Model-Based Design and Optimization of Blood Oxygenators

Blood oxygenators, also known as artificial lungs, are widely used in cardiopulmonary bypass surgery to maintain physiologic oxygen (O₂) and carbon dioxide (CO₂) levels in blood, and also serve as respiratory assist devices to support patients with lung failure. The time- and cost-consuming method of trial and error is initially used to optimize the oxygenator design, and this method is followed by the introduction of the computational fluid dynamics (CFD) that is employed to reduce the number of prototypes that must be built as the design is optimized. The CFD modeling method, while having progress in recent years, still requires complex three-dimensional (3D) modeling and experimental data to identify the model parameters and validate the model. In this study, we sought to develop an easily implemented mathematical models to predict and optimize the performance (oxygen partial pressure/saturation, oxygen/carbon dioxide transfer rates, and pressure loss) of hollow fiber membrane-based oxygenators and this model can be then used in conjunction with CFD to reduce the number of 3D CFD iteration for further oxygenator design and optimization. The model parameters are first identified by fitting the model predictions to the experimental data obtained from a mock flow loop experimental test on a mini fiber bundle. The models are then validated through comparing the theoretical results with the experimental data of seven full-size oxygenators. The comparative analysis show that the model predictions and experimental results are in good agreement. Based on the verified models, the design curves showing the effects of parameters on the performance of oxygenators and the guidelines detailing the optimization process are established to determine the optimal design parameters (fiber bundle dimensions and its porosity) under specific system design requirements (blood pressure drop, oxygen pressure/saturation, oxygen/carbon dioxide transfer rates, and priming volume). The results show that the model-based optimization method is promising to derive the optimal parameters in an efficient way and to serve as an intermediate modeling approach prior to complex CFD modeling.

Formulation of Generalized Mass Transfer Correlations for Blood Oxygenator Design

This paper numerically investigates non-Newtonian blood flow with oxygen and carbon dioxide transport across and along an array of uniformly square and staggered arranged fibers at various porosity (ε) levels, focussing on a low Reynolds number regime (Re < 10). The objective is to establish suitable mass transfer correlations, expressed in the form of Sherwood number (Sh = f(ε, Re, Sc)), that identifies the link from local mass transfer investigations to full-device analyses. The development of a concentration field is initially investigated and expressions are established covering the range from a typical deoxygenated condition up to a full oxygenated condition. An important step is identified where a cut-off point in those expressions is required to avoid any under- or over-estimation on the Sherwood number. Geometrical features of a typical commercial blood oxygenator is adopted and results in general show that a balance in pressure drop, shear stress, and mass transfer is required to avoid potential blood trauma or clotting formation. Different definitions of mass transfer correlations are found for oxygen/carbon dioxide, parallel/transverse flow, and square/staggered configurations, respectively. From this set of correlations, it is found that transverse flow has better gas transfer than parallel flow which is consistent with reported literature. The mass transfer dependency on fiber configuration is observed to be pronounced at low porosity. This approach provides an initial platform when one is looking to improve the mass transfer performance in a blood oxygenator without the need to conduct any numerical simulations or experiments.

Simulation of blood oxygenation in capillary membrane oxygenators using modified sulfite solution

Blood oxygenation is the main performance characteristic of capillary membrane oxygenators (CMOs). Handling of natural blood in in vitro investigations of CMOs is quite complex and time-consuming. Since the conventional blood analog fluids (e.g. water/glycerol) lack a substance with an affinity to capture oxygen comparable to hemoglobin's affinity, in this study a novel approach using modified sulfite solution is proposed to address this challenge. The solution comprises sodium sulfite as a component, simulating the role of hemoglobin in blood oxygenation. This approach is validated by OTR (oxygen transfer rate) measured using native porcine blood, in two types of commercially available CMOs. Consequently, the number of complicated natural blood investigations in the evolution procedure of newly developed oxygenators would considerably decrease. Moreover, the reassessing of failed devices, in clinics, would be performed more precisely using a modified sulfite solution than simple water/glycerol testing.

A theoretical model for evaluation of the design of a hollow-fiber membrane oxygenator

Geometric data are fundamental to the design of a contactor. The efficiency of a membrane contactor is mainly defined by its mass-transfer coefficient. However, design modifications also have significant effects on the performance of membrane contactors. In a hollow-fiber membrane oxygenator (HFMO), properties such as priming volume and effective membrane surface area (referred to as design specifications) can be determined. In this study, an extensive theoretical model for calculation of geometric data and configuration properties, and, consequently, optimization of the design of an HFMO, is presented. Calculations were performed for Oxyphan(®) hollow-fiber micro-porous membranes, which are frequently used in current HFMOs because of their high gas exchange performance. The results reveal how to regulate both the transverse and longitudinal pitches of fiber bundles to obtain a lower rand width and a greater number of windings. Such modifications assist optimization of module design and, consequently, substantially increase the efficiency of an HFMO. On the basis of these considerations, three values, called efficiency factors, are proposed for evaluation of the design specifications of an HFMO with regard with its performance characteristics (i.e. oxygen-transfer rate and blood pressure drop). Moreover, the performance characteristics of six different commercial HFMOs were measured experimentally, in vitro, under the same standard conditions. Comparison of calculated efficiency factors reveals Quadrox(®) is the oxygenator with the most efficient design with regard with its performance among the oxygenators tested.

In vivo MR imaging of pulmonary perfusion and gas exchange in rats via continuous extracorporeal infusion of hyperpolarized 129Xe

BACKGROUND

Hyperpolarized (HP) (129)Xe magnetic resonance imaging (MRI) permits high resolution, regional visualization of pulmonary ventilation. Additionally, its reasonably high solubility (>10%) and large chemical shift range (>200 ppm) in tissues allow HP (129)Xe to serve as a regional probe of pulmonary perfusion and gas transport, when introduced directly into the vasculature. In earlier work, vascular delivery was accomplished in rats by first dissolving HP (129)Xe in a biologically compatible carrier solution, injecting the solution into the vasculature, and then detecting HP (129)Xe as it emerged into the alveolar airspaces. Although easily implemented, this approach was constrained by the tolerable injection volume and the duration of the HP (129)Xe signal.

METHODS AND PRINCIPAL FINDINGS

Here, we overcome the volume and temporal constraints imposed by injection, by using hydrophobic, microporous, gas-exchange membranes to directly and continuously infuse (129)Xe into the arterial blood of live rats with an extracorporeal (EC) circuit. The resulting gas-phase (129)Xe signal is sufficient to generate diffusive gas exchange- and pulmonary perfusion-dependent, 3D MR images with a nominal resolution of 2×2×2 mm(3). We also show that the (129)Xe signal dynamics during EC infusion are well described by an analytical model that incorporates both mass transport into the blood and longitudinal relaxation.

CONCLUSIONS

Extracorporeal infusion of HP (129)Xe enables rapid, 3D MR imaging of rat lungs and, when combined with ventilation imaging, will permit spatially resolved studies of the ventilation-perfusion ratio in small animals. Moreover, EC infusion should allow (129)Xe to be delivered elsewhere in the body and make possible functional and molecular imaging approaches that are currently not feasible using inhaled HP (129)Xe.