Flow control and measurement
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. This article aims at presenting a short review of the existing techniques for flow control and flow rate measurement in microfluidics.
Flow Control Solutions in Microfluidics
The main flow control solutions can be divided in three sections: pressure based solutions (such as the Flow EZ™), volume displacement (such as syringe or peristaltic pumps) and passive techniques. All of these control techniques have different advantages. In order to make the best choice, 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.
1. Fluid Volume displacement in Microfluidics
Fluid volume displacement uses mechanical parts to directly displace a certain volume of fluid. The two main types of pumps using this approach, are the peristaltic and the syringe pump. They enable the user to do direct flow by indicating a flow rate as an input.
Microfluidic Peristaltic Pumps for flow rate control
The microfluidic peristaltic pump uses a mechanical rotor to squeeze a flexible tube containing the fluid resulting in alternative compressions and relaxations that will draw in the liquid and result in flow. This mechanical action moves the fluid from the inlet towards the outlet.
Nevertheless, the stability of this system can be quite bad due to internal friction and the functioning principle itself. Other issues, such as high shear stress and contamination of the fluid caused by the friction on the tube can be a problem in particular for biological experiments.
Syringe Pumps, as a microfluidic flow control system
This flow rate control solution is one of the most widely used in microfluidics. Ease of use is one of the main advantages of syringe pumps. A mechanical system, usually actuated by an electrical stepper motor, pushes the syringe filled with liquid at a fixed rate. By changing the size of the syringe or by regulating the motor speed one is able to produce different ranges of flow rate.
The main drawbacks to syringe pumps are the lack of stability and low responsiveness. Oscillations of the flow rate mainly occur because of the motor steps. The long settling time (time it takes to get and stay within ±5% of the target flow rate), can be caused by the compliance of the syringe walls and to some extent, the microfluidic tubing system.
2. Pressure Controller Solution
Our pressure controllers such as the MFCS™ or the Flow EZ™ use a compressed air source to pressurize a reservoir containing the fluid. The reservoir is connected to the microfluidic chip via a tube. 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. A very precise pressure source will thus deliver a precise and stable flow. The great responsiveness of pressure sources makes it one of the most powerful flow control methods on the market.
By using an additional flow rate sensor, one can even use a pressure controller to control flow rate directly. When a microfluidic channel is clogged, a syringe pump continues to perfuse leading to an increase of pressure or leakage and in the worst case to the burst of the chip. The LineUP system, controlling and monitoring both pressure and flow rate contains a simple way of limiting the pressure and/or flow rate. In a way, associating a pressure source with a flow rate sensor combines the main strengths of syringe pumps and pressure solutions. The user no longer has to calibrate or evaluate the fluidic resistance of its system, but can rely on a feedback loop to control the flow. The pressure controllers, sensors, and software sold by Fluigent are able to deliver stable and precise flow rates with very low settling times.
3. Passive methods for flow control and flow measurement in microfluidics
These last category moves the fluid by using non-mechanical energy present in the fluid or the system. This can be done using hydrostatic pressure, capillary forces, electromagnetic fields or electroosmosis (chemical energy).
Paper microfluidics is an emerging field that uses paper as a support for fluidic experiments. The microscopic structure formed by the paper fibers created a network of small capillary channels. A fluid is drawn in by the capillary forces and will thus spontaneously start moving . In a similar manner, we can also use small tubes in order to draw the liquid through channels.
Flow Rate Measurement in microfluidics
Live flow monitoring can be achieved using flow sensors. The solution sold by Fluigent is called the Flow Unit and can be used with the FRP or directly with the LineUP series for direct flow rate control.
An immense variety of microfluidic flow sensors using different fields of physics exist. Not all of them are suitable for flows in microchannels. Choosing the right microfluidic flow meter adapted to the flow regime and fluid is critical for accurate measurements. The next sections will describe some of the main technologies used for flow measurement. One can divide the solutions into thermal and non-thermal ones
1. Thermal sensors for flow measurement in microfluidics
A very common technology relies on the calorimetric method. A micro heater provides a minimal amount of heat to the medium monitored (around 1°C). Two temperature sensors, located on both sides of the heater, detect any temperature variation. The flow rate is then calculated based on the spread of heat, which is directly related to the flow rate.
This method of monitoring the flow is one of the simplest and least intrusive. It is quite easy to integrate in MEMS devices, since very small heaters and sensors already exist. Nevertheless it requires knowledge of the fluid density and specific heat capacity. These values also need to be constant for proper function of the sensor. In biological experiments, the presence of cells or particles in the fluid might affect the fluid properties and affect the measurement.
Several other thermal flow meters exist and function in a similar way . The hot wire uses a resistor as heater and sensing element. The resistance being dependent on the temperature, a relationship between applied tension, temperature and resultant resistance can be established. Other sensors use so-called “time-of-flight sensing”. This technique described in the figure uses only one sensor that is located downstream of the heater. By observing the heat distribution over time, it is able to deduce the fluid velocity and thus the flow rate.
2. Mechanical flow sensors for flow measurement in microfluidics
One of the most important groups of non thermal flow measurement techniques are the mechanical flow sensors. Since flow is usually laminar in microfluidics, laws to calculate the drag force on the channel walls and the pressure drop in the direction of flow are known. Both drag and pressure drop are directly related to the flow velocity. By using piezoresistive transducers or integrated pressure sensors, it is possible to deduce the flow velocity and thus the flow rate in the sensor .
3. Coriolis mass flow meters in microfluidics
The use of mass flow meters in microfluidics is growing as the technology is getting improved for microscale flows. In a mass flow meter operating on the “Coriolis principle” the fluid flows on a vibrating channel. The Coriolis force acting on the moving fluid will affect the frequency, phase shift or amplitude of the initial vibration proportionally to the mass flow rate. The main advantage of Coriolis mass flow meter is the independency between the measured flow rate and the properties of the liquid. These sensors can monitor gas or oil flows without any specific calibration. However, the technology remains expensive and the small inner diameter of the fluidic path might not be suitable for biological experiments.
4. Other technologies for flow control or flow measurement in microfluidics
Apart from mechanical technologies, a lot of different non-thermal solutions for flow measurement exist. Some of them involve optics, acoustics or electrochemical phenomena.
Conclusion of the review about the different technologies for flow control and flow measurement in microfluidics
We have seen that there is are a variety of flow rate control solutions when it comes to flow control and flow measurement. This review aimed to show strengths and weaknesses of the main technologies used in microfluidics. For controlling the flow, the main solutions, in microfluidics, are mechanical or pressure based. Mechanical solutions allow direct flow control but do not control the pressure that is applied to the fluid. The pressure based solutions ensure higher stability and faster responses and can even monitor and control the flow directly with the help of an additional flow sensor. Many different microfluidic flow sensor technologies have been studied and developed. The two main categories are thermal and mechanical sensors. Depending on the flow rates and regimes, one has to evaluate the right choice of sensor for his experiment.
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 X. Li et al, Biomicrofluidics, 2012, (6)
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