Hwa Liang LeoICBME Young Investigator Award Winner in Category: Biomechanics/BioMEMSI joined the Cardiovascular Fluid Mechanic lab of Georgia Tech, Atlanta in 1999 (biomedical engineering/mechanical engineering) after two years at the Nanyang Technological University (NTU), Singapore. My Masters (mechanical engineering) thesis at NTU focused on the design and development of a magnetically suspended centrifugal blood pump. My BSc (mechanical engineering) was at the University of Leeds, England, focusing mainly on the development of a vehicle braking system. Bioengineering is an interdisciplinary crossroad, which holds potential for many interesting and exciting discoveries. To me it is a perfect marriage of a discipline I thoroughly enjoy (Engineering), and a fascinating field of natural science which I have always loved, (Biology). From learning of the first heart transplant animal trial conducted in 1957, the fast-evolving field of Bioengineering continues to fascinate me with its continual progression and discovery. My research interest lies in the fluid mechanics of prosthetic heart valves. My objective is to explore the characteristics of the micro flow structures in the prosthetic valves and their relevance to hemolysis and thrombosis. My research work focuses on two types of valve designs, - the bileaflet mechanical heart valves (MHVs) and the tri-leaflet polymeric heart valve. With its superior hemodynamic performance and excellent durability, bileaflet MHVs have become one of the most widely transplanted valve designs. One of the interesting aspects of the bileaflet MHVs design is that a certain degree of regurgitant flow is usually incorporated at their hinge regions to allow better washout and to prevent the buildup of blood clots. However, recent studies have shown that these constricted hinge regions are susceptible to elevated velocity magnitude and high shear stresses, which could lead to the destruction of blood elements. Using 2D Laser Doppler Velocimetry technique, we were able to show that the Reynolds shear stresses measured during the leakage phase (~1500 - 6000 dynes/cm2) were approximately one magnitude higher than those during the forward phase (~100-500 dynes/cm2). Clinical studies have shown that hemolysis occurs at shear stress values of 1500-2500 dynes/cm2 and platelet damage as low as 100-200 dynes/cm2. Our past studies largely involved valves at the mitral position because of its more severe regurgitant conditions. At present, our lab is embarking on an investigation of the hinge flow structures in aortic position. This is to provide investigators with a more complete picture of the flow characteristics for different valves at various positions so that an optimal valve design can be worked towards. My next focus is in the fluid mechanics of the tri-leaflet polymeric valve, which is typically made from a polyurethane material known to have excellent non-thrombogeneic properties. Though this particular valve design is still at the developmental stages, studies have shown that it has excellent hemodynamic properties, equivalent to those of a tissue heart valve and that this valve promises durability equal to that of an MHV. Preliminary investigation of a prototype polymeric valve had revealed flow structures that are highly dependent on the valve's commissure design. In vivo experiments have shown that blood clots tend to build up at the valve's inflow region and the in vitro experiments performed in our lab suggested that this could be due to the closed commissure design of the prototype valve. The closed commissure design may have led to a low flow region, which propagates further upstream into the valve during leakage phase (Figure A and B). The low flow region increases the residence time of the blood elements and leads to the build up of the blood clots. The material development of a prosthetic heart valve is intimately linked to its flow characteristics. This is especially true for the tri-leaflet polymeric heart valve. More studies are needed to determine the optimal design for the polymeric valve's leaflet thickness, stent structure design and the gap size at both the commissure region and the coaptation point. For further information, please contact:
Hwa Liang Leo
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