Characterization of Core-Shell Alginate Capsules

A new droplets millifluidic/inverse gelation based process was used to produce core-shell alginate milli-capsules. Water-in-oil (W/O) emulsion dispersed phase containing Ca2+ ions was directly injected into a continuous alginate phase to generate a secondary W/O/W emulsion. Due to the cross-linking of alginate molecules by Ca2+ ions release, core-shell milli-capsules were formed with a very high oil loading. The influence of the curing time and of the storage conditions on capsules physico-chemical properties were investigated. It was first found as expected that alginate membrane thickness increased with curing time in the collecting bath. However, a plateau was reached for the higher curing times, in close relation with previous observations (Martins, Poncelet, Marquis, Davy, & Renard, 2017b) that an external oil layer surrounded the surface of W/O emulsion drops that acted as a barrier and hindered the release of aqueous CaCl2 droplets during curing time. Compression experiments on individual capsules revealed that alginate membrane thickness was inversely related to its mechanical properties, i.e. the thicker membrane, the lower surface Young modulus. Surface Young modulus ranged from 61 to 26 N/m at curing times of 3 and 45 min, respectively. This result was explained in terms of enhanced swelling properties of alginate membrane with curing time or storage conditions. Drying capsules led to much more resistant membranes due to the loss of water. Oil loading of 80 wt% was obtained for dry capsules whatever the conditions used.