Partitioning of hydrogels in 3D-printed microchannels. 
Hydrogels allow for controlling the diffusion rate and amount of solute according to the hydrogel network and thus have found many applications in drug delivery, biomaterials, toxicology, and tissue engineering.  This paper describes a 3D-printed microfluidic chip for the straightforward partitioning of hydrogel barriers between microchannels.  We use a previously-reported 3-channel architecture whereby the middle channel is filled with a hydrogel - acting like a porous barrier for diffusive transport - and the two side channels act as sink and source; the middle channel communicates with the side channels via orthogonal, small capillary channels that are also responsible for partitioning the hydrogel during filling.  Our 3D-printed microfluidic chip is simple to fabricate by stereolithography (SL), inexpensive, reproducible, and convenient, so it is more adequate for transport studies than a microchip fabricated by photolithographic procedures.  The chip was fabricated in a resin made of poly(ethylene glycol) diacrylate (PEG-DA) (MW = 258) (PEG-DA-258).  The SL process allowed us to print high aspect ratio (37 : 1) capillary channels (27 μm-width and 1 mm-height) and enable the trapping of liquid-phase hydrogels in the hydrogel barrier middle channel.  We studied the permeability of hydrogel barriers made of PEG-DA (MW = 700) (PEG-DA-700, 10% polymer content by wt.  in water) - as a model of photopolymerizable barriers - and agarose (MW = 120 000, 2% polymer content by wt.  in water) - as a model of thermally-gelled barriers.  We measured the diffusion of fluorescein, 10k-dextran-Alexa 680 and BSA-Texas Red through these barriers.  Fluorescein diffusion was observed through both 10% PEG-DA-700 and 2% agarose barriers while 10k-dextran-Alexa 680 and BSA-Texas Red diffused appreciably only through the 2% agarose hydrogel barrier.  Our microfluidic chip facilitates the tuning of such barriers simply by altering the hydrogel materials.  The straightforward trapping of selective barriers in 3D-printed microchannels should find wide applicability in drug delivery, tissue engineering, cell separation, and organ-on-a-chip platforms.