Droplet Detection and Measurement

The development of droplet-based microfluidics has had a significant impact on chemical synthesis and synthetic biology, which even created new fields such as high-throughput single-cell analysis. It involves generating small droplets from a fluid stream, which subsequently increases the convenience of droplet transfer. Microfluidic droplets can include, isolate and allow easy manipulation of reagents, particles, cells, or multicellular organisms.

The biggest advantage of droplet-based microfluidics is the uniformity or monodispersity of the droplets. Although the droplets prepared using microfluidics are more uniform, in some cases, higher droplet uniformity is required, such as for drug encapsulation or mixing expensive drugs. The most difficult and most demanding part of the process of monitoring droplets is droplet detection and size measurement. This article will focus on the three main methods of droplet detection and size measurement: optical imaging, detection initiated by laser or similar light sources, and electrical detection.

Optical Imaging of Microfluidic Droplets

When optical imaging is used for droplet detection, the droplet is photographed in the microfluidic channel, and the image is analyzed in real-time or offline. The main advantage of using real-time processing is the ability to adjust or stabilize the droplet size during the experiment. Real-time analysis requires a lot of processing power in terms of the number of images. Studies have shown that when analyzing each droplet using a line scan camera, real-time analysis of the full image and droplet control using a feedback loop can work at frequencies up to 250 Hz, which requires a fast response pressure pump.

Figure.1 Droplets sorted using an optical imaging system. One can observe cells inside the droplets by which they were sorted. (Girault M, et al. 2017)

In addition to droplet size measurement, this method can also sort droplets containing single cells or microorganisms, with an accuracy rate of up to 90%, and there is no need to mark the cells. In addition, it has also been shown that the system can separate empty droplets from occupied droplets, and can also distinguish the different cell types contained in the droplets.

Laser-initiated Microfluidic Droplet Detection

This type of system is based on a directional light source that enters the microchannel and detects the droplets. It can detect the droplets by exciting the fluorescent substances in the droplets, detecting backward or forward scattering or even reflected light. The main advantage is a very high detection rate, which allows the detection of very high droplet generation rates. The disadvantage is the lack of a baseline and the need for calibration for each batch of experiments.

Depending on the detection system, the detector is usually placed in a different position. For fluorescence detection, the detector is usually placed at a 90-degree angle to the incident light path, while avoiding backward or forward scattered light. On the other hand, when dealing with large droplets, a detector is usually placed behind the channel to collect backscattered light. In the case of reflected or backscattered light detection, the sensor is placed in the path of the incident light.

Figure.2 A schematic of the method to embed small laser droplets within larger aqueous microfluidic droplets. (Zheng L, et al. 2018)

The system can be upgraded to detect multiple fluorescence wavelengths so that different cells or enzymes with different fluorescence responses can be detected and sorted. The most common commercial device that uses the principle of laser detection is a flow cytometer. Here, laser detection is used to detect droplets or cells, count them, measure their size, classify the cells and analyze the inside of the cells (droplets) using fluorescent signals.

Electrical Technology of Microfluidic Droplet Detection

In the case of electrical technology, multiple sensor arrays are directly integrated into the microfluidic chip. In most cases, electrical detection methods based on capacitance, electrochemistry, impedance, and microwave are used. When liquid droplets or particles pass through the contacts, it emits electrical signals, which are then recorded and analyzed. The advantage is that the content of a single drop can be detected without any chemical or physical intrusion. On the other hand, the main disadvantage is that it is difficult to integrate electrical contacts in the chip, which increases the price of the chip and hinders rapid prototyping.

However, the results show that the presence, velocity, and size of droplets can be measured using this technique, avoiding any cross-contamination. Therefore, the technology shows high potential and further work in this field is needed.

References

• Zheng L, et al. (2018). "Embedding Liquid Lasers within or Around Aqueous Microfluidic Mroplet." Lab Chip. 18: 197-205.
• Girault M, et al. (2017). "An On-chip Imaging Droplet-sorting System: A Real-time Shape Recognition Method to Screen Target Cells in Droplets with Single-cell Resolution." Scientific Reports. 7: 40072.

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