort. Knowing the capabilities of genetic algorithms, they were then tested in two real industry problems to improve assembly-line functions. This resulted in increased efficiency in both production lines.
Open Access
Nanofiber reinforcement of a geopolymer matrix for improved composite materials mechanical performance
(Colorado State University. Libraries, 2015) Rahman, AKM Samsur, author; Radford, Donald W., advisor; Sampath, Walajabad S., committee member; Holland, Troy B., committee member; Heyliger, Paul, committee member
Geopolymers have the potential to cross the process performance gap between polymer matrix and ceramic matrix composites (CMC), enabling high temperature capable composites that are manufactured at relatively low temperatures. Unfortunately, the inherently low toughness of these geopolymers limits the performance of the resulting fiber reinforced geopolymer matrix composites. Toughness improvements in composites can be addressed through the adjustments in the fiber/matrix interfacial strength and through the improvements in the inherent toughness of the constituent materials. This study investigates the potential to improve the inherent toughness of the geopolymer matrix material through the addition of nanofillers, by considering physical dimensions, mechanical properties, reinforcing capability and interfacial bond strength effects. A process optimization study was first undertaken to develop the ability to produce consistent, neat geopolymer samples, a critical precursor to producing nano-filled geopolymer for toughness evaluation. After that, single edge notched bend beam fracture toughness and un-notched beam flexural strength were evaluated for silicon carbide, alumina and carbon nanofillers reinforced geopolymer samples treated at various temperatures in reactive and inert environments. Toughness results of silicon carbide and carbon nanofillers reinforced geopolymers suggested that with the improved baseline properties, high aspect ratio nanofillers with high interfacial bond strength are the most capable in further improving the toughness of geopolymers. Among the high aspect ratio nanofillers i.e. nanofibers, 2vol% silicon carbide whicker (SCW) showed the highest improvement in fracture toughness and flexural strength of ~164% & ~185%, respectively. After heat treatment at 650 °C, SCW reinforcement was found to be effective, with little reduction in the performance, while the performance of alumina nanofiber (ANF) reinforced geopolymer significantly reduced. By means of SEM, EDS and X-ray diffraction techniques, it was found that the longer and stronger SCW is more capable of reinforcing the microstructurally inhomogeneous geopolymer than the smaller diameter, shorter ANF. After heat treatment at 760 ºC, the effectiveness of SCW as reinforcement in both fracture toughness and flexural strength was reduced by ~89% and ~43%, respectively, while, the ANF filled materials performed worse than the neat geopolymer. A strong interaction was suggested between ANF and geopolymer at high temperature by means of chemical reactions and diffusion. SEM & X-ray diffraction results suggested the formation of Al₄C₃ on the SCW surface, which could reduce the interface strength between SCW and geopolymer. Therefore it is suggested that the interface strength should be as high as required for load transfer and crack bridging. Finally, to investigate the potential synergy of a nano-filled matrix material and the fiber/matrix interface toughening mechanism of a continuous fiber composite, composite specimens were produced and tested. Flexural and shear strengths of Nextel 610 continuous fiber reinforced 2vol% SCW filled geopolymer matrix composites were investigated. Specimens were produced with cleaned Nextel fiber and with carbon-coated fibers to investigate the combinations of nano-filled matrix with continuous reinforcement that is well bonded (cleaned fiber) versus poorly bonded (carbon-coated fiber) to the matrix. The results showed that flexural strength of cleaned and coated fiber composites improved by ~35% and ~21% respectively, while shear strength of the similar composite systems improved by ~39.5% and ~24%. The results verified the effectiveness of SCW in toughening not only the neat geopolymer, but also continuous fiber reinforced geopolymer matrix composites.
Open Access
Superhydrophobic titania nanoflowers for reducing adhesion of platelets and bacteria
(Colorado State University. Libraries, 2020) Montgomerie, Zachary Z., author; Popat, Ketul C., advisor; Li, Vivian, committee member; Sampath, Walajabad S., committee member
Thrombosis formation and bacterial infection are key challenges for blood-contacting medical devices. When blood components encounter a device's surface, proteins are adsorbed, followed by the adhesion and activation of platelets as well as an immune response. This culminates in clot formation via the trapping of red blood cells in a fibrin matrix, which can block the device's function and cause severe complications for the patient. Bacteria may also adhere to a device's surface. This can lead to the formation of a biofilm, a protective layer for bacteria that significantly increases resistance to antibiotics. Despite years of research, no long-term solutions have been discovered to combat these issues. To impede thrombosis, patients often take antiplatelet drugs for the life of their device, which can cause excess bleeding and other complications. Patients can take antibiotics to fight bacterial infection, but these are often ineffective if biofilms are formed. Superhydrophobic surfaces have recently been studied for their antiadhesive properties and show promise in reducing both thrombosis and bacterial infection. In this work, superhydrophobic titania nanoflower surfaces were successfully fabricated on a titanium alloy Ti-6Al-4V substrate and examined for both hemocompatibility and bacterial adhesion. The results indicated a reduction of protein adsorption, platelet and leukocyte adhesion and activation, whole blood clotting, bacterial adhesion, and biofilm formation, as well as surface stability compared to control surfaces.
Open Access
Titania nanotubes as potential interfaces for vascular applications
(Colorado State University. Libraries, 2015) Kelley, Sean Edward, author; Popat, Ketul C., advisor; Reynolds, Melissa, committee member; Sampath, Walajabad S., committee member
The primary fatality of the public worldwide is cardiovascular disease. Surgery is usually the modern answer to these complications including transplanting organs and artificial implants with the latter typically being most successful. Generating long term synergy between the transplants and the surrounding tissue continues to be a problematic causing the necessity for comprehending the complex interactions that occur between the two sides at the cellular level. New implants comprising of either purely cellular platforms or a mixture of synthetic and cellular frameworks have demonstrated tremendous potential for tissue restoration. Preferably, the surface of an implant should be suitable for cells to adhere, proliferate, and in many cases differentiate while performing their required functions as if they were in their own natural environment. The surface of these implants must also have a minimum but ideally no immune response. Titanium and its alloys are extensively employed in biomedical devices, due to their beneficial mechanical and relatively high biocompatible properties. Smooth muscle cells are one of the two major cells varieties that are in contact with vascular stents; consequently the interaction between the cells and the nanotube titania (TiO₂) surface is of the utmost importance. The objective of this research is to examine the cellular response of smooth muscle cells to titania nanotubes as a prospective surface modification to complement titanium vascular stents.