Droplet electrocoalescence as a robust tool for in situ examination of hydrogel microparticles in microfluidic...

Nowadays hydrogel microparticles find numerous applications in material science and biological engineering such as drug delivery systems, cell carriers, etc. 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 in situ 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 in situ characterization of the polyacrylamide, polyethylene glycol diacrylate and alginate microparticles during their crosslinking. We suggest that implementation of the droplet electrocoalescence for in situ 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.