Hollow fiber membranes are ubiquitously used in industrial separations, especially the filtration of drinking water. Hollow fibers are commonly used substrates for specialized bioreactor systems, with the ability of some hollow fiber cartridges to culture billions of anchorage-dependent cells within a relatively low (<100 mL) bioreactor volume. Hollow fibers can be used for drug efficacy testing in cancer research, as an alternative to the traditional, but more expensive, xenograft model.