In some examples, a membrane module includes a fiber bundle. The fiber bundle includes a tubular first region and a tubular second region positioned around the tubular first region. One of the first or second regions includes hollow fibers oriented along an axis of the fiber bundle. Another of the first or second regions includes hollow fibers that are not colinear in a radial direction from the axis of the fiber bundle. A void fraction of the second region is different from a void fraction of the first region.