Fig. 2. Synthesis of microcapsules by a water–gas microfluidic technique. Microdroplets bearing the anionic biopolymer, alginate as well as payloads of interest are generated by flowing the aqueous solution through a capillary. Pulses of gas (nitrogen or air) are sent through the annular region around the capillary. The frequency of the pulses is controlled by the function generator. Uniform aqueous droplets emerge from the tip of the capillary, and these are introduced into an aqueous reservoir solution containing the cationic biopolymer chitosan as well as the salt of a divalent cation (Ca²⁺). The droplets are thereby converted into microcapsules, with the shell being formed by electrostatic complexation between the anionic alginate and the cationic chitosan while the core is further strengthened by the Ca²⁺-induced cross-linking of alginate chains.