Flow Control in A Microfluidic Device

To conduct experiments in microfluidics effectively, it is necessary to master various available flow control techniques to use the most appropriate method to control the flow of microfluidics. This article briefly introduces the main microfluidic technologies.

• Applying pressure to the fluid in a sealed container with a fluid outlet.

According to the Hagen-Poiseuille equation: △p=8ηLQ/πr⁴=8ηLv/r²=32ηLv/D², The average flow rate of the liquid in the microfluidic channel is proportional to the applied pressure gradient. This is similar to Ohm's law of electricity: in the case of fluid flow, △P=R_fluidQ. The fluid resistance depends on the geometry of the channel and the fluid viscosity.

• Using volume control as a syringe pump or peristaltic pump.

The principle here is to use mechanical motion to modify the volume to apply the flow rate. For syringe pumps, different flow rates can be achieved by modifying the volume of the syringe and the infusion speed.

• Other technologies, such as electroosmotic pumps or integrated micropumps.

In order to obtain the best results in a microfluidic experiment, it is important to consider the following factors:

• The required flow or pressure range.
• How quickly do you need to set or change the flow rate?
• How stable is the flow required?

Based on the above, different flow control techniques can be used to manage microflows.

Hydrostatic Pressure

This is the easiest way to generate controlled flow in a microfluidic system. The pressure difference is obtained by changing the height of the interface between the liquid and the atmosphere in different reservoirs. The basic idea is to place the inlet water reservoir above the outlet water reservoir so that gravity flows fluid from the inlet to the outlet, just like a water tower.

Figure.1 Schematic of the microfluidic platform to enable the precise application of hydrostatic fluid pressure to the microwell chamber by varying the height difference between fluid level and microwell chamber. (Shang M, et al. 2021)

However, this technique is limited by the Laplace pressure generated at the gas-liquid interface. It is also limited by the lack of dynamic control of the pressure applied in the microchannel, which makes it difficult to implement any changes in pressure parameters. Another limitation is that as the liquid flows from one storage tank to another, the pressure drop will gradually change, causing the pressure drop to decrease linearly with time.

Pressure Generator/Pressure Pump

The simplest pressure generator consists of a pressure source, a static membrane pressure regulator, and a pressure gauge to monitor the pressure value of the atmospheric pressure. The working principle of this pump is to pressurize the sample container to control the pressure drop between the inlet and the outlet of the microfluidic system. The responsiveness of the generated flow depends on the responsiveness of the pressure pump.

Syringe Pump

The syringe pump was the first flow controller for microfluidics and was widely used in standard laboratories. It is based on a mechanical system usually driven by an electric stepper motor that pushes the syringe at a fixed rate. The main advantage of syringe pumps is that they can control the flow rate in the microchannel independently of fluid resistance.

The main disadvantage of the syringe pump is that it produces pulsating flow at low flow rates, and the time required to stabilize the effective flow rate when the flexibility of the tube cannot be ignored. To solve this, we provide a syringe pump stabilizer add-on kit designed to absorb and suppress flow rate fluctuations before affecting your experiment.

In Alfa Chemistry, you can find a variety of syringe pumps.

Peristaltic Pump

The liquid is contained in the hose, and additional compression and relaxation will suck in the liquid and cause flow.
A variety of techniques can be used, including:

• Using interchangeable hoses for high flow applications.
• Using the most compact pumps for medium flow (µL).

Electroosmotic Pump

The main idea is to generate electroosmotic flow in porous media. Due to the electrical pumping of liquids through nanoporous materials, electro-osmotic pumps do not have the problem of flow fluctuations. They can also withstand large back pressures, but they need to be used with liquids with low conductivity and lack repeatability.

Applications of Flow Control Microfluidics

• Bioassay
• Chemical synthesis
• Separation and analysis
• Single-cell biology
• Droplet microfluidics
• Microfluidic rheometer
• Light fluid

Reference

• Shang M, et al. (2021). "Microfluidic Studies of Hydrostatic Pressure-Enhanced Doxorubicin Resistance in Human Breast Cancer Cells." Lab Chip. 21: 746-754.

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