(543g) Microchannel Membrane Reactor Technologies for Artificial Lungs
A highly effective blood-oxygen exchange component of an artificial lung would mimic the structure of a natural lung, with blood-size channels about the size of a red blood cell. Calculations show that with 12 µm channels, such a device would require 140 million, 0.8 mm long channels, but the total size of the gas-exchange section would be only 57 mL and a blood prime of only 13 mL. Also attractive are 12 µm high broad open channels with support posts and 40 µm screen-filled rectangular channels. The total size of the former would be 250 mL with a blood prime of 13 mL, and the total size of the latter would be 280 mL with a blood prime of 27 mL.
A major challenge for developing such devices is the requirement that the multitude of channels must be uniform from channel to channel and along each channel in order to avoid preferential blood passages. One strategy is to fill broad rectangular channels with micro scale screens that can provide uniform support and stability. We have explored the effectiveness of 40 µm screen-filled blood-side channels and, as a comparison, 82 µm screen-filled channels. Small concept-devices, consisting of a single 69 mm wide and 3 or 6 mm long channel, were tested using 30% hematocrit blood and oxygen or air on the gas side. The measured oxygen fluxes in the devices were in the range of 4 to 9x10-7 moles/(min.cm2), with the latter close to the theoretical membrane limit. The pressure drop was in the range of 1 to 6 mmHg. Extrapolating the data to a device designed to process 4 L/min suggests a required blood prime of only 35 mL.
Another strategy is to employ lithographic techniques to develop patterned silicone rubber membranes that provide 15 µm high microchannels for artificial lungs. Two types of devices were fabricated: one had a series of parallel, straight, open rectangular channels that are each 300 µm wide, separated by 200 µm walls, and 3 mm long and the other is a wide rectangular channel with support posts, also 3 mm long. Experiments with 30% hematocrit, venous, bovine blood show average oxygen fluxes ranging from 11x10-7 moles/(min?cm2) at a residence time of 0.04 sec to 6.5 x10-7 moles/(min?cm2) at a residence time of 0.20 sec. The average oxygen flux vs residence time, which is due to transverse molecular diffusion, follows the same relation as for all membranes tested. The corresponding increase in hemoglobin saturation ranged from 9% at the residence time of 0.04 sec to 24% at the residence time of 0.20 sec. The support-post channel membranes are attractive for designers because they can be arbitrarily wide and would be less prone blockage.