Fiber Bundle Design for an Integrated Wearable Artificial Lung

Abstract

Mechanical ventilation (MV) and extracorporeal membrane oxygenation (ECMO) are the only viable treatment options for lung failure patients at the end-stage, including acute respiratory distress syndrome (ARDS) and chronic obstructive pulmonary disease (COPD). These treatments, however, are associated with high morbidity and mortality because of long wait times for lung transplant. Contemporary clinical literature has shown ambulation improves post-transplant outcomes in lung failure patients. Given this, we are developing the Pittsburgh Ambulatory Assist Lung (PAAL), a truly wearable artificial lung that allows for ambulation. In this study, we targeted 180 ml/min oxygenation and determined the form factor for a hollow fiber membrane (HFM) bundle for the PAAL. Based on a previously published mass transfer correlation, we modeled oxygenation efficiency as a function of fiber bundle diameter. Three benchmark fiber bundles were fabricated to validate the model through in vitro blood gas exchange at blood flow rates from 1 to 4 L/min according to ASTM standards. We used the model to determine a final design, which was characterized in vitro through a gas exchange as well as a hemolysis study at 3.5 L/min. The percent difference between model predictions and experiment for the benchmark bundles ranged from 3% to 17.5% at the flow rates tested. Using the model, we predicted a 1.75 in diameter bundle with 0.65 m surface area would produce 180 ml/min at 3.5 L/min blood flow rate. The oxygenation efficiency was 278 ml/min/m and the Normalized Index of Hemolysis (NIH) was less than 0.05 g/100 L. Future work involves integrating this bundle into the PAAL for which an experimental prototype is under development in our laboratory.

Figure 1

Schematic and prototype of an assembled test module. The schematic indicates components and flow paths through the test module. The bundle is assembled into a custom machined test housing shown in the image of the prototype.

Figure 2

The single pass loop system for measuring in-vitro gas exchange in blood. Clamps at the inlet and outlet reservoir are used to control blood flow through the circuit. Once conditioned, blood passed from the inlet reservoir through the loop into the outlet reservoir keeping the post device blood was separate from the conditioned blood.

Figure 3

Model calculations and experimental results of oxygenation efficiency (oxygenation per unit surface area) versus flowrate for the FB-1 (Figure 3A), FB-2 (Figure 3B), and FB-3 (Figure 3C).

Figure 4

Oxygenation performance of the FB-F fiber bundle determined using the validated gas exchange model. Model predictions are also shown for reference. The bundle meets the oxygenation target (180 ml/min) at 3.5 L/min.

Figure 5

Normalized index of hemolysis (NIH) of the full system and control loops. Both tests were run for 3h at a blood flow of 3.5 L/min. PfHb was measured once every 30 minutes.