Browsing by Author "Appa, Harish"
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- ItemOpen AccessDesign and development of an electrospun polymeric transcatheter heart valve for tissue engineering(2022) Guess, Rosslee Christie; Bezuidenhout, Deon; Appa, HarishThe growing global burden of valvular heart disease (VHD) remains a significant challenge to overcome. Despite the advancements in heart valve replacements and the ground-breaking introduction of transcatheter valve technologies, current solutions offer suboptimal therapy as they either require lifelong anticoagulation or have limited durability. The ideal replacement valve should mimic the functionality of its native counterpart, comprise of viable living tissue, and offer lifelong durability, also for young patients. The central aims of this project were to design, manufacture, and evaluate two types of electrospun trileaflet transcatheter heart valve (THV) substitutes, one with non-degradable Pellethane® leaflets (PEL valves) and the other type using degradable DegraPol15® leaflets (DP valves). PEL and DP scaffolds were successfully electrospun after implementing a humidity control system and optimising input (ambient and processing) parameters; and were characterised for their morphological (fiber geometry and architecture), mechanical (static and dynamic), and hydraulic (wettability and permeability) properties. Six distinct valve manufacturing concepts (leaflet suturing, leaflet bonding, sandwich spinning, trileaflet preforming, inverted trileaflet preforming and submould assembly) were designed and prototyped for feasibility screening and selection. The selected concept was further employed to produce PEL and DP valves that were subsequently assessed for hydrodynamic function and durability by means of pulsatile flow analyses and accelerated fatigue testing. The fibres of DP scaffolds were approximately three times thicker than those in PEL scaffolds (5.2 ± 1.3 vs 1.8 ± 0.5 µm) with correspondingly larger pores (16.3 ± 4 vs 8.0 ± 1.7 µm). All scaffolds were essentially randomly oriented, and porosities averaged at 75 ± 5%. PEL scaffolds were approximately three times stronger than DP counterparts with slightly lower extensibility (550 vs 730%). From the six valve manufacturing concepts, the method involving the stitching of pre-cut leaflets onto the expandable stent (CoCr, 23 mm) after heat bonding an electrospun PEL skirt was selected. Four groups of valves, differing in average leaflet thickness (n = 2–4 per group), were produced for each material type. Under simulated aortic conditions, the PEL valves (N = 10) achieved remarkably high accelerated fatigue life of up to 260 million cycles (equivalent to 2.5 years), with a positive correlation between leaflet thickness and durability. The hydrodynamic parameters obtained for the PEL valves were well within ISO standards (EOA > 1.25 cm2 , mean pressure gradient < 10mmHg, and RF < 20% at a cardiac output of 5 l/min). In the current embodiments, DP valves (N=14) were not able to withstand aortic pressures but demonstrated good hydrodynamic function at nominal pulmonary pressures. Although much further study is required, the concepts described in this project contribute valuable insight into the manufacturing, performance, and durability of devices intended for transcatheter heart valve tissue engineering.
- ItemOpen AccessMultiphase CFD modelling of stirred tanks(2007) Appa, Harish; Deglon, David; Meyer, CJStirred tanks agitated with Rushton turbines are commonly used in industry, for instance mixing processes and flotation systems. The need for more efficient systems in industries has led to the study of fluid flow within the tanks upon agitation; so that a better understanding of the phenomena can help in the optimisation of the tanks. In the recent years, efforts have been made towards the development of predictive methods using computational fluid dynamics (CFD). Among the various numerical works presented, emphasis was laid mainly on single phase systems. However, due to the various processes involving gas-liquid systems, the need for multiphase modelling of stirred tanks became increasingly important. This has led to more research studies involving multiphase flows. Most of the work reported showed good prediction of the velocity data and the power draw, reasonable turbulence parameters. But, the prediction of the gas hold-up was rarely well established. Therefore, the aim of this thesis, based on the numerical work presented by Engelbrecht (2006), is to investigate the discrepancies reported and to develop a multiphase model of a stirred tank agitated by a Rushton turbine. The commercially available CFD code FLUENT@ was used to model the agitated gas-liquid system. The results were validated with the numerical work of Engelbrecht (2006) and the experimental work presented by Deglon (1998). Two main cases were investigated, with a steady state and a transient approach. The QUICK scheme was used for the discretisation of the volume fraction and momentum and the first order upwind scheme for the discretisation of the turbulent kinetic energy and dissipation rate. The standard k - E turbulence model was used to account for the turbulent flow regime. A steady state MRF model was used for the investigation of the discrepancy reported by Engelbrecht (2006). The author reported that no convergence was achieved with such models. Solving the problem would have resulted in a good modelling approach for the prediction of gas dispersion, since steady state models are not computationally intensive. Three different boundary conditions, namely, a pressure outlet, an outflow and a velocity inlet, were used to model the outlet of the tank. The Euler-Euler multiphase model was used to simulate the gas-liquid system for the steady state model.
