A microchannel-based hemodialyzer offers a novel approach to hemodialysis practice and holds many promises for improving dialysis treatment efficiency and quality of life for kidney patients. The hollow fiber hemodialyzer, a conventional dialysis device, has certain limitations including non-uniformity of the dialysate flow path which necessitates the use of a high dialysate flow rate. In the microchannel-based design, successive stacked layers alternate between blood flow and dialysate flow. A flat porous membrane between these layers allows for the transport of toxins from the blood side to the dialysis fluid side. This design improves mass transfer characteristics and enables the use of lower dialysate to blood flow rate ratios suitable for a home hemodialysis system. One of the current issues in microchannel dialyzer is the emergence of bubbles through blood-outgassing and air ingress. This dissertation provides an in depth study of the bubble pinning phenomenon and the impact of the geometrical design of a single lamina of the microchannel dialyzer on bubble removal.
Our primary method to facilitate bubble mitigation in the system is increasing the surface hydrophilicity by applying polyethylene oxide (PEO) coating on internal surface of polycarbonate laminas. The effects of contact angle hysteresis and contact line (pinning) forces as well as the dewetting velocity on bubble stagnation have been studied. A theoretical model that estimates the required pressure drop along the length of a stagnant bubble to overcome pinning forces to stimulate motion has been developed. The model has been validated by experimental results. The model demonstrates that the use of highly hydrophilic surfaces is a robust approach to reducing both pinning and capillary forces and subsequently bubble clogging in microfluidic systems.
The geometrical design must exhibit low resistance for bubble mitigation while maintaining a relatively uniform flow distribution between channels for efficient mass transfer. A single lamina of the microchannel dialyzer under development at OSU is comprised of an inlet manifold, an outlet manifold and a parallel microchannel array; the manifolds feature a micropost arrangement that supports the membrane. The overall manifold geometry was designed for uniform flow distribution while the suggested arrangement of microposts inside the manifolds was aimed at exhibiting low bubble retention. For all the experiments in this dissertation, water has been used as the working fluid; however, for the selection of the microchannel size and velocity conditions, considerations for preventing blood hemolysis under shear stress were taken into account. Furthermore, the anticipated differences between an air-water system and an air-blood system have been discussed.