Optics in droplet microfluidics

Droplet-based microfluidics allows manipulating small volumes of liquids of two immiscible phases, e.g., water and oil. Effectively, this forms a small reactor in which a chemical reaction or biological process can be performed and observed over time. Microdroplets can be mixed, sorted, incubated, and analyzed. Those operations can be performed in specially designed microfluidic chips, creating a small lab-on-a-chip device. An added advantage, besides miniaturization, is that many replications of one experiment can be performed very quickly.

In our research, we aim to observe the behavior of various clinically relevant bacteria strains. More specifically, we are interested to see how they react to antibiotics. Antimicrobial resistance (AMR) is one of the most urgent threats to global health. It occurs when bacteria, viruses, fungi, and parasites transform over time and no longer respond to medicines. As a result, antibiotics or other antimicrobial drugs become unsuccessful and can no longer treat diseases effectively. World Health Organization (WHO) has recognized AMR as one of the top 10 threats for global public health.

Monitoring bacteria behavior, i.e., growth, is a challenging and time-consuming task, particularly when we need to follow thousands or millions of replicates of experiments. Optical methods combined with microfluidics allow us to address this problem. We can push droplets in front of a laser beam and analyze the light scattered on bacteria cells using specially designed chips. The intensity of scattered light is connected with a bacterial concentration in droplets, and we can follow it over time. We can monitor more than 1000 droplets per second and analyze them with dedicated software. Additionally, we can also make the system more compact and easier to use by using fiber optics.

Natalia Pacocha, Jakub Bogusławski, Michał Horka, Karol Makuch, Kamil Liżewski, Maciej Wojtkowski, and Piotr Garstecki, “High-Throughput Monitoring of Bacterial Cell Density in Nanoliter Droplets: Label-Free Detection of Unmodified Gram-Positive and Gram-Negative Bacteria,” Analytical Chemistry 93(2), 843-850 (2021).

DOI: doi.org/10.1021/acs.analchem.0c03408

Text: Jakub Bogusławski, PhD