Syringe-vacuum microfluidics: A portable technique to create monodisperse emulsions
Adam R. Abate and David A. Weitz
"Syringe-vacuum microfluidics: A portable technique to create monodisperse emulsions"
Entry by Fei Pu, AP 225, Fall 2012
Monodisperse drop formation is the central operation in droplet-based microfluidics but can be quite challenging due to the need for precise, steady pumping of reagents; forming monodisperse drops with controlled properties is thus a stringent demonstration of the effectiveness of a control system.A simple method for creating monodisperse emulsions with microfluidic devices is presented. Unlike conventional approaches that require bulky pumps, control computers,and expertise with device physics to operate devices, this method requires only the microfluidic device and a hand-operated syringe. The fluids needed for the emulsion are loaded into the device inlets, while the syringe is used to create a vacuum at the device outlet; this sucks the fluids through the channels, generating the drops. By controlling the hydrodynamic resistances of the channels using hydrodynamic resistors and valves, it is able to control the properties of the drops.This provides a simple and highly portable method for creating monodisperse emulsions.
Materials and Methods
The devices are fabricated in polydimethylsiloxane PDMS using soft lithography.The drop formation channels have dimensions of 25 micrometer in width and 25 mircometer in height. To enable production of aqueous drops in oil, hydrophobic devices are required, which achieved using an Aquapel chemical treatment. After this treatment, the channels are permanently hydrophilic, as is needed for forming aqueous-in-oil emulsions. To introduce reagents into the device, 200 microleter plastic pipette tips are inserted into the channel inlets. To apply the suction, a 10 ml Bectin-Dickenson plastic syringe coupled to the device through a 16 G needle and PE/5 tubing is set up. The other end of the tubing is inserted into the outlet of the device. A schematic is shown below
Results & Discussion
In many biological applications, drop size must be precisely controlled. This is essential, for example, when encapsulating molecules or cells in the drops, in which the number encapsulated depends on the drop size. With SVM, the drop size can be precisely controlled. A strategy to accomplish this is motivated by the physics of microfluidic drop formation. In microfluidic devices, the capillary number of the flow is normally small, Ca of 0.1; as a consequence, the drop formation physics follows a plugging/squeezing mechanism, dependent on a resistor setup. Figure 2 shows how droplets are squeezed and formed in monodisperse droplets. Figure 3 shows how droplets are controlled through the a similar concept to resistors.
Another way to control drop size is using parallel valves. Single-layer membrane valves allow the drop size to be varied in real time to screen for optimal reaction conditions. The valves are positioned on the inner and side inlets, as shown in Figure 4. By adjusting the actuation pressures of the valves, the flow rates in the drop maker are varied, thereby changing the drop size
This allows biological reactions to be performed with greatly enhanced speed and efficiency over conventional approaches: by reducing the drop volume, only picoliters of reagent are needed per reaction, while through the use of microfluidics, the reactions can be executed at rates exceeding hundreds of kilohertz. This combination of incredible speed and efficient reagent usage is attractive for a variety of applications in biology, particularly those that require high-throughput processing of reactions, including cell screening, directed evolution, and nucleic acid analysis.The same advantages of speed and efficiency would also be beneficial for applications in the field, in which the amount of material available for testing is limited, and results are needed with short turnaround.