WHY choose a microfluidic device instead a robot?
Microfluidics is a very attractive technology for both academic researchers and industrials since it considerably:
Microfluidic chips in silicon
Microfluidic chips in glass
Microfluidic chips in polymers
- Oxygen and gas permeability
- Optical transparency, robustness
- Non toxicity
One of the main drawbacks of PDMS chips is its hydrophobicity. Consequently, introducing aqueous solutions into the microchannels is difficult and hydrophobic analytes can adsorb onto the PDMS surface, thus interfering with analysis. There are now PDMS surface modification methods such as gas phase processing methods and wet chemical methods (or combination of both) to avoid issues due to hydrophobicity2. Another main issue of PDMS chips is that they are non-suitable for high pressure operation as it can alter channels geometry4.
Some key information:
- Transparent materials are favored to enable optical observation / analysis.
- Materials must be biocompatible for biological applications.
- Most of the chips need surface treatment to adapt their surface properties to the application and to limit non-specific adsorption.
How microfluidic can help you?
In other words it will deal with very precise control and fluids manipulation, under small volumes and space, typically sub-millimeter, scale. micro means one of the following features:
Active microfluidics refers to manipulation of fluid by active components as micropumps (See our MFCS™-EZ) or micro valves (see our ESS™ platform). Micro pumps supply fluids in a continuous manner or are used for dosing whereas the micro valves can determine the flow direction.
Microfluidic is often used and described in lab on chip technology, referencing also to organ on chip. However microfluidic, such as described before, can be applied to a wide range of application as example cosmetics (for mixed perfume sampling), 3D printing etc.
Every year, millions of individuals suffer from symptoms of pathogen related illnesses such as the flu, but new microfluidic chip technology may be able to decrease instances of these ailments.
Using chips anywhere from the size of microscope slides, to the size of quarters, researchers are developing a mechanism to read a patient sample and diagnose a variety of viral diseases in under an hour.
Currently, the RT-PCR laboratory test for flu diagnosis can take up to a week to deliver fairly accurate results. For severe cases of the flu, such as those during the 2009 H1N1 pandemic, a week without diagnosis can lead to unaggressive treatments as the virus advances further and worsens symptoms.
With microfluidic chips, a nasal sample can easily be taken from a patient and placed on a chip containing a network of intricate 3-dimensional tubing. These unique tubing designs resemble the environment a virus may encounter in the human body, such as the twists and turns of blood vessels.
By recreating the biological environment, a virus can be replicated within the chip, allowing an external reading device to recognize the virus’s genetic composition so a physician can make a proper diagnosis.
Microfluidic properties and notions
In microfluidic laminar flow occurs:
Figure 2: Coflow example: Operation of TWIST valves. Weibel D. B., Kruithof M., Potenta S., Sia S. K., Lee A., Whitesides G. M., Torque-actuated valves for microfluidics, Anal Chem 2005, 77:4726-4733. Copyright 2005 American Chemical Society. Figure 3: Illustration of turbulent and laminar fluid flows.
As the dimensions of the microfluidic devices are reduced, some physics characteristics are different compared to conventional laboratory-scale assay. For instance, physics laws for fluids at microscopic scale are different from laws at macroscopic scale, resulting in laminar fluid flows
For these reasons, dedicated microfluidic instruments have been developed to precisely control fluids (liquids or gas) inside microchannels. Some of these systems can be used directly inside the chip (electrodes, valves, etc) while some others are used as external actuators or accessories such as flow controllers (pressure pumps, syringe pumps, peristaltic pump, etc) or external valves.To learn more about these technologies: click here
Flow actuation system responsiveness
Characterizes by the RESPONSE TIME, the RISING TIME, the SETTLING TIME.
Increasing the flow resistance…
Increasing the external flow resistance improves flow stability and flow rate resolution
Microfluidic stability and resolutions
The ability to provide a stable flow has a key impact on the results.
Internal, dead and swept volume
You need to know what is the total volume of your fluidic circuit. Fitting like unions, adapters or tees can have an enclosed volume that will participate to the total volume of the circuit.
