Microfluidics application note
Published on 27 August 2019
For historical reasons peristaltic pumps have been widely used to handle liquid flows. However, researchers started to switch from peristaltic pumps to pressure-driven flow control systems. This application note explains the reasons of this drastic change which occurred in liquid flow control technologies.
Peristaltic Pump: Basic Principles
Peristaltic pumps are part of group of pump called “Liquid pumps”. Peristaltic pump cannot be modeled as perfect flow rate generators since the back pressure lowers the flow rate. Usually, the flow rate Q is expressed linearly as a function of the backpressure ∆Pback using a linear relation of the following type:
Where Qmax is the flow rate achieved when the pump is used without any exit fluidic resistance and ∆Pmax is the maximal backpressure that the pump can compete with.
Peristaltic pumps show the main advantage of using interchangeable flexible tubes (limiting the contamination problems) and are fitted for large flow rates applications. Another type of liquid pump called “piezo-electric pump” can be employed for intermediate flow rates (µL).
The control of flow rate requires the enslavement of the pump to a flow rate sensor and fluctuations can be observed for lower flow rates. HPLC pumps integrate all of these components while minimizing the fluctuations but reveal themselves as an expensive system for flow rate control.
Pressure-driven Flow Controller: Basic Principles
A pressure controller pressurizes a tank, such as Eppendorf, Falcon or bottle containing the sample, which is then smoothly and quasi-instantly injected into a microfluidic chip. In Figure 3, the reservoir is pressurized, the gas pushes on the fluid surface, the fluid flows through the outlet. Thus, controlling the input gas pressure of the tank allows for fine control of the liquid that flows out of the tank.
Thanks to piezoelectric pressure regulation, Elveflow’s systems can handle flows within 40 ms with a 0.005 % stability. One advantage of pressure driven flow control relies on the capability to handle fluid volumes of several hundreds of mL.
Figure 3: pressurized reservoir
By coupling our pressure controller with one of our flow sensors, you can perform an ultra precise and responsive flow control. You can request a flow rate value in the Elveflow Software and the pressure controller will automatically adjust pressure to reach the requested value thanks to a customizable PID Feedback loop.
Peristaltic Pump vs Pressure Control: Performances
Response time and stability
Peristaltic pumps offer the possibility to create a closed loop of liquid, which is less straightforward with other systems but can still be carried out with some experimental set-up adaptation. It is very helpful for long-term experiments. On the other hand, peristaltic pumps offer less stability on a long-term basis, which forces recurrent calibrations of its flow rate. The pulse issue at low flow rates is also ten times higher than with syringe pumps. For long-term experiments requiring flow stability, it is also now possible to use peristaltic pumps instead of pressure controllers which enable to work with a reservoir of several liters. When recirculation is required, as mentioned before, it is also possible to use a recirculation setup with a pressure controller and a MUX or a pressure controller and valves to perform dynamic cell culture.
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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Peristaltic Pump vs Pressure Control: Pros & Cons
PERISTALTIC PUMP
- Easy to set-up.
- An infinite amount of liquid can be dispensed.
- Ability to re-inject the same sample.
PRESSURE CONTROLLER
- Fast response time (down to 40ms)
- High stability and pulsless flow
- Possibility to handle fluid volumes of several liters
- Possibility to control fluids in dead-end channels
- Enables both flow and pressure control when used with a flow meter
(1) When pressure is not balanced, it is possible to have back flows, when performing flow switches with multiple inputs (association with valves is required to solve that problem). In order to prevent these backflows, one can add a microfluidic backflow blocker kit.
Both peristaltic pumps and pressure control systems have their drawbacks and advantages. Peristaltic pumps are convenient and have been used for a long period. Nevertheless, performances are limited (response time, pulses). It can be critical when the setup is complex or when a fine control is requested as it is usually the case in microfluidics.
Pressure control is more and more used because it was developed for microfluidics. It meets users expectations with its performances (responsiveness, stability, reproducibilty…). This technology is adapted to most microfluidic applications (97%*).
How to upgrade your current system with pressure control?
The system proposed here ensures an easy upgrading of your current system with all the advantages of piezoelectric pressure regulator technology.
This technology ensures fine handling of large fluid volumes (several liters) and allowing long-term experiments. The fast responsiveness & high stability of pressure-driven flow control offer critical features compared to peristaltic pumps for several applications from droplet generation, flow focusing, flow chemistry or synthesis. It is also possible to push & pull a fluid by controlling both pressure and vacuum on a same channel.
If interested, feel free to contact our microfluidics expert !
Tutorial: How to upgrade your peristaltic pump setup?
Video: Coupling our OB1 pressure controller + flow sensor and turn your system into a powerful peristaltic pump!
Schematic: How to upgrade your peristaltic pump setup?
Pressure driven flow control is a smart alternative to peristaltic pumps. It allows pulseless flow within subsec response time. It consists in using a gas input pressure within a hermetic liquid tank, in order to flow liquid from the tank to your microfluidic device.
List of components
*Please directly contact us for our peristaltic offer.
Microfluidics knowledge
Do you want tips on how to best set up your microfluidic experiment? Do you need inspiration or a different angle to take on your specific problem? Well, we probably have an application note just for you, feel free to check them out!
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