Core-shell polymer microcapsules are well known for their biomedical applications as drug carriers when they are filled with drugs and gas-filled microcapsules that can be used as ultrasound contrast agents. The properties of microcapsules are strongly dependent on their size (distribution); therefore, equipment that allows the preparation of small and well-defined microcapsules is of great practical relevance. In this study, we made polylactide microcapsules with a packed-bed premix emulsification system that previously gave good results for regular emulsions. Here, we tested it for applicability to a system in which droplets shrank and solidified to obtain capsules. The packed-bed column was loaded with glass beads of different sizes (30-90 μm) at various bed heights (2-20 mm), and coarse emulsions consisting of the polymer, a solvent, and a nonsolvent were pushed repeatedly through this system at selected applied pressures (1-4 bar). The obtained transmembrane fluxes (100-1000 m3 m-2 h-1) were much higher than those recorded for other membrane emulsification techniques. The average size of the obtained microcapsules ranged between 2 and 8 μm, with an average span of about 1; interestingly, the capsules were 2-10 times smaller than the interstitial voids of the beds. The droplets were larger when we used thicker beds and larger glass beads, and these effect correlated with the pore Reynolds number (Rep). Two breakup mechanisms were identified: spontaneous droplet snap-off dominated the system at low Reps, and localized shear forces dominated the system at higher Rep.
- biomedical applications
- drug-delivery systems