- ItemOpen AccessNumerical modelling of hydrodynamics, gas dispersion and mass transfer in an autoclave(2012) Appa, Harish; Deglon, David; Meyer, CJIncludes abstract. Includes bibliographical references.
- ItemOpen AccessStents for transcatheter aortic valve replacement(2017) Park, Kenneth Stuart; Bezuidenhout, Deon; Appa, Harish; Scherman, JacquesRheumatic heart disease (RHD) is the leading cause of aortic valve disease in the world. Surgery to repair or replace the diseased valves is the only means to save a patient's life once the disease becomes symptomatic. Transcatheter aortic valve replacement (TAVR) has revolutionised the treatment of age-related degenerative aortic valve disease, but is currently not suitable for the majority of RHD sufferers due to the rapid degeneration of flexible leaflet valves in younger patients, contraindications of commercial devices to regurgitant or non-calcific aortic valve disease, and also due to resource or funding limitations. The current research project aimed to develop and test novel compressible balloon-expandable stents suitable for patients with symptomatic rheumatic aortic valve disease, and which would allow for a percutaneous polymeric valve to be manufactured, be crimped onto balloon-based devices, and be expanded into a compliant or non-calcific native aortic valve. Several stent concepts were developed and evaluated using Finite Element Analysis (FEA) and two favoured concepts were selected for more complex FEA, in which the balloon was simulated using an Ogden material model, and rigorous testing. The stent material, a nickel-cobalt-chromium alloy, was modelled as an isotropic elasto-plastic material with isotropic hardening. The novel stent designs incorporated a native leaflet-mimicking crown shape for continuous leaflet attachment and mechanisms to anchor the stented valve within compliant aortic roots. The first of the favoured designs provided tactile location during delivery and anchored using self-expanding arms on a balloon-expandable frame of the same material ("self-locating stents"). The second design anchored using arms that protruded during deployment as a consequence of plastic deformation incurred during crimping ("expanding arm stents"). Prototypes were successfully manufactured through laser cutting and electropolishing and showed good surface quality. In vitro testing included determination of crimping and expansion behaviour and measurement of mechanical properties such as resistance to migration in the anatomy. Valve performance was evaluated through in vitro haemodynamics in a pulse duplicator and durability was tested in a high-cycle fatigue tester. Simulated use testing was performed using cadaveric animal hearts. Finally, valves were also implanted into the aortic valve position of pigs (in acute termination experiments) through a transapical approach in order to verify valve deployment behaviour and function in vivo, and determine the stent's ability to anchor in the native anatomy. Stents could be crimped to diameters below 6mm and deployed using commercial balloons and proprietary non-occlusive deployment devices. FEA simulations of stent crimping and deployment matched experimental behaviour well and provide a tool to optimise stent performance. Peak Von Mises stresses during deployment (1437 MPa and 1633 MPa for self-locating and expanding arm stents, respectively) were comparable to a "zig-zag" stent simulated for control purposes (1650 MPa). Radial strength, evaluated for expanding arm stents, was lower than the Control stent (116 N vs. 347 N). This design, although predicted to be safe under fatigue loading, had a lower fatigue safety factor than the Control stent. Stents resisted migration to forces of at least 22 N, which is four times greater than physiological loading on the valves. Polymeric valves incorporating the stents were constructed and demonstrated good in vitro haemodynamic performance (Effective Orifice Areas ≥2.0cm², ΔP<9 mmHg, regurgitation <6%) and durability of over 400 million cycles. Designs functioned as intended in simulated use tests. Valves constructed using self-locating stents could be successfully deployed without rapid pacing in eight of nine pigs, and valve position was correct in seven of these. Valves of expanding arm stents remained anchored in six of eight attempted implants in pigs. This study has demonstrated proof of concept for a novel balloon-expandable stent for a polymeric transcatheter heart valve that is capable of anchoring in a compliant native aortic valve.