In this study, we introduced an evaporation-driven method to fabricate Janus droplets based on phase-separation mechanism of aqueous two-phase system (ATPS). Specifically, phase transition of ATPS were driven by changes in the concentration of solutes at a constant temperature. Without any additional step, two-immiscible phases were induced to form at room temperature by evaporation of water. Taking advantage of this property of ATPS, we firstly generated single-phase droplets and then converted them into Janus ones by evaporation. Consequently, the single-phase droplets separated into two immiscible phases with clear interfaces.
In a typical experiment, a flow-focusing device was used to produce the homogeneous emulsion drops. An aqueous solution containing PEG and dextran was injected into the flow-focusing device as the dispersed phase, while FC-40 with 5% PFPE-PEG-PFPE surfactant was injected as the dispersed phase. The resultant droplets were single-phase initially due to the low concentration of PEG and dextran. Afterwards, we collected the emulsions on a hydrophobic plate and exposed them to air. Water inside emulsions evaporated through FC-40, resulting in the increase of the concentration of PEG and dextran. Consequently, the homogeneous droplets separated into two immiscible phases, and Janus droplets formed. The size of the Janus droplets were tuned from 4.4μm to 103μm, by adjusting the height of microfluidic device and the flow rate ratio of inner phase to outer phase. Volume ratio between different compartments was changed from 0.38 to 1.43 by altering the initial concentration ratio of PEG/dextran from 0.1 to 4.5.
This ATPS-based approach enabled the activity of encapsulated bio-ingredients to be preserved at a relatively high level, since phase separation was triggered under mild conditions by evaporating water. To further confirm the biocompatibility of the proposed method, we introduced horseradish peroxidase (HRP) and cells into the inner phase respectively. After droplet generation and phase separation, 71% of the relative activity of HRP was preserved, and 82.8% of the cells were still alive inside the Janus droplets. In comparison, the relative activity of HRP dropped to 51% and all the cells became dead with conventional organic-solvent-based phase-separation.
In conclusion, we successfully fabricated Janus droplets by the ATPS-based phase-separation method. The resultant Janus droplets were fabricated with clear interfaces and highly controlled volume ratio of the two compartments. The activity of encapsulated bio-gradients was highly preserved. Consequently, the proposed method potentially extended the applications of Janus droplets for encapsulating delicate biomolecules.
(This abstract is based on our published paper: H. Yuan, Q. Ma, Y. Song, Matthew Y. H. Tang, Y. K. Chan, H. C. Shum, Phase separation-induced formation of Janus droplets based on aqueous two-phase systems, Macromol. Chem. Phys., 2017, 218, 1600422)