Electrical analogy and ohm’s law
Microfluidic flows are characterized by the prevalence of the viscosity effects compared to inertia. From a physics point of view, this behavior is pointed out by a low Reynolds number. It leads to a drastic simplification of the complex Navier-Stokes equations describing fluid mechanics. Thus, a very simple equation linking :
- Mean flow-rate
- And microfluidic resistance can be deduced based on the electrical analogy (Ohm’s law)
Based on the electrical analogy (Ohm’s law).
Formula and definition of the microfluidic resistivity
This analogy points out the microfluidic resistance which quantifies an energy drop along the microfluidic channels. As in an electrical network, equivalent microfluidic resistances can be calculated to simplify modeling of complex microfluidic devices or Lab-On-Chip with multiple parallel and/or series channels.
- Geometrical parameters of the channel
- And the characteristics of the fluid that flows inside it
The microfluidic resistance formula for the two most common microchannel shapes is given below.
There are 2 kinds of microfluidic resistance…
In a microfluidic set-up, two kinds of flow resistances can be distinguished, depending on the impact they have on the experiment:
The external flow resistances
Induced by the tubing used to connect the microchip to the flow control system (pressure pump, syringe pump, peristaltic pump, etc). By applying the electrical analogy, this kind of resistance may become an easy and powerful solution to enhance the performances of flow control systems.
The internal flow resistances
Induced by the microchip design. Internal flow resistance management may be used such as a passive flow control system: in many applications, microfluidic circuits are designed to create dedicated functionalities such as gradient generator, droplet merging and splitting, passive valves, cell trapping, digital microfluidics(microfluidic droplets).
Increasing the flow resistance…
Based on the Ohm’s law analogy, increasing flow resistance minimizes the impact of any pressure fluctuation. This behavior can be used in synergy with a microfluidic flow control system based on pressure actuation. In this case, increasing the external flow resistance improves flow stability and flow rate resolution:
…Maximizes the flow stability
Each pressure flow controller has a given stability (often presented as a percentage of the full-scale pressure range). For any required pressure set-point, the applied pressure fluctuates around the pressure set-point modulo the stability. If the resistance of the set-up is very low, these pressure variations may lead to flow fluctuations conflicting with the requirements of the experiment. Increasing the flow resistance maximizes the flow stability.
…Allows to enhance the flow rate resolution
Each pressure flow control system has a given resolution based on its pressure sensor performances (often presented as a percentage of the full-scale pressure range). A small flow resistance may lead to unreachable flow rates. In this case, increasing the flow resistance allows to enhance the flow rate resolution for a given pressure actuator.
Of course, increasing the flow resistance leads to increase the pressure applied to the microfluidic device by the flow control system to generate a flow actuation.
How to modify your flow resistance?
The easiest way to modify the flow-resistance of a set-up is to change the external resistance. Indeed, as the resistance formula for circular shape tubing shows us, the inner diameter of the tubing has a key impact on the resistance. So the flow resistance can be drastically modified by selecting a tubing with the relevant inner diameter. Then, it can be adjusted sharply by modifying the length of the tubing, as these parameters are linearly linked. Thanks to this strategy you may avoid any modification on the microchip design.
Flow resistance management may be a powerful tool to increase the flow control performances of a flow control device based on pressure actuation. Fluigent provides pressure actuation systems with the highest stability and resolution (0.03% of the pressure range) that enables to reach the best performances without resistance management.
During microfluidic experiments, the ability to provide a stable flow has a key impact on the results. Unstable flow-rate often leads to poor experimental results and enhances the complexity of the experiment by adding undesired noise, pulses, poor repeatability or decreasing the experiment performances. Microfluidic flow control instruments are designed to provide superior enhance the flow stability.
Flow resistance impact on volumetric actuators
For the volumetric actuation systems, such as syringe pump or peristaltic pump, increasing the flow resistivity leads to enhance the flow rate stability but will drastically increase the flow rate settling time. You will find more information on the article: Stability & resolution.
