Microneedle arrays and drug delivery devices are provided for transdermally delivering a drug formulation to a patient. The microneedle array device includes a substantially planar substrate having an array of apertures; and a plurality of microneedles projecting at angle from the planar substrate, the microneedles having a base portion integrally connected to the substrate, a tip end portion distal to the base portion, and body portion therebetween, wherein each microneedle has at least one channel extending substantially from the base portion through at least a part of the body portion, the channel being open along at least part of the body portion and in fluid communication with at least one of the apertures in the substrate. In a preferred embodiment, each microneedle has a substantially rectangular cross-sectional shape and the channel is open to two opposing surfaces of the microneedle.