High-throughput millifluidics for monodisperse alginate milli-capsules filled with emulsions

Structured multiphase capsules based on biopolymers promise broad applications in life science related research and industry. For making these complex structures with high accuracy, various microfluidic emulsification techniques were used. However, the limited throughput of these methods remains a challenge. Within this study, millimetre-sized core-shell capsules, consisting of a liquid water-in-oil emulsion core surrounded by a gelled alginate shell, were produced by co-extrusion of two immiscible phases through coaxial capillaries. The inner capillary tip was placed upstream of the outer capillary tip which allowed high production throughputs. Droplet generation in a quiescent ambient air followed by external shell gelation yielded large, monodisperse capsules of around 2 mm in diameter with the capsule filling degree above 70 % and the encapsulation efficiency of 90–100 %. The throughput, capsule size, and dripping dynamics were controlled by the orifice size of outer capillary, fluid flow rates and alginate concentration. The critical Weber number and the breakup lengths of compound jet were significantly affected by the viscosity of alginate solution. The total flowrate in dripping regime was up to 240 ml/h using 1 wt% alginate solution and the coefficient of variation of capsule sizes was less than 5 %. Further experiments elucidated the influence of the gelation step on the shell and core volume. The developed model predicted the final capsule size and the loading capacity of capsules with an accuracy above 97 %. The findings of this study confirm the ability of in-air millifluidics to generate complex structured particles with precisely controlled attributes at high throughputs.