In vitro study of haemodynamic stresses and endothelialisation of artificial coronary arteries

Coronary artery bypass grafts (CABG) tend to fail after sometime from months to a few years, due to myointimal hyperplasia or intimal thickening. The thickening normally occurs at the heel, toe, along the suture line of the distal anastomoses and more frequently on the floor of the host artery. Moreover, many patients do not have veins suitable for grafting because of previous grafts to treat pre-existing vascular diseases or previous surgery. To date the clinical solution to this growing problem has been the use of artificial vessels of biocompatible polymer that could harmonize with the existing host artery. In addition endothelialisation of grafts has been recognised as a very important factor in sustaining the compatibility of the bypass graft. Determination of local haemodynamics in (CABG) has been recognised as a crucial factor for the general understanding of the cause of bypass failure. In this study an experimental and numerical programmes were carried out with the aim of optimising the geometry of the by pass graft from hemodynamics point of view. The optimized CABG was then designed and manufactured using Rapid Prototyping Fused Deposition Modelling, (FDM). The Electro-Spinning manufacturing technique was also used to construct the polyurethane CABG model. In the experimental part of the study, a scale of 40 degree anastomotic Pyrex glass model was designed and constructed to simulate the fluid flow and the shear stresses in the vicinity of anastomosis junction. Flow visualization method was used together with Particle Images Velocimetry (PIV) to gain a comprehensive picture of the flow characteristics inside the glass model under various flow rates. The data obtained were used for the validation of the numerical predictions In the numerical investigation the commercial ASNSYS package 11, was used for the 3 dimensional flow analysis of the anatomically realistic model. The pulsatile waveforms and flow rates were used as the boundary conditions and the governing equations namely Navier Stokes Equations were solved for various hemodynamics conditions. These included the effect of the graft-to-artery diameter ratio, and the intensity of the angle of attachment (20, 40 and 60 degrees) respectively. Particular emphasis was given on the effect of the anastomotic angle on the flow patterns and wall shear distributions at the distal anastomosis. The comparison between experimental and numerical data was observed to be fair. However, there were some differences in magnitudes between the measured and calculated velocity components particularly near anastomosis models. The difference was attributed to manufacturing limitations of the experimental model as it was not possible to reproduce the exact shape of Pyrex glass model in the computational analysis. The flow patterns and distributions of wall shear stress and shear strain rates were numerically evaluated and discussed. The numerical findings indicated that disturbed flows such as vortex and asymmetrical flows appeared in the localized regions of the proximal and distal artery segments and in the anastomotic domain. Moreover, dominating wall shear stresses occurred mainly on the bed, at the toe and at the heel respectively. The higher anastomotic angles the higher the vortex near the toe and the heel, and along the bed. For the flow rates and the anastomotic angles investigated the 40° proximal anastomosis model provided the best aerodynamics characteristics and would lessen the potential of intimal thickening more than other angles. Moreover, the 40° proximal anastomosis model appeared to have a lower variation range of averaged WSS and lower degree of asymmetrical flows. Various manufacturing techniques were reviewed and investigated to determine the most suitable technique for different biocompatible and biodegradable materials. These techniques included the conventional methods as well as the Free Form Rapid Prototyping techniques such as FDM. With the aid of FDM the optimized model of CABG was manufactured using thermoplastic Polyurethane. Moreover, a comprehensive review of the most likely biocompatible materials for CABG was carried out. The material selected for CABG applications needed to be robust, flexible and possess good mechanical properties to the host artery and should mimic and synchronize with the nature artery. Various materials such as poly-epsilon-caprolactone (PCL), Poly-L-lactic acid (PLLA) and Polyurethanes (PU) have been mechanically tested to determine the most suitable one. Polyurethane was found to be the most suitable polymer due to its elasticity, good mechanical strength and degree of compliance. Finally establishment of the endothelium on the developed biocompatible polyurethane artery to obtain an anti-thrombogenic was achieved under various in vitro flow conditions. In the cell culture study, the biocompatibility and cell retention of polyurethane was investigated under various flow conditions. Endothelial cells were seeded onto gelatin-chitosan polyurethane and were exposed to various flow rates. Various degree of high cell retention rate of more than 75% was observed in the gelatinchitosan polyurethane for all the flow rates tested.