Reducing each phase resistance will hence improve the overall gas transfer rate of the oxygenator. However, the magnitude of each resistance is not the same. For instance, the gas phase resistance is negligible 167 , while the blood phase boundary layer resistance is the highest and can be 100 times larger than the membrane resistance This phenomenon is particularly important in gas/liquid transfers in the laminar flow regimes. The absence of turbulences in the blood flow creates a boundary layer at the blood/membrane interface with increasing concentrations of gases that are not removed by convection (Figure ). This saturated layer at the membrane surface acts like a diffusional barrier to the gas transfer reducing the transfer rate and keeping the gas transfer efficiency below the technical possibilities of the membrane. This was confirmed also by the work of Vacanti's group 169 who tested the gas transfer in a microfluidic oxygenator across three different membranes: dense poly(dimethyl siloxane) (PDMS) membranes (63 µm and 12 µm thick) and porous polycarbonate (PC) with pore size of 1 µm. The gas transfer for all three membranes was in the same range, confirming that the bottleneck in the blood oxygenation performance is the mass transfer rate in the blood phase rather than the permeance of the membrane. This highlights the importance of an optimized and efficient blood module design.