Droplet electrocoalescence as a robust tool for <i>in situ</i> examination of hydrogel microparticles in microfluidic devices

Nowadays hydrogel microparticles find numerous applications in material science and biological engineering such as drug delivery systems, cell carriers, <i>etc.</i> Droplet microfluidics provides an efficient tool for producing monodisperse microparticles, however, optimization of synthesis conditions remains challenging. Here, we developed a simple and easy-to-use method for <i>in situ</i> visual assessment or quantitative characterization of hydrogel crosslinking inside water-in-oil droplets. It is based on the difference in the merging dynamics of water-in-oil emulsions and crosslinked hydrogel microparticles in an external electric field and is compatible with various designs of microfluidic devices, types of materials and crosslinking mechanisms. Integrating a metal electrode into a microfluidic device with a flow-focusing droplet generator, we investigated how water-in-oil droplet merging occurs and then demonstrated that electrocoalescence can be used for <i>in situ</i> characterization of the polyacrylamide, polyethylene glycol diacrylate and alginate microparticles during their crosslinking. We suggest that implementation of the droplet electrocoalescence for <i>in situ</i> control of hydrogel crosslinking technique paves the way to achieve efficient, stable and reproducible synthesis of hydrogel microparticles, which is highly demanded for biomedical applications.