Inverse micelle mediated calcium chloride transportation for facile alginate gelation in microdroplets

Block copolymer fluorosurfactants are frequently utilized to stabilize water-oil interfaces in droplet microfluidics, enabling parallel and compartmentalized biochemical reactions within individual droplets. Surfactants are able to self-assemble into inverse micelles with the concentration exceeding the critical micelle concentration (CMC), which has been identified as the main reason causing cross-contamination among droplets. This study explored the possibility to utilize the inverse micelles for passive cargo delivery from the fluorocarbon oil phase into the aqueous droplet interior, which has rarely been studied previously. We presented a novel strategy to load the molecular cargo, in this case calcium, into the inverse micelles and subsequently transport it into the water-in-oil droplets. Specifically, calcium chloride was firstly solvated with methanol and well-dispersed in fluorocarbon oil containing fluorosurfactants. Upon interaction with droplets containing un-crosslinked alginate stabilized by the same kind of fluorosurfactant, calcium ions were able to transport from inverse micelles through the water-oil interface and ultimately to the aqueous droplets, as observed by the successful production of alginate beads through ionic crosslinking of alginate in the microdroplets. The cytotoxicity of methanol was also validated to be minimal in two tested cell lines, suggesting the potential for broad adoption of alginate microbeads produced by the proposed approach.