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Abstract:
Emerging microfluidic technology has introduced new precision controls over reaction conditions. Owing to the small amount of reagents, microfluidics significantly lowers the cost of carrying a single reaction. Moreover, in two-phase systems, each part of a dispersed fluid can be treated as an independent chemical reactor with a volume from femtoliters to microliters, increasing the throughput. In this work, we propose a microfluidic device that provides continuous recirculation of droplets in a closed loop, maintaining low consumption of oil phase, no cross-contamination, stabilized temperature, a constant condition of gas exchange, dynamic feedback control on droplet volume, and a real-time optical characterization of bacterial growth in a droplet. The channels (tubing) and junction cubes are made of Teflon fluorinated ethylene propylene (FEP) to ensure non-wetting conditions and to prevent the formation of biofilm, which is particularly crucial for biological experiments. We show the design and operation of a novel microfluidic loop with the circular motion of microdroplet reactors monitored with optical sensors and precision temperature controls. We have employed the proposed system for long term monitoring of bacterial growth during the antibiotic chloramphenicol treatment. The proposed system can find applications in a broad field of biomedical diagnostics and therapy.

Keywords:
microfluidic loop, bacteria cultures, screening, antibiotic treatment, Escherichia coli

Abstract:
Herein, we describe a novel method for the assessment of droplet viscosity moving inside microfluidic channels. The method allows for the monitoring of the rate of the continuous growth of bacterial culture. It is based on the analysis of the hydrodynamic resistance of a droplet that is present in a microfluidic channel, which affects its motion. As a result, we were able to observe and quantify the change in the viscosity of the dispersed phase that is caused by the increasing population of interacting bacteria inside a size-limited system. The technique allows for finding the correlation between the viscosity of the medium with a bacterial culture and its optical density. These features, together with the high precision of the measurement, make our viscometer a promising tool for various experiments in the field of analytical chemistry and microbiology, where the rigorous control of the conditions of the reaction and the monitoring of the size of bacterial culture are vital.

Keywords:
droplet microfluidics, cell growth, viscosity, rheology, Escherichia coli

Abstract:
Blood is the richest source of diagnostic information. The growing interest in point-of-care analytics prompted several attempts to extract plasma from whole blood in simple diagnostic devices. The simplest method of separation is sedimentation. Here we show the first microfluidic system that uses sedimentation to extract plasma from undiluted blood and integrates execution of liquid assays on the extracted material. We present a microfluidic chip that accepts a small sample (27 μL) of whole blood, separates up to 6 μL of plasma, and uses metered volumes of plasma and of reagent (2-chloro-4-nitrophenyl-α-maltotrioside, CNP-G3) for a liquid enzymatic assay. With a custom designed channel, the system separates blood by sedimentation within few minutes of accepting the sample, mixes it with the reagent, and quantifies spectrophotometrically the product of the enzymatic reaction. As a model demonstration, we show a quantitative enzymatic α-amylase assay that is routinely used in diagnosis of pancreas diseases. The paper reports the design and characterization of the microfluidic device and the results of tests on clinically collected blood samples. The results obtained with the microfluidic system compare well to a reference bench-top analyzer.