Researchers use 4 types of microfluidic flow control systems:
Each kind of flow control system has advantages and drawbacks depending on the specific microfluidic application you are trying to achieve. In this review, we will study the pros and cons of each type of microfluidic flow control system.
Some researchers also use other kinds of flow control for very specific applications (hydrostatic pressure, electrosmotic pumps…). If you are interested in those systems, you can read this scientific tutorial about the physics of microfluidics flow control.
Ensure that the instruments can be set up exactly in the way you need them to be: we recommend that you always take the time to discuss your application and needs with company representatives before buying your microfluidic flow control setup.
Peristaltic pumps and recirculation pumps are used in microfluidics when researchers need samples to circulate continuously inside a device. Since this kind of pumps does not allow precise flow control, they are gradually becoming less used in microfluidic research. When recirculation and precision are required, researchers use more often a recirculation setup using pressure control or a syringe pump.
Strengths +
Easy to set-up
An infinite amount of liquid can be dispensed
The ability to re-inject the same sample
Weaknesses –
A strong pulse in the flow rate, vibrations and noise.
Weak reliability due to tube aging.
Syringe pumps are the most commonly used flow control systems in microfluidics even if in the last 5 years researchers have begun to use more alternative flow control systems (see our study on researchers’ opinions about microfluidics flow control).
Let’s say that syringe pumps can be divided in two categories. Classic syringe pumps, which are quite inexpensive but generate flow oscillations when dealing with microfluidics, and pulseless microfluidic syringe pumps, which are quite expensive but clearly offer better performances in terms of flow stability. In this tutorial, we will focus only on pulseless microfluidic syringe pumps. If you decide to use common syringe pumps, the information we provide in this tutorial will apply, but keep in mind the fact that your flow will not be stable at low flow rates.
The main advantage of syringes is that they are quite easy to use. The main weak point of pulseless syringe pumps is their low responsiveness, since it depends on the microfluidic setup. Flow changes inside chips can take seconds to hours (see our tutorial on syringe pump responsiveness in microfluidics). This lack of reactivity is one of the main limitations of syringe pumps for numerous applications. However, in 2013 and 2014, new solutions can help to overcome these problems (see our tutorial on how to upgrade your syringe pump to fit it to microfluidic needs).
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Syringe pumps generally allow fast setup for fluidic experiments.
New pulseless syringe pumps may give you a flow stability below 1%
The amount of dispensed liquid can be known for a long-term experiment (not during transient periods because of the flow rate uncertainty).
Maximum pressure generated by a syringe pump can be at several hundred bars. (High pressure syringe pumps are not pulseless, but could be very useful in nanofluidics).
The mean flow rate in the device does not vary with eventual changes in the fluidic resistance of the device (except if the syringe pump stalls due to high pressure).
Response time of the flow rate can vary from seconds to hours depending on the fluidic resistance and compliance. This effect can be minimized by using specific microfluidic tubing.
Without flow meters, users cannot know the real flow rate during the transient period (seconds to hours).
If the fluidic resistance of the device increases (due to channel clogging or dust, for example), the pressure generated by a syringe pump increases without limit and can lead to the device destruction.
Flow control of fluids in dead-end channels (like integrated valves) is impossible to obtain using syringe pumps.
The amount of fluid dispensed by the syringe pump is limited in volume.
Knowing the pressure inside the fluidic system requires a pressure sensor.
Even pulseless syringe pumps require to carefully choose the syringe size depending on your experimental conditions to avoid periodic pulsations on the flow rate due to the step-by-step motor of the syringe pump. This effect can be minimized using compliant microfluidic tubing.
Pressure controllers are flow control systems which pressurize the tank containing your sample. When pressurized, the sample is smoothly injected in your microfluidic chip. In microfluidics, researchers mainly use pressure controllers when they need responsiveness and stability, since pressure controllers can establish pulseless flows with short response times (80 ms) in microfluidic chips. Using pressure driven flows, pressure changes propagate within the fluidic setup without delay, leading to fast flow switch. Moreover, since there are no moving mechanical parts involved, pressure-driven flows remain smooth whatever your flow rates. Modern microfluidic pressure controllers also allow you to control both pressure and flow rate by integrating a flow meter with a feedback loop. Microfluidic researchers mainly use pressure controllers when they require high flow responsiveness and high flow stability and precision, as well as when they work with dead end channels or require large sample volumes.
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The pressure source enables pulseless flow
The amount of dispensed fluid can reach several liters
Response time is reduced to 80 ms
Enable the control of fluids in dead-end channels
Enable both flow and pressure control when used with a flow meter
Limitation of pressure at 8 bars maximum
When pressure is unbalanced, it is possible to have back flows when doing flow switches with multiple inputs (you can solve that problem with an association with valves.)
For some microfluidic applications, researchers use pressure controllers coupled with flow switch matrices. Researchers mainly use flow switch matrices when they need fast flow switches with no back flows (to avoid sample contamination or to instantaneously stop flows for observation). When a high-precision flow rate control is needed, researchers can also use quake valves or integrated PDMS peristaltic pumps because of their capacity to completely and instantaneously stop flow inside a microfluidic channel, and/or control flow in a high number of channels simultaneously while maintaining a reasonable setup cost.
Discover Elveflow brand of high performance flow switch matrices.
Low cost when controlling a large number of channels
High precision of flow control when used with a high precision pressure controller
The ability to control the pressure inside dead-end channels (such as quake valves)
The ability to instantaneously stop the flow inside microfluidic devices without any residual flow
The ability to perform fast sample switching without any back flow
The ability to control and monitor both flow rate and pressure (when coupled with a pressure controller with a flow rate feedback loop)
The price per channel can be expensive when dealing with a small number of channels
Setup complexity: the valves need a flow source.
A flow switch is an active element which enables the opening, closing or redirection of flow in a fluidic channel. Flow switches must be preferably used with pressure controllers because closing a channel connected to a working syringe pump may lead to an infinite pressure increase and the destruction of the fluidic system.
Pressure controllers enable fast sample switches (80 ms) in microchips but require a perfect equilibrium between all inlets to avoid back flow and sample contamination. The only way to achieve a clean and fast flow tuning/switching is to couple a pressure regulator with flow switches. Valves need to be placed between the liquid reservoir and the microchip. Since liquids are incompressible, pressure will instantaneously push the liquid into the chip and the flow change will have the reactivity of the valve opening (25 ms or less). Moreover, since the tubing between the microchip and the flow switch is full of liquid (which is incompressible), it avoids backflow and contamination between input capillaries.
When using a pressure controller, the variation of the hydrostatic pressure in the liquid tank makes it very hard to achieve a pressure equilibrium. The use of synchronized flow switches, such as microfluidic multiplexer, enables you to plug all channels simultaneously. Since liquids are incompressible, valves enable you to get a real time flow stop without any residual flows.
Lighter blue=> badly fitted — Darker blue => well fitted
For more reviews about microfluidics, please have a look at: «Microfluidics reviews». The photos in this article come from the Elveflow® data bank, Wikipedia or elsewhere if specified. Article written by Guilhem Velvé Casquillas and Timothée Houssin and revised by Lauren Durieux.
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