Syringe pumps and microfluidics
Introduction to the use of syringe pumps in microfluidics
In the past years, several new microfluidic syringe pump systems have changed the use of syringe pumps in microfluidic experiments.
In this tutorial, we will briefly explain the main advantages and drawbacks of using syringe pumps in microfluidics. We will also describe some newly available systems that eliminate these drawbacks and present flow control artifacts. Then, we will finish by discussing the performance of modern syringe pumps and alternative systems for handling fluids in microfluidic chips.
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A syringe pump is a great tool when it comes to fluid injection in microfluidic systems. Widely used in the medical field for long-term, constant injection of drugs, this tool has naturally transitioned into fluid injection research. It is now the most common instrument in microfluidic research (click here see our “researchers’ opinions” study about the types of flow control used by researchers in microfluidics). If you would like more information about the different types of existing microfluidic flow control instruments, see our tutorial on how to choose the right microfluidic flow control system for your research.
Syringe pump working principles
As indicated by its name, the main component of the syringe pump is the syringe. This tool has been widely used in medical settings for centuries. Nevertheless, by itself, the syringe involves a hand driven motion of the piston, which is not suitable for a controlled delivery of its contents. The syringe pump was invented to rectify this issue. It consists in a simple source of linear motion that controls the speed at which the piston is driven. Different models of syringe pumps fit for microfluidic work can be found at Darwin Microfluidics. They also offer peristaltic pumps selectioned to work with microfluidics.
If the diameter of the syringe is known, the instrument adapts its linear speed to the requested flow rate with the following formula:
Q= vS (Where Q is the flow rate, v the speed of the piston and S its section.)
Overview of technical properties of the syringe pump
One major advantage of syringe pumps lies in the fact that the user can easily adapt the working range of the instrument by changing the diameter of the syringe. A small syringe diameter enables better control at low flow rates but at smaller dispensable volumes. On the contrary, a bigger diameter enables larger volumes but decreases in performance at low flow rates. Another main advantage of syringe pumps is the ability to easily know the flow rate. Unfortunately, as we will see later, this is often not the case in microfluidics.
The market is full of basic instruments that provide basic performance and which are adequate for a wide range of applications. Some companies such as Harvard Apparatus, Cellix and others have developed high performance microfluidic syringe pumps approved by microfluidic experts.
Syringe pumps and settling time: do you really know the flow rate in your chip ?
A syringe pump, like other injection systems, can be mainly characterized by its settling time and its stability. The settling time of a syringe pump depends not only on the quality of its mechanics, but also, and more importantly, on the fluidic resistance and the fluidic compliance of the experimental setup. The elasticity and high fluidic resistance of syringes, tubing and chips are important parameters to obtain a stable flow rate in most microfluidic systems.
When changing flow rates, the piston pushes the syringe, the pressure increases in the fluidic system and deforms it instead of putting the liquid into motion. Depending on the fluidic resistance and elasticity of your system, the settling time can change from a hundredth of a ms, in a rigid fluidic system with a low resistance, to hours, for a soft fluidic system with a very high fluidic resistance.
To get the best responsiveness with a syringe pump, elasticity in the fluidic system must be avoided and the fluidic resistance of the chip should be minimized (to better understand this subject, see our tutorial on the responsiveness of syringe pumps). Nevertheless, it is also possible to improve the responsiveness of a syringe pump using additional tools (see syringe pump upgrades and alternatives below).
It is crucial when dealing with microfluidics and syringe pumps to get an estimation of the responsiveness of the flow rate in your experimental conditions since incertitude of the real flow rate in the microfluidic device is one of the main reasons for experimental failure and unexploitable scientific results.
Stability of syringe pumps : A question of flow rate
The flow stability of a syringe pump is determined by the minimal movement of its motor. Because the displacement of the piston and the volume injected are correlated, this minimal movement induces a minimal injected volume. Therefore, discrete phenomena which look like oscillations or pulses appear at low flow rates due to the motor step. Note that the minimal injected volume is proportional to the syringe diameter. That is why a smaller syringe diameter improves the stability of the flow at low flow rates.