Difference between resolution and stability
When we talk about stability, confusion can be made between resolution and stability. Actually, three parameters have to be distinguished:
The flow stability
is the ability of the flow control device to limit the fluctuations, or variation of the physical parameter controlled. The lower the value of the stability is, the more stable is your flow.
The dynamic resolution
is the lowest variation of a physical parameter that can be reached in control mode by a microfluidic flow control system. For example if you are using a control system based on pressure actuation, the dynamic resolution is the lowest difference between two pressure orders that can be performed by the device. The dynamic resolution is highly linked to the regulation algorithm of the microfluidic instrument and its internal actuators and sensors. The lower the value of the dynamic resolution is, the sharper you can tune your flow. Regarding microfluidic controllers based on pressure actuation, the dynamic resolution is often given as a percentage of the pressure full scale range.
The measurement resolution
is the lowest variation of a physical parameter that can be detected by a sensor. If you are using a flow control device based on pressure actuation, the measurement resolution is the lowest pressure variation that can be detected by the internal sensor. Flow control systems based on a feedback loop cannot provide dynamic performances (stability, dynamic resolution) better than the measurement resolution.
The flow controller 1 has a better measurement resolution and a better stability. Its dynamic resolution is the same as the dynamic resolution of the flow controller 2. But thanks to its better stability, the stability bands of the flow controller 1 (before and after t=50 unit of time) do not cross each other. A pressure band (from 200,5 to 201,5 pressure unit) is not covered by this controller. Enhancing the dynamic resolution would solve this problem.
The flow controller 2 has a worse stability and measurement resolution: its stability bands cross each other. There is no pressure band uncovered by this flow controller but it provides your experiment with lower performances.
Definition of the stability
When choosing a flow control system, it is important to independently analyze these three parameters:
The stability band
is the band defined by the maximum and the minimum value measured for a requested set-point. For a pressure driven flow control system, this band is often given as a percentage of the pressure range.
is a statistic tool used to quantify the variation of a parameter compared with the average value. Assuming that the physical parameter to be controlled fluctuates around its means value with a normal (Gaussian) distribution, the standard deviation can be used to define confidence intervals.
The coefficient of variation
is another way to use the standard-deviation. This parameter is given as the standard deviation divided by the mean value of the parameter measured. Contrary to the standard-deviation, the coefficient of variation is a dimensionless quantity.
Remarks about the microfluidics flow controllers based on mechanical displacement
Microfluidic flow controllers based on moving part, such as syringe pump and peristaltic pump, usually do not have internal flow rate or pressure sensor. Their operating principle is based on the control of a moving part and the mathematical relation between the mechanical movement and the flow-rate but not on real time measurements. It means that the only information available is theoretical flow-rate values. But these theoretical values may be impacted by a lot of internal and external parameters (see table 2). So, during the experiment, it will lead to a lack of stability, unrepeatability and decreasing performances without the user even knowing it. Besides, it doesn’t provide any information about whether the flow-rate orders have been reached or not.
For these reasons, all our microfluidic flow control solutions integrate high precision pressure or flow rate sensors. Besides their high stability (0.1%CV1) and high dynamic resolution (0.03%2), they are controlled by the powerful MAESFLO™ software which displays real time measured values with Flow Rate Control Module (FRCM).
Key factors of stability
DEFINITIONS: RESPONSE TIME VS SETTLING TIME
The response time
is the time elapsed between the inquiry of a set-point by the user and the first reaction of the microfluidic flow actuation system. It shows how fast the actuation system starts to react when a new order is requested. This time factor includes:
- The time elapsed to process information/the inquiry and to send it to the mechanical part of the microfluidic flow control instrument.
- The time elapsed by the mechanical part of the flow control instrument to do the first move.
The rising time
is the time elapsed to go from 10% to 90% of the set-point value requested. However, it does not give any information about the fact that the set-point is reached. This factor is linked to the transient state between the previous set-point and the new set-point requested. An actuation flow control system may have an excellent response time but provides you with long transient state.
