Design, manufacture and hydrodynamic evaluation of a prototype trileaflet mechanical heart valve for aortic valve replacement

Wium, Elsmari (2020-03)

Thesis (MEng)--Stellenbosch University, 2020.

Thesis

ENGLISH ABSTRACT: A rigid trileaflet heart valve for aortic valve replacement was designed and prototypes were developed for testing purposes. Due to the prevalence of rheumatic heart disease in developing countries, a need for a mechanical heart valve replacement that does not elicit a negative biological response exists. Tests were conducted to evaluate the hydrodynamic performance of the valve and to compare the results to that of a commercial bileaflet valve. The aim was to determine whether the prototype valve displays an enhanced performance. Design concepts were generated following a systematic approach. Concepts were evaluated analytically, and feasible concepts were developed using additive manufacturing techniques, well-knowing the limitations of this technology. Initial prototypes were subjected to a preliminary numerical and experimental assessment to determine whether the functional, material and manufacturing requirements are satisfied. Based on the outcome of the requirements evaluation, four trileaflet valve designs were selected for further investigation. Further investigation was required to determine whether the valves satisfy the performance requirements governed by regulatory bodies. Investigations included experimental and numerical evaluations of the trileaflet valves, a benchmark bileaflet valve and a commercial valve. The benchmark bileaflet valve, based on an existing commercial valve design, was manufactured using the same technique as the trileaflet valves, facilitating direct performance comparisons. Flow tests, using a pulse duplicator, and motion analysis tests, using a high-speed camera, were conducted. Flow test measurements were analysed to calculate standard heart valve performance parameters. Three of the four trileaflet valves exhibited a performance comparable to or exceeding that of the bileaflet valve. The additive manufactured valves behaved similar to the commercial valve. The highspeed images were analysed to calculate valve opening and closing times and leaflet velocities. Cycle-to-cycle variation, leaflet flutter, incomplete opening and asynchronous leaflet motion were observed. Considering asymmetries and variations, the trileaflet valves showed comparable closing times to the bileaflet valve, with one valve showing a distinct improvement. However, the trileaflet valve profile height contributed to corresponding leaflet tip linear velocities between the bileaflet and trileaflet valves. Steady state conditions were initially assumed for the numerical evaluation. The comparative study between trileaflet and bileaflet valves was extended and the effect of design variables was investigated. The results were used to formulate design recommendations and concurred that the valve identified in the experimental evaluation presented the best solution. The recorded leaflet motion was specified as input to an unsteady computational evaluation. A high-fidelity transient model enables a more detailed analysis of the three-dimensional flow field. There was good comparison to the experimental results and to some published results. However, the results showed the development of high shear stresses in some areas, inducing coagulation and haemolysis. A prosthetic heart valve design, evaluation and optimisation method was developed. The study demonstrated the feasibility of the concept but highlighted critical design and manufacturing aspects that require additional consideration. It also showed that prescribed kinematics computational fluid dynamics analyses offer an alternative solution for capturing asymmetrical valve and flow behaviour, without the computational expense of direct numerical simulations.

AFRIKAANSE OPSOMMING: ‘n Rigiede drie-seil hartklep vir aortaklepvervanging is ontwerp en prototipes is vir toetsdoeleindes geproduseer. As gevolg van die hoë voorkoms van rheumatiese hartsiekte in ontwikkelende lande, is daar ‘n behoefte vir ‘n kunshartklep met ‘n gunstige biologiese werking. Verskeie toetse is uitgevoer om die hidrodinamiese werking van hierdie klep met ‘n kommersiële twee-seilklep te vergelyk. Nadat ‘n sistematiese benadering gevolg is, is verskeie ontwerpskonsepte geproduseer. Hierdie konsepte was analities geëvalueer en uitvoerbare ontwerpe is met laagvervaardiging, met al sy beperkings in ag geneem, ontwikkel. Die inisiële prototipes is aan voorlopige numeriese en eksperimentele toetse onderwerp om te bevestig of die voorgestelde hartklep aan die funksionele, materiaal en vervaardiging vereistes voldoen. Vier drie-seil hartkleppe is, gebaseer op die uitkoms van die evaluering van vereistes, vir verdere toetse geselekteer om te bepaal of hierdie kleppe aan regulatoriese prestasie-vereistes voldoen. Die daaropvolgende toetse van die prototipe drie-seil hartkleppe, in vergelyking met ‘n twee-seilklep as standaard, vervaardig in dieselfde wyse as die toetskleppe, en ‘n kommersiële klep, het eksperimentele en numeriese evaluasies ingesluit. Vloeitoetse, deur gebruik te maak van ‘n polsdupliseerder en bewegingsanalisetoetse, met die gebruik van ‘n hoë-spoed kamera, is uitgevoer. Die metings van die vloeitoetse is geanaliseer om te bepaal of die kleppe aan die regulatoriese vereistes voldoen. Drie van die vier drie-seilkhartkleppe het vergelykbare of selfs ‘n beter werking as die toets twee-seilhartklep, getoon. Die laagvervaardigde kleppe het bewys om dieselfde eienskappe as die kommersiële klep te hê. Die beelde van die hoë-spoed klepbeweging is gebruik om die openings - en sluitingstye en seil-snelhede te bereken. Siklus-tot-siklus variasie, seilfladder, onvolledige oopmaak en ongesinchroniseerde beweging is opgemerk. Met die asimmetrie van die klepseile en die groot siklusvariasie in ag genome, het die drieseilkleppe vergelykbare, en die een klep selfs beter, sluitingstye as die tweeseilklep getoon. Alhoewel die drie-seilhartklep langer neem om te sluit, is die lineêre snelheid van die seilpunte, as gevolg van die klepprofiel-hoogte, vergelykbaar met die van die twee-seilklep. Gestadigde vloeidinamiese simulasies is inisiëel uitgevoer. Die resultate van dié simulasies is gebruik om die vergelykbare studie tussen die drie-seilklep en tweeseilklep uit te brei en om die effek van ontwerpsveranderinge te evalueer. Die uitkoms van hierdie studie is gebruik om ontwerpsaanbevelings te maak. Die drieseilklep, wat tydens die ekperimentele toetse as die bes ontwerpte klep geidentifiseer is, het ook positiewe numeriese gedrag getoon. Die opnames van die seilbewegings is gebruik om insetvoorwaardes vir ‘n transiënte simulasie te definieer. ‘n Betroubare berekeningsvloeidinamika model laat ‘n meer gedetaileerde analise van die driedimensionele vloeiveld toe. Die numeriese resultate is met die eksperimentele resultate en sekere gepubliseerde resultate vergelykbaar. Areas van hoë skuifspanning, wat stolling en hemolise veroorsaak, is waargeneem. ‘n Kunshartklep ontwerps-, evaluerings- en optimaliseringsmetode is ontwikkel. Die studie het die haalbaarheid van die drie-seilklep konsep gedemonstreer, maar het ook kritiese ontwerp- en vervaardigingsaspekte, wat bykomend in ag geneem moet word, beklemtoon. Dit het ook aangedui dat voorgeskrewe-kinematika berekeningsvloeidinamika ontledings 'n alternatiewe oplossing bied om asimmetriese klep- en vloeigedrag vas te lê, sonder die berekeningskoste van direkte numeriese simulasies.

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