However, the use of a smaller syringe limits the flow range you can achieve and the quality of a syringe becomes critical when the expected stability is on the order of magnitude of 0,1 µL/min. Also keep in mind that the elasticity in the system enables a smoother flow rate and enhances its stability, but decreases its responsiveness. It is also possible to improve the stability of a syringe pump using additional tools.
Conclusion: Find the balance between responsiveness and stability
To summarize, the performance of a syringe pump depends on its engine quality and the mechanical precision of its moving parts. This tool generally provides reliable performances. We have seen that some instability can occur at low flow rates. This can be improved by increasing the RC constant of the system, for instance by increasing the elasticity in the system, but this will decrease the responsiveness of the flow rate. When dealing with syringe pumps and microfluidics, the user must find the appropriate balance between stability and responsiveness. As shown below, it is also possible to overcome these problems by using recent commercial solutions provided by different companies.
If you want more information about the different types of existing microfluidic flow control instruments, please see our tutorial on how to choose the right microfluidic flow control system for your research.
How to upgrade your microfluidic syringe pump ?
In the last few years, several solutions have arisen to compensate for the main drawbacks of syringe pumps in microfluidic applications. In some cases, they enable the users to overcome stability and responsiveness issues and to know the real flow rate in their microfluidic device. When considering the use of syringe pumps in your microfluidic application, you will need to find the balance between performance, Plug & Play capacities, pricing and versatility.
Limit flow oscillation: Use pulseless microfluidic syringe pumps
High precision and pulseless syringe pumps have been developed for microfluidic applications. To reach this level of performance, the manufacturing companies upgraded the core of the syringe pump by adding motors with hundreds of thousands of steps, automatic motor gears to adjust the speed depending on the flow rate and fine mechanical contact between the moving mechanical pieces. These syringe pumps are generally expensive but are efficient enough to minimize flow oscillation for 90% of microfluidic applications. Nevertheless, flow responsiveness remains an issue for microfluidic applications even with this kind of syringe pumps.
For more information about pulseless syringe pumps, please see our tutorial about pulseless microfluidic syringe pump.
A selection of high precision pulseless syringe pumps can be found at Darwin Microfluidics.
You can also use the Elveflow syringe pump flow rate stabilizer kit, to learn more about it Syringe Pump Flow Stabilizer Microfluidic Kit
Monitor in real time the flow rate inside your chip: Use a flow meter
It is possible to overcome the uncertainty of the flow rate just by using a flow sensor. Indeed, the real flow rate is never really known with a standard syringe pump. However, by using a flow sensor, it is possible to measure the real flow rate in real time. The use of a simple flow meter eliminates much of the experimental failure that arises when dealing with syringe pumps. When using flow meters, some researchers can also manually adapt the flow rate of the syringe pump in order to improve its response time. This is a simple and efficient way to overcome the uncertainty of flow rates.
To learn more about Elveflow microfluidic thermal flow sensors click here
To learn more about Elveflow microfluidic masic flow sensors click here
Increase the responsiveness of your syringe pump: Use a flow meter with a feedback loop
To increase the responsiveness of the syringe pump in your microfluidic application, the most efficient way is to add a flow meter with a feedback loop. The principle is simple: the software adjusts the syringe pump speed depending on the information measured by the flow meter. Using this kind of feedback loop the syringe pump can achieve flow changes in as little as a hundredth of a millisecond. You can program the feedback loop yourself with labview or matlab using for example our microfluidic flow sensor library or buy a syringe pump pack which already includes a feedback loop. A well-fitted feedback loop will increase your responsiveness without any effect on the flow stability.
