All-aqueous multiphase microfluidics

Abstract

Immiscible aqueous phases, formed by dissolving incompatible solutes in water, have been used in green chemical synthesis, molecular extraction and mimicking of cellular cytoplasm. Recently, a microfluidic approach has been introduced to generate all-aqueous emulsions and jets based on these immiscible aqueous phases; due to their biocompatibility, these all-aqueous structures have shown great promises as templates for fabricating biomaterials. The physico-chemical nature of interfaces between two immiscible aqueous phases leads to unique interfacial properties, such as an ultra-low interfacial tension. Strategies to manipulate components and direct their assembly at these interfaces needs to be explored. In this paper, we review progress on the topic over the past few years, with a focus on the fabrication and stabilization of all-aqueous structures in a multiphase microfluidic platform. We also discuss future efforts needed from the perspectives of fluidic physics, materials engineering, and biology for fulfilling potential applications ranging from materials fabrication to biomedical engineering.

Figure 1

Microscopic images of w/w jets with induced structures in microfluidic channels. A straight w/w jet is perturbed by hydrodynamic vibration at the frequency of (a) 0 Hz, (b) 7 Hz and (c) 10 Hz. Reprinted with permission from Sauret et al., Lab Chip 12, 3380, (2012). Copyright 2012 The Royal Society of Chemistry. A folded viscous w/w jet is manipulated by electrical charging at applied DC voltages of (d) 0 V, (e) 20 V, and (f) 23 V. The positions of the cathode and the anode are illustrated by the symbols (+) and (−) in (f), respectively. Scale bar is 200 μm. Reprinted with permission from Song et al., Chem. Commun. 49, 1726 (2013). Copyright 2013 the Royal Society of Chemistry.

Figure 2

Forced breakup of a w/w jet induced by hydrodynamic perturbation. (a) and (b) Breakup behaviors of a w/w jet triggered by forced oscillations at different perturbation frequency, f. Within an optimal range of frequency, monodisperse w/w droplets are generated, without satellite droplets, as shown by the blue dots. Scale bar is 200 μm. Reprinted with permission from Sauret and Shum, Appl. Phys. Lett. 100, 154106 (2012). Copyright 2012 American Institute of Physics. (c) Perturbation amplitude, δr, increases with the intensity of vibration, which is regulated by the DC voltages applied to a piezoelectric vibrator. Reprinted with permission from Geschiere et al., Biomicrofluidics 6, 022007 (2012). Copyright 2012 American Institute of Physics.

Figure 3

Formation of a w/w emulsion induced by electro-hydrodynamic chopping. The width of channel is 200 μm. Reprinted with permission from Song et al., J. Chromatogr. A 1162, 180 (2007). Copyright 2007 Elsevier.

Figure 4

(a) Schematic of the formation of aqueous droplets in the all-aqueous electrospray approach; (b) optical microscopic images of monodisperse aqueous droplets produced by all-aqueous electrospray; (c) microscopic images of all-aqueous emulsions with a core-shell structure; droplets are collected on a glass slide. Scale bar is 200 μm.

Figure 5

Interfacial aggregation and assembly of macromolecules are affected by the local compositions of the aqueous phases. (a) Dextran-rich and PEG-rich phases selectively approach the different domains of a budding liposome upon extraction of water. Reprinted with permission from Cans et al., J. Am. Chem. Soc. 130, 7400 (2008). Copyright 2008 ACS Publications. (b) and (c) Conjunctions of the copolymers PEG-PCL and dextran-PCL aggregates at the interface of dextran-in-PEG emulsion, yielding asymmetrical vesicles. Scale bar is 10 μm. Reprinted with permission from Zhang et al., J.Controlled Release 147, 413 (2010). Copyright 2010 Elsevier.