The present invention is generally directed to improved microfluidic devices and systems, and particularly to a microfluidic device which comprises a planar body structure, at least a first channel network comprising intersecting channels disposed within the body structure wherein the first channel network is contained within an area of the planar body structure that is less than about 6 cm2; and a holder assembly configured to receive the body structure, the holder assembly having at least one aperture disposed therethrough from an upper surface to a lower surface thereof, and wherein the body structure is fixedly mounted to the holder assembly.