As previously described, we have integrated a flow sensor into a simple microfluidic experiment to demonstrate the benefits of adding a flow sensor to a syringe pump, as shown by the graph on the left. By adding a flow sensor to the syringe pump setup and programming a feedback loop, we show the resulting gap of responsiveness compared to systems without any flow sensors connected to a syringe pump. The use of a flow sensor with a feedback loop increases significantly the responsiveness of the flow control. In the current microfluidic experiment with a syringe pump, the settling time increased by 6 to 8 times with the use of a flow sensor.
To learn more about Elveflow microfluidic flow sensors : click here
To increase the responsivness you can also use the Elveflow syringe pump flow responsiveness kit : Syringe Pump Flow Stabilizer Microfluidic Kit for more info
Turn your syringe pump into a pressure pump: add a pressure feedback loop
In some cases, researchers want to keep their syringe pump while still being able to ensure a constant pressure in their microfluidic device (independently from the chip’s fluidic resistance or other pressure sources). Here too, you can program the feedback loop yourself, using our microfluidic pressure sensor library or by buying a syringe pump pack which already includes a feedback loop.
To learn more about Elveflow microfluidic pressure sensors : click here
Limit the flow oscillation of your syringe pump : Fluidic RC low-pass filter
A cheap way to overcome flow oscillation issues is to use a fluidic RC low-pass filter. A low-pass filter is just a calibrated elastic capacitance such as any elastic tubing coupled with a fluidic resistance. Since the efficiency of the low-pass filter depends on the flow oscillation frequency spectra, it is generally recommended to get a pack containing a set of different low-pass filters for your lab. The main drawback of low-pass filters is that they reduce the responsiveness of your system.
To learn more about Elveflow syringe pump flow stabilizer : Syringe Pump Flow Stabilizer Microfluidic Kit
Limit flow oscillations and increase responsiveness: add both a flow sensor feedback loop and a fluidic filter
An efficient way to overcome the two main drawbacks of syringe pumps is to use a flow sensor feedback loop with an RC fluidic filter. In this case the fluidic filter will smoothen the flow oscillation and the flow sensor feedback loop will adjust the flow rate to reach the desired rate as fast as possible. Here too, you can program the feedback loop yourself by using our microfluidic flow sensor library and RC filter or by buying a syringe pump pack which already includes a feedback loop.
To learn more about Elveflow microfluidic flow control modules : click here
Alternatives to syringe pumps for microfluidic flow control
Even if syringe pumps are the most used injection systems in research and microfluidics, other injection systems are becoming more and more popular when dealing with demanding microfluidic applications.
Need high flow responsiveness and stability : Microfluidic pressure controller
Pressure controllers are used in microfluidics when people need high flow stability and fast responsiveness. A pressure controller pressurizes a tank, such as Eppendorf, Falcon or bottle, containing the sample, which is then smoothly and quasi-instantly injected in a microfluidic chip. You can also easily use a pressure controller as a syringe pump (as shown in this tutorial).
To learn more about Elveflow microfluidic flow and pressure controllers brand : click here
Need the highest flow responsiveness : Pressure controller & flow switches
For some microfluidic applications, researchers use pressure controllers coupled with matrix valves. Researchers mainly use a flow switch matrix when they need fast flow switches with no back flow (to avoid sample contaminations) and high precision flow rate control. Flow switch matrices can also be applied to quake valves or integrated PDMS peristaltic pumps to completely and instantaneously stop the flow inside a microfluidic channel or to simultaneously control the flow within a high number of channels while maintaining a reasonable setup cost.
To learn more about Elveflow microfluidic flow switches matrices brand : click here
Need closed loop recirculation : Peristaltic pumps
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 adaptations (click here for an example with a pressure controller). 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 pressure controllers instead, 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.
For more tutorial about microfluidics, please visit our other tutorials here: «Microfluidics tutorials». The photos in this article come from the Elveflow® data bank, Wikipedia or elsewhere if precised. Article written by Guilhem Velvé Casquillas and Timothée Houssin and revised by Lauren Durieux.
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