Microfluidic Flow Control Technologies: Strengths and Weaknesses
To perform effective experiments in microfluidics, one needs to master the different flow control technologies available to use the most suitable way to control microfluidic flows. Physical properties and laws such as volumetric forces, weight or inertia are different in the micro scale. Considering these changes, some microfluidic technologies have been developed. This article aims at presenting a short review of the existing techniques and their strengths and weaknesses.
Main microfluidic flow control technologies
The main microfluidic flow control technologies are listed below:
1. Pressure is a great tool to control microfluidic flow
There are several ways to use pressure : you can use hydrostatic pressure or a pressure regulator, also named pressure pump.
- Using hydrostatic pressure is the simplest way to move fluids: the basic idea is to put the inlet reservoir higher than the outlet reservoir in order to let the gravity force move the fluid from the inlet to the outlet, just like a water tower.
- Pressure Pumps works by the pressurization of the sample reservoirs in order to control the pressure drop between the inlet and the outlet of the microfluidic system. The responsiveness of the generated flow rate depends on the responsiveness of the pressure pump.
Fluigent was the first company to introduce pressure-driven flow control to the microfluidic research market, as opposed to other conventional technologies. Our expertise in this field is translated in our LineUp™ series and MFCS series™. These microfluidics tools offer settling times down to 100ms and a resolution of 0.03% full scale (pressure sensor resolution) as well as a stability of 0.1% CV on measured values.
Pressure pumps in microfluidics have strengths and weaknesses. Some of them are listed below:
Strengths of pressure pumps in microfluidics
- Pulse-free flow control : there is no oscillation flow
- Both pressure and flow rate control in one device
- Very good stability : 0.005%
- Controls of the flow in dead-end channels is possible
- Ability to determine the exact pressure inside of the microfluidic component
Weaknesses of pressure pumps in microfluidics
- Limited to 8 bars
- Flow switches with multiple entry points can potentially cause back flow
By applying pressure to fluids in a sealed container with a fluid outlet, the fluids will move because of the pressure difference, according to a simple relation, similar to Ohm’s law for electricity (V=RI): In the case of fluid flow, P=RQ. The fluidic resistance is a function of the geometry of the channel and the viscosity of the fluid.
2. Using volumetric control as with syringe pumps is another way to control microfluidic flows
Syringe pumps are the most commonly used flow control system in microfluidics. They are based on a mechanical system usually actuated by an electric stepper motor that pushes a syringe at a fixed rate. However, in microfluidics, the use of syringe pumps is often problematic because the flow is not constant. This discontinuity is due to the mechanical system that creates an oscillating flow. This can be problematic for certain experiments such as biological ones.
Syringe pumps in microfluidics have strengths and weaknesses. Some of them are listed below:
Strengths of microfluidic syringe pumps
- Pulsatile flow
- Control of the flow in dead-end channels is impossible
- Difficult to determine the exact pressure inside of the microfluidic component
- High limit of pressure (depending on syringe material)
- No variation of the syringe rate
Weaknesses of microfluidic syringe pumps
3. Peristaltic pumps are another mechanical system for microfluidic flow control
This consists in exerting alternative compressions and relaxations on a flexible tube that contains the liquid. Those movements draw the liquid and result in flow. As with other technologies, peristaltic pumps have strengths and weaknesses.
Strengths of peristaltic pumps
for microfluidic flow control
- Easy to setup
- Large quantities of sample can be injected
Weaknesses of peristaltic pumps
for microfluidic flow control
- Low reproducibility of the experiment
- The flow rate is not constant due to stretching of the tubing
4. Other techniques such as electro-osmotic pumps or integrated micropumps are also a way to control microfluidic flow
However, those techniques are more specific.
The main idea of the electro-osmotic pump is to create an electro-osmotic flow in a porous media. This works due to an external electrical source that is applies to an electric double layer and this action generates high pressures and high flow rates.
The last flow control technology in microfluidics are integrated micropumps. Several kinds of integrated micropumps exist: there are mostly based on a peristaltic principle with flexible membranes made of PDMS.
To make the best choice concerning the microfluidic flow control technology that best fits your experiment, it is important to consider the following elements:
• The flow rate or the pressure range you need.
• How quickly you need to set or change the flow rate.
• How stable you need the flow rate to be.
TIPS FOR IMPROVING YOUR PRESSURE PUMP
Multiple solutions are offered in the microfluidic universe to help you to monitor your setup and your pressure pump. Fluigent has developed a large range of products to allow you to construct your system in a modular fashion.
If you want to use your pressure pump to:
- Control Flow Rate: you can add a microfluidic flow sensor that will measure the flow rate. Moreover, some software packages are available to allow you to adjust the pressure until you reach the desired flow rate.
- Stop the flow into the microfluidic chip: one can add a switch to the setup that can rapidly stop the flow in the microfluidic device without any backflow or residual flow.
- Improved flow switching: Switches can be added to the setup to allow for changing between two or more fluids. Those microfluidic valves are available with a large range of possibilities in terms of port number and allow one to employs multiple techniques such as sorting and recirculation.
- Monitor and control pressure: it’s possible to add a pressure generator to the microfluidic pressure pump to control the pressure inlet
- To obtain monodispersed droplets: you can add a surfactant to your samples in order to have long term stable microdroplets and to increase their frequency.
TIPS FOR IMPROVING YOUR MICROFLUIDIC SETUP
Some common issues are encountered when running a setup. Microfluidic experts have developed tools to solve those problems and help you to have the most efficient setup possible.
If you want to :
- Avoid bubbles in your microfluidic setup: gas bubbles in a liquid sample are a common problem encountered in numerous microfluidic experiment, and their removal in the sample of interest is quite often a major challenge for microfluidics. Indeed, gas bubbles circulating through a microfluidic system can damage equipment or the biological sample of interest and cause experimental errors. Multiple accessories are available on the microfluidic market such as our Bubble Trap Kit.
- Hand-free operation: tools are available to start/stop the flow or switch configuration of microfluidic devices with your foot.
- Automate your setup: The automation of your setup is possible thanks to some software tools that are able to monitor every compatible component of the setup.
- Unique software: has been designed to facilitate your manipulations and allows you to monitor your microfluidic devices in real time.
- Integrate microfluidic component to your setup: use OEM (Original Equipment manufacture) products to integrate them in your setup or devices.