In the attempt to improve the overall gas transfer efficiency of the microfluidic artificial lungs, in 2009, stacked two blood and four gas channel layers fabricating for the first time a microfluidic oxygenator with double-sided diffusion of oxygen into the blood channels. Successively, among others adopted this promising approach. In 2018, Dabaghi's group developed a microfluidic membrane oxygenator with double-sided gas transfer microchannels with the capability of being operated in a pumpless manner . Oxygen uptake in the double-sided device was increased by up to 343% compared to the single-sided device with the same design. They then assembled 4 devices to build a lung assist device (LAD) for preterm neonates with respiratory distress syndrome. The LAD could provide adequate oxygenation up to a blood flow rate of 60 ml/min using pure oxygen with a pressure drop low enough (10-51 mmHg) to enable a pumpless operation (Figure ). Recently, the same research group further ameliorated this approach by introducing for the first time a micro-oxygenator with four-sided oxygen diffusion 201 . Their novel design included embedded hollow air chambers between blood microchannels to involve also the side walls in gas exchange, increasing the gas exchange surface area without affecting blood flow characteristics or the channel's geometry. Compared with its equivalent double-sided gas exchange design, the new design's performance is enhanced by up to 223%. Although proof of concept has been demonstrated for this new configuration, a more reliable fabrication approach needs to be developed, and the robustness of the device must be further evaluated (Figure ).