The highly uniform shear forces at millimeter and smaller dimensions can produce monodisperse particles spontaneously during continuous flow of immiscible fluids. Microfluidic techniques have also recently been used to generate and organize arrays of solid monodisperse microparticles with various material properties. Combining these developments with a well-engineered “chip-to-world” interface will provide a new high-resolution 3-D printing technology for rapid prototyping and biotechnology.We produced hydrogel droplets at rates of 10-200 Hz in PDMS microreactors. Droplets were monodisperse, with 50-200 micron diameter dependent on flow rate, channel shape and the presence of surfactants. Droplets may be dispensed from the chip in their liquid state, or solidified by one of three mechanisms. Ultraviolet (UV) light was used to solidify photosensitized hydrogels. We also discuss other hydrogel systems that are solidified by a chemical reaction (sodium alginate and CaCl₂) or by thermal gelation (Pluronic F-127), methods to vary the droplets' chemical composition and mechanical properties, and methods for closed-loop electrical feedback on particle position for precision fabrication. Biocompatible hydrogels are of great interest for biomaterial applications such as drug delivery and tissue engineering.