Thermal, operational, and storage stability of immobilized carbonic anhydrase in membrane lungs

The significant improvement in CO2 removal rates from blood through membrane lungs with EMCO and ECCO2R is achievable by means of the immobilization of carbonic anhydrase (CA) onto the membrane surface. The practical application of this technology requires that the enzyme be maintained for long periods without loss of activity. Thus, studies were performed to evaluate the thermal, operational, and storage stability of CA in a cellulose nitrate-encapsulated, silicone rubber membrane-immobilized form. Cellulose nitrate microcapsules containing 1000 micrograms/ml CA were prepared using a modified version of the method of Chang, and immobilized onto a 0.06 m2 section of commercial silicone rubber membrane material. The extent of enzyme activity in free solution and in the encapsulated form was determined after long-term storage at 4 degrees C, 37 degrees C, and 50 degrees C. Likewise, in an in vitro test circuit, CO2 removal efficiency in both CA treated and untreated membrane lungs was measured for extended time periods of 10 hr over a 10 day period. The thermal stability tests showed a significantly greater degree of retained enzyme activity in the encapsulated form, over the free enzyme in solution, at all temperatures. This was especially evident at higher temperatures and when the enzyme was stored for extended periods. In the operational stability tests, the CO2 removal efficiency of the treated membrane was not degraded, and stayed significantly higher than the untreated membrane for extended time periods. This further illustrates the potential for the use of the immobilized enzyme, carbonic anhydrase, for improved CO2 removal efficiency.