Droplets Encapsulation for Biological Applications

With the in-depth study of the complexity of cell and molecular biology, researchers have found that many heterogeneities have been ignored in traditional batch analysis and experiments. Exploring these nuances without losing the speed and accuracy provided by traditional experiments has become an ideal application for microfluidics, especially droplet packaging.

Droplet-based microfluidics can be defined as an emulsion of micrometer-sized droplets produced in a microfluidic device. This feature makes mixing and heat transfer faster, thereby speeding up the reaction time. In addition, the droplets are isolated monodisperse chambers that act like reproducible microreactors to generate high throughput.

Commonly Used Droplet Packaging Technology

The most common form of encapsulation is to dilute the sample in solution into a dispersed phase.

Encapsulation during droplet formation

The most commonly used method in single-cell droplet encapsulation is to dilute the cells into the dispersed phase of the droplet before the droplet is formed, which is to dilute the cell suspension sufficiently so that it is impossible for two cells to be in the same droplet. This method is simple but produces many empty droplets. The probability of having one cell in a droplet is quite low: 36.8%.

If two different cell types are encapsulated in the same droplet, the probability of only one droplet in each entity drops to 13.5%. Two different sample inlets can be used as the dispersed phase in the co-flow design, and they will mix once they become droplets. It is also possible to break the surface tension of the surfactant by electrocoagulation to merge two droplets containing different agents.

Encapsulation after droplet formation

Reagents can be injected into existing droplets to provide greater operational flexibility after the droplets are formed.

• Plug-based microfluidics uses channels perpendicular to the flow of droplets to inject new reagents into the formed droplets by injecting droplets.
• Pick injection follows a similar procedure but uses a drive electric field to control the injection.

Figure.1 Encapsulation process based on alternating droplet generation. (Hirama H, et al. 2015)

Biological Applications of Droplet Encapsulation

High-throughput Screening Based on Droplet Encapsulation

• Optimized single-cell analysis in droplet-based microfluidics

In single-cell proteomics, whether by cell secretion or cell lysis in droplets, the detection of target molecules is usually done by fluorescently labeled antibodies. This analysis is called digital microfluidics.

Single-cell genomics allows for the targeting of DNA through single-cell encapsulation with primer-functionalized beads. It is therefore possible to use droplets to map the genome of entire cell subpopulations.

• Drop packaging can achieve ultra-high-throughput in directed evolution

Directed evolution is the process of limiting and accelerating protein evolution by imitating natural selection in vitro. The miniaturization improved control and homogeneity, and sorting capabilities of droplet-based microfluidics enable researchers to develop ultra-high-throughput directed evolution, which can screen 10⁸ proteins.

Droplets Used as Templates for Artificial Cells

The droplet compartmentalization can be used as a template for artificial cells. Depending on the research purpose, they can be used as membraneless compartments, templates for creating lipid bilayers, etc. More complex designs allow the droplets to be encapsulated in larger droplets to simulate the separation of real cells and provide better timely control of the response.

Figure.2 (a) Schematic of a biological cell encapsulated inside a vesicle-based artificial cell. (b) Schematic and microscopy images of the generation of cells-in-vesicles. (Elani Y, et al. 2018)

Droplet-based Drug Delivery System

In addition to acting as a microreactor, droplets can also be transformed into a drug delivery system (DDS). The drug is diluted in the dispersed phase, and its concentration is controlled by the flow rate. Depending on the route of drug administration, it may be necessary to turn the droplets into micro/nanoparticles through a solidification step. Similarly, some droplet-based DDS may functionalize their solution in response to a specific pH or temperature, acting as a controlled delivery system. For more information, please click on Microfluidic Drug Delivery Systems.

References

• Hirama H, et al. (2015). "One-to-one Encapsulation Based on Alternating Droplet Generation." Scientific Reports. 5: 15196.
• Elani Y, et al. (2018). "Constructing Vesicle-based Artificial Cells with Embedded Living Cells as Organelle-like Modules." Scientific Reports. 8: 4564.

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