The settling time
is the time elapsed to get and remain within an error band (±5%) of the final value. In other words, this factor gives information about how fast the flow actuation system reaches the set-point in a stable way. This factor is the time elapsed to reach the steady state induced by a new set-point.
- So when choosing a microfluidic instrument, you need to look not only at the response time but mainly at the settling time to conduct your microfluidic experiment at steady state as fast as desired.
- For these reasons, all our MFCS™ have not only a short response time (down to 40ms depending on user PC operating system) but they also offer a minimal settling time (down to 100ms) to always stay at a stable state.
INTERNAL VOLUME/ DEAD VOLUME / SWEPT VOLUME
Most of the time, you need to know what is the total volume of your fluidic circuit. Fittings like unions, adapters or tees can have an enclosed volume that will participate to the total volume of the circuit. This is referred in the specification sheets as “internal volume”. The internal volume is the sum of two components: the “swept volume” and the “dead volume”. The dead volume and the internal volume must not be considered as the same data.
- The swept volume is the portion of the internal volume that is directly in the flow pathway: fluids are bound to flow through this volume when flowing through the fitting. It is generally best to have internal volumes as low as possible.
- The dead volume is the portion of the internal volume that is out of the flow path. It means that the liquid that will go in this area – or the molecules that will diffuse there may not be recovered, or may be recovered later on. It is a kind of “buffer tank”. Of course, all manufacturers of fluidic parts try to minimize as much as possible the dead volumes in their fluidic paths.
If the connection with the tubing is not optimal, a supplementary internal volume can be created. To avoid this, ensure that all tubing are fully seated and tightened at all times. A good practice is also to try to match the tubing ID as closely as possible to the diameters of the port holes. This helps the liquid to flow directly and completely into the port hole from the tubing, and reduces the amount of turbulence experienced by the fluid.
Coline Lemang, R&D Engineer
- 1-All about fittings, A pratical guide to using and understanding fittings in a laboratory environment, John W. Batts IV. Copyright © 2011 IDEX Health & Science LLC.
- 2-IDEX Health & Science 2011-2012 catalog, Fluidic Products & Information for Laboratory Applications. Copyright © 2011 IDEX Health & Science LLC.
Presentation of microfluidic flow control technologies
In order to perform efficient experiments in microfluidics, you need to master the different technologies available to use the most suitable way to control microfluidic flows. The present article aims at presenting a short review of the existing techniques.
To do an appropriate choice, it is important to pay attention to the following elements:
- The flow rate or the pressure range you need. Generally speaking, droplets generation needs high pressure and high flow rate. On the contrary, cell manipulation or cell perfusion needs very low flow rates. Nanofluidics needs high pressure and low flow rate.
- How quickly you need to set or change the flow rate: it may be important for you to quickly change the flow rate in order to test more conditions of your experiment in a shorter time. For some applications as cell manipulation and cell capture (with optical or magnetic tweezers for example), a very short settlingtime is necessary to properly control the position of the cell in the capture area or to keep it in the observation area.
- How stable you need the flow rate to be: in some cases, the pulsation of the flow generated by poor micropumps can be more important than the phenomena you want to measure! It is particularly important for droplet generation where the droplet size standard deviation is in direct relation with stability of the flow.
Applying a pressure difference: the fluids will move because of...
- Applying a pressure difference: the fluids will move because of the pressure difference, according to a simple relation, similar to the Ohm law for electricity: Flow rate = Pressure / Resistance. The fluidic resistance is a function of the geometry of the channel and the viscosity of the fluid. For more information on the microfluidic resistance, click here
- Using volumetric pumps as syringe pumps or peristaltic pumps: here the principle is to use a mechanical movement that modifies a volume to apply a flow rate. For syringe pump, modifying the syringe volume and the infusion rate enable to achieve different flow rates.
- Others techniques as electro-osmotic pumps or integrated micropumps are more specific and are described below.
