## 4 Discussion
However, the relative contributions of each manufacturing step to the final precision distributions of the two models remain ambiguous. Figure 9 shows a comparison of the PIV achieved by the two phantoms. The PIV algorithm yields zero flow at the wall boundaries. Hence both models show expected boundary behaviour. However, the second model yields more coherent near-wall velocities that are more conducive to consistent WSS calculation. The reverse flow observed in the brachioccephalic branch of the second phantom is most likely due to the positive pressure gradient caused by constriction of the aortic arch during deceleration. The same flow pattern was observed by [PERSON] et al. [11]. The recirculation and the reverse flow observed are indicative of low and intermittent WSS. These flow patterns are risk factors for CVD, atherosclerotic plaque formation, and endothelial cells development [50; 51]. [PERSON] et al. [52] produced an ex vivo study which showed stenosis risk areas in the aortic arch at the same regions where reverse flow and recirculation were observed in this study. A computed tomography examination of the aortic arch illustrated the same areas for atherosclerosis lesions [53]. PIV of the first phantom yielded erratic velocity vectors that were not indicative of coherent flow, and the second phantom showed clear expected flow patterns. It is believed this is due to the variation in wall thickness caused by the misalignment of the internal core. The phantom would move laterally during experimentation due to the increased expansion of the thinner section and the force generated by the interaction of the fluid and the phantom. This led to large out of plane components of velocity that were not resolved with the PIV set up. These components of velocity would be greater than the thickness of the light sheet used. Furthermore, the poor surface finish of the initial phantom may have caused difficultly in tracking particles (Figure 6). Thus, the images obtained were not sufficient  
for the PIV analysis and the cross-correlation algorithm yielded some arbitrary velocity vectors ranging between 0-0.05 m/s. As a result of these erratic vectors, the velocity scales shown are different. This analysis was based on the outcomes of only two phantoms. Hence, the consistency of results cannot be assured. The cost of mould generation and time-consuming nature of casting limits the ability to generate more phantoms and a statistical analysis cannot be undertaken. The current geometry has a high level of curvature and contains a branch. These are the parameters which may limit the successful production of the phantom. Since the second method overcame these limitations, there is a degree of confidence that the method may be successful in more sophisticated cases. However, further conclusive studies are required to determine how wall thickness precision will be maintained in larger, more complex, arterial geometries. The model experiences a sudden transition from compliant to rigid at the connection points. However, this transition does not mitigate the results at the arch and BCA junction. The current model was enlarged 50%, but the results' applicability was maintained by dynamic similarity and compliance matching. Thin wall phantoms require high manufacturing precision. Therefore, for narrower arteries such as the coronary artery, the model will need a larger scale to achieve a wall thickness suitable for the manufacturing method developed here. This phantom size will also require an increased field of view for PIV analysis. PIV analysis has been established as the leading methodology in experimental fluids [54]. However, the methodology has certain limitations in cases when fluid-solid boundary interaction must be modelled. In particular, most applications of PIV require solid boundary materials to be optically transparent and have a refractive index that matches the working fluid. Furthermore, when specific wall compliance is required, the wall thickness of the solid boundary must be precisely achieved.