Formation Dynamics of Cell-Loading Alginate Droplets in the Microtube Dripping and Cryo-Cross-Linking Process for Cell-Entrapped Cryogel Beads as the Biocatalysts toward Phenyllactic Acid Biosynthesis

The dynamic formation of alginate droplets containing Lactobacillus buchneri cells by the microtube dripping method was investigated experimentally. Supermacroporous cell-entrapped cryogel beads were then fabricated via cryo-cross-linking of these cell-loaded droplets under freezing conditions and used as the biocatalysts for the synthesis of phenyllactic acid by bioconversion. The evolution of the interface, the dripping velocity, the microthread length, and the droplet diameter during the formation process of cell-loaded droplets were studied by the high-speed imaging method together with computational fluid dynamics modeling. The results showed that the formation and detachment of cell-loaded droplets were controlled by the interaction of the gravity, the surface tension, and the viscous drag forces. With the increase of the microtube size from 0.5 to 2.2 mm, the length of the microthread, the dripping velocity and the mean diameter of the cell-loaded droplets increased by 5.9, 3.7, and 1.7 times, respectively. There is a good agreement between the simulation and experimental results regarding the dynamic performance of the formation and detachment of cell-loaded droplets. The cell-entrapped cryogel beads prepared by the microtube dripping and cryo-cross-linking methods have narrow diameter distributions, macroporous structures, and high porosity of 88.1%. These beads had remarkable biocatalysis ability for the synthesis of phenyllactic acid by conversion of the substrate phenylpyruvic acid. Compared with the free cells, cell-entrapped cryogel beads had the high biotransformation ratio of about 95.9% after recycled for two times, indicating that these beads could be employed as a new class of biocatalysts for the production of phenyllactic acid and thus have potential application in bioconversion processes.