Droplet Generation in Microfluidics: Using the mp6 Micropump

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This article is based on a case study we conducted on droplet generation using the mp6 micropump. Since then, we have developed a successor to it: The Bartels Pump | BP7. Everything demonstrated in this article is possible using the BP7, if not easier.

Close-up view of microfluidic droplet generation on a chip, showcasing precision fluid manipulation in a lab setting
Droplet generation on a chip: A snapshot of advanced microfluidic processes

The article will introduce you to droplet generation and show you two example systems we used to achieve this exciting tech. The cornerstone of this study were two droplet generator chips by our partner microfluidic ChipShop.

Droplet-based microfluidic systems have found their way into laboratories as it is possible to generate (monodisperse) droplets in the femto- to nanoliter scale. They opened up unlimited experimental possibilities like digital PCR and single cell experiments where the compartmentalization and surface to volume ratio are important.

This means that by utilizing droplet generation in microfluidics, researchers can achieve high-throughput screening for drug testing and enable rapid, sensitive diagnostics for medical applications, thereby revolutionizing personalized medicine and point-of-care testing.

Fundamentals of Droplet Generation

Droplet generation in microfluidics uses two immiscible phases: the continuous phase (oil, the medium in which droplets flow) and the dispersed phase (water, the droplet). For generating droplets, microfluidic systems generally include a microfluidic chip, a fluid handling system and tubing. This system is usually connected to a computer and a microscope to visualize droplet formation.

Shown is a droplet generator with two channel crossings one right after the other. With this desing three phases are used to generate droplets, two water based and one oil. In this image the oil phase is trancslucent and the two water phases are blue and yellow. Because of its design the droptlets are generated with yellow on the edge while the middle of the droplet is blue,
Three phase droplet generation on a double cross droplet generator

Utilizing the BP7 Micropump

At Bartels Mikrotechnik, the Bartels Pump | BP7 micropump is a key component for droplet generation. This positive displacement membrane pump uses piezo actuators to efficiently pump both liquids and gases. With a dynamic range flow rate for water from 0 to 14000 μl/min and for gases up to 35000 μl/min, the BP7 micropump provides the versatility needed for various experimental setups. Additionally, it operates at pressures up to 500 mbar for water and 140 mbar for air, ensuring precision in microfluidic applications.

Many mp6 micropumps are being produced in Bartels Mikrotechnik's production lab in Dortmund, Germany.
Many mp6 micropumps are being produced in Bartels Mikrotechnik's production lab in Dortmund, Germany.

Microfluidic Chips in Use

This study utilized two specific microfluidic chips to showcase their droplet generation capabilities.

  1. Fluidic 912 – Single Cross Geometry:

The droplet generator chip Fluidic 912 provides eight identical droplet generator units with a nozzle size of 80 μm on the chip. One inlet introduces the continuous phase, which separates into two channels.

To operate one unit of the Fluidic 912, you need a control unit capable of managing two individual flows: one for the continuous phase and one for the disperse phase. The mp-Multiboard, as explained in the experimental setup, can achieve this.

The chip Fluidic 912 with eight 80 µm nozzles, requiring mp-Multiboard for phase control
The chip Fluidic 912 with eight 80 µm nozzles, requiring mp-Multiboard for phase control

2. Fluidic 1032 – Double Cross Geometry:

Researchers specifically developed the droplet generator chip with three identical droplet generation units of double cross geometry and a flow focusing nozzle size of 100 μm for use in single cell sequencing experiments, where they need to introduce single cells and beads/lysis buffer into a single droplet in an oil phase.

The fluidic Chip 1032 features Mini Luer interfaces and its use requires a microfluidic pump setup with the
ability to control three individual flows.

However, you can also use this droplet generator to generate water in oil droplets. Simply close one inlet and supply a liquid stream to the remaining two inlets.

The chip Fluidic 1032 with three 100 µm nozzles for single cell sequencing, requiring three flow controls
The chip Fluidic 1032 with three 100 µm nozzles for single cell sequencing, requiring three flow controls

Experimental Setup

The experimental setup included:

  • Microfluidic droplet generation chips (Fluidic 912 and Fluidic 1032)
  • Fluidic accessories such as Mini Luer connectors and tubing
  • Reagents including surfactant-containing droplet oil and water
  • A pump setup with the mp-Multiboard and mp-Highdriver4, using 2-3 micropumps

The mp-Highdriver4 controlled the micropumps, allowing users to adjust the voltage individually and precisely control the flow rates of water and oil, which was crucial for generating droplets of varying sizes. To emphazise, if the flow rate is not properly controlled, it will be impossible to form droplets.

Droplet Generation from the schematic representation to the final system, illustrating the output of the mp-Multiboard
Droplet Generation from the schematic representation to the final system, illustrating the output of the mp-Multiboard

In our case study, we used two different microfluidic chips for droplet generation. The setup can be easily adjusted; adding an extra mp6 micropump allows for the application with chip 1032. Controlled by the mp-Highdriver4, up to four mp6 micropumps can be managed simultaneously with individual voltage settings. This allows precise control over water and oil flow rates. Additionally, pressure sensors help adjust droplet size, with pressures ranging from 50 to 100 mbar for DI-water and 80 to 195 mbar for oil. To conclude, as the water flow rate increases, droplet diameter and generation rate also increase, while reducing oil flow rate leads to larger droplet diameters.

Different Chips, different Droplets

Droplet generator 912

With the Droplet generator 912 it can be observed that with increasing water-flow-rate, respectively water pressure, and constant oil flow rate, the droplet diameter and the generation rate increase. By decreasing the oil flow-rate, respectively oil pressure and keeping the water flowrate  constant, the droplet diameter increases.

This chip is somewhat limited in that it can only generate droplets the width of the fluidic channel.

Droplet generator 1032

Due to the different channel widths and flow focusing junction, the system can generate droplets smaller than the channel width. The droplet size depends on the pressure applied through the mp6. As water pressure increases while keeping oil pressure constant, both the droplet size and the generation rate increase.

Droplet Generator Chip Fluidic 912 in action
Droplet Generator Chip Fluidic 912 in action
Droplet Generator 1032 in action. Generating droplets the size of the fluidic channel and smaller
Droplet Generator 1032 in action. Generating droplets the size of the fluidic channel and smaller

Results and Observations

The experiments demonstrated that adjusting the flow rates and pressures of the water and oil phases could generate uniform droplets of various sizes. For instance, increasing the water flow rate while maintaining a constant oil flow rate resulted in larger droplets and higher generation rates.

This case study illustrates the capability of the mp6 micropump to generate uniform droplets for a range of laboratory applications, showcasing its effectiveness in microfluidic setups.

Furthermore, with this setup it can be shown that it is possible to generate monodisperse droplets in various sizes by using the micropump mp6 and microfluidic chips by microfluidic ChipShop.

If this article sparked your interest in micro pumps, visit our shop or contact us via our contact form so we can help you integrate micro pumps perfectly into your system.