Based on these methods, different techniques and technologies are available to manage microfluidic flows:
Based on these methods, different techniques and technologies are available to manage microfluidic flows:
1. Using hydrostatic pressure
This is the simplest way to move fluids: the basic idea is to put the inlet reservoir higher than the outlet reservoir in order to let the gravity force move the fluid from the inlet to the outlet.
Using water, 1cm (1 inch) of level difference generates a pressure difference of 0.981 mBar (2.49mBar). Because of obvious practical constraint, it is hard to get a level difference larger than 100 cm or 3 feet, limiting the pressure to around 100 mBar. The stability is quite good in the short time range but bad in the long time range: the flow will empty the inlet reservoir and fill in the outlet reservoir, modifying the level difference and so the pressure difference over time. It means that you need to move the reservoirs while the liquid flows if you want to obtain a stable flow rate.
Besides, the responsiveness is also dependant on how quickly it is possible to move the reservoirs during the manipulation. Another limitation is the sensibility to air bubbles: typically an air bubble in a 50μm microchannel generates a Laplace pressure of 28 mBar for water, equivalent to 27.4cm. So large modifications of the level difference are necessary to keep the flow rate constant in presence of air bubbles in your microfluidic chip. Due to these constraints this technique is not adequate for complex microfluidic network and is very difficult to automate. Besides, it is only suitable for pressure lower than 100mBar.
2. Pressure generator or pressure pump
Figure 1: Microfluidic set-up with MFCC™-EZ pressure controller
The resolution of the generated flow rate depends on the resolution of pressure sensor in the pressure pump. For instance, all our MFCS™ (Microfluidic Flow Control Systems) offer a resolution of 0.03% full scale (pressure sensor resolution) as well as a stability of 0.1% CV on measured values. In term of responsiveness, the typical response and settling times of a pressure pump are less than 1 minute but it can be lowered depending on the technology of the pressure controller. For instance, based on the FASTAB™ technology, the MFCS™ serie have a response time < 40ms and a settling time < 100ms. Besides, pressure generators can be controlled by computer and the different steps of the experiment can thus be automated. The pressure pumps, also called pressure controllers or pressure generators allow efficient experiments with complex microfluidic network with several inlets and outlets. This technique can be associated with flow sensors and adequate algorithm in order to reach the desired flow rate by automatically tuning the applied pressure. To learn more about this solution, have a look at the Flow Rate Control Module (FRCM™) to control your flow rates while keeping the benefits of the pressure actuation.
3. Syringe Pumps
4. peristatic pump
The liquid is contained in a flexible tube and alternative compressions and relaxations will draw in the liquid and propel it away. The pulsation of the flow is very high leading to stability around 20% CV, and mainly dependant on the number of rollers compressing the flexible tube. Also it is necessary to calibrate at least once a day the peristaltic pump to get a precision better than 5 %. Compared to syringe pumps, the flow pulsations are more important but they can draw in liquids. To achieve different flow rates, it is necessary to change the inner diameter of the flexible tube.
5. Electrosmotic pump
The main idea is to create an electro-osmotic flow in a porous media made of glass: an electric potential is applied between the porous media and depending on the ionic force of the liquid, a flow rate will be generated. The flow rate and pressure are typically limited to 150μl/min and 10 PSI. The main advantage is that there is no moving part and it can be controlled directly with an electric signal. The main drawback is that it works only with water or alcohol and the relation between the flow rate and the applied voltage depends on the ionic force: in practice, it can only be used for liquids with constant chemical properties and a calibration is needed.
6. Integrated micropump
Several kinds of integrated micropumps exist: there are mostly based on a peristaltic principle with flexible membrane made of PDMS. The flow rate range is typically low but it can achieve 10 to 100μl/min. The main advantage is the opportunity to control fluids down to the pl (10^-12 l) range. It is particularly useful for applications where a very low quantity of product is to be tested: for example when testing drugs or when screening for protein crystallization conditions.
The integration in the microsystem needs complex steps of micromachining and needs a specific design dedicated to the application.