What is Droplet Microfluidics?

Author: Burcu Yaldız, University of Ege Department of Bioengineering 

  Lab-on-a-chip is a technology used for miniaturization in small-scale studies.  Fluids with a volume between microliter and picoliter in this platform are controlled and manipulated in networks of channels with dimensions 1-1000 micrometers. One of the subcategories of this technology is droplet-based microfluidics. In this platform, micrometer-sized droplets are generated and manipulated by the immiscible multiphase flows inside microchannels. This technology has been developing rapidly since it was introduced due to its fundamental features. One important feature of this platform is that it reduces the amount of critical sample volume, reagent and waste. On the other hand, it increases the portability and controllability of the assays. It also enables flexible operational, easy automation, shorter analysis times and improved sensitivity. Because of its considerable features, this technology offers a highly efficient platform for various application areas.

The working principle of droplet generation

   Droplet microfluidics can be divided into two types as flow based and electrowetting-based, actually latter is also known as digital microfluidics. In the flow system, droplets are generated by combining two immiscible phases (continuous and dispersed), such as oil and water. In digital fluidic (electrowetting) system, droplets are formed as the electric field changes the interface tension between the liquid and the hydrophobic surface. In this writing, we will focus on flow-based droplet formation technique. There are various geometries and techniques to generate droplets in the desired properties and frequencies. Droplets are produced by either active or passive method in all these geometries and techniques. The active method for generation of droplets requires an external energy input for droplet manipulation. Conversely, the passive method uses simpler devices and achieves similar results with the active method. Therefore, the passive method is more preferred for droplet formation. The T-junction, where droplets are created by the passive method, is the most common geometry used to create droplets but the other channel designs are hydrodynamic focusing and co-flow. The T- junction is highly preferred as it is easy to operate and control as well as to generate droplets with high monodispersity. In this geometry, two different fluid streams are pumped perpendicularly to each other. And as a consequence of the T-junction geometry, streams are incessantly split into dilute stream layers and finally combined. When the dispersed phase and continuous phase encounter at the junction, shear forces extend the head of the dispersed phase. Finally, spherical droplets or liquid plugs are formed due to interfacial tension. In a study, it was found that the capillary number (Ca) in the T- junction had an effect on droplet formation. The capillary number demonstrates the ratio of viscosity to the interfacial tension. According to this ratio, spherical droplets are formed at the high Ca where the viscous forces are dominant. However, liquid plugs are formed at the low Ca where the interfacial tension effects are dominant.

The Parameters affecting the success of the operation

   There are important parameters that should be considered in order to perform a successful operation with droplet chips. Surface chemistry is one of the parameters effective on the generation, manipulation, and control of droplets. The desired surface chemistry can be obtained by choosing the right material. Until now, various materials such as glass, PMMA, COC, FEP, and PDMS have been used. PDMS is one of the most preferred materials in researches as it has features such as biocompatibility, transparency, and hydrophobicity. The surface functionalities of the channels and the concentration of surfactants in the continuous phase are other important parameters. Previous studies have shown that the low concentration of surfactant in the continuous phase causes the droplets to merge. Material leaks out of the droplets in such a case. Conversely, the high concentration of surfactants causes the droplets to be divided due to low surface tension.

Application areas

   As mentioned in the introduction, the droplet-based technology is used in a wide range of areas because of its considerable advantages. This platform has been used in chemical reactions, cell biology, medical diagnostics, cell manipulation, food, medicine, and other applications. Initially, only ions and small molecules were analyzed using this platform. However, it has been proved that the analysis of macromolecules can also be done. Furthermore, new droplet platforms have been designed for molecular biology techniques such as polymerase chain reaction (PCR), enzyme-linked immunosorbent assay. Combination of PCR devices has important advantages such as portability, low reagent consumption, rapid heating/cooling, and short assay time. For enzymatic assays, it reduces the number of mixing and washing steps by holding molecules and reactions into a small volume. Single cell transcriptome and proteome analysis, or drug effect analysis at single cell level are also a promising application of this technology. Various controlled chemical reactions can be carried out with very small amounts of material using droplet-based technology as microreactors. The common problem encountered in macro-scale reactions is the low efficiency of heat and mass transfers. Micron-sized droplet reactions allow heat and mass transfer much more efficiently. In addition, working with small volumes reduces the potential hazards of working with hazardous reagents. By now, this technology has been used in studies such as titration, precipitation, hydrolysis, single cell nükleic acid and metabolism analysis, and 3D cell culture microreactor assays. This platform is also preferred to produce nano and micro-sized particles. Reaction kinetics and thermodynamic parameters are effectively controlled during particle synthesis in this platform. Thus, particles with adjustable size and crystal structure can be synthesized. Moreover, since droplets can be controlled, the efficiency of production and particle monodispersity are quite high compared to conventional methods.

Disadvantages

   The droplet-based microsystem is successfully applied to basic problems in various application areas. However, there are still some problems, limiting the use of this technology. One of these problems is related to obtain maximum information from droplets in the volume of picoliter during the analysis. In such a case, any detection method which has high sensitivity needs to be integrated into the device.

   Another problem is the liquid composition. The liquid composition may comprise complex components such as additives, especially in the fields of cosmetics and food. Such impurities can cause undesirable changes in surface tension, interfacial stability, and rheological behavior. Although there are  problems that need to be overcome, successful results have been obtained from many studies using droplet microfluidics, since it had emerged. Considering its advantages and achievements, it is thought that the use of this technology will be indispensable for studies in the field of biology and chemistry.

References

1. Chou, W., et.al. (2015), doi: 10.3390/mi6091249
2. Shang L., et.al. (2017), doi: 10.1021/acs.chemrev.6b00848
3. De Menech, M., et.al. (2008), doi: 10.1017/S002211200700910X
4. Mashaghi, S., et.al. (2016), doi: 10.1016/j.trac.2016.05.019
5. Solvas, X.C., et.al. (2011), doi:  10.1039/C0CC02474K
6. Baret, J.C., et.al. (2009), doi: 10.1021/la9000472
7. Mazutis, L., et.al. (2012), doi:  10.1039/C2LC40121E
8. Rosenfeld, L., et.al. (2014), doi: 10.1007/s10404-013-1310-x