We describe a one-step passive strategy to create concentration gradients in static droplet arrays. The technique exploits controlled exchange of materials between moving plugs and stationary drops. The concentration of soluble reagents can be varied from drop-to-drop in the presence of other soluble reagents or insoluble materials (e.g.cells) at well-defined time points.

We report a simple technique to separate plasma from blood in a flowing immiscible plug. We investigate the effect of various control parameters such as blood dilution, injection flow rate, observation time and fluid properties on plasma separation. We find that the technique works best for diluted blood samples at low plug velocities and long observation times. We postulate that the main mechanism responsible for efficient separation is the sedimentation of blood cells in the plug and their subsequent collection by the moving plug causing a significant accumulation of cells at the rear of the plug. We discuss the time scales determining the sedimentation, advection and collection of a blood cell in the immiscible plug and propose a phase diagram that is able to predict the operating space for effective plasma separation. We demonstrate that the technique allows for the extraction of more than 60% of the plasma by volume from 1 μL of diluted blood. We show the practical significance of this method by compartmentalizing the separated plasma into discrete microfluidic droplets and detecting cholesterol. This technique features low consumption of blood (nL-scale) and low shear rate (∼1 s⁻¹). It is inexpensive, easy to use, and has the potential to be developed as an efficient point-of-care device for blood diagnostics in resource-poor environments. More advanced applications could also be envisioned by integrating our plasma separation method into existing microfluidic drop manipulation techniques.

We developed a droplet-based microfluidic screening system with an on-chip sampling probe integrating multi-channels for sample introduction, reagent merging and nanolitre-scale droplet generation, and a slotted-vial array sample presenting system. The present system was applied in protein crystallization conditions screening with an ultra-high sampling throughput up to 6000 h⁻¹ for different samples.

Multiwell plate and pipette systems have revolutionized modern biological analysis; however, they have disadvantages because testing in the submicroliter range is challenging, and increasing the number of samples is expensive. We propose a new microfluidic methodology that delivers the functionality of multiwell plates and pipettes at the nanoliter scale by utilizing drop coalescence and confinement-guided breakup in microfluidic parking networks (MPNs). Highly monodisperse arrays of drops obtained using a hydrodynamic self-rectification process are parked at prescribed locations in the device, and our method allows subsequent drop manipulations such as fine-gradation dilutions, reactant addition, and fluid replacement while retaining microparticles contained in the sample. Our devices operate in a quasistatic regime where drop shapes are determined primarily by the channel geometry. Thus, the behavior of parked drops is insensitive to flow conditions. This insensitivity enables highly parallelized manipulation of drop arrays of different composition, without a need for fine-tuning the flow conditions and other system parameters. We also find that drop coalescence can be switched off above a critical capillary number, enabling individual addressability of drops in complex MPNs. The platform demonstrated here is a promising candidate for conducting multistep biological assays in a highly multiplexed manner, using thousands of submicroliter samples.