Biocompatible surfactants for water-in-fluorocarbon emulsions

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Original entry: Xu Zhang, APPHY 225, Fall 2009

Reference

Biocompatible surfactants for water-in-fluorocarbon emulsions C. Holtze, A. C. Rowat, J. J. Agresti, J. B. Hutchison, F. E. Angilè, C. H. J. Schmitz, S. Koester, H. Duan, K. J. Humphry, R. A. Scanga, J. S. Johnson, D. Pisignano and D. A. Weitz, Lab Chip 8,1632-1639 (2008).

Keywords

microfluidics, fluorosurfactant, IVT (in vitro translation), PDMS

Summary

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In this paper, the development of a novel class of fluorosurfactants is introduced as a powerful tool to stabilize the aqueous droplets in fluorocarbon oil while maintaining compatibility with biological systems. This new surfactant is synthesized by coupling oligomeric perfluorinated polyethers(PFPE) with polyethyleneglycol (PEG). Oligomeric PFPEs are soluble in fluorocarbon oils and are sufficiently large to provide good steric stabilization of the emulsion. The PEG moieties are soluble in water and prevent the adsorption of biological compounds to interfaces. For best emulsion stability and biocompatability, it is achieved to have a dense PFPE brush on the outside, and a dense PEG brush on the inner side of the interface by tuning surfactant size and geometry (Fig.2). The surfactant system is chosen by coupling product of 600 g <math>mol^{-1}</math> PEG and 6000g <math>mol^{-1}</math> PFPE. Emusions stabilized with this surfactants are found to remain stable for weeks.

This new surfactant is found to be able to stabilize water-in-fluorocarbon oil emulsions during all necessary steps of a drop-based experiment including drop formation, incubation, and reinjection into a second microfluidic device.

1. Drop formation: By altering the relative flow rates of the oil to the aqueous phases, the volume fraction of 80% is achieved and no coalescence is observed. Increasing the oil flow rate decreases drop size (Fig.3c). Monodisperse drops can be generated and stabilized at rates of up to 30kHz.

2.Incubation: After a three-day incubation on-chip in the channel of a PDMS microfluidic device, compressed drops remain stable and do not exhibit coalescence (Fig.4). Despite the drops being closed packed, they remain monodisperse, as shown by their hexagonal packing.

3.Reinjection: The drops are flown through a flow-focusing device and they become more widely spaced as fluorocarbon oil flows through feed-channels on either side of the reinjection channel(Fig.5).The drops again remain intact and monodisperse. This confirms that the surfactant also provides stability under conditions of shear.

To test for biocompatability of the drop interface, in vitro translation(IVT) of plasmid DNA encoding the enzyme <math>\beta</math>-galactosidase in drops is performed. Moreover, the compatibility with cell is also tested by encapsulating an aqueous dispersion of yeast cells in growth medium in drops.

Soft Matter Connection

This novel surfactant makes a breakthrough in biological assays. The conventional surfactant could not play the role of stabilizing the drops and providing a biological inert interior surface for the water drops simultaneously. Surfactants with short fluorotelomer-tails have been used, but do not provide sufficient long-term emulsion stability. Fluorosurfactants with longer fluorocarbon tails offer long-term stabilization but have ionic headgroups that may interact with oppositely charged biomolecules like DNA, RNA causing them to lose their activity. There were no fluorosurfactants with non-ionic headgroups before this.

The introduction of this novel non-ionic fluorosurfactant gives people new ideas on synthesis and new hope for a broad range of high-throughput, drop-based applications.