Oxygenator with stacked fiber membrane

A blood oxygenator comprising:

• a housing having a first end opposite a second end with a sidewall extending between the first end and the second end along a longitudinal axis, the housing defining an interior chamber having a liquid inlet at the first end and a liquid outlet at the second end;
• and
• a gas exchange assembly positioned within the interior chamber, the gas exchange assembly comprising:
• a retainer having an upper cap spaced apart from a lower cap by one or more spacers;
• and
• a gas exchange medium disposed between the upper cap and the lower cap, wherein the gas exchange medium comprises a plurality of subunits stacked on top of each other, each subunit comprising a plurality of layers of hollow fiber mats, wherein the plurality of subunits are stacked offset from one another such that an axis extending through a centroid of each of the subunits is at an acute angle relative to the longitudinal axis of the housing.