Theoretical investigation of mass transfer in membrane oxygenators

Two-Dimensional Finite Element Model for Oxygen Transfer in Cross-Flow Hollow Fiber Membrane Artificial Lungs

A predictive, two-dimensional model with good absolute accuracy for flow and mass transfer in cross-flow hollow fiber membrane artificial lungs is developed. The proposed model is able to predict the gas transfer to water flowing outside and perpendicular to hollow fibers in the artificial lung. The model uses a finite element technique to solve the Navier-Stokes equations and the convection-diffusion equation on the computational domain of a unit fiber cell. Subsequent stream-wise and cross-wise unit fiber cells are then coupled/assembled to the relationship between the oxygen transfer rate and flow rate of a cross-flow hollow fiber membrane artificial lung. The model is compared to experimental water data obtained by perfusing three commercial artificial lungs with water.

Pulsatile Flow and Mass Transport Over an Array of Cylinders: Gas Transfer in a Cardiac-Driven Artificial Lung

The pulsatile flow and gas transport of a Newtonian passive fluid across an array of cylindrical microfibers are numerically investigated. It is related to an implantable, artificial lung where the blood flow is driven by the right heart. The fibers are modeled as either squared or staggered arrays. The pulsatile flow inputs considered in this study are a steady flow with a sinusoidal perturbation and a cardiac flow. The aims of this study are twofold: identifying favorable array geometry/spacing and system conditions that enhance gas transport; and providing pressure drop data that indicate the degree of flow resistance or the demand on the right heart in driving the flow through the fiber bundle. The results show that pulsatile flow improves the gas transfer to the fluid compared to steady flow. The degree of enhancement is found to be significant when the oscillation frequency is large, when the void fraction of the fiber bundle is decreased, and when the Reynolds number is increased; the use of a cardiac flow input can also improve gas transfer. In terms of array geometry, the staggered array gives both a better gas transfer per fiber (for relatively large void fraction) and a smaller pressure drop (for all cases). For most cases shown, an increase in gas transfer is accompanied by a higher pressure drop required to power the flow through the device.

Assessment of oxygen transfer in membrane oxygenators during clinical cardiopulmonary bypass

Although functional replacement of the heart and lungs by a pump and oxygenator is a widespread surgical procedure, no widely accepted technique for describing gas exchange in oxygenators exists. In this study, 8 types of commercially available membrane oxygenator (2 flat sheet membrane, 4 gas in hollow fibre membrane and 2 blood in hollow fibre membrane) have been studied during clinical cardiopulmonary bypass. O2 transfer increased with blood flow rate but the O2 transfer at a given blood flow was lower than that obtained by the manufacturers in laboratory studies. Overall O2 transfer coefficients were calculated from the ratio of O2 transfer rate to an O2 difference expressed either as an O2 partial pressure or an O2 concentration. Specific O2 transfer coefficients (overall coefficient divided by membrane area) were similar for oxygenators with a flat sheet or gas in hollow fibre membrane configuration. The two types of oxygenator with blood in hollow fibre membranes had significantly lower (P less than 0.01) specific O2 transfer coefficients. This study shows that oxygenator gas transfer characteristics can be studied in the clinical environment and that O2 transfer coefficients can be related to oxygenator design features.

Computationally two-dimensional finite-difference model for hollow-fibre blood-gas exchange devices

The goal of this research is to develop a predictive model with good absolute accuracy for blood-gas exchange devices. The proposed model, unlike existing models, is able to predict gas transfer to blood flowing outside oxygenating fibres without experimental data. The proposed model uses a finite-difference numerical technique to solve computationally two-dimensional gas exchange problems such as gas transfer to blood outside hollow fibres. The model is compared to bovine and human experimental data from the small test cells with microporous polypropylene fibres. The test cell flow rates range from 1 m litre min-1 to 5 m litre min-1 for a 72-fibre device. Shear-augmented oxygen diffusion appears to be present, although good accuracy is obtained with a nonaugmented diffusion model, particularly at lower flows. The maximum deviation of oxygen saturation predicted by a shear-augmented bovine blood model from the experimental regression line was 1.7 per cent.