Kinetics of CO2 exchange with carbonic anhydrase immobilized on fiber membranes in artificial lungs

Artificial lung devices comprised of hollow fiber membranes (HFMs) coated with the enzyme carbonic anhydrase (CA), accelerate removal of carbon dioxide (CO2) from blood for the treatment of acute respiratory failure. While previous work demonstrated CA coatings increase HFM CO2 removal by 115 % in phosphate buffered saline (PBS), testing in blood revealed a 36 % increase compared to unmodified HFMs. In this work, we sought to characterize the CO2 mass transport processes within these biocatalytic devices which impede CA coating efficacy and develop approaches towards improving bioactive HFM efficiency. Aminated HFMs were sequentially reacted with glutaraldehyde (GA), chitosan, GA and afterwards incubated with a CA solution, covalently linking CA to the surface. Bioactive CA-HFMs were potted in model gas exchange devices (0.0119 m(2)) and tested for esterase activity and CO2 removal under various flow rates with PBS, whole blood, and solutions containing individual blood components (plasma albumin, red blood cells or free carbonic anhydrase). Results demonstrated that increasing the immobilized enzyme activity did not significantly impact CO2 removal rate, as the diffusional resistance from the liquid boundary layer is the primary impediment to CO2 transport by both unmodified and bioactive HFMs under clinically relevant conditions. Furthermore, endogenous CA within red blood cells competes with HFM immobilized CA to increase CO2 removal. Based on our findings, we propose a bicarbonate/CO2 disequilibrium hypothesis to describe performance of CA-modified devices in both buffer and blood. Improvement in CO2 removal rates using CA-modified devices in blood may be realized by maximizing bicarbonate/CO2 disequilibrium at the fiber surface via strategies such as blood acidification and active mixing within the device.