A portable droplet microfluidic device for cortisol measurements using a competitive heterogeneous assay.

Point-of-care monitoring of chemical biomarkers in real-time holds great potential in rapid disease diagnostics and precision medicine. However, monitoring is still rare in practice, as the measurement of biomarkers often requires time consuming and labour intensive assay procedures such as enzyme linked immunosorbent assay (ELISA), which pose a challenge to an autonomous point-of-care device. This paper describes a prototype device capable of performing ELISA autonomously and repeatedly in a high frequency using droplet microfluidics. Driven by a specially designed peristaltic pump, the device can collect liquid samples from a reservoir, produce trains of droplets, complete magnetic bead based ELISA protocols and provide readouts with colourimetric measurement. Here, cortisol was chosen as a target analyte as its concentration in the human body varies on a circadian rhythm which may be perturbed by disease. The prototype device draws in and analyses 350 nL of the sample containing free bioactive cortisol every 10 seconds, with a sample-to-signal time of 10 minutes, and measures favourably in the analytical range of 3.175-100 ng ml-1, with reliably lower variability compared with the well plate based assay. As most ELISA type assays share similar procedures, we envisage that this approach could form a platform technology for measurement or even continuous monitoring of biomarkers in biological fluids at the point-of-care.

A Droplet Microfluidic-Based Sensor for Simultaneous in Situ Monitoring of Nitrate and Nitrite in Natural Waters.

Microfluidic-based chemical sensors take laboratory analytical protocols and miniaturize them into field-deployable systems for in situ monitoring of water chemistry. Here, we present a prototype nitrate/nitrite sensor based on droplet microfluidics that in contrast to standard (continuous phase) microfluidic sensors, treats water samples as discrete droplets contained within a flow of oil. The new sensor device can quantify the concentrations of nitrate and nitrite within each droplet and provides high measurement frequency and low fluid consumption. Reagent consumption is at a rate of 2.8 mL/day when measuring every ten seconds, orders of magnitude more efficient than those of the current state-of-the-art sensors. The sensor's capabilities were demonstrated during a three-week deployment in a tidal river. The accurate and high frequency data (6% error relative to spot samples, measuring at 0.1 Hz) elucidated the influence of tidal variation, rain events, diurnal effects, and anthropogenic input on concentrations at the deployment site. This droplet microfluidic-based sensor is suitable for a wide range of applications such as monitoring of rivers, lakes, coastal waters, and industrial effluents.

Nitrate measurement in droplet flow: gas-mediated crosstalk and correction.

In droplet microfluidics, droplets have traditionally been considered discrete self-contained reaction chambers, however recent work has shown that dissolved solutes can transfer into the oil phase and migrate into neighbouring droplets under certain conditions. The majority of reports on such inter-droplet "crosstalk" have focused on surfactant-driven mechanisms, such as transport within micelles. While trialling a droplet-based system for quantifying nitrate in water, we encountered crosstalk driven by a very different mechanism: conversion of the analyte to a gaseous intermediate which subsequently diffused between droplets. Importantly we found that the crosstalk occurred predictably, could be experimentally quantified, and measurements rationally post-corrected. This showed that droplet microfluidic systems susceptible to crosstalk such as this can nonetheless be used for quantitative analysis.

Phased peristaltic micropumping for continuous sampling and hardcoded droplet generation.

Droplet microfluidics has recently emerged as a new engineering tool for biochemical analysis of small sample volumes. Droplet generation is most commonly achieved by introducing aqueous and oil phases into a T-junction or a flow focusing channel geometry. This method produces droplets that are sensitive to changes in flow conditions and fluid composition. Here, we present an alternative approach using a simple peristaltic micropump to deliver the aqueous and oil phases in antiphase pulses resulting in a robust "chopping"-like method of droplet generation. This method offers controllable droplet dynamics, with droplet volumes solely determined by the pump design, and is insensitive to liquid properties and flow rates. Importantly, sequences of droplets with controlled composition can be hardcoded into the pump, allowing chemical operations such as titrations and dilutions to be easily achieved. The push-pull pump is compact and can continuously collect samples, generating droplets close to the sampling site and with short stabilisation time. We envisage that this robust droplet generation method is highly suited for continuous in situ sampling and chemical measurement, allowing droplet microfluidics to step out of the lab and into field-deployable applications.