Researchers’ opinion on flow control for microfluidics
One of the greatest challenges of researchers using microfluidics is miniaturizing analysis processes in very small microchips. Whether it is named MEMS, lab on chip or microTAS, miniaturization presents several advantages in reducing the size of analysis processes: Analysis is getting cheaper, faster and more efficient.
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Issues of having a suitable instrumentation
For on-chips applications, instrumentation for flow control is a key element as assays performances mainly depends on the instruments used.
There are as many instruments available as there are applications. Since each instrument has specific strengths and weaknesses, researchers using microfluidics must be aware of all of the pros and cons of each instrument to choose the most suitable system for their applications.A hundred of researchers using on-chip microfluidics were interviewed about their instrumentation and their opinion about it.
Choice of technology for microfluidic flow control
The same way as droplet-based microfluidics, the majority of researchers interviewed uses syringe pumps technology for on-chip flow control. It is the most common device for flow control and the choice of using syringe pumps is mainly based on their habits and equipment of their lab.
(*)This study is based on the kind answers given by researchers using on-chip microfluidic instruments [1-36]
However, a significant part of these researchers has also recently moved to pressure-driven flow for their on-chip application. Capillary and valves systems are valued for several reasons detailed in the next paragraph.
Vaccuums are systems used by researchers who want to control their experiments at the outlet of their channels, or researchers who intend to generate reverse flows.
Advantages & disadvantages of flow control instruments
1) Syringe pumps
Syringe pump is the most commonly used device for flow control. Inspired by medical devices, syringe pumps have been widely applied for microfluidic applications. Most of the syringe pumps are based on a syringe driven by a motor and a rotary screw.
- Easy to setup and control
- Precise flow rate control at high flow rate
- Wide commercial availability due to variety of providers
- Good reproductivity of the assays
- High cost of the syringe
- Generation of cyclic variations in the flow rate due to the rotary screw
- Poor responsivness at low flow rate
- Occlusions may occur and increase the fluidic resistance until the device burst
- Limited amount of fluid
- Incompatible with a valve-based closing system
2) Pressure controller
Pressure controller is an alternative to syringe pumps and is based on a simple concept. Reservoirs of fluids connected to the chip are pressurized thanks to an on-air pressure controller. As fluids are incompressible, flow control is perfectly operated thanks to the pressure controller.
- Ease of setup and control
- Fast response time (< 10 ms) for a large range of flow rates
- Pressure sources enable flow without oscillation
- The amount of dispensed fluid can vary between a few mL and several liters with the same performances
- Control of fluid in dead-end channels
- Using a pressure controller does not enable to know the flow rate (*)
- Flow rate varies with fluidic resistance when controlling flow in pressure(*)
(*) can be overcome with pressure source including flow rate feedback loop (more informations)
Micropumps are commonly based on valves systems. They usually result of a sequential opening and closing of various valves operated by a mechanical, pneumatical or electrokinetical system.
- Low-cost and fast fabrication
- Easy operation and maintenance
- Little dead volume
- Require external hardware to control valves opening / closing sequences
- Oscillating flow rate due to valves opening / closing sequences
Capillary flow control does not require any external device. Thanks to their hydrophilic walls, microchannels of the chip spontaneously fill with liquids.
- No need any external pumping system
- Useful to reduce the amount of external hardware
- Little dead volume
- Flow must already be preset
- Lack of interactivity: it is almost impossible to change the flow rate
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