Syringe pumps and microfluidics

A syringe pump is a liquid handling device used to deliver a precise amount of liquid in highly sensitive research and industrial environments. The applications of a syringe pump can range from chemistry, biology in the research field to food processing & micro-dispensing requirements for industrial applications.

A syringe pump consists of a motor, its driver and also has a reciprocating screw and nut. The nut connects this driver to the piston of the syringe, which contains the liquid inside it. This mechanism helps to regulate the amount of liquid being injected to achieve a precise and smooth delivery.

Introduction to the use of syringe pumps in microfluidics

Working principle of a syringe pump & its types

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 of 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)

Syringe pumps are generally of two types:

• Medical syringe pumps : The medical syringe pump is used to deliver a regulated volume of liquid (medicine, nutrients) to treat a patient. They are perfectly suited for in vivo diagnosis, treatment & research. Professionals in the medical field are trained to operate these pumps.
• Research Syringe Pumps : The fluid handling pumps are used for research purposes that demand delivery of small quantities of liquids. These types of pumps are designed for small liquid volumes and also feature additional features that are widely used in research.

Design & overview of the technical properties of a syringe pump

The design of a syringe pump primarily depends on its application. Although, all syringe pumps consist of these basic design elements shown in the the figure above.

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.

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 & stability

Industrial applications of syringe pumps

The ease of use for syringe pumps and the ability to change the working range of the pump by changing the diameter of the syringe, makes them an easy choice for many industrial applications. Here are some industrial applications where syringe pumps are widely used:

Microfluidics is a vast discipline of research that demonstrates how liquids interact in micrometer scale channels. Learn more about Microfluidic applications here.

In Neuroscience, syringe pumps and other flow control pumps are used to introduce drugs and other agents in order to observe the structure of neurons, and its effect on macroscopic brain functions

Microdispensing & Manufacturing : Syringe pumps are widely used for dispensing in industrial applications such as – water treatment, food processing, emulsions, and inks.

Chemical development has seen an increasing use of syringe pumps and other flow control pumps in the last two decades. This includes performance optimization in industrial applications such as nano-materials, energy storage, pharmaceutical and much more.

Alternatives to syringe pumps for microfluidic flow control

Even though syringe pumps are the most widely used injection systems in research and microfluidics, other injection systems are becoming more and more popular when dealing with demanding microfluidic applications, and where the primary objectives are as follows:

Need for 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’s range of microfluidic flow switch matrices 

Recirculation : A major limitation of syringe 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 can 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.

How to upgrade your syringe pump?

In the last few years, several solutions have emerged to compensate for the main drawbacks of syringe pumps in microfluidic applications. In some cases, they allow 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 oscillations 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.

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 mass 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

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.

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 the Elveflow microfluidic flow control modules : click here