Pascaline Mary

High-throughput injection with microfluidics using picoinjectors

Adding reagents to drops is one of the most important functions in droplet-based microfluidic systems; however, a robust technique to accomplish this does not exist. Here, we introduce the picoinjector, a robust device to add controlled volumes of reagent using electro-microfluidics at kilohertz rates. It can also perform multiple injections for serial and combinatorial additions.

Microfluidic Droplet-Based Liquid−Liquid Extraction

We study microfluidic systems in which mass exchanges take place between moving water droplets, formed on-chip, and an external phase (octanol). Here, no chemical reaction takes place, and the mass exchanges are driven by a contrast in chemical potential between the dispersed and continuous phases. We analyze the case where the microfluidic droplets, occupying the entire width of the channel, extract a solute-fluorescein-from the external phase (extraction) and the opposite case, where droplets reject a solute-rhodamine-into the external phase (purification). Four flow configurations are investigated, based on straight or zigzag microchannels. Additionally to the experimental work, we performed two-dimensional numerical simulations. In the experiments, we analyze the influence of different parameters on the process (channel dimensions, fluid viscosities, flow rates, drop size, droplet spacing, ...). Several regimes are singled out. In agreement with the mass transfer theory of Young et al. (Young, W.; Pumir, A.; Pomeau, Y. Phys. Fluids A 1989, 1, 462), we find that, after a short transient, the amount of matter transferred across the droplet interface grows as the square root of time and the time it takes for the transfer process to be completed decreases as Pe-2/3, where Pe is the Peclet number based on droplet velocity and radius. The numerical simulation is found in excellent consistency with the experiment. In practice, the transfer time ranges between a fraction and a few seconds, which is much faster than conventional systems.

Monodisperse Colloids Synthesized with Nanofluidic Technology

Limitations in the methods employed to generate micrometric colloidal droplets hinder the emergence of key applications in the fields of material science and drug delivery. Through the use of dedicated nanofluidic devices and by taking advantage of an original physical effect called capillary focusing, we could circumvent some of these limitations. The nanofluidic (i.e., submicrometric) devices introduced herein are made of soft materials, and their fabrication relies upon rapid technologies. The objects that we have generated are simple droplets, multiple droplets, particles, and Janus particles whose sizes lie between 900 nm and 3 microm (i.e., within the colloidal range). Colloidal droplets have been assembled on-chip into clusters and crystals, yielding discrete diffraction patterns. We illustrate potential applications in the field of drug delivery by demonstrating the ability of multiple droplets to be phagocytosed by murine macrophage-type cells.

Analysis of gene expression at the single-cell level using microdroplet-based microfluidic technology

In the present work, we have measured the messenger RNA expression of specific genes both from total RNA and cells encapsulated in droplets. The microfluidic chip introduced includes the following functionalities: RNA∕cell encapsulation, lysis, reverse transcription and real-time polymerase chain reaction. We have shown that simplex and duplex gene expression measurements can be carried out over a population of 100 purified RNA samples encapsulated simultaneously in 2 nl droplets in less than 2 h. An analysis of 100 samples containing one to three cells has shown excellent consistency with standard techniques regarding average values. The cell-to-cell distributions of the E-cadherin expression suggest fluctuations on the order of 80% in the number of transcripts, which is highly consistent with the general findings from the literature. A mathematical model has also been introduced to strengthen the interpretation of our results. The present work paves the way for the systematic acquisition of such information in biological and biomedical studies.

DNA sequence analysis with droplet-based microfluidics

Droplet-based microfluidic techniques can form and process micrometer scale droplets at thousands per second. Each droplet can house an individual biochemical reaction, allowing millions of reactions to be performed in minutes with small amounts of total reagent. This versatile approach has been used for engineering enzymes, quantifying concentrations of DNA in solution, and screening protein crystallization conditions. Here, we use it to read the sequences of DNA molecules with a FRET-based assay. Using probes of different sequences, we interrogate a target DNA molecule for polymorphisms. With a larger probe set, additional polymorphisms can be interrogated as well as targets of arbitrary sequence.

On-chip background noise reduction for cell-based assays in droplets

Droplet-based microfluidics provides an excellent platform for high-throughput biological assays. Each droplet serves as a reaction vessel with a volume as small as a few picolitres. This is an important technology for a high variety of applications. However this technology is restricted to homogeneous assays as it is very difficult to wash reagents from the reaction vessel. To help overcome this limitation, we introduce a method to effectively dilute the content of a droplet while retaining the high throughput. We use electrocoalescence to merge the parent drop with a much larger drop containing only solvent, thereby increasing the volume of the drop by as much as a factor of 14. Three T-junctions then break the larger drop into eight smaller droplets. This dilution and break-up process can be repeated, thus leading to many drops comparable in size to the original one but with much lower concentration of reagents. The system is fully integrated in a PDMS device. To demonstrate its power, we perform a labelling reaction at the surface of the cells by coencapsulating yeast cells expressing S6 peptide tags with the enzyme SFP synthase and the fluorescent substrate CoA 488. After reaction, the droplets are diluted twice using the system and the intensity of their fluorescence is measured. This noise reduction method enables us to more easily distinguish the fluorescence at the surface of a single cell from the fluorescent background inside the droplet.

Controlling droplet incubation using close-packed plug flow

Controlling droplet incubation is critical for droplet-based microfluidic applications; however, current techniques are either of limited precision or place strict limits on the incubation times that can be achieved. Here, we present a simple technique to control incubation time by exploiting close-packed plug flow. In contrast to other techniques, this technique is applicable to very short and very long incubation times.