In one aspect, apparatuses for providing chemical gradients are described herein. In some embodiments, an apparatus described herein comprises a conduit having a first end and a second end, one or more microchannels disposed in the conduit and extending from the first end toward the second end, and a fiber coiled around the exterior of at least one microchannel, wherein the fiber comprises an active agent that is operable to diffuse into the interior of the microchannel.