Controlling uniformity of photopolymerized microscopic hydrogels

This paper studies hydrogels created by photopolymerization with a uniform beam of light. Under some conditions the density profiles of the resulting hydrogels were uniform cylinders, mirroring the illumination profiles. However, under other conditions, gels with hollow cylindrical shapes were formed. We studied the photopolymerization of poly-N-isopropylacrylamide (pNIPAAM), a hydrogel that has been widely used in tissue engineering and microfluidic applications, and examined how the size and uniformity of pNIPAAM microscopic gels can be controlled by varying parameters such as exposure time, exposure area, exposure intensity, monomer concentration, photoinitiator concentration and terminator concentration. A simplified reaction-diffusion model of the polymerization process was developed and was found to describe the experiment for a wide range of parameters. This general framework will guide attempts to establish optimal conditions for the construction of microscopic hydrogels using photolithography, which is a method that has found applications in fields such as microfluidics, drug delivery, cell and tissue culturing, and high resolution 3D printing.