Merging drops in a Teflon tube, and transferring fluid between them, illustrated by protein crystallization and drug screening

Abstract

The ability to manipulate drops with small volumes has many practical applications. Current microfluidic devices generally exploit channel geometry and/or active external equipment to control drops. Here we use a Teflon tube attached to a syringe pump and exploit the properties of interfaces between three immiscible liquids to create particular fluidic architectures. We then go on to merge any number of drops (with volumes of micro- to nano-liters) at predefined points in time and space in the tube; for example, 51 drops were merged in a defined order to yield one large drop. Using a different architecture, specified amounts of fluid were transferred between 2 nl drops at specified rates; for example, 2.5 pl aliquots were transferred (at rates of ~500 fl s(-1)) between two drops through inter-connecting nano-channels (width ~40 nm). One proof-of-principle experiment involved screening conditions required to crystallize a protein (using a concentration gradient created using such nano-channels). Another demonstrated biocompatibility; drugs were mixed with human cells grown in suspension or on surfaces, and the treated cells responded like those grown conventionally. Although most experiments were performed manually, moderate high-throughput potential was demonstrated by mixing ~1000 different pairs of 50 nl drops in ~15 min using a robot. We suggest this reusable, low-cost, and versatile methodology could facilitate the introduction of microfluidics into workflows of many experimental laboratories.

Biocompatibility of fluids for multiphase drops-in-drops microfluidics

“…Having established which combination of fluids to use (i.e., HFE-7500, 5-cSt silicone oil + 0.25% Abil EM180, medium containing cells), we performed a proof-of-concept drug screen (Feuerborn et al 2015; supplementary information). DRB (6-dichloro-1-β-D-ribofuranosyl-benzimidazole) is a general transcriptional inhibitor, and – when a drop containing it is merged with another containing Jurkats as in Fig.…”

“…M: 100-, 200-, 300-bp markers. This Figure was redrawn using data from supplementary information in reference (Feuerborn et al 2015) …”

“…An alternative microfluidic method utilising a Teflon tube and fluid mechanics, rather than the more common approach of relying on the geometry of micro-scale channel networks, has recently been developed (Feuerborn et al 2015; Walsh et al 2016). This method exploits the interfacial tension between three or more immiscible liquids to create specific fluidic architectures.…”

“…(iv) Internal vortices within the merged drop mix contents. This Figure was adapted from (Feuerborn et al 2015) …”

“…1. The time needed for the second drop to catch up the first one depends on the initial spacing between drops (established when the two drops first enter the tube) and their relative motion thereafter (Feuerborn et al 2015). This relative velocity is influenced by the thickness of the fluidic film surrounding the drops, which varies with Capillary number (Ca  = ηU/γ , where η  = carrier fluid viscosity and U  = average velocity) (Bico and Quéré 2000; Bretherton 1961).…”